Ionization Chambers

Introduction

These "ion chambers" are nothing more than a bare wire stuck through a hole into a metal can! No special gas or sealing is required. For best performance it is probably a good idea to add a desiccant to the inside of the can to keep the humidity low. (I didn't!) Build one; its really simple!

When ionizing radiation (ultra-violet light, x-rays, etc.) pass through a gas, collisions with the gas molecules produces ion pairs, typically charged molecules and free electrons. If an electric field is present, the ions will move apart, each moving in opposite directions along the electric field lines until they encounter the conductors that are producing the electric field.

An ion chamber is an extremely simple device that uses this principle to detect ionizing radiation. The basic chamber is simply a conducting can, usually metal, with a wire electrode at the center, well insulated from the chamber walls. The chamber is most commonly filled with ordinary dry air but other gasses like carbon dioxide or pressurized air can give greater sensitivity. A DC voltage is applied between the outer can and the center electrode to create an electric field that sweeps the ions to the oppositely charged electrodes. Typically, the outer can has most of the potential with respect to ground so that the circuitry is near ground potential. The center wire is held near zero volts and the resulting current in the center wire is measured.

The voltage required to sweep the ions apart and to the center wire or outer can before a significant number of them recombine or stick to a neutral molecule is usually under 100 volts and is often just a few volts. In fact, if the voltage is above a couple of hundred volts, the speeding electrons will produce additional ion pairs called "secondary emissions" giving an enhanced response. Geiger tubes operate at even higher voltages with a special mixture of gasses and exhibit a sudden and very large discharge for each ionizing particle. But below 100 volts the only current is the ions produced by the radiation. The resulting current is extremely low in most situations and detecting individual x-rays is difficult, especially with ordinary air at atmospheric pressure. Usually the capacitance of the electronics connected to the center wire smoothes the individual pulses too much for detection even when feedback is used to greatly reduce the time constant. These room-pressure chambers therefore respond to the average level of ionizing radiation and do not provide "clicks" like a Geiger counter tube.

Homebrew

Sensitive homemade ion chambers for detecting nuclear radiation are fairly easy to build but the circuitry is tricky and should only be attempted by "seasoned" experimenters - the currents are likely to be well below 1 pA unless there is a serious nuclear war in progress! (The simple version is "beginner friendly"!) Special electronics is needed at the front end, typically called an "electrometer" circuit, which produces an output voltage in proportion to the input current. The electrometer must have a very low bias or leakage current to avoid masking the desired signal and the intrinsic impedance of the amplifier must be extremely high. The input impedance of the electrometer may be fairly low, however, using feedback to convert the tiny current into a usable voltage.

Older designs used special electrometer tubes like the 5886 which requires only 10 mA at 1.25 volts for the filament and about 10 volts for the plate. These tubes are great for the experimenter because they are relatively immune to static discharge and they consume about the same amount of power as a typical transistor stage. Some electrometers use vibrating capacitors or mechanical choppers to convert the tiny DC currents into AC before amplification to avoid DC bias and leakage problems. Newer circuits typically use MOSFETs or Electrometer grade JFETs in the front-end. MOSFET op-amps usually contain protection diodes which can be responsible for several picoamperes of leakage at room temperature and a fairly steep increase in leakage as the temperature increases but in some ion chamber applications this extra leakage is tolerable. Non-protected MOSFET front-ends are easily damaged by static electricity and special low-leakage protection diodes are usually added. Low current JFETs like the 2N4220 give respectable performance and the types intended for electrometer applications like the 2N4117A are quite impressive, exhibiting leakage well below 1 pA. They have the added benefit of being significantly less sensitive to static electricity than unprotected MOSFETs. Full ESD precautions must be observed with any of these approaches!

Big Resistors!

As mentioned earlier, most electrometer circuits use feedback to reduce the effective input impedance and to direct the tiny input current through a very large feedback resistor such that a reasonable voltage is produced at the output. The feedback resistor must be quite large, however. If the input current is 1 pA and the feedback resistor is 100 megohms, the output voltage will only be 100uV. Special resistors measuring in the millions of megohms are available but are usually difficult for the experimenter to obtain (see http://www.ohmite.com/catalog/v_rx1m.html , for example).  Recently, I stumbled upon a quantity of 13,000 megohm resistors - send me an email if you want one. That same 1 pA would give 13 millivolts which is easily amplified with ordinary op-amp circuitry.

An Experimental Circuit

An extremely sensitive circuit was desired that didn't require special resistors and that didn't fail every time a slight ESD mistake was made and the result is the experimental circuit shown below. It uses a 2N4117A  as the input amp and another as the feedback resistor. If one studies the tiny curves supplied in the data books and uses a little "extrapolation" and imagination, the leakage of the 2N4117A with the drain and source connected together can be seen to have a slope equivalent to about 75 million megohms! There is unit-to-unit variation and it is necessary for the input JFET to have lower leakage than the feedback JFET so the circuit is not for everyone. (If the input JFET leakage is higher, the output voltage will be very low. Simply swap the JFETs.) The FETs are easily damaged, too, which can lead to frustration when the "best one" gets zapped. The circuit will not be particularly accurate since the actual feedback resistance is not known (nor linear) but the experimental ion chamber is not easily characterized anyway. Despite the circuit's shortcomings, it is extremely sensitive and surprisingly stable. A "good one" might drift only 0.1 fA in a day if ambient conditions are relatively constant. (That corresponds to about 10 mV drift on the output.) The short-term variation is below 1 mV which corresponds to 0.01fA! If the ion chambers really work, this circuit should be able to see the current!

The input FET (the one on the right) and the two transistors form an "error" amplifier that attempts to maintain the drain voltage at 10 volts (set by the resistor divider in the emitter of the NPN). If current flows out of the ion chamber causing the gate voltage to rise, the drain will begin to drop and the voltage on the collector of the NPN will go up. This rise will decrease the current in the PNP and thus lower the output voltage. The voltage drop across the first "resistor" FET will increase and more current will flow through it - nearly all of the ion chamber current, in fact. There is sufficient loop gain that the input voltage does not change very much and most of the ion chamber current flows through the feedback FET. The zener in the source of the input FET moves the gate voltage operating point up above ground so that dual polarity supplies are not needed. The output voltage should be a few volts, perhaps 3 or 4, depending upon the relative leakage of the FETs. If the voltage gets too near 6 volts, the sensitivity will drop and the response will become more logarithmic (which might be useful for some applications). If the voltage is too low, the circuit might "bottom out" and loose control. There isn't much that can be done to set the operating point expect swap out FETs! The glass around the FET leads must be VERY clean. Use a good solvent to remove any contaminants.

Ion Chambers

The first experimental ion chamber was made with a zinc can from a D-cell battery and an old 8-pin glass-to-metal header as seen in the photo below. The two FETs were mounted inside the chamber with the theory that this would eliminate the problem of connecting the extremely high impedance probe to the outside world without creating leakage paths to ground. The problem with the concept is that the transistor bodies and leads compete with the wire for the free ions! Carefully painting the transistor bodies and legs with conformal coating helped but the circuit will not tolerate the coating around the base of the transistor - it is too conductive! (In retrospect, the transistors should reside in their own can with the sense wire passing though a hole into the ion chamber which is what the schematic shows.) The pickup wire should be thin and near the center of the can to keep the capacitance low so the response time is as short as possible. When power is first applied, it can take a very long time, maybe 20 minutes, before the circuit settles out to a steady reading. At first, about 150 volts was applied to the can but it was soon discovered that only a few volts are adequate and a 9 volt battery was used instead. The 15 volt power supply voltage should be fine for most ion chamber sizes. If the voltage is too low, the readings will be low as the ions have time to recombine before being swept to the electrodes.

To test the chamber, a 1.3" diameter disk of radioactive material (a calibration disc from an old Geiger counter) was leaned against the can. The voltage changed a few 10s of millivolts but I quickly lost interest in this chamber when the lid slipped and zapped the FETs.  I had already spotted an old mint can at the back of the workbench which I liked better for a chamber. (See pictures below.)  An audio connector was added to the center of the 3" dia. tin can and the FETs were mounted directly to the pins. A ring of wire was used for the center electrode. The insides were washed well with a solvent and then dried with a hot air gun before the base was added and tack-soldered. (I should have removed the plastic coating on the inside of the can.)

Little feet were added to the bottom of the can so that I could easily slip my radiation disk underneath without disturbing the chamber. This ion chamber gave gratifying result: the little radiation source gave an output voltage change of about 70 mV which was very large compared to the meter wander of about 2 mV.

At this point in the festivities, I decided to try a crude calibration. Really crude. My calibration reference was a Heathkit Geiger counter which has a meter that reads counts per minute and mR/ hour. The scales are a little suspicious since the CPM scale is an exact power of 10 bigger than the mR/hr  scale. (0.3 mR/hr = 300 CPM on the X1 scale, for example.) It is entirely possible that the Geiger tube dimensions were selected to achieve just this result. In the past I had compared this Geiger counter against another "bomb shelter" type and obtained surprisingly close readings - maybe within 10%. The radiation disk gives 1500 CPM when held directly against the Geiger tubes mylar window and 500 CPM when the lid of a mint can is placed in between (to simulate the ion chamber walls). The background radiation measured about 13 CPM. Now here is where the calibration gets a bit "iffy". The disk is large compared to the Geiger tube window but it is small compared to the diameter of the ion chamber. To make a long story short, the ion chamber will read low by some factor - maybe 4. What I think it all means is that the ion chamber gives about 6 mV for a radiation level that causes about 10 CPM in the Heathkit unit corresponding to 0.01 mR/hr. The background radiation should give a reading just above 2 mV which is about how much the readings wander from minute to minute. This calibration may be within one order of magnitude!

But now I am hooked. I want an ion chamber that can easily see the background radiation. To make a bigger chamber, I chose a 4.5" by 4" diameter peanut can (see below). I also decided to move the FETs into their own compartment. For this compartment, I used a steel wheel from the center of an electronic component reel. Any small can would work here, but this piece fits nicely on the bottom of the peanut can and it had a hole perfect for the 8-pin header that I happen to have in large quantities. A hole was drilled in the bottom of the peanut can for the electrode. No insulator was used - just the air gap. You can see the wire sticking up from the FETs in the first picture and the wire is visible passing through a hole into the chamber in the second picture. The end of the peanut can was sealed with aluminum foil to keep out air currents and electric fields but to allow less energetic or larger particles in.

The voltage on the outer can was increased to 22.5 volts by using an old 'B' battery on the theory that a larger chamber would need a larger field to sweep out the ions quickly enough.

After power was applied and sufficient settling time went by (about 15 minutes), the meter reading was seen to be significantly more jumpy. The FETs were the same ones from the previous chamber and great care was taken to keep everything clean so I immediately suspected that I was seeing individual ion trails. When I slipped the radiation disk under the aluminum window, the reading climbed to a whopping 1.5 volts! And what a coincidence! The Heathkit gives a count of 1500 CPM for this same source when covered with aluminum foil. (Actually, the foil hardly attenuates the radiation coming from the disk.) So now I have a direct readout of mR/hr: 1 volt = 1 mR/hr. Unfortunately, I have not yet corrected for the much larger detector area of the new chamber but it works out to be nearly 10! So the sensitivity of the new chamber is 1 volt per 0.1 mR/hr which is pretty sensitive! The radioactive element from a smoke detector was held up to the Geiger counter and the count soared to about 22,000 CPM but placing a piece of aluminum foil in between dropped the count to 200 CPM. The ion chamber gave a reading of 200 mV which is in perfect agreement. But I didn't expect agreement since this source is small relative to the Geiger tube, also. The mylar window on the Geiger tube blocks the alpha particles some and this may account for the agreement. These calibrations are really coarse! By using the Geiger counter to measure the background radiation it was determined that the ion chamber should be indicating 13 mV but since the zero setting is arbitrary, it was hard to confirm this level. Reversing the polarity on the outer can caused a shift of about 30 mV (after several minutes of settling) which is about what is expected if the background is near 15 CPM (plus 15 to minus 15 is a total of 30). The experimental setup is shown below:

More Experiments:

I tried a long chamber made from a section of air duct with a thin wire stretched between two Styrofoam plugs:

But then:

Another chamber was constructed with a large cookie tin similar to the peanut can design above. The performance of this much larger chamber was excellent. A single Coleman lantern mantle nearly "pegged" the output. The background radiation gives about 4 mV (400 mV after amplification) which corresponds to 40 fA current. (Some CMOS opamps have input current below 40 fA like the LMC6001 and would work fine without the JFET.) Even though the circuit was given a low frequency response to reduce 60 Hz response, the meter jitters in response to individual ion trails. (The superior shielding of the cookie tins would probably allow for a faster response, if desired, but watch out for circuit instabilities.)

The new circuit includes several improvements. The feedback FET is replaced with a Victoreen 100,000 megohm resistor which is the long glass tube in the photo. A zener diode was added to the emitter of the 2N4401 to increase the loop gain and a .01 uF Miller capacitor was added to reduce the amplifier frequency response (for stability and to reduce 60 Hz gain). An op-amp (OP-07) was added to boost the output by a factor of 100. The "zero" pot is used to set the output to a few volts since the OP-07 cannot swing below 1 or 2 volts out without a negative supply. This pot must be able to be adjusted to the gate voltage and with some FETs the voltage may not go low enough. The symptom will be a high op-amp output voltage. If so,  just lower the 10k resistor or add a 1k above the pot. An additional zero pot for the meter could be added as in the first schematic to get a near-zero reading for the background radiation, if desired.

Notice that the drain resistor was reduced to 125k. This value was experimentally determined by finding the drain current that gives the 2N4117A a near-zero temperature coefficient. The test circuit is simple: connect a sensitive current meter from +10 volts to the drain, ground the gate, and connect the source to ground through a 500k pot. The current is observed at room temperature then the FET is warmed and the current change is noted. The pot is adjusted until little or no change occurs. I heated the FET by touching a warm PTC to the can - probably reaching about 65 degrees Celsius and the final current change was below the current change caused by a 100 uV gate voltage change. (Corresponds to less than 1 fA ion chamber current for 40 degrees.) Room temperature may vary by +-4 degrees which would correspond to a wander of 0.1 fA which is well below the 40 fA background current from the chamber. The bias current that gave this wonderful temp-co was 40uA and since the drain resistor will have 5 volts across it, the desired resistor value is 5/40 uA = 125k. "Your results may vary." Actually, the FETs are surprisingly stable at all currents and the whole procedure may be unnecessary; just use 125k!

Also consider the circuit used in the CDV-715 mod below. The mod is hard but the circuit is easy. Also, my circuits use ultra-low leakage JFETs because I have about a thousand of them but there are also op-amps that can do the job directly. Investigate the LMC6001 (25 fA, tested!). Just leave the FET and source resistor out of the CDV-715 circuit below and connect the negative input of the op-amp directly to the sense wire. Don't connect the 33k battery test resistor.

A 22.5 volt battery was insufficient to capture all of the ions but two batteries (45 volts) seemed to do the job - in other words, higher voltage did not result in a higher reading. Higher voltages may be desired for observing individual events, however, since the ions will be swept to the electrode faster.

Parts Notes:

A commercial Geiger tube is pretty hard to beat for general radiation monitoring but the simplicity and versatility of a homemade ion chamber that requires no special gasses or pressures makes it an attractive alternative for many experiments.

Interesting email from readers

Ion Chamber Bias Supply (Battery Topper)

After discovering that one of my bias batteries was jumping around a few volts, wreaking havoc with the readings, I decided to build a floating, regulated high voltage supply. The result is a micro-power 110 volt supply that runs on an ordinary 6V lantern battery. The circuit is similar to my Geiger counter supply but without the additional voltage multiplier on the output. Current consumption in a typical ion chamber setup is only 150 uA (average) so a lantern battery will last a decade, if the shelf life doesn't get it first. No power switch is needed.

The circuit has an output filter consisting of a 100k resistor and a .22 uF capacitor and a typical experiment will have additional capacitance across the chamber, too.  To get rid of the last bit of wander as the circuit pulses, increase the 100k to 10 megohm since ion chambers don't draw significant current. I left the value lower in case this device is used to drive a heavier load and added an external 10 meghom in series with the output with a 10 uF polyester capacitor to ground right at the ion chamber. The load can't be too heavy, however. This circuit can just barely drive a 22 megohm resistor with the full 110 volts and a 10 megohm multimeter will load it down to about 85 volts. The .22 uF will still bite you when it's charged and a charged 10 uF will really get your attention! So be careful!

The circuit bolts right onto the terminals of the battery so I have dubbed this type of circuit a "Battery Topper". Sorry, I couldn't help it! I actually like the idea of mounting handy circuits right on a lantern battery for quick lab circuits. Solder lugs were added to make contact with the battery terminals and narrow nuts were screwed onto the battery first to raise the circuit up a bit so that the hand wiring underneath doesn't press against the top of the battery. The circuit was built on a piece of countertop laminate.

 Experimenter's Ionization Chamber

Here is a truly simple experimenter's chamber made from an ordinary cookie tin:

This experimenter's chamber is made from a 4" (10 cm) diameter, 5.5" (14 cm) tall tin with a tight-fitting lid. The inside of the can is conductive and does not appear to have the typical plastic coating. A 5-way binding post is mounted in the center of the can and a 4" (10 cm) wire is suspended from the post inside the can. The wire length is short enough to insure that it doesn't touch the lid. Another all-metal binding post and pin are installed in the bottom of the can, and a sheet of gray insulating plastic is glued into place to keep hastily constructed experiments from contacting the can. The electrometer circuitry will be extremely sensitive to stray electric fields, so a shield is mandatory. Another can previously containing mints is pressed into service:

A pin jack is soldered to the tin so that it can be plugged onto the pin on the chamber, and tape is applied to the lip of the can so that the pin is the only connection point. The inside surfaces of the shield are also insulated with tape to prevent accidental contact with the circuitry. The method of connecting the shield isn't critical, and a clip lead will also work; the main culprits are the ever-present, low frequency, line-related electric field and changing electrostatic fields due to movement near the chamber. The wires from the test circuitry can simply slip between the shield and the chamber, or a small notch may be made in the shield to make a little room for a few conductors. The opening of the chamber may be covered by the original lid, aluminum foil, or wire screen, depending on the experiment. Leaving the end open will let in too much stray electric field in most environments.

Here's a simple starter experiment:

The can is connected to the positive battery voltage through a 4.7k resistor, and the meter is connected between the collector of the transistor and the positive terminal of the battery. The meter is on the 1 volt scale for most measurements.

The transistor is an ordinary NPN Darlington type like the MPSAW45A. The resistor can be any value above 1k; it simply limits current in the event of a short circuit. A little piece of double-sided foam sticky tape holds the battery in position.

When a ray passes through the chamber, several atoms are ionized and the positive voltage on the can attracts the electrons. The positively charged atoms wander to the more negative center wire and, upon contact, reclaim their missing electrons. This process results in a current flow in the base of the transistor which is amplified by a factor near 30,000. This higher current flows through the 10 megohm resistance of the meter, producing the indicated voltage. As a point of reference, a reading of 10 mV would correspond to roughly 200,000 electrons per second, so even weak radioactive sources produce large numbers of ions. 

To observe the background and leakage level, the lid is placed on the bottom, the top shield is added and the reading is allowed several minutes to stabilize. The meter settles to a little over 30 mV and exhibits an occasional jump. A camping lantern mantle known to contain radioactive thorium is place in the lid of the chamber, and the lid is secured on the open end of the can such that the mantle is inside the chamber. The meter reading climbs to over 600 mV:

Placing the item to be tested inside the chamber in this manner gives the ultimate sensitivity, but care must be taken to avoid touching the center wire.

This very simple detector demonstrates how easily an effective radiation sensor may be made with a minimum of effort. Below is shown another way to build this simple circuit with even less effort. First, solder the 4" wire directly onto the base of the transistor:

Drill a hole in the can right in the center of the bottom and epoxy the transistor, face-down, such that the wire protrudes into the can without touching the sides. Make sure that no epoxy touches the center lead of the transistor (base lead). The epoxy is too conductive!

Connect two wires to the collector and emitter leads. The picture shows a length of solid copper telephone cable used for the connections. The two blue conductors are connected to the transistor legs and the two orange wires are connected to the can. The blue wires are given a little slack so that the cable pulls on the can connection and not on the transistor legs.

The resistor is connected on the other end of the orange wires. A mint tin is tacked to the coffee can in a couple of places to act as a shield and a wiring reminder is written on the back of the mint tin. A little optional notch is seen near the bottom of the mint tin that allows the phone cable to exit the tin without being pinched. The opening of the coffee can is covered with ordinary kitchen aluminum foil held in place by an elastic band. 

Without a radioactive source, the meter reading settles to 50 mV and placing the lantern mantle on the foil gives a reading of just under 150 mV. The reading is lower because the foil is blocking a significant amount of the radiation from the mantle.

This simple circuit has serious limitations. It is extremely sensitive to the ambient temperature and a slight warming will cause a large increase in the zero reading and the gain. A modern small-signal transistor should be chosen since older or larger die transistors will probably exhibit too much leakage current. Despite these limitations, the simple circuit can be used for sensing unusually high radiation levels, observing sudden changes as when bringing a radioactive item near the chamber window or even for making functional but silly gadgets simply for the entertainment value! Seeing such a simply chamber work so well can provide the motivation to build a more sophisticated circuit, too.

An Improved Transistor Circuit for the Ionization Chamber

The single Darlington circuit could use more gain for detecting weaker sources, but simply adding another transistor exacerbates the temperature drift problem. By building a differential amplifier with one side not connected, a temperature-compensated circuit results: 

If the room temperature increases a little, both sides leak more current but the voltage across the meter stays about the same. The 10k resistors are not necessary but are included to protect the transistors from inadvertent shorts during testing. Another 10k resistor between the battery voltage and the outer can is probably a good idea in the event the can is shorted to the base of the transistor. The schematic shows 12 volts but a 9 volt battery works fairly well. The higher voltage sweeps the ions out of the chamber faster, before they have a chance to recombine so the meter reading will be a little higher at 12 volts. The sensitivity is good; the photo shows the response to a radioactive mantle held about an inch away from an aluminum foil window on the end of the chamber. Construction of the chamber is similar to the earlier devices except the metal end cap from a spool of resistors is used for the shield. This end cap accommodates a multi-pin header that is used to hold the components and also acts as a feedthru. The ion chamber sense wire is soldered directly onto the base of the transistor and is suspended in air by the transistor without additional support. It might be a good idea to use a plastic bead to hold the wire more securely as described above but this unit is well-behaved as-is. The meter should read upscale slightly but, if it doesn't, try connecting the sense wire to the other base and reverse the meter leads. If it still doesn't read a little above zero, clean the transistors with acetone. If that doesn't work, try new Darlington transistors since you probably have a leaky one. Hold a radiation source near the aluminum foil window and the reading should quickly climb. The meter can be a 1 volt digital panel meter or even a digital multimeter set to the 1 volt scale.

Since the original construction of this meter, a couple of useful modifications have been made. A wire shelf was added to allow items to be inserted into the chamber through a small hole in the foil without fear the item will touch the center pin. (The foil is peeled back to reveal the shelf in the photo to the left.)

Additionally, a 100k potentiometer was connected across the battery and the wiper was connected through a 1 megohm resistor to the base of the Darlington that was originally not connected. This potentiometer becomes an effective zero control. The picture to the right shows the reading produced by a 2% thoriated tungsten welding rod inserted into the chamber such that it rests on the new internal wire shelf. The meter was zeroed with the new potentiometer before inserting the rod. The reading climbs quickly to over half-scale and drops slowly back to zero when the rod is removed.

  Modify a CDV-715 Survey Meter for High Sensitivity

This project is not for the faint hearted. It's way up there on the difficulty scale and I only give enough information here for the more advanced experimenter. The circuit would be great for homemade chambers like the cookie tin version above.

This project endeavors to modify an ion chamber style radiation meter, the CDV-715,  so that it has useful sensitivity, taking advantage of the internal ion chamber, high value resistor, and nice packaging. The new circuit increases the electrical gain by 1000, converting the scales from R to milli-R.  I decided to abandon virtually all of the existing electronics for several reasons. The chamber current is converted into a voltage by switched resistors but the ceramic switch will simply leak too much; it can only be used for range switching after amplification. There is no need for 1.5 volts since there will be no filament to heat so the inverter that converts 1.5 volts to 50 volts and 10 volts will be unnecessary. A 9 volt battery will power the electronics and a unique version of the Cockcroft-Walton voltage multiplier will generate the 50 volts for ion chamber bias. The electrometer tube will be replaced by a very low leakage JFET and a micro-power op-amp to save power. According to the manual for the CDV-715, the output should be about 14 fempto-amperes per mR/hour, a pretty tiny current. Using the 220,000 megohm resistor already in the instrument, the corresponding voltage will be about 3 mV per mR/hour which isn't completely unmanageable. If the 100X switch setting is to become 500mR full-scale, the voltage will still only be 1.5 volts when the meter pegs. So, no range switching is needed at the input.

In order to take advantage of the instrument's calibration, the ion chamber should be biased at 50 volts as is done by the original circuit.  A modified Cockcroft-Walton voltage multiplier driven by an astable two-transistor flip-flop boosts the 9 volts to 50 volts. This inverter circuit draws only 100uA but for even less overall power consumption, five 9 volt batteries could be added in series with the main battery to get 45 volts. These batteries could be tiny 10A size batteries with no power switch since there is virtually no current required. I potted my inverter with wax in a little plastic box:

The voltage output is a fraction of a volt below 50 volts with 9 volts power; just right! A very high impedance voltmeter is required to accurately measure the voltage; an ordinary 10 megohm meter will load the voltage down to about 35 volts. As the wax was cooling, I dropped a piece of tinned copper-clad board on top of the wax to serve as a bottom cover and I soldering a couple of wires onto the copper that line up with some of the holes on the PCB for mounting.

I removed all but a couple of the components on the original circuit board, leaving only the 5.6 meg resistor and 0.1uF capacitor used for filtering the 50 volts. I don't show them on the schematic but they are between the 50 volt inverter and ion chamber. Feel free to leave them out; they aren't really doing much.  I mounted the 220,000 megohm resistor to a new terminal bolted to one of the switch mounting holes. The other end of the resistor has a large diameter socket for the pin of the ion chamber and a smaller socket soldered on sideways for the JFET gate. If you decide to not use sockets for the JFET, you must turn power off and discharge the chamber to the negative of the battery every time or you will lose the JFET! (I lost several - bad habits die hard.) I used two traces right below the JFET for its drain and source, cutting them to make two isolated pads. The sockets are sticking up because they will hit the ion chamber if they stick out the bottom of the board.

The op-amp circuit is built on a piece of tinned copper-clad board. One op-amp is used to generate V/2 (4.5 volts) and the other is the feedback amplifier. I used two single micropower op-amps but a dual CMOS type would be fine. I like micropower types to give the best battery life but an ordinary LM358 should be fine. 

I used the switch section with the terminals sticking up (where the high-value resistors were connected) for the power switch and the switch terminals mounted in the PCB for the range switch. The three resistors that set the range may be mounted in the holes where the cal pots were located. However, cut away the trace on the wiper terminal located near the ion chamber pin because that trace goes other places. Feel free to figure out a completely different switching scheme.

If you use the chamber as-is you might have a roughly calibrated unit but if you want maximum sensitivity to any radiation, you may want to modify your chamber. I unsoldered the chamber holding it over the stove (after removing the solder plug in the center to let out pressure). It took two oven mitts and a bit of effort. The heat partially melted the internal plastic liner and the exhaust fan was quickly turned on. I removed the liner and the metal plate, drilled a large hole in the lid, nearly the full diameter, and replaced the disc electrode with a wire loop:

To make the foil window I cut aluminum foil to the outside diameter so that it folded up at the edges when inserted from the back of the can. I then added a band of tin (piece of ground buss) to press the foil against the side and bottom of the can and tack-soldered the tin band into place while pressing. A little conductive silver paste insured good electrical contact and a little bead of glue sealed the seam. In order to take advantage of the thin foil window, I cut a 3" round hole in the bottom of the case directly beneath the ion chamber and covered the hole with some wire mesh.

I reinstalled the battery holder after installing a metal 9 volt battery clip inside. A battery snap connects the battery to the circuit. Total current is about 400uA.

The result is a very sensitive detector! On the X10 scale a single lantern mantle will give a reading of 1 on the meter and there are two more sensitive ranges! (There is a mantle beneath the unit in the picture above.) On the X0.1 scale the meter occasionally jumps up to mid-scale when a "big one" strikes and it gives a 1/2 scale reading for a mantle 5 inches away! There is no provisions for zeroing the meter but setting it near the bottom of the scale on the X0.1 scale seems adequate. The zero pot is touchy on the most sensitive scale and you may wish to tame it down by increasing the series resistors once you know the required voltage. I soldered a wire from the case to the 50 volts, making it long enough to allow the case to be set to the side with the unit open. Alternately, a spring contact could be added to make contact between the case and lid. The round metal plate that was in the ion chamber could be used as a beta blocker by placing it between the aluminum window and the wire mesh. It would be a good idea to add a fresh desiccant packet to the inside of the ion chamber since humidity impacts the sensitivity. I didn't solder the ion chamber back together since it was such a snug fit so it is relatively easy to open for changing desiccant packets. I think I'll add a clip on the inside  of the chamber to hold one. You could just seal the chamber really well but pressure changes might cause problems with the aluminum window. I just had an idea: there is a little solder-covered pinhole in the middle of the back of the ion chamber that could be used as a breather hole. Seal the chamber really well, leaving that little hole open. Now, somehow affix a little metal or plastic box over that hole big enough to hold a desiccant packet. Poke a little breather hole in the box and the air will have to pass through the desiccant to get into the chamber. I'll work on that.

I don't know if all this is worth the trouble but now we know it can be done!

The Polonium Pen

Recent events point to the need for a simple device for testing cocktails and beers for excessive quantities of polonium. The Polonium Pen is a pocket-sized ion chamber with LED readout that is perfect for the job. Simply hold the Polonium Pen over your drink and, if the LED lights up, order something else.

The circuit is similar to the single transistor detectors above and only requires two Darlington transistors, an LED, and one or two resistors along with a battery, power switch and tiny homemade ionization chamber. This prototype was a bit difficult to build, but the pen may be built in any number of ways as long as a couple of critical requirements are met. The wire probe sticking into the ion chamber must not touch anything that is ever-so-slightly conductive, not even most glasses, and the electronics must be surrounded by a metal package to shield it from stray electric fields. Because the ion chamber is so small, the LED will only light in the presence of a significant amount of radiation, most likely alpha particles.

The prototype will light up brightly when held near the Americium bit in a smoke detector's ion chamber and a polonium-laced drink should produce the same reaction. But most radioactive sources the experimenter is likely to have will not produce any response at all, placing this in the novelty category for most of us. The LED also comes on when the button is first pressed for a couple of seconds before fading out, hence the lit LED in the photo! This "feature" insures that the pen is working properly.

The first prototype required a high-value resistor (66 megohms) across the emitter-base of the PNP darlington to get the LED to go out but the second unit didn't need one, possibly due to superior insulation of the sensor wire. A value as low as 22 megohm is fine and even lower is probably OK for detecting really "hot" drinks! The prototype's ion chamber is made from a 1.5" long piece of 11/32" dia. brass tubing and any similar size will work. The first probe wire was a pin from one of those "pin art" bed-of-nails toys. The insulator in the photo is a glass necklace bead, but it turned out to be far too conductive. It was replaced with a similar sized plastic bead. The new bead was slightly too large for the brass tube, but by heating the tube with a soldering iron while pressing the bead into the tube, the bead was easily cut down to size. (Place masking tape on your fingers to prevent burns!) The bead was easily popped back out with a screwdriver. A nail was glued into the bead with a little length at the head end for making the transistor connection and then the bead was glued into the brass tube.  Another bead was temporarily slipped into the other end to hold the nail in the center of the tube while the glue dried. I switched to a nail for the probe because it fit in the new plastic bead better. Tin and clean the nail before inserting in the bead, especially if steel flux is needed. Later, solder the base lead on quickly; the plastic beads melt easily. The epoxy technique mentioned earlier will also work well.

A fine mesh screen was wrapped over the open end of the chamber to keep out wind currents and electric fields. Rolling the screen between the fingers makes it conform to the shape of the tube nicely. Solder the screen to the tube in one spot. Some screen materials may require steel flux, but the screen should be tinned with the flux and then washed with hot water and detergent before placing it on the tube; that flux is highly corrosive! Copper screen is recommended; the screen shown is plated copper and it takes solder nicely. Make sure the sensor wire doesn't touch the screen and is fairly well centered in the tube (not critical). The circuitry was constructed using point-to-point wiring and is held in place by the base wire soldered to the nail's head and the emitter of the PNP soldered to the brass tube.  One end of a battery connector was fashioned from another glass bead (long, dark tube on the left in the photo below) and a copper-plated nail. A white plastic bead was used to cover the end of the copper nail where the wire connects.

Some thin Kapton tape was carefully added to insulate the circuit from the outer tube but no tape was allowed to touch the NPN's base lead. Even electrical tape is too conductive!

The outer tube was fashioned from 1/2" stainless steel tube, but copper would have been much easier to machine. The assembly was placed next to the tube to locate the hole for the LED, and a cotton swab was marked for depth with a black marker for applying epoxy to the inside of the tube to hold the glass bead battery terminal.

The swab was dipped in epoxy, inserted up to the black mark, and the epoxy was swabbed along the inside wall of the outer tube. After the epoxy was applied to the inside of the steel case, the assembly was slid into place (bead first) and the LED was positioned to face out the hole. After the epoxy set, conductive silver paint was gingerly applied to the edges of the screen to hold the ion chamber in position and to ensure a good electrical connection to the outer tube.

The other battery contact was made from a spring stolen from a AA cell battery holder fastened to a 1" steel standoff. The screw fits inside the spring and holds the narrow end of the spring to the standoff. The wide end of the spring friction fits in the steel tube to make contact. It is nicely centered by the tight fit and doesn't touch the center contact of the battery.  When the button is pressed, the head of the screw makes contact with the battery's positive terminal, completing the circuit. The standoff "button" is held in place by a homemade nylon bushing that screws into the steel tube. (Threading the stainless steel was difficult! And fashioning the bushing from a nylon rod was no picnic.) The end of the standoff that protrudes was ground down to match the steel case after cutting off enough to eliminate the threaded hole. The steel case was given a coarse sanded finish, too, to hide all the scratch marks from the threading process! The brushed finish actually looks nicer than the original shiny finish and it hides fingerprints. This isn't the easy way to build such a pen but it gets the idea across.

A tiny 10A 9 volt alkaline battery was installed and the threaded nylon bushing was tightened. A little clear epoxy fills the LED hole and the pen is complete:

Every spy and "diplomat" should have one!

Note: It has been suggested that this pen might not detect polonium in a drink at concentrations that are lethal. Polonium emits copious quantities of alpha particles, really a mind-bending amount, but alphas cannot travel through a liquid. They barely make it through a few inches of air. So, the pen will only receive alphas from the very surface of the liquid. Polonium is ridiculously toxic and it doesn't take much to kill. On the other hand, polonium really pours out alphas. Bottom line is I don't really know if it works and I'm fresh out of polonium to try.

High-Value Resistor from Neon Lamp

(One reader reports no conductivity with his neons so 'your results may vary'.)

I was intrigued by the idea of using an ion chamber with a bit of radioactive material inside as a resistor-like circuit element for very high impedance circuits. But as I was getting ready to try it, I remembered various discussions about NE-2 and similar neon bulbs containing a radioactive element to help the bulbs start. So it seemed that a simple neon lamp might be a nice resistor. I connected a bulb to a leakage meter and measured the current for various voltages across the lamp. The bulb was kept in the dark during the experiment. The "resistance" measured about 300 gigohms with 4 volts across the bulb. To make sure I wasn't measuring leakage along the glass, I snipped off the top of the bulb to "let out the radioactive gas". To my surprise the resistance dropped in half! It turns out that the electrodes in the bulb are slightly radioactive, probably due to added thorium. My Geiger counter jumped from 10 CPM to 60 CPM when the elements were held near the mylar window of the tube. The air must provide a better "target" than the rarified neon gas for the radiation to make ions which is why the resistance dropped when I snipped off the top of the bulb.

With a low voltage across the neon lamp, most of the ions have time to recombine before reaching the electrodes and this gives the bulb a resistor-like characteristic curve. I would expect the current flow to level out at some potential as all the ions reach the electrodes. So this resistance isn't particularly linear and probably varies from bulb to bulb but many projects just need a really big resistor to pull some low voltage node toward a known potential. For example, you might want to pull a gate of an FET to ground with a really big resistor to maintain a very high input Z and a neon lamp like this might by perfect.

To prepare the bulb, clean it with a powerful solvent like lacquer thinner or acetone and keep it in the dark when in use. I have successfully painted a bulb with black enamel which helps but I had to mask off the area near the leads because the paint is too conductive. A black plastic box would be a good idea. Close the box on the leads and then heat the leads with a soldering iron so that they melt into the plastic, allowing the box to close fully. Most plastics are excellent insulators. Also, keep the inside of the entire project's case dark.

Oh, yes. There was one last experiment to perform. I wiggled the electrodes of the neon bulb back and forth until they broke off. Now, all that was left was the leads and the glass base. I measured the resistance and got a reading in excess of 20,000 gigohms so the surface of the glass isn't a significant player, at least for that bulb. The resistance might be even higher than that; I just didn't have the patience to wait for the slowly drifting meter! The leakage meter really slow down when the resistance gets that high.

I checked a few more lamps and found great variation between them. They seem to vary between a couple of hundred gigohms to over 5000 gigohms. That's a lot of variation within a batch of identical-looking lamps! Keep these variations in mind. After testing some other types of lamps, I've concluded that there is a wide variation in the amount of radioactive element in the lamps. I was somewhat surprised to be able to easily measure the radioactivity from those tiny electrodes of the first lamp I tested! That bulb was unusually hot! More info: I tested the two tiny electrodes individually and found that only one of them is really hot, probably due to manufacturing variations.

So, there you have it; a box of neons with radioactive electrodes is actually an assortment of resistor values!

I have a lot of the bulbs in the picture. (charles@wenzel.com). (The series resistor is insignificant and it was left in place.)

Nuclear War Detector

Being caught off guard by a nuclear war can be inconvenient. This project should alert the utterly unobservant individual to any significant nuclear exchange or radiation spill in the immediate vicinity, giving the owner time to kiss various body parts goodbye. The detector consists of a small ionization chamber as described above with the single Darlington transistor amplifier and a sensitive level detector that activates a flashing LED when the chamber current reaches a sufficient level.

The device is built into two empty spice cans, one serving as the ion chamber and one as the circuitry shield and battery case. The sensitivity is set so that the lamp will begin to flash with approximately 3 pA of chamber current, about the amount one might expect if two lantern mantles were stuffed into the can. The mantles would be providing mostly alpha radiation in close proximity to the sense wire but in a nuclear exchange, the chamber would be responding to beta particles and gamma rays. The volume of the ion chamber is about 1/8 that of an old CDV-715 survey meter and there are no elements within the chamber to flatten the response for different energy particles. A wild guess for the sensitivity is between 2 r/hr and 10 r/hr for lamp flashing. In other words, the lamp will start flashing when the old CDV-715 is reading on the second or third scale. Suffice it to say, a blinking light isn't a good situation.

The schematic features a front-end like the experimental chamber above but without the current-limiting resistor connecting the can. It should be a less likely to accidentally touch the base of the transistor to the can since the unit is sealed and connecting the can directly to the battery makes wiring the rest of the circuit a bit simpler. A 1 megohm resistor can be used in place of the wire to connect the base to the pickup loop to protect the transistor, if desired. Instead of a meter, the MPSW45 pulls current from the base of a 2N4403 which further amplifies the current and turns on the BS170 by raising the gate voltage. When the FET is on, the LED with the built-in flasher begins to flash.

When the unit isn't flashing it draws virtually no current, so the battery will last its normal shelf life. The unit may be tested by warming the chamber with a hot air gun or hair dryer. Heat it gently and give time for the heat to reach the transistors. The heat will cause leakage within the transistors, simulating radiation and the lamp will flash until the transistors cool again.

The prototype circuit was built right onto the bottom of the chamber can with the help of a small piece of PCB material. The can is at the positive battery potential and the PCB is at the negative potential. The base of the Darlington is connected to a black insulated wire (see picture below) that passes through a generous hole to a Teflon standoff inside the can. (That black wire could be a 1 meg resistor as mentioned previously.) The standoff also holds a bare wire loop inside the chamber for collecting the ions. The side of the loop isn't critical, and straight wire a couple of inches long is also fine.

After the pickup loop and wire are installed, the can is closed with a piece of tinned PCB material soldered in a few places. Other solderable metal will also work.

(The views above are the same can but the top photo is after paint.)

The Teflon standoff may be replaced by a homemade plastic terminal. Those little beads for making necklaces make great insulators although they do have the tendency to melt during soldering. One way to limit the tendency to melt is to secure the electrical terminal in the bead with epoxy putty. The putty insulates the plastic from the soldering heat. Choose a terminal that doesn't conduct heat very well such as a copper-clad steel nail (sold in most home improvement stores). Brass nails also work. Make sure the epoxy doesn't get on the outside of the bead. A wipe with a little alcohol on a paper towel will remove any epoxy residue. To mount the bead, drill a hole in the metal slightly smaller than the bead and run a soldering iron around the hole to heat it on all sides. Press the bead into the hot hole and the plastic will melt. With a little effort, the terminal will be securely mounted with plastic insulation between the nail and metal inside and out. The photos below show the raw materials and a finished terminal.

The nail can be pushed all the way through the epoxy if a feedthru is desired. For this project, push the nail all the way through and the base of the transistor may be connected on one side and the little pickup loop on the other side, eliminating the need for the large hole and black wire.  

Mounting a bead in this manner without the epoxy and nail makes a great insulated hole for wires, too. Just melt the bead into the hole and pass the wire through. (Most wire insulation leaks too much for ion chamber connections and it shouldn't be allowed to touch metal surfaces.) This larger bead is nice in that it is tapered and has a shoulder in the middle. Heat was applied with a soldering iron held in the right hand while the can was rotated with the left hand. The left index finger pushed the bead down with a steady force, requiring a bit of dexterity. The shoulder seated nicely and the plastic flowed into the sharp edges of the hole; the bead isn't going anywhere! (These bead photos are to illustrate the idea but were not used in the Nuclear War Detector prototype.)

The larger pepper can houses the battery and LED. The battery holder is fashioned from balsa wood strips secured with epoxy and a battery door is cut into the bottom of the can with an abrasive cutting wheel (see photo below). The LED is held in position by its anode lead which is soldered directly to the can. A blue plastic "mosaic" piece from the art store covers the LED hole.  

It is easier to paint the cans first then touch up the solder connections later. Try a Q-tip dipped in a little of the spray paint. The finished unit shown below is ever-vigilant on the shelf of the author's bomb shelter, at least long enough to make the picture, anyway.

Super-Sensitive Ionization Chamber

Improved performance may be realized with an electrometer JFET or ultra-low leakage CMOS op-amp in place of the Darlington transistor. This detector features an old-fashioned electrometer JFET transistor like those commonly used in early smoke detectors, pH meters and electrometer amplifiers. The leakage in these FETs is remarkably low, perhaps below 25 fA, depending on the package and surface contamination.

One of the op-amps in the LM358 is used to generate 1/2 the battery voltage near 4.5 volts. When the meter reads zero, the gate of the FET will also be at 4.5 volts. The source will be at a slightly higher voltage due to the FET characteristics and the zero pot applies a voltage to pin 3 that is set to that source voltage. This voltage will depend on the individual FET. As a starting point, short the large value resistor and measure the source voltage. Select the resistor to get that same voltage on pin 3 with the pot at its center position.  Don't be concerned if you need a much larger resistor than 5k, even approaching 82k, but the source voltage shouldn't be much above 4.5 volts, perhaps 6.5 volts at the most with higher Idss parts. Reduce the 121k (not a critical value, by the way) for experimenting with higher Idss FETs to keep the source voltage down to a couple of volts or less.

Specialized op-amps are also available with similar leakage properties to electrometer JFETs (see the National LMC6001, for example). To use such an op-amp, simply leave out the FET and connect pin 2 directly to the 1 megohm resistor. However, extreme caution must be used to prevent the wire from touching the can or the op-amp may be destroyed. The 1 megohm resistor protects the FET, but it may not protect many op-amps. With only an op-amp, it may be desirable to connect the selected resistor from the pot to zero volts instead of 4.5 volts since the required zeroing voltage will be near 4.5 volts. Another 100k resistor would be a good choice. In fact, the pot may not be necessary at all; simple connect pin 3 to the 4.5 volts from the other op-amp.

In order to convert the very tiny chamber current into a reasonable voltage, a very large resistance is needed. This prototype uses a 100,000 megohm (10^11 ohm) glass Victoreen resistor which will convert 1 fA into 100 uV at the junction of the 1 meg and 10k resistor (10 mV at pin 1)  making it reasonably easy to see extremely small changes in chamber current. The 1 meg and 10k between pins 1 and 7 add additional gain and that ratio may be changed to accommodate a different high value resistor. For example, a Victoreen version of the CDV-715 has a 220 gigohm resistor and the 10k could be raised to 22k to give roughly the same sensitivity as the prototype. A much lower value, say 10 gigohm, can also work, but temperature drift from the FET may become annoying. (An ultra-low current op-amp without the FET will exhibit less drift.) It is also possible to use an op-amp with relatively poor bias specifications as long as the leakage remains fairly constant and the offset drift is low. For example, the TLC2652 chopper-stabilized op-amp used without the FET might appear to be a poor choice due to its relatively large 4 pA bias current, but that current is internally compensated and remains fairly constant for modest changes in temperature around room ambient. This leakage will require the zero pot be adjusted to a different voltage by only about 400 mV. Don't bother with op-amps that have input bias currents above 10 pA (which, by the way, is most of them).

The chamber is made from an empty 3.8" (10 cm) diameter, 6.8" (17 cm) tall tin can. Removing the chocolate cappuccino wafers from the tin required several cups of coffee but was well worth the effort. Seriously, tins like these take solder easily and make great enclosures for a variety of quick scientific and hobby projects. The insulator is made with another large tapered plastic necklace bead as described previously. The hole was drilled small and then widened carefully with a tapered reamer until the bead almost rested on its shoulder when inserted. The metal around the hole was heated with a soldering iron and the bead was pushed into the hole until the shoulder itself started to melt a little. The plastic oozes into the splits in the metal from the reaming process and becomes solidly anchored. These beads have a hole large enough for a #4 bolt so two standoffs were mounted on each side of the hole. The interior standoff holds a stiff piece of 14AWG copper wire salvaged from a piece of Romex house wiring. The photo to the right shows the internal copper wire soldered to one bent standoff and the outside view of the other standoff. The bolt was eventually put in the other way because of the difficulty in threading the nut onto the bolt down in the chamber. This simple insulator is quite effective and seems to perform as well as exotic Teflon standoffs. The only drawback could be the large amount of exposed plastic that might accumulate a charge in some instances. The insulation properties are excellent and a thoroughly melted-in standoff is very strong. A piece of copper PCB material was cut to fit well inside a mint can cover, and a center hole was drilled to clear the plastic insulator. The PCB will be at zero volts potential and the tin will be at a higher positive voltage, so it is important to make sure the two don't touch. The mint can shown below was coated on the inside with electrical tape (not shown) and the bottom, which is normally concave, was pressed out to make the interior of the can larger. The rounded end of the red handle of a large X-Acto knife makes a good tool for pressing the metal out. Remove the blade first! Two notches are eventually cut for the wires and access to the zeroing potentiometer. The PCB material was secured in place with spray adhesive. The circuit is built directly onto the copper board using point-to-point wiring.

The large glass resistor is held in position by two more necklace beads, the new insulator of choice in the lab. The beads were partially melted by holding a soldering iron tip in the hole for a minute or so, taking care not to touch the plastic. While the plastic was still soft, the bead was pressed onto the resistor. The beads were melted onto the PCB by heating the PCB with the soldering iron and a little solder while gently pressing the bead down. This connection isn't particularly strong, and a little plastic glue was added to the now-flat side of the beads when they came loose. Keep the resistor clean and free of fingerprints, and make sure the wire from the insulator to the resistor doesn't touch anything. Construction is crowded simply to allow for more circuitry in the future. 

The cans were painted yellow, but provisions must be made to make good electrical contact between the chamber can and the end cap which can be aluminum foil, wire screen, etc. The prototype has a short length of tinned copper shield braid soldered to the inside of the can and folded over the lip of the opening:

A single piece of bare copper wire would also work. An ordinary pipe clamp is used to secure the foil or screen but a rubber-band would also suffice. The clamping action forces the foil or screen against the grounding conductor. Alternatively, the paint could be removed from the end of the can.

The actual unit deviates from the schematic somewhat and the finishing touches evolved. Firstly, the digital meter was replaced with an analog current meter with a custom face. In order to achieve good sensitivity, a range switch was added as were connections for an external meter or computer data taker. For "normal" sensitivity, a 3k resistor is connected in series with the meter (3 volts gives a full scale current of 1 mA). The toggle switch connects a 220 ohm across the 3k to give a total resistance near 300 ohms (adding the meter's resistance) for a X10 scale. The large meter was mounted directly to the top of a box that once held a pair of calipers by its electrical terminals instead of mounting it in the conventional manner. The chamber was mounted to the box with a couple of sheet metal screws screwed right into the sides of the chamber. 

So far, these are pretty ordinary modifications, but now the project deviates from "normal engineering practices".  In order to get a very long averaging time for the meter, a 1 farad super capacitor was connected across the terminals of the meter. This unorthodox technique actually works pretty well. The meter resistance is about 100 ohms so the time constant is about 100 seconds. By keeping the resistance across the super cap low, any problems associated with dielectric adsorption, etc. are minimized. Another RC was added for an analog voltage output (the two banana plugs at the top of the photo above). The capacitor is 0.22 farad and the series resistor is 500 ohms (two 1k resistors in parallel). The capacitor has two 5 volt zeners head-to-head across it to prevent over-voltage which will quickly damage a super capacitor.  

The power supply is a bit of an after-thought to avoid batteries. The two black wires come from an unregulated molded "wall-wart" power supply that supplies about 20 volts. A 390 ohm resistor and 9 volt zener form a 9 volt supply to run the op-amp and a 10k resistor, and a 16 volt zener provide 16 volts to bias the chamber. The modified schematic is shown below:

Not shown in the schematic is a last-minute power indicator LED added in series with the 390 ohm resistor. If the circuit will not zero on the sensitive scale, the 9 volt power supply is changing voltage too much (due to the extra current required to charge the super caps). In that case, switch to a three-terminal regulator and set the ion chamber voltage a little lower to assure the regulator has enough voltage drop across it. A 100 uA meter movement will also eliminate the problem, allowing the meter resistors to be raised by a factor of 10, from 3 k to 30 k and 220 ohm to 3 k.

The ionization chamber works about as well as can be expected without significantly more care in chamber design. The long averaging time makes it possible to see radiation from very weak sources. Below is a plot of the response when a radioactive lantern mantle is rested up against the foil window then removed five minutes later:

The long super cap time constant is clearly seen and the absence of noise is refreshing. Such extra-long averaging times would be useful for testing samples slipped directly inside the chamber, too, but the casual experimenter may wish to shorten the response time a bit by choosing lower value caps. Although the schematic doesn't show it, a double-pole switch was eventually added to disconnect one leg of each super capacitor for those times when a fast response is desired. Also, keep in mind that the circuit will take a long time to settle after power is first applied and much longer when the super caps are used. The meter will peg for several minutes and adjusting the zero with the caps switched in can be tricky since it takes many minutes to see the results of an adjustment. With experience, the speed of the drifting can allow the zero to be set with the caps connected. Simply try to stop the meter. Then very, very slowly adjust the needle to a low reading, perhaps 20% of full-scale. Expect to be "lost" once or twice with such a long time constant!

 Other Ideas

Using a wire mesh instead of aluminum foil for the window allows alpha and beta particles to enter the chamber more easily. The mesh also allows air to be blown into the chamber with an induction motor or brushless motor fan. (Brush motors will probably generate too many ions of their own.) The photo shows a fan positioned off to one side to create a bit of airflow through the chamber. This arrangement blows ions in the surrounding air into the chamber where they are separated by the chamber potential, producing an upward meter deflection. Lighting a match or butane lighter anywhere near the fan's input pegs the meter. Alpha emitters such as the element from a smoke detector or a lantern mantle give very high readings if held nearby. Turning the fan on and off might provide some idea about the level of ionization in the room, but leaving the fan on and blocking the air with cardboard might give more consistent results since the electric and magnetic fields from the motor may influence the reading. The basement lab where this technique was tried showed a consistent 100 mV increase with airflow (the super caps and computer plotter were indispensable for the measurement). Perhaps a radon test is in order!

 A wire tray was built onto the original lid and a hole was cut so that items could be inserted into the chamber right above the center wire without opening the chamber. Power should be removed when installing or removing the tray to prevent shorting the outer can to the center wire. The tray is useful for observing the radiation from long objects like thoriated tungsten electrodes or test tubes filled with radioactive substances. If a coarse mesh is used the circuit will respond to the slight change in electric field when an item is inserted, so some settling time will be required. Experience has shown that observing the change when the item is removed instead of inserted gives better results. Heavy items will cause the tray to bend toward the wire slightly, so it is best to flip the lid 180 degrees so that the inserted item rests on the bottom of the can with the screen above. If the chamber is mounted with sheet metal screws through the bottom as with this prototype, twist the lid slightly so that inserted items miss the screws. An open paper tray allows the observation of radiation from alpha and beta emitters. The tray in the photo below is made from thin cardboard and is filled with potassium chloride, a beta emitter. The low level of radiation is just detectable using the long averaging and a computer plot, giving about a 20 mV change. The discrete steps in the plot are due to the limited resolution of the commercial data taker that has a 10 volt, 11 bit input (about 5 mV per step).

Fairly weak sources can be measured with the wire mesh in place. A thick piece of pink granite was found to give a Geiger-counter reading about 60% higher than the background reading using a long averaging time. The slab gives a noticeable reading increase when placed in front of the ion chamber as shown below. The chart starts with a "hot" part of the slab in front of the chamber, then the slab is removed, and lastly, the slab is replaced but with a less radioactive portion in front of the chamber. 

The two large spikes are from the vibration produced when moving the slab; it is heavy! The voltage change from the high reading to background level is about 160 mV. With these long averaging times, the chamber performs well, even when compared to a large tube Geiger-counter. (The chart runs for 30 minutes.)

Radon Accessory

Here's an interesting accessory to the ion chambers above or other radiation detectors that will allow for the indirect detection of radon gas. Radon is a noble gas and is hard to detect directly; it doesn't react chemically or easily stick to anything and it is usually present in very small amounts. It has a short half-life of under four days so the concentration in a home is due to a constant replenishment as the gas seeps in from the ground or the structure's building materials. Its decay products are much easier to detect because they readily stick to dust in the air and typically have a short half-life, making them more radioactive. This accessory draws air through a filter, catching dust laden with the radioactive "daughters" of the radon present in the air.

An ordinary computer case fan is mounted in one end of a 4" PVC plumbing coupler as shown. This particular fan is held in place by friction but any technique is fine, as long as it pulls air through the coupler. A white poster board doughnut is secured with some foam caulking to seal the gaps along the edges of the fan.  The fan is oriented to pull air through a filter affixed to the other end of the coupler with tape. The filter in the picture is a piece of 3M SV-DF01 dusting cloth but other thin dusting or cleaning cloths that will allow a little airflow will also work.

When the fan is running, radioactive dust collects on the outside of the filter. The isotopes of interest all decay within about a couple of hours so they are decaying as they are building up on the filter. At some point, there is enough radioactive material on the filter that the rate of decay equals the rate of collection. This "equilibrium point" is seen as a flattening in the increase in radioactivity.

The filter end of the contraption is placed very near the end of an ion chamber but with a little room for air flow. To get consistent readings, use a single layer of corrugated cardboard as a spacer. Remove the spacer when the filter and ion chamber are both flush against it.

This accessory may be used in two ways. If you have a data taker for your ion chamber, it is interesting to plot the output vs time after you turn on the fan. The radiation will build up, exponentially reaching a steady level. Turn off the fan and the opposite exponential will be plotted.  Here's a typical plot:

The faint dotted lines are an hour apart and the vertical sensitivity corresponds to about 10 fA of chamber current per division. The fan is turned on at the first marker and turned off at the second marker just after the middle of the plot. The height of the response will depend on the level of radon in the vicinity, the velocity of air through the filter, and a host of other variables. It would be hard to calibrate the response but it could be useful for checking the relative levels in different parts of a house or for periodically checking for significant level changes. Seeing the exponential helps to validate the detection.

Another way to use the gadget is to simply place it in the room to be tested and let it run for a few hours. Then position it up against the ion chamber and make a measurement. It can touch in this case, since no air is flowing. The reading will settle after a couple of minutes. The radiation level will begin to drop after about 15 minutes so don't take too long. Since these isotopes are all but exhausted after a few hours, the filter may be used over and over, until it becomes dusty enough to restrict air flow. The filter should not cause a reading increase when held against the ion chamber at the beginning of a test before the fan has run. If it does, it may be time to change the filter. I simply affixed mine with scotch tape.

Either way you go, a long averaging time is best for the ion chamber so the readings are not bouncing around. My squirrel cage fan pulls a much better vacuum and collects a much higher density of radioactive particles. The radiation was easily detected with an ordinary Geiger counter despite the thick walls of the Geiger tube! Where's my gas mask?

If you want to try a simple experiment to see if this accessory is worth the trouble, try taping a little piece of dusting cloth over the opening of a squirrel cage fan. Let it run for three hours then gently affix it to the aluminum foil window of an ion chamber. I used more tape attached to the original tape because the dusty side of the filter should be against the foil. These experiments are detecting beta particles which have little penetrating power. 

The meter reading jumped from near zero to two-thirds scale on my simple darlington chamber. And all this time I thought my house didn't have radon!

Components

The following is a discussion of the components used in the ionization chambers above. Most of the parts are ordinary resistors and capacitors with no particularly critical specifications, but a few must be chosen carefully. 

JFETs 

The JFETs used above are "electrometer" types, typically from the 2N4117 family. They have very low IDSS, usually below 250 uA. Here are a few examples:

The plastic parts seem to have lower leakage. Mosfets are also useful for ion chambers but the vulnerability of the gate to excessive voltage makes them difficult to handle, especially when a long wire is connected directly to the gate. Using a positive voltage on the outer can and adding a resistor in series with the JFET gate helps to prevent damage. If the outer chamber is accidentally shorted to the sense wire, the JFET diode forward-biases, and the resistor limits the current. Larger die JFETs might make interesting front-ends. One obvious choice is the die for the family including the 2N4220 to 2N4224, 2N3821 to 2N3824, 2N5556 to 2N5558, 2N5457 to 2N5459, MPF109 and MPF 111. These FETs will exhibit more leakage than those listed above but leakage should still be well below 1 pA at room temperature. The higher IDSS suggests a higher resistor in the source to set the current to a low level, perhaps selected for a couple of hundred uA. The FET current can be chosen to reduce the temperature coefficient, but the technique can be tricky. Temporarily ground the gate and select the source resistance that gives a minimum change in source voltage when the FET is gently heated with a warm PTC or resistor. Don’t overheat the part to avoid erroneous results. Freeze spray cools the parts too much and the condensation leaks too much, so stick with heating.

Op-Amps 

Electrometer-grade op-amps are available with leakage currents competitive with the best JFETs. One of the best is the LM6001, tested to 25 fA! Most CMOS op-amps will have leakage currents low enough to be useful in ion chamber projects, but offsets of a few pA are to be expected with many. The op-amp used with a JFET front-end is not particularly critical, and the ordinary LM358 dual op-amp is a typical choice. The circuits above use a "synthetic ground" set to 1/2 the battery voltage and most op-amps that will operate properly on only 9 volts will work well.

 Resistors 

High-value resistors are often a problem for experimenters. Companies that manufacture such resistors include Ohmite, Ohmcraft, SRT Resistor Technology, IRC, IMS, Micro-Ohm, and others. High value resistors can also be found in older equipment like the CDV-715 survey meter. Most experimenters collect such resistors by cannibalizing old equipment or by finding them on an auction site. The prototypes above use either carbon film or metal film resistors for the lower values. Metal film resistors are hard to beat, exhibiting excellent accuracy and temperature stability, but most of the resistors aren't critical. The power dissipation is low for all of the resistors, usually well below 1/4 watt. The 390 ohm resistor supplying the 9 volt zener dissipates a little over 300 mW.

Transistors

The Darlington transistors used above are garden-variety types but very old parts may not exhibit the amazingly low leakage of modern devices. The gain of one MPSAW45A was verified to be over 30,000 down to picoampere inputs and the leakage might be as low as 100 fA at room temperature.

 Capacitors

Large non-polar mylar capacitors, perhaps 1 uF, are suitable for bypassing the chamber voltage to the battery negative voltage ("ground"). The use of a non-polar capacitor allows the polarity to be reversed for experimental purposes. For example, the background radiation can be distinguished from zero offset or leakage by reversing the voltage on the outer can and measuring the change in the output. A microprocessor or computer could change the voltage with a relay then measure the current change after sufficient settling time.

The super capacitors used to filter the meter and output can be expensive if purchased new, but they are fairly common on relatively modern surplus circuit boards. Their application above was more of an afterthought, but it is fun to use such components in unexpected ways. They feature very low leakage so the time constants can be extremely long without significant error.

Mechanical Parts

Teflon terminals are excellent for these projects but the plastic beads work great, even the ones with the metallic glitter inside. Other phenolic or ceramic terminals should be avoided or at least thoroughly tested. Most glasses have too much surface conductivity, too. The softer plastic 5-way binding post used for the first chamber is questionable but seems to work well enough.

The best choice for the chamber is the tin alloy typically used for cookie tins. They are easy to work and take solder beautifully. Watch those sharp edges, however. Many of these cans have a plastic lining that forms an insulation layer. Several chambers with that plastic coating seem to work fine but it might be a good idea to choose a can without it or to sand it out. The plastic could block alpha emissions from the metal alloy but the lighter alloys probably don't emit many, anyway. Avoid steel cans and keep soldering to a minimum on the inside since some solders have significant quantities of radioactive contaminants.

These simple ionization chambers are the "crystal radios" of radiation detection. They are truly simple to build and really work. They respond to any ionizing radiation that can get inside the chamber from 100 nm ultra-violet light through X-rays and gamma-rays. They also respond to alpha (helium nuclei) and beta (free electrons or positrons) radiation if provisions are made to allow them inside the chamber. Alphas can't even make it through a piece of paper so a wire mesh or placing the sample in the chamber is best. The serious hobbyists can find a wealth of literature for improving these simple designs, converting them into serious scientific instruments.