Ionization Chambers

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:

The internal wire was soldered to both ends, the plugs taped in place and the tube stretched to tighten the wire

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The finished detector was pretty awful! First of all, the electrostatic shielding on the ends was inadequate, causing a huge 60 Hz signal.  And, the wire would vibrate with the slightest bump causing wild swings in the output. Maybe this thing might be the start of a seismometer, but it stinks as an ion chamber!

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.)

This tin measures about 10.5" across and about 6" tall (a "regular" height tin should work as well). The center portion of the lid was cut out with scissors to make a frame to hold the aluminum foil window. The circuit is housed in a smaller cookie tin tack-soldered to the bottom. Connections are made via a 5-pin audio connector.

The electrode looks like a lasso.	The electrode is a 5" dia. wire ring mounted to a Teflon standoff and a short piece of stiff telephone wire connects the electrode to the circuitry. The wire passes through a large hole to reduce the chances for leakage currents. The circuitry is a modified version of the first schematic featuring a resistor for the feedback and an op-amp for boosting the output signal. The transistor circuit was also modified to increase the loop gain and improve the stability (see ckt. desc. below).
A word of caution: the metal sure looks like ground and a person (um, like me) might start soldering  components  that go to ground to it. The can will actually be connected to +45 volts or more so the "ground" connections are made above the metal. The only components that connect to the can in the photo are a large yellow cap and a couple of white caps used to support my elevated ground buss.

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:

	The 4.7uF capacitor should be a non-polar film type with a voltage rating above the voltage used (45 volts in the schematic). A non-polar type allows the voltage polarity to be reversed for experimental purposes.
	The 100,000 megohm resistor is a specialty device which may be hard to obtain.
	The 2N4117A is an unusual electrometer-grade JFET which has few substitutes.
	The other components are not particularly critical.

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.

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.