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Make a Cheap and Effective Radiation Detector ( Ion Chamber )

Make a Cheap and Effective Radiation Detector ( Ion Chamber )

News: April 16, 2011 just added a PCB for a much easier to make version of the ion chamber kit, now available in the products section. This PCB has been setup so that leakage from the PCB to the ion chamber probe is completely minimized.

So why would I build an ion chamber instead of a Geiger counter?
Given the recent events in Japan (Let us all hope and pray for the safety of Japan), interest in Geiger counters have gone through the roof. Geiger tubes have all of a sudden become harder to get and the prices for tubes and Geiger counters have gone up considerably. I even hear that they have sold out of Geiger counters in France. The parts to build an ion chamber are cheap and readily available. Also most of the tubes (like the CI-3BG) that I see available on ebay are only sensitive to gamma radiation, some like the SBM20 will detect beta and gamma radiation. While this project will not tell you exactly how much radiation the source has, it will detect gamma and beta sources without any trouble, and if you want to detect alpha sources you can put a sample inside of the chamber. So it will tell you with relative certainty if something is radio active. This ion chamber is so sensitive that it detects the lantern mantles from over 6 inches away, which I was very surprised to see.

Just below this you will find the step by step instructions on how to make this simple and cheap but very effective radiation detector (ion chamber). The original source of this information is from techlib. I am just providing a step by step instruction on how to assemble this and check out if the unit is working correctly. We will also provide a $9.99 kit (the kit contains the resistors, trimmer pot, transistors, battery snap, and hookup wire).

The kit maker will have to provide a 9V battery, cookie tin or tea tin, aluminum foil, digital multimeter or analog multimeter with a 200-400mV range, soldering iron, and hand tools required to complete the build.

Any ion chamber kit shipped to Japan will be sold for $3.75 each plus postage. For sales to Japan please contact

日本への特別価格は各イオン部屋のキット$3.75。 プラス郵送料。 電子メール:

And as we always support education, there are discounts for school projects. Please contact

See the video of this detector in action (this unit took me less than an hour to build ):

Are you asking yourself how does this detect radiation?
Here is a simple description: The ion chamber indirectly detects ionizing radiation by measuring ion pairs created when the radiation passes through the chamber. Radiations such as alpha, beta, gamma, and x-ray are energetic enough to dislodge electrons from atoms and molecules creating ion pairs. One side of each ion pair is positively charged and the other side is negatively charged. In order to detect the ion pairs inside of the can we need a way to collect them. The can body is positively charged to about 9V and forms the anode of the circuit that attracts the negatively charged side of the pair. The probe in the can is attached to the base of a darlington pair transistor and it forms the cathode of the circuit that attracts the positive charged side of the ion pair. As the radiation passes through the chamber, air in the chamber becomes ionized, the more radiation that passes through the chamber results in higher ionization levels. As ions pairs get collected at the anode and cathode it causes current flow in the circuit that is amplified and measured by the meter.

Here is the schematic, listing parts values:
To see a full size image click on this image then click the image on the following page.

Ion Chamber Schematic
Simple Ion Chamber

After looking at the assembly instructions you might be asking yourself why is this assembled like this?
So the reason this kit is air wired ( not on an a PCB ) is that the method used to detect the radiation requires extremely low leakage paths. In the end we are trying to detect very low currents that a PCB could introduce and cause the circuit to malfunction. I am not saying that this cannot be built on a PCB but that it adds another level of problems to the circuit.

Step 1) Get all of the parts:
3 ea. 10K resistors 2% or better
2 ea. 2.2K resistors 2% or better
1 ea. 1M resistor
1 ea. 100K trimmer potentiometer
1 ea. cookie tin or tea tin
about 16 inches of hookup wire
1 ea. 9V battery
1 ea. 9V battery snap
2 ea. MPSW45A NPN darlington transistors
2 ea. MPSA64 PNP darlington transistors

Hint for the resistors, if you cannot read the values from the resistor by the color bands just measure them with a multimeter.

Step 2) Start the assembly:
Connect the emitters of the two MPSA64 transistors together. As you can see from the drawing that if the transistor has the face with the writing towards you and the legs pointing down, the transistor face is EBC, emitter on the left, base in the middle, collector on the right.
To make life easy I used my helping hands aligator clip to hold one of the MPSA64.

Package drawing showing the EBC Emitter Base Collector pins:

MPSA64 Pinout
MPSA64 Pinout

I bent a little loop into the loose MPSA64 emitter leg and hooked it to the emitter of the clamped MPS64A, then soldered it

Step 3) Bend little hook into two of the 10K resistors then snip the excess leads

Step 4) Hang the 10K resistors from the base leads of the MPSA64, then solder in place

Step 5) Bend a hook into the ends of the 2.2K resistors, snip, then hang from the collectors of the MPS64A and solder into place

Step 6) Flip the assembly over, take the two long leads on the 2.2K resistor bend them over each other

Step 7) Solder the leads on the 2.2K resistor then clip the excess from the leads

Step 8.) Bend the legs of the emitter and collector on both of the MPSW45A transistors. The MPSW45A has the same pinout as the MPSA64 (EBC as the transistor faces you)

Step 9) Solder the emitter of one MPSW45A to the 2.2K resistors legs that are soldered together, then solder the collector leg to the 10K resistor leg, then repeat for the other MPS45W45A on the other half of the circuit.

Step 10) Clip the excess leads from the collector and emitters of the MPSW45A and 10K resistor, make sure to leave the base lead long

Step 11) Cut about 4 inches of two wires and strip the ends. These will be used to connect the meter to the ion chamber.

Step 12) Solder one of the wires to the MPSA64 collector where it attaches to the 2.2K resistor, then repeat with the other wire for the other half of the circuit

Step 13) Cut a piece of wire to use for the ion collector. I am using a tea tin that is about 6 inches by 4 inches, so I cut the wire so that it will extend about 3/4 of the way into the can, see some of the later pictures for more clarity.

Step 14) Solder the wire onto one of the MPSW45 base legs.

Info picture:
Here is a picture of the tea tin I used.

Step 15) Scrape the bottom edge of the tin can, the lid will be tack soldered to this later as a shield for the circuit

Step 16) Time to cut a hole into the tin, carefully put a small hole about 1/4 inch across into the bottom of the can.

Step 17) Scrape a small section of the bottom of the tin, then heat and add solder (pre-tin this section). This is where a resistor will be soldered later.

Step 18) Take the circuit and bend the ion probe lead away so that it can be inserted into the hole in the can. Insert into can.

Step 19) Cut the end off of one of the legs on the remaining 10K resistor and leave about 0.2 inches

Step 20) Solder the short leg of the 10k resistor to the emitter junction of the two MPSA64 transistors

Step 21) Solder the long leg of the 10K resistor to the can, then cut the excess of the lead off. Make sure the the ion probe is centered into the hole at this point.

Step 22) Cut a about a 4 inch piece of wire, strip the ends and solder one end to the unconnected base lead of the MPSW45A. This wire will be used for zeroing the circuit.

Step 23) Cut another piece of wire about 4 inches and strip the ends. Solder one end to the junction that connect the 2.2K resistors and the emitters of the MPSW45A transistors (this is the ground junction) This wire will also be used for the zero circuit.

Step 24) Put a hole in the lid of the can about 1/4 inch wide. Feed the battery clip wires through the lid.

Step 25) Solder the red lead from the battery clip to the junction of the emitters on the MPSA64 transistors and 10K that goes to the can body. Solder the black battery lead wire to the ground junction where the 2.2K resistor and the emitters of the MPSW45A transistor meet.

Step 26) Feed the wires through the hole in the lid

Step 27) Solder one end of the 1M ohm resistor to the trimmer pot right lead.

Step 28) Carefully center the ion probe then tape a wire down to “hold” it in place when the lid gets put on.

Step 29) Carefully align the lid to the bottom of the can and tack in three places around the can to hold in place. You may have to take it back apart if something is not working so do not go crazy and solder the whole lid in place.

Step 30) Attach the longer end of the 1M resistor used for zeroing to the edge of the can, I used a point on the lid tacking.

Step 31) Connect the zero circuit. The wire from the base of the MPSW45A needs to be soldered to the center of the trimmer potentiometer. The wire from the ground junction will need to be soldered to the reaming leg of the trimmer pot.

Step 32) Double check that the ion probe is centered. You can move it around very gently if you have to.

Step 33) Cut a piece of aluminum foil and use a rubber band to put it on the chamber.

Step 34) Connect a meter. If you are using a digital DMM the lead connection will not matter, since it will read positive or negative. I soldered this one to the circuit.

Step 35) OK here comes the make or break point. De-solder the center connection on the zero pot and leave it hanging in the air not touching anything.
Set your meter so that it can measure at least 9 volts.
Connect a battery to the ion chamber wait about a minute.
Watch the meter it should start at about about +/-4V then slowly start settling after about a minute it should be less than +/-200mV of zero
if this happened great, if not skip to the debug section.
OK it seems to be working at this point.
Disconnect the battery.
Solder the zero wire back on to the center lead of the zero pot.
Connect the battery wait about a minute
The meter is probably reading +/-8V now and not settling, do not panic, this is what is supposed to happen. Slowly adjust the pot, being careful not to touch the leads and hold the pot only from the sides away from the leads.
At some point the reading on the meter will start to head towards zero, keep adjusting the pot so that you get closer to zero. Change the range on your meter lower as you get closer to zero. Do not get upset if you cannot reach zero volts, it is OK to be +/-20mV from zero. Let the unit settle for a minute, make sure you are not touching it anywhere as your body can inject current into the can.
If you watch the meter you will see it bounce around as background radiation is passing through the chamber.
If you are satisfied that it is +/-20mV now it is time to test it.
If you have a radioactive source ( I used a radioactive lantern mantle) place it in front of the aluminum foil, place it so that it is not touching the foil and watch the meter, it should climb as the ions build in the chamber this can take tens of seconds for the ions to build, after you take the source away it will also take tens of seconds for the meter to settle back to background levels again as all of the ion pairs are collected. All you are looking for is the change (delta) in the reading from background with the unit sitting for say a minute with nothing near the chamber to the maximum amount it read with a source near the chamber.

Things I have noted about the chamber. If you pull a source away from the chamber the meter reading will climb in the seconds after the source is pulled away then start to drop. Likewise If you place a source next to the chamber the readings will drop in the seconds after the source was placed next to the chamber then it will start to climb.

Congrats you have a functioning ion chamber!

Be sure to visit the teclib site for some real cool experiments, including detecting radon.

Debug section
Do not worry it is usually very simple to figure out what happened.
I built two units the first worked right off the bat, the second did not.
OK so you are not getting the meter to settle, it is staying pegged at a high level or reading 0.
First rule of troubleshooting is measure your power source. Measure your battery and it should read about 9V, if it is good then move on to debugging, if not replace the battery with a fresh one and go back to step 35
De solder the tack points around the lid.
Carefully lift the lid a little bit and look inside to see if there is anything from the circuit touching the can body, if there is anything is touching lift the lid and try to adjust the circuit so that the only thing touching is the 10K resistor connected to the can body then gently put it back together and try step 35.
If it did not work lift the lid again and make sure all of your solder connections are in place and then make sure the circuit is connected like the schematic. Fix the oops then try going back to step 35.
Another method for debugging is to disconnect the zero lead, lift the lid and power the circuit up. Watch the meter it should start at about about +/-4V then slowly start settling after about a minute it should be within several hundreds of mV of zero. While it is powered up carefully move the circuit with something that does not conduct electricity like a plastic pen body to see if anything changes, if you see a change you probably have a short or bad solder joint.

Another issue you might have is that your solder flux is contaminating the circuit and causing leakage paths, remove all power and clean the circuit with a high grade alcohol let the circuit dry completely before applying power again.

One more issue I have seen is that the meter seems to jump around a lot as you move around when you are close to the unit, I have seen this happen on very dry days ( humidity levels below 20% ) when I am wearing clothes that seem to cause static like a polyester blend shirt. If you are seeing an issue where the meter seems to jump around (like 50 or more Milli-Volts) even if you are three feet away, try moving farther away and see if the unit settles down, if it does then static is probably the cause.