Saturday, May 18, 2013

New ion chamber

A prototype simple ion chamber radiation detector built by my girlfriend and me:

It is an ion chamber based on LMP7721 operational amplifier. LMP7721 is very convenient for building ion chambers, as it has very low input offset current ( 3fA typical ), and unlike other such sensitive chips has two back-to-back diodes for input protection, which help you not fry the chip when you turn on the chamber bias voltage or touch the input wire.

Special thanks to National Semiconductor / Texas Instruments for generously providing engineering samples for this project, and to Eduard for 10 TOhm feedback resistor.

Circuit diagram:

Errata: 47k adjustable resistor should be 4.7k (the dot got lost after scanning..). Note: the voltage stabilizer (part on the right) is fairly inefficient, and is built this way because I didn't have any better parts to use. I recommend making something more efficient to conserve the battery life.

Construction notes: we mounted the LMP7721 dead-bug style (upside down), using air to insulate the input wire (the circuit board would leak too much). The 10M resistor at the negative input of the LMP7721 protects it from over-voltage during power-on or in the event that you touch the input wire.

I recommend to first built the circuit without the LMP7721, then tin the wires that will connect to LMP7721, making sure to evaporate any remaining flux, then solder them to LMP7721 without using any flux (to avoid depositing any flux residue on the LMP7721). First bend the wires so they touch the relevant pins, then gently solder them with the very tip of the soldering iron. I used lead-free solder (96.5%Sn, 3%Ag, 0.5%Cu if I recall correctly), which in addition to being legal and less toxic than lead based solder, has an advantage of being less likely to be contaminated with radioactive lead 210 (which is a decay product of U-238). I do not know, though, if lead-free solder is more likely to be contaminated with something else. I tested this solder in my other ion chamber to make sure it is not radioactive. In the final version of this circuit I will probably drill a special hole at the #8th input of the op-amp to keep it air-insulated.

After construction, you have to leave it on overnight to let all static charges on the dielectrics dissipate - until they dissipate, they mess up the readings.

A few things I may add/change about it:

1: The lid of this project box is too thin and the chamber reacts wildly if you push on the lid (like a condenser microphone). I'm going to add extra foil cover which would not be mechanically coupled to the lid to minimize this effect. Ideally, I should ground the shielding and add a separate ion chamber box, which would also decrease potential effects due to charge deposition on the plastic parts inside the chamber.

2: Add a packet of desiccant.

3: Add a charger circuit and seal the box against moisture.

4: Add magnetically activated reed switches for shorting over the 10T resistor and the filtering resistor in the bias supply, thus permitting easier zero adjustment.

Peek inside:

This project was inspired by