Current Amplifier 20fA to 20uA range

Maybe this will help.

Design femtoampere circuits with low leakage, part 1 - EDN

Electronic Design News (EDN) is a good source.

Thanks for the link.

Radiation interactions are more like pulse events than any linear signal. So why not use gas amplification and pulse counters? Detecting a single ionizing event is quite simple in the Geiger-Muller region.

Ion Chambers represent true exposure and are primary standard devices with larger dynamic range for 6 decades. Results are better with ion chamber. Radiation therapy uses Thimble Ion Chambers.

GM will turn out of ions quickly due to 10E+4 amplification. These are good for contamination monitoring.

This seems like an ordinary Geiger counter would work just replace the power source with a nuclear battery...or did I miss something?

https://www.ptwdosimetry.com/en/support/downloads/?type=3451&downloadfile=1503&cHash=390ad915264205500518c46e0bb18874

LMC6001 Ultra, Ultra-Low Input Current Amplifier....

https://www.ti.com/lit/ds/symlink/lmc6001.pdf?ts=1666812505139&ref_url=https%253A%252F%252Fwww.google.com%252F

Yes, I agree, these parts LMC6001 are good that were made by National Semiconductor long ago for 10fA leakage.

AD549 of ADI and OPA128 of Burr Brown were 100fA leakage at best.

Few new parts are of similar level at 100fA leakage current. Reaching 2fA leakage is huge problem.

Yes, these instruments are excellent for Radiation Therapy Ion Chamber dose profile calibration but are very expensive instruments few thousands of dollars range. I usually find them in cancer hospitals.

I am looking for $100 range solution that can be used by university students.

A little late for a suggestion. Leakage current is very temperature dependant. Minus 5 Celsius may reduce leakage versus data sheet 25'C to 1/10. +1 'C to avoid ice formation?

1. Use a peltier cooler? About 20 dollars.
2. Run the whole head amplifier in the laboratory freezer?
3. Scavenge the cooler works from a "scrap" freezer - for many the fault may not be in the cooler.

When leakage is reduced significantly, feeding in an opposing offset current?

Peltier device Tec1-12706 about 5$. 40 x 40 mm - enough for head amplifier? Cooling whole shebang should reduce surface insulation leakage as well as semiconductor leakage. Consider head parts connected in air on the "quick/ugly" wiring method to minimise solid insulation.

Yes. See reply to similar #2 is #9 from me.

The equipment in beta gauging systems for on line inspection of metals and paper should be the design you are looking for. Some of the lower strength radioactive sources are krypton gas, directed into ion chambers. Not sure just how the intensity compares to your application, but the operating conditions are severe, think hot rolled aluminum plate or 70 mph wet & dirty paper machine web.

Pointer: the circuitry in chlorine instruments used in the clean water industry convert typically 100nA to a 4-20mA signal representing typically 0-2ppm free chorine.

Worthy of study, perhaps?

100nA is large current and easier to detect.

20fA level detection requires 2fA bias current circuits.