FAQ- Home built preamp using the Cremat CR-110

[Richard Hull]

I have always wanted to try building a proportional detector preamp as is commonly demanded for 3He and BF3 neutron detector tubes. The CR-110 preamp module has been mentioned many times here. It is pretty straightforward to use in a battery powered, compact metal case.

As such, good preamps are a key to neutron detection with proportional tubes, I decided to make this build a FAQ rather than have it lost amid the normal postings.

All preamps used in fusion detection are of the charge delivery type. Thus, we need a charge sensitive preamp. The discharge in proportional nuclear detectors are producing charges in the nanocoulomb or less range. As such, very tiny energy collection and amplification is demanded to be able to deliver a strong enough voltage signal to drive follow-on instrumentation to show and record a detection. The first thing such a preamp must do is offer an ultrahigh impedance to the the detector that will not swamp-out the weak detection charge. This is fulfilled with an FET in modern preamps. After the FET, a secondary amplifier allows most premaps to put out a voltage signal pulse on the order of 50 millivolts or more. The CR-110 has all of this fully wired into a nice little package.

I attach the pdf file by Cremat showing the setup I used at the bottom of the pictures. I attach some images of my simple build and a scope shot of the output detection pulse.

Pictures are worth a thousand words and the adroit among us can see how easy it is to build around this pre-done module. Prices range from $45 to over $100 for this key component. Nucleon has the best price I have seen. I bought two. Note: I made the CR-110 mount in a gold augat strip for ease of insertion in a non-solder critical pin connection.

Note: All attempts to build such a sensitive device must be fully enclosed in an all metal box!!! (See photos) Such sensitivity can easily pickup and heavily amplify tiny stray voltages and currents so prevalent in today's world. Fully shielded cables are needed for all inputs and outputs.

The key here is to keep the HV/High voltage out of the CR-110. Good HV capacitors are a must. I lowered the input capacitance from Cremat's diagram's .01 to .001ufd @ 3kv. 500pf might have been even better. The odd BNC HV selection was based on the cables I had on hand, but proved sufficient to the voltages I planned to use. (I hate making up cables)

Richard Hull