Discriminator Help

[Aidan_Roy]

Unexpectedly, I had enough time today to really take a look at my NIM stuff and conclude that something is going on with my discriminator. Now, I do say something because it could be an issue or it’s something I am doing wrong. I don’t strongly suspect that second option due to the behavior I’ve seen from this unit.

To start off I’ll describe the electronics I have in the bin. It is a tennelec tc243 amplifier set to unipolar, positive, Gaussian output. The input is fed by a tennelec tc 171 preamp and SI-19N He3 proportional tube. The output of the main amp goes into the input of the nuclear semiconductor 532 high energy discriminator. The output of the discriminator is to the input of my nuclear Chicago counter. I also have the second output of the amp connected directly to an ortec 441ja rate meter.

Now onto the issue. When everything is turned on and the tube is being fed the 1310 volts that I found results in approximately 1 cps from the tube when exposed to uranium ore supposedly measured at 92kcpm by two Ludlum 2200s, if the discriminator is set to anything less than 8.5 out of 10 the counter picks up several 10s of thousands of counts per second. Meanwhile, the ratemeter is counting just the 1. If I connect the rate meter to the discriminator and the counter directly to the amp output the opposite happens. 10s of thousands on the ratemeter and approximately 1 cps on the counter. At a discriminator setting of approximately 9.33, the counter and ratemeter seem to agree with one another on the reading. If I max out the discriminator, I will still get close to 25 counts over a period of 10 minutes when there is no gamma source present near the tube.

The amp is set to a course gain of 200, a fine gain of 0.65, and a peaking time of 4 microseconds.

Pictures of the discriminator and it’s very simple circuitry are shown below. I couldn’t find any issues just by looking, none of the components seem missing or damaged. Any input is welcome.

Aidan

[Richard Hull]

You need to start near the beginning. Scope the output of the preamp! I assume that is where the tens of thousand of counts are coming from. Adjust the voltage to the 3He tube, (probably down) while scoping the output of the preamp. Do this first. You should not need too much gain in the amp after the preamp. I am a bit "put-off" and puzzled by the term "high energy discriminator". One would normal want a windowed spectroscopy amp NIM module. (combo amp and discriminator)

Richard Hull

[Aidan_Roy]

Hi Richard,

If I had an o-scope, the first thing I would do is check the preamp and make the rise time adjustment. I don’t really suspect that the preamp is the source however, due to the behavior described in the original post (those counts only appear once the counting device is connected to the discriminator). I also do not think that the tube bias should be turned down much more. Too far below where it is currently and the rate meter will no longer have the precision to detect any counts at my current gain setting. According to the seller, the optimal voltage for proportional mode is 1400 volts.

While following the procedure listed in the FAQs, as well as in the Carl Willis videos, I found that around 1390v will give the approximately 5 cps desired from a gamma source. And after very slowly adjusting that down, 1310 was the best and most reliable level to receive roughly 1 cps from the gamma source.

I agree that the name “high energy discriminator” isn’t sitting right with me. When I had purchased all of the gear a few months ago, there were no good looking SCAs available at a reasonable price so I purchased the amp and discriminator as separate units. I assumed that this one would be okay because it looked to be in great condition and, since it’s a simple component on its own, I hadn’t assumed that it would do anything more than discriminate signals for me. I have been completely unsuccessful at find any information about this unit online.

What I am thinking is that the discriminator might be seeing ringing in the signal resulting in it producing a cascade of extraneous output responses. I think it has an automatic gain control and this could be causing it. If it is the case, adjusting the rise time would be the only way to stop it. This is just my best guess however and I’m not confidant in it.

Aidan

[Richard Hull]

That's the reason I said I was also suspicious of that discriminator. If you are confident in all the electronics before it, look for another discriminator. However, a scoping is almost a must, starting at the head end. You are shooting rather blind here even though it sounds like you did the right thing based on Carl's superb video's.

Richard Hull