FAQ - NIM SCA? Tuning an SCA for Neutron detection

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Richard Hull
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FAQ - NIM SCA? Tuning an SCA for Neutron detection

Post by Richard Hull »

The mystery of using and setting up an SCA (Single Channel Analyzer) as a component module in a NIM neutron counting system is the topic here.
Its purpose is to create a narrow window of pulse voltage ranges for output as a suitable and valid detection pulse that fits the "window". If a pulse does not reside within the window, in will not make it to the output of the SCA. Variable amplification is another feature of the SCA. Some pulses on the input are too weak for proper analysis. Boosting their voltage will help set the window with more ease.

You will need the following
1. NIM bin
2. Neutron detection tube ( 3He, BF3, other)
3. NIM bias supply (0-2500 volt variable)
4. External current sensitive preamp with cabling to and from NIM system modules
5. SCA module (example here is a Tennelec 246)
6. Counter/timer NIM module or a simple external digital counter with a stop watch.
7. A good oscilloscope would be a big help in this setup. If you do not have one, find someone who does and ask them to help out.
8. Hot gamma source. Ideal (old WWII heavily loaded radium instrument). Ultra hot piece of Uranium ore, Ultra hot piece of Monazite (Thorium ore)
You can readily purchase a 10uCi Cs137 source

I attach the basic setup diagram in the first image below.

The first order of business is to use and oscilloscope and set the bias voltage for the tube.
Set the SCA as follows.....
Set the lower detect level on the knob to 000
Set the upper level knob to 999
Set the gain to 50 or 100
Put a scope on the output of the SCA....Set the scope voltage knob to 50 or 100millivolts per division, (.1volt/div). Set the time base on the scope to about 200 usec/div.

Place a very hot gamma source touching the detector tube.

You should have an idea, based on the tube type, and data sheet, what the maximum bias voltage should be. Ideally, some manufacturers give a typical bias voltage operating range.

Turn the bias supply to zero volts and slowly bring up the voltage, approaching but never exceeding the maximum.
Well before you hit maximum you should see some 10 -20millivolts or even 100 millivolts of gamma detection "hair" , on the scope as in the second image below.
Now you are very close to the correct bias setting. Leave this bias setting alone now. Record this bias setting.

Now back to the SCA
Watching the hair just above the scope's zero volt line, raise the lower limit control slowly until the gamma hair completely disappears. Let us say this occurs at 132 on the dial. Advance it to 200 on the dial.
Now set the scope voltage level to 2 volts/division.
Carefully over a number of minutes, watch the scope. You should see an occasional real spike show up on the screen. This is a super hot cosmogenic particle detection.
You are probably close to proper bias and SCA detection level settings for neutron detection.
The scope has really helped here. You are now done setting up the SCA


WAIT! What about the upper level setting? I tend to leave my SCA upper level knob at 999. However once you have a whole lot of neutrons (200cpm or more) feel free to lower the 999 down until your neutron count drops noticeably. Now, you can inch worm it back up until your counts are fully restored.

I attach a cut sheet on the Tennelec 246 so you can see the typical controls found on an SCA.

Richard Hull
Attachments
NIM neutron block diag.jpg
preamp to SCA.jpg
tc246a.jpg
tc246b.jpg
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Finn Hammer
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Re: FAQ - NIM SCA? Tuning an SCA for Neutron detection

Post by Finn Hammer »

I think, perhaps, that in the opening paragraph, it would be beneficial to put emphasis on the "analyzer" aspect of an SCA, instead of that of the amplifier.
Although it does amplify, it is the variable discriminating window that warrants its place in a bin for neutron detection.
I may be wrong.

Cheers, Finn Hammer
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Richard Hull
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Re: FAQ - NIM SCA? Tuning an SCA for Neutron detection

Post by Richard Hull »

Thanks Finn. My mistake. I knew the "A" in SCA was for analyzer but my brain was working on what was left of the sugar in a chocolate bar when I composed the verbiage for the acronym. Probably my mind was racing ahead to what I was going to say about the pre-amplifier. Funny thing, the human brain. Of course, mine is 76 years old and has seen a lot of coming and going. I tend to write while thinking 6 sentences ahead. I do not mentally multitask as I once did.

This gave me a chance, in hindsight to expound a bit more on what it does to a more suitable degree.

It is now correctly edited to the true verbiage for SCA.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ - NIM SCA? Tuning an SCA for Neutron detection

Post by Rex Allers »

Seems the "you will need" list should include the hot gamma source too.

Might also mention somewhere kinds of things that might be appropriate for that source.
Rex Allers
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Richard Hull
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Re: FAQ - NIM SCA? Tuning an SCA for Neutron detection

Post by Richard Hull »

Done! The best gammas come from Radium's many daughters and old Thorium chemicals and especially hot Thorium ores 2.7 meV gammas. Naturally, store any of these materials and items out of doors in gasket sealed containers. The two radon isotopes, "emanation" and "thoron" pour out of these sources. No problem indoors during the short time they are used for testing and adjusting the SCA.

Thanks go to both Finn and Rex for helping me update and correct the original post with their fine point observations. These are the type responses that feed any FAQ to make it better.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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