Fast Neutron Counter

28 10 2010

Dear community,

Please help me evaluate this fast neutron counter:

http://www.drct.com/neutron_detection/Pug-7_Fast_Neutron.htm

Is this appropriate for measuring fast neutrons from deuterium-deuterium fusion?

Accurate neutron counts are crucial for both the research and safety.

This meter comes calibrated and is in my budget (barely).

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One response

29 10 2010
Mark Suppes

Erik Snyder over several emails:

Uhhh, hmm, I’d be a bit skeptical on that one. The base instrument is multifunctional to be used with GM tubes and I guess proportional counting tubes (by changing the bias voltage?). The ZnSAg scintillator would be used with some kind of photomultiplier tube. I’m not sure how a 2 wire interface would work with a pm tube…maybe it has an on-board voltage divider/power supply?

At any rate, if you want to use a simple base unit, I’d just go with a proton recoil tube for your wand like an LND 281, it acts a lot like a GM tube does.You’d have to shield with some fairly thick PE or PP plastic to keep some of the background counts down from cosmic rays and terrestrial beta.

I do think a scintillator is the more accurate way to go though, in which case I don’t think the PUG is the best unit for that, I’d get something that’s really intended to use with scintillator heads. The reasons are varied, but suffice to say that the pulse shape and amplitude is not a “one size fits all” to just detect with some simple monostable as they are almost certainly doing in the PUG 7. In fact, the shape and amplitude of your PMT output TELL YOU SOMETHING about your neutron spectrum, why throw that information away with a “dumb” detection circuit! If you are at all handy and want to keep the budget under control, you can just make your own base unit in a weekend.

Well, so what I’d do if I were you is determine WHY you need a real-time fast neutron counter or fluxometer. If it is to tell if you’re about to be dead in 72 hours from radiation poisoning, the PUG will tell you that. If it is to do a quantitative measurement, find a way to get a decent scintillation system working.

-Erik-
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Well, if you need to conserve cash but do good research, the rule is fix up something old or make it yourself.

You might find an old but functioning portable scintillator in a “scientific junkyard” like the ones out in the Boston area. You could also re-purpose an old 96 well scintillating counter like they use for tritiated assays in biomedical type work…just pull out the PMT and power supply and make your own signal processing and interface circuitry. By signal processing, the only must have is a transimpedance amplifier, which you can make for about $10 (my fave is based on the LF 411 op amp). Other signal processing can be done via post-processsing (MATLAB, C, etc.) or DSP. Regarding interface, that’s up to you in terms of doing A->D conversion of the waveform, counting, or just a meter. A good middle ground is to do proportional counting (counting in impulse bins) so you get at least some idea of the incident energy of the neutrons.

If you REALLY want to start out cheap, buy some scintillation material and a photodiode. The ZnSAg scintillator material can be mixed with paraffin wax and cast into a window for a photodiode, or you might be able to buy pre-cast “crystals”. There are photodiodes out there with on-silicon T.I. amps, all you provide is a power supply, and a couple of resistors and caps to set the gain and phase/frequency response. This method might take $200 to do well enough to be considered lab grade. If you go this route, be sure to buy a photodiode that has good response at 450 nm, which is the peak output wavelength of the phosphor. Your photdiode also will need at least 100 MHz of bandwidth to accurately follow individual events. Lower bandwidths will work, but individual counting will become impossible, and since you’re dealing with low count rates, that could make your measurements a lot less accurate.

Like I said earlier, the PUG will work, but the trouble with these multipurpose machines is that it is pretty dumb when it comes to triggering – it’s just looking for a rising edge. Such a system is pretty touchy when it comes to noise (just moving the cable around can trigger it owing to capacitive microphonics), and it cannot tell you anything about the energy spectrum of your neutrons. Furthermore, you have to turn a 1-turn pot on the front to CRUDELY adjust the bias voltage, and all it takes to blow out your PMT is for someone to accidently fiddle with that knob. Does it work well enough to tell you if your machine is on…yeah. Can it tell you whether a situation is Immediately Threateding to Life and Health (ITLH)? Yes. But can it tell you whether you’re dealing with 50 millirems/hour versus 40 millirems/hr? Pretty doubtful. Scintillators do not at all behave like gas discharge tubes, and anyone that tells you that there’s no difference in operation is either a liar or doesn’t know a blasted thing.

By the way, if you want to know about gas discharge tubes (GM or otherwise) give LND a call. The two guys that run the place are old physicists, and while they can be hard to get hold of, they are really good people and they know what the heck they’re doing. They’re located pretty close to you too, just a ways out further east on Long Island.

-Erik-
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Here’s some ideas for a near plug-and-play system:

Thorlabs PD-10A Amplified Silicon photodiode ($248- cheap but a little bit noisy):

http://www.thorlabs.com/thorProduct.cfm?partNumber=PDA10A

Hammamatsu PM modules (lower noise, a bit higher cost):

http://sales.hamamatsu.com/assets/pdf/parts_H/m-h7360e.pdf

http://sales.hamamatsu.com/en/products/electron-tube-division/detectors/photomultiplier-modules/part-h7732p-11.php

Saint Gobain scintillator disc:

http://www.detectors.saint-gobain.com/uploadedFiles/SGdetectors/Documents/Product_Data_Sheets/BC436-Data-Sheet.pdf

or the BC 720:

http://www.detectors.saint-gobain.com/product_detail.aspx?id=161938

You put it together in an hour or two, and you save a lot of money and end up with a calibrateable, laboratory grade instrument instead of a civil defense safety tool.
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One thing I’ll say right now though, is you’d better get used to DIY, you are rapidly moving into territory where you’ll have no choice. Your quadrapole magnets aren’t COTS and you sure as heck aren’t going to get shimmed properly until you learn the art of magnet shimming (and winding) yourself.

Turnkey solutions are nice in that they CAN save you time and money, but don’t get too comfy: they often aren’t “as advertised”. I have experienced this a lot lately, both in terms of quality and in features/functionality of a lot of different equipment.

I realize you want to get to a solution fast but let me tell you, unless you can hire on REALLY experienced help, you’ll just have to run up the learning curve hill yourself eventually. There is just no way around it, you are in an area that requires putting in a lot of dues in terms of ancillary knowledge. Don’t be intimidated by it though, it’s lots of fun, and you might just discover some unique things along the way you wouldn’t have otherwise. The building of a scientfic apparatus is an excellent education that few ever get, and it will help to discipline your thoughts in much the same way that learning to program did.

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