Neutron Scintillation Tube/Counter

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Dennis P Brown
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Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Tue Nov 14, 2017 9:35 pm

Again, I am getting done one of the last delayed projects from a year ago: this case, the neutron scintillation detector system.

I have neutron sensitive plastic (3 inch diameter, 2 1/4 inch thick; this plastic rejects gamma's; I believe it is BC-412 or the like.) Also, I have a photo-multiplier tube and the oil required for optical mating of the surfaces. And of course, a mating connector for the PMT.

I just assembled the main tube/scintillation plastic with oil and sealed (silicon sealant) this assembly in a plastic tube that the PMT/scintillator with its tape just fits into (see picture below.)

The end of the scintillation plastic (opposite to the PM tube) is covered with Al foil and I used the same oil to mate this to the plastic/PMT assembly. The foil is both highly reflective and smooth; with the oil, this should provide a high reflective surface to increase any scintillation events within the plastic back into the PMT. I am only interested in signal strength and not calibration. My driver/reader will be near the best voltage but barely.

This complete assembly is sitting on a square aluminum plate. A matching aluminum tube snugly fits over the PMT-scintillator & plastic tube. Then the lower plate bolts to that of the end of the square Al tube.

I need to shorten the square Al tube by six inches before I do the final assembly. The other end plate has a coaxial cable connector for the high excitation voltage & signal feed out. The PMT ground is the Al case and connecting coaxial cable jacket.

Hopefully, this both works and my ST-360 does not have noise issues like it does for my He-3 tube. That is an issue I will have to address later.

Since the ST-360 works fine with my gamma/x-ray scintillation tube I am hopeful it will work with this neutron scintillation detector.

My gamma ray scintillation detector end socket has a capacitor across the high voltage and ground pin. I was wondering if I should do the same for this tube (and wonder why this is done at all?) If anyone knows, please weigh in - thanks!


MVC-016L.JPG
Neutron Scintillation Detector Assembly

Silviu Tamasdan
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Re: Neutron Scintillation Tube/Counter

Post by Silviu Tamasdan » Tue Nov 14, 2017 10:57 pm

I actually have a BC-412 scintillator very similar to yours. It's 3inx3in and mated to an appropriate PMT.

It will have a gamma problem. I don't know why you think it rejects gammas. It responds to gamma radiation quite nicely. I actually never used it for neutrons (though I could). It should respond to fast neutrons, but you will have difficulty separating those events from ambiental gammas. In fact it is sensitive enough that I can discern a few ounces of potassium chloride with it - higher counts when it's close that when it's away.

Here are some quick stats from a recent run with my BC412.

Background

Code: Select all

Interval, min: 10:00
Trigger, mV: -15.0
Range, mV: 1,000.0
CPS
Average: 80.8
Deviation: 9.4
Total: 48,468
Double Events: 6,146
Rejected: 2,910
Noise: 19,524
With KCl sample (about 5 ounces)

Code: Select all

Interval, min: 10:00
Trigger, mV: -15.0
Range, mV: 1,000.0
CPS
Average: 98.8
Deviation: 10.7
Total: 59,262
Double Events: 7,293
Rejected: 4,682
Noise: 22,636
With 10uCi of 137-Cs

Code: Select all

Interval, min: 10:00
Trigger, mV: -15.0
Range, mV: 1,000.0
CPS
Average: 782.0
Deviation: 29.6
Total: 469,190
Double Events: 90,741
Rejected: 8,084
Noise: 133,559
With a 2x2x2cm cube of LYSO (lutetium yttrium oxyorthosilicate, contains 176-Lu)

Code: Select all

Interval, min: 10:00
Trigger, mV: -15.0
Range, mV: 1,000.0
CPS
Average: 128.4
Deviation: 21.6
Total: 77,049
Double Events: 10,686
Rejected: 2,956
Noise: 31,528
My BC-412 detector is the fat one next to the corona tubes in the picture below.

Let us know how the testing with yours goes; but AFAIK there is no plastic scintillator that responds only to neutrons, not gamma.

Also regarding decoupling capacitors, what I've seen done is a capacitor from the cathode to the last dynode (the one next to the anode); I've not seen it going from the cathode to the anode. I'm not an expert on those things.
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detectors.jpg
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Wed Nov 15, 2017 9:59 am

The data sheet on the plastic via the company indicates it does not see gamma's above 5 MeV; whether that is an issue for my system is yet to be determined. (here is the source for the data: http://www.hep.phy.cam.ac.uk/~lester/te ... _Sheet.pdf)

Still, I can only drive it up to 1200 volts (it can handle up to 1500 volts) so not sure if I will reach its optimum plateau or not.

Also, still wondering if I should put a capacitor across the HV (cathode) to last dynode pin (thanks for that correction, Silviu; as best I can tell, most do.) Just I'll add the caps as per most sites suggest. Not difficult to do since I have 2 kV caps in the correct size - assume it helps reduce depletion of charge on the dynodes.

Found a site where someone uses my specific scintillation tube and wires it up for use with a std geiger counter: http://www.diyphysics.com/wp-content/up ... -Probe.pdf

Here is where they convert a V-700 (or not so cheap anymore - unless you have one, hardly worth the cost considering the step-up transformers tend to fail and have no replacement; so definitely not worth buying for the conversion.) So, if you have such a unit, converting this surplus geiger counter into a scintillation reader/driver might be worth the effort (not adding a counter, however ... ): http://www.prutchi.com/pdf/radiation/CD ... torial.pdf
Last edited by Dennis P Brown on Wed Nov 15, 2017 2:50 pm, edited 10 times in total.

Silviu Tamasdan
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Re: Neutron Scintillation Tube/Counter

Post by Silviu Tamasdan » Wed Nov 15, 2017 12:27 pm

I don't think gamma above 5MeV will be a concern; I'd be surprised if you'd generate any of that with a fusor. But 50-100keV Xray/gamma will be a problem. And background radiation. As you see from my data above, BC412 reads about 80cps of background in my location.

As for the optimum voltage, my probe uses a XP5312 which works from 800 to 1100V. I found that it behaves in a fairly linear way from 900 to 1050V, but not so well behaved at the ends (800 or 1100V). If your PMT goes to 1500V, then 1200V will likely be enough IMO to put it in the sweet spot or close.

Capacitor: I've wired 2 PMTs on my own, and in each case the instructions I got with the tubes had me put a capacitor not from anode to ground, but from the last dynode before the anode to ground. The anode itself was not decoupled. They both work as expected. Not sure if the capacitor is required, but since the instructions had me put one there, I did.

You may want to look up seller "iradinc" on ebay, he sells a number of PMTs and kits at much better prices than the site you linked to above. He's the one I got most of my scintillation stuff from. For instance a Hamamatsu R10133 3-inch PMT complete with wiring kit and mounting hardware was about $60.
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Wed Nov 15, 2017 1:31 pm

Thanks for the comments and explanation on the PMT! Fixed my post.

Silviu Tamasdan
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Re: Neutron Scintillation Tube/Counter

Post by Silviu Tamasdan » Wed Nov 15, 2017 3:34 pm

I did a few tests with my BC412 probe and a Po/Be source to see exactly how well it works with neutrons. The gamma radiation is a big problem with trying to use this as a neutron detector. I'm pasting some results below, draw your own conclusions.

I used my 3-inch BC412, in a lead castle (pretty flimsy because I don't have enough lead). For the testing I checked 5 minutes each the background and the PoBe source with 3cm lead, 1.5cm lead and no lead between the probe and the source (or the place where the source would have been for the background tests). A caveat is that because I didn't have enough lead, I had to remove one whole side of the lead castle to use in the 3cm tests; and for that reason I decided not to put that side back when running the tests with the other lead thicknesses but rather leave it open so conditions varied as little as possible.

I then checked a gamma spectrum of the Po/Be source with a NaI(Tl) detector. Attaching the gamma spectrogram at the end (with background subtracted). The peaks I'm seeing are approx. 11.7keV, 83.3keV and 847keV.

Background, 5min, 3cm lead

Code: Select all

Interval, min: 5:00
Trigger, mV: -20.0
Range, mV: 2,000.0
CPS
Average: 21.0
Deviation: 4.9
Total: 6,310
Double Events: 800
Rejected: 319
Noise: 2,546
PoBe source at 10cm, 3cm lead, 5 min

Code: Select all

Interval, min: 5:00
Trigger, mV: -20.0
Range, mV: 2,000.0
CPS
Average: 25.3
Deviation: 5.5
Total: 7,575
Double Events: 990
Rejected: 302
Noise: 3,181
Background, 1.5cm lead, 5 min

Code: Select all

Interval, min: 5:00
Trigger, mV: -20.0
Range, mV: 2,000.0
CPS
Average: 22.7
Deviation: 5.0
Total: 6,815
Double Events: 988
Rejected: 296
Noise: 2,903
Po/Be at 10cm, 1.5cm lead, 5 min

Code: Select all

Interval, min: 5:00
Trigger, mV: -20.0
Range, mV: 2,000.0
CPS
Average: 31.8
Deviation: 6.7
Total: 9,550
Double Events: 1,399
Rejected: 310
Noise: 4,029
Background, no lead, 5 min

Code: Select all

Interval, min: 5:00
Trigger, mV: -20.0
Range, mV: 2,000.0
CPS
Average: 28.9
Deviation: 6.1
Total: 8,656
Double Events: 1,350
Rejected: 293
Noise: 3,834
Po/Be at 10cm, no lead, 5 min

Code: Select all

Interval, min: 5:00
Trigger, mV: -20.0
Range, mV: 2,000.0
CPS
Average: 54.7
Deviation: 7.8
Total: 16,409
Double Events: 2,507
Rejected: 311
Noise: 7,309
My interpretation is: the background does increase a bit as I removed more lead, but the counts that the source generate raise a lot faster. The neutrons shouldn't be affected (much) by the lead, so the raise in counts is likely due in large part to gammas that the source generates and which the lead shields. To elucidate I took the gamma spectrum of the Po/Be source with a NaI(Tl) probe which does not count neutrons at all. Picture below (with background subtracted). The source does generate gammas with energies around 11.7, 83.3 and 847keV (the numbers on the X-axis are not energies but channel numbers; I calculated the energies of the peaks by comparing with a 137-Cs spectrum taken with the same probe).

In my opinion if you want to use a BC412 probe for neutrons, be prepared to use some heavy lead shielding. I don't think 3cm would be enough.
Attachments
PoBe gamma spectrum.png
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Bruce Meagher
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Re: Neutron Scintillation Tube/Counter

Post by Bruce Meagher » Wed Nov 15, 2017 4:36 pm

If you want to detect fast neutrons properly with these plastic scintillators you need to implement pulse “shape" discrimination. The shape of the light output from a gamma event compared to a neutron event (longer tail) is sufficiently different that it is relatively straight forward to design a circuit to discriminate these events. See my previous post viewtopic.php?f=13&t=11444&p=75597#p75597

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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Wed Nov 15, 2017 4:57 pm

I would think that Fusor's produce a low gamma ray output compared to the neutron signature (when thermalized.) In any case, I have some massive lead shielding if I need it (a very heavy/thick Pb brick so, that is covered ... as long as I don't drop it ... lol.)

Fast neutrons, are an issue since the detector is sensitive to those but I'd think sensitive is a misleading term relative to number of actual events in the scintillation material. Pulse shape discrimination is, of course, the proper procedure as you point out.

Silviu Tamasdan
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Re: Neutron Scintillation Tube/Counter

Post by Silviu Tamasdan » Wed Nov 15, 2017 5:07 pm

You will need lead not only on the fusor side, but on all sides... you want to shield not only from fusor gammas/Xrays but background radiation as well. Reduce the noise as much as possible _before_ discrimination etc.
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Re: Neutron Scintillation Tube/Counter

Post by Rex Allers » Thu Nov 16, 2017 12:38 am

I'm inclined to agree with Silviu and Bruce that your BC-412 may be fairly sensitive to other things besides neutrons. Maybe some lead around the detector will help blocking things other than neutrons. But leaving that, you also seemed to not be too sure about the how and why of capacitors in a PMT divider chain.

Photonis doesn't make PMTs anymore, but I found this link that still seems to work to a general document they wrote about using PMTs.

http://lmu.web.psi.ch/docu/manuals/bulk ... basics.pdf

Page 24 of that document (I think pg 18 in the pdf file) has a section titled "Decoupling (Reservoir capacitors)" that covers this subject. I think it is one of the clearest and most detailed I have seen.

Hamamatsu also has a PMT handbook that you can find on the web that is good and covers similar material.

For a scintillation-type detector that is more specific to neutrons I think you could look at the ZnS-based detectors like this page from Eljen
http://www.eljentechnology.com/products ... -detectors

That page also shows commercial equivalents from other manufacturers. I think these don't show up very often in places like ebay and they tend to be expensive. They are also not very sensitive, so maybe not an ideal alternative.

Keep us posted if you continue down this path.
Rex Allers

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