Fusor Forums

The neutron sensitive plastic I have (a rather large piece) has a bit of crazing on its surfaces. I have been using plastic polish and a buffing wheel to remove the more shallow "cracks" but the deeper stuff will require that I create shallow depressions in my detector surface - no way I will regrind and polish this block flat; just not interested in that level of effort. As such, does anyone know what is the best contact gel or silicone sealant to use as an interface for the plastic and photo-multiplier and where to get said material? I require such a material to both provide a good optical transmission interface between the plastic and detector, as well as fill in any depressions.

Also, since I have a large hole saw (same diameter as my photo-multiplier tube face) I would like to stack a few cylinders of cut plastic together to increase the collection volume and create a good optically clear interface between the stacked cylinders.

The plastic block I have is BC-412 - its peak emission spectra occurs in the 420-460 nm; so, rather blue when it is stimulated by neutrons.

Here is an old post:
viewtopic.php?f=13&t=6255&hilit=+coupling+gel

If you follow the links in there is another set of post about coupling gel.

The "real" coupling gel is quite thick and expensive. I had a scintillation block that was quite crazed and I had to machine most of the surfaces out but not all. Used NOVUS platic polisher kit.
Someone here pointed out thick oil for RC differential gears used in carts. It's available in many hobby shops. It worked quite well and was drawn in the surface cracks after a few days.
I did not have any issues with the oil seeping out after several years either. Just be careful on how much you use.

After all these years the interface between block is near perfect.

Hope that helps.

Dennis,

The following link has some info about different optical coupling greases http://www.logwell.com/tech/servtips/op ... rease.html

I saw this on eBay http://www.ebay.com/itm/262765111357?, but I'm not too sure if it's really a grease (vs oil). Ideally you want to match the refractive index (RI) of the optical coupling grease to the RI of the PMT window material, or the RI of the detector material, to minimize light losses.

Are you thinking of using pulse shape discrimination to differentiate the neutron interactions from gamma interactions in the BC-412?

Bruce

Very nice posts; I'll order some of the oil - looks promising since it could help reduce the effects of the crazing. Thank you both for the help!

Rich, Bruce
I remember Richard stated that plastic scintillators weren´t adequate for fusor work. I supposed was related to lower sensitivity of plastic scintillators.
Am I wrong?
Roberto

Here is the quote:

Re: FAQ - Choosing a Gamma Spec Scintillator detector

Unread postby Richard Hull » Thu Mar 24, 2016 11:44 am

Plastic scintillaors can never be used for Gamma Spec work. Absolutely unacceptable.

Richard Hull

This plastic is for neutron detection and BC-412 plastic is sensitive to neutrons; it is doped to fluorescence efficiently upon impact by fast neutrons. The company specifically states in the technical paper that it is insensitive to gamma's (5 MeV >.) Relative spectral resolution, Richard is correct. Of course, insensitivity to gamma's is a desirable feature for a neutron detector.

Thanks Dennis!

Roberto

Bottom line......Use the 412 and put an extremely hot source of gammas near it. A Cs137 10 uCi source is good...(easily purchased). You should see nothing. If you see something it is no good. Still, A gamma detection might be able to be windowed out, electronically, provided the neutron signal is much more intense. What is the efficiency of 412 for neutron detection???? Will it need a moderator or is it detecting fast neuts directly???

The BC720 I struggled with between 1998 and 2002 detected only fast neutrons and was about .5% to 1% efficient. You need a torrent of neutrons to get a few detects/ second.
I spent over $600 with Bicron on neutron plastic detectors in the early days and was grossly disappointed. Any plastic other than the BC720 in a detection scenario would be suspect in amateur hands due to a very low efficiency and proper windowing warranted by the user having an instense gamma and neutron source at hand to convince a critcal review of their neutron detection results via proper level detect isolation.

I know ...........Been there and barely done that successfully for only $600.00 spent.

Richard Hull

Thanks Richard!

Since my fusor puts out at least 3*10^4 neutrons/sec and I suspect a good bit higher I would hope that is enough; so, while hardly "hot" I again hope that my $120 worth of parts (plastic/photo-multiplier/interface board) and with my ST-360 counter I could then obtain enough counts so that I am above any noise floor. If not ...well, not a big loss but I am hoping this plastic works because noise issues from my Russian tube is a huge problem now. (My other scintillation detector system is only for x-rays/gamma but does work well - even picks up Beta (rather shocked by that ability.) But that system is solely used as a radiation safety detector system.)

Dennis,

BC-412 is sensitive to both gammas and fast neutrons according to the Saint-Gobin datasheet. I believe pulse shape discrimination can be used to separate the two types of events.

Check out:
viewtopic.php?t=9019&p=61738,
http://www.pnl.gov/main/publications/ex ... -21609.pdf, and
"Plastic Scintillators with Efficient Neutron/gamma Pulse Shape Discrimination” by N. Zaitseva et. el.

I too have a ton of this material and hope to do some PSD experimenting in the distant future.

Bruce

Quick update on my scintillation neutron detector project:

I cut out three "cylindrical pieces" from the neutron plastic "plate" I had obtained. One was a disaster (it moved while cutting); one turned out mostly ok; and the one was a perfect cut. Polished the perfectly cut one (after I melted it free from the circular saw bit but that is another story) and it is now crystal clear. Will try the same polishing methodology on the second cylinder that was just ok. Also, I received the oil to couple the detector to the plastic cylinder (and I will also use some of that oil to mate together two of the cut cylinders together in order to increase my total effective volume.)

Have to decide the best way to hook up the output to my power supply/amplifier/counter. Apparently, there are two methods (ground the cathode or the anode.) I have a metal shell to put the detector in for shielding (mostly for EMF but will stop low energy x-rays.) Just have to free up some time to tackle these last assembly steps.

Bruce; thanks for the information. My oscilloscope's sweep feature failed and no way I am getting another one. Still, my fusor supplied 25 - 30 k's of neutrons (using a russian BF3 detector) so I would hope I can detect that level above a background from cosmic rays (hopefully, not famous last words ... lol.)

Hope to get some results in the not too distant future.

I use Parker Super-O-Lube for coupling gel on the secondary electron detector for my SEM.

http://www.skygeek.com/parker-super-lub ... Akxl8P8HAQ