Proton recoil scintillation fast neutron detectors have been around now for quite some time, ever since the Hornyak button, it has had some improvements, and we know that they all pretty much work. The complaint however has always been the same: "Not sensitive enough" and the recommendation to follow is usually: "Go get an He3 tube, with a moderator". While this is true, and the advice is spot on, I am not completely satified that this proton recoil scintillation concept has been taken to anywhere near its limits.
First came the Hornyak, then Bicron came along with the target shaped rings of ZnS(Ag) scintillator, inside of a 5/8 x 2" clear disk. Finally came PRECILA, here ludlum took 4 of the 2" Bicron style scintillators, and attached them to a square Lucite block to combine the light. Ludlum also added a small Li doped plastic ZnS(Ag) thermal neutron section, to now cover the entire neutron energy range up to 20 MEV. This design is of course for commercial niche markets like weapons labs, and reactor facilities, etc, and not the home lab.
For the home lab, fast neutrons are what we are mainly after, like for fusors, potential drop accelerators etc. The new PRECILA detector head hits a sensitivity of .1mrem/hr, which is pretty darn good, but still nowhere near as good as an He3 tube with a moderator. This brings me to my point that the proton recoil scintillator, may still have a lot of room for optimization, and increased fast neutron sensitivity. The sensitivity of these is directly related to the scintillation light collecting area geometry, of the ZnS(Ag) doped scintillator, within the clear plastic matrix. All of the approaches that I have seen use only one dimension, maybe 2 if you include depth, of the clear plastic matrix, to fill with the ZnS scintillaton material. Yes they now have larger, up to 5" diameter units, for $750, all for $20 worth of plastic, and 10 bucks worth of scintillator. Below is an idea of how I propose to boost the sensitivity of a proton recoil scintillator, to a higher level:
Specifically I was thinking: What if you instead made a similar 5-6"D version of the target ring style button, and then you machined a Lucite cone shaped reducing adapter / light guide section. To collect all of the scintillation light, and to couple this to a common size 2" PMT, it could all even be a one piece design, with the target rings cut into the end of the Lucite cylinder, to be filled later with the optical epoxy 5% ZnS(Ag) mixture. This should help, but it is still not what is possible.
To take this a step further, what if your cut linear grooves into the side barrel surface of the Lucite cylindrical section, which attaches to the PMT, and then filled these same grooves with the scintillation mixture, that fills the end rings. These scintillator filled linear grooves on the side of the barrel, would be in addition to the target style ZnS filled ring grooves on the end away from the PMT, which normally contain the ZnS(Ag) epoxy scintillation mixture?
Various different end geometry ZnS(Ag) fill methods have been tried, but none have used the outside barrel surface, of a longer length clear cylinder. Figuring the ideal length and groove geometry would not be that hard, and this longer length collecting area should work, since Lucite is pretty clear to the blue scintillator wavelength.
The final idea is: Progressively deeper linear groove’s cut into the cylinder, instead of just the radial end ones, which deepen as you go away from the PMT end of the cylinder, going towards the end. Fabrication would be simple: A simple table saw, with an angle jig, could be used to cut the progressively deeper grooves into the sides of the Lucite cylinder. To boost the scintillation collecting area even more, alternating grooves could be cut to a different depth. The grooved cylinder could then be placed into an internally polished Al tube, with one end closed. It could then be back filled under vacuum, with the ZnS(Ag) optical epoxy mix. The only thing left would be to mount it to the PMT.
(Since posting this, I have learned that Doug already had an idea like this before: By packing plastic rods as a light guides, and then filling the space between them with the ZnS/Epoxy scintillation mix. On this variation do you frost the plastic rods with sandpaper first, for better filling and coupling to the epoxy scintillator mix? I would also think that there would be an optimum ratio between the rod diameter and the total length, do you remember the size used? It would seem that hexagon shaped plastic rods would be ideal in order to to better regulate the scintillation layer thickness? I am now thinking of using this method between my 2" Bicron copy, and the PMT for some added umpfff. Since Doug mentioned that the ZnS Has a high index of refraction, which drops the light transfer efficiency: Maybe a liquid filling with an high index gelled ZnS containing fluid would increase the light collection efficiency even more.)
(Sorry Doug, Now I get it "Meterology" Sorry I did not get the spelling, I thought this column was for weather related stuff, seriously. This is why I put the post where it is, I will pay more attention in the future.)
These ideas are just some thoughts on the matter, maybe someone else could think of a different approach to increase the available ZnS(Ag) epoxy scintillator filled area. Thanks for your thoughts.....Lutz
PS: Please let me know if you have 20 g to an ounce or so of ZnS(Ag) to sell, or if you want to combine an order, thanks. Attached are the detail specs for the Ludlum PRECILA device.