MWD detectors.

[Adburhoe]

I'm trying to get a bit more information on the construction of scintillation detectors used on a drill head. I bought a MWD detector mostly for the crystal, but I assumed it would have a nice rugged PMT, and I could reuse that as well, but when I took it apart I got a surprise. No PMT. Can someone fill me in on what type of photodetector I have in the picture ? There are no manufacturer markings. I looks like it is coupled to a calumniator or similar. I'm not sure, it may just be a light pipe designed to allow for relative motion between the crystal and photodetector.

And lastly, is there a rule of thumb for how long a Nal crystal can be before the transmissibility begins to decrease the lower energy particles?

[Mike S]

Sorry for what may be a silly question because I can't tell from the picture, but could there be a PMT in that housing? What does the other end look like?

[Adburhoe]

I think it's possible, but unlikely. I was incorrect regarding the lack of manufacturer markings on it. The number "2616" is written by hand, in blue ink on the back of the photo detector, and it is also machine engraved on a metallic surface that is encased about an inch beyond the front glass window, adjacent to what is either a second smaller light aperture, or a circular array. It looks like a solid state detector of some type, but I cant fine any pictures of SiPMs that look similar. (I have no experience with SiPMs).

It may be a better choice for this crystal, (12" long NaI(Tl), I would expect some decay in pulse height that may be outside what a typical PMT is optimized to detect???? I'd like to find information on how to test it, drive it, or at least make an intelligent decision if I'm going to use a PMT instead. Given the application, the manufacturer may have chosen to use a SiPM to decrease noise when subjected to vibration, heat, high magnetic field, or similar situation that is only of concern when used in the intended application. In those cases, I may swap it out for a PMT. But if it was chosen because it is better that a standard PMT because of the signal decay in a big crystal, then I'm better off figuring out how to drive this thing.

Attached are some more pics. Also, going by the part number hand engraved on the exterior casing of the assembly, the detector was a Bicron, 2MWD12/1.2L-X. It was a 2mm thick 300 series stainless steel tube, with turned (as in machined on a lathe) endcaps circumferentially welded in place. The endcaps are over 8mm thick stainless, so I don't think they were too concerned about low energy particles!