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Drone-borne HPGe - sense or nonsense?

One of our clients asked us our opinion about the feasibility (and need) of a high-resolution HPGe system as a sensor for drone-borne gammaray mapping. Companies like Ortec sell "handheld"HPGe systems nowadays that can be used in the field. Check for instance their "Micro-Detective Ultra light" - a system for outdoor use in security applications. A very nice system if you need to have high-res spectra to identify cocktails of (man-made) nuclides. However, to hang 100k+ system underneath a drone...well, I don't know. 

Modern scintillators like LaBr3 or CeBr3 also provide pretty high resolution spectra, but are much simpler in use. To get a grip on the options I did a little study into into the comparison of a HPGe detector vs a high-res scintillator like CeBr3 or LaBr3, for “outdoor” surveying purposes like on a drone.

There are a number of factors that determine the applicability of a HPGe in the field. Main “pro” of course is the high resolution of a HPGe system. Another “pro” is the ability to measure low-energy lines. 

However, in the field the situation is very different from the lab:

  1. The HPGe needs cooling which is one way or the other, implying much more energy use (several 10s of W) than a scintillator (less than 2W);
  2. Low-energy lines can only be measured if the system has a very thin “window” in front of the Ge crystal. This seems unachievable in the field, because of the need for proper robust housing;
  3. Weight is an issue – the Ortec handheld device (Micro-Detective Ultra light) is 7 KG. For that you get a 50x33mm Ge detector which has about 15% efficiency w.r.t a 3x3 NaI
  4. Detection efficiency is an even "bigger" issue - the said 15% efficiency w.r.t a 3x3 inch NaI translates to less than 2.5% of the efficiency of our MS-1000 drone detector. Which is less in weight (6kg) and power consumption (few W). To achieve a proper spectrum, the HPGe data acquisition will take 50 times longer than the MS-1000;
  5. The resolution of a HPGe is unparalleled. It is about 0.1% at for instance 1.3 MeV (Co-60). LaBr has about 2.1% resolution, CeBr about 2.8%, CsI about 6.5%. However, for many if not most applications “in the field” the resolution of the LaBr or CeBr  is by far good enough to separate the peaks – for instance for radon studies, out-of equilibrium Uranium or large parts of the man-made "cocktails" of interest to people in the security domain;
  6. Overall robustness and price (>150k) may make it difficult to accept flying a HPGe under a drone. Who is going to insure this?
  7. Operability. The HPGe is really made for “stand still” measurements. Which makes sense in view of the long acquisition times needed to achieve some accurate readings. This will basically prohibit use of a HPGe as a real “surveing” instrument mapping area’s “on the fly”

All in all we think that - at least for "roving" or "mapping" applications, a HPGe seems not the way to go. A (much less expensive) modern crystal-based system like CeBr3 or LaBr3 would do the job much more efficiently.



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