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RE: New Detector
Superconducting tunnel junction sensors (TJS) and
transition edge sensors (TES) are remarkable for their energy resolution for
soft x-rays. These are important technologies which have been under
development for many years, and have applications in x-ray fluorescence,
beam diagnostics and astronomical instrumentation. TES devices achieve
very good energy resolution, but at very limited count rates only. TJS
devices sacrifice some resolution, but are capable of rates in the kHz.
These devices are typically thin films (<1 micrometer) with areas of 1
mm2 or less to limit the main source of noise (leakage current). Area may
be scaled by using arrays.
For
gamma spectrometry, LLNL has reported a microcalorimeter made by coupling
a high Z absorber to a TES, which responds to the temperature rise due
to photoelectric absorption. A thick (250 micrometer) tin absorber layer
gives >70% stopping power out to 60 keV. But a fundamental
limitation is the need for small absorber mass to produce the rise in
temperature. With such a small mass, higher energy gammas (e.g. 662 keV)
cannot produce a photopeak, since Compton scattering dominates, and in the
unlikely event of PE absorption the photoelectron range is comparable to or
greater than the absorber dimensions.
Actually, I think you are looking at two
different systems. Simon's tunnel junction detectors are rather large,
or at least they were a year and a half ago when I left LLNL.
Also, for those of you questioning wether someone is simply jumping on
the homeland security bandwagon, you should probably consider whether this
work has been in progress long before homeland security was even a phrase
(which it has).