HotSpotter? Neutron/Gamma Detector Page: 4 of 10
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HotSpotterTM Neutron/Gamma Detector
Z. W. Bell
Y-12 National Security Complex
Oak Ridge, TN 37831-8084, USA 865/574-6120
D. E. Smith, W. H. Andrews
Oak Ridge National Laboratory
Oak Ridge, TN 37831-6002
The HotSpotterTM Neutron/Gamma Detector combines in a single detecting module high sensitivity
to gamma rays up to 3 MeV and sensitivity to neutrons. Using a 15 mm cubic CdWO4 (cadmium
tungstate) crystal mounted on a 25 mm photomultiplier, the instrument realizes a factor of 5
increased photopeak efficiency over NaI(Tl) at 1 MeV, and a factor of 2 improvement over CsI(Tl).
The addition of a 0.5 mm layer of 10B- impregnated epoxy covering the crystal provides neutron
sensitivity without sacrificing gamma ray spectroscopic characteristics. Neutrons are detected by
the presence of the 478 keV gamma from the 10B(n,cL)7Li* reaction. In this paper, we describe the
electronics and software of the instrument, and some of its characteristics.
The detection and characterization of radioactive material is an important aspect of safeguards and
security. Instruments performing these functions find application in screening packages for
radioactive materials, personnel or area dosimetry, criticality alarm systems, portal monitoring, and
tamper indicators for stored uranium and plutonium. The usual implementation is via sodium
iodide scintillator, and although these detectors are fairly robust (7% resolution at 661 keV) and
inexpensive, sodium iodide is sufficiently inefficient above 1 MeV that other crystals become
Cadmium tungstate is a dense (7.9 g/cc) inorganic crystal with high average atomic number. It is
not hygroscopic (and therefore does not require hermetic sealing), and is readily available as
rectangular prisms. At 1 MeV, the photoelectric contribution to its linear attenuation coefficient is
0.0590 cm1, while it is 0.0129 cm1 in sodium iodide. Thus, at high energies, an improvement in
photopeak efficiency by a factor of 4.6 results from the substitution of a cadmium tungstate crystal
equal in size to a sodium iodide crystal. This improvement in photopeak efficiency makes it
possible to detect peaks in pulse height spectra with smaller, lighter detectors.
Cadmium tungstate's high photopeak efficiency enables thermal neutron detection through the
detection of the 478 keV gamma ray resulting from the 10B(n, a)7Li* reaction. When clad with a
thin, 10B-rich material (such as 0.5 mm of boron-impregnated epoxy), the gamma rays from the
reaction have approximately a 50% probability of entering the crystal, where they are detected with
a probability dependent on the size of the crystal. For 15 mm cubes (the size used in HotSpotterTM)
the efficiency is approximately 37%, resulting in an overall thermal neutron detection efficiency of
about 18%. The presence of a thin layer of low-Z, low-density cladding does not significantly
affect the spectroscopic properties of the crystal.
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Bell, Z. W. HotSpotter? Neutron/Gamma Detector, report, April 1, 2003; Oak Ridge, Tennessee. (digital.library.unt.edu/ark:/67531/metadc734451/m1/4/: accessed December 11, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.