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Selecting Your Neutron DetectorNucsafe offers systems with either 3He pressurized gas tubes or a unique neutron sensitive scintillating fiber originally developed by Pacific Northwest National Laboratory for the Department of Energy. No single detector technology is ideal for all applications. However, specific detector technologies may out perform or provide inherent benefits to a given application. Each neutron detector type offers some unique features. 3He Pressurized Gas Tube Proportional Detectors The most popular detector for thermal neutrons has been the gas-filled sensitive proportional detector, charged to a pressure of several atmospheres with 3He gas. 3He is expensive and these detectors are limited to count rates of < 105/sec by gas recombination effects. 3He filled proportional counters come in a variety of active length and diameters and pressures. They offer many of the performance characteristics required by both safeguards and spectroscopy applications: high sensitivity, moderate operating voltage and excellent spectral resolution. Gas mixtures can be tailored for optimization of combined parameters including operating voltage, pulse rise time, pulse jitter time, gas gain, spectral resolution, neutron sensitivity and gamma ray sensitivity. Multiple detectors can be arrayed with a single high voltage and counting electronics channel.
3He and 10B gas detectors have a better gamma ray into neutron discrimination capability than solid state neutron detectors because of the lower electron density of a gas tube relative to the higher effective atomic number of a silicate glass. The neutron capture cross section is also a factor of 4 to 5 higher in 10B and 3He respectively.
Nucsafe uses a variety of advanced technologies in our products. However,
our core technology is a scintillating neutron sensitive glass fiber for neutron
detection. It represents the first compelling new technology for thermal neutron
detection since 3He and 10BF3 tubes. Its key
advantages over gas tubes are its sensitivity, ruggedness, flexible geometry,
fast timing, and wide dynamic range. The scintillating glass fibers work by
incorporating 6Li and Ce3+ into the glass bulk
composition. The 6Li has a high cross-section for thermal neutron capture. The
capture reaction produces a tritium ion and an alpha particle and kinetic
energy. The triton will likely interact with a Cerium ion through Coulombic
(electrical) interactions. This interaction results in the excitation of one of
the Cerium atom’s electrons. The resulting de-excitation of the electron
produces a flash of light. This scintillation propagates through the glass fiber
which acts as its own wave guide. The fibers are optically coupled to a
photo-multiplier tube. At this point, the light is multiplied and converted to a
electronic pulse that can be processed and counted.
Although the neutron capture cross section of 3He and 10B are 4 to 5 times larger than 6Li, there are many more atoms in a solid glass ribbon than a pressurized gas tube. When comparing the technologies for sensitivity, the product of the number of atoms times the cross section for neutron capture provides a reasonable method. The following figure presents a comparison for a portable system. For a typical 500 cm2 Guardian PRST system, it would take about 7 150 cm3 3 atmosphere 3He tubes to provide the sensitivity.
The neutron-sensitive scintillating glass fibers spatially distribute these atoms more efficiently. This results in larger active areas of neutron detection with higher sensitivity at lower cost. The 6Li is difficult to separate from the glass matrix while 3He can be easily extracted from a gas tube and then purified to enhance the yield of a nuclear device in concentrated forms. As a result, the glass fibers pose a low and acceptable nuclear proliferation risk. The solid state nature of the glass fiber technology is inherently safer. There are no explosive hazards with glass fiber, as there is with a pressurized tube of gas ranging from 3 to 20 atmospheres of pressure. As there are no transport hazards, glass fiber systems can be shipped on commercial carriers as regular cargo, can be transported as passenger luggage or even used as in-situ detectors built into the aircraft; in fact the glass fibers have been space qualified by NASA. Glass fibers are also less sensitive to vibration and other microphonic sources so it may be used while being moved in mobile and aerial measurement systems or when operated in industrial environments. The neutron sensitive scintillating glass fiber detector is a good example of a scintillation detector. The detection process of the fiber is depicted in this figure. For every thermal neutron captured, about 6000 photons are produced because of the high reaction energy (Q = 4.78 MeV). The elegant part of this technology is that the fibers act as their own light guide to direct the light created to the photo-multiplier tubes that are the light sensitive device in the detector. Only a fraction of the light produced is actually detected by the PMT as some light is lost in the photon transport along the fiber length and from losses of photons through the fiber.
· Fast neutrons are thermalized by hydrogen-rich moderator · Thermal neutron capture by 6Li · Alpha particle and Triton are produced · Triton particle excites Ce3+ · Ce3+ fluoresces · Light guided to PMTs |
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