Filters For Thermal Neutron Radiography

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Abstract
A comprehensive study of data helpful in the design of filters for thermal neutron radiography has been undertaken. Many existing or proposed facilities might benefit from optimization of temporary or permanent filters designed to transmit thermal neutrons, attenuate fast neutrons, attenuate gamma, and soften the Maxwellian spectrum to maximize detail contrast. Each situation will require a different filter design, but in each case the design should consider directly measured filter data, rather than relying on fundamental cross section libraries and computer calculations. A review of measured data shows that quasi-single crystal sapphire (Al203) at ambient temperature can perform better than liquid nitrogen cooled silicon (Si), quartz (Si02) and bismuth (Bi) for discrimination between thermal and fast neutrons or for softening the Maxwellian spectrum. If discrimination against gamma radiation is also needed measurements for bismuth showing thermal neutron transmission, which is poor relative to Al2O3 should be considered. The use of thick bismuth filter cooled by liquid nitrogen is found to be attractive primarily for cold neutron beam optimization, not for thermal neutron beams. Single crystal lead fluoride (PbF2) is an alternative to quasi-single crystal bismuth in those situations that demand gamma reduction.

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