Upgrade of the diane: performance improvement in thermalization of fast neutrons for radiography

摘要

In a small neutron radiography system such as DIANE, which is basedo n a sealed-tube neutron generator, the maximum possible efficiency of fast neutron thermalization must be achieved, consistent with realistic industrial and manufacturing practices. To this end, MCNP simulations and experiments have been performed for further enhancing the performance of the moderator/collimator assembly in the DIANE. These calculations and experiments have shown that a fast neutron reflector/multiplier can be useful in increasing the number of fast neutrons entering the moderator. Various heavy metals with high inelastic cross sections for fast neutrons and low capture cross sections have been tested. Results indicate that the best moderating materials for this application are beryllium, zirconium hydride, and high-density polyethiene, and the best reflector/multiplier material for this use is tungsten. The MCNP calculations incdicate that for a fast neutron output of 4 x 10~(11) n s~(1-) in 4 #pi# steradians, a DIANE can be fabricated which produces a thermal netron beam for radiography having a flux of approximately 2.1 x 10~5 n cm~(-2) s~(-1) at an effective collimator ratio of 30, or about 7.5 x 10~4 n cm~(-2) s~(-1) at a collimator ratio of 50.