DEVELOPMENT OF AN ACTIVE EPITHERMAL NEUTRON MULTIPLICITY COUNTER (ENMC)

Year
2000
Author(s)
J.E. Stewart - Los Alarnos National Laboratory
W. H. Geist - Los Alamos National Laboratory
H.O. Menlove - Los Alamos National Laboratory
N. Ensslin - Los Alamos National Laboratory
C. Shonrock - Los Alamos National Laboratory
Abstract
The Epithermal Neutron Multiplicity Counter (ENMC) has been modified for active assay. Endplugs that can be easily inserted into the ENMC for active assay were designed based on Monte Carlo modeling using the Monte Carlo N-Particle (MCNP) code. The die-away time, neutron detection efficiency for both fission and AmLi neutrons, and the coupling were determined for several different endplug configurations. The endplugs are constructed out of polyethylene and have a hole for insertion of AmLi interrogation sources. Running the ENMC in active mode will reduce the assay time for uranium samples by a factor of approximately ten over the standard Active-Well Coincidence Counter (AWCC). This is because the ENMC uses higher pressure 3He tubes than the AWCC resulting in a MCNP calculated efficiency near 50% and a die-away time around 20 ?s. This large reduction in the die-away time reduces the number of accidental coincidences from the AmLi interrogation neutron source. This paper will discuss the results of the MCNP modeling of the active endplugs as well as some initial results of calibrations using the ENMC in active mode.