Development of a high efficiency neutron detector array at HRIBF

A new high-efficiency neutron detector array is being developed at the HRIBF to complement existing experimental equipment for the beta decay studies of neutron-rich fission fragments. The neutron detector array will consist of 2 foot long $^{3}$He tubes at 10 atmosphere surrounded by a polyethylene moderator. With an expected efficiency of $\sim$70\% the array will greatly aid in the determination of absolute beta-delayed neutron branching ratios when combined with the ``ranging-out'' of postaccelerated radioactive ion beams [1]. Geant4 [2] and MCNP calculations have been performed to simulate the efficiency of the array in a variety of geometries and results were compared to the efficiencies of existing neutron detectors. Simulation results will be presented. Work supported by DOE grants DE-AC05-00OR-22725, DE-AC05-76OR00033 and DE-FG02-96ER41006. \begin{thebibliography}{99} \bibitem{ref1} C.~J.~Gross, {\em et al.} Proc. of ENAM Conf., EPJDirect, in press (2005). \bibitem{ref2} S. Agostinelli, {\em et al.}, Nucl. Instrum. Methods Phys. Res. A506, (2003), 250. \end{thebibliography}