LAUSR

We present a new analysis of the sensitivity of a water Cherenkov-based neutron multiplicity counter for nondestructive assay of trace quantities of ²⁴⁰Pu. The multiplicity counter [water-based neutron detector (WaND)] contains 1.0 m³ of water doped with 0.2% gadolinium in the form of a gadolinium salt (0.4% GdCl₃). It can be used for testing the presence of trace quantities of ²⁴⁰Pu within a sample of unknown material by observing the correlated timing behavior of neutron capture candidates in the detector. The new analysis takes advantage of the fact that the detector is comprised almost entirely of a homogeneous medium, which permits the use of a simple model to reproduce the observed neutron capture timing behavior. The results of this analysis indicate that the detector has an absolute neutron detection efficiency of 23.4 ± 1.1% and is sensitive to a ²⁴⁰Pu mass as small as 2.3 mg (at $1\sigma $ ) after a 1-h data acquisition.