Use of Gd in gas counters as neutron detectors in SNO

Abstract

The Sudbury Neutrino Observatory (SNO) will observe solar neutrino interactions in D 2O. Photomultiplier tubes (PMT) surrounding the D 2O will detect Cherenkov light from these interactions. The neutral current (NC) interactions of the neutrinos disintegrate the deuterons producing neutrons. Cherenkov light from the subsequent capture-gamma-ray shower (CGRS) will register in SNO. The default method uses the Cl of dissolved salt as the neutron capture agent. Since the neutron absorption mean path in D 2O is very long, a small amount of efficient neutron absorber can capture a large fraction of the neutrons. Therefore it is possible to construct discrete neutron counters based on this absorber. We describe the conceptual design of a detection system using Gd in proportional wire chambers (PWCs). Gd neutron capture produces 8 MeV of capture-gamma-ray energy and, with high probability, low energy conversion electrons. The proposed method uses the coincidence between conversion electrons detected in the PWCs, and the CGRS, detected by the SNO PMTs, to register the neutron capture event. This coincidence makes it possible to observe a positively identified background free neutron signal. Further it makes the system insensitive to spurious signals from the PWCs and the PMTs of SNO. We present a conceptual design in which 28% of the neutrons will produce a coincidence. The estimated fraction of neutrons from all sources which will give a detectable signal in the SNO PMTs is 45%. In this design, we require about 6.7 kg of 4 μm natural Gd foils in 870 m long by 6 cm diameter PWCs on a 90 cm lattice. Photon losses due to counter shadowing can be kept to < 20%. We have determined experimentally that the neutron capture conversion electron detection efficiency in Gd foils in a PWC is 85% in the limit of an infinitely thin foil.