A Large Volume (1m3) Spherical Proportional Counter for Atmospheric Neutron Measurements

Abstract

A large volume (1m3) spherical proportional counter has been developed at CEA/Saclay, for low flux neutron measurements. Neutrons can be measured successfully, with high sensitivity, using 3He gas in the detector. The proton and tritium energy deposition in the drift gaseous volume, from the reaction 3He(n,p)3H, can provide the neutron spectra from thermal neutrons up to several MeV. The α resolution of the detector has been measured using 222Rn gas, in order to have the response of the total active volume of the detector. The three α peaks from 222Rn decay and 222Rn daughters decay have been measured with FWHM less then 2%, with gas mixture Ar 98% and CH4 2% at a pressure of 200 mbar. The neutron sensitivity has been tested inserting in the sphere 100mgr of 3He. The 0.765 MeV peak of p and 3H is well separated form the cosmic ray background after irradiation with 252Cf neutron source. The ground level atmospheric thermal neutrons have been also measured after several hours of exposure. The very low noise, the limited wall effect, the high resolution and the high sensitivity are some of the advantages of the detector.