A neutron multiplicity detector for rare spontaneous fission events

Abstract A neutron multiplicity detector designed to register rare events of spontaneous fission is described. The detector consists of 56 3 He-filled counters. The detection efficiency for single neutrons is ϵ n = 0.483 ± 0.005 for a ten liter sensitive volume. The intrinsic background of the detector was determined in a room with 15 m of water equivalent (m.w.e.) shielding by using a blocking scintillation system against cosmic rays. The parameters of the multiplicity distribution of prompt neutrons for the spontaneous fission of 256 Fm were measured to be as follows: ν = 3.59 ± 0.06, σ ν 2 = 2.16 ± 0.05 and Γ 2 = 0.89 ±0.03 .