Development of etched track detector system for low fluxes of thermal neutrons

A new etched track detector system for measuring low fluxes of thermal neutrons was developed. The detector consists of two foils of CR-39 etched track detector in close contact. One of the foils has a thin layer of 10B implanted just below the surface. The detection of thermal neutrons through products of 10B(n, α)7Li reaction with a pair of CR-39 etched track detectors in close contact during irradiation and chemical etching of latent tracks was studied. After counting, only tracks, produced by the same absorbed neutron, which leaves tracks of alpha particle on surface of one detector and tracks of lithium ions on surface of other detector, were taken into account. In this way, by utilising coincidence detection method, the background due to objects that can not be separated from tracks by automatic counting system was significantly reduced. A lower neutron detection limit was found to be <105 neutrons/cm2 (for 1 cm2 detector's area) and is two orders of magnitude lower than that obtained by an increase of the implanted 10B density or by an increase of the counted detector area. (author)