Combined composite scintillation detector for separate measurements of fast and thermal neutrons

We propose and study a new composite scintillation detector. It is the combined detector for separate detection of fast neutrons and thermal neutrons in the presence of gamma background radiation. The combined detector consists of two scintillation parts. It is the organic composite scintillator that is made from stilbene crystalline grains (grains with sizes from 2.5 to 3.0 mm) those we introduce in organosilicone matrix. The second part of the detector is a thin inorganic composite detector of thermal neutrons that contains Ce:GPS (Ce-doped gadolinium pyrosilicate) crystalline grains those are in the matrix of the same type. To detect thermal neutrons with minimal gamma background the single-layer composite material was used (grains with sizes in the following ranges: from 0.06 to 0.1, from 0.1 to 0.3 and from 0.3 to 0.5 mm). For chosen values of the sizes of the grains the efficiency of detection of gamma radiations with energies lower than 150 keV is negligible. The comparison of the values of scintillation amplitudes in the range of thermal neutron detection and in the range of recoil proton spectrum shows that the signals from thermal and fast neutrons do not overlap. Therefore the analysis of these two spectra can be run separately in different energy windows. The combined composite detector has no technological limitations on area of it input window. It needs screening from gamma radiations of low energies (corresponding to 33 keV peak from Gd conversion electrons or corresponding to the total range of 33 keV and 77 keV peaks). With increase of a grain size the efficiency of thermal neutrons detection growths. On the other hand the detection of the signals not only in the range of 33 keV peak but in the range of 77 keV peak as well causes the increase of the thickness of gamma-shielding and use a witness detector.