Study On The Radiation Detection Performance And The Gamma Rejection For Neutron Detection Of GdI₃:Ce Scintillator

Gadolinium has the largest capture cross-section to thermal neutrons among all known elements,and the mean free path of the secondary particles produced by its neutron absorption is quite short.Therefore,it comes possible for a Gd-based scintillator of very small thickness to achieve a high neutron detection efficiency and spatial resolution,and a lot of novel Gd-based scintillating materials have been developed and applied for micro imaging with neutrons in recent years.GdI3:Ce is a new kind of Gd-based scintillator,it has excellent scintillation properties such as high cross-section for neutron absorption,high light yield,short decay time constant and high optical transparency,hence receives sustained attention and research in the field of neutron detection and imaging in recent ten years.The high density and effective atomic number of GdI3:Ce makes it sensitive to gamma-rays.Because there is usually a strong gamma background around a neutron source,effective measures need to be taken to reject gamma-rays while detecting neutrons.The main work in this thesis is to study the neutron and gamma detection performances of GdI3:Ce scintillator and improve its gamma rejection performance while detecting neutrons.Both the simulations and the experiments are carried out.The main work of the simulations includes following parts:?1?For the quantitative analysis of the gamma detection performance of GdI3:Ce,the linear attenuation of gamma rays for it was calculated by Monte Carlo simulations,and the energy spectrum of 250?m GdI3:Ce scintillator to gamma-rays in an energy ranged from 20 keV to 1000 keV was simulated.The simulation demonstrates that the GdI3:Ce scintillator has high detection efficiency to low-energy gamma rays,which will cause interference to the neutron signals converging at 80 keV.?2?The IC electron detecting capability versus the thickness of GdI3:Ce scintillator was analyzed by simulating the interaction process of IC electrons in GdI3:Ce scintillator,whose thickness ranged from 50?m-300?m.It shows the detection efficiency of thermal neutrons was almost 100%and the total energy deposition of IC electrons was nearly saturated when the thickness of GdI3:Ce scintillator was 250?m.The neutron detection performance was hardly improved by still raising the thickness of the scintillator while the interference from gamma-rays kept increasing.?3?The methods of adding lead enclosures and lowering the scintillator thickness are proposed for GdI3:Ce to reject gamma-rays while detecting neutrons,and the feasibility of adding lead enclosures was studied by simulations.The result shows the scattering and absorption phenomenon of thermal neutrons was not strong with lead enclosures of 1-2mm thickness.Lead enclosures of proper thickness can be used for neutron detection and imaging to reduce gamma background.Experimental work includes:?1?The GdI₃:Ce spectroscopy system was built and calibrated using 137Cs and 24124IAm sources.?2?In order to study the effect on gamma-ray rejection by adding lead shielding,the pulse height spectra of 252Cf source were measured by GdI3:Ce detector with various thickness lead shield.The result shows there was not a Gaussian-shaped neutron peak in the pulse height spectra when the frontage of the detector was not shielded by lead.When the thickness of the frontal lead shield of the detector was lmm,the gamma background was mostly shielded and the neutron peak came to appear.The contribution to reducing the interference of low-energy gamma background was hardly enhanced when the thickness of the lead shielding grew further whilst the neutron detection efficiency became decreasing.?3?The peak in the pulse height spectra was confirmed as the neutron peak by absorbing the thermal neutrons from 252Cf source using a boron sheet.