GROWTH AND CHARACTERIZATION OF SCINTILLATORS FOR THERMAL NEUTRON DETECTION

Abstract

The dwindling supply of 3He demands the development of detectors alternative to 3He based detectors for the detection and measurement of thermal neutrons. In the present thesis work, we report the growth and detailed characterization of Gd3Ga3Al2O12(GGAG) and LiI-based single crystal scintillators for detecting thermal neutrons and discriminating thermal neutrons in the presence of a gamma background. These scintillators have been chosen due to high thermal neutron capture cross sections of 155Gd, 157Gd, and 6Li isotopes. The thesis work can be divided into two parts. In the first part, the Gd3Ga3Al2O12:Ce (GGAG:Ce) single crystal scintillator was grown using the Czochralski method. A disk of ~ 1 mm thickness was used for thermal neutron detection using a thermalized Am-Be source. Gd isotopes with high thermal neutron capture cross-sections ensure almost 100% stopping efficiency in such a low thickness. On the other hand, a lower thickness provided better discrimination from the gamma background. The capture of thermal neutrons by Gd isotopes produces low-energy conversion electrons and X-rays, which could be successfully observed in the form of well-discriminated photopeaks around 34 and 74 keV. These photopeaks could also be measured in nuclear reactor beam lines with high gamma background of 300 mR/h. In addition, a spacing of 100 μ is found to be well resolved using this GGAG:Ce detector. As GGAG:Ce has similar decay time for gamma and thermal neutron, it cannot be employed for pulse shape discrimination. Therefore, the discrimination from the gamma background was achieved using a phoswich detector made up of GGAG:Ce and CsI:Tl single crystal scintillators coupled to a photomultiplier tube (PMT). Pulse shape discrimination was done using a digitizer by the charge integration method, and a high FoM ~ 6.5 was obtained for thermal neutron and gamma discrimination. This phoswich was also tested in Dhurva Nuclear Reactor, having a gamma background of 1 R/h. The performance of the phoswich detector was also tested by replacing PMT with a silicon photomultiplier (SiPM). Also, GGAG:Ce,B single crystal coupled to Si-based photosensors such as avalanche photodiode (APD) and SiPM was found to detect thermal neutrons efficiently. The results have clearly demonstrated the potential of compact detector for detecting thermal neutrons and the discrimination of thermal neutrons from gamma backgrounds. In the second part, we explored the commercially available LiI thermal neutron detector for pulse shape discrimination by choosing different materials for doping and co-doping.LiI:Eu,Sr, LiI:Tl and LiI:Eu,Tl single crystals were grown by the Bridgman method. "Sr" co-doping causes a redshift in the emission of LiI:Eu. The thermoluminescence glow curve of LiI:Eu showed two bands at around 65 ֯C and 100 ֯C. "Sr" co-doping offers lower self-absorption, improved energy resolution and the possibility of thermal neutron and gamma discrimination which was not present in LiI:Eu single crystals. However, "Sr" co-doping resulted in the reduction of the light output. "Tl" doping to LiI fastens the gamma scintillation decay time compared to "Eu" doped LiI and introduces PSD ability. However, "Tl" co-doping to LiI:Eu crystal did not affect the gamma scintillation decay time and could not introduce PSD ability in LiI:Eu single crystal scintillator. The results clearly suggest the use of the co-doping method in the development of LiI-based thermal neutron detectors.