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Title: GAMBE - GAMma Blind neutron Efficient detector
Author: Ahmed, Ahmed Mohamed Omar
ISNI:       0000 0004 7658 1868
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Thermal neutron detectors, which are based on semiconductor material such as silicon coated with neutron reactive material like 10B and 6Li have been discussed for many decades. The performance of the thermal neutron detector system, GAMBE, which is based on two silicon sensors in a sandwich configuration is investigated. The results show that a single sandwich design with 6LiF film of (1.5 ± 0.6) mg/cm2 thick can achieve a total ("tn) and a coincidence ("cn) detection efficiency of 4% and 1% respectively. While, 6Li foil of (40 ± 10) μm thick is able to attain a ("cn) of (1.5 ± 0.9)% and a ("tn) of (9.2 ± 1.4)%. The coincidence that defines a true neutron hit is the simultaneous signal recorded by the two sensors facing the conversion layer. These coincidences provide a very good method for rejecting spurious hits coming from gamma-rays, which are usually present in the neutron field under measurement. This methodology results in a high gamma-ray rejection factor of 108. However, the price to pay is a reduction of the detection efficiency of the single sandwich detector. The thermal neutron detection efficiency of the detector is enhanced by using a stacked detector configuration and highdensity polyethylene (HDPE) sheets, as neutron moderators and reflectors. The GAMBE detector is positioned inside a box of HDPE with a lead window in the direction of the neutron flux for neutron moderation and a reduction of the effect of gamma-rays on the detector. The experimental layout was modeled in MCNP4C to investigate the contribution of HDPE to the thermal neutron flux (n/s/cm2). In this research, a stack of 4 silicon semiconductor sensors with two 6LiF films of an average thickness of (2.8 ± 0.6) mg/cm2 in a configuration of two sandwiches is shown to achieve a total and a coincidence detection efficiency of (27 ± 3)% and (4 ± 1)% respectively. This represents a significant improvement compared to a single detector. The effect of these stacked detectors for the development of a handheld thermal neutron detector, using 4 coated Si detectors is shown to have a 22% efficiency. Finally, this information is used to inform the optimised design of the handheld detector. The results based on GEANT4 and MCNP4C simulations indicate that the total detection efficiency of this portable detector with a stack of 7 sandwich detectors will increase up to 52% by using an optimal thickness of a 6LiF film of 17 μm (3.95 mg/cm2). This handheld detector has a highest total detection efficiency of 69% when using a 6Li foil of 36 μm thick.
Supervisor: Van Zalinge, Harm ; Burdin, Sergey ; Gianluigi, Casse Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral