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Title: Silicon carbide and diamond neutron detectors for active interrogation security applications
Author: Hodgson, Michael
ISNI:       0000 0004 5923 2527
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2016
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A thorough investigation has been carried out in order to determine the suitability of diamond and silicon carbide for active interrogation applications. This included electrical and radiological characterisation of single crystal diamond (D-SC) and polycrystalline diamond (D-PC) detectors; epitaxial silicon carbide (SiC-EP) and semi-insulating silicon carbide (SiC-SI); all compared against the performance of a commercial silicon PIN photodiode (Si-PIN) from Hamamatsu. This work aided in determining whether the detectors were suitable for radiation detection purposes, as well as obtaining the operational criteria for use. Characterisation work was also conducted on semi-insulating silicon carbide detectors from three different suppliers, as well as on detectors fabricated via different techniques. This work demonstrated the robustness of the material, as the charge collection properties were unaffected by contact fabrication technique. Changes in current-voltage characteristics were observed for different contact fabrication methods, but were generally still low (≈nA) over the ranges tested (±500V). Following this work the performance of selected detectors was measured against criteria for the AWE active interrogation project. Radiation dose dependent performance deterioration was observed in the SiC-SI and D-PC detectors, with decreased charge collection efficiency (-45±4%) and intrinsic efficiency (-40% at -400V) observed respectively. It is not clear as to whether these effects are a result of bulk material damage or contact/surface/mount damage, but an increase in the current-voltage relationship was also observed on these detectors, as well as the Si-PIN (SiC-SI≈+25% and D-PC≈+20% at -400V; Si- PIN≈+300% at -25V). Instability of the peak position and/or counting rate with irradiation time was observed in D-SC, D-PC and all the semi-insulating SiC (polarisation effect). For D-SC this was primarily with alpha particles and stability would be maintained after a period of time, with that period decreasing as the incident flux increases. For D-PC and the semi-insulating SiC, this effect was observed on most radiation types tested (alpha, beta, X-ray, gamma, neutron and protons) with polarisation rate increasing as the the number of charge carriers created per incident particle increased. However, it has been shown that combinations of ambient light and 0V bias could depolarise a semi-insulating SiC detector and even decrease its polarisation rate for future irradiations. D-SC, SiC-EP and semi-insulating SiC material were also shown to operate from -60◦C to +100◦C. For D-SC and SiC-EP the charge collection efficiency was similar (±10%) over the entire range, apart from at +100◦C for D-SC where it was ≈50% down. For SiC-SI, the charge collection efficiency peaked at room temperature, but became more stable at +100◦C (lower polarisation rate). All the detectors demonstrated the ability to detect and discriminate between both different energy neutrons and ionising photon (gamma) energies using simple energy threshold discrimination. Comparison of the endpoint energy for AmBe (< 4.1MeV >) and Cf-252 (< 2.1MeV >) or mono-energetic 1MeV and 5MeV neutrons, give ratios (Emax(High Energy)/Emax(Low Energy)) of ≈3.5, 2.5, 5.0, 4.9 and 2.0 for D-SC, D-PC-, SiC-EP, SiC-SI and Si-PIN respectively. Similarly comparison of the endpoint energy for Co-60 gammas (1.2MeV and 1.3MeV) and AmBe neutron (Emax(AmBe)/Emax(Co − 60)) give ratios of 8.1, 16.0, 6.4, 6.9 and 9.1 respectively. It was also shown that the neutron-gamma detection ability can be improved through simple design optimisation techniques, including: the use of high atomic number filtration to reduce gamma detection; hydrogenous proton conversion layers to improve neutron detection; and large area detection arrays to improve counting statistics.
Supervisor: Lohstroh, A. Sponsor: Science and Technology Funding Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available