Ph.d. degree
Weizmann Institute of Science Rehovot, Israel, 2017

Itamar Israeli (Israelashvili)

Supervisor: Professor Amos Breskin

A novel liquid-Xenon detector concept for combined fast-neutron and gamma-ray imaging and spectroscopy

Keywords:

• Noble liquid detectors (scintillation, ionization, double-phase)
• Detector modelling and simulations (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc)
• Micropattern gaseous detectors (MSGC, GEM, THGEM, RETHGEM, MHSP, MICROPIC, MICROMEGAS, InGrid, etc)
• Photon detectors for UV, visible and IR photons (gas) (gas-photocathodes, solid-photocathodes)

Abstract:

This Ph.D. thesis is dedicated to the design and development of a new detector concept for simultaneous imaging and spectroscopy of fast-neutrons and gamma radiation. The work was motivated by the aim of developing a single efficient radiographic imager for scanning cargo and containers, in the search of small, operationally-relevant quantities of concealed special nuclear materials (SNM), such as highly enriched uranium (HEU) and 239Pu and explosives. The reason to search for rather small quantities (500 g) of SNM is to interdict the scenario of terrorists smuggling small pieces of weapon grade uranium into targeted area, in order to assemble and deploy a crude improvised nuclear device. These requirements influence the design of detector parameters, such as position resolution and detection efficiency.
The new detector combines a liquid-xenon (LXe) scintillator contained in “fiber-like” Tefzel capillaries, coupled to a UV-sensitive Gaseous Imaging Photomultiplier (GPM). The research focused on validating this new idea for simultaneously detecting hidden explosives, predominantly of low-Z materials, and high-Z fissile materials - utilizing fast neutrons and gamma radiation, respectively. Imaging of both radiations, in the energy range of 0-14MeV, relies on their induced UV scintillation-light localization from in a LXe converter with a UV-sensitive GPM - a cascaded Thick-Gas Electron Multiplier (THGEM) coated with a cesium iodide (CsI) photocathode and equipped with a patterned readout anode electrode.
A comprehensive computer-simulation (GEANT4) study was performed aiming at the optimization of the LXe converter configuration and geometry. Simulations were also carried out in order to evaluate the expected performance for gamma-ray and fast-neutron radiography of the LXe in Tefzel capillaries versus a plain-volume LXe scintillator. The simulation results were used to determine the capillary LXe convertor configuration.
Characterization of a 100 mm in diameter triple-THGEM GPM detector, with CsI photocathode deposited on its first element, has been performed at room temperature (RT) and at LXe cryogenic conditions - in the Weizmann Institute Liquid Xenon cryostat (WILiX); the detector was investigated thoroughly in different counting gases and operation pressures. The imaging performances, at RT and at cryogenic temperature, were studied with a segmented, 61-pads readout electrode and APV25-SRS CERN-RD51 readout electronics designed to operate at cryogenic temperature, using a dedicated software.
Gamma and neutrons imaging experiments were performed at the laboratory, using a 60Co gamma source (1.17 and 1.33 MeV) and a AmBe neutrons source yielding a mixed field of 4.4 MeV gamma and 0-11 MeV fast neutrons.
The localization properties of low-energy gamma-rays (60Co) and mixed fast-neutrons/gamma (AmBe) in the present, not optimal detector geometry, derived from irradiation of a Pb edge object, yielded spatial resolutions of 12±2mm (FWHM) for gamma and 10±2mm (FWHM) for the mixed gamma/neutron field. The experimental results are in good agreement with GEANT4 simulations.
For preferable detector geometry, in which the photocathode is closer to the LXe converter, the expected ultimate pencil-beam resolutions, for the energy ranges foreseen for the gamma/neutron radiography, e.g. 4.43 and 15.1 MeV gamma-rays and 1-15 MeV neutrons, are 2-4 mm and ~2 mm (FWHM), respectively. The expected detection efficiencies for a 50 mm thick converter would be ~35% and 20%, respectively.
The results indicate that the novel mixed radiation-field detection concept has the potential of use in fast-neutron resonant transmission (FNRT) radiography and in dual discrete gamma-ray radiography (DDGR). While the energy resolution of the detector would be sufficient for gamma spectroscopy, that of neutrons, by time-of-flight, would require further improvement of the GPM's time resolution.