JINST Instrumentation Theses Archive

M.Sc. degree thesis
accepted by Hebrew university of Jerusalem, Israel, in 2008

Michal Brandis

Supervisor: Eliahu Friedman and Mark Goldberg

Particle Identification via Track Imaging in Liquid-Scintillator-Core Capillary Arrays

 Keywords:

• Detector modelling and simulations I
• Scintillators and scintillating fibers and light guides
• Particle identification methods
• Particle tracking detectors

 Abstract:
This work describes the development of a high-spatial-resolution detector for an explosives detection system (EDS) that is based on the method of gamma resonance absorption (GRA) in 14N. Apart from its imaging capabilities, the main requirements of such a detector are: high nitrogen content and the ability to distinguish, on an event-by-event basis, between internally produced protons and electrons / positrons.

The segmented detector comprises a glass capillary matrix (capillary diameter ~20�m) filled with a nitrogen-rich, high-refractive-index liquid scintillator. Gamma rays incident on the detector can create 1.5 MeV protons via the nuclear reaction 14N(γ,p)13C if they impinge on 14N nuclei at the resonant energy of 9.17 MeV; Off-resonance gamma rays create only electrons and positrons via atomic interactions. All secondary charged particles generate scintillation light emission during the slowing down process in the active detector medium. When the index-of refraction of the liquid core is higher than that of the glass matrix cladding, part of this light will be trapped in the capillary in which it was created, providing spatial information about the point of interaction. The array face is attached to an optoelectronic readout that amplifies the signals and displays the track projection.

Differentiation between particle types is based on the difference in stopping power between proton and electrons / positrons. The latter create tracks that can be up to a few centimeters long, while the protons in question generate much shorter tracks of only ~50 �m. Furthermore, protons produce much more light per unit length along their tracks. Thus, they give rise to intense light pulses in one or two adjacent capillaries. These are very distinct from the long, faint tracks left by electrons and positrons. The work encompassed three parts: a) development of a suitable scintillator; b) a preliminary experiment and c) Monte-Carlo simulations. The scintillator cocktail developed was based on 1-Methyl-Naphthalene. A preliminary experiment was conducted in which the capillary array was irradiated by gamma rays and neutrons from 137Cs and 239Pu/Be sources to study the tracks they induce. The results are encouraging and will be presented. Simulations of the detector were carried out with the Geant4 code. In these simulations, the interactions, tracks and energies of protons, electrons and positrons (including secondaries, tertiaries, etc.) within the detector are followed, in order to quantify the ratio between electron background noise and proton events