2008 JINST TH 003
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:
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Detector modelling and simulations I
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Scintillators and scintillating fibers and light guides
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Particle identification methods
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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
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