Verlag des Forschungszentrums Jülich
JUEL-4076
In order to reach this aim, first, the storage phsosphors had to be optimised. Investigations
on BaFBr:Eu2+ have shown that its sensitivity can be raised significantly by doping it with 1
mol% calcium. Additionally, the calcium doping increased the stimulability of the phosphor at
the wavelength of 635 nm, which is important for practical applications . It was demonstrated
that this increased stimulability is due to a calcium-induced formation of disturbed colour centres,
i .e. FA(Br, Ca2+ ), whose stimulation maxima are shifted by about 80 nm towards longer
wavelengths compared to undisturbed colour centres. Furthermore, the method of linearly
modulated photostimulated luminescence (LM-PSL) was used for the first time on
BaFBr:Eu2+ . With this method, which is using a linearly increasing intensity of the stimulating
light, it was possible to determine the optical cross-sections of the centres responsible for the
photostimulated luminescence (PSL).
Additionally, the two storage phosphors KCLEu2+ and KBr:Eu2+ were investigated . It was
found that the optimum activator concentration for highest photostimulated luminescence
output is 0.05 mol% Eu2+ . Doping experiments analogous to BaFBr:Eu2+ were performed on
KCLEu2+ for the purpose of shifting the stimulation maximum to a longer wavelength and a
linear correlation between the maximum and the bromine content was found. However, although
the stimulation maximum is shifting in a favourable direction, the doping of KCLEu2+
with KBr is no feasible or practicable way for a optimisation of this storage phosphor, because
the sensitivity of those KCI1-xBrx:Eu2+ mixed crystals is considerably lower than of
KCI:Eu2+ . Using results from LM-PSL experiments it was possible to show that this reduced
sensitivity is not caused by a diminished optical cross-section, i .e . a lower stimulability, but a
decreased number of photostimulable centres . Hence, KCI:Eu2+ without other dopants was
used for all subsequent experiments.
While the sensitivity of the storage phosphors for x-rays was examined in the first part of the
optimisation, neutrons were used for the second part . It was possible to demonstrate that a
maximum PSL-intensity after neutron irradiation is achieved when GdF3 with a fraction of 35
mol% or LiF with a fraction of 70 mol% is added as a neutron converter material . Moreover, it
was found that samples containing LiF feature a PSL-signal per absorbed neutron that is
twice as high as those samples containing GdF3 as converter. This advantage results from a
much higher usable energy output of LiF alter a neutron absorption reaction. Especially
KCI:Eu2+ has proven to be a suitable material for neutron image plates because it shows a
sensitivity for secondary particles from the neutron converter which is comparable to
BaFBr:Eu2+ while at the same time the sensitivity for γ-quanta is about 1 order of magnitude
lower. This leads to a neutron equivalent for KCI:EU2+-6LiF image plates of 1/20 neutron per
photon for soft γ-rays (< 300 keV) while the equivalent for hard γ-quanta (> 300 keV) is 1/400
neutron per absorbed photon . Another advantage of KCIEu2+ is that this phosphor displays
an reduced neutron activation compared to BaFBr:Eu2+ . These results demonstrate that a
combination of KCI:Eu2+ and 70 mol% LiF fulfils all requirements for the fabrication of highly
efficient, low γ-sensitivity neutron image plates . Therefore three different morphologies of
image plates were fabricated based on KCI:Eu2+ -LiF: polymer supported (P-NIPs), ceramic
(C-NIPs) and pixelated (Pix-NIPs) neutron image plates . In order to determine factors influencing
the spatial resolution of these NIPs, simulations on some optical properties were performed
with a Monte-Carlo based algorithm which show a good agreement with experimental
results.
A scanner was built as part of this work in order to experimentally determine the spatial resolution
of the image plates . Using this scanner, it was found that the resolution of C-NIPs and
P-NIPs is diminishing with increasing thickness of the plate because of an increased lateral
scattering of the stimulation light within the image plate. At the same time, the sensitivity is
increasing due to a growing fraction of absorbed neutrons. The demanded spatial resolution
of ≤ 1 mm2 was reached with all fabricated image plates while a higher resolution can only be
achieved by a reduced thickness leading to a lower sensitivity.
Schlapp, Michael
Entwicklung hocheffizienter, gamma-insensitiver Detektormaterialien und Bildplatten für Neutronen
111 S., 2003
Neutron image plates provide the means for two-dimensional, position-sensitive detection of
neutrons. Commercially available neutron image plates consist of a mixture of a neutron
converter and storage phosphor dispersed in an organic binder supported by a flexible sheet.
The aim of this dissertation was to find a system of converter and storage phosphor which
shows a high efficiency for the detection of neutrons while at the same time features a low γ-sensitivity.
Additionally, the technology to fabricate neutron image plates based on this system
with a spatial resolution of ≤ 1 mm2 was to be developed.
Pix-NIPs represent a novel morphology of image plates in which the scattered light is confined
to the cell of a honeycomb structure embedded in the image plate. Due to this confinement,
the resolution of such an image plate is nearly independent of scattering properties of
the NIP and is primary a function of the size of the honeycomb structure used .
Determining the Detective Quantum Efficiency DQE for a C-NIP consisting of KCI:Eu2+ 6LiF
with the above mentioned scanner resulted in a value of 38 % which corresponds to an efficiency
comparable to commercially available neutron image plates .
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