Speaker
Dr
Jacques Bourgoin
(GESEC R&D)
Description
X-ray detectors require large Z materials in order to absorb X-ray radiations
sufficiently. In case of X-ray imaging, fluorescence becomes one of the limiting
factors for spatial resolution and contrast at X-ray energies above the ka threshold
(typical average values are 10 keV, 25 keV and 30 keV for GaAs, CdTe and CsI,
respectively). Since both the energy and yield of the fluorescence for a given
material increase with its atomic number Z, there is an optimum value of Z. We
selected GaAs corresponding to this optimum, which, at the present time, can be
epitaxially grown in self-supported thick layers to fulfil the requirements (in
particular the homogeneity of the electronic properties) for imaging. Image
properties obtained with this material have been evaluated and experimentally
measured in terms of Line Spread Function and Modulation Transfer Function (MTF),
both in cases of high and low contrast, and compared with CdTe and CsI. The spatial
frequencies (lp/mm) are 7.5, 3.0 and 1.0 for GaAs, CdTe and CsI, respectively, for a
MTF amplitude of 0.5, a pixel size of 50 µm and an energy of 40 keV. The image
contrast obtained for a given object contrast is better for GaAs than for CdTe and
CsI, at low energy and for identical thicknesses of the materials. For instance, at
32 keV, the image contrasts are 0.92, 0.65 and 0.4 for an object contrast of 1 and
0.16, 0.09 and 0.02 for an object contrast of 0.5 for GaAs, CdTe and CsI (200 µm
thick), respectively. We shall describe the dependences of the image contrast versus
pixel size and energy. Finally, we shall discuss the minimum detectable object size
in the conditions of mammography for these materials and demonstrate that an object
of a given size can be detected using GaAs with a dose at least 100 times less than
using CsI.
Author
Dr
Guocai SUN
(GESEC R&D)
Co-authors
Dr
Alain Mauger
(Uinversité Pierre et Marie Curie)
Dr
Jacques Bourgoin
(GESEC R&D)
Dr
Samira Makham
(GESEC R&D)
