UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

108 7. CIX module performance under X-ray irradiation
7.3 Average photon energy
The third key feature of CIX 0.2 is its ability to measure the average energy of the
absorbed spectrum. In order to evaluate this characteristic, the average photon energy
reconstruction was tested on a homogeneous wedge-shaped aluminum absorber. Like this
it is possible to exactly simulate the transmission spectrum behind the absorber as well as
the resulting absorption spectrum in the detector. The simulation used for this purpose
is the same as in section 6.4.
Fig. 7.15 shows the average photon energy behind the absorber as a function of the Al
thickness for a Si (a) and a CdZnTe (b) module. The two plots contain simulation data
as well as measurement results.
Starting with the simulations, the topmost curves in Fig. 7.15 show the simulated average
photon energy of the transmission spectrum behind the Al absorber. In this context the
term average energy is defined as the average energy of those photons with energies above
10 keV. The 10 keV lower limit reflects the fact that the impinging X-ray spectrum does
not contain any photons below this value due to the filters installed in the X-ray setup (see
simulated tube spectrum in Fig. 6.24). The definition furthermore gives a measure of the
average energy, independent of the large number of low energy hits introduced by charge
sharing and electronic noise. In a perfectly absorbing sensor, these average photon energies
should be reproduced by the measurement. However, it has been mentioned before that
Si suffers from a low X-ray absorption coefficient at higher X-ray energies. The low X-ray
absorption of the Si sensors is reflected by the triangular data markers in Fig. 7.15(a)
A v e r a g e p h o to n e n e r g y [k e V ]
5 5
5 0
4 5
4 0
3 5
3 0
2 5
2 0
a ) 0 .3 m m S i
S im : A l tra n s m is s io n s p e c tru m
S im : A b s o rp tio n c o e ff. o n ly
S im : F u ll
M e a s u re d s p e c tru m
0 1 2 3
A l th ic k n e s s [m m ]
b ) 3 m m C d Z n T e
S im : A l tra n s m is s io n s p e c tru m
S im : W ith o u t X -ra y flu o r.
S im : F u ll w ith X -ra y flu o r.
M e a s u re d s p e c tru m
0 1 2 3
A l th ic k n e s s [m m ]
Fig. 7.15: Average simulated (filled symbols) and measured (open symbols) photon energy
reconstruction measured on an Al-absorber of varying thickness. (a): 300 µm Si sensor
(Si03). (b): 3 mm CdZnTe sensor (CZT04). The modules were irradiated at a focal
spot to detector distance of 12.5 cm, a tube endpoint energy of 90 keV and tube currents
of 100 µA and 50 µA for Si and CdZnTe, respectively. In the simulation, photons with
energies below 10 keV have been ignored in order to be independent of charge sharing and
detector effects like electronic noise. The measured average energies were calculated from
the deposited charge (integrator) and the number of detected photons (counter) obtained
at standard chip settings. For Si and CdZnTe two different constant scaling factors are used
to reduce the integrator current and thereby correct for the low energy hits registered by
the integrator. The error bars show the error of the module-wide average.
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2 0