MIXS on BepiColombo and its DEPFET based focal ... - MPG HLL

J. Treis et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) 540–547 547
framerate of around 2.5 kHz. For <strong>MIXS</strong>, the framerate will be by a
factor of two higher, the integration time will be r200 ms. As
can be seen from Fig. 8, the sensor shows excellent homogeneity
and noise. The visible structures in the offset and noise maps
are due to the ASTEROID channel-to-channel offset. The noise
distribution is very flat, no noisy pixels have been observed.
The noise dispersion of 7% is due to pixels close to the sensor rim
at the bottom having a slightly higher noise. This is caused by the
proximity of the ASTEROID IC inducing additional heat load,
probably in form of infrared radiation.
The sensors have been exposed to an 55 Fe source. The devices
show excellent spectral resolution. The resulting spectra are
shown in Fig. 9. The energy resolution is with 126 eV FWHM at
5.9 keV close to the Fano limit, and the slightly worse energy
resolution of the integral spectra taking into account also splits, is
with 128 eV within the expectations. The overlay of the spectra of
the various multiplicities gives an impression of the quality of the
gathered spectra. 76.9% of the recorded patterns are singles, 20.6%
are doubles and the remaining 2.5% distribute evenly among
triples and quadruples, a split behavior which is in agreement
with the simulations. The peak-to background ratio of C3000
qualifies the devices as suitable for spectroscopy applications and
is a proof of the good quality of the entrance window.
Imaging tests have been done using a 450 mm thick silicon
baffle. The resulting images are shown in Fig. 10. the smallest
feature size for this baffle was 0.4 mm. Although a quantitative
analysis using MTF structures still need to be done, these images
give an impression of the homogeneity and excellent imaging
properties of the sensors.
6. Summary and outlook
DEPFET based Macropixel detectors have been designed
and produced for the focal plane instrumentation of the <strong>MIXS</strong>
spectrometer on board BepiColombo’s MPO. The devices are in the
process of being die-tested for flight qualification. Prototypes of
even larger area and pixel sizes have been successfully taken into
operation. The tested devices are homogeneous and defect free
and show excellent imaging and spectroscopic properties.
As soon as the die testing is finished, device qualification will
continue with quantitative testing of the imaging properties, a
quantitative analysis of the entrance window quantum efficiency
and the radiation hardness of the device to confirm the results of
the diode test irradiation. The radiation qualification of the
ASTEROID and SWITCHER ICs is also a matter of concern.
In addition, prototype devices have been taken to the BESSY II
beamline at the PTB facility in Berlin, Germany, for calibration
purposes. The calibrated sensors are going to be used for X-ray
fluorescence analysis measurements on Iron samples at the
ELETTRA synchrotron facility in Trieste, Italy.
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