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Development of CdTe Pixel Detectors for Hard X/Gamma-Ray Imaging
JL. Gálvez*a, M. Hernanza, L. Álvareza, D. Karelina, B. Artigues a, M. Lozanob, G. Pellegrinib, M. Ullánb, E. Cabrujab,
R. Martínezb, M. Chmeissanic, C. Puigdengolesc
a Institut
de Ciències de l’Espai (CSIC-IEEC), Campus UAB, E-08193 Barcelona, Spain;
b Centro Nacional de Microelectrónica (IMB-CNM(CSIC)), Campus UAB, E-08193 Barcelona;
c Institut de Física d’Altes Energies (IFAE), Campus UAB, E-08193 Barcelona, Spain
Our research and development activities aim to study a gamma-ray imaging spectrometer in the MeV range based on CdTe detectors, suited either for the focal plane of a focusing mission or as a
calorimeter for a Compton camera. In addition, our undergoing detector design is proposed as the baseline for the payload of a balloon-borne experiment dedicated to hard X- and soft gamma-ray
polarimetry, currently under study and called CµSP (CZT µ-Spectrometer Polarimeter). Other research institutes such as INAF-ISAF, DTU Space, LIP, INEM/CNR, CEA, are involved in this proposal. We
will report on the main features of the prototype we are developing at ICE (CSIC-IEEC), a gamma-ray detector with imaging and polarimetry capabilities in order to fulfil the combined requirement of high
Encuentro RIA (Red de Infraestructuras de Astronomía)-AstroMadrid , Madrid, 25-27 Septiembre 2013
detection efficiency with good spatial and energy resolution.
.
CdTe pixel detector module
Daughter board prototype (HV bias voltage, CdTe
pixel detector, read-out ASIC, input/output
connection to data acquisition board)
New CdTe pixel detector module prototype:
 CdTe pixel detector: 11 x 11 pixels, 12.15mm x 12.15mm x 2mm,
Zoom: ASIC to daughter
board wire-bond
Al-schottky /CdTe/Pt for electron collection.
 Read-out ASIC: 128 channel low noise and power VATAGP7.1
ASIC (trigger signal; serial, sparse and sparse with neighbour
channels read-out mode).
 Alumina (Al2O3) fan-out board: will be fabricated in the IMB(CNM-CSIC)
clean room facilities. Two masks required: metallisation and passivation.
Bump-bonding procedure at IMB(CNM-CSIC) :
Attachment of the CdTe detector to the fan-out board following these steps:
 low melting point solder paste deposited by screen printing
 pick & place machine to place detector onto fan-out board
 reflow step in a controlled atmosphere
Fan-out board for a 11 x 11
pixel detector based on a
glass substrate fabricated
at IMB(CNM-CSIC)
CdTe pixel detector module
design as a stratospheric
balloon-borne payload
to operate as a scattering
polarimeter between 100 and
500 keV [1], [2]
POLARIMETER CAPABILITY
 Why? Provide key information about the geometry, magnetic fields, and emission
mechanisms in a wide variety of cosmic sources.
 How? X-POSIT proposal/CµSP(CdTe micro-Spectometer Polarimeter) for hard Xand soft gamma-ray astrophysics.
CdTe pixel detector module as focal plane detector
of the next generation of hard X and soft gamma ray
telescopes based on Laue lenses and/or advanced
Compton instruments [3]:
+ photon collecting area with - detector area
and - volume and - instrumental background
HARD X/GAMMA-RAY DETECTOR CONCEPT
Challenge: Reaching E ~ 1MeV with high detection efficiency,
keeping a good spatial and energetic resolution.
Trade-off detector design:
 Performances? Operate as a scattering polarimeter between 100 and 500 keV;
detector with 3D spatial resolution.
E
in
E
1
E2
CdTe thickness Detection efficiency Energy resolution
 Task? Measurement of the polarization status of the Crab pulsar, i.e. the
Scheme of the detector
prototype
polarization level and direction.
Experimental set-up and spectra measurement
The experimental setup consists basically of a vacuum
chamber, an oil-sealed rotary vane vacuum pump, a
freezer and a controllable high voltage power supply. The
vacuum chamber is made of Aluminum and has a volume
of 350mm x 300mm x 350mm.
SUMMARY
A new CdTe pixel detector prototype is under development. It consists of a new Al-schottky//CdTe//Pt pixel
detector (size 12.15mm x 12.15mm x 2mm) and a new read-out ASIC with trigger signal capability. This
detector design is suited for the focal plane of a focussing mission or as a calorimeter for a Compton camera
or hard-X/gamma-ray polarimeter. By now we have some working layers with readout electronics based on the
NUCAM ASIC, which have been tested successfully with some radiation sources. First promising results,
regarding spatial uniformity and energy resolution, are presented.
Experimental conditions:
 Detector HV bias: -400V
 Cathode irradiation
 Temperature: -10ºC, Acquisition time: 5 hours
 Radio-isotope: 133Ba@1μCi and 241Am@10μCi
 Distance between detector and radioisotope: 30cm
Proposal:
 stack of CdTe pixel detectors with increasing thicknesses
 take advantage of both approaches to find an optimal trade-off between the thickness and the energy resolution
 complex read-out electronics to handle the interactions within thin and thick layers
Experimental set-up inside the
vacuum chamber
REFERENCES
[1] Caroli, E., Gálvez, J., et al, A 3D CZT hard x-ray polarimeter for a balloon-borne payload, SPIE Conference
Series, 8443, 2012, sep, 10.1117/12.928326.
[2] Alvarez, J., Gálvez, J.. et al, CdTe/CZT pixel detector for gamma-ray spectrometry with imaging and polarimetry capability in astrophysics, (NSS/MIC), 2011 IEEE, pp. 4611-4615, 2011,oct.,10.1109/NSSMIC.2011.6154744.
Read-out chip settings:
[3] Gálvez, et al, Development and performance of a gamma-ray imaging detector, SPIE Conference Series, 8443,
 Leakage current comp.: 2nA
2012, sep, 10.1117/12.925979.
 Threshold: 2.4V/1.8V
 Shaping time: 7.5μs
 ADC amplitude: 350keV
 Collection time clock: 12μs
An energy resolution of 9.2keV (FWHM) at 356keV (133Ba source)
and 5.47keV (FWHM) at 59.5keV (241Am source) for one pixel
This work was supported by project AYA2011-24704 of the Spanish MINECO and
FEDER funds.
Fan-out fabrication is granted by project GICSERV-269 of the “Programa de Accesos a
las ICTS” (IMB-CNM (CSIC)).
We acknowledge also financial support from AGAUR through grant 2009 SGR 315.