behaviour of the CdTe diode must be deeply studied in order to know
the response of the detector when it is operated under different
conditions of temperature, bias voltage, humidity or magnetic field.
This response is characterized by the energy resolution, the time
resolution, the coincidence time resolution between two detectors
operated in coincidence mode and the spatial resolution (in case of
having a pixelated diode).
One key feature of semiconductor detectors is their thickness in the electric field direction, this value has a strong impact on the energy resolution, time resolution and coincidence time resolution due to the trapping and detrapping effects suffered by charge carriers and their drift time along the diode. In the VIP module design, the thickness for each pixel is 2 mm. A CdTe diode of dimensions 4x4x2 mm3 has been used to evaluate the energy resolution under different values of temperature, humidity and bias voltage.
Spectroscopy of 22Na at -1000 V/mm bias voltage, 8ºC and <5% humidity shows an energy resolution of 1.2% at 511 keV, see the publication in JINST, February 2013: "Energy and coindidence time resolution measurements of CdTe detectors for PET", G. Ariño-Estrada, et al.
The time resolution has been measured with the CdTe detector working in coincidence mode with a photomultiplier tube coupled to a scintillator crystal, whereas for the coincidence time resolution measure two identical CdTe detectors have been used. Results show 12.5 ns FWHM for a 25 keV minimum acceptance energy and an improving down to 6 ns for 500 keV minimum acceptance energy, see the publication in JINST, February 2013: "Energy and coindidence time resolution measurements of CdTe detectors for PET", G. Ariño-Estrada, et al.
size is of crucial importance for the spatial resolution performance,
which, together with related effects such as small pixel effect and
charge sharing, has been measured using a pixelated diode of 10x10x2 mm3 divided into 100 pixels of 1x1 mm2 surface each. The
results of these measurements were shown in iWoRID 2013, 27th
June, in Paris.
Future targets are, on the one hand to measure the effect of strong magnetic fields on these setups to proof their capability to operate simultaneously with an MRI scanner and on the other hand, to build a stack of pixel arrays to create a 3D detector and repeat the same characterization process.