Journal article

Depth of interaction and bias voltage dependence of the spectral response in a pixellated CdTe detector operating in time-over-threshold mode subjected to monochromatic X-rays

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Author list: THUNGSTRÖM G, NORLIN B, FRÖJDH C, Fröjdh E, FRÖJDH C, NORLIN B, THUNGSTRÖM G

Publication year: 2012

Start page: Art. no. C03002

ISSN: 1748-0221

DOI: http://dx.doi.org/10.1088/1748-0221/7/03/C03002

URL: http://www.scopus.com/inward/record.url?eid=2-s2.0-84858777567&partnerID=40&md5=7a0a5dc1429312c17646a93526e5dfd6

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Abstract

High stopping power is one of the most important figures of merit for X-ray detectors. CdTe is a promising material but suffers from: material defects, non-ideal charge transport and long range X-ray fluorescence. Those factors reduce the image quality and deteriorate spectral information. In this project we used a monochromatic pencil beam collimated through a 20Όm pinhole to measure the detector spectral response in dependance on the depth of interaction. The sensor was a 1mm thick CdTe detector with a pixel pitch of 110Όm, bump bonded to a Timepix readout chip operating in Time-Over-Threshold mode. The measurements were carried out at the Extreme Conditions beamline I15 of the Diamond Light Source. The beam was entering the sensor at an angle of ∌20 degrees to the surface and then passed through ∌25 pixels before leaving through the bottom of the sensor. The photon energy was tuned to 77keV giving a variation in the beam intensity of about three orders of magnitude along the beam path. Spectra in Time-over-Threshold (ToT) mode were recorded showing each individual interaction. The bias voltage was varied between -30V and -300V to investigate how the electric field affected the spectral information. For this setup it is worth noticing the large impact of fluorescence. At -300V the photo peak and escape peak are of similar height. For high bias voltages the spectra remains clear throughout the whole depth but for lower voltages as -50V, only the bottom part of the sensor carries spectral information. This is an effect of the low hole mobility and the longer range the electrons have to travel in a low field. © 2012 IOP Publishing Ltd and Sissa Medialab srl.


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