Wide-gap CdTe Strip Detectors for High-Resolution Imaging in Hard X-rays

Kavli Affiliate: Tadayuki Takahashi

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| Summary:

We propose a new strip configuration for CdTe X-ray detectors, named
"Wide-gap CdTe strip detector", in which the gap between adjacent strips is
much wider than the width of each strip. It has been known that the observed
energies of an incoming photon in adjacent strips can be utilized to achieve a
position resolution finer than the strip pitch, if and only if the charge cloud
induced by an incoming X-ray photon is split into multiple strips and their
energies are accurately measured. However, with existing CdTe strip detectors,
the ratio of such charge-sharing events is limited. An idea for a potential
breakthrough to greatly enhance the ratio of charge-sharing events is to widen
the gaps between strips on the detector. To test the concept, we developed a
wide-gap CdTe strip detector, which has 64 platinum strip electrodes on the
cathode side with some variations in strip pitches from 60 um (30 um strip and
30 um gap width) to 80 um (30 um strip and 50 um gap width). We evaluated the
performance depending on the strip pitches by irradiating X-rays from Am-241 on
the detector. The charge loss due to the wider gaps on the detector was found
to be significant to the extent that the assumption that the energy of an
incoming photon for a charge-sharing event was the simple sum of the energies
detected in adjacent strips lead to a significant degradation in the energy
resolution in the accumulated spectrum, compared with those obtained with its
predecessor having standard gap-widths. We then developed a new
energy-reconstruction method to compensate for the charge loss. Application of
the method to the data yielded a spectrum with a comparable spectral resolution
with that of the predecessor. The ratio of the charge-sharing events for 17.8
keV events was doubled from that of the predecessor, from 24.3 to 49.9 percent.

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