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Position Sensitive Proximity Charge Sensing Readout of HPGe Detectors

  • Author(s): Priest, Anders Peterson
  • Advisor(s): Vetter, Kai
  • et al.
Abstract

Electrode segmentation is a necessity to achieve position sensitivity in semicon-

ductor radiation detectors. Traditional segmentation requires decreasing electrode

sizes while increasing channel numbers to achieve very fine position resolution. These

electrodes can be complicated to fabricate, and many electrodes with individual

electronic channels are required to instrument large detector areas. To simplify

the fabrication process, we have moved the readout electrodes onto a printed cir-

cuit board that is positioned above the ionization type detection material. In this

scheme, charge from radiation interactions will be shared amongst several electrodes,

allowing for position interpolation. Because events can be reconstructed in between

electrodes, fewer electrodes are needed to instrument large detector areas. The prox-

imity charge sensing method of readout promises to simplify detector fabrication

while maintaining the position resolution that is required by fields such as home-

land security, astrophysics, environmental remediation, nuclear physics, and medical

imaging.

We performed scanning measurements on a proof of principle detector that we

fabricated at Lawrence Berkeley National Laboratory (LBNL). These measurements

showed that position resolution much finer than the strip pitch was achievable using

the proximity charge readout method. We performed analytic calculations and Monte

Carlo modeling to optimize the readout electrode geometry for a larger detector to

test the limits of this technology. We achieved an average position resolution of 288

μm with eight proximity electrodes at a 5 mm pitch and 1 mm strip width, set 100

μm away from the detector surface by a Kapton spacer. To achieve this resolution

using standard technologies, 300 μm pitch strips are necessary, and would require

100 channels to instrument the same area. Through our optimization calculations,

we found that there is a trade-off between position resolution and energy resolution,2

and this system provided comparatively poor energy resolution by HPGe standards,

with 4.7 keV FWHM at 59.5 keV. With another electrode geometry, we were able

to achieve 2.9 keV FWHM at 59.5 keV. This dissertation describes the work we

completed to achieve these results.

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