Abstract:
A new method for determining the entry point of gamma-rays in closed ended HPGe
detectors has been developed. Exploiting the position dependence shown by the
current pulses generated when a gamma-ray interacts with the detector, it is possible
to electronically divide the crystal in the radial coordinate and thus increase the
effective granularity of the detector.
Position resolution is particularly important for correcting the Doppler peak
broadening observed in many in-beam gamma-spectroscopy experiments. Position resolution
within coaxial crystals is accomplished by combining the angular information
gained by segmenting the outer contact with the determination of the event
radius by current pulse shape analysis.
With increasing gamma-energy, more than one interaction is in general required
to completely stop a gamma-ray in a germanium detector. The concept of a main
interaction, defined as that depositing the largest fraction of the original gamma-energy,
is introduced and seen to be the dominant contribution to the event current pulses.
A Monte Carlo simulation for the positions and energies of the interactions in an
event has been performed in order to establish the physical limits for the position
resolution that can be measured in a segmented CLUSTER module. A varying
fraction of events, from 55% at a gamma-energy of 400 keV to 85% at 1800 keV, have
their main interaction within 5 mm from the entry point. The position of the
main interaction can therefore be successfully used to measure the entry position
of the gamma-ray in the detector.
In order to provide high quality charge/energy and current outputs from the
detector signal, a new preamplifier for large volume HPGe detectors has been
developed. The intrinsic equivalent noise contribution from the preamplifier was
measured at 0.65 keV + 35 eV /pF. The measured energy resolution when the
input FET is operated at cryogenic temperature is 2.30 keV at 1333 keV with
3 μs shaping time.
Using this preamplifier and the first prototype of a two-fold segmented
CLUSTER module, a radial resolution of ±4mm has been measured with the
new method both at 662 and 1333 keV. The method can be incorporated into
an analogue electronic circuit and is therefore directly applicable in in-beam gamma-
spectroscopy experiments.
