Highly segmented HPGe
detectors will be used in the future generation of Tracking detector arrays.
The Perkin Elmer (Ortec) 24-fold prototype detector,
called TIGRE (Tracking the Interaction of Gamma Ray Events ) was delivered to the University of Liverpool in February 2001, with the aim of testing the feasibility of PULSE SHAPE ANALYSIS as a tool to improve the position resolution and to
enable Gamma Ray Tracking.
It consists of a coaxial high-purity n-type germanium crystal, 65 mm
diameter by 80 mm length, inside a 1 mm thick alluminium can of the
same shape and housed into a vacuum vessel. The inner contact is
lithium drifted, while the outer contact is boron implanted.
The outer electrode is segmented in two dimensions, 6 ways
longitudinally and 4 ways in depth,
this results in a total of 25 output electronic channels.
The transverse segmentation planes are at different relative distance,
namely 15 mm, 15 mm, 20 mm and 30 mm. This choice reduces the volume
of the segments in which the electric field is not homogeneous, i.e.
the front segments, and allows to obtain a more equal interaction
rate among all the parts. The front face of the detector has rounded
corner to minimize the crowding of the electric field lines at sharp
edges. Each contact has been fitted with fast charge sensitive preamplifiers based on the University of Cologne design and provided with warm FETs.
Even if warm, these FETs provide low noise (about 2.5 mV
peak-to-peak, i.e. about 10 keV) and good energy resolution
performance.
The preamplifiers are mounted on a cylindrical motherboard, 250 mm
diameter by 98.5 mm length, in the back of the detector. Special care
was taken in reducing length and crossing
of the electric wires, main causes of cross-talk problems.Cross-talk
is one of the
most common problem in the segmented detector technology.
It consists in a coupling of the signals from adjacent segments,
resulting in a net charge observed in the segment that has not been
hit by the gamma-ray too.
It is due to an insufficient shielding and a possible crossing
of the wires from different electronic channels.
In spite of the fairly complex electronics of the 6x4 detector,
no cross-talk has been observed so far.
The operating voltage of the detector is +3200 V and it is applied on
the centre contact.
There is only one preamplifier power supply requiring a +/-12 V line.
The detector is constantly cooled to a temperature of 95 K.
The cryostat has a nominal lasting time of 72 hours.
DETECTOR PERFORMANCE : Energy resolution and Efficiency measurements have been performed using conventional anlogue electronics. The energy resolution of the side channels range from 2.1 to 2.5 keV at 1332 keV gamma-ray energy. The resolution of the central contact is 3.9 keV. This results were obtained with 6 us shaping time constant.
A scanning apparatus has been developed.
More information about
tracking detectors can be found in Digital Pulse Processing and Gamma Ray Tracking
Document Last Modified Thu Feb
26 17:01:05 2002
Maintained by Martina Descovich