Pulse Shape Discrimination for INTEGRAL's Spectrometer Instrument
R. T. Skelton, J. L. Matteson
S. A. Slassi-Sennou, R. P. Lin
N. W. Madden
P. von Ballmoos, J. Knoedlseder
Abstract
NASA is providing a Pulse Shape Discrimination (PSD) system for the
Ge detectors of the Imaging Spectrometer (SPI) on ESA's INTEGRAL
mission. This will reduce the background and improve the
sensitivity between approximately 400 keV and 2 MeV. The dominant
background contributor for the detectors in this energy range will be
beta-minus decays from Ge isotopes activated by cosmic rays. This
induced activity has mostly single-site energy losses, while photons in this
energy range usually deposit energy in multiple sites. The differences
in these types of events' pulse shapes can be exploited to reject
single-site events. The effectiveness of this technique is enhanced by using
Ge detectors with small central bores and a high-bandpass preamplifier output
(separate from the spectroscopy output); pulses from single-site events are
strongly peaked, and pulse duration is around 400 ns. This signal is sampled
at 100 MHz, digitized to 9 bits, and analyzed on the basis of its resemblance
to a single-site event versus a linear combination. Recent progress includes
an analysis algorithm that runs fast enough in the flight processor yet still
approaches the theoretical increase in sensitivity, rejecting more than 95%
of the single-site events while accepting about 60% of the multiple-site
events. For the background expected in the interplanetary INTEGRAL orbit,
sensitivity should be improved by a factor of 2.1 at 845 keV. The Pulse Shape
Discriminator's electronic systems and analysis algorithm are described, and
results on discrimination between single-site and multiple-site events are
presented.
File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:19.