IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
Fig.2 Timing spectrum
mm thick NaI(Tl) crystal
coupled with 203 mm, 50 mm
CsI(Tl) crystal and optically
coupled to three matched 3"
dia photomultiplier tubes (ETL
9255). The detector has an
ultra low background 0.5 mm
thick beryllium entrance
window. The new PSD, based
on scintillation time
comparison, uses the rise time
difference in the signals of
NaI(Tl) (250 ns) and CsI(Tl)
(1100 ns) to produce a gating
pulse to selectively choose the
pulses due to LE photon
interactions in NaI(Tl) alone.
The schematic of the PSD
electronics with typical signals
is shown in Fig.1.
The Pulse Shape Analyser
(PSA), analyses the input pulses
and generates two negative
pulses, one at 90% peak
amplitude labeled as "A" and
another at selected fraction (B)
of the peak amplitude labeled
as "B". The outputs "A" and "B"
from PSA are fed to the inputs
of Time to Amplitude Converter
(TAC) as "Start" and "Stop"
pulses, respectively. This results
in TAC unit generating a square
pulse whose amplitude is
proportional to the time
difference between A & B.
Hence, the LE photon
interaction from NaI(Tl)
generates pulses with lower
amplitudes compared to the
pulses from CsI(Tl). Once the
amplitude of TAC pulses
corresponding to NaI(Tl) or CsI
is known, any of them can be
selected for gating by using a
SCA with properly selected LLD
& ULD.
Processing of pulses for
risetime analysis results a time
delay in generating gate pulse.
Typical time delay is observed
to be 3.75 µs for NaI(Tl) pulses
and 5.75 µs for CsI pulses.
Every bipolar pulse from delay
line amplifier is precisely
delayed and then presented to
MCA for analysis. One of the
inputs of MCA coincidence
circuit is the delayed pulse from
delay amplifier and the other
input is the gating pulse from
the SCA. The input pulses to the
delay amplifier are accepted
for spectrum analysis only when
the gating pulses occur
simultaneously. This results in
rejection of most of the
unwanted high energy
interactions, which will have a
time characteristic of CsI(Tl)
detector, thereby reducing the
background considerably.
The timing spectrum obtained
with 137Cs source, with
optimized PSD parameters, is
shown in Fig.2. The figure of
merit (M) for the obtained
timing spectrum is estimated to
be 3.0.
Comparison of the peak
position and the FWHM values
obtained for 17 keV, 60 keV
and 122 keV with and without
PSD electronics clearly shows
that PSD has not affected
spectrum parameters. In order
to quantitatively estimate the
background reduction, spectra
were obtained with and without
PSD (Figure-3). The inclusion of
PSD electronics has reduced
the background from 9.5 cps to
0.28 cps in 17 keV region and
5.8 cps to 0.3 cps in 60 keV
region. This results in an
improvement in the MDA values
by a factor of 2.
A state of art pulse shape
discriminator employing readily
available nuclear electronics
Fig.3 Background reduction with
PSD electronics
ENABLING TECHNOLOGIES 133