IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
shown along with two-detector
coincidence spectrum. As can
be seen, the peak (511 keV) to
background (around 530 keV)
ratio is about 10 2 for single
detector, while it is about 10 5
for coincidence mode. Thus,
the Coincidence Doppler
method allows to analyze the
high momentum regions with
much better precision, enabling
chemical sensitivity.
A two detector coincidence
system has been setup
consisting of a pair of HPGe
detectors, spectroscopy
amplifiers and ADC's and a
MCA capable of acquiring the
coincident spectrum. The Ge
detectors are of 30 % efficiency
having an energy resolution of
1.27 keV and 1.5 keV at 662
keV γ-ray. Fig. 2 shows a twodimensional
spectrum recorded
on Si (100). The channel
numbers (converted to
annihilation energies) of the
two detectors form the
horizontal and vertical axes of
the figure. The horizontal and
the vertical bands correspond
to the intensities of the
annihilation gamma rays of the
individual detector. The intense
peak at the center corresponds
to the counts for E 1 =E 2 = 511
keV. The elliptical region
extending diagonally with
E 1 +E 2 = 1022 keV originates
from the coincidence events
and corresponds to the
annihilations with high
momentum electrons. This
region is nearly background
free.
Coincidence Doppler
measurements have been used
to investigate Nickel Silicides.
The ratio of the measured
Doppler curve in Ni, NiSi and
NiSi 2 , with respect
to Si is shown in
Fig. 3. The Ni curve
shows a maximum around
p L = 13 x 10 -3 m 0 c which
corresponds to annihilation of
4
positrons with 3d electrons and
the broadening of the curve is
due to the contribution from the
more localized 3p electrons.
Unlike the Ni curve, the NiSi
and NiSi 2 do not exhibit a
maximum around 13 x 10 -3
m o c, implying that the
contributions from 3d electrons
have decreased. It may be
noted that there is a distinct
maxima arising around 27 x
10 -3 m 0 c for NiSi and NiSi 2 .
This may be attributed to
positron annihilation with 3p
electrons thus bringing out the
potential of coincidence
Doppler measurements in
distinguishing annihilations
from both NiSi and NiSi 2 .
These experiments illustrate the
efficacy of coincidence Doppler
broadening technique, that
facilitates measurement of
Doppler broadening spectrum
with a high peak-tobackground
ratio to probe the
high momentum regions.
Ni
NiSi
NiSi 2
Ratio to Si
3
2
1
Fig.2 A two dimensional display of the coincident events
collected on Si (100). A total of 3 x 10 7 counts are
accumulated in the spectrum.
0 10 20 30 40
p L
(10 -3 m o
c)
Fig.3 Ratio of the experimental curves
with respect to Si.
BASIC RESEARCH 159