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{\Large The Gas-Silicon-Scintillator Hybrid Detectors}

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{\large N. Curtis}

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{\large 2nd October, 1996}

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\noindent{\large Introduction} 

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A preliminary study of the 170 MeV $^{24}$Mg + 377 $\mu$gcm$^{-2}$ $^{12}$C
target data taken at the Australian National University in June 1996 has been
made. This analysis has concentrated on the performance of the CsI scintillator
detectors mounted at the rear of the hybrid telescopes. This is to allow
a comparison with the the data taken using the MEGHA array during the same
running period, as discused by R.~Cowin in the report ``Effects seen in the
MEGHA telescopes.''

The experimental setup for this run consisted of two of the new Gas-Silicon-Scintillator hybrid detectors placed at 170mm from the target
position in the 2m chamber. For the 170 MeV $^{24}$Mg + $^{12}$C target data
analysed the detector telescopes were centred at 16$^{\circ}$/16$^{\circ}$.
Both the in-plane and out-of-plane angular coverage of each of the detector 
telescopes was approximately $\pm$ 8.0$^{\circ}$.

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\noindent{\large Gas and Silicon Particle Identification} 

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The upper data in Figure 1 shows the particle identification spectrum obtained
by plotting the
energy measured in a silicon detector against the corresponding
gas ionisation chamber for the hybrid run. Although the calibrations of the silicon detector energy 
are currently at a preliminary stage (degrading the resolution of this plot), loci can be seen for particles
ranging from He to Si. The data shown includes singles, coincident and pulser
events. Due to the preliminary state of the analysis, no TDC gating has been
applied to any of the hybrid data presented in this report.

In the lower graph the corresponding particle identification spectrum obtained
from the analysis of data taken with MEGHA is seen (this is discussed
in greater detail in the report ``Effects seen in the
MEGHA telescopes''). This has been expanded
along both the gas and silicon energy axes (with respect to the hybrid data) to emphasis the
un-physical gas energy signal corresponding to Li and He ions. There is no
evidence for such un-physical gas signals in the hybrid data, suggesting that
the cross-contamination of gas and scintillator signals observed in the
MEGHA data is a feature of the signal read-outs on the MEGHA back plates,
and not a physical feature of the gas and scintillator detectors themselves.
This idea is supported by the gas energy against scintillator
energy spectra, which are shown in a later section. 

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\noindent Figure 1: Particle identification spectra obtained from
170 MeV $^{24}$Mg + $^{12}$C target data. In the top spectrum the data
were taken using a Gas-Silicon-Scintillator hybrid telescope. In the lower
spectrum the data were obtained from MEGHA.

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\noindent{\large Silicon and Scintillator Particle Identification}

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In Figure 2 light ion particle identification spectra obtained from
the silicon and scintillator detector energy signals are shown (hybrid data). 
In the upper plot the data were taken with the amplifier used for the 
scintillator signal set to a positive input, in the lower graph the
amplifier was set for a negative signal. It is seen that the ``negative 
signal'' data displays the same overall form as the ``positive signal'' data,
but that the scintillator signal covers a greater range of channels (voltage).
This suggests that the correct setting is for negative signals, and that in the
positive signal case it is only the lower amplitude, second, part of the bi-polar signal that
is being amplified, and not the correct, full-amplitude, first peak.


In Figure 2 it is possible to see a clear locus of events tracking across
the lower left hand of the data, and these are attributed to events where an 
$\alpha$-particle has been detected. To confirm this a second scintillator-silicon particle identification spectrum has been produced, and this
is shown in Figure 3. This was obtained by gating on the
gas-silicon particle identification spectrum (shown in Figure 1), so that
only events corresponding to $\alpha$-particles were incremented into this
new scintillator-silicon spectrum (these data were taken with the amplifier
used for the scintillator signal set to a negative input). The locus of events is clearly seen in
this new plot (Figure 3), confirming that the identification of the 
particles as alphas was correct.


One feature of the data shown in Figure 2 is the smooth fall-off of events with increasing
scintillator energy, indicating that saturation of the signal or
signal cut-off was not occurring
at any stage of the signal processing.
In the MEGHA data taken in July 1996 evidence was seen for the saturation of 
signals obtained from the CsI detectors, which has been suggested to be a result
of an insufficient current supply to the scintillator pre-amplifiers.
In the hybrid data the scintillator pre-amplifiers were powered by the
MEGHA scintillator power supply, effectively five separate
supplies, each designed to power one module. In the hybrid data two of the five supplies
were used, so that each was loaded by one scintillator only. The lack of
evidence for saturation in the hybrid data, but appearance in the
MEGHA data, therefore suggests that the power supplies used can correctly
supply one scintillator, but may be overloaded when used to power
an entire module. It is clear that this problem warrants further investigation.

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\noindent Figure 2: Particle identification spectra obtained by plotting 
the energy deposited in a CsI detector against that lost in the silicon
strip detector for a hybrid run. In the upper plot the
amplifier corresponding to the scintillator signal was set to receive a
positive input. In the lower plot the amplifier was switched to a negative
input.

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\noindent Figure 3: Particle identification spectra obtained by plotting 
the energy deposited in a CsI detector against that lost in the silicon
strip detector for a hybrid run. The data have been filtered by placing 
a software gate on the silicon-gas particle identification spectrum
shown in Figure 1. Only particles identified as alphas in the silicon-gas
spectrum have been incremented into the above plot. This confirms that the
clear locus of events seen in Figure 2 corresponds to $\alpha$-particles.

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\noindent{\large Gas and Scintillator Signal Cross-talk}

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In Figure 4 the energy deposited in the CsI scintillator is plotted against that
lost in the gas ionisation chamber. In the upper plot the data are those obtained
with a Gas-Silicon-Scintillator hybrid, whilst those in the lower graph
were taken with MEGHA. The hybrid data is as expected, with the majority of
events (80\%) lying along the Y-axis, corresponding to particles which deposited
energy in the gas ionisation chamber and which were then stopped by the silicon detector. A further 11.5\% of the data lie along the X-axis, corresponding to
high energy light ions which are stopped in the scintillator, but which
leave insufficient energy in the gas to produce a gas detector signal.
Only 8.5\% of the events produce both a gas and scintillator signal, either
a ``genuine'' coincidence (it has already been noted that no TDC gates have been applied to any of the spectra taken with the hybrids) or events where
a light ion and heavy ion are detected at the same time (such events are seen
in the gas-silicon particle identification plot shown in Figure 1 as a 
broad locus of data running vertically between silicon energy channels 10
to 24). 


In the MEGHA data a clear correlation between gas and
scintillator energy can be seen as a diagonal band tracking across the 
spectrum. The saturation of the scintillator signal may also be seen
as the upturn in the data at the end of this locus. This correlation
is seen as an indication of cross-talk between the signals from the gas and
CsI detectors.
However, there is no evidence for such a correlation in the hybrid data,
indicating that the problem with the MEGHA data lies with the MEGHA
feedthroughs and cabling, and not the detectors themselves. It is noted that
the scintillator signals are pre-amplified inside the MEGHA modules,
as the scintillator pre-amplifiers are mounted directly above the photodiodes
on the rear of the scintillators. The gas signals, however,
are not \, pre-

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\noindent Figure 4: Spectra of scintillator energy against gas energy.
The upper data were obtained during the hybrid run, and the lower data
were taken with MEGHA.


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\noindent amplified until outside the MEGHA module, and are thus
extremely susceptible to noise pick-up inside the detector housing (before
pre-amplification the gas detector signals are very weak, due to a combination
of the low energy deposited in the gas by passing ions and the relatively
large amount of energy needed to create an ion/electron pair). This suggests
that it is more likely to be the 
gas signals which pick up noise from the scintillator signals, rather than
the opposite.





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