Although
is much better than Freon-12 for
electron detection, the crucial question is how it performs for
pion rejection. Preliminary estimations[2] of the
pion
detection efficiency show that it is sufficiently small
for Freon-12 gas.
An important mechanism for the detection of slow (with velocity under the Cerenkov gas threshold) pions is due to the detection of fast knock-on electrons.
Figure 5:
Energy distribution of secondary electrons
in
interactions.
Fig. 5 shows the number of knock-on electrons with energies above threshold as a function of the threshold energy. This value is approximately proportional to the probability of pion detection. Thresholds for different gases are shown as well.
The number of knock-on electrons also depend on the gas density, which varies significantly for different gases.
Table 2: Estimation of
- meson detection efficiency for
different gases
Table 2. shows the threshold 
- factor:
,
gas density and estimated value of relative pion detection efficiency
with respect to Freon-12 gas.
The pion detection efficiency is discussed in
more detail in section 6.3.
We conclude that
can be used as the radiator
gas for the CLAS Cerenkov detector.
Another factor which affects the performance of the detector is
light cone opening angle, which is greater for
than for Freon-12.
Fig. 6 shows the difference in
for
and Freon-12
as a function of Theta and Phi,
computed using the CC package.
Figure 6:
Difference in mean number of photo-electrons for
and Freon-12 gases as a function of Theta
and Phi for 0.8 GeV electrons.
We note the net gain of 
additional
photoelectrons in case of
gas
for the most part of the sensitive region.
This is in good agreement with estimations from Table 1.
