Efficiency of Gas Cherenkov Mirrors on the Left Arm

The efficiency of each channel is a combined result of mirror efficiency, PMT efficiency and also the alignment condition. First of all, the mean value of number of photoelectrons can be found by fitting the adc_c signal with distribution or gaussian distribution. From figure can see that mirror #7 and #8 have lower mean value for number of photoelectrons (around 6), compared to other mirrors.

Secondly, to check the efficiency for each channel, plot of adc_sum vs. det_xgeom for each channel of gas cherenkov on the left arm is given in Figure . Here a cut in adc_c[i;i] $>$chan$_{single}$ is used to choose the $i^{th}$ mirror, where chan$_{single}$ is the channel number of the upper edge of single photoelectron peak in adc_c spectra. Part of the espace_kumac file is shown as follows:

define/variable 'eadcgas:spec_e.gas.adc_sum_t'
define/variable 'egasxg:spec_e.gas.xdetgeom'
define/variable 'egasyg:spec_e.gas.ydetgeom'

do i=1,10
define/cut/1d spec_e.gas.adc_c[[i];[i]]-1 130. 6000. /* cut #1 */
define/cut/1d spec_e.gas.adc_c[[i];[i]]-2 380. 6000. /* cut #2 */
define/cut/1d spec_e.gas.adc_c[[i];[i]]-3 130. 380. /* cut #3 */
define/logical egc1[i] spec_e.gas.adc_c[[i];[i]]-1
define/logical egc2[i] spec_e.gas.adc_c[[i];[i]]-2
define/logical egc3[i] spec_e.gas.adc_c[[i];[i]]-3
enddo

do i=1,10
set/ntuple on
spectra/save event_type/eadcgas/egasxg/egasyg egc1[i]
spectra/save event_type/eadcgas/egasxg/egasyg egc2[i]
spectra/save event_type/eadcgas/egasxg/egasyg egc3[i]
set/ntuple off
enddo

The plot shows that mirror # 10 has a high single photo-electron peak, which means it probablly have problem in alignment. Figure shows the performance of mirror # 9 and # 10 during E94-010 (GDH) experiment in 1998. Can see that mirror # 9 had alignment problem during E94-010. Obviously when redoing the alignment after E94-010 mirror # 9 was fixed but # 10 was affected. The overall efficiency is probablly not optimized and furthur work needs to be done if high pion rejection efficiency is required.

Figure: Gas cherenkov ADC_SUM vs. position on the left arm

Figure: Gas cherenkov ADC_SUM vs. position on the left arm during E94-010 (h-arm)

Figure shows the event distribution for each gas cherenkov mirror on the left arm. For the $i^{th}$ mirror, the grey region shows the events which has only one photoelectron in it (cut#3: adc_c[i;i]$<$chan$_{single}$). Here the grey region shows the edge of each mirror.

From these plots can see that the outgoing particles are not pointing to the center of the detector, mirror #1 and #2 are not fully covered. The overall efficiency could be improved by moving the whole detector lower by 1/4 of the mirror size (vertical). By doing this the bad performance of mirror #10 can also be partially avoided.

Figure: Event distribution of gas cherenkov on the left arm