Log entry time 05:53:54 on November 08, 2008

Entry number 247525

I just took a few runs with different configurations.

1600 V + 3.5 + 4.4 + 4.4V

beam current 2,4,6,8,10,12 uA

1. Dark Current The dark current 2~23uA 4~43uA 6~70uA 8~90uA 9~115uA 10~140uA The limit is 200uA. It seems that the wire chamber is quite stable during the 140uA period. However, the 140uA is quite close to the 200uA limit. I think that we should try to combine the two channel together (in parallel). I am not sure whether this is ok? Does anybody have any suggestions?

2. Background rate The background rate is about 9-10MHz per 2uA. At 12 uA the background rate is about 60MHz. It is higher than my expectation 25MHz by more than a factor of 2. The reason for this discrepancy is due to the background from both target region and the downstream beam pipe.

2.1 Background from target It is clear that there are huge amount events coming from the up stream window. It seems that a lot of pions are punching through the upstream beam pipe window collimator. I suggest to make a thicker collimator to shield the background.

I also suggest to add in another background shielding collimator for the upstream He4 gas.

2.2 Background from downstream beam pipe We can see clearly that the background rates for chamber 1 and chamber 3 are quite similar at this kinematics. This is not the case for the previous two pass. This suggests that the downstream shielding plate are not that effective at this energy rate. The background become more energetic, and start punching through the shielding plate. We can see clearly from the tracking eff defined by "track num"/"trigger".

At 2uA with 1600 V, our tracking eff is about 0.12, this is very different from the 0.7 that we get from 1 pass. This means a lot of trigger from shower are not coming from the target region.

Another possible explaination is that the direct energetic photon are starting hitting shower. They may not leave hits on the wire chamber.

The expected rates should around 25-35MHz. In this case, I prefer the first explaination, since the shielding in my models are much thicker than what we have now. I suggest that we should plan to put more shielding for the downstream pipe.

3. Tracking eff.

3018 2 3.5 10.3815 9.43255 0.121533 0.00311031

3019 4 3.5 18.9571 17.2504 0.131031 0.00468902

3020 6 3.5 29.1015 26.6706 0.14424 0.00661045

3021 8 3.5 39.3595 36.2396 0.161165 0.00874506

3022 10 3.5 47.5341 44.016 0.173073 0.0114784

3023 12 3.5 47.8428 43.9743 0.172098 0.01165

It is clear that the tracking efficiency increase with the increase of the beam current. This is due to the increase of the background rates. Including the shower and improving the tracking should help. However, I suggest that we do everything possible to reduce the background from both target region and downsteram pipe.

4. Hitting eff. RUN beam HV rate1+2 rate3 tracking eff multi track

3032 2 1600 11.4283 10.3231 0.122457 0.00345644

3033 2 1650 11.8536 10.4125 0.122606 0.00407765

3034 2 1700 14.0716 12.0248 0.124238 0.00442659

3035 1 1700 8.90605 7.58549 0.120534 0.00332048

3036 1 1650 7.37986 6.50915 0.119568 0.00308429

3037 1 1600 5.24161 4.64528 0.115835 0.00300985

3038 1 1550 4.44435 4.01126 0.0864287 0.00162635

It seems that the tracking efficiency is quite stable among the HV settings except the 1550 V. I guess it is safe to set it at 1600 V for electrons and current threshold. We can set it a little bit higher if necessary.

A copy of this log entry has been emailed to: jiang@jlab.org,jpchen@jlab.org,doug@jlab.org,brads@jlab.org,camsonne@jlab.org,gao@jlab.org,cisbani@jlab.org,xqian@jlab.org

Figure 1

Figure 2