This is interesting. There is a dramatic 3x increase in the rates at low thresholds with the latest configuration. Another oddity is that the beam-off rate on the witness PMT is 6x higher than it used to be...
There are three changes made since the previous configuration:
1) A much thicker upstream target collimator was installed at the same time
the extra lead plate was added. (This likely accounts for the difference in
T1, T6 rates.)
2) An additional 1" of Pb plate was added to the beamline side of the witness
box. Two 4" thick wood blocks were also added to the beamline side of the
witness box.
3) The Cerenkov PMTs were turned on by accident for ~2 hours at production
currents during owl shift on Nov. 30
(runs 4046--4052, see Halog
251429). (I only mention this in case the 2 hours at full current may
have damaged something. I really doubt it, but who knows.)
Assuming the PMT isn't screwed up somehow, then perhaps the upstream collimator is doing more harm than good? Since it didn't seem to make a significant difference in the trigger rates, perhaps we could revert to the old collimator and see if the rates go back down?
Started 6:33pm, Thursday Dec 4, Run 4181
Beam current: 11.9 uA
87.2 kHz T1 rate (about 10% lower than with previous 3He cell + beam tune)
1.3 kHz T3 rate (about 10% lower than with previous 3He cell + beam tune)
0.040 kHz T5 rate
18.6 kHz T6 rate
(T1) Threshold for 'BB_Sh_2' is -0.080 V (1925)
(T6) Threshold for 'BB_Sh' is -0.120 V (1833)
! NOTE: A thicker upstream target collimator was also installed at
! the same time the extra lead plate was added. This accounts
! for the difference in T1, T6 rates.
### This is with the 1" of Pb on all sides but that facing the target.
### Target side has 2+" of Pb.
### 1" thick Al plates on bottom, 1" Pb plate on top
### All sides covered with 2--4" of borated plastic
### Added 1" more of Pb on beamline side of witness box (vs. last config)
### Added 4" of wood to beamline side of witness box (vs. last config)
T7 rates w/ HV on 19 on (witness PMT @ 2100V); 11.8 uA beam
Thresh Rate (kHz)
-30 524.
-50 477.
-100 384.
-300 86.5 (~1.8 kHz during trip)
-500 10.3 (~140 Hz during trip)
-700 3.1
-900 1.8
Done 6:44pm, Run 4181
### Previous configurations:
### This is with the 1" of Pb on all sides but that facing the target.
## Side facing target is unshielded.
### 1" thick Al plates are on top and bottom.
### Done on Tuesday, run 3654
Beam current: 11.8 uA
103. kHz T1 rate
1.4 kHz T3 rate
0.051 kHz T5 rate
22.5 kHz T6 rate
T7 rates w/ HV on 19 on (witness PMT @ 2100V); 11.8 uA beam
Thresh Rate (kHz)
-100 104.
-300 27.5 (~300 Hz during trip)
-500 9.3
-700 6.3
-900 4.3
### This is the first (unshielded) run
T7 rates w/ HV on 19 on (witness PMT @ 2100V); 11.8 uA beam
Thresh Rate (kHz)
-100 168.
-300 48.6
-500 17.6
-700 11.8
-900 8.8