R&D status report of LST technology (2). Changguo Lu Princeton University BaBar IFR Workshop SLAC, December 8, 2002. Outlines. LST cell size study; Aging test results; LST rate capability; QA at US assembling sites. LST cell size study.
BaBar IFR Workshop
SLAC, December 8, 2002
Criterion for the comparison:
Relative variation of the E-field on the anode wire surface along the circumferential direction.
Coverless LST chamber has one distinguish feature: its E-field on the anode wire surface varies along the circumferential direction.
9 x 9 mm2 cell with conductive cover
V contour plot
V contour plot
9 x 9 mm2 cell size, 100m anode wire, Vanode = 4700V
For all-covered cell E-field on the anode wire surface is constant, but for coverless cell E-field varies with .
The criterion for comparison:
(Emax – Emin)/Emax
* V = 4700* (Emax – Emin)/Emax
The differences among three geometries are small, 9 8 mm2 cell size won’t be a problem.
Safe gas mixture (ZEUS)
Standard binary gas mixture.
Both of them can get good single’s rate plateau curve.
No one system is a duplication of the other, we have to conduct our own aging test.
So far the accumulated charge dose has reached 170 mC/cm, no aging sign has shown up yet.
The test chamber used in this test was made of PVC, anode is 100m Au-coated Cu-Be wire, a ceremic end board used to distribute high voltage to eight wires, PVC profile was painted with standard graphite paint. The chamber was heat sealed and the wires were heat staked onto plastic supports. No glue was used.
We are proposing to use Noryl instead of PVC as the profile and envelop building material. According to ZEUS aging test results Noryl certainly will be much better than PVC. SMC experience shows that their PVC LST aging was well beyond 1C/cm, so we believe that the Noryl LST should be even better than 1C/cm.
LST signal size is sensitive to temperature and gas pressure.
Aging test chamber with Sr-90 source. During this period the room temperature was quite stable, ~ 240.5C.
Qrpc = 1000pC Q = 20C/cm2
Qlst = 100pC Q = 2C/cm2
The spectra of 61Hz/cm and 111Hz/cm are generated by Sr-90 source with different absorbers.
Very similar to cosmic ray background LST can handle the rate of BaBar IFR endcap layer 18.
Further increase the source intensity we can see the charge spectrum shifting towards left:
Aging performance up to 2C/cm2 for Noryl LST is still unknown. But so far no any negative evidence. We are going to perform this test as long as we get final version of LST from PolHiTech.
Never overestimate the importance of QA for a successful large LST system.
PolHiTech will continue to use the QA procedure for Phenix LST mass production. They will continue to monitor the anode wire current for one month. The final selection of good chamber will be based on the current history of the chamber. After receiving the LST chambers at US assembling sites (Princeton and Ohio State), before making the final assembly unit, second round of QA will take place.
The proposed QA procedure at assembling site is based on successful OPAL LST system. That procedure had been used to test more than 28000 LST tubes (3609 8-cell chambers of length 3.0 m to 7.3 m) at CERN. Similar QA program was used at University of Maryland during that time.
Due to graphite coating non-uniformity, some isolated islands may exist on the cathode surface, under the normal background rate those bad spots may not cause trouble, but under high rate condition it would trigger self-sustaining discharge, and reveal this type of defect.
Even w/o source the high current is still remaining.
F. Fabbri et al. CERN/EP 87-134, 29 July, 1987)