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This report outlines the findings from the R&D of LST technology, including a study on cell size, aging test results, rate capability, and QA procedures at assembling sites. The report discusses the comparison of different cell geometries and the effects of gas mixtures. It also provides updates on the aging test, proposes the use of Noryl material for the LST profile, and evaluates the suitability of LST for the BaBar endcap IFR. Finally, the report outlines the proposed QA procedure at the Princeton assembling site.
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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 • Comparison of three different cell geometries: • Standard cell size — 9 9 mm2,ST type; • Small size, anode wire is located at the center of the cell — 9 8 mm2, SC type; • Small size, anode wire is equally distant from three conductive walls — 9 8 mm2, SE type. Criterion for the comparison: Relative variation of the E-field on the anode wire surface along the circumferential direction.
E-field variation on anode wire surface 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 E-field lines
Coverless cell with normal cell size V contour plot E-field lines
E-field distribution on the anode wire surface 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
… E-field distribution * V = 4700* (Emax – Emin)/Emax The differences among three geometries are small, 9 8 mm2 cell size won’t be a problem.
Plateau curve of different gas mixture Safe gas mixture (ZEUS) Standard binary gas mixture. Both of them can get good single’s rate plateau curve.
LST for ZEUS, SLD, Phenix and our proposed one No one system is a duplication of the other, we have to conduct our own aging test.
Update on 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.
Effect of the gas pressure on the streamer signal size 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.
Is LST suitable for BaBar endcap IFR? • Two key performances need to be checked (based on Jerry’s estimation): • Rate capability - 100Hz/cm; • Aging performance – 2C/cm. Qrpc = 1000pC Q = 20C/cm2 Qlst = 100pC Q = 2C/cm2
Rate capability of LST 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.
… rate capability 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.
Proposed QA procedure at Princeton assembling site 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.
QA procedure • Stability of the anode current — at the maximum voltage continuously monitor and record the current drawn by each chamber for one month. Criterion for acceptance: high current activity above 100nA < 5% of the time in the first half of the history, no activity in the second half of the time; • Pulse-form test — decent pulse shape should be seen on the scope: 30 ~ 50 mV, 40 ns. Criterion fro rejection: anomalously shaped pulses or no pulse at all, indication of construction faults; • Cathode scanning with a radioactive source — using radioactive source to produce exceptionally high rate of charge exchange on the cathode to detect defect on the graphite coated cathode. Garfield calculation shows at 4700V the total cathode charge ~ 55nC/m, if one anode wire current is 55nA, the source moving speed is 10cm/s, then within 10 seconds we can scan 1m long chamber, and make 10 times cathode charge exchange.
Cathode scan 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)
Conclusion • LST E-field study by Garfield for three different cell’s geometries shows that the small cell size (9x8mm2) has very little affect on the E-field distribution, will not deteriorate the performance. A real test will be conducted on the new prototypes; • Aging test on an old PVC LST chamber shows that up to 170mC/cm charge dose, no aging symptom can be seen, which indicates even with PVC LST should have no problem for barrel IFR; • Rate test shows that up to 110Hz/cm the charge spectrum shows no difference from cosmic ray background, which indicates that LST should be able to handle the rate for BaBar IFR endcap layer 18 @ 4x1034/cm2sec PEP II luminosity.
