General Meeting Summary 9/18/14
- Gain Monitoring System (GMS) was tested with ~12 hr long runs.
- The input light was provided by a blue LED driven at frequency 0.8 Hz and supply voltage to the prototype PMTs was 1.3 kV.
- The system's performance was stable, but the PMT signal widths are 2-3% larger than what one would expect from the Poisson distribution.
- The GMS was used to calibrate the prototype detector
- The faulty base in channel 2 had been replaced before these calibrations
- Compared to previous tests the PMT voltages were raised to 1.4 kV allowing for cleaner separation of the SPE from the pedestal
- Measured amplitude of PMT signals from cosmics passing at 12.5 cm from the PMT is 40-50 PE and the summed signal is about 150 PS.
- As in previous tests a significant fraction of low amplitude signals were detected. This may be due to light leaking from the rear of the crystals into adjacent PMTs. Optimization studies to eliminate these low amplitude signals are ongoing.
- Test of the crystal curing system
- A 3x3x16 cm^3 crystal was irradiated to dose 271 krad. No sign of spontaneous curing was observed after 20 days.
- The crystal was then exposed to IR light of 940 nm from a set of 4 TSAL-7400 LEDs and then from a set of 2 blue and 2 red LEDs (in both cases curing was done from the irradiated end of the crystal)
- The transparency of the crystal was measured in the transverse direction at ~1cm from the end before curing and after 3 hours of IR curing, after 20 hours of IR curing, and after 48 hours of blue light curing.
- No definite sign of curing after exposure to the different LED systems (different wavelengths) was observed.
- Discussion about crystal irradiation and curing
- See no effect of curing for either wavelength
- The accuracy of the transmittance measurement is 0.5-1% for an irradiated crystal (60 krad) of 3 cm thickness. For a non-irradiated crystal the uncertainty is not as well known.
- Good news in general is that the effect of 200 krad does not appear to be very strong
- Perhaps see only relatively small effect and with large uncertainties, so that there is no observed net effect of the curing. Based on transmittance measurements with 20 cm long crystals at Caltech (see Figs. 9 and 10) the expected change in transmittance is 1.5-2% for the 3 cm thick crystals used in the present studies. This is marginal given the accuracy of the present setup.
- Comparison to measurements through thickness of 20 cm suggests that the observable effect would be larger (~10%), so that effect of change in transmittance should be clear with the ~1% accuracy of the setup. Tests will be done to check the effect of irradiation using a crystal recently purchased from SIC (Jan 2014)
- Discussion about the path forward with crystal radiation studies
- The observed effect on transmittance for crystals (those tested so far) irradiated with instantaneous doses of 20-30 rad/hr are small. For comparison, the estimated dose rates for experiments reach 1 krad/hour for crystals closest to the beam line. Next steps in the irradiation/curing studies should verify if the effect is really so small: 1) check procedure and possibly improve accuracy of setup, 2) check the effect on different crystals
- A next step to prove that curing works could be tests at higher radiation doses. This will require faster irradiation. Discussion about logistics of such a measurement, e.g., planning for shipping crystals to an irradiation facility like Idaho, when and with what setup transmittance measurements would be taken (e.g., one right after irradiation with a setup TBD and others when crystals are back at JLab), and how many crystals should be tested. Note that for lead glass after a dose of 230 krad there was no significant residual radiation and blocks could be shipped back immediately.
- NSF MRI: summary of discussion with program officer. Upshot is that re-submission at same funding level is encouraged. Next steps will include implementing the suggestions and check agreements with universities.
COLLABORATION MEETING: 19 NOVEMBER AT JLAB
- Discussion about agenda items
- Ideas on science, e.g., polarized target experiments with NPS
- Presentation on PbWO4 crystal studies by Rainer Novotny (Rainer will also give a seminar on Monday, 17 Nov, and will be available for informal discussions on Mon/Tue)
- NPS crystal irradiation/prototype studies
- NPS electronics
- Funding for NPS
- additional topics...
ITEMS FOR NEXT MEETING:
- NPS prototype and PWO studies
- Next collaboration meeting agenda
NEXT MEETING: THUR, 9 OCTOBER at 9:00 AM (EST)
Following two meetings planned for:
- THUR, 30 OCTOBER at 9:00 AM (EST)
- COLLABORATION MEETING: WED, 19 NOVEMBER