General Meeting Summary 7/10/14
- Discussion of TAC report pi0 proposal
- add two-photon detection efficiency - can estimate from original NPS document, which showed two-photon detection efficiency as function of pion energy at distance of 4 m from the target (quick estimate for expected high-energy pions is ~70% for 8 GeV, ~30% for 3 GeV pions
- Discussion about most recent results of PbWO4 crystal radiation tests - one of the goals of these tests is to test the efficiency of curing
- Transmittance measurements after 60 krad (accumulated at ~200 rad/hour) shows no sign of degradation.
- Reproducibility of the transmittance measurements is 3-5%.
- Conclusions and path forward with crystal radiation studies
- PbWO4 crystals can handle doses of 50-60 krad without noticable changes in their optical performance
- Radiation damage is conditioned by dose rate. At dose rates 50-300 rad/hr crystal degradation may reach equilibrium, where the effect on the transmittance is only 10-15%. Spontaneous crystal recovery (~0.05%/hr) partially compensates radiation damage. The exptected effect of radiation damage for a 1.5 cm crystal sample for dose rate ~100 krad is on the order of ~1% (extrapolated from higher dose rates, see figure from R-Y Zhu paper)
- To detect such small effects of degradation the accuracy of the measurement has to be improved to better than 0.5% (literature value is 0.25%)
- May also consider to irradiate to higher doses. Limitation at JLab is the availability of high doses over reasonably short times. Alternatives may be irradiation at BNL or Caltech facilities. We may also want to test a crystal of the actual size that will be used in NPS.
- Prototype construction is ongoing
ITEMS FOR NEXT MEETING:
- NPS prototype and PWO studies
NEXT MEETING: THUR, 31 JULY at 9:00 AM (EST)