General Meeting Summary 11/19/14
NPS COLLABORATION MEETING - 19 NOVEMBER 2014
JLAB: S. Abrahamyan, P. Brindza, A. Camsonne, M. Carmignotto, R. Ent, T. Horn, C. Hyde, A. Mkrtchyan, H. Mkrtchyan, C. Munoz-Camacho, R. Novotny, J. Roche, B. Wojtsekhowski, S. Wood, S. Zhamkochyan, J. Zhang
PHONE: D. Day, D. Dutta, P. Nadel-Turonski, V. Tadevosyan, J. Wagner, Z. Zhao
Work in progress ...
- TCS theory: Theoretically, the quantity of interest are the transverse target asymmetries A_ux and A_uy. Numerical studies using the VGG model show that these may be sensitive to Htilde. Also of interest if the double asymmetry with polarized beam that can be accessed in double DVCS and is sensitive to the unknown Etilde. DVCS and TCS are complementary (universality, spacelike-timelike crossing) and measuring both reactions is of great interest as well.
- Status on PWO: the PANDA EMC requires ~8000 high-quality PWO crystals for the endcaps. The crystal performance requirements are satisfied by PWO-II and improved version of the PWO-I crystals used for CMS. The requirements of NPS are similar to those of Panda. There are two methods to produce PWO: Czochralsky (used by BTCP and Crytur) and Bridgeman (used by SICCAS). The Bridgeman growing method is slower than the Czochralsky method and requires great care due to evaporation. Crystal quality can be described by three main parameters: optical transmittance, light output and count rate, and radiation hardness. The latter can be defined by the induced absorption coefficient, delta_k, for which values of better than 1.1 m^-1 have been achieved at Panda. The refractive index and so the transmittance is different along each crystal axis. Measuring the light yield as a function of temperature can help identify slow component contribution due to impurities - requirement of a few % of slow component is reasonable in terms of chemical composition. Crystal irradiation tests may best be done at low temperatures since that way the statistical recovery is reduced (see slide #17 in Rainer's talk). However, there is a trade-off between light output and resolution due to pile-up when considering to operate at lower temperatures. Stimulated recovery with difference wavelengths of light is most useful to for recovery from radiation damage due to electrons/photons (slides #19-#21 in Rainer's talk provide an overview). Radiation damage due to hadrons as observed for the CMS EMC (large fluence ~10^13 cm^-2) is best recovered through annealing with heating. After BTCP shut down SICCAS/China and more recently CRYTUR/Czech Republic are the only manufacturers of PWO worldwide. Newer SICCAS produced crystals show large fluctuations in optical properties, light output, and the radiation induced absorption coefficient (slides #30-33 in Rainer's talk show recent results). The fluctuations may be due to problems with quality control in the crystal production process. The R&D phase at CRYTUR started in June 2014 and knowledge and raw materials are available for crystal production. The first long crystals are expected to be produced before the end of the year.
- Alternative crystals: PWO is the most compact crystal and has a small Moliere radius. LYSO is next, but manufacturing long, homogeneous crystals (compared to those that are used for PET) may prove difficult. Radiation hard BGO crystals may be available, but production of long crystals while keeping them homogeneous could be difficult as well. Furthermore, the timing for BGO is worse compared to PWO. Next in size are fluorides like CeF3 and BaF2. No technology to grow large crystals exists. BaF2 has a slow component that could be removed with a good filter, but this would make this crystal choice more costly. CsI(Ta) is too slow. For CsI there is no manufacturer available and one would have to eliminate slow components. In summary, there are not many choices beyond PWO.
- NPS PWO studies: the crystals tested in irradiation studies at JLab were produced by SICCAS in 2014. The requirement on radiation hardness was delta_k < 1.6 m^-1. A Cs-137 source was used producing dose rates of 260 rad/h.
- EIC R&D: proposal was submitted and approved in July 2014. The main goal is to develop procedures to determine PWO quality regarding light yield and radiation hardness. Current activities focus on setting up infrastructure and collaboration with Giessen on the crystal tests (slides #3-5 in Carlos's talk provide the details).
- Irradiation of PbF2 crystals at IAC for DVCS in 2010: the PbF2 crystals tested were produced by SICCAS. The test results on radiation hardness show large variation among the crystals (see slide #11 in Carlos's talk). Optical transmittance measurements were in good agreement with visual inspection.