Long Term Recovery of Seven PWO Crystals Ren-yuan Zhu California Institute of Technology CMS ECAL Week, CERN
Introduction 20 endcap and 5 barrel PWO crystals went through (1) thermal annealing at 200 o C, (2) irradiations by J ray at 15, 100, 400 and 9k rad/h until equilibrium and (3) short term (300 hours) recovery after 9 krad/h. As reported on Mar 25, 2003, 25 PWO crystals belong to three types. While type II crystals are good for the barrel dose rates (15 rad/h), type I crystals are more radiation hard and good enough for the endcaps (500 rad/h). One type III crystal has problem for monitoring with 440 nm light, so should be rejected. All samples recover after irradiation. Two issues discussed during last ECAL week: (1) stability of PWO crystal after 200 o C thermal annealing, and (2) long term recovery time constant. 7 samples (5 type I and 2 type II) went through long term (160 days) recovery test. 15 crystals returned to CERN.
Thermal Annealing Rigorous temperature control both in amplitude and slope: From RT to 200 o C: 200 minutes; Maintain at 200 o C: 240 minutes; From 200 o C to 25 o C: 400 minutes. Crystals are kept in dark at RT (18 o C) after annealing. The minimum time between annealing and the 1st measurement is 48 hours. Question: Does the length of waiting time between the measurement and the end of annealing affect measurement result?
Summary of Light Output Type I: 5 20% loss @15 rad/h Type II: 10 25% loss @15 rad/h Waiting time: at least 48h
LO loss versus Waiting Time No Correlation between the LO loss and the time between the ending of the thermal annealing and the start of the initial 1 st measurement.
PWO Stable after Annealing No variations in either light output or longitudinal transmittance was observed 8 hours after the thermal annealing procedure.
Longitudinal Transmittance Recovery Short Term (two weeks) Recovery ECAL monitoring takes 30 minutes to cover entire calorimeter in situ. The maximum recovery speed and time constant are measures of the monitor-ability of the crystal.
Summary: Short Term LT Recovery All samples recover!!! RT at Caltech
Long Term Recovery under 18 o C Two type II samples, B2375, and B2376, and five type I samples went through a long term recovery test for 160 days. Since February, they are stored in a cooler, which keeps temperature at 18 o C with 0.1 o C precision.
LT Recovery: Type I Small Damage and 2 Time Constants: ~40 h and ~1,950 h
LT Recovery: Type II Large Damage and 2 Time Constants: ~30 h and ~1,300 h
Emission Weighted RIAC (I)
Emission Weighted RIAC (II) 25 Crystals: 3 Types Type I: 2456, 2466 and 2467 and all 5 barrel samples: P7467, P7557, P7566, P7654, P7903. Type II: All other endcap samples, except that in I and III; Type III: 2465.
EWRIAC Recovery: Type I Small Damage and 2 Time Constants: ~40 h and ~1,950 h
EWRIAC Recovery: Type II Large Damage and 2 Time Constants: ~30 h and ~1,300 h
Summary of Long Term Recovery Recovery of longitudinal transmittance at 440 nm and EWRIAC are consistent. A residual component with longer recovery time constant is at 20 to 30% level for type I and II crystals respectively
Summary No variations in either light output or longitudinal transmittance for measurements 8 hours after the 200 o C thermal annealing. PWO recovery measured under 18 o C in 160 days can be described by two time constants: few tens hours and few thousands hours. Type I crystal has longer recovery time constant. A residual component at 20/30% level may have even longer recovery time constant.