Peter Hobson - Brunel University, UK3 July 2003 VPT faceplate testing  Samples of faceplate glass are irradiated to approximately 20 kGy gamma ray dose in our facility at Brunel.  At least 3 faceplate samples per glass batch are tested.  Samples are irradiated in the dark, at a temperature around 20°C, at a dose rate of abour 300 Gy per hour.  Induced absorption is measured within two hours of irradiation.  Irradiated samples are stored in the dark at 20±1°C.  Spectrophotometer is recalibrated every 12 months.

Peter Hobson - Brunel University, UK3 July 2003 VPT faceplate testing  We use the latest measured PbWO 4 emission spectrum appropriate for end-cap crystals  This is convolved with the induced absorption over the wavelength range 350 nm to 610 nm  The integrated transmission after irradiation must be > 90% of the unirradiated transmission for all sample in one batch.

Peter Hobson - Brunel University, UK3 July 2003 VPT faceplate testing  Irradiated causes the growth of an absorption band centred around 330 nm. This band is well known in the literature for borosilicate glasses.  Even after 20 kGy dose there is no induced absorption at 800 nm. This allows a measurement consistency check.  The induced absorption shape is very similar in different batches, peak height does however vary.

Peter Hobson - Brunel University, UK3 July 2003 VPT faceplate testing  Recent faceplates (June 2003) were generally very good quality, but note batch Glass Post irradiation internal transmission (%) after 20.0 kGy

Peter Hobson - Brunel University, UK3 July 2003 VPT faceplate testing  One sample from batch showed very poor radiation tolerance.

Peter Hobson - Brunel University, UK3 July 2003 VPT faceplate testing  Long term annealing (in dark at 20°C) shows increase in absorption in the band centres at 330 nm.

Peter Hobson - Brunel University, UK3 July 2003 Summary  Pre-testing of faceplate glass is an essential part of ensuring that production tubes will be able to operate over the full lifetime of CMS.  We have a standard protocol for determining which glass batches can be used.  A new Hitachi U4100 spectrophotometer, used for all glass batches in the production run, has improved our measurement accuracy repeatability.  Recent glass batches have been very good.  The anomalous result for one sample of a batch (29992) is unique – all other batches have shown very little variation between samples.