Abstract
The well-established technology of electromagnetic calorimetry using Lead Tungstate crystals has recently seen an upheaval, with the closure of one of the most experienced large-scale suppliers of such crystals, the Bogoroditsk Technical Chemical Plant (BTCP), which was instrumental in the development of mass production procedures for PWO-II, the current benchmark for this scintillator. Obtaining alternative supplies of Lead Tungstate crystals matching the demanding specifications of contemporary calorimeter devices now presents a significant challenge to detector research and development programmes.In this paper we describe a programme of assessment carried out for the selection, based upon the performance under irradiation, of Lead Tungstate crystals for use in the Forward Tagger device, part of the CLAS12 detector in Hall B at Jefferson Lab. The crystals tested were acquired from SICCAS, the Shanghai Institute of Ceramics, Chinese Academy of Sciences. The tests performed are intended to maximise the performance of the detector within the practicalities of the crystal manufacturing process.Results of light transmission, before and after gamma ray irradiation, are presented and used to calculate dk, the induced radiation absorption coefficient, at 420nm, the peak of the Lead Tungstate emission spectrum. Results for the SICCAS crystals are compared with identical measurements carried out on Bogoroditsk samples, which were acquired for the Forward Tagger development program before the closure of the facility.Also presented are a series of tests performed to determine the feasibility of recovering radiation damage to the crystals using illumination from an LED, with such illumination available in the Forward Tagger from a light monitoring system integral to the detector.
Highlights
Results of light transmission, before and after gamma ray irradiation, are presented and used to calculate dk, the induced radiation absorption coefficient, at 420 nm, the peak of the Lead Tungstate emission spectrum
This paper describes the programme of evaluation undertaken to assess the SICCAS crystals acquired for the Forward Tagger Calorimeter (FT-Cal) in terms of the light transmission and radiation hardness requirements of operations within CLAS12
No significant variation in the light yield measurements was seen in comparison to the Automatic Crystal quality Control System (ACCOS) studies, or the quoted values from the manufacturer, all crystals tested in this work for radiation hardness are considered to have met the light yield requirements of the FT-Cal
Summary
Including the Forward Tagger Calorimeter (FT-Cal), a subsystem of the Forward Tagger device, part of the CLAS12 facility being constructed in Hall B at Jefferson Lab [5]. For this reason, a further programme of quality control is required to assess whether their characteristics are within acceptable values. The third part is the assessment of the ability to recover radiation damage to the crystals by means of optical annealing using visible light illumination from an LED, exploiting the availability of such illumination from the light monitoring system of the FT-Cal. A total of 370 crystals, including spares, were initially acquired from SICCAS, and for each crystal the induced absorption coefficient was measured. The BTCP examples chosen represent a typical range of properties and response to irradiation of this type of crystal
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