Mikhail Korjik

Stimulation of Radiation Damage Recovery of Lead Tungstate Scintillation Crystals Operating in a High Dose-Rate Radiation Environment

Scintillation crystals of the lead tungstate family - PWO, PWO-II - became widely used in electromagnetic calorimeters in high energy physics experiments at high-luminosity accelerator facilities. During the operation of electromagnetic calorimeters a degradation of the optical transmission of these crystals occurs due to creation of color centers. In addition to the recharge by γ-radiation of the point structure defects, which exist a priori in the crystals, additional damage occurs to the crystal matrix due to the interaction of hadrons. Thus radiation induced optical absorption can limit the energy resolution of the calorimeter. To reduce the recharge by γ-radiation we have both minimized the concentration of point structure defects during manufacture, and used light from visible to infrared to stimulate the recovery of the color centers. In this paper we show that method of stimulated recovery is also applicable to recover from degradation of the crystals optical transmission caused by hadron interactions. The mechanisms of the damage under γ- and hadron-irradiation are discussed.

Comparison of Radiation Damage Effects in PWO Crystals Under 150 MeV and 24 GeV High Fluence Proton Irradiation

Radiation damage effects induced by the hadronic part of ionizing radiation in experiments at high-energy and high-luminosity accelerators will play a significant role as limiting factor of the long term stability when operating an experimental setup. Measurements of the deterioration of the optical transmission of lead tungstate (PbWO 4, PWO) scintillation crystals were performed after irradiation with high and low energy protons. One sample with CMS specification was irradiated in 2010 with a 24 GeV/c proton beam at the CERN Proton Synchrotron (PS) with a flux of about 109 p/(cm2s) up to an accumulated fluence of 3.0 103 p/cm2. Eight more crystals of PWO-II quality with dimensions of 2×2×5 cm3 were selected from the set of crystals produced for the PANDA electromagnetic calorimeter at the future FAIR facility at Darmstadt. Four crystals were produced by the Bogoroditsk Technical Chemical Plant (Russia) and the others by the Shanghai Institute of Ceramics (China). These samples were irradiated in 2012 with a 150 MeV proton beam at the AGOR accelerator at KVI (Groningen, The Netherlands) with a flux of 3 109 up to an integral fluence of about 1.0 1012 p/cm2 and 1.8 1013 p/cm2, respectively. Due to the proton induced activation of long-lived radioactive secondaries the optical inspection of all samples had to be performed several months after irradiation for safety reasons. However, the samples were kept continuously cold to minimize thermal recovery. Both irradiations produced a similar set of induced absorption bands. Moreover, a shift of the fundamental absorption edge at short wavelength appears even after irradiation with low energy protons. The contribution will show in detail the set of experimental data and discuss a possible mechanism for an understanding and interpretation of the observed effects.

Radiation damage and recovery of medium heavy and light inorganic crystalline, glass and glass ceramic materials after irradiation with 150 MeV protons and 1.2 MeV gamma-rays

Detector systems based on inorganic heavy scintillator materials are limited in their performance due to severe radiation damage caused by hadrons. The present paper investigates a series of medium heavy crystalline and glass ceramic samples using 150 MeV protons for irradiation studies. New materials, such as DSB:Ce show very promising features.

Radiation Damage of Oxy-Orthosilicate Scintillation Crystals Under Gamma and High Energy Proton Irradiation

Radiation damage effects in lutetium, yttrium and gadolinium oxy-orthosilicates, Lu₂SiO₅:Ce, Y₂SiO₅:Ce, Gd₂SiO₅:Ce under γ- and protons irradiation have been investigated. Crystals were irradiated with a ⁶⁰Co gamma source to a total dose of 2000 Gy and a high-rate 24-GeV proton beam at CERN PS, to a fluence of ~3.6 ×10¹³ p/cm². Color centers created under irradiation, phosphorescence and its mechanism in the crystals are discussed.

Novel Glass Ceramic Scintillator for Detection of Slow Neutrons in Well Logging Applications

One of the neutron detection techniques is based on the scintillation glasses enriched with the 6Li isotope. A limitation to increase the light yield (LY) in glass scintillators is imposed by the absence of long-range order in their atomic structure. As a result, the mean free path of excitons in glasses is very small and the delivery of electronic excitations to luminescent centers through the exciton migration has very low efficiency. The only way toward creating effective scintillation in glasses is to use direct excitation of the luminescent centers, which are dopants in the form of Ce3+ ions. However, the synthesis of the glasses heavily doped with cerium possesses significant technological challenge due to heterovalent properties of cerium. These limitations can be overcome by the impregnation of the glass matrix with scintillator nanocrystals with average size less than the emitted light wavelength. The controlled recrystallization of the glass enables control of the nanocrystal dimension and their distribution in the glass matrix. Details of the method are described in US patent application 13/242 839. By using this approach we have synthesized glass ceramic scintillation material obtained through partial crystallization of Ce-doped lithium-silica glass with the formation of the petalite nanocrystals in the glass matrix. Samples containing 16.5 at.% of lithium demonstrate bright radio-luminescence with maximum at 410 nm, fast scintillation with an average decay time constant of 70 ns, and LY up to 240% of LY of GS20 scintillation glass. LY of the obtained material is almost constant between 20° C and 70 °C and depends weakly on the temperature decreasing by ~30% with the temperature increase from 70 °C to 170 °C. Scintillation detectors based on this material can replace He3 neutron counters in many applications, including wireline and logging-while-drilling (LWD) “sourceless” neutron porosity measurement that requires the neutron detectors capable to operate at temperatures above 150 °C.

On the development on scintillation materials operating at high temperature

High light yield and its stability at high temperature are required conditions to exploit scintillation material for a well logging applications. High light yield causes sensitivity and expressivity of the measurements and its stability at high temperature gives an opportunity to simplify design of well logging tools. A combination of high light yield and its stability at high temperature in the material occurs not too often and depends on the favourable concurrence of the properties of the matrix host and luminescent centre. Among all scintillation materials available the most promising are those which scintillation properties aie due to Ce3+ and Pr3+ activating ions. They emit fast and high light yield scintillation due to interconfiguration luminescent transitions 5d → f(Ce3+) 4f5d → f2 (Pr3+). In this article we discuss factors which affect temperature dependence of light yield of Pr and Ce doped scintillation materials. The influence of the processes of nonradiative quenching and thermoionisation on the temperature dependence of scintillation yield are analysed. It is concluded that Pr3+ activator has higher potential to construct scintillators capable to operate at high temperature.

New Start of Lead Tungstate Crystal Production for High-Energy Physics Experiments

Presently, there is a demand to apply high-quality lead tungstate (PbWO4, PWO) scintillation material for electromagnetic calorimetry (EMC). Unfortunately, the mass production of lead tungstate using the Czochralski method was stopped after shut down of Bogoroditsk Technological Chemical Plant (Bogoroditsk, Russia). CRYTUR (Turnov, Czech Republic) having long time experience and the necessary technology in the development and mass production of oxide crystals expressed their interest in meeting PWO requirements for the high-energy physics community. Last year the development of lead tungstate crystals was started by CRYTUR. Several series of samples were produced under different technical conditions. All test crystals are being characterized with respect to light yield, scintillation kinetics, photoluminescence, optical transmittance and radiation hardness studied by γ-ray irradiation in laboratories at Giessen and Minsk. The obtained results confirmed that the technological approach of CRYTUR will allow to produce PWO crystals with properties very close to the PWO-II specifications of the PANDA experiment at FAIR (Darmstadt, Germany).

Comparison of damage effects in plastic scintillators due to irradiation with γ-rays, 190 MeV and 24 GeV protons

Plastic scintillation materials play a crucial role in the construction of large area detectors in high energy physics experiments. Future detector concepts for HEP experiments, particularly at collider facilities, will require an unique combination of the material features and of affordable price. Mandatory is a minimal level of radiation damage due to the electromagnetic part of ionizing radiation and energetic hadrons as well: tolerable deterioration of the optical transmission, negligible impact on the scintillation mechanism and small contribution of radio-luminescence due to secondary reaction products in the detector material. A systematic study of the radiation hardness of inorganic optical and scintillation materials has been performed by us since several years. It resulted in the understanding of the damage effects in particular in self activated, Ce3+-doped and cross-luminescent crystalline materials. Here we report on first results for the industrially produced plastic scintillation material EJ260 (ELJEN). This plastic is a bright, fast and green light emitting scintillator with properties similar to other materials such as BC428 and NE103, respectively. Samples of a thickness of 1.25 cm were tested before and after irradiation with γ-quanta (1.2 MeV, Giessen) and protons of 190 MeV (KVI, Groningen) and 24 GeV (CERN, PS), respectively. No damage of the material properties was observed after irradiation with γ-quanta up to an absorbed dose of 1 kGy consistent with similar studies. However, the exposure to 190 MeV protons with a fluence of 5×1013p/cm2 showed a large reduction of the optical transmission and a slowing down of the scintillation kinetics. In this study we also focused on the impact of high energetic protons. We obtained evidence that light fragments of the (p,12C) reactions play a significant role in the damage of the matrix and the luminescent organic dye centers.

New lead tungstate crystal production for high-energy physics experiments based on the Czochralski technique

In spite of the demand to consider high-quality lead tungstate scintillation crystals for electromagnetic calorimetry (EMC) the successful mass production of lead tungstate using the Czochralski method had to be terminated due to the shutdown of the Bogoroditsk Technical Chemical Plant (Bogoroditsk, Russia). CRYTUR (Turnov, Czech Republic) having long time experience and the necessary technology in the development and mass production of oxide crystals has re-started as a new optional manufacturer the development of lead tungstate crystals based on available pre-mixed raw material using the Czochralski technique. After producing several samples under different technical conditions first full-size crystals have been produced and are characterized with respect to light yield, scintillation kinetics, photoluminescence, optical transmittance and radiation hardness studied by γ-irradiation in laboratories at Giessen and Minsk in comparison to the quality limits of PWO-II, the standard for the PANDA-EMC at FAIR. The most recent samples come very close to the required parameters.

Study of the glass and glass ceramic stoichiometric and Gd 3+ heavy loaded BaO∗2SiO 2 :Ce (DSB:Ce) scintillation materials for calorimetry application

Application of crystalline materials in ionizing radiation detectors has played a crucial role in the discovery of properties of matter. Future detector concepts at HEP experiments will require a tolerable level of radiation damage in particular caused by energetic hadrons: minor deterioration of the optical transmission, low level of afterglow and radio-luminescence. From systematic studies of the radiation hardness of inorganic optical and scintillation materials we concluded that both oxide and fluoride crystals composed of atoms with atomic numbers below 60 should be reasonably survivable. In this study we focuse on cheap glass (BaO*2SiO2) and (DSB: Ce) glass ceramics capable for a mass production. Loading this glass by admixing gadolinium oxide (Gd3+) provides a two times larger light yield. Both type of the materials can be produced in fiber and bulk geometry.