DEPARTMENT OF RADIATION
PHYSICS AND DOSIMETRY - NZ63
Head of the Department: Prof. Paweł Bilski
English   Język polski

Luminescent detectors of ionizing radiation based on silicate crystals


Research project NCN PRELUDIUM 2012/05/N/ST8/03334

realized in years 2013-2015

Research team:
Anna Kilian (Twardak) - project manager
Paweł Bilski - research supervisor

The aim of the project was to investigate the optically and thermally stimulated luminescence properties of silicate crystals produced by different methods.

Widespread occurrence and use of ionizing radiation entails the evaluation of radiation exposure of both employees professionally exposed to radiation and the general population, as well as environmental monitoring. In practice, for this purpose usually passive detectors are used. Passive detectors are instruments which do not require external power supply. The vast majority of passive detectors are based on luminescent phenomena such as thermoluminescence (TL), optically stimulated luminescence (OSL) and radiophotoluminescence (RPL). No need for external power supply can both reduce the size of the detectors and allows much longer measurements. Furthermore, using passive detectors allows storing information about the accumulated radiation dose for a long time. A multitude of applications of this type of detectors generates an increase in demand for new detectors with different properties based on a variety of materials.


The purpose of this project was to investigate optically stimulated luminescence and thermoluminescent properties of doped and undoped oxy orthosilicates crystals such as lutetium orthosilicate (LSO), yttrium (YSO), gadolinium (GSO) and others, with a view to optimize their properties in order to use them as ionizing radiation dosimeters. The main motivation of this study was that the preliminary investigations of luminescent properties of these materials indicated that some of them exhibit significant thermoluminescent signal. The examined crystals have been produced using various crystal growth methods such as: the Czochralski method, liquid phase epitaxy (LPE), and a relatively new Micro-Pulling-Down method (MPD).


Within the project, glow curves of thermally and decay curves of optically stimulated luminescence of doped and undoped orthosilicates were collected. Systematic studies of received data allowed determining, i.a. the influence of dopants on the luminescent signal. The significant differences in the thermoluminescent properties of crystals produced by the Czochralski and LPE methods were found. Undoped LSO and YSO crystals produced by the Czochralski method have a high concentration of oxygen vacancies, as compared to single crystalline films produced by LPE method (result of lower crystallization temperature of the latter), resulting in the differences in the glow curves (change of the position and intensity of peaks). In the case of single crystalline films doped with Ce3+ ions significant increase in signal intensities can be explained by the formation of centers of Ce4+-Pb2+ (the presence of Pb2+ ion is the result of flux PbO-B2O3 used in LPE). The dependence of the luminescent properties of crystalline films growth using LPE method with different fluxes (PbO-B2O3 and Bi2O3) was examined. It was found that the observed differences in the glow curves result from the different dominant luminescence centres: Ce3+-Bi3+ for LSO: Ce and YSO: Ce grown on Bi2O3 flux and Ce4+-Pb2+ for films based on PbO-B2O3 flux.


From all of the tested samples, a LSO: Ce crystal characterized by the highest sensitivity OSL signal was selected for more detailed study of the dosimetric properties. The sample showed good reproducibility and the linear dependence of the detector response in the whole investigated dose range (100 Gy to 1 Gy). These are some of the most important parameters which determine the suitability as a detector of ionizing radiation. Furthermore signal stability depending on the temperature and the storage time of the detector was examined.


An important part of this work was to produce oxy silicate crystals using Micro-Pulling-Down method. Several crystal growth processes were conducted and undoped YSO crystals were achieved. The optimal process conditions such as temperature and pulling rate were determined.


Publications:

A. Twardak, P. Bilski, Y. Zorenko, V. Gorbenko, O. Sidletskiy, OSL dosimetric properties of cerium doped lutetium orthosilicates, Radiat. Meas., 71 (2014) 139-142;

A. Twardak, P. Bilski, Y. Zorenko, T. Zorenko, V. Gorbenko, E. Mandowska, A. Mandowski, O. Sidletskiy, J. Mares, Thermoluminescent properties of undoped and ce-doped lutetium orthosilicate and yttrium orthosilicate single crystals and single crystalline films scintillators, IEEE T. Nucl. Sci., 61 (2014) 276-281;

A. Twardak, P. Bilski, Y. Zorenko, V. Gorbenko, A. Mandowski, E. Mandowska, O. Sidletskiy, Comparative study of TL and OSL properties of LSO and LSO:Ce single crystals and single crystalline films, Radiat. Meas., 56 (2013) 196-199;