Radiation Physics and Thermoluminescence Yigal Horowitz, Professor of Physics Ben Gurion University of the Negev Beersheva, Israel, 84105 Academic Education: 1965: M.Sc. in Nuclear Physics, McGill University, 1968: Ph.D. in Nuclear Physics, McGill University Research Areas: Thermoluminesence and Thermoluminescent Dosimetry: Supralinearity and efficiency of TL materials as a function of ionisation density, the Unified Interaction Model for dose response in TL materials, Heavy Charged Particle response, kinetics of thermoluminesnce, defect studies via optical absorption, optical bleaching and spectral emission, development of advanced TL materials. Application of computerised glow curve deconvolution to TL mechanisms and radiation dosimetry, space dosimetry, mixed-field dosimetry, solid-state nanodosimetry. Radiation Detector Physics and Applications: Photon general cavity theory, application of Monte Carlo calculations to detector response, development of advanced Q-nanodosemeters, development of advanced solid state beta ray spectrometers. Research Group Dr. Leonid Oster, Senior Lecturer, Negev Academic College Shlomo Biderman, Ph.D. Student Nail Issa, Ph.D. Student General Information The scientific study of thermoluminescence has an extremely rich history spanning many centuries and interacting with many other fields of endeavor: archaeology, geology, medicine, solid-state physics, biology and organic chemistry, to name just some of the mainstream areas of study. At Ben Gurion University we are interested in thermoluminescence application to ionising radiation dosimetry as well as in the development of radiation response theories for solid state systems. These areas of endeavor are of major importance in fundamental questions of radiation ecology, which in turn can have great impact on questions concerning the future of nuclear energy and the effects of interaction of radiation, both beneficial and harmful, with the human organism. Our efforts at BGU, spanning three decades, have culminated recently in a major effort initiated at BGU to develop a Q- nanodosimeter; a solid-state dosimeter, of miniscule dimensions, capable of at least partially mimicking the double-strand response of DNA to ionising radiation. Graduate Students and Research Associates: Research in these areas of interest was initiated by Prof. Horowitz in the mid -1970s and has continued vigorously and unabated to the present time. In these twenty five years, twenty three research students have studied for graduate degrees in the Radiation Physics Laboratory; nine graduate students were awarded a PhD degree and two PhD students are currently engaged in research for their degrees. Three of the graduated PhD students went on to highly successful academic careers and are currently tenured Professors of Nuclear Engineering at Ben Gurion University (Dubi), and Medical Physics (Moscovitch - Georgetown University Washington D.C.) and the University of Ionnina (Kalef-Ezra-Greece); three are employed in research and development in the Israeli Defense Industries (Elta and IRR-1) and two as high school and college teachers. One of the recently graduated PhD students (Y. Weizman) was recently awarded the Intel Prize for excellence in his PhD research, the only student in 1999, from the Faculty of Natural Sciences, so honoured. Current International Collaborations are with: (i) Professor M. Zaider on the development of a Q-dosemeter based on ionisation density thermoluminescence (TL) phenomena in LiF;Mg,Ti, (ii) Professor M.E. Brandan - UNAM- Mexico on the study of heavy charged particle efficiency in LiF:Mg,Ti using the UNAM Pelletron accelerator and (iii) Dr. A. Semones, Houston, NASA – on the study of the HCP response of the peak 5a nanodosimeter for space applications Research Funding:The various research projects and the Radiation Physics Laboratory have been supported over the years by four contracts with the International Atomic Energy Agency (60 K USD); three contracts with the U.S.- Israel Binational Science Foundation (370 K USD); one contract with the Israeli Cancer Association (15 K USD), four contracts with the Canadian Owned Group of Nuclear Reactors- CANDU (475 K): The Rashi Foundation (L. Oster) and various contracts in support of the UNAM collaboration: total worth of awarded contracts approximately 1 M USD. Description of Research Acitvity. I. The Unified Interaction Model: We have developed a theory of dose response, the Unified Interaction Model (UNIM) which is capable of explaining all the important features of the supralinearity and sensitisation of the various glow peaks in LiF:Mg,Ti (TLD-100) and other TL materials (1-4). The model combines the physical concepts of the Defect Interaction Model (DIM) for gamma rays (uniformly ionising radiation) first proposed by Fain and Monnin and elaborated by McKeever, with features of the Track Interaction Model (TIM) (developed at BGU by Horowitz and collaborators) for densely ionising heavy charged particles (HCPs), into a unified mathematical framework. The UNIM is the only radiation effects model of solid state systems capable of explaining the ionisation density dependence of peak 5 supralinearity (both gamma ray energy and radiation type). The UNIM incorporates a localised trapping entity (the track for HCPs, the spatially correlated TC/LC pairs for gamma rays and electrons) which dominates the dose response at low dose. The spatial features of the occupation density of the trapping centers and luminescent centers sets the scene for the relative efficiencies of the competitive mechanisms and leads to the linear /supralinear behaviour and to the dependence of the supralinearity on ionisation density. The ability of the UNIM to describe the supralinearity (TL efficiency) as a function of dose (ionisation density) for both gamma rays and HCPs in a unified mathematical and physical framework is a singular and unique achievement among the many previous models proposed to explain TL supralinearity. 1. S. Mahajna and Y.S. Horowitz "The Unified Interaction Model applied to the gamma induced supralinearity and sensitisation of peak 5 in LiF:Mg,Ti (TLD-100) ", J. Phys. D. Appl. Phys., 30, 2603-2619 (1997). 2. Y.S. Horowitz et al, Invited Paper, "The Unified Interaction Model applied to the gamma induced supralinearity and sensitisation of peak 5 in LiF:Mg,Ti (TLD-100", Radiat. Prot. Dosim., 78, 169-193 (1998). 3. Y.S. Horowitz: "Theory of thermoluminescence gamma dose response: The unified interaction model", Nucl. Instrum. B.,184, 68-84 (2001). II. The Track Interaction Model (TIM) and Modified Track Structure Theory The study of the properties of thermoluminescent (TL) materials to HCPs has been of particular interest in recent years due to their possible applications to dose measurements during radiation therapy treatments with heavy ions. The microscopic processes that lead to the emission of light are highly complex due to the highly localised ionisation density in the HCP track. During the last twenty years Horowitz and collabrators have performed both theoretical and experimental studies of the HCP response of LiF:Mg,Ti (TLD-100). The interpretation of the experimental results has led to a continuing development of models based on the structure of the HCP track to understand relative efficiencies (4) and the interaction between spatially correlated entities along the HCP track to explain supralinearity (5-7). More recently the BGU and UNAM groups have studied the TL response of LiF:Mg,Ti to 3 and 7.5 MeV helium ions and have interpreted the results in terms of a combined theory using both a modified Monte Carlo Track Interaction Model (MCTIM) and modified track structure theory (MTSM) (8-9). These experiments and their succesful interpretation has led to a far deeper understanding of the HCP relative TL efficiencies as a function of ionisation density, energy and particle type. 4. Kalef-Ezra, J. and Horowitz, Y.S., "Heavy Charged Particle thermoluminescence dosimetry:Track structure theory and experiments", Int. J. Appl. Radiat. Isot.,33, 1085-1100 (1982). 5. Horowitz, Y.S., Moscovitch, M. and Dubi, A., "Response curves for the thermoluminescence induced by alpha particles using track structure theory", Phys. Med. Biol., 27, 1325-1338 (1982). 6. Moscovitch, Y.S. and Horowitz, Y.S. "Microdosimetric track interaction model applied to alpha particle induced supralinearity and linearity in LiF:Mg,T", J. Phys. D. Appl. Phys., 21, 804-814 (1988). 7. Horowitz, Y.S., et al., "The Track Interaction Model for alpha particle induced supralinearity:Dependence of the supralinearity on the vector properties of the alpha particle radiation field", J. Phys. D. Appl. Phys., 29, 205-217 (1996). 8. Y.S. Horowitz et al and M.E. Brandan et al UNAM), "The Extended Track Interaction Model: supralinearity and saturation He-ion TL fluence response in sensitised TLD-100", Radiat. Meas., 33, 459-473 (2001). 9. Y.S. Horowitz, O. Avila and M. Rodriguez-Villafuerte, "Theory of heavy charged particle response (efficiency and supralinearity in TL materials", Nucl. Instrum. Meths., B184, 85-112 (2001). III. Development and Characterisation of Advanced TL Materials In collaboration with the NRC-Negev and with funding from the IAEA and the U. S.-Israel BSF, the BGU group was instrumental in the development of super- sensitive LiF:Mg,Cu, P TL materials with superior dosimetric properties. These included a material 30-50 times more sensitive than LiF:Mg,Ti as well as an additional material 15-20 times more sensitive than LiF:Mg,Ti with a negligible residual signal following conventional readout (10). The BGU group was the first to characterise the exceptionally low neutron sensitivity of LiF: Mg,Cu, P (11) as well as the first to systematically study the properties of the material at maximum glow curve heating temperatures between 240oC and 280oC (12,13) These studies were instrumental in the acceptance of LiF:Mg,Cu,P in environmental and personnel applications. 10. Y.S. Horowitz, Invited paper, "LiF:Mg,Ti versus LiF:Mg,Cu,P:The competition heats up", Radiat. Prot. Dosim., 47, 135-141 (1993). 11. Y.S. Horowitz and B. Ben Shachar, "Thermoluminescent LiF:Mg, Cu, P for gamma ray dosimetry in mixed fast neutron-gamma radiation fields", Radiat. Prot. Dosim., 23,. 401-404 (1988). 12. L. Oster, Y.S. Horowitz et al., "Further studies of the stability of LiF:Mg, Cu,P (GR-200) at maximum readout temperatures between 240oC and 280oC", Radiat. Prot. Dosim., 65, 159-162 (1996). 13. G. Ben-Amar, Y.S. Horowitz et al "Investigation of the glow peak parameters, reusability and dosmetric precision of LiF:Mg,Cu,P at high heating rates up to 20 K s-1", Radiat. Prot. Dosim., 84, 235-238 (1999). IV. Computerised Glow Curve Deconvolution (CGCD): Applications to TLD The BGU group has pioneered the implementation and development of advanced CGCD routines and their application to dosimetric problems (14). From the early 1980s we have improved the "state-of-the-art" using computerised analysis techniques: In increased precision and lowered minimum measurable dose(15, 16); In high dose dosimetry (17);In the optimsiation of anealing procedures (18, 19); In fading properties (20); In kinetic analysis (21,) and in the retrieval of dosimetric information (22). 14. Y.S. Horowitz and D. Yossian, "Computerised Glow Curve deconvolution: Application to thermoluminescence Dosimetry", Monograph, Radiat. Prot. Dosim., 60, 1-115 (1995). 15. M.Moscovitch, Y.S. Horowitz et al "LiF thermoluminescence dosimetry via computerised first order kinetics glow curve analysis", Radiat. Prot. Dosim., 6, 1-4 (1984). 16. Y.S. Horowitz and M. Moscovitch "Computerised glow curve deconvolution applied to ultra low dose LiF thermoluminescence dosimetry", Nucl. Instrum. & Meths. A244, 556-564 (1986). 17. Y.S. Horowitz and M. Moscovitch, "Computerised glow curve deconvolution applied to high dose (103 - 105) TL dosimetry", Nucl. Instrum. Meths., A243, 207-214 (1986). 18. Y.S. Horowitz, Invited Paper, "The annealng characteristics of LiF:Mg,Ti", Radiat. Prot. Dosim., 30, 219-230 (1990). 19. B. Ben Shachar and Y.S. Horowitz, "Thermoluminescence in annealed and unannealed LiF:Mg,Ti (TLD-100, Harshaw) as a function of glow curve heating rate and using computerised glow curve deconvolution", J. Phys. D. Appl. Phys., 25, 694-703 (1992). 20. Y.S. Horowitz et al "Study of the long-term stability of peaks 4 and 5 in TLD-100: correlation with isothermal decay measurements at elevated temperatures", J. Phys. D. Appl. Phys., 26, 1475-1481 (1993). 21. D. Yossian and Y.S. Horowitz, "Computerised glow curve deconvolution applied to the analysis of the kinetics of peak 5 in LiF:Mg,Ti (TLD- 100)", J. Phys. D. Appl. Phys., 28, 1495-1508 (1995). 22. D. Yossian and Y.S. Horowitz, "Retrieval of dosimetric information from distorted glow curves using computerised glow curve deconvolution", Radiat. Prot. Dosim., 66, 75-78 (1996). V. The LiF:Mg,Ti System We are carrying out a multi-pronged investigation of the LiF:Mg,Ti system using a variety of experimental techniques including optical absorption, spectral emission analysis using an advanced CCD spectrphotometer, HCP studies, glow curve kinetic analysis as a function of dopant concentration as well as optical bleaching. The studies are aimed at the investigation of the nature of the spatially correlated TC/LC pair (23) responsible for the unique/complex behaviour of the TL efficiency of LiF:Mg,Ti as a function of ionisation density. We have recently established, using Tm-Tstop techniques, that peak 5 is a composite of three peaks (peaks 5a,5 and 5b)(24). The discovery of this "fine-structure" has led to an on-going revolution in our understanding of the LiF:Mg,Ti system. Optical bleaching at 310 nm has revealed that the conversion efficiency of peak 5a to peak 4 is unusually high at 30%, and that of peak 5 is much lower, of the order of a few per-cent. We have proposed that the high conversion efficiency of peak 5a is due to the doubly trapped e-h characteristics of the TC/LC complex giving rise to peak 5a and the hole-only trapping characteristics of peak 4. Ionisation of an electron from the e-h occupied complex leaves behind the hole-only occupied complex, which gives rise to peak 4 (25). The next stage in our research has established the geminate nature of the recombination process of peak 5a-this in order to develop a Q- nanodosemeter based on the two-hit trapping characteristics of the TC/LC structure giving rise to peak 5a. (26). On-going research is aimed at the characterisation of the peak 5a nanodosimeter in order to establish its use in space nanodosimetry and clinical applications (27). 23. Y.Weizman, Y.S. Horowitz et al "Mixed-order kinetic analysis of the glow curve characteristics of single crystal LiF:Mg,Ti as a function of Ti concentration", Radiat. Meas., 29, 517-525 (1998). 24. Y.S. Horowitz et al "Ionisation density effects in the thermoluminescence of TLD-100:Computerised Tm-Tstop glow curve analysis", Radiat. Prot. Dosim., 84, 239-242 (1999). 25. Y. Weizman, Y.S. Horowitz and L. Oster "Investigation of the composite structure of peak 5 in the thermoluminescent glow curve of LiF:Mg,Ti (TLD-100) using optical bleaching", J. Phys. D. Appl. Phys., 32, 2118-2127 (1999). 26. Y.S. Horowitz, L. Oster, D. Satinger, S. Biderman and Y. Einav, "The composite structure of peak 5 in the glow curve of LiF:Mg,Ti (TLD- 100): Confirmation of peak 5a arising from a locally trapped electron-hole configuration", Radiat. Prot. Dosimetry, (2002) in press 27. Y.S. Horowitz, "Thermoluminescence radiation dosimetry in space:A critique of current practise and future perspectives" Abstract (D12, p.59), 2nd Int. Workshop on Space Radiation Research (IWSSRR-2) 2002, Nara, Japan.