Radiation Physics and Thermoluminescence

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					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.

				
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