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Analysis of luminescence from common salt (NaCl) for application
to retrospective dosimetry
N.A. Spooner
a
,
b
,
*
, B.W. Smith
a
, O.M. Williams
a
, D.F. Creighton
b
, I. McCulloch
c
, P.G. Hunter
a
,
D.G. Questiaux
a
, J.R. Prescott
b
a
Defence Science and Technology Organisation, Edinburgh, South Australia 5111, Australia
b
Institute for Photonics and Advanced Sensing, School of Chemistry and Physics, University of Adelaide, Adelaide, South Australia 5005, Australia
c
Research School of Earth Sciences, Australian National University, ACT 0200, Australia
article info
Article history:
Received 9 November 2010
Received in revised form
27 June 2011
Accepted 28 June 2011
Keywords:
Salt
Emission spectra
Luminescence imaging
Retrospective dosimetry
TL kinetics
TL
OSL
IRSL
abstract
Thermoluminescence (TL), Optically-Stimulated Luminescence (OSL) and Infrared-Stimulated Lumines-
cence (IRSL) emitted from a set of 19 salt (NaCl) samples were studied for potential application to
retrospective dosimetry. TL emission spectra revealed intense TL emissions from most samples, centred
on 590 nm; UV and blue emissions were also found. Significant thermally-induced sensitivity changes
were observed and TL, OSL and IRSL growth curves were measured. Pulse anneal analysis was performed,
as was quantitative imaging of the TL, OSL and IRSL to assess sample heterogeneity. Kinetic analysis
found lifetimes at 20
C of the 200
C and 240
C TL peaks to be 0.6 ka and 4 ka respectively; sufficient for
application to retrospective dosimetry.
Ó2011 Elsevier Ltd. All rights reserved.
1. Introduction
Common salt (NaCl) is an alkali halide long known to be
a sensitive thermoluminescence (TL) phosphor; early work is
summarised by McKeever (1985) and subsequent reports including
those by Gartia et al. (2004) and Murthy et al. (2006) concluded
that NaCl has properties suitable for dosimetry. Similarly, the
optically-stimulated luminescence (OSL) from NaCl was reported
by Bailey et al. (200 0) to have good potential for application to
dating and dosimetry. Tanir and Bölükdemir (2007) reached similar
conclusions from a study of the dose response of infrared-
stimulated luminescence (IRSL).
Here we report an investigation into the potential use of
luminescence from NaCl for measurement of recent exposure to
ionising radiation on the timescale of weeks to years, for retro-
spective dosimetry and radiological event analysis. This work
included the detection and characterisation of TL glow peaks in
the 100e300
C range, the UV OSL emissions stimulated by
470 nm blue light, and blue-to-orange emissions stimulated by
880 nm IR illumination.
Characterisation commenced with measurement of emission
spectra, then analysis of the trap kinetic parameters, dose response
behaviour, signal-of-formation, pulse-annealing behaviour of OSL
and IRSL, and a brief inspection of grain-to-grain heterogeneity
using the Photon-Counting Imaging System (PCIS), McCulloch et al.
(2011).
2. Samples
A set of 19 samples was collected from around the world from
locations including Australia, Canada, Pakistan, Poland, the UK and
USA. A disparate range of origins of the NaCl crystals was sought,
with the majority being salts commercially sold for domestic use
and variously produced by evaporation from sea water, saline lake
water or saline river water, or crushing of rock salt. Samples of salt
damp crystals and specimen rock salt were also included. Details
will be presented elsewhere. In all cases, samples were prepared as
180 e250
m
m grains by grinding by pestle and mortar then sieving,
*Corresponding author. Institute for Photonics and Advanced Sensing, School of
Chemistry and Physics, University of Adelaide, Adelaide, South Australia 5005,
Australia. Tel.: þ61 8 8303 4852; fax: þ61 8 8303 4384.
E-mail address: nigel.spooner@dsto.defence.gov.au (N.A. Spooner).
Contents lists available at ScienceDirect
Radiation Measurements
journal homepage: www.elsevier.com/locate/radmeas
1350-4487/$ esee front matter Ó2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.radmeas.2011.06.069
Radiation Measurements 46 (2011) 1856e1861
and were attached as monolayers of grains to the central 5 mm
portion of stainless steel discs using silicone oil.
3. Sample characterisation - luminescence emission spectra
Characterisation commenced with measurement of the emis-
sion spectra of the natural TL (NTL) and artificially-dosed TL (ATL) of
each sample. The aims were to identify any TL signal-of-formation
that may be present, and to reveal any intense or ubiquitous TL
emissions with dosimetric potential, for further study. The
University of Adelaide Fourier-transform thermoluminescence
spectrometer (Prescott et al., 1988) was used for this work; spectra
were measured in a wavelength range of 250e720 nm and over
a temperature range of 50e400
C, with heating rate 5 K/s and
reheats subtracted.
NTL in salts such as studied here may originate either as a signal-
of-formation, as a photo-induced signal due to exposure to
daylight, or, in the case of rock salt, as a result of dose accrual during
burial. Significant NTL in the temperature range below 280
C
would constitute a problematic background for retrospective
dosimetry application; however, no significant NTL was observed
other than from rock salt, and there at temperatures sufficiently
high as to not interfere with the proposed application.
ATL was measured following Sr
90
/Y
90
beta irradiations ranging
from 2 to 10 Gy depending on sensitivity. Fig. 1(a) & (b) are two
examples chosen to illustrate the principal TL emission peaks
observed.
Several emission peaks were observed: a prominent and
frequently intense emission peak centred at 590 nm was observed
from most, but not all, samples, and many samples also exhibited
a UV (370 nm) peak, though typically of lower intensity than the
590 nm emission. The examples shown in Fig. 1 illustrate these
peaks, and a less intense broad blue band centred at w440 nm. The
intense emission at 590 nm has been attributed to Mn
2þ
sites
(López et al., 1977).
The complete set of NTL and ATL emission spectra will be pub-
lished elsewhere.
The near-ubiquitous TL emission centred on 590 nm (hereafter
called the “590 nm”emission) was notable for showing generally
high dose-sensitivity, and as the TL peaks emitting in this band
include those occurring in the 100e300
C temperature range of
interest, work then focussed on characterising the TL peaks asso-
ciated with this emission. Sample #3 was selected for detailed
Fig. 1. TL emission spectrum of two selected salt samples (a) Sample #10, Himalayan
rock salt, Pakistan; (b) Sample #3, Table salt (evaporated sea water), “Woolworth’s
Homebrand”, Australia.
Tem
p
erature (ºC)
0.1 K/s
5 K/s
0.02 K/s
0.01 K/s
0.002 K/s
2 K/s
0.2 K/s
0.5 K/s
1 K/s
0.05 K/s
Area-normalised TL (A.U.)
300
200100
0
0
0.05
0.1
Fig. 2. Area normalised TL glow curves for sample #3, measured at various heating
rates as shown, with reheats subtracted. The TL is dominated by the 590 nm emission.
Doses were 0.15 Gy.
Fig. 3. Glow curves measured following pre-heating of salt #3 before irradiation.
N.A. Spooner et al. / Radiation Measurements 46 (2011) 1856e1861 185 7
study, both because it exhibits strong 590 nm emission and because
it was produced from evaporated seawater and so is representative
of a common type of salt. Results are reported below.
4. Kinetic analysis
The method of variation of heating rates was used to measure
the electron trap depth E, frequency factor s, and thermal
quenching behaviour of the 590 nm TL. The apparatus was
a modified Alldred glow oven capable of heating rates of
10e0.001 K/s. TL detection was by an EMI 9635QA PMT with 3 mm
Schott GG 455 colour glass filter: this apparatus and the protocols
used are described in Spooner and Questiaux (2000).
The family of TL glow curves measured at various heating rates
are presented in Fig. 2, as area normalised data to assist visual-
isation of the TL peak behaviours given the presence of significant
glow curve shape changes during measurement.
The principal TL peak, at 240
C (5 K/s), was found to have
E¼1.45 eV and s ¼7.9 10
13
s
1
, giving a lifetime at 20
Cof
approximately 4 ka. The two other readily identifiable TL peaks, at
approximately 100
C and 200
C (5 K/s), were similarly analysed
and found to have lifetimes at 20
C of approximately 7 h and 0.6 ka
respectively.
Mass normalisation was also performed, and revealed no
evidence for thermal quenching (note that the apparent increase in
lightsum seen in Fig. 2 for the 100
C and 200
C (5 K/s) TL peaks is
an artefact of this form of data presentation).
The Isothermal Decay method was then applied, but significant
sensitivity changes occurred in the course of the measurements
which modified the TL decay curve shapes, invalidating the
assumption that only thermal erosion was occurring and hence
precluding conventional analysis. These data will be presented
elsewhere. These sensitivity changes are consistent with the
observed glow curve shape changes seen in Fig. 2, and lead to
a need for the sensitivity changes to be investigated.
a
b
Fig. 4. Mass-normalised first-glow 590 nm TL glow curves measured after beta doses;
(a) no bleaching, (b) the TL residual measured following preheating to 160 C (2 K/s)
then 2000 s exposure to 880 nm IR, using the Risø IR LED module.
Fig. 5. Shows the mass-normalised 590 nm TL integrated over the 180e280 C range:
triangles represent unbleached TL, squares the residual TL.
a
b
Fig. 6. (a): unbleached mass-normalised first-glow UV TL glow curves measured after
beta doses; (b) the bleached TL measured following preheating to 160 C (2 K/s) then
2000 s exposure to 470 nm light from the Risø LED module.
Fig. 7. TL growth curves for 180e280 C, for both unbleached and bleached (2000 s,
470 nm light).
N.A. Spooner et al. / Radiation Measurements 46 (2011) 1856e18611858
5. Thermally-induced sensitivity changes
The TL peak seen in Fig. 2 at 240
C (5 K/s) diminishes in
intensity as heating rate is reduced, to the extent that it is virtually
indistinguishable at the slowest rate, 0.002 K/s. An exploratory
annealing experiment (not shown here) confirmed that the sensi-
tivity changed as result of exposure to elevated temperature. The
590 nm TL sensitivity was therefore analysed in a follow-up
experiment designed to identify the effects of pre-heating the salt
before any radiation is applied. Steps were as follows:
1) w8 mg of salt #3 grains (180e250
m
m) loaded onto each of 48
stainless steel discs;
2) Pairs of discs were heated at 2 K/s to each of 20 different
temperatures from 30 to 400
C;
3) 0.13 Gy
90
Sr/
90
Y beta dose to each disc;
4) 590 nm TL emissions measured to 360
C at 2 K/s, with reheat,
using a Risø TL-DA-8, an EMI 9635QB PMTand 3 mm Schott OG
530 colour glass filter.
The data are presented in Fig. 3: each mass-normalised pair of TL
glow curves contributes a transect at its specific annealing
temperature.
A major decrease in sensitivity of the 240
C 590 nm TL peak
occurs as the annealing temperature approaches 200
C, consistent
with observations from the kinetics study. This coincides with an
increase in sensitivity of the 100
C TL peak, suggesting that
transfer or transformation of centres is occurring. However, as
readout of the most stable TL requires exceeding the apparently
critical 200
C temperature it was concluded that only first glow TL
data may be reliable for dosimetry. Hence the doseeresponse
behaviours of UV TL, IR-stimulated emissions, and OSL (470 nm
stimulation) UV emissions were next analysed, to assess their
suitability for dosimetry.
6. Luminescence doseeresponse
OSL, IRSL and TL doseeresponse curves for UV and blue-to-
orange emissions were constructed following beta irradiation
(
90
Sr/
90
Y). These experiments were performed using a Risø TL/OSL
DA-20 reader with an EMI 9235QB PMT; 590 nm TL emissions
stimulated by heating or blue-to-orange IRSL stimulated by 880 nm
illumination were isolated using 3 mm thickSchott BG 39 þGG 455
colour glass filters. UV emissions stimulated by heating or 470 nm
illumination were isolated using a 7 mm thick Hoya U 340 filter. All
heating was performed at 2 K/s in flowing nitrogen. In all cases 10
sample aliquots were used and given doses of 0, 0.14, 0.27, 0.54, 1.1,
2.2, 4.4, 8.7,17.5 and 35 Gy
90
Sr/
90
Y beta irradiation; data shown are
mass-normalised (counts/mg).
Fig. 8. (a) OSL and (b) IRSL shine curves, measured with 2000 s exposures, following
doses then preheating to 160 C.
Fig. 9. Dose response curves for Salt #3, OSL (UV emissions) and IRSL (blue-to-orange
emissions). Plotted are integrals of the first 100 s minus the final 100 s.
a
b
Fig. 10. Pulse Anneal response for OSL and IRSL. Measurement conditions are as above;
data were collected at 30 C following incremental heatings with 30 Cor10C
intervals; depletion corrected.
N.A. Spooner et al. / Radiation Measurements 46 (2011) 1856e1861 1859
6.1. 590 nm TL
The TL dose response curve derived from the data of Fig. 4(a)
shows useful sensitivity and dose response characteristics over
this range of doses. It is also apparent from Fig. 4(b) and 5 that
prolonged exposure to IR produced little bleaching of the
590 nm TL.
6.2. UV TL
The family of UV TL glow curves are shown in Fig. 6 and the dose
response curves in Fig. 7. It is seen that 470 nm illumination
strongly bleaches the 200
C UV TL peak, reducing it to w10% of
initial intensity, and effectively fully removes the 240
CUVTL
peak.
6.3. OSL and IRSL
Fig. 8 shows the OSL and IRSL shine curves. Note that the initial
decay rates are both rapid, but the OSL intensity continues to
decline whereas the IRSL reaches an elevated “background”from
which further decline is slow. This suggests slow depletion of
a large trapped charge population, possibly the traps giving rise to
the 200
C and 240
C TL peaks.
Fig. 9 shows the corresponding dose response curves. The IRSL
emission is markedly less sensitive than is the UV OSL emission,
and furthermore as the IRSL has both a higher background and
exhibits supralinear growth, the UV OSL appears the more useful.
Our measurements are broadly consistent with those of Bailey et al.
(2000), who also concluded that the UV OSL emission showed
promise for dosimetric applications.
7. OSL and IRSL origin
The Pulse-Anneal method was used to investigate the
temperature regime in which the traps giving rise to UV OSL and
blue-to-orange IRSL are thermally drained (Fig. 10). The intensities
of both IRSL and OSL decline rapidly as the 100
C TL peak is
drained, suggesting a common trap origin for these three signals.
The IRSL then shows further decline towards background as the
200
C and 240
C peaks are removed, again suggesting a common
origin but also suggesting that there is no significant deeper trap
population contributing to IRSL (although the possibility that the
heatings are desensitising deeper traps cannot be excluded). In
contrast, the OSL shows significant sensitisation as the 200
CTL
peak is removed, suggestive of trap transfer or transformation as
noted above.
8. Spatially-resolved luminescence
The PCIS (McCulloch et al., 2011) enabled spatially-resolved
luminescence to be studied across the wavelength range
Fig. 11. Sample #3 (a) UV OSL (3 mm U 340), lightsum ¼310
5
counts/Gy (b) blue-to-orange IRSL (BG 39 þGG 455), lightsum ¼210
6
counts/Gy. Measurements followed 6 Gy
beta irradiation and 160 C preheating.
Fig. 12. Salt #3. (a) 695e1050 nm TL integrated from 200 to 300 C following 6 Gy beta
90
Sr/
90
Y dose. TL lightsum is 5.2 10
7
counts, corresponding to 1.7 10
6
counts/mg/Gy. (b)
Reheat: the grains, now drained of TL, appear as “shadows”due to lower emissivity than the discs and cooling by N
2
.
N.A. Spooner et al. / Radiation Measurements 46 (2011) 1856e18611860
200e1050 nm, including 695e1050 nm (Red-Near-IR) TL emis-
sions not previously observed. TL, OSL and IRSL was imaged from
various samples: Fig. 11 shows frames captured for Sample #3
under (a) 470 nm stimulation and (b) 880 nm stimulation, and
Fig. 12 shows Red-Near-IR TL from the 695e1050 nm waveband
(3 mm thick Schott RG 695 colour glass filter). The range of grain-
to-grain variation is low, consistent with the origin of this
material.
9. Conclusions
Luminescence sensitivity of 590 nm TL and blue-stimulated UV
OSL is sufficient to enable dose detection limits of <1 mGy to be
readily achievable for the evaporated sea water sample reported
here. Furthermore, these emissions were observed from most
samples studied, suggesting the general applicability of salt for
opportunistic dosimetry, particularly if 590 nm TL or blue-
stimulated UV OSL are utilised. Findings include:
1. Significant degradation of 590 nm TL sensitivity is apparent
following heating beyond w180
C.
2. The higher-temperature 590 nm TL peaks, observed at
approximately 200
C and 240
C (measured at 5 K/s) have
thermal stabilities giving them lifetimes at 20
Cofw0.6 ka and
4 ka respectively.
3. Thermal desensitization mandates that only first glow data is
usable for dosimetry.
4. 590 nm TL exhibits much greater sensitivity (w210
6
counts/
Gy/mg) than does UV TL, even for when predominantly blue/
UV sensitive bialkali PMTs are used for detection of both
signals.
5. OSL UV emissions stimulated by 470 nm illumination show
much greater sensitivity than blue-to-orange emissions stim-
ulated by 880 nm IR illumination.
6. Thermal Quenching was not detected.
Acknowledgements
The authors acknowledge the support of the Defence Science
and Technology Organisation and the School of Chemistry and
Physics, University of Adelaide.
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