optically stimulated luminescence and thermoluminescence in some cu+ doped alkali fluoro-silicates

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Radiation Measurements 38 (2004) 59–70www.elsevier.com/locate/radmeas

Optically stimulated luminescence and thermoluminescencee#ciencies for high-energy heavy charged particle irradiation

in Al2O3:C

E.G. Yukiharaa, R. Gazaa, S.W.S. McKeevera ;∗, C.G. SoaresbaDepartment of Physics, Arkansas-Oklahoma Center for Space and Planetary Sciences, Oklahoma State University,

145 Physical Sciences Building, Stillwater, OK 74078-3072, USAbNational Institute of Standards and Technology, Gaithersburg, MD 20899-8460, USA

Received 18 May 2003; received in revised form 27 June 2003; accepted 1 July 2003

Abstract

The thermally and optically stimulated luminescence (TL and OSL) response to high energy heavy-charged particles (HCPs)was investigated for two types of Al2O3:C luminescence dosimeters. The OSL signal was measured in both continuous-wave(CW) and pulsed mode. The e#ciencies of the HCPs at producing TL or OSL, relative to gamma radiation, were obtainedusing four di;erent HCPs beams (150 MeV=u 4He; 400 MeV=u 12C; 490 MeV=u 28Si, and 500 MeV=u 56Fe). The e#ciencieswere determined as a function of the HCP linear energy transfer (LET). It was observed that the e#ciency depends on the typeof detector, measurement technique, and the choice of signal. Additionally, it is shown that the shape of the CW–OSL decaycurve from Al2O3:C depends on the type of radiation, and, in principle, this can be used to extract information concerningthe LET of an unknown radiation Beld. The response of the dosimeters to low-LET radiation was also investigated for dosesin the range from about 1–1000 Gy. These data were used to explain the di;erent e#ciency values obtained for the di;erentmaterials and techniques, as well as the LET dependence of the CW–OSL decay curve shape.c© 2003 Elsevier Ltd. All rights reserved.

Keywords: Optically stimulated luminescence; Thermoluminescence; Aluminum oxide; Heavy charged particle; Space dosimetry

1. Introduction

Thermoluminescence (TL) and optically stimulated lumi-nescence (OSL) dosimeters o;er many advantages over cur-rently used dosimeters in radiotherapy with heavy chargedparticles (HCPs), and in space dosimetry. These include:small sizes and low mass, zero power consumption, low ef-fective atomic number, no necessity for data storage, anda large dynamic range (GeiC et al., 1998a, b; Benton andBenton, 2001).

However, the spatial distribution of absorbed dose follow-ing HCP irradiation di;ers from that from low-LET radia-tion such as gamma or X-rays. While, in the latter, the dose

∗ Corresponding author: Tel.: +1-405-744-5802; fax: +1-405-372-5645.

E-mail address: [email protected](S.W.S. McKeever).

is, disregarding attenuation, nearly uniformly deposited inthe irradiated material, the dose due to heavy charged par-ticles is deposited along tracks of high ionization density.Theoretical studies demonstrate that the dose can reach val-ues as high as 105 Gy at the core of these tracks (Waligorskiet al., 1986; Katz et al., 1990; KrHamer, 1995; Avila et al.,1999) and decrease with r−2, where r is the radial distancefrom the center of the track (Butts and Katz, 1967; KrHamer,1995). Due to this basic di;erence in the spatial distribu-tion of the energy deposited, the response of the dosimeterscan be signiBcantly di;erent for di;erent types of radiation.It is essential, therefore, to understand how the dosimetersrespond to di;erent HCPs and energies, and to identify theimportant parameters deBning the dosimeter response.

To characterize the luminescence response of a dosime-ter in an HCP Beld, it is useful to deBne a relative e#ciency�HCP; � as the sensitivity of the dosimeter to HCP radia-tion compared to that of a low-LET reference source (for

1350-4487/$ - see front matter c© 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S1350-4487(03)00251-8

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60 E.G. Yukihara et al. / Radiation Measurements 38 (2004) 59–70

example, 60Co gamma rays). The sensitivities are deBned asthe mass normalized signal divided by the dose (S=D). Therelative e#ciency of HCP radiation to 60Co gamma rays istherefore:

�HCP; � =SHCP=DHCP

S�=D�; (1)

in which SHCP represents the mass normalized signal (TLor OSL) due to an HCP dose DHCP in the linear responserange, and S� is the signal due to a dose D� from a referencelow-LET gamma source, also in the linear response range.

Previous studies with materials such as LiF indicate areduction in the luminescence e#ciency for high-LET radi-ations, deBned for purposes of this paper as ¿ 10 keV=�m(see, for example, the data compiled by Horowitz, 1981).According to track structure theories (Butts and Katz, 1967;Kalef-Ezra and Horowitz, 1982), the response of the ma-terial is a convolution of the dose distribution caused bythe HCP irradiation and the e#ciency of the material toa uniformly distributed dose of ionizing radiation, repre-sented by the dose response to a low-LET radiation source.Typically, the latter response has a shape schematicallyillustrated in Fig. 1 and exhibits saturation for doses above102–103 Gy, depending on the material. The lack of sen-sitivity to HCP radiation arises from the fact that the dosesclose to the track generally exceed these saturation doses,sometimes by 2–3 orders of magnitude. The dose responseof the dosimeters to a low-LET reference radiation has beenused in conjunction with models of radial dose distributionsaround HCP tracks to predict the e#ciency for di;erentparticles and energies with some degree of success (GeiCet al., 1998a; Avila et al., 1999; RodrMNguez-Villafuerteet al., 2000; Gamboa-deBuen et al., 2001).

Al2O3:C, initially developed as a TL dosimeter, has ex-ceptional dosimetric properties. These include very high sen-sitivity to ionizing radiation, a relatively simple glow curvewith a peak at ∼177◦C (450 K), a linear response over

Log (Dose)

Log

(Res

pons

e)

Fig. 1. Schematic dose response in ideal materials.

6 orders of magnitude in dose, low background, low fadingduring storage in the dark, a simple emission band centeredat 420 nm, and a relatively low e;ective atomic number(10.2) (Akselrod et al., 1990, 1993). However, the use ofthis material as a TL dosimeter has two main drawbacks:the total output is strongly inOuenced by the heating ratedue to the drastic reduction in the e#ciency of the lumines-cence centers caused by thermal quenching over the temper-ature range 100–250◦C (Milman et al., 1995; Akselrodet al., 1998a), and the TL shows light-induced fading (Musk,1993; Moscovitch et al., 1993; Walker et al., 1996; Whitleyand McKeever, 2000). This last observation prompted in-vestigators to explore the potential of optically stimulatedluminescence (OSL) of Al2O3:C for dosimetry (Markeyet al., 1995).

The use of Al2O3:C as an optically stimulated lumines-cence (OSL) dosimeter has proved to be more advantageousthan its use in TL mode. With the OSL technique, the highoptical sensitivity can be used to obtain a fast, all-opticalmeasurement of absorbed dose (McKeever et al., 1996). Theimproved sensitivity occurs for two reasons: Brst, the stimu-lation rate is a maximum from the beginning of the measure-ments and, second, the optical stimulation can be done atroom temperature, therefore avoiding problems of reducedluminescence e#ciency caused by thermal quenching. Al-though OSLmeasurements are usually performed using con-tinuous wave (CW) stimulation, in the case of Al2O3:C fur-ther advantages, such as higher sensitivity and re-readability,can be obtained with the pulsed OSL technique (POSL)(McKeever et al., 1996). In this technique, the stimulationlight source is pulsed at a high frequency, typically 4 kHz,and the luminescence from the dosimeter is read betweenthe pulses (McKeever and Akselrod, 1999). The associationbetween the Al2O3:C material and the POSL technique isalready well-established, and the combination is used suc-cessfully as a passive radiation protection dosimetry method(McKeever, 2001). In addition, the all-optical nature of theOSL measurements allows technically innovative, fast andreal-time applications to be developed (Huston et al., 2001;Polf et al., 2002). New dosimetric applications such as theones mentioned in the Brst paragraph will require a parallelcharacterization of the TL and OSL properties of Al2O3:Cin various radiation Belds, including HCP radiation.

Until now, the only study on the e#ciency of Al2O3:Cto HCPs has been by Yasuda and Kobayashi (2001).They looked at the OSL response of Al2O3:C LuxelTM

dosimeters irradiated in the Heavy Ion Medical Acceler-ator in Chiba (HIMAC) with selected high-energy ions(150 MeV=u 4He; 290 MeV=u 12C; 500 MeV=u 40Ar, and500 MeV=u 56Fe), covering the range of LET∞ in H2Ofrom 2.2 to 198 keV=�m. Their data show that the e#ciencyis slightly increased (1.04) for the lowest LET particle(2:2 keV=�m), and drops continuously thereafter reaching avalue below 0.4 for the highest LET particle (198 keV=�m).The OSL measurements were performed in CW modeand the OSL signal was integrated over the time of the

E.G. Yukihara et al. / Radiation Measurements 38 (2004) 59–70 61

stimulation. The authors speculated that the increasede#ciency at 2:2 keV=�m may be associated with thesupralinear dose response reported in earlier TL stud-ies of Al2O3:C (Akselrod et al., 1990). In additionto that, Yasuda et al. (2002) noted that the shape ofthe CW–OSL decay curves of Al2O3:C was di;er-ent for di;erent types of radiation (137Cs gamma-rays,4He 150 MeV=u; 12C 290 MeV=u; 40Ar 500 MeV=u), withan increasing OSL decay rate as the LET increased.

The objective of the present study is to investigate theTL and OSL properties of Al2O3:C dosimeters irradiatedwith high-energy HCPs (150 MeV=u 4He; 400 MeV=u 12C;490 MeV=u 28Si, and 500 MeV=u 56Fe). Al2O3:C TL andOSL e#ciencies relative to 60Co gamma rays were deter-mined for the various HCP beams, for di;erent materialsand measurement techniques (TL, CW–OSL or POSL). Itwas observed that the shape of the CW–OSL decay curvevaries with the type of radiation, as Brst observed by Yasudaet al. (2002). The results are compared and discussed basedon additional results on the dose response of the material toa low-LET irradiation (90Sr beta rays). It is shown that theobtained e#ciency values for HCPs depend on the materialand choice of signal (TL, CW–OSL initial intensity, totalCW–OSL intensity, or POSL), and are associated with thecorresponding dose response to low-LET radiation.

2. Experimental details

2.1. Detector packages: description and preparation

The detector packages used in this study were composedof two types of Al2O3:C dosimeters: 9 unpolished singlecrystal dosimeters of the type generally known as TLD500,with an approximate diameter of 5:0 mm and a thickness of0:9 mm, grown in the Urals Polytechnic Institute (Russia);and 2 LuxelTM dosimeters (powdered Al2O3:C crystals be-tween two thin plastic layers) produced by Landauer Inc.(see disclaimer at the end of this paper). The dosimeterswere mounted in polycarbonate (LexanTM) holders of di-mensions 7:0 cm × 3:5 cm × 0:7 cm. The sensitive area ispractically the same area as the holder (7:0 cm × 3:5 cm)and the thickness of polycarbonate traversed by the ion

beam before reaching the dosimeters is 2:0 mm. The hold-ers were enclosed in black plastic to avoid exposure ofthe light-sensitive dosimeters to light during irradiation andtransportation.

Before irradiation, the dosimeters were subjected to ap-propriate annealing or bleaching procedures. The Al2O3:Csingle crystal dosimeters were annealed at 900◦C for 15 minand cooled in contact with a metal plate at room tempera-ture. The LuxelTM dosimeters were bleached overnight us-ing light from a halogen lamp Bltered by a yellow glass BlterKopp 3-69. A summary of the dosimeters used in this studyis presented in Table 1.

2.2. Irradiation

The detector packages were irradiated at the Heavy IonMedical Accelerator in Chiba (HIMAC) as part of theSecond Inter-Comparison for Cosmic-rays with HeavyIon Beams at NIRS (ICCHIBAN-2). The data presentedhere were obtained using four beams (150 MeV=u 4He;400 MeV=u 12C; 490 MeV=u 28Si, and 500 MeV=u 56Fe)and four doses for each beam (1, 10, 50 or 100 mGy,indicated as absorbed dose in water). In total, 16 detectorpackages (4 beams × 4 doses) plus 3 control detectors wereanalyzed. The characteristics of the beams are presented inTable 2.

Post-HCP irradiation calibration of the individual dosime-ters was done using one of two 90Sr beta sources: a lowactivity source delivering an average volume dose rate of0:624 mGy=s and a high activity source delivering an av-erage volume dose rate of 57:7 mGy=s to the Al2O3:C sin-gle crystal dosimeters. The dose rates of the sources wereobtained through calibration of the sources against a 60Cosource at the National Institute of Standards and Technol-ogy (NIST) using the Al2O3:C single crystal dosimeters. In-dependent calibration against NIST standards was also per-formed to obtain the dose rates to the LuxelTM dosimeters.Thus, the reference doses are quoted as gamma dose in wa-ter. The dose received during transportation and storage ofthe dosimeters was evaluated through a set of control de-tector packages and appropriately subtracted from the dosesobtained from the HCP-irradiated dosimeters. The dose fromthe control detectors was generally smaller than 0:23 mGy.

Table 1Characteristics of the dosimeters used in this study and annealing/bleaching procedures

Dosimeter Al2O3:C single crystal Al2O3:C LuxelTM

Type TL/OSL OSLQuantity measured Absorbed dose Absorbed doseSensitivity range ∼1 �Gy to 10 Gy ∼1 �Gy–10 GySensitive area 5:0 mm diameter 1:7 cm × 2:0 cmSize 5:0 mm diameter by 0:9 mm thickness Thin Blm 1:7 cm × 2:0 cmMass ∼70 mg ∼160 mgAnnealing/bleaching 900◦C=15 min Yellow light overnight


Table 2Ion beam parameters for the irradiations with heavy charge particles performed at the HIMAC facility

Ion Nominal energy (MeV/u) Estimated energy (MeV/u) Range in H2Oa (cm) LET∞ H2Oa (keV/�m)

4He 150 144 14.68 2.26312C 400 390 26.84 10.828Si 490 469 15.02 55.5156Fe 500 464 8.76 189

aCalculated using the estimated energy.

2.3. TL and OSL measurements

ContinuousWave OSL (CW–OSL) and TL readouts wereperformed using a RisH TL/OSL-DA-15. For the CW–OSLreadout, the samples were stimulated with light from greenlight emitting diodes (LEDs), with emission centered at525 nm and a power of ∼10 mW=cm2 at the sample. Thediscrimination between the stimulation light and the stim-ulated luminescence from the dosimeter was accomplishedusing a Hoya U-340 Blter pack of 7:5 mm thickness (cen-tered at 340 nm with a full width at half maximum of 80 nm)at the detection window. TL readouts were performed with aheating rate of 1◦C=s using broad band Corning 5-58 Blters(centered at 410 nm; FWHM = 80 nm) to select the mainemission band of Al2O3:C at 420 nm. In both CW-OSL andTL readouts, apertures of appropriate diameters were usedto protect the photomultiplier tube (PMT). Examples ofTL and CW–OSL curves are presented in Fig. 2a and b,respectively.

For the TL readouts, analyses using the TL peak heightor integrated TL area were found to be equivalent. The peakheight was used throughout the analysis presented in thiswork. For the CW–OSL readouts, the analyses using theCW–OSL signal integrated over the period of stimulation(300 s) or using the initial CW–OSL intensity (lumines-cence integrated over the Brst 3 s of stimulation) were notequivalent. Therefore, we present the results based on boththe CW–OSL area and initial intensity.

The POSL readouts of the LuxelTM dosimeters were per-formed using the second harmonic light (at 532 nm) froma Nd:YAG laser as the stimulation source. The laser isQ-switched, delivering short pulses (300 ns) at a frequencyof 4 kHz. The luminescence was detected between the pulsesusing a gated photon-counting system. A Nd:YAG 532 nmlaser line Blter and Corning 5–58 Blters were placed be-tween the sample and the PMT to avoid overexposure of thePMT during the laser pulses. In the current setup, the totalstimulation energy delivered to the sample during one singlemeasurement depletes the signal by ∼30%. Fig. 2c showsan example of a POSL data set. During the Brst second inwhich the pulsed stimulation is on, the luminescence sig-nal shows a quick build-up, related to the charge build-upin the excited state of the luminescence centers and in shal-low traps, followed by a decrease due to exhaustion of the

trapped charge population (Akselrod et al., 1998b). At 1 s,the stimulation is turned o; and the luminescence decays.The POSL signal was taken as the integrated signal duringthe 1 s of laser stimulation.

Table 3 summarizes the experimental parameters for thedi;erent techniques used in this study.

2.4. Calibration method and calculation of the e;ciency

The individual calibration procedure used for eachdosimeter consisted of: (i) readout of the TL, CW–OSL orPOSL signal S; followed by (ii) annealing or bleaching, (iii)irradiation with reference dose D�, and (iv) readout of theresultant signal S�. From this, the gamma dose D necessaryto produce a signal of intensity S, is determined by

D =SS�D�: (2)

To calculate the experimental relative e#ciency for irra-diation with a given HCP (Eq. (1)), it is necessary to sub-tract the signal due to the dose received during transporta-tion and storage Sc, which can be obtained from the controldetectors, from the signal S, in order to obtain the signal dueto irradiation in the HIMAC (SHCP):

SHCP = S − Sc: (3)

The problem is that the signals S and Sc are not comparable,since they were obtained with di;erent detectors that maydi;er in mass and sensitivity. In this case, it is more conve-nient to calibrate each dosimeter individually and work withthe dose D obtained from Eq. (2), which is proportional tothe signal but independent of the mass or sensitivity of thedosimeter.

Substituting Eq. (3) in (1), we obtain

�=(S − Sc)=DHCP

S�=D�(4)

and

�=(S=S�)D� − (Sc=S�)D�

DHCP=D − Dc

DHCP: (5)

From this equation, the e#ciency can be calculated based onthe values D; Dc and DHCP. Here D, calculated by Eq. (2),is proportional to the signal S due to HCP irradiation anddose received during storage and transportation; Dc is the


50 100 150 200 250 3000

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(arb

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Fig. 2. Example of (a) TL curve of Al2O3:C single crystal dosime-ters, and (b) CW–OSL and (c) POSL signals from LuxelTM

dosimeters. The inset shows schematically the temperature or in-tensity of the stimulation light as a function of time in each mea-surement mode.

dose received during storage and transportation, determinedusing the control detectors; and DHCP is the actual dose de-livered by the HCP to the dosimeters, determined by the re-searchers responsible for the irradiations for ICCHIBAN-2using a calibrated plastic scintillation counter and calibratedion chambers for low and high dose rates, respectively.

3. Results

3.1. Linearity of the HCP dose response

The evaluation of the relative e#ciency by Eq. (1) re-quires the doses DHCP and D� to be in the linear range of thedose response. To check if the signal SHCP is linearly pro-portional to DHCP, the dosimeters were irradiated with nom-inal doses DHCP (in water) equal to 1, 10, 50 and 100 mGyfor each beam, and the resultant TL, CW–OSL and POSL(S = SHCP + Sc) signals were measured. As discussed inSection 2.4, each dosimeter was calibrated individually andEq. (2) was used to calculate D = S=S� × D�. The valuesD−Dc, which are proportional to SHCP, can then be obtained.The values D−Dc calculated from the TL peak height andintegrated CW–OSL signals for the Al2O3:C single crystalsdosimeters are presented in Fig. 3 as a function of the knowndose DHCP delivered to the samples. The values calculatedfrom the integrated CW–OSL and POSL for the LuxelTM

dosimeters are presented in Fig. 4 as a function of the knowndelivered dose DHCP. In the graphs, each data point repre-sents the average value D − Dc from three dosimeters andthe error bars represent the standard deviation (±1) of themean value. It can be seen that all signals exhibit good lin-earity for the range of doses used in this study, with the ex-ception of the TL signal due to irradiation with the Fe beam,which may be exhibiting a small degree of supralinearity.The OSL signal with the Fe beam irradiation was found tobe linear.

Table 4 presents the complete data set of D − Dc valuesfor the 16 dosimeters. These values represent the gammadose necessary to produce the same signal as that producedby HCP irradiation, as discussed in Section 2.4. For exam-ple, the information in column 6 (single crystals TL), row4 (detector 4) of Table 4 indicates that, for the Al2O3:Csingle crystals, a dose of 89:4 mGy from gamma radiationproduces the same TL signal as irradiation with a dose of100 mGy in the He beam (4He; 150 MeV=n). If the signal tobe considered is the CW–OSL of the single crystal dosime-ters (column 4), a dose of only 81 mGy from gamma radi-ation will produce the same signal as the HCP irradiation.

Examining Table 4, the main conclusion is that the gammadose necessary to produce the same signal as that from HCPirradiation depends on the sample type and on the readouttechnique. In other words, each sample type and techniqueis characterized by a di;erent e#ciency, as will be shownin the next section.

3.2. Determination of relative e;ciency

The data presented in Table 4 were used to obtain thee#ciency of the dosimeters to HCPs relative to gamma-dosein water by applying Eq. (5). The Bnal result is presented inTable 5, where the e#ciency for each HCP and technique isshown as the weighted average based on the e#ciencies forthe four di;erent doses. The values for the Al2O3:C single


Table 3Summary of the experimental parameters for the di;erent techniques used in the present study

Dosimeter Method Heating rate Stimulation Detection Signal

Al2O3:C single crystals TL 1◦C/s — 5–58 Blters Peak height at 225◦C(centered at 410 nm,FWHM = 80 nm)

CW–OSL — 10 mW=cm2 at 525 nm U340 Blters Area (integrated over(green LEDs) (centered at 340 nm, 300 s)

FWHM = 80 nm) I0 (initial intensity,integrated over Brst 3 s)

Al2O3:C LuxelTM CW–OSL — 10 mW=cm2 at 525 nm U340 Blters Area (integrated over(green LEDs) (centered at 340 nm, 300 s)

FWHM = 80 nm) I0 (initial intensity,integrated over Brst 3 s)

POSL — 532 nm (Nd:YAG laser) 5–58 Blters Integration over 1 s ofPower not recorded (centered at 410 nm, POSL signal(depletion of 30% FWHM = 80 nm)of the initial signal)

crystal (Fig. 5a) and for the LuxelTM (Fig. 5b) dosimetersare plotted as a function of the corresponding LET in waterfor each beam.

As expected, a general decrease in the e#ciency athigh-LET values is observed in Fig. 5. However, di;erenttechniques or types of dosimeters resulted in e#ciencyvalues that are signiBcantly di;erent. In some cases, anover-response is observed.

Fig. 5a shows that, for the case of Al2O3:C single crystaldosimeters, the TL e#ciency is similar to the e#ciency ofthe integrated CW–OSL, whereas the e#ciency obtained us-ing the initial CW–OSL intensity is signiBcantly higher. Forthe CW–OSL from LuxelTM dosimeters, a similar situationis observed (Fig. 5b), but with a signiBcant over-reponse forthe initial CW–OSL intensity. It is also seen that the e#-ciencies are lower for the Al2O3:C single crystal than for theLuxelTM dosimeters. The POSL e#ciency was lower thanthat obtained from either of the CW–OSL signals (Fig. 5b).

The over-response is observed mainly for the He beam.The relative e#ciency for the He beam (2:23 keV=�m) isabove 1 for the initial CW–OSL intensity of the Al2O3:Csingle crystal dosimeters (Fig. 5a) and for the CW–OSLdata (integrated and initial intensity) of the LuxelTM dosime-ters (Fig. 5b). It is to be noticed that there is a strongover-response of the initial intensity of the CW–OSL fromLuxelTM dosimeters for both He and C (Fig. 5b).

3.3. LET information contained in the OSL decay curves

It was shown in the previous sections that, for the CW–OSL readouts, the doses and e#ciencies obtained are dif-ferent if one uses the total area or the initial intensity of theCW–OSL decay curve in the analysis. As shown in Fig. 6,

this is caused by a dependence of the CW–OSL decay curveshape on the type of radiation. In Fig. 6, the normalizedCW–OSL decay curves of LuxelTM obtained after a dose of100 mGy of di;erent types of irradiation are compared. Therate of decay is slow for the beta irradiation, intermediatefor irradiation with He beam, and fast for irradiation with C,Si and Fe (the curves practically overlap in the latter cases).

To quantify the changes in the CW–OSL curves, we de-Bne a ratio R as the total CW–OSL area (integrated intensity)after beta irradiation divided by the total CW–OSL area af-ter irradiation with HCP beams, after the curves are normal-ized so that the initial intensity is 1 as in Fig. 6. Expressingthis deBnition in another way, R = (A�=I�)=(AHCP=IHCP), inwhich A represents the total CW–OSL area and I the initialintensity after irradiation with beta and HCPs. In Fig. 7 theratio R is plotted against the radiation LET. The data pointcorresponding to the LET of beta irradiation is, by deBni-tion, equal to 1. The other data points are an average ofthe R-value for three readouts. Only the glow curves cor-responding to 100 mGy were used in the analysis to mini-mize the e;ect of the low-LET background radiation on theR value and because of the low noise level in these curves.The CW–OSL curves from the same sample, obtained afterthe HCP and beta irradiation, were used to calculate the Rvalues.

No signiBcant di;erence was noted in the shape of the TLcurves of Al2O3:C single crystal dosimeters with di;erenttypes of irradiation.

3.4. Dose response to beta radiation

In order to understand the response of Al2O3:C toHCPs, it is helpful to investigate the dose response of the


1 10 100

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DHCP (mGy)

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He 144MeV/u C 390MeV/u Si 469MeV/u Fe 464MeV/u

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Al2O3:C single crystal dosimeters

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D -

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(a)

(b)

Fig. 3. (a) TL and (b) OSL dose response of Al2O3:C singlecrystal dosimeters irradiated with di;erent beams as a functionof the HCP dose (DHCP). The signal SHCP is represented by thequantity D − Dc, which is proportional to SHCP but cancels massand sensitivity variations among the dosimeters. The error barsrepresent the standard deviation of the values, as indicated in Table4. The indicated doses are in water.

di;erent materials and methods to high doses of low-LETradiation. In this experiment, Al2O3:C single crystal andLuxelTM dosimeters were irradiated with doses up to about600–1000 Gy using a 90Sr beta source and the signalwas measured using various techniques, as described inSection 2.3.

In Fig. 8a we present the CW–OSL and TL beta-doseresponse of the Al2O3:C single crystal dosimeters, wherethe CW–OSL signal was considered as either the totalCW–OSL area (integrated intensity over 300 s of stimu-lation) or the initial CW–OSL intensity (integrated overthe Brst 3 s of stimulation). In each case, the dosesquoted are gamma doses to water, obtained by cali-bration of the 90Sr source against a 60Co standard, as

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Fig. 4. (a) CW–OSL and (b) POSL dose response of Al2O3:CLuxelTM irradiated with di;erent beams as a function of the HCPdose (DHCP). The signal SHCP is represented by the quantity D−Dc,which is proportional to SHCP but cancels mass and sensitivityvariations among the dosimeters. The error bars represent the stan-dard deviation of the values, as indicated in Table 4. The indicateddoses are in water.

already noted. It is observed that each technique has adistinct dose-response curve, with di;erent degrees ofsupralinearity and di;erent saturation levels. Further-more, it is also important to observe that in each casethe signals decrease after reaching saturation. The lat-ter phenomenon was investigated in detail by Yukiharaet al. (2003) and was shown to be associated with a decreasein the concentration of recombination centers (F+-centers)at very high doses.

The inset of Fig. 8a presents the supralinearity factor SF,deBned as the ratio between the dose response curve and thelinearity curve. This can be written as SF = (S=D)=(S0=D0),in which S is the signal obtained following a dose D, andS0 is the signal after a chosen dose D0 in the linear range of


Table 4Values of D − Dc for the 16 detectors evaluated from Al2O3:C single crystal and LuxelTM dosimeters using di;erent techniques

Beam Detector Nominal dose Single crystals Single crystals LuxelTM

DHCP CW–OSL TL CW–OSL POSL

Area l0 Area l0(mGy) (mGy) (mGy) (mGy) (mGy) (mGy) (mGy)

He 1 1.01 0.85(3) 1.04(4) 0.91(6) 0.98(16) 1.53(13) 0.859(12)2 10.04 8.0(3) 9.5(5) 8.50(14) 10.27(15) 14.1(6) 8.74(10)3 50.22 42.7(12) 52.2(22) 45.1(6) 51.7(6) 68.0(8) 43.9(6)4 100.19 81(2) 100(4) 89.4(8) 104.9(12) 139(3) 85.7(5)

C 5 0.98 0.53(4) 0.74(5) 0.522(17) 0.86(21) 1.54(20) 0.600(13)6 10.22 5.5(2) 7.4(2) 5.94(4) 7.5(4) 12.9(3) 6.36(7)7 50.21 26.9(11) 36.6(11) 28.9(9) 39.6(6) 66.2(8) 32.7(3)8 100.26 53(2) 73(5) 53.7(5) 80.8(9) 134.3(16) 65.4(5)

Si 9 0.99 0.295(26) 0.42(3) 0.349(13) 0.38(12) 1.00(12) 0.400(14)10 10.05 3.22(21) 4.3(4) 3.69(6) 4.77(22) 8.63(23) 4.02(4)11 50.65 16.8(5) 21.7(19) 18.9(3) 24.9(3) 43.8(9) 20.18(20)12 100.27 32.6(11) 41.4(20) 36.6(4) 48.1(6) 84.4(10) 39.9(4)

Fe 13 1 0.31(3) 0.46(8) 0.288(15) 0.51(12) 0.95(10) 0.369(24)14 10.07 2.95(9) 4.00(12) 3.12(7) 4.07(19) 6.64(14) 3.40(4)15 50.17 14.9(4) 18.8(5) 16.6(5) 20.02(25) 31.9(4) 16.6(2)16 100.16 29.9(9) 37.6(11) 35.7(17) 39.5(5) 64.1(7) 33.5(4)

The values were obtained from application of Eq. (2) to obtain D and subtracting the background dose Dc obtained from the controldosimeters. The table entries represent the mean value and respective standard deviation (of the mean) of at least three readouts with di;erentdosimeters. The values are indicated in the following notation: 0.85(3) meaning 0:85± 0:03. The indicated doses are in water.

Table 5Relative e#ciencies �HCP; � for each type of dosimeter and technique calculated using Eq. (5) and the data presented in Table 4

Beam LET (keV/�m in H2O) Al2O3:C single crystals Al2O3:C single crystals Al2O3:C LuxelTM

CW–OSL TL CW–OSL POSL

Area l0 Area l0

He 2.26 0.825(13) 1.008(20) 0.885(6) 0.999(7) 1.366(13) 0.809(4)C 11.2 0.534(11) 0.726(14) 0.564(3) 0.763(7) 1.322(10) 0.603(3)Si 55.7 0.326(7) 0.420(15) 0.367(3) 0.465(4) 0.849(8) 0.3755(22)Fe 193 0.297(5) 0.382(6) 0.318(5) 0.380(3) 0.641(5) 0.3144(19)

The values represent the weighted average of the e#ciencies obtained for the four doses (1, 10, 50 and 100 mGy) for each beam, thestandard deviation being indicated in the same notation of Table 4.

the dose-response curve (Chen and McKeever, 1997). Thecurve of supralinearity factor versus dose (inset of Fig. 8)is also called the dose-response function f(D) (Horowitz,1981). This function represents the e#ciency of the ma-terial in producing luminescence at a given dose. For lowdoses, this e#ciency is 1, while at high doses the e#ciencydecreases due to saturation of the material. That is exactlywhat is happening during HCP irradiation in the micro-scopic scale. The total dose delivered to the material is low,

but the dose inside the track is very high and, consequently,the material is locally saturated, resulting in reducedluminescence.

Fig. 8b shows the dose-response curve of CW–OSL andPOSL obtained with LuxelTM dosimeters. Again each tech-nique exhibits a distinct dose-response curve. The satura-tion of the LuxelTM dosimeters, however, seems to occur atdoses higher than that of the single crystal dosimeters and,for the dose range used in the study, no clear decrease in


0.1 1 10 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

β

FeSi

C

He

Al2O3:C single crystal dosimeters

Integrated CW-OSL Initial CW-OSL intensity TL peak height

Effi

cien

cy

LET (keV/µm H2O)

0.1 1 10 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

β

FeSi

C

He


POSL Integrated CW-OSL Initial CW-OSL intensity

Eff

icie

ncy

LET (keV/µm H2O)

(a)

(b)

Fig. 5. E#ciencies for (a) Al2O3:C single crystal and (b) LuxelTM

dosimeters obtained with di;erent techniques. The error bars rep-resent the standard deviation of the values, as indicated in Table 5.

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

C, Si, Fe

He

β

β irradiation He 144 MeV/u (2.26 keV/µm H2O) C 390 MeV/u (10.8 keV/µm H2O) Si 469 MeV/u (55.5 keV/µm H2O) Fe 464 MeV/u (189 keV/µm H2O)

CW

-OS

L (n

orm

aliz

ed)

Time (s)

Fig. 6. Normalized CW–OSL decay of LuxelTM dosimetersirradiated with a dose of 100 mGy from di;erent beams.

the signal intensity was observed after saturation. Again, theinset presents the supralinearity factor.

In both cases, single crystal and LuxelTM dosimeters, it isto be noted that the CW–OSL area and the initial intensity

1 10 100

1.0

1.2

1.4

1.6

1.8

Fe

Si

C

He

β

Rat

io R

LET (keV/µm H2O)

Fig. 7. Ratio R between the total CW–OSL area of beta-irradiatedsamples and HCP-irradiated samples, after normalization of theCW–OSL curves to the initial intensity as in Fig. 6, as a function ofradiation LET. (In other words, R=(A�=I�)=(AHCP=IHCP), in whichA represents the total CW–OSL area and I the initial intensity afterirradiation with � rays and HCPs.) The data points correspondto three measurements and the error bars represent the standarddeviations of the R values.

are proportional to each other at very low doses, but thisproportionality breaks down for doses higher than about 5–20 Gy. Plotting the normalized CW–OSL curves at thevarious doses (Fig. 9), it can be seen that the CW–OSL decayrate becomes faster as the dose is increased. It is importantto point out that the same stimulation power was used in allreadouts. For doses6 10 Gy the CW–OSL curves overlap,as represented in Fig. 9. For higher doses, the CW–OSLcurve shape changes gradually (only representative curvesare displayed in the graph), the decay rate becoming fasterwith higher doses, until the limit represented by the curveof 500 Gy.

4. Discussion

The Al2O3:C OSL and TL e#ciencies for HCP relativeto 60Co gamma rays obtained in this study show that thevalues are dependent on the material type and readout tech-nique (TL, CW–OSL, POSL) and, in the case of CW–OSL,even on the choice of signal (total area or initial intensity).This result is not surprising, considering that the e#ciency,as discussed in the introduction, is a convolution of the re-sponse of the material (dose response to a low-LET radia-tion) and the particular radial dose distribution around theHCP track. Since the dose deposition for a particular HCPis uniquely deBned for a particular material and beam, thedi;erences in the e#ciency curves shown in Fig. 5 shouldonly be the result of di;erences in the dose responses.

Fig. 8 shows clearly that each material and techniquehas its own dose-response curve when irradiated with


1 10 100

100

101

102

Al 2O3:C single crystal dosimeters

Dose (Gy)

Lum

ines

cenc

e (n

orm

aliz

ed)

TL peak height Initial CW-OSL intensity Integrated CW-OSL intensity Linearity

1 10 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

f(D

)

Dose (Gy)

1 10 100 1000

100

101

102

Al 2O3:C LuxelTM dosimeters

Integrated CW-OSL Initial CW-OSL intensity POSL Linearity

Lum

ines

cenc

e (n

orm

aliz

ed)

Dose (Gy)

1 10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

f(D

)

Dose (Gy)

(b)

(a)

Fig. 8. Dose response of (a) Al2O3:C single crystal and (b)LuxelTM dosimeters obtained using di;erent techniques. The insetspresent the supralinearity factor (SF), deBned as the ratio betweenthe luminescence signal and the linearity curve.

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

≥ 500 Gy

100 Gy

50 Gy

≥ 10 Gy


CW

-OS

L (n

orm

aliz

ed)

Time (s)

Fig. 9. Normalized CW–OSL decay of LuxelTM dosimetersirradiated with various doses.

low-LET 90Sr beta radiation. It is, therefore, expectedthat the e#ciency values will reOect this. This can be ob-served, for example, by comparing the e#ciency valuesand the dose-response function f(D) curves for Al2O3:Csingle-crystals (Figs. 5a and 8a). Fig. 8a shows that the TLand integrated CW–OSL intensity dose-response functionsare similar. Consequently the e#ciency values for thesetechniques are also similar (Fig. 5a). On the other hand, theinitial CW–OSL intensity has a higher response (Fig. 8a),and, as should be expected, also a higher e#ciency whencompared to the other techniques (Fig. 5a). The same com-parison applies to the case of LuxelTM dosimeters (Figs. 5band 8b). The higher dose-response function (initial CW–OSL intensity) corresponds to the higher e#ciency, whilethe lower dose-response function (POSL) corresponds tothe lower e#ciency. These comparisons make evidentthe connection between the e#ciency to HCPs and thedose-response function to low-LET radiation.

The results presented demonstrate that the properties ofAl2O3:C irradiated with low-LET radiation are important notonly to explain the e#ciencies to HCPs, but also to under-stand the shape of the CW–OSL curve after irradiation withHCPs. It was shown that the shape of the CW–OSL decaycurves with doses higher than ∼10 Gy is dose-dependent,the decay becoming faster as the dose is increased (Fig. 9).Since in crystals irradiated with HCP most of the signaloriginates in regions of high ionization densities along thetracks, we can expect the signal after HCP radiation alsoto decay faster than the CW–OSL due to a low dose oflow-LET radiation. This was indeed observed, as shown inFig. 6. The exact shape of the CW–OSL decay after HCP ir-radiation would actually depend also on the particular radialdose distribution for that HCP and, therefore, on the type ofradiation.

Related to this change in the shape of the CW–OSL curveswith type of radiation, the results on the CW–OSL tech-nique show that the initial part of the CW–OSL curve (initialintensity) and the total CW–OSL area (total intensity) havedi;erent e#ciencies to HCPs, the ratio between them beingLET-dependent (Fig. 7). The di;erent e#ciencies of parts ofthe signal from the same material or from di;erent materialshave been used to explore the possibility of dose equivalentestimation in at least two approaches. The TL of other typesof dosimeters, such as LiF and CaF2:Tm (TLD-300), forexample, are also known to have a certain degree of depen-dence on the LET (Kalef-Ezra and Horowitz, 1982; Vanaet al., 1996; Yasuda, 2001; Hajek et al., 2002). Vana andcolleagues exploit this dependence in LiF and proposed theso-called high-temperature ratio (HTR) method to obtain the“average LET” for dose assessment of aircrew and in spacein mixed LET Belds (Vana et al., 1996; Berger et al., 2002;Hajek et al., 2002). The problem with this method is that itis questionable whether the “average LET” could be appliedto estimate the e;ective quality factor of space radiation ata low-earth orbit (Yasuda and Fujitaka, 2000). A di;erentapproach was taken by Yasuda and Fujitaka (Yasuda, 2001,


2002; Yasuda and Fujitaka, 2002), in which an estimation ofthe dose equivalent is obtained by approximating the qual-ity factor function Q(LET ) with a combination of the e#-ciencies �(LET ) of two TL peaks of LiF (Yasuda, 2001).The method can also be used combining the e#ciencies ofvarious luminescent dosimeters (Yasuda, 2002; Yasuda andFujitaka, 2002). Although these methods are still being de-veloped and tested, the LET dependence of the CW–OSLcurves of Al2O3:C certainly o;er an extra piece of informa-tion that may prove to be useful for dosimetry of radiationBelds involving HCPs on earth and space.

5. Conclusions

In this paper, the TL and OSL response of Al2O3:C tohigh-energy heavy-charged particles (HCP) was investi-gated and the e#ciency curves as a function of LET weredetermined based on the data for four di;erent HCP beams.The main conclusion is that the type of material, measure-ment technique and choice of signal are important factorsin determining the e#ciency. In the case of the CW–OSLsignal from Al2O3:C, it was shown that the shape of thedecay curve depends on the type of radiation. In principle,this LET-dependence could be used for extracting infor-mation relative to the LET of an unknown radiation Beld.The di;erences in e#ciency between di;erent techniquesand materials were correlated with the corresponding doseresponse to high doses of low-LET radiation from a 90Srbeta source. It is observed, for example, that a mate-rial/technique combination with high dose response is alsocharacterized by high e#ciencies. It was further observedthat the CW–OSL decay curve shape changes at high betadoses, suggesting that the LET dependence of the CW–OSLcurves is due to di;erences in the dose distribution along theHCP tracks and the particular dose response of the material/technique.

Acknowledgements

The experiments were performed as part of the IC-CHIBAN international inter-comparison project usingHeavy Ions at NIRS-HIMAC. The authors thank Dr. Y.Uchihori and Dr. E. Benton for the opportunity of par-ticipating in the ICCHIBAN project, Landauer Inc. forproviding the LuxelTM dosimeters used in this study, andDr. Emico Okuno for valuable discussion. This research issupported by NASA contract NAG9-1332.

DisclaimerIn this paper, certain commercially available products are

referred to by name. These references are for informationalpurposes only and do not imply endorsement by NIST.

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optically stimulated luminescence and thermoluminescence in some cu+ doped alkali fluoro-silicates

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