KCl: Eu2+ as a solar UV-C radiation dosimeter. Optically stimulated luminescence and thermoluminescence analyses

JOURNAL OF RARE EARTHS, Vol. 27, No. 4, Aug. 2009, p. 579

KCl: Eu2+ as a solar UV-C radiation dosimeter. Optically stimulated luminescence and thermoluminescence analyses I. Aguirre de Cárcer1, H.L. D´Antoni2, M. Barboza-Flores3, V. Correcher4, F. Jaque1 (1. Departamento Física de Materiales, Universidad Autónoma Madrid, Spain; 2. NASA Ames Research Center, Moffett Field, California, USA; 3. Centro de Investigación en Física, Universidad de Sonora, México; 4. Centro Investigaciones Energía y Medioambientales (CIEMAT), Madrid, Spain) Received 25 September 2008; revised 1 March 2009

Abstract: The KCl:Eu2+ system response to UV-C was investigated by analyzing the optically stimulated luminescence (OSL) and thermo-luminescence (TL) signal produced by ultraviolet light exposure at room temperature. It was found that after UV-C irradiation, OSL was produced on a wide band of visible wavelengths with decay time that varied by several orders of magnitude depending on the Eu2+ aggregation state. In spite of the low intensity of solar UV-C reaching the Earth’s surface in Madrid (40º N, 700 m a.s.l.), it was possible to measure the UV-C radiation dose at 6:48 solar time by using the TL response of the KCl:Eu2+ system and differentiate it from the ambient beta radiation dose. Keywords: thermo-luminescence; rare-earth-doped alkali halides; solar ultraviolet dosimeter; environmental radiation

KCl:Eu2+ is an extremely sensitive dosimeter for UV-C and ionizing radiation that can be used to measure environmental radiation doses. The KCl:Eu2+ crystal system allows monitoring the most dangerous ultraviolet photons that impinge upon terrestrial living organisms. It has been assumed that those photons are mostly absorbed by the stratospheric ozone layer[1], but a measurable amount of UV-C reaches the Earth surface and has been detected with state of the art spectrophotometers[2]. It is important to measure the UV-C flux reaching the surface of our planet because the ozone shield is expected to continue decreasing over the next few decades thus produce a potentially dramatic increase in the amount of UV-C that reaches the surface. Sensors, durable and reliable must be developed in order to monitor incoming solar UV-C radiation and validating other records. The KCl:Eu2+ crystals are known to respond to UV-C excitation by producing photoconductivity, afterglow or post-luminescence, optically stimulated luminescence (OSL) and thermoluminescence (TL)[3] so they might be a good active material for such sensors. The use of OSL and TL of pre-irradiated KCl:Eu2+ has been proposed for detection of the UV-C flux and dose estimation. Nanto et al. described a linear dose relation between UV-C dose and optically stimulated emission. In that study, the KCl:Eu2+ system was reset to initial conditions by heating to 300 ºC for 30 min[4] before each measurement.

Aguirre de Cárcer et al.[5] studied the TL at different irradiation temperatures for an easy and reliable measurement of the UV-C (230–280 nm) dose impinging the crystal. The annealed KCl:Eu2+ system contains free Eu2+ ionscation vacant dipoles[6]. The UV-C photons with energy close to the high energy excitation band of Eu2+ in the KCl matrix, and to the material conduction band, are absorbed (4f7 (8S7/2)ĺ4f65d) and a modification of the material results. The system is damaged by UV-C irradiation in such a way that the crystal TL response is linear to the UV-C dose received. In order to develop a portable UV-C dosimeter for field measurements we try to identify a single technology (either heat ramp or light illumination) that helps make easier the maintenance of the experimental measuring conditions and allows the assembly of compact and durable devices to be able to reliably work in ambient environmental conditions. The aim of this work was to determine which technology (OSL or TL) will have better properties for solar UV-C measurements with KCl:Eu2+.

1 Experimental 1.1 Synthesis and characterization In this study, small KCl:Eu2+ single crystals with Eu2+

Foundation item: Project partially supported by MCyT Grant (MAT2005-05950) (Spain) Corresponding author: I. Aguirre de Cárcer (E-mail: [email protected]) DOI: 10.1016/S1002-0721(08)60292-6

580

concentration of about 0.15mol.% (in the melt) were used. The samples were cleaved from a crystal that was grown under a controlled atmosphere (100 torr) of dry argon, at the Crystal Growth Laboratory of the Institute of Physics, Universidad Nacional Autonoma de Mexico, using the Czochralski technique. Before irradiation, samples of about 30 mg were heated up to 500 °C in order to dissolve the rare earth impurity in the KCl matrix and recover initial conditions before UV-C irradiation. Measurements of TL were carried out with an automated Risø TL system model TL DA-12[7]. This reader was fitted with an EMI 9635 QA photomultiplier and the emission was observed through a blue filter (FIB002, Melles-Griot Company) where the wavelength peaks at 320–480 nm; FWHM was (80±16) nm and peak transmittance (minimum) was 60%. Model TL DA-12 was also provided with a 90Sr/90Y beta source with a dose rate of 0.012 Gy/s calibrated against a 60Co photon source in a secondary standards laboratory[8]. All TL measurements were performed using a linear heating rate of 5 °C/s from RT up to 500 °C at a N2 atmosphere. The samples were not powdered to avoid triboluminescence processes[8].

2 Results and discussion 2.1 Optical stimulation retrieval of UV-C flux Before the OSL and TL dose response analysis began, the crystal was optically characterized. The material system emission at 420 nm, corresponding to the presence of Eu2+vacancy dipoles, was registered when the crystal was excited at the divalent europium low energy excitation band (320–380 nm) as well as when the crystal was excited at the higher energy band centered around 260 nm. After UV-C irradiation of the crystal, the excitation in the 450–700 nm range was analyzed fixing the emission at 420 nm. For that emission of the KCl:Eu2+ UV-C pre-irradiated sample, two prominent up-conversion excitations bands are distinguished, centered at 521 nm ( 28,4 a.u. relative intensity) and 686 nm (65,1 a.u. relative intensity) which corresponds to the F (521 nm) centers and FZ (646 nm) centers bands. Their excitation provoked the Eu2+ emission (420 nm) of the UV-C pre-irradiated crystal. After UV-C pre-treatment, it was observed that the intensity of the optically stimulated luminescence at 420 nm by illumination at 521 nm did not loose all its signals after prolonged excitation. In fact, the sample could be further stimulated with light at 521 nm and responded by emitting in the divalent Europium 420 nm broad emission band. A USB-4000 spectrometer by Ocean Optics[2] was used to analyze the time evolution of the Eu2+ with the optical

JOURNAL OF RARE EARTHS, Vol. 27, No. 4, Aug. 2009

stimulation of the KCl:Eu2+ sample. The 420 nm emission was registered several times after optical stimulation with green light (521 nm) in the time range 0–1000 s. It was observed that the time required to measure a significant decay was very large for an environmental sensor. Furthermore, the UV-C dose (sample pre-irradiate with 250 nm radiation for 7, 8, 10 and 30 min) versus 521 nm up-conversion emission showed a poor linear relation. The OSL is produced after UV-C irradiation on a wide band of visible wavelengths with a decay time that varies in several orders of magnitude with the rare earth impurity state of aggregation. An intense photo-stimulated light at about 420 nm has been reported when the UV light irradiated specimen was stimulated with 560 nm light at room temperature[4]. It has been proposed that part of the electrons excited from Eu2+ ions to the conduction band by UV irradiation are trapped at anion vacancies to produce F centers and Eu3+ ions[4] but the optical signal of the trivalent ion was not register in that study. Trivalent europium detection difficulty can be caused by the transitory nature of the europium ion in that oxidized state; the hole can stabilize as a FZ centre (Eu2+-hole) which have a broader absorption band than Eu3+ in alkali halides matrices. The time decay response of our samples were orders of magnitude higher than previously reported[4]. We have ascertained that a single excitation band centred at 521 nm can not photobleach all the traps and regenerate the crystal to its initial measuring conditions. In the experiment reported[4] the photobleaching excitation band is very broad with a summit at 560 nm. Perhaps this broad band results from the addition of clusters of holes trapped at slightly different energies which reorganize when some holes are set free by optical stimulation and recombine at Eu2+ sites, thus avoiding an extinction of the emission. At variances with Nanto et al.[4] our results of the OSL emission of as-grown KCl:Eu2+ show that the pre-treated sample has a long decay (Fig. 1) which resembles a 1/t decay,

Fig. 1 Time evolution of the pre-irradiated UV-C sample, 420 nm emission of Eu2+ when optically stimulated with 521 nm light

I. Aguirre de Cárcer et al., KCl: Eu2+ as a solar UV-C radiation dosimeter. Optically stimulated luminescence and…

a characteristic we judge inappropriate for UV-C dose retrieval. 2.2 A process model for KCl:Eu2+ TL and OSL Melendez et al.[9] have studied the thermo-luminescence glow curves of KCl:Eu2+ after 1 min of UV-C (240 nm) irradiation, showing two glow peaks: an intense TL peak at low temperature (390 K) and a less intense one at higher temperature (470 K)[9]. Chernov et al. after strong irradiation of KCl:Eu2+ with a deuterium lamp (12 h) measured the absorption bands of FZ (which corresponds to the 360 K glow peak) and F centres absorption band (the 470 K TL peak)[10]. The TL signal is strongly reduced when samples are quenched from high temperature[6] which indicates that the electron trapping, and consequently the formation of Eu3+, is connected with the existence of europium precipitates. The proposed damage mechanism[4,6] involves the photoionization of divalent europium creating an electronhole pair. The electron travels by the conduction band until it reaches a minimum of potential energy. The hole is hypothesized to rest close to the UV-C excited divalent europium ion. The system reorganizes its electrical charge density locally by dipole hopping and dipole diffusion[11] until a new equilibrium state is reached which contains information of the UV-C dose absorbed, according to the following model: damage thermal treatment Eu2++ hȖUV-CĺF+Eu3+ or FZ+Eu3+ĺe+Eu3+ĺ (Eu2+)*ĺ Eu2++ hȞlocal environment F centers are electrons trapped at a cation vacancy. Fz are divalent impurity F centers neighbors . The dose related signal is manifested when the captured electrons are liberated by optical stimulation or thermal treatment and recombines with a hole, transferring the e-h recombination energy to a nearby divalent ion, which emits a photon whose energy informs the precipitation ions environment[6] or the crystal precipitation stage. It should be remarked that Eu3+ ions have been detected by absorption measurements only after high dose of X-irradiation[12]. Under ultraviolet irradiation the induced damage is much lower and Eu3+ has not been detected so far, by means of absorption or emission spectra, neither at room temperature nor at liquid nitrogen temperature[9]. After 20 min of 230 nm photon excitation of the KCl: Eu2+ crystal, the Eu3+ 610 nm emission was weakly detectable in the spectrum at 304 nm excitation of the UV-C irradiated sample. In this work we have been able to optically measure the presence of Eu3+ by optical stimulation of the pre UV-C irradiated as grown samples. A Hitachi SPEX Fluorolog F-2500 fluorimeter was used

581

to pre-irradiate the samples during the exposure time by setting the luminescence subprogram on Ȝexcitation at 270 nm which triggers the instruments excitation, without running the data acquisition step. Excitation and emission spectra were obtained after the UV-C sensibilitation step (cycles of UV-C dose and thermal readout are done to the samples until constant values are obtained for the same UV-C dose). Fig. 2 presents the emission of the KCl:Eu2+ crystal after UV-C damage when illuminated with Ȝ=521 nm. The 420 nm broad emission band is ascribed to the Eu2+ (4f65dĺ4f7 (8S7/2) emission from different precipitates[6]. Fig. 3(a) presents the emission of the pre-irradiated UV-C crystal when excited at 304 nm. The narrow and structured band centred at 780 nm emission is ascribed to Eu3+ (intermediate product of the UV-C damage recover mechanism) 5 D1ĺ7F6[13], and the 610 nm light emission attributed to Eu3+ 5D0ĺ7F2[14]. The Eu3+ emissions support the damage model mechanism mentioned before. Fig. 3(b) shows the excitation spectra of the pre-UV-C irradiated KCl:Eu2+ 610 nm emission. The excitation spectrum of the 780 nm emission (not shown) is within the broad OSL excitation band of the crystal[4] that triggers the electron-hole recombination and is also in the trivalent Eu ion luminescence excitation band. 2.3 Environmental radiation dosimeter To explore the viability of the UV-C TL dosimeter we contrasted data from different techniques in a cross validation effort. The clear sky radiation spectral intensity in UV-C was recorded with a spectrophotometer[2] at the same time and location as the KCl:Eu2+ solar exposure (Fig. 4) For environmental measurements crystals were heated to 500 ºC prior to the solar exposure and kept in a sealed dark recipient which also contained hygroscopic silicon gel to avoid

Fig. 2 Emission of the KCl:Eu2+ crystal illuminated with Ȝ=521 nm after UV-C damage (420 nm emission band is ascribed to the Eu2+ ( 4f65d-4f7 (8S7/2))

582

JOURNAL OF RARE EARTHS, Vol. 27, No. 4, Aug. 2009

Fig. 3 Emission of the pre-irradiated UV-C crystal when excited at 304 nm (780 nm emission is ascribed to Eu3+ 5D1-7F6[14] and the 610 nm light emission has been attributed to Eu3+ transition 5D0-7F2[15])(a) and excitation spectra of the UV-C pre-irradiated sample emission at 610 nm (b)

diation source used. On the other hand, this fact is of great interest since it is possible to define a specific UV dosimetry region (at temperatures higher than 200 ºC in the TL signals), scarcely affected by the ionising radiation.

Fig. 4 Scattered solar light irradiance at the TL dose measurement, registered with Ocean Optics Spectrometer USB 4000[2] (Madrid, June 11, 2008 at 06: 48 a.m.)

water condensation. The crystals were exposed to the skylight for several minutes and kept later in a dark receptacle which was labelled and kept in a bucket of ice until the TL readout was done. Another crystal was used as a control corresponding its TL signal to the beta irradiation present in the city air at the time of measurement. The TL spectrum after solar irradiation shows several peaks centred at 110, 190, 350 and 425 ºC, being the more intense TL peak at 350 ºC (Fig. 5). This result reveals that the KCl:Eu2+ crystals can measure small doses of solar UV-C present in the atmospheric scattered field while the sky is not yet illuminated by direct visible radiation. The KCl:Eu2+ UV-C dosimeter can be calibrated comparing the TL response to high energy photons (270 nm) and the TL response under a fixed beta radiation dose. However, we observed (Figs. 5 and 6) that the shape of both TL glow curves are quite different depending on the type of the irra-

Fig. 5 KCl:Eu2+ glow curve after 3 min of sky light exposure (Madrid 40 _ N, 650 m asl, June 11th 06:48 solar time)

Fig. 6 The same crystal sample (as in Fig. 5) glow curve after 60 mGy beta irradiation

I. Aguirre de Cárcer et al., KCl: Eu2+ as a solar UV-C radiation dosimeter. Optically stimulated luminescence and…

3 Conclusion For a portable solar UV-C sensor, TL dose retrieval is preferred rather than the optically stimulated luminescence because of the large luminescence decay time of the latter and the need of heating the sample to reset initial measuring conditions in the optically stimulated luminescence alternative device. We should be able to calibrate the registered UV-C dose by the TL dosimeter with a known flux in the 265–285 nm. The UV-C damage mechanism to KCl:Eu2+ was further elucidated by confirming the presence of Eu3+ in the damaged crystal. A portable solar UV-C dosimeter could be developed with KCl:Eu2+ crystals that registered doses as low as 0.24 ȝW/cm2 of solar UV-C (250–280 nm) using the TL of sensitized crystals. Acknowledgements: We are thankful to Prof. L. Arizmendi and Prof. M.D. Petit (UAM) for the use of their equipment and suggestions.

References: [1] World Meteorological Organization, Global Ozone Research and Monitoring, Scientific Assessment of Ozone Depletion: 1998. NOAA, NASA, UNEP, WMO, EC. Report No. 44. Geneva. [2] D´Antoni H L, Rothschild C, Schultz S, Burgess S, Skiles J W. Extreme environments in the forests of Ushuaia, Argentina. Geophys. Res. Lett., 2007, 39: L22704. [3] Cordoba C, Muñoz J A, Cachorro V, I Aguirre de Carcer, Cussó F, Jaque F. The detection of solar ultraviolet-C radiation using KCl:Eu2+ thermoluminescence dosemeters. J. Phys. D: Appl. Phys., 1997, 30: 3024.

583

[4] Nanto H, Murayama K, Usuda T, Endo F, Hirai Y, Taniguchi S, Takeuchi N. Laser-stimulable transparent KCl:Eu crystals for erasable and rewritable optical memory utilizing photostimulated luminescence. J. Appl. Phys., 1993, 74: 1445. [5] Aguirre de Carcer I., Rowlands A P, Jaque F, Townsend P D. Thermoluminescence of KCl:Eu2+ under ultraviolet irradiation at different temperatures. Radiation Measurements, 1998, 29(2): 203. [6] Aguirre de Cárcer I, Cussó F, Jaque F. Afterglow and photoconductivity in europium doped alkali halides. Phys. Rev. B, 1988, 15: 10812. [7] Bøtter-Jensen L, Duller G A T. A new system for measuring optically stimulated luminescence from quartz samples. Nucl. Tracks Radiat. Meas. Part D., 1992, 20: 549. [8] Correcher V, Delgado A. On the use of natural quartz as transfer dosimeter in retrospective dosimetry. Radiat. Meas., 1998, 29(3-4): 411. [9] Melendrez R, Perez-Salas L P, Pashchenko R, Aceves T M, Piters M, Barboza-Flores M. Dosimetric properties of KCl:Eu2+ under Į, ȕ, Ȗ, x ray, and ultraviolet irradiation. Appl. Phys. Lett., 1996, 68(24). [10] Chernov V, Piters T M, Barboza-Flores M. Behaviour of F and Fz centres under thermal stimulation in KCl:Eu2+ irradiated with ionizing and UV radiation. Radiation Effects & Defects in Solids, 2003, 158: 269. [11] Pooley D, Ruciman W A. Recombination luminescence in alkali halides. J. Phys. C. Solid State Physics, 1990, 3: 1815. [12] Aguilar M, Murrieta Garcia-Sole H, Rubio J. Trivalent europium in X-irradiated NaCl:Eu. Phys. Rev. B, 1982, 26: 4507. [13] Chen X Y, Liu G K. The standard and anomalous crystal-field spectra of Eu3+. Journal of Solid State Chemistry, 2005, 178: 419. [14] Liu Huangqing, Wang Lingling, Chen Shuguang, Zou Bingsuo. Optical properties of nanocrystal and bulk ZrO2: Eu3+. Journal of Alloys and Compounds, 2008, 448: 336.