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Sensors and Materials, Vol. 36, No. 2 (2024) 607–621
MYU Tokyo
S & M 3550

Radio-photoluminescence in Ag-doped Na–Al Borate Glass
Hiroki Kawamoto,* Yutaka Fujimoto, and Keisuke Asai
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,
6-6-07, Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan(Received October 30, 2023; accepted January 22, 2024)

Keywords: radio-photoluminescence, dosimeter, valence change of Ag, clustering of Ag, borate glass

In this paper, we report on the radio-photoluminescence (RPL) properties and RPL center
formation mechanism in Ag-doped Na–Al borate glass (BG:Ag). In BG:Ag, broad emission
bands at 550–800 and 450–800 nm with excitation wavelengths at 320 and 360 nm appeared
after X-ray irradiation, respectively. The electron spin resonance (ESR) signal intensity of Ag2+
and Ag2+ increased with absorption dose. These results suggest that BG:Ag exhibits RPL from
Ag2+ and Ag2+. Furthermore, RPL emission bands and the ESR signals of Ag2+ and Ag2+
disappeared after heating at 773 K for 1 h. These results indicate that RPL centers can beeliminated by heating and that BG:Ag is a candidate reusable RPL material. On the basis of thevariation of the ESR spectra with time from immediately after irradiation, we assumed thefollowing RPL center formation mechanism. First, electron–hole pairs are generated by ionizing
radiation. Some of the electrons are trapped at Ag+ and form Ag0. Subsequently, Ag2+ ions areformed by the association of Ag+ and Ag0. On the other hand, holes are trapped at the boronoxygen network to form a boron oxygen hole center (BOHC). Subsequently, the trapped holestransfer to Ag+ from BOHC and form Ag2+.

1.

Introduction

Radio-photoluminescence (RPL) (1,2) is an emission phenomenon induced by thephotoexcitation of luminescence centers (RPL centers) formed by irradiation with ionizing
radiation, such as X-rays or gamma rays. Because the RPL intensity increases with absorptiondose, RPL is applied to luminescent-type dosimeters. Similar phenomena applied in luminescenttype dosimeters include thermoluminescence(3,4) (TL) and optically stimulated luminescence(OSL).(5) In TL and OSL, electrons and holes formed by ionizing radiation are trapped separately
at different sites, and the number of trapped electrons and holes corresponds to the amount of
cumulative dose. Subsequently, trapped electrons and holes recombine to induce TL or OSL
after excitation by heat or light, respectively.(6,7) Trapped electrons and holes, in other words,
information of cumulative dose, are lost by reading the absorption dose. In contrast, in an RPLprocess, electrons and holes formed by ionizing radiation are confined at trapping centers toform RPL centers. In RPL, the number of RPL centers corresponds to the amount of cumulative*Corresponding author: e-mail: hiroki.kawamoto.c7@tohoku.ac.jp

https://doi.org/10.18494/SAM4766

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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dose. RPL is caused by the radiative transition of RPL centers by photoexcitation. In otherwords, an RPL process does not involve the recombination of electron–hole pairs formed byionizing radiation, and the number of RPL centers does not change after emission. In addition,
RPL has some unique characteristics such as high stability of RPL centers at room temperatureand restoration of initial state by heating. Because of these characteristics, the absorption dosecan be read and measured repeatedly using RPL dosimeters. Hence, RPL dosimeters are widelyused in applications such as personal use,(8) environmental monitoring,(9–11) medical
dosimetry,(12) and dose imaging.(13–16)
Previous researchers have reported some RPL materials(17) such as Ag-doped phosphateglass,(18) Ag-doped alkali halide,(19) Ag-doped SrF2,(20) Sm-doped CsBr,(21) Sm-doped CaSO4,(22)Eu-doped CaF2,(23) Eu-doped CaSO4,(24) and nondoped CaF2.(25) Among these materials, Agdoped phosphate glass is the most famous RPL material because this glass has been used inpersonal dosimeters sold as “glass badges” since the 1960s.(26) In Ag-doped phosphate glasses,
Ag0, Ag2+, and Ag2+ are attributed to be RPL centers.(26–28) In addition, recently, high-ordermolecular silver clusters Agmn+ formed in the high dose range have also been attributed to be
RPL centers.(29) According to previous reports,(26–30) the RPL center formation mechanism is asfollows. First, electrons and holes are generated by ionizing radiation. Some of the electrons aretrapped at Ag+ and form Ag0.
Ag+ + e− → Ag0(1)

Subsequently, Ag2+ ions are formed by the association of Ag+ and Ag0 via the diffusion of Ag+.
Ag+ + Ag0 → Ag2+

(2)

On the other hand, holes are trapped at PO43− tetrahedra and form a phosphorous-oxygen-hole
center (POHC).
PO34− + h + → POHC

(3)

Subsequently, holes trapped at POHC transfer to Ag+ and form Ag2+.
Ag + + POHC → Ag 2+ + PO34−(4)

The processes of Ag2+ (2) and Ag2+ (3) formation proceed with time at room temperature. As
a result, the RPL intensity increases with time at room temperature. This effect is known as“build-up”.(28,30,31) In addition, Ag0, Ag2+, and Ag2+ can be eliminated by heating at 673 K for 1h.(26,32) Therefore, Ag-doped phosphate glasses have been used in reusable personal dosimeters.
Although some RPL materials have been reported, the development of new RPL materials with
RPL properties superior to those of Ag-doped phosphate glasses has not been achieved. To
improve RPL materials, new RPL materials should be developed.

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Considering the example of the Ag-doped phosphate glasses, materials wherein the silver
valence changes and cluster formation occurs when energy is applied would be candidates of
new Ag-doped RPL materials. In this study, we focused on Ag-doped Na–Al borate glass(BG:Ag) as such a material. It was previously reported(33) that luminescent silver clusters (Ag3+)
were formed by irradiation with a femtosecond laser. In addition, the silver clusters were deletedby heating at 773 K for 1 h. Therefore, if a valence change or cluster formation of silver is
induced by ionizing radiation, BG:Ag exhibits RPL, and BG:Ag is a candidate reusable RPL
material. In this study, we aimed to develop a new Ag-doped RPL material and investigated the
RPL properties and RPL center formation mechanism in BG:Ag.

2.

Materials and Methods(100 − x)(30Na2O–5Al2O3–65B2O5)–xAg2O (x = 0 and 1) glass samples were synthesized by themelt-quenching method in air. The raw materials Na2CO3 (99.9%; Rare Metallic Co. Ltd.), Al2O3(99.99%; High Purity Chemicals Co., Ltd.), H3BO3 (99.99%; High Purity Chemicals Co., Ltd.),
and Ag2O (99.0%; Wako Pure Chemical Industries Ltd.) were mixed, loaded into an alumina
crucible, and melted at 1373 K in an electric furnace (Barnsted International, 1300 Furnacemodel FB1314M). The liquid melt was poured onto a stainless-steel plate heated to 573 K andthen the liquid melt was cooled to room temperature in air.
The glass samples were irradiated using an X-ray generator (Rigaku, RINT 2200) with a Cu
X-ray tube operated at 40 mA and 40 kV. The samples were irradiated to 1–1000 Gy at theposition where the dose rate was measured to be 1, 100, and 500 Gy/min using a reference
dosimeter (RAMTEC Light, Toyo Medic).
First, to investigate the RPL properties, UV–Vis absorption, photoluminescence (PL), andelectron spin resonance (ESR) spectra were measured at each dose. UV-Vis absorption spectra in
the range of 200–800 nm were measured using a spectrometer (UV-2700, SHIMADZU). The
excitation–emission mappings were measured in the unirradiated and 1-kGy-irradiated samples
with an L42 optical filter (HOYA) in the excitation and emission wavelength ranges of 240–400
and 400–800 nm, respectively. In addition, the PL spectra with the excitation wavelengths of
320, 360, and 560 nm were measured with a spectrometer (F-7000, Hitachi). The ESR spectra
were measured with a JES-X330 spectrometer (JEOL). Second, the variations of the PL and ESR
spectra over time at ambient temperature (~298 K) were measured for 1 and 2 h immediately
after 1 kGy of irradiation (500 Gy/min, 2 min) to investigate the RPL center formation
mechanism, respectively. Finally, we measured the heating temperature dependences of PL and
ESR spectra of BG:Ag after 1 kGy irradiation of X-rays (100 Gy/min, 10 min) to investigate theeffect of heating on RPL properties. The irradiated samples were heated at 298, 373, 473, 573,
673, and 773 K with an electric furnace for 1 h. In addition, to investigate reusability, the PL
spectra were measured in BG:Ag subjected to two cycles of heating and irradiation.

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Sensors and Materials, Vol. 36, No. 2 (2024)

Results and Discussion

Figure 1 shows the absorption spectra and absorption difference spectra of the samples. In
the nondoped borate glass sample (BG), the absorption band at 500–700 nm with the peak at 560
nm appeared after irradiation of ionizing radiation. On the basis of a previous report,(34) theabsorption band with the peak at 560 nm is attributed to the boron oxygen hole center (BOHC).
These results suggest that BOHC was formed by ionizing radiation in BG. On the other hand, abroad absorption band composed of two or more bands at 260–550 nm appeared in BG:Ag. In
BG, no significant change in absorption at 260–500 nm was observed. Therefore, the broad
absorption band at 260–500 nm can be attributed to Ag species. Although the optical absorption
bands of Ag species have been investigated for a long time, the origin of absorption bands ofsilver species in various glasses is difficult and has not yet been established. According to aprevious report,(35) absorption bands at 270, 310, and 340 nm are attributed to Ag2+, Ag2+, andAg0 in a Ag-doped Na–Al phosphate glass, respectively. Other researchers reported that theabsorption band at 320 nm is attributed to Ag2+ or Ag32+, and the band at 370 nm is attributed toFig. 1.

(a)

(b)

(c)

(d)

(Color online) Absorption spectra and absorption difference spectra in (a), (b) BG and (c), (d) BG:Ag.

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Ag0 in Ag-doped Zn–Ga phosphate glass.(36) Although the origins of the absorption bands of Ag
species have not yet been established, previous researchers reported the appearance of newabsorption bands due to valence change and the clustering of Ag. These results indicate thatvalence change and the clustering of Ag were induced by ionizing radiation in BG:Ag.
Figure 2 shows the excitation–emission mapping of the difference in the luminescenceintensity between unirradiated and 1-kGy-irradiated BG:Ag. Figure 2 also shows that a broad
emission band at 450–800 nm was generated after the irradiation of ionizing radiation with theexcitation wavelength range of 300–390 nm. The broad emission band seemed to have peaks at
the excitation wavelengths of 320 and 360 nm. Therefore, we determined the excitation
wavelengths to be 320 and 360 nm in BG:Ag.
Because the absorption band peak at 560 nm was generated after the irradiation of ionizing
radiation in BG, emission spectra with an excitation wavelength of 560 nm were measured inaddition to the excitation wavelengths of 320 and 360 nm. Figure 3 shows the emission spectra of
BG. In all the spectra, no new emission band was observed after irradiation with X-rays. This
result indicates that BG does not exhibit RPL.
Figure 4 shows the emission spectra of BG:Ag. New broad emission bands with the peak at
approximately 650 nm appeared in both spectra after the irradiation of X-rays. According to aprevious report,(28) in the case of Ag-doped phosphate glasses, the emission at 650 nm is
attributed to Ag2+ and Ag2+. These results suggest that BG:Ag exhibits RPL from Ag2+ andAg2+.
Figure 5 shows the dose dependence of the RPL intensity. With both excitation wavelengths,
the RPL intensity increased linearly in the dose range of 1–300 Gy and became slightly saturated
with a dose higher than 500 Gy. This saturation of PL intensity in the high dose range isconsidered as concentration quenching. On the basis of the 3σ method, detection limits with theexcitation wavelengths of 320 and 360 nm were estimated to be approximately 20 and 5 Gy,

Fig. 2. (Color online) Excitation–emission mapping of the difference in luminescence intensity between
unirradiated and 1-kGy-irradiated BG:Ag.

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(a)

(b)

(c)
Fig. 3.

(Color online) Emission spectra with excitation wavelengths of (a) 320, (b) 360, and (c) 560 nm in BG.

respectively. These results indicate that BG:Ag can be a candidate RPL dosimeter in the dose
range of 5–300 Gy with the excitation wavelength of 360 nm.
Figure 6 shows the ESR spectra of the glass samples. In both glasses, the four-split signal wasobserved at 335 mT after irradiation. According to previous reports,(34,37) this signal is attributedto BOHC. In the PL measurement result, BG did not exhibit RPL. These results indicate that
BOHC does not act as an RPL center. In BG:Ag, the ESR signals were observed at approximately
310, 325, 335, and 370 mT after irradiation. The four-split signal at 335 mT is the same as the
ESR signal observed in BG. Hence, the ESR signal of BOHC was generated by irradiation alsoin BG:Ag. On the basis of previous reports,(28,32) the ESR signals at 310 and 370 mT are
attributed to Ag2+. On the other hand, the ESR signal at 325 mT is attributed to Ag2+. These
results indicate that in BOHC, Ag2+ and Ag2+ were formed by ionizing radiation in BG:Ag. As
mentioned above, BOHC does not act as an RPL center. Therefore, Ag2+ and Ag2+ can be
attributed to RPL centers in BG:Ag. Figure 7 shows the dose dependence of the ESR signal

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(a)

(b)
Fig. 4.

(Color online) Emission spectra with excitation wavelengths of (a) 320 and (b) 360 nm in BG:Ag.

(a)

(b)

Fig. 5. (Color online) Dose dependences of RPL intensity with excitations at (a) 320 and (b) 360 nm. The dottedline indicates the 3σ value.

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Sensors and Materials, Vol. 36, No. 2 (2024)(a)Fig. 6.

(a)Fig. 7.

(b)(Color online) ESR spectra in (a) BG and (b) BG:Ag.

(b)

(c)

(Color online) Dose dependences of ESR signals of (a) BOHC, (b) Ag2+, and (c) Ag2+ in BG:Ag.

intensity. All of the ESR signal intensities corresponding to the formation yields of BOHC, Ag2+,
and Ag 2+ increased linearly in the dose range of 1–1000 Gy. This result supports theconcentration quenching of PL intensity in the high dose range.
Figure 8 shows the variation of the RPL spectra with time. In both spectra, the intensity at
400–800 nm increased with time. This increase in the RPL intensity is known as “build-up”, asmentioned in Sect. 1. These results indicate that the RPL center formation reaction proceededwith time at room temperature.
Figure 9 shows the time dependences of the RPL intensities at 650 nm. In the case of theexcitation wavelength of 320 nm, the RPL intensity increased for 24 min and almost saturated
after 26 min. On the other hand, in the case of the excitation wavelength of 360 nm, the RPL
intensity increased continuously for 58 min. These results suggest that the different RPL centerswere excited with different excitation wavelengths. In addition, the formation of the RPL center
excited at 320 nm was completed in 24 min at room temperature.
Figure 10 shows the variations of the ESR spectra and ESR signal intensity with time. Shortly
after irradiation, the doublet ESR signals at 300 and 370 mT and the four-split ESR signal of
BOHC were observed. On the basis of a previous report,(38) the doublet ESR signals are
attributed to Ag0. The doublet ESR signals of Ag0 became broad ESR signals of Ag2+. As shownin Figs. 10(b) and 10(c), the ESR signal intensity of BOHC decreased and the ESR signal

Sensors and Materials, Vol. 36, No. 2 (2024)

615

(a)
Fig. 8.

(b)(Color online) Variations in RPL spectra over time with excitations at (a) 320 and (b) 360 nm.

(a)Fig. 9.

(b)(Color online) Time dependences of RPL intensity with excitations at (a) 320 and (b) 360 nm.

intensity of Ag2+ increased with time. These results indicate that Ag2+ and Ag2+ formation
proceeded with time at room temperature. On the basis of the results of the variation of ESR
spectra, we discuss the RPL center formation mechanism in BG:Ag. As mentioned above, the
ESR signal intensities of BOHC and Ag2+ decreased and increased with time, respectively. In
addition, the doublet ESR signal of Ag0 changed into the broad ESR signal of Ag2+. On the basisof these results, we assumed the following RPL center formation mechanism. First, electron–
hole pairs are formed by ionizing radiation. Electrons are trapped at Ag+ and form Ag0.
Ag+ + e− → Ag0

(1')

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(a)

(b)

(c)

Fig. 10. (Color online) Variation of (a) ESR spectra and ESR signal intensities of (b) BOHC and (c) Ag2+ over time.

Subsequently, Ag2+ ions are formed by the association of Ag+ and Ag0.
Ag+ + Ag0 → Ag2+

(2')

On the other hand, holes are trapped at the boron-oxide network and form BOHC.
BO3 or BO4 + h+ → BOHC

(3')

Subsequently, holes trapped at BOHC transfer to Ag+ and Ag2+.
Ag+ + h+ → Ag2+

(4')

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Figure 11 shows the temperature dependences of the PL spectra and PL intensity of 1-kGyirradiated BG:Ag. At both excitation wavelengths, the PL intensity at 450–750 nm increased anddecreased upon heating at 298–573 K and 573–773 K, respectively. After heating at 773 K, thePL intensity decreased to the same level as that of the unirradiated sample. These results
revealed that the minimum temperature to eliminate RPL centers was 773 K.
Figure 12 shows the temperature dependences of the ESR spectra and ESR signal intensitiesof the 1-kGy-irradiated sample. The ESR signal intensities at 310, 370, and 335 mT, which are
attributed to Ag2+ and Ag2+, were decreased with increasing heating temperature. The decreasein the four-split ESR signal intensity of BOHC was observed in the temperature range of 298–
473 K. Then, the singlet signal was clearly observed after heating at 573 K. The singlet signal
intensity at 340 mT decreased after heating at 673–773 K. Finally, the ESR spectra became thesame as that of the unirradiated sample after heating at 773 K. This result indicates thatparamagnetic species generated by ionizing radiation were completely eliminated by heating at
773 K. To determine the origin of the singlet ESR signal, we focused on the hyperfine splitting

(a)

(b)

(c)

(d)

Fig. 11. (Color online) Heating temperature dependence of PL spectra and PL intensity with excitation wavelengthsof (a), (c) 320 and (b), (d) 360 nm. The dotted line indicates the PL intensity of the unirradiated sample.

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(a)

(b)

(c)

Fig. 12. (Color online) Heating temperature dependences of (a) ESR spectra and ESR signal intensities of (b) Ag2+
and (c) Ag2+. The dotted lines indicate the ESR signal intensity of the unirradiated sample.

(hfs) of the ESR signal. Hfs is defined as splits in the energy levels caused by the interaction of
the nuclear spin with the electron spin. If I is the nuclear spin of the nuclei interacting withelectrons or holes, the number of hyperfine splittings n of the ESR signal can be expressed asn = 2 * I + 1. Hence, the singlet signal can be attributed to electron or hole nuclei with the nuclear
spin of 0. In BG:Ag, the only nucleus with a nuclear spin of 0 is that of 16O. Therefore, the singlet
signal that appeared after heating at 573 K can be attributed to electrons or holes trapped at
O-related centers. Figure 12(b) shows that the ESR signal intensity of Ag2+ monotonicallydecreased to the same level as that of the unirradiated sample upon heating at 298–573 K. In thetemperature range of 573–773 K, no significant variation of the ESR signal intensity of Ag2+ was
observed. These results indicate that Ag2+ was eliminated completely by heating at 573 K.
Figure 12(c) shows that the ESR signal intensity of Ag2+ was increased by heating at 373 K. As

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mentioned above, the ESR signal of BOHC decayed in the temperature range of 298–473 K.
These results indicate that the Ag2+ formation reaction (4) was promoted by heating at 373 K.
Subsequently, the Ag2+ signal intensity was decreased to almost the same level as that of theunirradiated sample by heating at 473–573 K. In 573–773 K, no significant variation of the ESR
signal intensity of Ag2+ was observed. These results showed that Ag2+ was completely
eliminated by heating at 773 K.
We now discuss the difference in the heating temperature dependences of PL and ESR signal
intensities. PL intensities increased in the temperature range of 298–373 K. On the other hand,the ESR signal intensities of Ag2+ and Ag2+ decreased in the temperature range of 373–573 K. If
the RPL centers in BG:Ag are only Ag2+ and Ag2+, the PL intensities should decrease after
heating at 373–573K. However, the results were not so. The temperature dependence of the PL
spectra shows that the emission peak wavelength seemed to shift to the short wavelength region.
This result suggests that other luminescence centers were formed by heating after irradiation. In
the ESR spectra, the singlet ESR signal of electrons or holes trapped at oxygen-related centers
appeared after heating at 573 K. Hence, the oxygen-related paramagnetic species may act as theluminescence center. In addition, there is the possibility of the formation of new RPL centers
without any unpaired electrons that are ESR-undetectable species. According to a previous
study,(33) Ag3+ clusters are formed by irradiating a femtosecond laser into glass with the same
composition as that of the glass sample used in this study. Furthermore, the formation of muchlarger Ag clusters in glass irradiated with ionizing radiation has been reported. (39,40) Therefore,
other luminescent Ag clusters undetectable by ESR might be formed by heating at 373–573 K
after X-ray irradiation and function as RPL centers.
Figure 13 shows the RPL and ESR spectra of BG:Ag after heating. In Fig. 13, the RPL
emission band was repeatedly eliminated by heating and generated by irradiation. This result
indicates that BG:Ag is a candidate reusable RPL material.

(a)

(b)

Fig. 13. (Color online) RPL spectra with excitations at (a) 320 and (b) 360 nm in BG:Ag after heating at 773 K for 1
h following irradiation. The unirradiated and 1-kGy-irradiated sample after n times of repeated heating is expressed
as n_0 Gy and n_1 kGy in the legend, respectively.

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Sensors and Materials, Vol. 36, No. 2 (2024)

Conclusions

In this study, we investigated the RPL properties and RPL center formation mechanism in
Ag-doped Na–Al borate glass. Broad emission bands with the peak at 650 nm appeared after theirradiation of X-rays. Therefore, it was revealed that BG:Ag exhibits RPL. On the basis of the
ESR measurements, Ag2+ and Ag2+ were attributed to the RPL centers in BG:Ag. In addition,these RPL centers could be deleted by heating at 773 K for 1 h and repeatedly regenerated by
X-ray irradiation after being eliminated. These results indicate that BG:Ag is a candidate
reusable RPL material. Finally, we elucidated the RPL center formation mechanism in BG:Ag on
the basis of the variation of the ESR spectra with time immediately after irradiation. The
proposed RPL center formation mechanism is as follows. First, electron–hole pairs are generated
by ionizing radiation. Some of the electrons are trapped at Ag+ and form Ag0. Subsequently,
Ag2+ ions are formed by the association of Ag+ and Ag0. On the other hand, holes are trapped at
the boron-oxide network to form BOHC. Subsequently, the trapped holes transfer to Ag+ from
BOHC and form Ag2+.

Acknowledgments
This work was supported by a Grant-in Aid for Scientific Research (A) (Grant No. 22H00308,
2022–2026) and a Grant-in-Aid for Scientific Research (B) (Grant No. 21H01853, 2021–2024)
funded by the Japan Society for the Promotion of Science, the Kazuchika Okura Memorial
Foundation 2022 research grant, and the Nakatani Foundation for the Advancement of
Measuring Technologies in Biomedical Engineering 2022 General Research grant.

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