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Sensors and Materials, Vol. 30, No. 7 (2018) 1585–1590
MYU Tokyo
S & M 1614

Scintillation Characteristics of Pr:CaF2 Crystals
for Charged-particle Detection
Noriaki Kawaguchi,1* Hiromi Kimura,1 Masaki Akatsuka,1 Go Okada,1Naoki Kawano,2 Kentaro Fukuda,3 and Takayuki Yanagida1
1Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan
2Akita University, 1-1 Tegata Gakuen-machi, Akita 010-8502, Japan
3Tokuyama Corporation, 1-1 Harumi-cho, Shunan, Yamaguchi 745-0024, Japan(Received January 22, 2018; accepted June 8, 2018)

Keywords: scintillator, luminescence, scintillation, single crystal, fluoride

The scintillation properties of Pr:CaF2 were studied in comparison with those of Eu:CaF2
under α-ray excitation using a sealed source of 241Am. The relative light yields of the Pr:CaF2crystals were approximately 0.6–13% of the Eu:CaF2 crystal that is known as a conventional
scintillator for charged-particle detection. The scintillation decay times of the Pr:CaF2 crystals
were significantly faster than that of the Eu:CaF2 crystal. Therefore, the Pr:CaF2 crystals canbe used for charged-particle measurements with high count rates.

1.

Introduction

Some phosphors can be used for measurements of ionizing radiation such as X-rays,γ-rays, neutrons, and charged particles. Such phosphors convert ionizing radiation to visible
light and are coupled with a conventional photodetector. Inorganic phosphors for radiation
measurements can be classified as either scintillators(1) or storage phosphors. Storage phosphors[also known as thermoluminescence,(2) optically stimulated luminescence,(3–6) and radiophotoluminescence(7–11)] are used to record information on radiation dose whereas scintillators
immediately convert ionizing radiation to visible light.(12,13) Among these materials,scintillators are particularly suitable for online measurements.
The required characteristics of the scintillator are a high light yield, a fast decay time, and ahigh discrimination capability. The light yield affects the energy resolution of the scintillation
detector, and it is required by some applications. The fast decay time is required because
it allows fast responses of scintillation detectors. To discriminate ionizing radiations, theeffective atomic number (Zeff) of the scintillator is important. Scintillators for γ-ray detection
are required to consist of heavy elements that efficiently interact with γ-rays. In contrast,
scintillators for thermal neutron and charged-particle detection are required to consist of lightelements in order to decrease the rate of erroneous detection due to the interaction with γ-rays.
Additionally, thermal neutron scintillators have to contain elements with high cross sections*

Corresponding author: e-mail: n-kawaguchi@ms.naist.jp
http://dx.doi.org/10.18494/SAM.2018.1926
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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for the (n, α) and (n, γ) nuclear reactions (e.g., 6Li or 10B). For charged-particle detection, themost common materials are Ag:ZnS and Eu:CaF2 that contain relatively insensitive elements
for both γ-rays and thermal neutrons. The conventional scintillators for γ-ray and thermal
neutron detection were reviewed by van Eijk,(12) who comprehensively covered, for example,
Tl:NaI, Tl:CsI, Bi4Ge3O12, Ce:Gd2SiO5, and Ce:Lu2SiO5 as γ-ray scintillators, and LiF/Ag:ZnS,
Eu:LiI, and GS20 (Ce-doped lithium silicate glass) as thermal neutron scintillators. The γ-rayscintillators have high marketability owing to their medical and security applications; therefore,
many materials were developed. The Ce:LaBr3(14) and SrI2(15) scintillators, which show
extremely high light yields, are recent attractive materials because the energy resolution of theγ-ray detector is important for its applications. In the case of the thermal neutron scintillator,
one of the most common materials is the GS20 glass owing to its non-hygroscopicity andacceptable light yield (~6000 photons/neutron). In addition, some new products are available,
for example, the Ce:Cs2LiYCl6 single crystal(16) that is hygroscopic but shows high lightyield and excellent discrimination capability. Another example is the Eu:LiCaAlF6 single
crystal,(17–21) which shows non-hygroscopicity and higher light yield (typically ~20000 photons/neutron) than GS20. As mentioned above, whereas many γ-ray and thermal neutron scintillators
have been developed, relatively few studies were performed for charged-particle scintillators,
and thus we are interested in the further study of charged-particle scintillators.
Transition-metal-ion-doped CaF2 crystals are used for the charged-particle scintillators andthermoluminescent materials for radiation dosimetry. Among them, Eu:CaF2 is well known as
a commercial scintillator for charged-particle detection. In the α-ray or β-ray measurements (e.g.,for dust monitor), erroneous signals by background γ-rays must be suppressed, and in that sense,
CaF2 is attractive as a host crystal. The advantage of the Eu:CaF2 crystal is its high light yield,
but on the other hand, the disadvantage is its slow decay time. The α/β ratio of the scintillator isalso important for charged particle detection. Fluoride materials tend to show a lower α/β ratio
than other inorganic materials.(16) Therefore, the Eu:CaF2 crystal is more suitable for β-ray
detection than α-ray detection. Other transition-metal-ion-doped CaF2 scintillators have notbeen studied extensively for α-ray detection, although many dopants (for example, Tm:CaF2,
Dy:CaF2, and Mn:CaF2) are studied for radiation dosimetry. The X-ray-induced luminescenceand photoluminescence properties of other transition-metal-ion-doped CaF2 have been studied,and it is known that excited Ce3+, Pr3+, and Nd3+ ions can emit fast luminescence with the 5d–4ftransition.(22–24) The vacuum ultraviolet (VUV) luminescence of Nd3+ is the fastest among
these ions, but typical photodetectors cannot detect VUV light. Luminescence with the 5d–4ftransition of Pr3+ ions is faster than that with the 5d–4f transition of Ce3+ ions and acceptablewavelength. In this study, we have investigated the α-ray response of Pr:CaF2 crystals.

2.

Materials and Methods

The Pr:CaF2 and Eu:CaF2 crystals were prepared by the following method. Starting
materials were prepared from a stoichiometric mixture of CaF2 and EuF3 or PrF3 powders(purity < 99.9%) produced by Stella Chemifa Corporation. They were mixed and put into
graphite crucibles. The crucibles set in the stainless used steel (SUS) chamber are enclosed by

Sensors and Materials, Vol. 30, No. 7 (2018)1587

a carbon heater. The chamber was evacuated up to 10−4 Pa. After baking, the chamber wasfilled with high-purity Ar gas (99.99%) and CF4 gas (99.99%) until ambient pressure. Then, thecrucible was heated up to 1450 ℃ and kept at this temperature for 30 min and cooled to room
temperature. The obtained crystals were cut and polished.
We measured the X-ray-induced emission spectra and transmittance spectra using aspectrometer equipped with a nitrogen-purged sample chamber, a deuterium lamp as a lightsource, and an X-ray tube as an X-ray source. We also evaluated scintillation decay curves
and pulse height spectra under α-ray excitation from a sealed source of 241Am by using aphotomultiplier tube (PMT; Hamamatsu, R7600U-200) that is sensitive across the UV andvisible wavelength range. Scintillation decay curves were obtained using an oscilloscope(Tektronix, TDS3052C). The obtained decay curves were fitted byI(t) = A0 + A1 exp(−t/τ1) + A2 exp(−t/τ 2),(1)where I(t) is the luminescence intensity as a function of time t, τ1 and τ 2 are the scintillation
decay times, and A0, A1, and A2 are constants.
Output signals from the PMT were analyzed using a preamplifier (ORTEC, 113 Scintillation
Preamplifier), a shaping amplifier (ORTEC, 672 Spectroscopy Amplifier), and a multichannel
analyzer (Amptek, MCA8000A) for the pulse height spectra.

3.

Results and Discussion

Figure 1 shows aspects of the cut and polished Pr:CaF2 crystals. All crystal pieces containno visible cracks and opaque phases. Figure 2 shows the transmittance spectra of the samples.
The transmittances at the wavelength of 230–370 nm were more than 80%. Two absorption
bands are observed.  These bands can be ascribed to the 4f → 5d transition of Pr3+.(25)
Figure 3 shows the X-ray-induced spectra of the Pr:CaF2 and Eu:CaF2 crystals. Emission
peaks with the 5d–4f transition of Pr3+ or Eu2+ ions are observed, and broad peaks are alsoobserved at around 280 nm when the dopant concentrations are low. These broad peaks

Transmittance [％]

100
80
60

0.1％Pr:CaF2
1％Pr:CaF2
5％Pr:CaF2
10％Pr:CaF2
15％Pr:CaF2
20％Pr:CaF2

40200150200250

300350

Wavelength [nm]Fig. 1. (Color online) Cut and polished Pr:CaF2crystals.

Fig. 2. (Color online) Transmittance spectra ofPr:CaF2 crystals.

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2+Intensity [arb. unit]

10000

Eu 5d-4f transition

8000
6000
4000
2000
0200

CaF2 STE
300

400(a)
Fig. 3.

500(b)(Color online) Scintillation spectra of (a) Eu:CaF2 and (b) Pr:CaF2 crystals under α-ray irradiation.

indicate the intrinsic luminescence of CaF2 originating from the self-trapped exciton (STE),
and these STE emissions are quenched as the Pr3+ concentration increased. The emission
wavelength of Pr3+ ions is overlapped with that of the STE emission of CaF2. The overlapping
emission is clearly observed in the case that the Pr3+ concentrations are 0.1 and 1%. Emission
peaks with the 4f–4f transition of Pr3+ions are also observed from the X-ray-induced spectra ofthe Pr:CaF2 samples in the wavelength range between 450 and 550 nm. The intensities of theseemission peaks are decreased with decreasing Pr3+ concentration, and it can be explained by theconcentration quenching.
Figure 4 shows the scintillation decay curves of the Pr:CaF2 and Eu:CaF2 crystals under
α-ray excitation from the sealed source of 241Am. These decay curves can be fitted by the sumof two exponentials and a constant. The obtained scintillation decay times of the Pr:CaF2 andEu:CaF2 crystals are shown in Table 1. The scintillation decay curves of the Pr:CaF2 crystals
contain both fast and slow components because these are mixed luminescences that originatedfrom the faster 5d–4f transition of Pr3+ ions and the slower 4f–4f transition of Pr3+ ions orthe STE of CaF2. The scintillation decay times of the 5, 10, and 20% Pr:CaF2 crystals are
significantly shorter than that of the Eu:CaF2 crystal.
Figure 5 shows pulse height spectra of the Pr:CaF2 crystals under α-ray excitation. Fullenergy deposited peaks were obtained by all the samples. Their pulse heights decreased withincreasing Pr3+ concentration. Figure 6 shows the relative light yields of the Pr:CaF2 crystals
calculated from the peak channel numbers of the pulse height spectra of the Pr:CaF2 andEu:CaF2 under α-ray excitation. Here, we defined the relative light yield of Eu:CaF2 as 1. As
a result, the relative light yields of the Pr:CaF2 crystals were around 0.6–13% of that of theEu:CaF2 crystal. Pr:CaF2 samples with higher dopant concentration show lower light yields
because of the concentration quenching of luminescence with the 5d–4f transitions of Pr3+ ions.
In summary, the scintillation decay times of the 5, 10, and 20% Pr:CaF2 crystals weresignificantly faster than that of the Eu:CaF2 crystal. From pulse height spectra under α-rayexcitation, full-energy deposited peaks were obtained by all the Pr:CaF2 crystals. Therefore,
the Pr:CaF2 crystals can be used for charged-particle measurements with high count rates.

Sensors and Materials, Vol. 30, No. 7 (2018)

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Intensity [arb. unit]

0.1％Eu:CaF2
0.1％Pr:CaF21％Pr:CaF25％Pr:CaF210％Pr:CaF220％Pr:CaF20Fig. 4.

500

1000
Time [ns]1500

Scintillation decay curves of Eu:CaF2 and Pr:CaF2 crystals under α-ray irradiation.

counts102100.1％Pr:CaF21％Pr:CaF25％Pr:CaF210％Pr:CaF2
20％Pr:CaF2

110001000

2000 3000 4000
channel number5000

Fig. 5. (Color online) Pulse height spect ra ofPr:CaF2 crystals under α-ray irradiation.

4.

Relative light yield [arb. unit]

Table 1
Scintillation decay times of Eu:CaF2 and Pr:CaF2 crystals under α-ray irradiation.
Scintillation decay times (ns)τ2
τ10.1% Eu:CaF2
140.0
391.2
36.9
536.8
0.1% Pr:CaF2
30.9
455.2
1% Pr:CaF2
14.8
284.7
5% Pr:CaF2
  6.2
158.2
10% Pr:CaF2
  7.5
138.2
20% Pr:CaF2
0.150.10.05

005101520Pr concentration [％]Fig. 6. (C olor on l i ne) D op a nt c onc e nt r at ion
dependence of relative light yields (Eu:CaF2 = 1) of
Pr:CaF2 crystals under α-ray irradiation.

Conclusions

We have successfully obtained transparent and noncracked crystals of Pr:CaF2. The
transmittances of the Pr:CaF2 crystals were more than 80% at the wavelength of 230–370 nm.
The Pr3+ concentration dependence of X-ray-induced luminescence was investigated. We haveconfirmed that the STE emission of CaF2 was quenched with increasing Pr3+ concentration.
The scintillation decay times of the 5, 10, and 20% Pr:CaF2 crystals were shorter than that ofEu:CaF2. The scintillation decay times of high-Pr3+-concentration samples were shorter thanthose of the other samples. The pulse height spectra of the Pr:CaF2 crystals were measured andfull-energy deposited peaks were obtained. The relative light yields of the Pr:CaF2 crystals
were around 0.6–13% of the Eu:CaF2 crystal.

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Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research (A) (17H01375) and a
Grant-in-Aid for Early-Career Scientists (18K14158) from the Ministry of Education, Culture,Sports, Science and Technology of the Japanese government (MEXT), as well as by the A-STEPand Matching Planner Program of the Japan Science and Technology Agency (JST). The
Cooperative Research Project of the Research Institute of Electronics, Shizuoka University,
TEPCO Memorial Foundation, Murata Science Foundation, Izumi Science and Technology
Foundation, Iwatani Naoji Foundation, Kazuchika Okura Memorial Foundation, Terumo
Foundation for Life Sciences and Arts, SEI Group CSR Foundation, and Taisei Foundation arealso acknowledged.

References
12345678910111213141516171819202122232425

G. F. Knoll: Radiation Detection and Measurement (Wiley, New York, 2010) 4th ed.
S. W. S. McKeever: Thermoluminescence of Solids (Cambridge University Press, Cambridge, 1985).
E. G. Yukihara and S. W. S. McKeever: Optically Stimulated Luminescence (Wiley, Chichester, 2011).
S. W. S. McKeever: Nucl. Instrum. Methods Phys. Res., Sect. B 184 (2001) 29.
G. Okada, K. Fukuda, S. Kasap, and T. Yanagida: Photonics 3 (2016) 23.
N. M. Winch, A. Edgar, and C. M. Bartle: Nucl. Instrum. Methods Phys. Res., Sect. A 763 (2014) 394.
H. Nanto, Y. Miyamoto, T. Oono, Y. Takei, T. Kurobori, and T. Yamamoto: Procedia Eng. 25 (2011) 231.
G. Okada, B. Morrell, C. Koughia, A. Edgar, C. Varoy, G. Belev, T. Wysokinski, D. Chapman, and S. Kasap:
Appl. Phys. Lett. 99 (2011) 121105.
G. Okada, J. Ueda, S. Tanabe, G. Belev, T. Wysokinski, D. Chapman, D. Tonchev, and S. Kasap: J. Am.
Ceram. Soc. 97 (2014) 2147.
Y. Miyamoto, H. Nanto, T. Kurobori, Y. Fujimoto, T. Yanagida, J. Ueda, S. Tanabe, and T. Yamamoto: Radiat.
Meas. 71 (2014) 529.
G. Okada, Y. Fujimoto, H. Tanaka, S. Kasap, and T. Yanagida: J. Rare Earths 34 (2016) 769.
C. W. E. van Eijk: Nucl. Instrum. Methods Phys. Res., Sect. A 460 (2001) 1.
T. Yanagida: Opt. Mater. 35 (2013) 1987.
E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. W. Kramer, and H. U. Gudel: Nucl. Instrum. Methods
Phys. Res., Sect. A 486 (2002) 254.
N. J. Cherepy, G. Hull, A. D. Drobshoff, S. A. Payne, E. Van Loef, C. M. Wilson, K. S. Shah, U. N. Roy, A.  
Burger, L. A. Boatner, W. S. Choong, and W. W. Moses: Appl. Phys. Lett. 92 (2008) 083508.
C. W. E. Van Eijk, A. Bessière, and P. Dorenbos: Nucl. Instrum. Methods Phys. Res., Sect. A 529 (2004) 260.
N. Kawaguchi, T. Yanagida, A. Novoselov, K. J. Kim, K. Fukuda, A. Yoshikawa, M. Miyake, and M. Baba:
Proc. Nuclear Science Symp., Ed. P. Sellin (IEEE, 2008) 1174–1176.
N. Kawaguchi, T. Yanagida, Y. Fujimoto, Y. Yokota, K. Kamada, K. Fukuda, T. Suyama, K. Watanabe, A.
Yamazaki, V. Chani, and A. Yoshikawa: Nucl. Instrum. Methods Phys. Res., Sect. A 652 (2011) 351.
T. Yanagida, N. Kawaguchi, Y. Fujimoto, K. Fukuda, Y. Yokota, A. Yamazaki, K. Watanabe, J. Pejchal, A.
Uritani, T. Iguchi, and A. Yoshikawa: Opt. Mater. 33 (2011) 1243.
T. Yanagida, A. Yamaji, N. Kawaguchi, Y. Fujimoto, K. Fukuda, S. Kurosawa, A. Yamazaki, K. Watanabe, Y.
Futami, Y. Yokota, A. Uritani, T. Iguchi, A. Yoshikawa, and M. Nikl: Appl. Phys. Express 4 (2011) 106401.
T. Yanagida: J. Lumin. 169 (2016) 544.
T. Yanagida, Y. Fujimoto, K. Watanabe, K. Fukuda, N. Kawaguchi, Y. Miyamoto, and H. Nanto: Rad. Meas.
71 (2014) 162.
K. Y. Loong, G. D. Jones, and R. W. G. Syme: J. Lumin. 53 (1992) 503.
F. Nakamura, T. Kato, G. Okada, N. Kawaguchi, K. Fukuda, and T. Yanagida: J. Eur. Ceram. Soc. 37 (2017)
4919.
K. D. Oskam, A. J. Houtepen, and A. Meijerink: J. Lumin. 97 (2002) 107.