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Sensors and Materials, Vol. 36, No. 2 (2024) 443–449
MYU Tokyo
S & M 3530

Relationship between Lifetime and Emission Wavelengthin Scintillation and Photoluminescence from the Same Excited State
Takayuki Yanagida,* Takumi Kato, Daisuke Nakauchi, and Noriaki Kawaguchi
Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma City, Nara 630-0192, Japan(Received October 20, 2023; accepted December 27, 2023)

Keywords: scintillation, photoluminescence, decay time, lifetime, emission wavelength

The relationship between the lifetimes and emission wavelengths of scintillation andphotoluminescence, when the initial excited state was the same, was studied. When the rateequation is dominant, lifetime should be the same at different emission wavelengths. However,
we have observed the dependence of lifetime on emission wavelength in common scintillators.

1.

Introduction

Photoluminescence (PL) and scintillation (SC) are close but different phenomena. When theluminescence center is excited, dynamics including excitation and emission (relaxation)
processes at the localized luminescence center can be observed, and such a phenomenon is PL.
In SC, after the absorption of ionizing radiation, the excitation of the host and the transportationof energy from the host to luminescence centers occur in addition to the PL process.(1–3) In terms
of practical applications of scintillators, SC lifetime (decay time) is important since it is adominant factor of the timing resolution of radiation detectors.(4) Despite such importance, notheories for scintillation lifetime have been established, and that for PL(5) has been used for along time instead of the original theory. In PL, the lifetime can be expressed as
12

2n  n2 + 2  ∑f f µ i ,3 Γ =− ∝ 3 
τ λ 

(1)

where G, t, n, and l represent the decay rate of an excited state, decay time, refractive index, andemission wavelength, respectively. The matrix element connecting the initial state |i> with thefinal state |f> via the dipole operator m will only be of appreciable size for transitions between
states of different parities. Although Eq. (1) is offered for PL, the relationship has also been
confirmed in SC.(6) In our previous work,(6) Eq. (1) was confirmed only for scintillators withdifferent excited states, such as 5d–4f transitions of Ce3+, Pr3+, and Nd3+. However, if one
considers the common rate equation, one may consider that lifetime will be the same when the
excited state is the same. The common and simple rate equation is expressed as*Corresponding author: e-mail: yanagida.takayuki@naist.ac.jp

https://doi.org/10.18494/SAM4748

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

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where N, t, and k mean the number of excited states (excited electrons), time, and rate constant,respectively. In this relationship, lifetime should be the same when the excited state is the same.
In other words, the rate equation does not care about excitation processes.
The aim of the current work is to study whether Eq. (1) is still dominant for different initial
excitated states. If Eq. (1) is dominant, a correlation between emission wavelength and lifetime
can be observed. To exemplify them, the generality of samples is important. In terms of material
forms, single crystals,(7–23) ceramics,(24–34) and glasses(35–54) are common forms of scintillators,
and samples with these forms are selected.

2.

Materials and Methods

Sample materials are Ce3+-doped Lu3Al5O12,(55) Gd3Al2Ga3O12,(55) Y3Al5O12,(55) LuAlO3,(56)YAlO3,(56) Y2SiO5,(57) Gd 2SiO5,(58) Lu 2SiO5,(59) (Lu,Y)2SiO5,(59) LiCaAlF6,(60) Eu 2+-doped
CaAl2O4,(15) SrAl2O4,(15) BaAl2O4,(15) LiCaAlF6,(61) and LiSrAlF6(62) in single crystal or ceramic
forms. Except for CaAl2O4, SrAl2O4, BaAl2O4, and LiSrAlF6, all the samples are commerciallyavailable. PL spectra and lifetimes were evaluated using Quantaurus-QY and Quantaurus-τ
(both by Hamamatsu), respectively. SC spectra(63) and lifetimes(64) were measured using ouroriginal setups.
Figures 1 and 2 show examples of results in this work. After PL and SC spectra were
measured, 2–4 monitoring wavelengths for lifetimes were determined. In PL, the excitation levelof 5d1 was selected as the excitation wavelength, which was determined to be 280 nm for mostmaterials and 470 nm for Ce-doped garnet materials. The number of monitoring points depended
on the width of the emission peak. As exemplified in Fig. 1, each monitoring band was separated
without any overlapping as much as possible. Technically, perfect separation was impossible
since such a broad spectrum contained several emission peaks with a Gaussian-like shape, andtail parts of Gaussian overlapped each other. However, in such a case, the dominant lifetime
originated from the target emission band. Moreover, strictly, some materials such as rare-earth
silicate materials (e.g., Lu2SiO5) have different sites for dopant Ce3+, and emissions fromdifferent sites show slightly different lifetimes.(65) In such a case, since the observed values will
be an average lifetime of every component, such an effect was not considered. After selecting
the monitoring wavelengths, wavelength-resolved PL and scintillation time profiles were
measured, as shown in Fig. 2, and the obtained time profiles were analyzed by single exponential
approximation.

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

Fig. 1. (Color online) PL spectrum of Ce-doped
Y2SiO5 crystal under 280 nm excitation. Hatchedranges were monitoring points of lifetimes.

3.

445

Fig. 2. (Color online) Time profiles of Ce-doped
Y2SiO5 crystal monitoring at various wavelengths
under 280 nm excitation.

Results

Figures 3(a)–3(c) show the relationship between emission wavelength and lifetime. Since themeasurement ranges of Ce- and Eu-doped samples were markedly different, they were separately
plotted in different panels. In Ce-doped materials, a relationship of t ~ Al1.5 was observed, anda proportional relationship was confirmed. Equation (1) assumes a perfect crystal with an
ideal energy structure, and differences with actual materials (e.g., defects) will be areason for the difference between theory and experiments. Proportion was also confirmed
in Eu-doped AEAl 2O 4 (AE = alkali earth) samples. Since the number of data points waslimited, the multiplier (n) increased. In panel (c), no proportional relationship wasobserved. The main reason can be blamed for technical matters. Eu-doped LiCaAlF6 and
LiSrAlF6 showed similar emission spectra peaking at 370 nm with an FWHM of ~10 nm.
In these samples, the wavelength separation was not enough.
Figure 4 shows the current results in addition to previous results of Ce3+-, Pr3+-, andNd 3+-doped samples in PL,(6) and Fig. 5 is the same plot for SC. Here, a clear correlation
between emission wavelength and lifetime was confirmed. Throughout the work, Eq. (1) is
confirmed to be dominant even when the phenomenon starts from the same excitated state.
Since the rate equation does not consider any states before excitation, the present result
suggests that excitation processes will affect a behavior of lifetime. Even in PL, theexcitation source has a few ns pulse width, and the condition changes from an ideal rateequation, which assumes the full occupation of the excited state under the initial condition.

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(a)

(b)

(c)
Fig. 3. (Color online) Relationship between emission wavelength (horizontal axis) and lifetime (vertical axis) of
PL and SC in (a) Ce-doped scintillators, (b) Eu-doped AEAl2O4, and (c) Eu-doped LiAEAlF6. Circles and triangles
represent PL and SC, respectively. In (a) and (b), approximation by τ = Aλn is also plotted, where A and n are positive
number constants.

Fig. 4. Distribution of PL emission wavelength(horizontal axis) and PL lifetime (vertical axis).

Fig. 5. Distribution of scintillation emission
wavelength (horizontal axis) and scintillation
lifetime (vertical axis).

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

4.

447

Conclusions

When the initial excited state was the same, the relationship between emission wavelengthand lifetime in PL and SC was studied. Their clear correlation was observed in both phenomena.
In addition to different excited states, even in the same excited state, emission wavelength andlifetime were confirmed to be correlated.

Acknowledgments
This work was supported by MEXT Grants-in-Aid for Scientific Research A (22H00309),
Scientific Research B (21H03733, 21H03736, and 22H03872), and Exploratory Research(22K18997), and Cooperative Research Project of the Research Center for Biomedical
Engineering.

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