Neutron Storage Phosphor

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Characteristics of a SrBPO5:Eu2+ Material as a
Neutron Storage Phosphor
K. Sakasai, M. Katagiri, K. Toh, H. Takahashi, M. Nakazawa, and Y. Kondo
problem is to search phosphors that contain neutron sensitive
materials such as atomic boron or atomic lithium in the base
matrix. Recently, new storage phosphors for thermal neutron
detection have been reported [2]-[4], where the phosphors
contained atomic boron in the base matrix. The phosphor may
be candidates for the purpose.
On the other hand, the authors have been studying
characteristics of a SrBPO5:Eu2+ phosphor [5] for the purpose
[6]-[8], where the phosphor shows photoluminescence and
photostimulated luminescence (PSL) due to a Eu2+ transition at
390 nm after X-ray irradiation. The phosphor has a low
density compared to that of BaFBr:Eu2+, which is usually used
as a phosphor material of IP. It will be favorable for reducing
gamma-ray influence on the signal in the field where both
neutrons and gamma-rays exist because phosphors with lower
densities are less sensitive to gamma-rays. In this paper,
characteristics of the SrBPO5:Eu2+ material as a neutron
storage phosphor are described.

Abstract-- Characteristics of a SrBPO5:Eu2+ material have been
investigated as a neutron storage phosphor. The authors found
this phosphor shows photostimulated luminescence by
illumination with 635 nm laser light after neutron irradiation.
The neutron sensitivity was proportional to E-0.5, where E is
neutron energy. The Sγ/Sn ratio of the phosphor using enriched
boron was better than that of a commercially available imaging
plate, where Sγ and Sn are the gamma and neutron sensitivities,
respectively. A collimated neutron beam image was clearly
obtained with the phosphor sample by scanning focused laser light
on it.
I.

INTRODUCTION

neutron imaging plate (NIP) made of storage phosphors
T(BaFBr:Eu
) and neutron sensitive materials (Gd O ) has
HE

2+

2

3

made a great success in the field of neutron scattering studies
[1]. However, there is a disadvantage when the NIP is used for
neutron detection in a field where both neutrons and gammarays exist. Since the NIP is sensitive not only to neutrons but
also gamma-rays, it is difficult to discriminate the neutron
signals from the gamma-ray when the NIP is read out. To
overcome this problem, it is necessary to search new storage
phosphors consisting of light materials. Adding neutron
sensitive materials such as 10B to conventional storage
phosphors, however, is pessimistic for making neutron storage
phosphors because the ranges of charged particles from
10
B(n,α)7Li reactions are very short in the phosphors and they
cannot make enough amounts of electron-hole pairs to store the
irradiated neutron information.
One of ways to solve the

II. EXPERIMENTAL
2+

The SrBPO5:Eu powder samples were prepared in the
following way: appropriate amounts of SrCO3, H3BO3,
(NH4)2HPO4, and EuCl3·6H2O were mixed in a mortar for 20
minutes. The purity of these materials was higher than 99.5%.
The powder samples were obtained by firing them in a muffle
furnace in a nitrogen atmosphere with a heating schedule at
600 ºC for 2 hours and at 800 ºC for 2 hours. The firing was
repeated two times to obtain homogeneous powder samples.
Photoluminescence and PSL spectra were measured using a
Hitachi F-2500 spctroflourometer. PSL decay characteristics
were measured using an experimental setup shown in Fig.1.
For this purpose, the samples were illuminated by a He-Ne

Manuscript received October 25, 2002. This work was supported in part by
the Japan Ministry of Education, Culture, Sports, Science and Technology
under Grant-in-Aid for Scientific Research (KAKENHI 14350044).
K. Sakasai and M. Katagiri are with the Japan Atomic Energy Research
Institute, Ibaraki, 319-1195 JAPAN (telephone: +81-29-284-3519, +81-29282-6547,
e-mail:
[email protected],
[email protected]).
K.Toh is with the Institute for Material Research, Tohoku University,
Sendai, 980-8577 JAPAN (telephone: +81-22-215-2063, e-mail:
[email protected]).
H. Takahashi and M. Nakazawa are with the Department of Quantum
Engineering and System Science, University of Tokyo, Tokyo, 113-8656
JAPAN
(telephone:
+81-3-5841-6972,
+81-3-5841-7007,
e-mail:
[email protected], [email protected]).
Y. Kondo is with the Department of Applied Physics, Tohoku University,
Sendai,
980-8579
JAPAN
(telephone:
+81-217-7964,
e-mail:
[email protected]).

0-7803-7636-6/03/$17.00 ©2003 IEEE.

Dark Room
PMT

Laser

Filter
PSL
(390nm)

Photon
Counting
Unit
Laser
Controller

Fast
Counter

Personal
Computer

635nm
Sample

Fig. 1. The experimental setup for PSL measurement.

455

GP-IB

5000

5000

Optical Intensity

PSL counts (/0.1s)

2%

4000

5%

3000

1%
2000

0.5%

1000
0.1%
0
-1000
300

350

400

450

1.2

PSL (a.u.)

0.6
0.4

10

20

30

40

50

350

400

450

Fig.2 shows photoluminescence spectra of samples excited
by 290-nm UV light. The concentrations (mol %) of Eu in the
samples were 0.1%, 0.2%, 0.5%, 1%, 2%, and 5%. There was
a broad peak at 390 nm that is attributed to the 5d-4f transition
of Eu2+. The spectra were similar to that of BaFBr:Eu2+. The
lifetime of the 390-nm emission from the sample was estimated
to be 0.75 µs, while that from a commercially available IP
(Fuji Film Co. Ltd.) containing BaFBr:Eu2+ was 0.80 µs in the
same experiment [9]. Although an optimal Eu concentration of
1% was reported, it may exist between 2% and 5% in our
experiment.
Fig.3 (a) and (b) show PSL emission and PSL excitation
spectra from the sample after neutron, respectively. The
neutron irradiation was carried out for 30 minutes at the Cold
Neutron Radiography Facility (CNRF) of JRR-3M at Japan
Atomic Energy Research Institute (JAERI). The neutron flux
and neutron wavelength were about 108 n/cm2/s and 0.5 nm,
respectively. One can see a 390-nm emission spectrum from
the sample. Although the 630-nm excitation light was used for
measurement of the PSL emission spectrum, light with shorter
wavelength is more efficient to read out radiation information
accumulated in the sample from the results of PSL excitation
spectrum measurement. Fig.4 shows a PSL output decay curve
of luminescence at 390 nm of the neutron-irradiated sample,

500

W avelength (nm)
20

Intensity (a.u.)

0

III. RESULTS AND DISCUSSION

0.2

(b)

15

10

0
450

1000

MellesGriot) was set in front of the photomultiplier tube
(Hamamatsu R647P). The signal from the photomultiplier tube
were amplified and discriminated from noise by a photoncounting unit (Hamamatsu C3866). The output pulses from the
unit were counted by a fast counter (HP53131A). The system
was controlled by a personal computer through the GP-IB
interface.

E xcitation: 630 nm
Source: CNRF
Irrad. time: 30 min.

0.8

5

SB P:Eu (2%)
S ource: CNRF at JRR-3M

Fig.4. The PSL output decay curve of luminescence at 390 nm of the
neutron-irradiated sample. In the figure, results with a Li block (1-cm
thickness) are also shown, where the block was set in front of the
sample to shield it from neutron irradiation.

Fig. 2. Photoluminescence emission spectra of SrBPO5:Eu materials
when the Eu concentration in the sample was changed.

0.0
300

2000

Elapsed time (sec)

500

W avelength (nm)

(a)

with Li block
without Li block

3000

0

Excitation: 290nm

1.0

4000

Emission: 390 nm
Source: CNRF
Irrad. tim e: 30 m in.

500

550

600

650

700

W avelength (nm )
Fig.3 (a) PSL emission spectrum and (b) PSL excitation spectrum of
SrBPO5:Eu (2%) after neutron irradiation

laser (Audio Technica SU-31E, 6 mW) after irradiation with
neutrons. To eliminate the background light, a filter (made by

0-7803-7636-6/03/$17.00 ©2003 IEEE.

456

TABLE I
THE NEUTRON SENSITIVITIES OF SAMPLES

Sensitivity
(PSL/mm2/neutron)

Ratio

SrBPO5:Eu2+ (1%)

1.14 x 10-2

0.69

SrBPO5:Eu2+ (2%)

1.66 x 10-2

1

10

Sr B PO 5 :Eu(2%)

S n /S γ (a.u.)

1

E -0.5 fitting

2+

SrBPO5:Eu (5%)

0.1
Sr

nat

0.2

5

3

Neutron energy (meV)
Fig.5. The Sn/Sγ ratio of the SrBPO5:Eu2+ (2%) as a function of
neutron energy, where Sn and Sγ are the neutron and gamma (5.9-keV
55
Fe gamma-ray) sensitivities, respectively.

30
1% : M=0.020244t
2% : M=0.029418t

PSL, M (x10 3 counts)

25

5% : M=0.015871t
20
15
10
at JRR-3M SANS-J
λ =0.5nm
4
2
φ =3.1x10 n/cm /s

5
0

0

200

400

600

800

1000

1200

Irradiation tim e, t (sec)

Fig.6. The total PSL output linearity measurement when the Eu
concentrations were 1%, 2%, and 5%.

which was obtained with the experimental apparatus shown in
Fig.1. The concentration of Eu of the sample was 2%. . The
neutron irradiation time was 9.6 s at CNRF. In the figure,
results with a Li block (1-cm thickness) are also shown, where
the block was set in front of the sample to shield it from the
neutron irradiation. The figure clearly shows that the
SrBPO5:Eu2+ sample itself was sensitive to neutrons for PSL
production without adding any neutron sensitive materials such
as Gd.
Fig.5 shows the Sn/Sγ ratio of the samples (Eu concentration:
2%) as a function of neutron energy, where Sn and Sγ are the
neutron and gamma (5.9-keV 55Fe gamma-ray) sensitivities of

0-7803-7636-6/03/$17.00 ©2003 IEEE.

0.45

the sample. The results of a sample using enriched boron
(enrichment: 96%) are also shown in the figure. The neutron
irradiation was carried out at the Small Angle Neutron
Scattering Facility (SANS) of JRR-3M at JAERI. As seen in
the figure, the Sn/Sγ ratio was increased by a factor of 4.6 by
using enriched boron instead of natural boron. The factor of
4.6 is almost the same as the ratio of the enrichment to 10B
natural abundance. One can also see the ratios of both samples
were proportional to E-0.5, where E is neutron energy. This is
clear evidence that the samples are sensitive to neutrons and
the PSL output is proportional to the number of nuclear
reactions of 10B atoms with neutrons because the neutron cross
section of 10B(n,α)Li is proportional to E-0.5 in this neutron
energy range.
Fig. 6 shows the results of total PSL output linearity
measurement when the Eu concentrations were 1%, 2%, and
5%. The neutron flux and neutron wavelength were 3.1x104
n/cm2/s and 0.5 nm, respectively. In this experiment, all
samples were made by using enriched boron. The PSL outputs
from the samples were well proportional to the irradiation time.
In the figure, also shown are results of linear fitting curves
obtained by the Least-Square method. Since the slopes
correspond to neutron sensitivities of the samples, relative
neutron sensitivities of Sr10BPO5:Eu2+ (1%) and Sr10BPO5:Eu2+
(5%) samples were 0.69 and 0.54, respectively, when the
neutron sensitivity of Sr10BPO5:Eu2+ (2%) sample was
normalized to be unity.
Next the authors have measured neutron sensitivities of the
samples. In this experiment, the samples were irradiated
neutrons and then illuminated with 635-nm laser light through
a filter sheet which has a small hole with a diameter of 1 mm.
With this sheet, only the small area was illuminated with the
laser. In other words, the PSL signal from this area can be
measured. The neutron irradiation was carried out under the
same condition in Fig. 6 and the irradiation time was 300 s.
Table I lists measured neutron sensitivities of the samples. In
this experiment, relative neutron sensitivities of Sr10BPO5:Eu2+
(1%) and Sr10BPO5:Eu2+ (5%) samples were 0.69 and 0.45,
respectively, when the neutron sensitivity of Sr10BPO5:Eu2+
(2%) sample was normalized to be unity. These results were
almost agreed with those of Fig.6. From the results of Table I
and Fig.6, the optimal Eu concentration is 2 % in our
experiments. Therefore, the authors have investigated the

BPO :Eu(2%)
0.4 0.6 0.8 1

7.37 x 10

-3

457

TABLE II
THE NEUTRON AND GAMMA-RAY SENSITIVITIES OF A Sr10BPO5:Eu2+ (2%)

(a)

SAMPLE AND COMMERCIALLY AVAILABLE NEUTRON IMAGING PLATE

Sr10BPO5:Eu2+ (2%)

BAS-ND

Neutron
sensitivity (Sn)

1.66 x 10-2

1.24

Gamma-ray
sensitivity (Sγ)

3.47 x 10-5

2.19 x 10-2

Sγ/Sn

2.09 x 10-3

1.77 x 10-2

PSL (a.u.)
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0

Unit: Sn (PSL/mm2/neutrons), Sγ (PSL/mm2/γ’s)

8mm
10

2+

gamma sensitivity of Sr BPO5:Eu (2%) sample to estimate
the Sγ/Sn ratio. The gamma-ray sensitivity was measured in the
same manner of neutron sensitivity measurement, using a
gamma-ray source (662keV-137Cs source) instead of neutron
beam of SANS. Table II lists measured neutron and gammaray sensitivities of the samples compared with those of a BASND neutron imaging plate. One can see that the neutron
sensitivity of a Sr10BPO5:Eu2+ (2%) sample is two orders lower
than that of BAS-ND, while the gamma-ray one is three orders
lower. These results may come from the fact that the BAS-ND
sheet contains Gd atoms which have a large neutron cross
section and consists of relatively heavy materials (BaFBr:Eu2+
and Gd2O3). However, the Sγ/Sn ratio of the Sr10BPO5:Eu2+
(2%) sample is ten times better than that of BAS-ND. The low
Sγ/Sn ratio is favorable for reducing the gamma-ray influence
on the signal.
Finally the authors have tried to take a collimated neutron
beam image with a Sr10BPO5:Eu2+ (2%) sample with a
diameter of 12 mm and a thickness of 1 mm. The sample was
made by the Spark Plasma Sintering (SPS) method. In the SPS
method, the Sr10BPO5:Eu2+ (2%) powder was set in a carbon
vessel and then fired for 10 minutes at 800 ºC. The powder
sample was pressed with a stress of 5 kN in a vacuum
atmosphere throughout the firing. After the sample was
irradiated with a collimated neutron beam with a diameter of
1.5 mm, its surface was scanned with focused laser light by
using an X-Y stage controller. The neutron flux and neutron
irradiation time were 3.1 x 104 n/cm2/s and 30 minutes,
respectively. The scanning was carried out every 0.25 mm on
the sample surface. The results were shown in Fig. 7 (a). The
beam intensity image can be clearly obtained. Fig.7 (b) shows
the edge spread function measured with the sample, where the
half of the surface (left side) was irradiated with neutrons.
Although Lorentzian line spread function (LSF) is usually
applied to photographic film, the imaging plate detector’s LSF
was reported to have a strong similarity to Gaussian [10].
Therefore the authors fitted the data in Fig.6 (b) by ESF of a
Gaussian LSF and estimated the spatial resolution to be about
0.4 mm when FWHM of LSF is defined as a spatial resolution.

0-7803-7636-6/03/$17.00 ©2003 IEEE.

8mm

Irradiated area

PSL intensity (a.u.)

600

12 mm

500
400
300

Scanning points

200

(b)

100
0

0

1

2

3

4

5

6

7

8

Position (mm)
Fig.7. (a) A collimated neutron beam image obtained with the sample.
The beam size was 1.5 mmφ. (b) The measured the edge spread
function.

IV. CONCLUSIONS
The characteristics of a SrBPO5:Eu2+ material have been
investigated as a neutron storage phosphor. The phosphor
showed PSL at 390 nm by a He-Ne laser excitation after
neutron irradiation. It was confirmed that the total PSL output
was proportional to the number of nuclear reactions of 10B
atoms with neutrons. The Sγ/Sn ratio of this phosphor using
enriched boron was better than that of a commercially available
imaging plate, where Sγ and Sn are gamma and neutron
sensitivities, respectively. A collimated neutron beam image

458

“Storage phosphors for thermal neutron detection,” Nucl. Instr. and
Meth. A, vol. 486, pp. 160-163, 2002.
[5] A. Karthikeyani and R. Jagannathan, “Eu2+ luminescence in stillwellitetype SrBPO5- a new potential X-ray storage phosphor,” J. Lumin., vol.
86, pp.79-85, 2000.
[6] K. Sakasai, M. Katagiri, K. Toh, T. Nakamura, H. Takahashi, and M.
Nakazawa, “A SrBPO5:Eu2+ phosphor for neutron imaging,” in
Proceedings of the Fifteenth Meetings of the International
Collaboration on Advanced Neutron Sources ICANS-XV, 6-9 November
2000, vol.I, pp.639-644.
[7] K. Sakasai, M. Katagiri, K. Toh, H. Takahashi, M. Nakazawa, and Y.
Kondo, “Characteristics of a SrBPO5:Eu2+ storage phosphor for neutron
imaging,” Denki-gakkai Kenkyu-kai Siryo, NE-01-8, August 2001. (in
Japanese.)
[8] K. Sakasai, M. Katagiri, K. Toh, H. Takahashi, M. Nakazawa, and Y.
Kondo, “A SrBPO5:Eu2+ storage phosphor for neutron imaging,” Appl.
Phys. A, in press.
[9] K. Sakasai, M. Katagiri, K. Toh, H. Takahashi, M. Nakazawa, and Y.
Kondo, “2-dimensional imaging characteristics of a New
Photostimulable Phosphor SrBPO5:Eu2+,” unpublished.
[10] Jens Hofmann and Christian Rausch, “Performance of a prototype
detector system for thermal neutrons based on laser stimulated
luminescence,” Nucl. Instr. and Meth. A, vol. 355, pp. 494-500, 1995.

was clearly obtained by scanning focused laser light on the
sample. The phosphor might be a candidate for a new neutron
imaging plate, though the neutron detection efficiency will be
by two orders of magnitude lower compared to Gd containing
phosphors.
V. REFERENCES
[1]
[2]

[3]

[4]

Y. Karasawa, S. Kumazawa, and N. Niimura, “The character and
application of a neutron imaging plate,” Physics B, vol. 241-243, pp.
139-141, 1998.
M. J. Knitel, V. R. Bom, P. Dorenbos, C. W. E. van Eijk, I.
Berezovskaya, and V. Dotsenko, “The feasibility of boron containing
phosphors in the thermal neutron image plate, in particular the systems
M2B5O9X:Eu2+ (M=Ca, Sr, Ba; X=Cl, Br). Part I: simulation of the
energy deposition process,” Nucl. Instr. and Meth. A, vol. 449, pp. 578594, 2000.
M. J. Knitel, B. Hommels, P. Dorenbos, C. W. E. van Eijk, I.
Berezovskaya, and V. Dotsenko, “The feasibility of boron containing
phosphors in the thermal neutron image plate, in particular the systems
M2B5O9X:Eu2+ (M=Ca, Sr, Ba; X=Cl, Br). Part II: experimental results,”
Nucl. Instr. and Meth. A, vol. 449, pp. 595-601, 2000.
A. V. Sidorenko, A. J. J. Bos, P. Dorenbos, N. J. M. Le Masson, P. A.
Rondnyi, C. W. E. van Eijk, I. V. Berezovskaya, and V. P. Dotsenko,

0-7803-7636-6/03/$17.00 ©2003 IEEE.

459

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