Thermal annealing effects on the luminescence and scintillation properties of CaMoO4 single crystal grown by Bridgman method

Article abstract of DOI:10.1016/j.jallcom.2017.11.005

CaMoO₄ single crystal with high quality and large size was grown by Bridgman method. The as-grown CaMoO₄ crystal presents blue-black color, and the annealed CaMoO₄ crystal presents optically transparent. The mechanisms of crystal fading by thermal annealing were explained in detail. The annealing effects on the optical transmission, photoluminescence spectra, X-ray luminescence spectra and luminescence decay time were investigated. The results indicate that thermal annealing not only has positive effects on reducing oxygen vacancies and alleviating the self-absorption phenomenon, but also the luminescent intensity can be enhanced greatly. The annealed CaMoO₄ crystal has moderate Stokes shift (1.46 eV). The light output of annealed CaMoO₄ single crystal by the excitation of 662 KeV X-ray from a ¹³⁷Cs source was measured to be 57.9 p.e. per MeV. This work offers detailed and significant data of scintillation properties of CaMoO₄ single crystal, which are very helpful for potential applications in searching for rare events especially in neutrinoless double beta decay area.

Full text of DOI:10.1016/j.jallcom.2017.11.005

Accepted Manuscript
Thermal annealing effects on the luminescence and scintillation properties of
CaMoO single crystal grown by Bridgman method
4
Linwen Jiang, Zhenhai Wang, Hongbing Chen, Yaping Chen, Peng Chen, Zizong Xu
PII:
S0925-8388(17)33737-4
10.1016/j.jallcom.2017.11.005
JALCOM 43698
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 26 August 2017
Revised Date: 22 October 2017
Accepted Date: 1 November 2017
Please cite this article as: L. Jiang, Z. Wang, H. Chen, Y. Chen, P. Chen, Z. Xu, Thermal annealing
effects on the luminescence and scintillation properties of CaMoO single crystal grown by Bridgman
4
method, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.005.
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ACCEPTED MANUSCRIPT
Thermal annealing effects on the luminescence and scintillation properties of
CaMoO single crystal grown by Bridgman method
4
a,b
a
a,b,*
a
a
Linwen Jiang , Zhenhai Wang , Hongbing Chen , Yaping Chen , Peng Chen ,
c
Zizong Xu
a
State Key Laboratory Base of Novel Functional Materials and Preparation Science,
Ningbo University, Ningbo 315211, PR China
b
Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province,
Ningbo University, Ningbo 315211, PR China
c
Department of Modern Physics, University of Science & Technology of China, Hefei
2
30027, PR China
Abstract: CaMoO single crystal with high quality and large size was grown by
4
Bridgman method. The as-grown CaMoO crystal presents blue-black color, and the
4
annealed CaMoO crystal presents optically transparent. The mechanisms of crystal
4
fading by thermal annealing were explained in detail. The annealing effects on the
optical transmission, photoluminescence spectra, X-ray luminescence spectra and
luminescence decay time were investigated. The results indicate that thermal
annealing not only has positive effects on reducing oxygen vacancies and alleviating
the self-absorption phenomenon, but also the luminescent intensity can be enhanced
greatly. The annealed CaMoO crystal has moderate Stokes shift (1.46 eV). The light
4
output of annealed CaMoO single crystal by the excitation of 662 KeV X-ray from a
4
1
37
Cs source was measured to be 57.9 p.e. per MeV. This work offers detailed and
significant data of scintillation properties of CaMoO single crystal, which are very
4
helpful for potential applications in searching for rare events especially in neutrinoless
double beta decay area.
Keywords: CaMoO ; Crystal growth; Bridgman method; Luminescence properties
4
1
. Introduction
Scintillation properties of many crystals within the ABO family (A = Mg, Ca, Sr,
4
Cd, Zn, Pb, Ba; B = Mo, W) have been studied extensively over a wide temperature
range for decades owing to their technological importance in a variety of potential
applications in fields of electro-optical devices, phosphors, and scintillators [1-9].
Research on these compounds intensified recently due to newly emerging applications,
providing different scintillation targets for cryogenic particle physics experiments
searching for rare events, such as interaction with dark matter particles, neutrinoless
*
Corresponding author.
E-mail address: chenhongbing@nbu.edu.cn (Hongbing Chen).

ACCEPTED MANUSCRIPT
double-beta decay (0νββ) or radioactive decay of very longlived nuclei [10]. Thus, the
identification, characterization and optimization of potentially useful scintillation
materials for cryogenic applications are important tasks, driven by the requirements of
experiments searching for rare events.
The searching for 0νββ is a key experiment for evaluating the mass of neutrino
and determining the Majorana/Dirac nature of neutrinos. 0νββ is a special case of
double beta decay that occurs without the emission of two neutrinos. Several
inorganic scintillators containing molybdenum were developed in the last years. The
most promising of them are ZnMoO [10-12], CdMoO [13,14], PbMoO [15,16],
4
4
4
Li MoO [17,18] and CaMoO [19-22]. As the CaMoO crystals possess the relatively
2
4
4
4
bright scintillation light at room temperature, it has been concluded that CaMoO4,
displays excellent performances at cryogenic temperatures [19]. Meanwhile, this
1
00
crystal contains Mo isotope that is considered to be the most promising nucleus for
the detection of 0νββ owing to the nearly 10% natural abundance and the high
transition energy (Q=3034keV) [20]. CaMoO is one promising candidate materials
4
for meeting the requirements of experiments searching for rare events.
For a long time, owing to the high melting point (about 1430℃ ~1440℃ ) of
CaMoO crystal, the conventional Czochralski [20-22] method was applied in the
4
growth of CaMoO single crystal, and no other methods were reported in references.
4
In this work, we grew successfully the CaMoO single crystal with large size using
4
the Bridgman method. By contrast to the Czochralski (Cz) method, there are many
advantages for the Bridgman method, such as preventing the volatilization of melt and
controlling the shape of single crystal. In this paper, we reported in detail the growth
process and scintillation properties of CaMoO single crystal grown by Bridgman
4
method. In case of CaMoO single crystal not much has been reported about its defect
4
equilibrium and annealing effects on crystal luminescence, thus the crystals have been
subjected to thermal annealing under air atmospheres at elevated temperatures and
their optical and luminescence properties.
2
. Experimental section
2
.1. Preparation of polycrystalline powders and single crystal growth
Polycrystalline CaMoO powders were synthesized by conventional solid state
4
reaction. The following reagents were taken as raw materials: CaCO (99.95%),
3
MoO (99.999%). Firstly, the raw materials were weighed at a stoichiometric ratio. In
3
addition, 1.5 mol% excess MoO was added into the mixtures in order to compensate
3
for the volatilization of MoO (melting point: 795ꢀ and boiling point: 1155 ꢀ) in
3
2

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the process of polycrystalline synthesis (800 ꢀ) and crystal growth (1440 ꢀ). Two
raw materials were mixed and ground manually in a corundum mortar for 1.5 hours.
Afterwards, the mixture was calcined at 600 ℃ for 6 hours. Subsequently, the bulk
substance was ground evenly to powder. The obtained powders were calcined again at
8
00 ℃for 4 hours. Eventually, the polycrystalline CaMoO powders with white color
4
were obtained.
CaMoO single crystal was grown by Bridgman method in a Bridgman furnace. Fig.
4
1
shows a schematic illustration of CaMoO single crystal growth using Bridge
4
method. The temperature of furnace chamber was set at 1440ꢀ. The platinum
crucible was used as container of CaMoO single crystal. The crystal seed with Φ
4
2
1
1.5×60 mm was obtained by spontaneous nucleation. The crystal seed was put into
the seed well, and then polycrystalline CaMoO powders were filled in the platinum
4
crucibles. The platinum crucible was sealed completely for decreasing the
volatilization of melt. To reduce the remnant thermal stress and alleviate the color
centers of the CaMoO crystal, the as-grown CaMoO single crystal was annealed in
4
4
air atmosphere.
Fig. 1. Schematic illustration of CaMoO single crystal growth using Bridge method
4
2
.2. Characterizations
The as-prepared powders were characterized using X-ray diffraction (XRD), which
was recorded using a Bruker D8 diffractometer with Cu K radiation (λ=1.5406 Å).
α
Transmission spectra were measured using an American Perkin Lambda 950
UV-VIS-NIR Spectrometer. The emission spectra of CaMoO single crystals under
4
X-ray excitation were recorded on a homemade XEL spectrometer. The emission
spectra under ultraviolet excitation (λ =300 nm) were measured using FluoroMax-4
ex
spectrometer. Luminescence decays were measured using the nanosecond
hydrogen-filled coaxial and microsecond flash lamps (both IBH Scotland) applying
the methods of time-correlated single photon counting and multichannel scaling,
3

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respectively. The decay fits were obtained by the least-square sum fitting procedure
(spectra Solve software package, Ames Photonics Inc.) after performing the
convolution of the considered function with the instrumental response.
3
. Results and discussion
CaMoO polycrystalline powders were used as the raw materials for CaMoO4
4
crystal growth. The purity and crystallinity of CaMoO polycrystalline powders are
4
crucial factors for the next crystal growth. Fig. 2 shows the XRD patterns of CaMoO4
polycrystalline powders at different calcination temperatures. It can be seen that the
sample at 600 ꢀ calcination temperature exhibits characteristic CaMoO phase.
4
However, some extra XRD peaks are also observed, suggesting incomplete reaction
below 600 ꢀ. With the temperature increasing (700 ꢀ), it can be clearly observed
that the diffraction peaks of CaMoO samples were well-indexed to CaMoO phase
4
4
(PDF# 29-0351) with the tetragonal phase structure, and no peaks of impurities can be
observed in XRD patterns, implying the high purity and good crystallinity of products
above 700 ꢀ. It can be concluded that the well-crystallized tetragonal CaMoO have
4
been successfully prepared under the present calcination temperature of 800 ꢀ.
Fig. 2. XRD patterns of CaMoO polycrystalline powders at different calcination temperatures
4
The XRD pattern for the grown CaMoO4 single crystal was shown in Fig. 3. It can
be seen that the XRD pattern is in good accord with CaMoO phase (PDF# 29-0351).
4
Diffraction peaks corresponding to the secondary phase and shift peaks were
undetected, indicating clearly the successful growth of CaMoO crystal. The lattice
4
constants determined from the CaMoO XRD result are a=b=0.5222 nm and
4
c=1.1425 nm, which is very close with the standard lattice constants (a=b=0.5226 nm
4

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and c=1.143 nm, PDF#29-0351). Meanwhile, the lattice constants also were
compared with those in other researches, as shown in Table 1. It can be seen that the
lattice constants of CaMoO powders in Ref. [23-25] are also very close with those in
4
this work.
Fig. 3. XRD pattern of CaMoO crystal grown by Bridgman method
4
Table 1 Comparison of lattice constants with other researches
lattice constants
CaMoO4)
Standard
card
This work
Refer. [23]
Refer. [24]
Refer. [25]
(
Microwave
radiation
Solid state
reaction
Mechanochemical
method
Bridgman
Preparation method
a=b (nm)
—
0
1
.5222
.1425
0.5226
1.143
90
0.5212
1.1438
90
0.5225
1.1451
90
0.52292
1.14428
90
c (nm)
α=β=γ(°)
9
0
In this work, 1.5 mol% excess MoO was added into the mixtures in order to
3
compensate for the volatilization of MoO in the process of both polycrystalline
3
synthesis and crystal growth. In order to confirm the MoO components of powders
3
on crucible sidewall, the XRD patterns were analyzed, as shown in Fig. 4. The XRD
patterns are in good agreement with the MoO standard card (PDF#05-0508),
3
indicating clearly the existence of MoO component on crucible sidewall. Therefore,
3
it can be concluded that the excessive MoO has been squeezed out of CaMoO in
3
4
the process of crystal growth.
5

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Fig. 4. XRD patterns of powders on crucible sidewall
The CaMoO single crystal was grown through Bridgman method using c-axis
4
oriented seed. The as-grown CaMoO single crystal was shown in Fig. 5(a). The size
4
2
of obtained CaMoO single crystal is about Φ 40×180 mm . It can be observed that no
4
low angle boundaries, inclusions and other macroscopic defects consist in the
as-grown crystal. Table 2 shows the comparison of sizes of CaMoO ingots prepared
4
in other research groups. It can be observed that the size of CaMoO crystal in this
4
work have a great enhancement. Fig. 5(b) shows the annealed and polished CaMoO4
crystal, it can be seen clearly that the annealed crystal presents optically transparent.
Fig. 5. (a) as-grown CaMoO crystal with blue-black color. (b) Annealed and polished CaMoO4
4
crystal.
Table 2 Sizes of CaMoO ingots prepared in other research groups
4
cylindrical diameter
Length
(mm)
Growth
method
Place of crystal growth
reference
(
mm)
Czochralski
technique
Institute for Theoretical and
Experimental Physics, Russia
3
0
80
130
—
[
20]
Czochralski
method
Institute of Materials SRC Carat,
Ukraine
3
8
[21]
Czochralski
method
Institute for Nuclear Research of
Russian Academy Science, Russia
≈
20
[22]
5
0
180
Bridgman
method
Ningbo University, China
This work
6

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The CaMoO single crystal was cut into several regular cylinder-shaped wafers
4
with Φ25 mm×5 mm along the (001) direction. Meanwhile, the intersection interfaces
were polished. The unannealed sample and the annealed sample at 1200 ꢀ are shown
in Fig. 6(a) and (b), respectively. It can be observed that the annealed CaMoO crystal
4
wafers became transparent and optically clear, which is distinct obviously with the
bule-black color of unannealed crystal wafers. V. B. Mikhailik et al. also reported the
similar phenomenon of CaMoO crystal after annealing in a flow of oxygen at 1250ꢀ
4
[
21].
Fig. 6. (a) unannealed and (b) annealed CaMoO wafers
4
The annealing effects on the color of CaMoO crystal can be explained as below: In
4
the process of CaMoO crystal growth using Bridgman method, plenty of Ca, Mo and
4
O atoms with accurate stoichiometric ratio will rearrange in an ordered and regular
way accompanied with devitrification appearance, forming perfect CaMoO crystal
4
structure in an ideal growth condition. Nevertheless, complicated environments of
actual crystal growth, such as temperature disturbance, local unhomogeneous
components and asynchronous rearranged rate of dispersed atoms, will give rise to the
imperfect atom array, forming defective microstructures in crystal, such as oxygen
vacancy related defects, the valence state reduction of molybdenum ions, interstitial
atoms, stacking faults and dislocation of lattice planes. Those micro-defects can be
utilized to be the color centers of crystal. Therefore, the blue-black color may be an
integrated result of dispersed color centers originated from those metastable
micro-defects. When the as-grown CaMoO crystal was annealed, plenty of
4
micro-defects in crystal were rehabilitated, and thus crystal presents optically
transparent. Fig. 7 gives an illustration clarifying the role of oxygen vacancies in
crystal color. The closed crystal-growth condition induced the forming of oxygen
vacancies due to the lack of oxygen [26]. Annealing at air atmosphere is likely to
induce oxygen filling the unoccupied vacancies by high-temperature diffusing way.
Therefore, the color centers of oxygen vacancies were eliminated, and finally the
crystal became transparent and optically clear.
7

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Fig. 7. Schematic illustration of thermal annealing effects on the oxygen vacancies
The recent investigations on high temperature thermal treatment of PbWO crystal
4
have demonstrated the role of thermal annealing on optical properties of crystal
[
27-29]. Moreover, Mohit Tyagi et al. [26] also reported that the BaWO crystals
4
turned yellowish on annealing in air or oxygen, and turned dark by a similar treatment
at 700 ꢀ under vacuum. The thermal treatment is thought to affect the oxygen
chemical surrounding or coordination from normal crystal lattice. Totally speaking,
the annealing process is an effective way for eliminating color problems. Meanwhile,
it is also beneficial for preventing crystal cracking from intensive interior stress.
Both annealed and unannealed CaMoO single crystals with dimensions of Φ 25×5
4
2
mm were used for measuring the transmission spectra. The transmission spectra are
presented in Fig. 8. Obviously, the annealed CaMoO possessed higher optical
4
transparent properties in contrast with the unannealed CaMoO . Table 3 show the
4
comparison of transmission of CaMoO and ZnMoO crystals prepared in different
4
4
research groups. The discrepancy of transmission in crystals may be attributed to the
additional uncertainties in the transmittance measurements arise because of its
birefringent nature, and due to macroscopic defects, such as voids and scattering
centers in the crystals. R. Y. Zhu et al. [30] reported that the uncertainty due to the
birefringence was determined to be less than 10 % for different azimuthal orientations
of the crystal. To avoid this effect, a particular azimuthal orientation should be chosen
which gave the maximum longitudinal transmittance. In this work, the optical
absorption edges of both annealed and unannealed CaMoO single crystals are located
4
at the same position 318 nm wavelength, which is almost consistent with 320 nm
reported in other literatures [20, 21].
8

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Fig. 8. Transmission spectra of CaMoO single crystal
4
Table 3 Comparison of transmission of CaMoO and ZnMoO crystal in other research groups
4
4
Transmits Wavelength
Crystal species
Measured devices
reference
-ivity
range (nm)
CaMoO4
CaMoO4
ZnMoO4
ZnMoO4
75-80 %
77-81 %
67-82 %
71-83 %
78-81 %
71-79 %
400-800
a Varian Cary1 spectrophotometer
—
[20]
[21]
400-700
400-1000
400-700
Commercial fiber Optic spectrophotometer
SUPERLUMI station
[31]
[32]
CaMoO (annealed)
400-1500
400-1500
UV-VIS-NIR spectrometer
UV-VIS-NIR spectrometer
This work
This work
4
CaMoO (unannealed)
4
Fig. 9 shows the emission spectra of both annealed and unannealed CaMoO single
4
crystals under ultraviolet excitation (λ =300 nm). It can be observed that the emission
ex
peaks of two samples are almost in the same position. The luminescence peaks exhibit
a broad emission band with a maximum at 523 nm, while the emission intensity of
unannealed sample is inferior in contrast with the annealed sample. The broad green
emission centered at 523 nm can be attributed to the electronic transition within
2
-
intrinsic emission of [MoO ] anion complex, where the valence O 2p electrons are
4
promoted to the empty 4d molybdenum states in the course of charge transition,
similar with the other molybdate crystals [31,32].
Fig. 9. Emission spectra of both annealed and unannealed CaMoO crystal under ultraviolet
4
excitation (λ =300 nm)
ex
9

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Among the variety of inorganic Mo-containing materials, only CaMoO shows
4
relatively bright scintillation at room temperature since the conducting band of
CaMoO is split and narrow subzone occurs at the bottom part of the conducting zone.
4
Other molybdates, with structure of sheelite or wolframite, show relatively weak and
quite slow scintillation at low temperature. The emission spectra of annealed CaMoO4
crystal are represented in Fig. 10(a). Under excitation from 300 nm to 345 nm, an
intense broad emission peak around 495-525 nm is observed. The sample exhibits a
green emission. Fig. 10(b) shows the excitation spectra of annealed CaMoO crystal.
4
All excitation spectra collected at the emission wavelength of 480 nm, 500 nm, 530
nm, 550 nm exhibit a strong and broad band centered around 325–330 nm. It is
proposed that MoO octahedral structure are the luminescent centers and a
6
charge-transfer transition between the O 2p orbital and the empty d orbital of the
central Mo ion is responsible for this luminescence.
Fig. 10. (a) Emission and (b) excitation spectra of annealed CaMoO crystal
4
Table 4 shows the comparison of position of emission and excitation peak of
tungstate and molybdate crystals. It indicates that the position of the emission and
excitation peak is very strongly influenced by the symmetry associated with crystal
structure and closest surroundings associated with cation species. For the molybdate
system, though the mechanism of observed luminescence in CaMoO , MgMoO and
4
4
ZnMoO is believed to be the same, the position of the emission and excitation peak
4
displays obvious distinction. The position of the emission peak shifts from 523 nm for
CaMoO to 564 nm for ZnMoO and to 600 nm for α-MgMoO . This shift is
4
4
4
associated with the different value of the crystal field in these crystals. That can also
be explained in terms of the crystal field effect of oxygen on d-state of Mo in the
2
−
[
MoO4] centre. The values of Stokes shift (difference in energy between the
absorption and emission band maxima) also are shown in Table 4. It can be observed
that CaMoO has the moderate Stokes shift (1.46 eV), suggesting the moderate
4
self-absorption phenomenon in annealed CaMoO crystal.
4
1
0

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Table 4 Comparison of the position of emission and excitation peak of tungstate and molybdate
crystals in other research groups (at 298 K)
Crystal
structure
λem
(nm)
λex
(nm)
Stokes shift
(eV)
Crystal species
reference
β-MgMoO4
α-MgMoO4
MgWO4
sanmartinite
cuprosheelite
wolframite
wolframite
scheelite
520
600
490
564
425
438
535
551
523
320
310
310
302
270
276
369
355
324
1.55
2.05
1.50
1.90
1.68
1.67
1.04
1.24
1.46
[33]
[33]
[33]
ZnMoO4
BaWO4
[32]
[26]
CaWO4
scheelite
[34]
CdMoO4
scheelite
Previous work
Previous work
This work
Li MoO4
2
Rhombohedral
scheelite
CaMoO4
The main goal of this study is to measure the potential of the CaMoO crystal for
4
application in scintillation detection of ionizing radiation, and x-ray luminescence
techniques is the most relevant for this purpose. Since the high penetration depth of
the x-ray photons is hardly sensitive to surface phenomena [35], the bulk emission
properties are little affected by the excitation conditions in contrast with experiments
that use ultraviolet or visible light excitation, where this effect can be a serious
concern [33]. Fig. 11(a) and (b) exhibit the emission spectra of both annealed and
unanealed CaMoO crystals under X-ray excitation. The emission spectra profiles of
4
two samples are very similar, while the emission intensity of unanealed CaMoO is
4
inferior compared with that of annealed CaMoO . The peak of emission spectra
4
deviates about 5 nm wavelength from the ultraviolet excitation peak. The slight
difference may result from X-ray excitation much deeper in degree and the more
serious self-absorption.
Fig. 11. Emission spectra of (a) unannealed and (b) annealed CaMoO crystal under X-ray
4
excitation
11

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It is well known that the luminescence decay time is an important performance
parameter for scintillators. This quantity is determined by the response of a scintillator
and the corresponding detector. The smaller decay time implies a faster response in a
neutron detector. Fig. 12 shows the luminescence decay time of unannealed and
annealed CaMoO single crystals at room temperature, respectively. The solid curve
4
of the two samples can be fitted with a single exponential decay I(t)=A+I exp(-t/τ),
0
where I(t), I and τ represent the luminescence intensity, initial luminescence intensity
0
2
and decay time, respectively. R can be used to indicate the fitted level of data point,
2
Here, The R values are 0.995 and 0.996, indicating a good single exponential decay
fit. The fitting decay time is 12.16 ꢀs and 20.35 ꢀs for unannealed and annealed
CaMoO crystal, respectively. The values are near with that of 17 ꢀs reported by S.
4
Belogurov [20]. The shorter decay time for unannealed CaMoO crystal may be due
4
to the micro-detects in crystal, which induced the quenching of energy transition in a
shorter time.
Fig. 12. Luminescence decay times of (a) unannealed and (b) annealed CaMoO single crystal
4
The photoelectron numbers were determined by the peak ADC channel obtained by
a Gaussian fit (not including the Compton edge), multiplying by a calibration constant
corresponding to the number of photoelectrons per ADC channel. Samples of volume
1
5 mm×10 mm×5 mm were cut from the ingot and mechanically polished to optical
grade. The sample was placed into an optical cryostat and exposed to alpha-particle
2
41
excitation from Am source (33 kBq). Fig. 13 shows the shaping time of single
photoelectron peak. The photoelectron peak is centered at 65 channels for 10ꢀs
shaping time and 143 channels for 0.5 ꢀs shaping time.
1
2

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Fig. 13. The single photoelectric peak shaping time of CaMoO single crystal
4
Fig. 14(a) shows the X-ray spectra of annealed CaMoO single crystal by the
4
2
41
excitation of 59 KeV X-ray from a Am source. The full energy peak of CaMoO4
single crystal centered at 430 channels for 10 ꢀs shaping time, and the photoelectric
peak centered at 143 channels, so the light output of CaMoO single crystal is
4
~
430/143/0.059 =51 p.e./MeV. Fig. 14(b) shows the X-ray spectra of CaMoO single
4
137
crystal by the excitation of 662 KeV X-ray from a Cs source. When the gain of
main amplifier is 50*0.5, the full energy peak of CaMoO single crystal centered at
4
4
30 channels (0.5 ꢀs shaping time). When the gain of main amplifier is 20*0.6, the
full energy peak centered at 1600 channels (0.5ꢀs shaping time), and the photoelectric
peak centered at 143 channels, so the light output of CaMoO crystal is
4
~
1600/12/87/*25/0.662=57.9 p.e./MeV. The measured ratio value is ~1100/1600=0.68,
and the theoretical ratio value is ~0.478MeV/0.662MeV=0.722, suggesting that the
theoretical value is very near with the measured value. Totally speaking, the light
output of CaMoO single crystal is not prominent with that of other crystals, such as
4
NaI, BaF and Bi Ge O . However, it still is very significant to fundamental
2
4
3
12
researches and potential applications in 0νββ areas.
2
41
137
Fig. 14. Light output of CaMoO single crystal by (a) Am source exciting and (b) Cs source
4
exciting
1
3

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Conclusions
CaMoO single crystal with high quality and large size was grown by the
4
Bridgman method. The as-grown CaMoO crystal presents blue-black color, which
4
can be attributed to the closed crystal-growth condition at high temperatures, giving
rise to the forming of color centers of oxygen vacancies. The thermal annealing in air
atmosphere not only has positive effects on reducing oxygen vacancies and alleviating
the self-absorption phenomenon, but also the luminescent intensity can be enhanced
greatly. The annealed CaMoO crystal has the moderate Stokes shift (1.46 eV). The
4
light output of CaMoO single crystal by the excitation of 662 KeV X-ray from a
4
1
37
Cs source was 57.9 p.e. per MeV. In conclusion, the light output of CaMoO single
4
crystal is not prominent with that of other crystals, but it still is very significant to
fundamental researches and potential applications in 0νββ areas.
Acknowledgements
This work is supported by Natural Science Foundation of China (No.51701098),
and Natural Science Foundation of Ningbo municipality (No. 2017A610100). This
work is also supported by the Opening Project of State Key laboratory of Crystal
Material (Shandong University), and K. C. Wong Magna Foundation in Ningbo
University.
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Highlights
•
•
•
•
CaMoO crystal with large size was grown successfully by Bridgman method.
Mechanism of crystal fading by thermal annealing was proposed.
Thermal annealing effects on the optical properties were investigated in detail.
4
Significant data of scintillation properties of CaMoO crystal were presented.
4