- Email: [email protected]
Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors
Author’s Accepted Manuscript Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors P. Schuster, E. Brubaker
www.elsevier.com/locate/nima
PII: DOI: Reference:
S0168-9002(16)31134-2 http://dx.doi.org/10.1016/j.nima.2016.11.016 NIMA59435
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 7 July 2016 Revised date: 27 October 2016 Accepted date: 6 November 2016 Cite this article as: P. Schuster and E. Brubaker, Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.11.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2
Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors
3
P. Schustera,∗, E. Brubakerb
1
a Department
4 5
6
of Nuclear Engineering, University of California, Berkeley, CA, USA b Sandia National Laboratories, Livermore, CA
Abstract This paper reports a series of measurements that characterize the directional dependence of the scintillation response of crystalline melt-grown and solution-grown trans-stilbene to incident DT and DD neutrons. These measurements give the amplitude and pulse shape dependence on the proton recoil direction over one hemisphere of the crystal, confirming and extending previous results in the literature for melt-grown stilbene and providing the first measurements for solution-grown stilbene. In similar measurements of liquid and plastic detectors, no directional dependence was observed, confirming the hypothesis that the anisotropy in stilbene and other organic crystal scintillators is a result of internal effects due to the molecular or crystal structure and not an external effect on the measurement system.
7
Keywords: stilbene, organic scintillator, neutron detection, scintillation anisotropy;
8
1. Introduction
9
For many decades, the trans isomer of the organic crystal scintillator stilbene has been used for radiation detection.
10
Recently, a new solution-based growth method has been developed that produces large crystals with high light output
11
and excellent neutron-gamma pulse shape discrimination (PSD) [1, 2, 3]. This solution-grown stilbene is receiving
12
substantial interest over liquid and plastic alternatives as its performance is better than liquid and PSD-capable plastic
13
scintillators [2, 4, 5], and its solid form is often easier to work with than liquid scintillators that are subject to thermal
14
expansion and risk of leaks.
15
Crystal organic scintillators are known to have a directionally dependent scintillation response for heavy charged
16
particle interactions. The directional dependence of anthracene has been widely characterized [6, 7, 8, 9], but only
17
limited measurements have been reported for the directional dependence of stilbene. Previous measurements of proton
18
recoil events in stilbene include the magnitude of change in light output at 3.7, 8, and 22 MeV [10, 8], and the light
19
output vs. angle in two planes at 3.7 MeV [10] and in one plane at 14 MeV [7]. Thus far, no measurements of the ∗ Corresponding
author Email addresses: [email protected] (P. Schuster), [email protected] (E. Brubaker)
Preprint submitted to Nuclear Instruments and Methods in Physics A
November 18, 2016
20
effect across a full hemisphere of crystal axes have been provided. Additionally, all previous measurements were
21
of melt-grown stilbene crystals. Given the rising popularity of solution-grown stilbene as a detection material, it is
22
important to thoroughly characterize the directional dependence and understand if it depends on crystal size or growth
23
method.
24
This paper characterizes the scintillation anisotropy in melt-grown and solution-grown stilbene for 14.1 MeV and
25
2.5 MeV proton recoil events. The magnitude of change in the light output and pulse shape has been quantified, and
26
the light output and pulse shape have been characterized as a function of direction for a full hemisphere worth of
27
proton recoil directions. In order to investigate how consistent the anisotropy effect is across stilbene detectors, four
28
stilbene samples have been characterized. These include two solution-grown samples with the same dimensions, a
29
solution-grown sample of different dimensions, and a melt-grown sample of different dimensions.
30
Characterizing the effect across these four detectors will provide information on whether the effect depends on
31
crystal geometry or growth method, and will evaluate whether the effect varies significantly between two seemingly
32
identical stilbene samples. The geometry of a crystal has been hypothesized to affect the anisotropy as it impacts the
33
light collection efficiency and PSD performance [11]. The growth method and quality of the crystal has also been
34
hypothesized to impact the anisotropy since the growth method does impact the light output and PSD performance [1,
35
2].
36
Previous authors stated that no anisotropy effect was observed in amorphous plastic and liquid materials [12,
37
p. 261], but no measurements have been published to support that statement. This paper reports measurements of
38
plastic and liquid scintillators that verify no anisotropy is observed, supporting the leading hypothesis that the physical
39
mechanism that produces the anisotropy comes from the molecular or crystal structure within the material and is not
40
caused by an external effect on the measurement system [6].
41
2. Measurement Techniques
42
In order to characterize the directional dependence in these materials, the detector response to proton recoil events
43
at different directions in the detector was measured. The method for measuring proton recoil events at a fixed energy
44
as a function of recoil direction was the same as in a previous paper which contains a more detailed description
45
than will be presented here [6]. To summarize, monoenergetic neutrons were produced using a Thermo Electric MP
46
320 DD (D+D→3 He+n; En =2.5 MeV) or DT (D+T→4 He+n; En =14.1 MeV) neutron generator. Events in which the
47
neutron deposited approximately its full energy to the recoil proton were selected in order to choose proton recoils with
48
energy Erecoil ≈ En traveling in the same direction as the incident neutron. A motor-driven rotational stage was used
49
to position the detector at any angle in 4π with respect to the incident neutron direction. In the DT measurements, the 2
50
detectors were placed approximately 152 cm from the neutron generator, controlling the incident angle of the neutrons
51
on the detector within 2o . In the DD measurements, the detectors were placed approximately 39 cm from the neutron
52
generator, controlling the incident neutron angle within 8o .
53
Events were recorded using a Struck SIS3350 500 MHz 12-bit digitizer that recorded 384 samples per event. The
54
digital signal processing technique was the same as described in [6]. For each event the light output L and pulse shape
55
parameter S were calculated. L was calculated as the sum of the baseline-subtracted digitized samples xi in each pulse
56
multiplied by a calibration factor as shown in Eq. (1). S was calculated as the fraction of the light in a defined delayed
57
region of the pulse relative to the total pulse as shown in Eq. (2). The boundaries of the delayed region are set between
58
i p + Δ1 and i p + Δ2 samples to the right of the peak sample of the pulse i p . The boundaries of the total region for the
59
S calculation were i p − 10 and i p + Δ2 .
L=C
i=384
xi
(1)
i=1
iP +Δ2 i p +Δ1
xi
i p −10
xi
S = i +Δ p 2
(2)
60
The boundaries of the delayed region were selected for each material independently via an iterative process to
61
maximize separation in the S vs. L distribution of the neutron and gamma-ray events. Table 1 shows the step sizes
62
Δ1 and Δ2 used in calculating the boundaries of the delayed and total regions. The same boundaries were used for all
63
solution-grown stilbene detectors, and different boundaries were used for each of the melt-grown stilbene, liquid, and
64
plastic detectors. Table 1: Integration window step sizes Δ1 and Δ2 used for calculating S values.
Material Solution-grown stilbene Melt-grown stilbene EJ309 liquid LLNL plastic
65
An approximate calibration was performed using a
22
Δ1 10 16 10 10
Δ2 60 70 40 60
Na source to convert the light output into electron equiva-
66
lent light output units, keVee. This calibration is approximate as the energy deposited by the proton recoils in the
67
neutron generator measurements is much higher than the energy deposited by the electron recoils in the calibration
68
measurements. If the light output response is nonlinear, a different calibration factor should be calculated in different
69
energy ranges. This calibration is not meant to provide an absolute measurement of light output, but rather to provide
70
a rough energy scale for reference. This calibration does not impact the magnitude of the anisotropy presented in the 3
71
following sections as the anisotropy is quantified as the relative magnitude of change.
72
Neutron events were selected using a pulse shape discrimination cutoff based on the S vs. L distribution. The
73
light output spectrum for neutron events was produced and the following fit function was applied to approximate the
74
detector response to a monoenergetic neutron-proton scattering interaction. This function is built from a negatively
75
sloped line with a sharp cutoff that represents the light output produced in the detector, incorporating nonlinearity up
76
to the maximum energy that a neutron can deposit in the scattering interaction. The sloped line with sharp cutoff is
77
convolved with a Gaussian function that represents detector resolution. mσ −(L−L)ˆ 2 L − Lˆ mL + b − √ e 2σ2 1 − erf f (L) = √ 2 2π σ 2
(3)
78
Lˆ represents the expected light output produced by a full energy proton recoil. Events with light output in the range
79
Lˆ ± σ were selected as full-energy proton recoils. This selection widens the distribution of proton recoil directions
80
selected around the forward direction. Though this selection does not produce a sharp cutoff on proton recoil directions
81
selected due to energy resolution fluctuations, the lower energy boundary of this range corresponds to a proton recoil
82
traveling approximately 12o -15o from the forward direction in all measurements presented in this paper. To calculate
83
the expected pulse shape parameter Sˆ at a given angle, a distribution of the S values for the full energy events is
84
produced. Sˆ is calculated as the centroid of a Gaussian fit to that S distribution.
85
The proton recoil direction is expressed in three-dimensional space with respect to a defined set of axes in (θ,
86
φ) spherical coordinates. θ represents the angle between the proton recoil direction and the positive z axis, and φ
87
represents the angle between the positive x axis and the projection of the proton recoil direction on the xy-plane.
88
Due to crystal symmetry, only one hemisphere worth of interaction directions was measured in each material. For
89
the crystalline stilbene materials measured, the relationship between the defined axes and the stilbene crystal axes
90
directions will be discussed in Sec. 3.
91
3. Anisotropy Measurements on Stilbene
92
The directional response to 14.1 MeV and 2.5 MeV proton recoils was characterized in four stilbene samples.
93
Three were produced using the solution-growth method. Of these three samples, two are cubic samples with 1.9 cm
94
edge lengths and known crystal axes directions, shown in Fig. 1a. The third is a rectangular prism with approximate
95
dimensions 4.3 cm x 2.5 cm x 0.9 cm. The crystal axes directions in the third sample are unknown. The fourth sample
96
is an older stilbene crystal with unknown history and considerable wear, shown in Fig. 1b. It was produced using a
97
melt growth technique, so it is unlikely to be a perfect monocrystal. The crystal is cloudy and the surface has been 4
(a) Bare solution-grown cubic stilbene B with crystal axes indicated.
(b) Melt-grown cylindrical stilbene wrapped in teflon tape.
Figure 1: Photos of solution-grown and melt-grown stilbene samples.
Figure 2: Solution-grown cubic stilbene A mounted to the photomultiplier face before the outer plastic sleeve was attached. The crystal is wrapped in an inner layer of teflon tape and an outer layer of electrical tape.
98
polished numerous times. This crystal is a cylinder of size approximately 2.5 cm height and 2.5 cm diameter.
99
Each crystal was polished and wrapped in teflon tape and then in electrical tape for easier handling before mounting
100
to the face of a 60 mm Hamamatsu H1949-50 photomultiplier tube (PMT) assembly using Visilox V-788 optical
101
grease. Fig. 2 shows this for cubic stilbene A. In mounting both cubic samples with known axes, the a axis was
102
positioned parallel to the ground, and the c axis was positioned perpendicular to the PMT face. A plastic sleeve was
103
placed over the crystal and wrapped in black tape to block out external light. The coupling of the crystal and PMT
104
were fixed for all measurements to control the light collection efficiency.
105
An alternate set of axes, shown in Fig. 3, has been established for expressing the proton recoil direction within
106
the crystal so that the axes directions and features of interest are on the interior of the two-dimensional visualiza-
107
tions and not on the edges. Fig. 4 shows the directional response of the cubic solution-grown stilbene sample B to
108
14.1 MeV proton recoil events. This data is provided in the supplementary files accompanying this paper. This vi-
110
sualization displays a hemisphere worth of directions essentially as if it had been flattened and viewed from above √ with r = 1 − cos θ increasing radially outward from 0 to 1 and φ increasing counter-clockwise from 0◦ to 360◦ . In
111
all such plots, the black points indicate measurements and the color scale represents a smooth interpolation between
112
measurements. The length of the vertical bar on the colorbar indicates the average statistical error.
109
5
Figure 3: Visualization of the crystal axes directions within the alternate set of axes established for the directional distribution plots of Lˆ and Sˆ in stilbene cubic B.
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 4: Response of solution-grown cubic stilbene crystal B at various recoil directions to 14.1 MeV protons. Radial distance is r =
√
1 − cos θ.
113
The measurements on the cubic solution-grown samples with known crystal axes confirm statements by previous
114
authors that the proton recoil direction of maximum light output is along the b axis and minimum light output is
115
along the c axis. The Lˆ distribution shows a distinct maximum region at approximately (θ, φ) = (50◦ , 210◦ ) which
116
is confirmed to be the direction of the b axis in the crystal axes. The minimum region is at approximately (θ, φ) =
117
(60◦ , 330◦ ) which is the c axis, and a saddle point is at approximately (θ, φ) = (45◦ , 90◦ ) which corresponds to the a
118
axis.
119
The features are very similar between the Lˆ and Sˆ distributions. The locations of the maximum, minimum, and
120
saddle points are approximately the same in both distributions. This is different from anthracene, whose maximum
121
and minimum directions are opposite in the Lˆ and Sˆ distributions [6]. This is likely due to differences in molecular 6
122
and crystal structure between stilbene and anthracene. One difference between the Lˆ and Sˆ distributions in stilbene is
123
that the gradient from larger to smaller values is much steeper in the Sˆ distribution, making for wider “valleys” in the
124
Sˆ distribution than in the Lˆ distribution.
125
Fig. 5 shows the directional response of the cubic solution-grown stilbene sample B to 2.5 MeV proton recoil
126
events. This data is also provided in the supplementary files accompanying this paper. Although the resolution in
127
this figure is worse than in the measurements at 14.1 MeV, the qualitative features appear the same as in Fig. 4.
128
Both distributions show maximal and minimal regions at the same region. These features were the same also for the
129
measurements on the other two solution-grown stilbene samples at 14.1 MeV and 2.5 MeV. Thus, these measurements
130
showed that the qualitative features observed in the directional distribution of the light output and pulse shape are
131
consistent across solution-grown stilbene samples with different geometries and at different proton recoil energies.
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 5: Response of solution-grown cubic stilbene crystal B at various recoil directions to 2.5 MeV protons. Radial distance is r =
√
1 − cos θ.
132
Fig. 6 shows the directional distributions for the melt-grown stilbene detector. The defined axes in the melt-
133
grown stilbene have been oriented so that the features approximately line up with the corresponding features in the
134
solution-grown stilbene. The height axis of the cylinder, which is perpendicular to the face of the PMT, is located at
135
approximately (θ, φ) = (60◦ , 330◦ ). The qualitative features on the distributions in melt-grown stilbene at 2.5 MeV
136
(not pictured) agree with the features shown in Fig. 5. These distributions show that the qualitative features in the
137
directional light output and pulse shape responses are consistent between a melt-grown and solution-grown stilbene
138
sample. To quantify the magnitude of the directional dependence, two metrics are calculated for each detector. The first metric is the magnitude of change across all measurements, calculated as the ratio of the maximum to minimum Lˆ 7
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 6: Response of melt-grown stilbene crystal at various recoil directions to 14.1 MeV protons. Radial distance is r = were oriented to produce a distribution of features similar to Fig. 4, as the true crystal axes were unknown.
√
1 − cos θ. The axes
and Sˆ values: AL =
Lˆ max Lˆ min
AS =
Sˆ max Sˆ min
139
Table 2 shows the AL and AS values and their statistical errors calculated for the four stilbene materials measured.
140
The AL and AS values are on the same order across all materials at 14.1 MeV, although the AS value is not directly
141
comparable between the solution-grown materials and the melt-grown materials because the timing windows for
142
defining the delayed region in calculating S are different. The statistical error is propagated from the error produced
143
by the fit function. Table 2: Magnitude of change in Lˆ and Sˆ values measured for stilbene detectors with statistical errors. *Indicates only maximum and minimum angles from DT measurements were measured at DD energies.
AL Cubic A Cubic B Rectangular Melt-grown
Solution-grown
14.1 MeV 1.200(7) 1.191(8) 1.191(4) 1.187(5)
2.5 MeV 1.468(50)* 1.365(39) 1.327(26)* 1.401(55)
AS 14.1 MeV 2.5 MeV 1.071(1) 1.034(5)* 1.100(1) 1.058(7) 1.078(1) 1.039(5)* 1.066(1) 1.048(3)
144
Systematic limitations exist in these measurements that are not accounted for in the errors provided in Table 2. One
145
source of systematic uncertainty is the range of proton recoil directions being selected due to the geometric width of
146
the incident neutron direction and the spread in proton recoil directions being selected in the light output range Lˆ ± σ.
147
Additionally, the distribution of proton recoil directions being measured within each crystal is not exactly the same. 8
148
Another systematic effect is the possibility of fluctuations due to temperature dependence in these detectors, which has
149
been observed in other organic scintillators such as anthracene [6]. Although the temperature dependence in stilbene
150
has not been characterized, it is not expected to be a significant effect on these stilbene anisotropy measurements
151
because the room temperature remained within a 1o C range for the DT measurements and a 2o C range for the DD
152
measurements. The possibility that external effects such as magnetic field effects on the PMT are responsible for
153
the anisotropy have been ruled out as the effect is not observed for gamma-ray interactions on anthracene [6] or for
154
neutron interactions on liquid and plastic detectors, which will be shown in Sec. 4.
155
Another systematic limitation is the use of an approximate fit function on the light output spectrum. AL and AS
156
vary more for measurements at 2.5 MeV than at 14.1 MeV, likely due to the difficulty in fitting the light output
157
spectrum at 2.5 MeV. Fig. 7 shows the neutron light output spectra for neutrons produced in DT and DD collisions
158
at a given angle in the solution-grown cubic stilbene B sample. The edge in the light output spectrum is much clearer
159
to the naked eye in the DT distribution than in the DD distribution for several reasons. First, the DD measurements
160
suffer from fewer events as the cross section for the DD reaction is much lower than that for the DT reaction. Also,
161
there are fewer optical photons produced per DD event, making for worse energy resolution due to photostatistics.
162
The fit function is much more robust for the DT measurements, and consistently locates the same Lˆ value regardless
163
of bin size. Although the fit function locates Lˆ in the DD light output spectrum, it is not a very robust process as
164
changing the binning or light output range can change the final Lˆ value significantly. In the DD measurements on the
165
cubic B and melt-grown samples, 45 proton recoil directions were measured, and the distribution of Lˆ and Sˆ values
166
showed features similar to the DT distributions, lending confidence that the fit function selected a consistent feature
167
on the light output spectra. In the DD measurements on the cubic A and rectangular samples, however, measurements
168
were only taken at the angles of maximum and minimum light output from the corresponding DT measurements so no
169
assessment of the overall features could be made, giving less confidence that the fit function performed consistently
170
on those measurements, so the AL and AS values calculated from those two measurements are considered less reliable.
171
Those values are indicated with an asterisk (*) in Table 2.
172
The magnitude of change of the light output in melt-grown stilbene samples has been reported by previous au-
173
thors [10, 8]. Fig. 8 shows the AL value reported by previous authors and in this paper at different proton recoil
174
energies. All measurements are consistent with the trend that the magnitude of change in the light output decreases
175
as the proton recoil energy increases. Comparing these results to similar measurements of anthracene made with the
176
same detection system [6] confirms previous authors’ statements that the magnitude of the light output anisotropy AL
177
in stilbene is on the same order as that in anthracene, but the magnitude of the pulse shape anisotropy AS is much
178
greater in anthracene than in stilbene [8]. 9
Number of counts
Number of counts
ˆ L
1500 1000 500 0 4000
6000
8000
ˆ L
1000
500
0 500
10000
L (keVee)
600
700
800
900
1000
L (keVee)
(a) DT neutrons.
(b) DD neutrons.
Figure 7: Light output spectrum fit as measured for DT and DD neutrons incident on the cubic solution-grown stilbene sample B.
1.6
Brooks and Jones Tsukada and Kikuchi* This Paper
AL
1.4
1.2
1 10 1
Proton Erecoil (MeV) Figure 8: Magnitude of change in light output for proton recoil events at energies from 2.5-22 MeV in melt grown stilbene detectors as reported by Tsukada and Kikuchi [10], Brooks and Jones [8], and in this work. *AL reported in [10] is for a polycrystalline sample, so it is likely lower than the value for single crystal stilbene.
Another metric for quantifying the anisotropy is the variability introduced to the Lˆ and Sˆ values by the effect. If the distribution of directions measured is approximated to be evenly distributed over the hemisphere, the observed standard deviation σobs of measured Lˆ and Sˆ values is related to the variability σanis introduced by the anisotropy effect. The contribution from statistical variance is subtracted in quadrature: σanis =
σ2obs − σ2stat ,
179
where σ2stat is the average statistical variance from the set of measurements at different recoil directions. In all cases,
180
the correction for σstat resulted in a <5% difference between σobs and σanis . Values of σanis for the solution-grown
181
cubic B and melt-grown stilbene are given in Table 3, normalized to the average measured value μ. Table 3: Normalized variability, σanis /μ, introduced by the anisotropy in directional measurements of solution-grown cubic B and melt-grown stilbene samples.
Erecoil (MeV) Lˆ Sˆ
Solution-grown 14.1 2.5 4.4(4)% 7.4(8)% 2.3(2)% 1.4(2)% 10
Melt-grown 14.1 2.5 4.3(3)% 7.8(8)% 1.7(2)% 1.0(1)%
182
These measurements show that the variability introduced by the anisotropy at 14.1 MeV and 2.5 MeV is on the
183
same order in solution-grown and melt-grown stilbene samples. Again, comparing the variability in the pulse shape
184
is not perfectly fair as different timing windows were used for calculating S in the solution-grown and melt-grown
185
detectors. Regardless, this serves as an approximate comparison of the variability in the pulse shape as calculated for
186
optimal neutron-gamma PSD in these materials.
187
4. Measurement of Plastic and Liquid Scintillators
188
Directional measurements were also made on plastic and liquid scintillator detectors. The plastic sample was
189
an LLNL PSD-capable plastic cylinder approximately 1.25 cm tall and 2.5 cm in diameter. The plastic sample was
190
wrapped and coupled to the PMT using the same procedure as described for the stilbene detectors. The liquid scintil-
191
lator was EJ-309 in a 5 cm tall 5 cm diameter cylinder.
192
The same process was used for calculating Lˆ and Sˆ as was used for the stilbene measurements. In the liquid and
193
plastic measurements, the detector resolution σ in Eq. (3) was fixed at a constant value. That constant value was
194
determined by taking the mean of the σ values produced in a preliminary analysis with σ allowed to vary as one of
195
the fit parameters. Fig. 9 shows the directional response of the EJ309 liquid to 14.1 MeV protons. Fig. 10 shows the
196
same measurements on the PSD-capable plastic scintillator. Additional measurements were taken of 2.5 MeV proton
197
recoils, and similar qualitative features were observed in the directional responses.
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 9: Response of EJ309 liquid at various recoil directions to 14.1 MeV protons. Radial distance is r =
√
1 − cos θ.
198
These measurements do not show any directional dependence correlated to detector orientation. The strongest
199
evidence supporting that conclusion is the lack of qualitative features that are observed in the stilbene detectors. 11
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 10: Response of plastic scintillator at various recoil directions to 14.1 MeV protons. Radial distance is r =
√
1 − cos θ.
200
These measurements do not show a maximum, minimum, and saddle point region in the hemisphere with smooth
201
transitions in between as was observed in the stilbene measurements. Instead, the directional dependence appears to
202
be random variability from statistical and other non-statistical effects. Table 4 shows the observed variability σobs and
203
the average statistical variability σstat in the liquid and plastic measurements. For all cases, σobs is on the order of
204
σstat , indicating that statistical fluctuations dominate the observed variability. Sources of systematic variability such
205
as those described in Sec. 3 may also exist in these measurements. Again, temperature dependence may be one of
206
these, but it is expected that such effects are not significant as the room temperature varied within a range of 1◦ C for
207
the DT measurements and within 2◦ C for the DD measurements. Additionally, no distinct correlation was observed
208
between Lˆ or Sˆ and the room temperature.
209
Thus, both qualitative and quantitative observations confirm statements made by previous authors that no direc-
210
tional dependence exists in liquid and plastic organic scintillators. The lack of directional variability correlated to
211
detector orientation in these amorphous materials indicates that the directional dependence observed in crystalline
212
materials is in fact due to an internal effect and not produced by the measurement system or external factors. Table 4: Variability in directional measurements of liquid and plastic scintillators. μ is the average measured value of the expected light output Lˆ or pulse shape Sˆ .
Lˆ Sˆ
Erecoil (MeV) σobs /μ σstat /μ σobs /μ σstat /μ
EJ309 Liquid 14.1 2.5 0.64(05)% 1.04(11)% 0.39(05)% 0.74(04)% 0.16(01)% 0.59(06)% 0.05(01)% 0.16(02)%
12
Plastic 14.1 2.5 0.93(07)% 1.42(15)% 0.53(02)% 1.41(12)% 0.27(02)% 0.86(09)% 0.10(04)% 0.33(02)%
213
5. Conclusion
214
The measurements reported in this paper characterize the directional dependence of the scintillation produced by
215
14.1 MeV and 2.5 MeV proton recoil events across a hemisphere worth of directions in melt-grown and, for the first
216
time, solution-grown stilbene detectors. The light output and pulse shape produced by 14.1 MeV and 2.5 MeV proton
217
recoils at a hemisphere worth of directions in a solution-grown stilbene sample is provided in the supplementary files
218
accompanying this paper. The scintillation was confirmed to be maximal for proton recoils along the b crystal axis
219
and minimal along the c axis. The magnitude of change in the light output and pulse shape was found to be consistent
220
across four stilbene detectors of different geometries and growth methods. The relationship between magnitude of
221
change in light output and proton recoil energy agreed with measurements by other authors at different proton recoil
222
energies.
223
The directional response of liquid and plastic scintillator detectors was also measured and results were provided to
224
demonstrate that no anisotropy was present. Those measurements support the leading hypothesis that the scintillation
225
anisotropy in crystalline materials is caused by an internal effect and is not the result of an external effect on the
226
measurement system.
227
6. Acknowledgements
228
229
The authors wish to thank John Steele for his assistance in building the motor driven rotational stage, and Natalia Zaitseva of Lawrence Livermore National Laboratory for providing the solution-grown stilbene and plastic samples.
230
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship
231
Program under Grant No. DGE 1106400. This material is based upon work supported by the Department of Energy
232
National Nuclear Security Administration under Award Number DE-NA0000979 through the Nuclear Science and
233
Security Consortium. Sandia National Laboratories is a multi-program laboratory managed and operated by San-
234
dia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s
235
National Nuclear Security Administration under contract DE-AC04-94AL85000.
236
[1] L. Carman, N. Zaitseva, H. P. Martinez, B. Rupert, I. Pawelczak, A. Glenn, H. Mulcahy, R. Leif, K. Lewis, S. Payne, The effect of material
237
purity on the optical and scintillation properties of solution-grown trans-stilbene crystals, Journal of Crystal Growth 368 (2013) 56–61.
238
doi:10.1016/j.jcrysgro.2013.01.019.
239
URL http://www.sciencedirect.com/science/article/pii/S0022024813000584
240
[2] N. Zaitseva, A. Glenn, L. Carman, H. Paul Martinez, R. Hatarik, H. Klapper, S. Payne, Scintillation properties of solution-grown trans-stilbene
241
single crystals, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated
242
Equipment 789 (2015) 8–15. doi:10.1016/j.nima.2015.03.090.
13
243
[3] M. Bourne, S. Clarke, N. Adamowicz, S. Pozzi, N. Zaitseva, L. Carman, Neutron detection in a high-gamma field using solution-grown
244
stilbene, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
245
806 (2016) 348–355. doi:10.1016/j.nima.2015.10.025.
246
URL http://www.sciencedirect.com/science/article/pii/S0168900215012322
247
[4] S. Pozzi, M. Bourne, S. Clarke, Pulse shape discrimination in the plastic scintillator EJ-299-33, Nuclear Instruments and Methods in Physics
248
Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 723 (2013) 19–23. doi:10.1016/j.nima.2013.
249
04.085.
250
URL http://www.sciencedirect.com/science/article/pii/S016890021300524X
251
[5] D. Cester, G. Nebbia, L. Stevanato, F. Pino, G. Viesti, Experimental tests of the new plastic scintillator with pulse shape discrimination capa-
252
bilities EJ-299-33, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated
253
Equipment 735 (2014) 202–206. doi:10.1016/j.nima.2013.09.031.
254
URL http://www.sciencedirect.com/science/article/pii/S0168900213012655
255
[6] P. Schuster, E. Brubaker, Investigating the Anisotropic Scintillation Response in Anthracene through Neutron, Gamma-Ray, and Muon
256
Measurements, IEEE Transactions on Nuclear Science 63 (3) (2016) 1942–1954. doi:10.1109/TNS.2016.2542589.
257
URL http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=7465791
258 259 260
[7] E. Brubaker, J. Steele, Neutron imaging using the anisotropic response of crystalline organic scintillators, IEEE Nuclear Science Symposium Conference Record (2010) 1647–1652doi:10.1109/NSSMIC.2010.5874055. [8] F. Brooks, D. Jones, Directional anisotropy in organic scintillation crystals, Nuclear Instruments and Methods 121 (1) (1974) 69–76. doi:
261
10.1016/0029-554X(74)90141-4.
262
URL http://www.sciencedirect.com/science/article/pii/0029554X74901414
263 264 265 266
[9] K. Tsukada, S. Kikuchi, Y. Miyagawa, Directional anisotropy in the characteristics of the organic-crystal scintillators, II, Nuclear Instruments and Methods 37 (1965) 69–76. doi:10.1016/0029-554X(65)90339-3. URL http://www.sciencedirect.com/science/article/pii/0029554X65903393 [10] K. Tsukada, S. Kikuchi, Directional anisotropy in the characteristics of the organic-crystal scintillators, Nuclear Instruments and Methods
267
17 (3) (1962) 286–288. doi:10.1016/0029-554X(62)90007-1.
268
URL http://www.sciencedirect.com/science/article/pii/0029554X62900071
269 270 271
[11] M. M. Bourne, A. DiFulvio, S. D. Clark, S. A. Pozzi, N. Zaitseva, S. Payne, Scalability of Stilbene for Use in Nuclear Nonproliferation Applications, in: Institute of Nuclear Materials Management Annual Meeting Proceedings, Indian Wells, CA, 2015. [12] J. Birks, The Theory and Practice of Scintillation Counting, Pergamon Press, Oxford, 1964. doi:10.1016/B978-0-08-010472-0.
272
50001-X.
273
URL http://www.sciencedirect.com/science/article/pii/B978008010472050001X
14
www.elsevier.com/locate/nima
PII: DOI: Reference:
S0168-9002(16)31134-2 http://dx.doi.org/10.1016/j.nima.2016.11.016 NIMA59435
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 7 July 2016 Revised date: 27 October 2016 Accepted date: 6 November 2016 Cite this article as: P. Schuster and E. Brubaker, Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.11.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2
Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors
3
P. Schustera,∗, E. Brubakerb
1
a Department
4 5
6
of Nuclear Engineering, University of California, Berkeley, CA, USA b Sandia National Laboratories, Livermore, CA
Abstract This paper reports a series of measurements that characterize the directional dependence of the scintillation response of crystalline melt-grown and solution-grown trans-stilbene to incident DT and DD neutrons. These measurements give the amplitude and pulse shape dependence on the proton recoil direction over one hemisphere of the crystal, confirming and extending previous results in the literature for melt-grown stilbene and providing the first measurements for solution-grown stilbene. In similar measurements of liquid and plastic detectors, no directional dependence was observed, confirming the hypothesis that the anisotropy in stilbene and other organic crystal scintillators is a result of internal effects due to the molecular or crystal structure and not an external effect on the measurement system.
7
Keywords: stilbene, organic scintillator, neutron detection, scintillation anisotropy;
8
1. Introduction
9
For many decades, the trans isomer of the organic crystal scintillator stilbene has been used for radiation detection.
10
Recently, a new solution-based growth method has been developed that produces large crystals with high light output
11
and excellent neutron-gamma pulse shape discrimination (PSD) [1, 2, 3]. This solution-grown stilbene is receiving
12
substantial interest over liquid and plastic alternatives as its performance is better than liquid and PSD-capable plastic
13
scintillators [2, 4, 5], and its solid form is often easier to work with than liquid scintillators that are subject to thermal
14
expansion and risk of leaks.
15
Crystal organic scintillators are known to have a directionally dependent scintillation response for heavy charged
16
particle interactions. The directional dependence of anthracene has been widely characterized [6, 7, 8, 9], but only
17
limited measurements have been reported for the directional dependence of stilbene. Previous measurements of proton
18
recoil events in stilbene include the magnitude of change in light output at 3.7, 8, and 22 MeV [10, 8], and the light
19
output vs. angle in two planes at 3.7 MeV [10] and in one plane at 14 MeV [7]. Thus far, no measurements of the ∗ Corresponding
author Email addresses: [email protected] (P. Schuster), [email protected] (E. Brubaker)
Preprint submitted to Nuclear Instruments and Methods in Physics A
November 18, 2016
20
effect across a full hemisphere of crystal axes have been provided. Additionally, all previous measurements were
21
of melt-grown stilbene crystals. Given the rising popularity of solution-grown stilbene as a detection material, it is
22
important to thoroughly characterize the directional dependence and understand if it depends on crystal size or growth
23
method.
24
This paper characterizes the scintillation anisotropy in melt-grown and solution-grown stilbene for 14.1 MeV and
25
2.5 MeV proton recoil events. The magnitude of change in the light output and pulse shape has been quantified, and
26
the light output and pulse shape have been characterized as a function of direction for a full hemisphere worth of
27
proton recoil directions. In order to investigate how consistent the anisotropy effect is across stilbene detectors, four
28
stilbene samples have been characterized. These include two solution-grown samples with the same dimensions, a
29
solution-grown sample of different dimensions, and a melt-grown sample of different dimensions.
30
Characterizing the effect across these four detectors will provide information on whether the effect depends on
31
crystal geometry or growth method, and will evaluate whether the effect varies significantly between two seemingly
32
identical stilbene samples. The geometry of a crystal has been hypothesized to affect the anisotropy as it impacts the
33
light collection efficiency and PSD performance [11]. The growth method and quality of the crystal has also been
34
hypothesized to impact the anisotropy since the growth method does impact the light output and PSD performance [1,
35
2].
36
Previous authors stated that no anisotropy effect was observed in amorphous plastic and liquid materials [12,
37
p. 261], but no measurements have been published to support that statement. This paper reports measurements of
38
plastic and liquid scintillators that verify no anisotropy is observed, supporting the leading hypothesis that the physical
39
mechanism that produces the anisotropy comes from the molecular or crystal structure within the material and is not
40
caused by an external effect on the measurement system [6].
41
2. Measurement Techniques
42
In order to characterize the directional dependence in these materials, the detector response to proton recoil events
43
at different directions in the detector was measured. The method for measuring proton recoil events at a fixed energy
44
as a function of recoil direction was the same as in a previous paper which contains a more detailed description
45
than will be presented here [6]. To summarize, monoenergetic neutrons were produced using a Thermo Electric MP
46
320 DD (D+D→3 He+n; En =2.5 MeV) or DT (D+T→4 He+n; En =14.1 MeV) neutron generator. Events in which the
47
neutron deposited approximately its full energy to the recoil proton were selected in order to choose proton recoils with
48
energy Erecoil ≈ En traveling in the same direction as the incident neutron. A motor-driven rotational stage was used
49
to position the detector at any angle in 4π with respect to the incident neutron direction. In the DT measurements, the 2
50
detectors were placed approximately 152 cm from the neutron generator, controlling the incident angle of the neutrons
51
on the detector within 2o . In the DD measurements, the detectors were placed approximately 39 cm from the neutron
52
generator, controlling the incident neutron angle within 8o .
53
Events were recorded using a Struck SIS3350 500 MHz 12-bit digitizer that recorded 384 samples per event. The
54
digital signal processing technique was the same as described in [6]. For each event the light output L and pulse shape
55
parameter S were calculated. L was calculated as the sum of the baseline-subtracted digitized samples xi in each pulse
56
multiplied by a calibration factor as shown in Eq. (1). S was calculated as the fraction of the light in a defined delayed
57
region of the pulse relative to the total pulse as shown in Eq. (2). The boundaries of the delayed region are set between
58
i p + Δ1 and i p + Δ2 samples to the right of the peak sample of the pulse i p . The boundaries of the total region for the
59
S calculation were i p − 10 and i p + Δ2 .
L=C
i=384
xi
(1)
i=1
iP +Δ2 i p +Δ1
xi
i p −10
xi
S = i +Δ p 2
(2)
60
The boundaries of the delayed region were selected for each material independently via an iterative process to
61
maximize separation in the S vs. L distribution of the neutron and gamma-ray events. Table 1 shows the step sizes
62
Δ1 and Δ2 used in calculating the boundaries of the delayed and total regions. The same boundaries were used for all
63
solution-grown stilbene detectors, and different boundaries were used for each of the melt-grown stilbene, liquid, and
64
plastic detectors. Table 1: Integration window step sizes Δ1 and Δ2 used for calculating S values.
Material Solution-grown stilbene Melt-grown stilbene EJ309 liquid LLNL plastic
65
An approximate calibration was performed using a
22
Δ1 10 16 10 10
Δ2 60 70 40 60
Na source to convert the light output into electron equiva-
66
lent light output units, keVee. This calibration is approximate as the energy deposited by the proton recoils in the
67
neutron generator measurements is much higher than the energy deposited by the electron recoils in the calibration
68
measurements. If the light output response is nonlinear, a different calibration factor should be calculated in different
69
energy ranges. This calibration is not meant to provide an absolute measurement of light output, but rather to provide
70
a rough energy scale for reference. This calibration does not impact the magnitude of the anisotropy presented in the 3
71
following sections as the anisotropy is quantified as the relative magnitude of change.
72
Neutron events were selected using a pulse shape discrimination cutoff based on the S vs. L distribution. The
73
light output spectrum for neutron events was produced and the following fit function was applied to approximate the
74
detector response to a monoenergetic neutron-proton scattering interaction. This function is built from a negatively
75
sloped line with a sharp cutoff that represents the light output produced in the detector, incorporating nonlinearity up
76
to the maximum energy that a neutron can deposit in the scattering interaction. The sloped line with sharp cutoff is
77
convolved with a Gaussian function that represents detector resolution. mσ −(L−L)ˆ 2 L − Lˆ mL + b − √ e 2σ2 1 − erf f (L) = √ 2 2π σ 2
(3)
78
Lˆ represents the expected light output produced by a full energy proton recoil. Events with light output in the range
79
Lˆ ± σ were selected as full-energy proton recoils. This selection widens the distribution of proton recoil directions
80
selected around the forward direction. Though this selection does not produce a sharp cutoff on proton recoil directions
81
selected due to energy resolution fluctuations, the lower energy boundary of this range corresponds to a proton recoil
82
traveling approximately 12o -15o from the forward direction in all measurements presented in this paper. To calculate
83
the expected pulse shape parameter Sˆ at a given angle, a distribution of the S values for the full energy events is
84
produced. Sˆ is calculated as the centroid of a Gaussian fit to that S distribution.
85
The proton recoil direction is expressed in three-dimensional space with respect to a defined set of axes in (θ,
86
φ) spherical coordinates. θ represents the angle between the proton recoil direction and the positive z axis, and φ
87
represents the angle between the positive x axis and the projection of the proton recoil direction on the xy-plane.
88
Due to crystal symmetry, only one hemisphere worth of interaction directions was measured in each material. For
89
the crystalline stilbene materials measured, the relationship between the defined axes and the stilbene crystal axes
90
directions will be discussed in Sec. 3.
91
3. Anisotropy Measurements on Stilbene
92
The directional response to 14.1 MeV and 2.5 MeV proton recoils was characterized in four stilbene samples.
93
Three were produced using the solution-growth method. Of these three samples, two are cubic samples with 1.9 cm
94
edge lengths and known crystal axes directions, shown in Fig. 1a. The third is a rectangular prism with approximate
95
dimensions 4.3 cm x 2.5 cm x 0.9 cm. The crystal axes directions in the third sample are unknown. The fourth sample
96
is an older stilbene crystal with unknown history and considerable wear, shown in Fig. 1b. It was produced using a
97
melt growth technique, so it is unlikely to be a perfect monocrystal. The crystal is cloudy and the surface has been 4
(a) Bare solution-grown cubic stilbene B with crystal axes indicated.
(b) Melt-grown cylindrical stilbene wrapped in teflon tape.
Figure 1: Photos of solution-grown and melt-grown stilbene samples.
Figure 2: Solution-grown cubic stilbene A mounted to the photomultiplier face before the outer plastic sleeve was attached. The crystal is wrapped in an inner layer of teflon tape and an outer layer of electrical tape.
98
polished numerous times. This crystal is a cylinder of size approximately 2.5 cm height and 2.5 cm diameter.
99
Each crystal was polished and wrapped in teflon tape and then in electrical tape for easier handling before mounting
100
to the face of a 60 mm Hamamatsu H1949-50 photomultiplier tube (PMT) assembly using Visilox V-788 optical
101
grease. Fig. 2 shows this for cubic stilbene A. In mounting both cubic samples with known axes, the a axis was
102
positioned parallel to the ground, and the c axis was positioned perpendicular to the PMT face. A plastic sleeve was
103
placed over the crystal and wrapped in black tape to block out external light. The coupling of the crystal and PMT
104
were fixed for all measurements to control the light collection efficiency.
105
An alternate set of axes, shown in Fig. 3, has been established for expressing the proton recoil direction within
106
the crystal so that the axes directions and features of interest are on the interior of the two-dimensional visualiza-
107
tions and not on the edges. Fig. 4 shows the directional response of the cubic solution-grown stilbene sample B to
108
14.1 MeV proton recoil events. This data is provided in the supplementary files accompanying this paper. This vi-
110
sualization displays a hemisphere worth of directions essentially as if it had been flattened and viewed from above √ with r = 1 − cos θ increasing radially outward from 0 to 1 and φ increasing counter-clockwise from 0◦ to 360◦ . In
111
all such plots, the black points indicate measurements and the color scale represents a smooth interpolation between
112
measurements. The length of the vertical bar on the colorbar indicates the average statistical error.
109
5
Figure 3: Visualization of the crystal axes directions within the alternate set of axes established for the directional distribution plots of Lˆ and Sˆ in stilbene cubic B.
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 4: Response of solution-grown cubic stilbene crystal B at various recoil directions to 14.1 MeV protons. Radial distance is r =
√
1 − cos θ.
113
The measurements on the cubic solution-grown samples with known crystal axes confirm statements by previous
114
authors that the proton recoil direction of maximum light output is along the b axis and minimum light output is
115
along the c axis. The Lˆ distribution shows a distinct maximum region at approximately (θ, φ) = (50◦ , 210◦ ) which
116
is confirmed to be the direction of the b axis in the crystal axes. The minimum region is at approximately (θ, φ) =
117
(60◦ , 330◦ ) which is the c axis, and a saddle point is at approximately (θ, φ) = (45◦ , 90◦ ) which corresponds to the a
118
axis.
119
The features are very similar between the Lˆ and Sˆ distributions. The locations of the maximum, minimum, and
120
saddle points are approximately the same in both distributions. This is different from anthracene, whose maximum
121
and minimum directions are opposite in the Lˆ and Sˆ distributions [6]. This is likely due to differences in molecular 6
122
and crystal structure between stilbene and anthracene. One difference between the Lˆ and Sˆ distributions in stilbene is
123
that the gradient from larger to smaller values is much steeper in the Sˆ distribution, making for wider “valleys” in the
124
Sˆ distribution than in the Lˆ distribution.
125
Fig. 5 shows the directional response of the cubic solution-grown stilbene sample B to 2.5 MeV proton recoil
126
events. This data is also provided in the supplementary files accompanying this paper. Although the resolution in
127
this figure is worse than in the measurements at 14.1 MeV, the qualitative features appear the same as in Fig. 4.
128
Both distributions show maximal and minimal regions at the same region. These features were the same also for the
129
measurements on the other two solution-grown stilbene samples at 14.1 MeV and 2.5 MeV. Thus, these measurements
130
showed that the qualitative features observed in the directional distribution of the light output and pulse shape are
131
consistent across solution-grown stilbene samples with different geometries and at different proton recoil energies.
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 5: Response of solution-grown cubic stilbene crystal B at various recoil directions to 2.5 MeV protons. Radial distance is r =
√
1 − cos θ.
132
Fig. 6 shows the directional distributions for the melt-grown stilbene detector. The defined axes in the melt-
133
grown stilbene have been oriented so that the features approximately line up with the corresponding features in the
134
solution-grown stilbene. The height axis of the cylinder, which is perpendicular to the face of the PMT, is located at
135
approximately (θ, φ) = (60◦ , 330◦ ). The qualitative features on the distributions in melt-grown stilbene at 2.5 MeV
136
(not pictured) agree with the features shown in Fig. 5. These distributions show that the qualitative features in the
137
directional light output and pulse shape responses are consistent between a melt-grown and solution-grown stilbene
138
sample. To quantify the magnitude of the directional dependence, two metrics are calculated for each detector. The first metric is the magnitude of change across all measurements, calculated as the ratio of the maximum to minimum Lˆ 7
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 6: Response of melt-grown stilbene crystal at various recoil directions to 14.1 MeV protons. Radial distance is r = were oriented to produce a distribution of features similar to Fig. 4, as the true crystal axes were unknown.
√
1 − cos θ. The axes
and Sˆ values: AL =
Lˆ max Lˆ min
AS =
Sˆ max Sˆ min
139
Table 2 shows the AL and AS values and their statistical errors calculated for the four stilbene materials measured.
140
The AL and AS values are on the same order across all materials at 14.1 MeV, although the AS value is not directly
141
comparable between the solution-grown materials and the melt-grown materials because the timing windows for
142
defining the delayed region in calculating S are different. The statistical error is propagated from the error produced
143
by the fit function. Table 2: Magnitude of change in Lˆ and Sˆ values measured for stilbene detectors with statistical errors. *Indicates only maximum and minimum angles from DT measurements were measured at DD energies.
AL Cubic A Cubic B Rectangular Melt-grown
Solution-grown
14.1 MeV 1.200(7) 1.191(8) 1.191(4) 1.187(5)
2.5 MeV 1.468(50)* 1.365(39) 1.327(26)* 1.401(55)
AS 14.1 MeV 2.5 MeV 1.071(1) 1.034(5)* 1.100(1) 1.058(7) 1.078(1) 1.039(5)* 1.066(1) 1.048(3)
144
Systematic limitations exist in these measurements that are not accounted for in the errors provided in Table 2. One
145
source of systematic uncertainty is the range of proton recoil directions being selected due to the geometric width of
146
the incident neutron direction and the spread in proton recoil directions being selected in the light output range Lˆ ± σ.
147
Additionally, the distribution of proton recoil directions being measured within each crystal is not exactly the same. 8
148
Another systematic effect is the possibility of fluctuations due to temperature dependence in these detectors, which has
149
been observed in other organic scintillators such as anthracene [6]. Although the temperature dependence in stilbene
150
has not been characterized, it is not expected to be a significant effect on these stilbene anisotropy measurements
151
because the room temperature remained within a 1o C range for the DT measurements and a 2o C range for the DD
152
measurements. The possibility that external effects such as magnetic field effects on the PMT are responsible for
153
the anisotropy have been ruled out as the effect is not observed for gamma-ray interactions on anthracene [6] or for
154
neutron interactions on liquid and plastic detectors, which will be shown in Sec. 4.
155
Another systematic limitation is the use of an approximate fit function on the light output spectrum. AL and AS
156
vary more for measurements at 2.5 MeV than at 14.1 MeV, likely due to the difficulty in fitting the light output
157
spectrum at 2.5 MeV. Fig. 7 shows the neutron light output spectra for neutrons produced in DT and DD collisions
158
at a given angle in the solution-grown cubic stilbene B sample. The edge in the light output spectrum is much clearer
159
to the naked eye in the DT distribution than in the DD distribution for several reasons. First, the DD measurements
160
suffer from fewer events as the cross section for the DD reaction is much lower than that for the DT reaction. Also,
161
there are fewer optical photons produced per DD event, making for worse energy resolution due to photostatistics.
162
The fit function is much more robust for the DT measurements, and consistently locates the same Lˆ value regardless
163
of bin size. Although the fit function locates Lˆ in the DD light output spectrum, it is not a very robust process as
164
changing the binning or light output range can change the final Lˆ value significantly. In the DD measurements on the
165
cubic B and melt-grown samples, 45 proton recoil directions were measured, and the distribution of Lˆ and Sˆ values
166
showed features similar to the DT distributions, lending confidence that the fit function selected a consistent feature
167
on the light output spectra. In the DD measurements on the cubic A and rectangular samples, however, measurements
168
were only taken at the angles of maximum and minimum light output from the corresponding DT measurements so no
169
assessment of the overall features could be made, giving less confidence that the fit function performed consistently
170
on those measurements, so the AL and AS values calculated from those two measurements are considered less reliable.
171
Those values are indicated with an asterisk (*) in Table 2.
172
The magnitude of change of the light output in melt-grown stilbene samples has been reported by previous au-
173
thors [10, 8]. Fig. 8 shows the AL value reported by previous authors and in this paper at different proton recoil
174
energies. All measurements are consistent with the trend that the magnitude of change in the light output decreases
175
as the proton recoil energy increases. Comparing these results to similar measurements of anthracene made with the
176
same detection system [6] confirms previous authors’ statements that the magnitude of the light output anisotropy AL
177
in stilbene is on the same order as that in anthracene, but the magnitude of the pulse shape anisotropy AS is much
178
greater in anthracene than in stilbene [8]. 9
Number of counts
Number of counts
ˆ L
1500 1000 500 0 4000
6000
8000
ˆ L
1000
500
0 500
10000
L (keVee)
600
700
800
900
1000
L (keVee)
(a) DT neutrons.
(b) DD neutrons.
Figure 7: Light output spectrum fit as measured for DT and DD neutrons incident on the cubic solution-grown stilbene sample B.
1.6
Brooks and Jones Tsukada and Kikuchi* This Paper
AL
1.4
1.2
1 10 1
Proton Erecoil (MeV) Figure 8: Magnitude of change in light output for proton recoil events at energies from 2.5-22 MeV in melt grown stilbene detectors as reported by Tsukada and Kikuchi [10], Brooks and Jones [8], and in this work. *AL reported in [10] is for a polycrystalline sample, so it is likely lower than the value for single crystal stilbene.
Another metric for quantifying the anisotropy is the variability introduced to the Lˆ and Sˆ values by the effect. If the distribution of directions measured is approximated to be evenly distributed over the hemisphere, the observed standard deviation σobs of measured Lˆ and Sˆ values is related to the variability σanis introduced by the anisotropy effect. The contribution from statistical variance is subtracted in quadrature: σanis =
σ2obs − σ2stat ,
179
where σ2stat is the average statistical variance from the set of measurements at different recoil directions. In all cases,
180
the correction for σstat resulted in a <5% difference between σobs and σanis . Values of σanis for the solution-grown
181
cubic B and melt-grown stilbene are given in Table 3, normalized to the average measured value μ. Table 3: Normalized variability, σanis /μ, introduced by the anisotropy in directional measurements of solution-grown cubic B and melt-grown stilbene samples.
Erecoil (MeV) Lˆ Sˆ
Solution-grown 14.1 2.5 4.4(4)% 7.4(8)% 2.3(2)% 1.4(2)% 10
Melt-grown 14.1 2.5 4.3(3)% 7.8(8)% 1.7(2)% 1.0(1)%
182
These measurements show that the variability introduced by the anisotropy at 14.1 MeV and 2.5 MeV is on the
183
same order in solution-grown and melt-grown stilbene samples. Again, comparing the variability in the pulse shape
184
is not perfectly fair as different timing windows were used for calculating S in the solution-grown and melt-grown
185
detectors. Regardless, this serves as an approximate comparison of the variability in the pulse shape as calculated for
186
optimal neutron-gamma PSD in these materials.
187
4. Measurement of Plastic and Liquid Scintillators
188
Directional measurements were also made on plastic and liquid scintillator detectors. The plastic sample was
189
an LLNL PSD-capable plastic cylinder approximately 1.25 cm tall and 2.5 cm in diameter. The plastic sample was
190
wrapped and coupled to the PMT using the same procedure as described for the stilbene detectors. The liquid scintil-
191
lator was EJ-309 in a 5 cm tall 5 cm diameter cylinder.
192
The same process was used for calculating Lˆ and Sˆ as was used for the stilbene measurements. In the liquid and
193
plastic measurements, the detector resolution σ in Eq. (3) was fixed at a constant value. That constant value was
194
determined by taking the mean of the σ values produced in a preliminary analysis with σ allowed to vary as one of
195
the fit parameters. Fig. 9 shows the directional response of the EJ309 liquid to 14.1 MeV protons. Fig. 10 shows the
196
same measurements on the PSD-capable plastic scintillator. Additional measurements were taken of 2.5 MeV proton
197
recoils, and similar qualitative features were observed in the directional responses.
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 9: Response of EJ309 liquid at various recoil directions to 14.1 MeV protons. Radial distance is r =
√
1 − cos θ.
198
These measurements do not show any directional dependence correlated to detector orientation. The strongest
199
evidence supporting that conclusion is the lack of qualitative features that are observed in the stilbene detectors. 11
(a) Light output Lˆ (keVee).
(b) Pulse shape parameter Sˆ .
Figure 10: Response of plastic scintillator at various recoil directions to 14.1 MeV protons. Radial distance is r =
√
1 − cos θ.
200
These measurements do not show a maximum, minimum, and saddle point region in the hemisphere with smooth
201
transitions in between as was observed in the stilbene measurements. Instead, the directional dependence appears to
202
be random variability from statistical and other non-statistical effects. Table 4 shows the observed variability σobs and
203
the average statistical variability σstat in the liquid and plastic measurements. For all cases, σobs is on the order of
204
σstat , indicating that statistical fluctuations dominate the observed variability. Sources of systematic variability such
205
as those described in Sec. 3 may also exist in these measurements. Again, temperature dependence may be one of
206
these, but it is expected that such effects are not significant as the room temperature varied within a range of 1◦ C for
207
the DT measurements and within 2◦ C for the DD measurements. Additionally, no distinct correlation was observed
208
between Lˆ or Sˆ and the room temperature.
209
Thus, both qualitative and quantitative observations confirm statements made by previous authors that no direc-
210
tional dependence exists in liquid and plastic organic scintillators. The lack of directional variability correlated to
211
detector orientation in these amorphous materials indicates that the directional dependence observed in crystalline
212
materials is in fact due to an internal effect and not produced by the measurement system or external factors. Table 4: Variability in directional measurements of liquid and plastic scintillators. μ is the average measured value of the expected light output Lˆ or pulse shape Sˆ .
Lˆ Sˆ
Erecoil (MeV) σobs /μ σstat /μ σobs /μ σstat /μ
EJ309 Liquid 14.1 2.5 0.64(05)% 1.04(11)% 0.39(05)% 0.74(04)% 0.16(01)% 0.59(06)% 0.05(01)% 0.16(02)%
12
Plastic 14.1 2.5 0.93(07)% 1.42(15)% 0.53(02)% 1.41(12)% 0.27(02)% 0.86(09)% 0.10(04)% 0.33(02)%
213
5. Conclusion
214
The measurements reported in this paper characterize the directional dependence of the scintillation produced by
215
14.1 MeV and 2.5 MeV proton recoil events across a hemisphere worth of directions in melt-grown and, for the first
216
time, solution-grown stilbene detectors. The light output and pulse shape produced by 14.1 MeV and 2.5 MeV proton
217
recoils at a hemisphere worth of directions in a solution-grown stilbene sample is provided in the supplementary files
218
accompanying this paper. The scintillation was confirmed to be maximal for proton recoils along the b crystal axis
219
and minimal along the c axis. The magnitude of change in the light output and pulse shape was found to be consistent
220
across four stilbene detectors of different geometries and growth methods. The relationship between magnitude of
221
change in light output and proton recoil energy agreed with measurements by other authors at different proton recoil
222
energies.
223
The directional response of liquid and plastic scintillator detectors was also measured and results were provided to
224
demonstrate that no anisotropy was present. Those measurements support the leading hypothesis that the scintillation
225
anisotropy in crystalline materials is caused by an internal effect and is not the result of an external effect on the
226
measurement system.
227
6. Acknowledgements
228
229
The authors wish to thank John Steele for his assistance in building the motor driven rotational stage, and Natalia Zaitseva of Lawrence Livermore National Laboratory for providing the solution-grown stilbene and plastic samples.
230
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship
231
Program under Grant No. DGE 1106400. This material is based upon work supported by the Department of Energy
232
National Nuclear Security Administration under Award Number DE-NA0000979 through the Nuclear Science and
233
Security Consortium. Sandia National Laboratories is a multi-program laboratory managed and operated by San-
234
dia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s
235
National Nuclear Security Administration under contract DE-AC04-94AL85000.
236
[1] L. Carman, N. Zaitseva, H. P. Martinez, B. Rupert, I. Pawelczak, A. Glenn, H. Mulcahy, R. Leif, K. Lewis, S. Payne, The effect of material
237
purity on the optical and scintillation properties of solution-grown trans-stilbene crystals, Journal of Crystal Growth 368 (2013) 56–61.
238
doi:10.1016/j.jcrysgro.2013.01.019.
239
URL http://www.sciencedirect.com/science/article/pii/S0022024813000584
240
[2] N. Zaitseva, A. Glenn, L. Carman, H. Paul Martinez, R. Hatarik, H. Klapper, S. Payne, Scintillation properties of solution-grown trans-stilbene
241
single crystals, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated
242
Equipment 789 (2015) 8–15. doi:10.1016/j.nima.2015.03.090.
13
243
[3] M. Bourne, S. Clarke, N. Adamowicz, S. Pozzi, N. Zaitseva, L. Carman, Neutron detection in a high-gamma field using solution-grown
244
stilbene, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
245
806 (2016) 348–355. doi:10.1016/j.nima.2015.10.025.
246
URL http://www.sciencedirect.com/science/article/pii/S0168900215012322
247
[4] S. Pozzi, M. Bourne, S. Clarke, Pulse shape discrimination in the plastic scintillator EJ-299-33, Nuclear Instruments and Methods in Physics
248
Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 723 (2013) 19–23. doi:10.1016/j.nima.2013.
249
04.085.
250
URL http://www.sciencedirect.com/science/article/pii/S016890021300524X
251
[5] D. Cester, G. Nebbia, L. Stevanato, F. Pino, G. Viesti, Experimental tests of the new plastic scintillator with pulse shape discrimination capa-
252
bilities EJ-299-33, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated
253
Equipment 735 (2014) 202–206. doi:10.1016/j.nima.2013.09.031.
254
URL http://www.sciencedirect.com/science/article/pii/S0168900213012655
255
[6] P. Schuster, E. Brubaker, Investigating the Anisotropic Scintillation Response in Anthracene through Neutron, Gamma-Ray, and Muon
256
Measurements, IEEE Transactions on Nuclear Science 63 (3) (2016) 1942–1954. doi:10.1109/TNS.2016.2542589.
257
URL http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=7465791
258 259 260
[7] E. Brubaker, J. Steele, Neutron imaging using the anisotropic response of crystalline organic scintillators, IEEE Nuclear Science Symposium Conference Record (2010) 1647–1652doi:10.1109/NSSMIC.2010.5874055. [8] F. Brooks, D. Jones, Directional anisotropy in organic scintillation crystals, Nuclear Instruments and Methods 121 (1) (1974) 69–76. doi:
261
10.1016/0029-554X(74)90141-4.
262
URL http://www.sciencedirect.com/science/article/pii/0029554X74901414
263 264 265 266
[9] K. Tsukada, S. Kikuchi, Y. Miyagawa, Directional anisotropy in the characteristics of the organic-crystal scintillators, II, Nuclear Instruments and Methods 37 (1965) 69–76. doi:10.1016/0029-554X(65)90339-3. URL http://www.sciencedirect.com/science/article/pii/0029554X65903393 [10] K. Tsukada, S. Kikuchi, Directional anisotropy in the characteristics of the organic-crystal scintillators, Nuclear Instruments and Methods
267
17 (3) (1962) 286–288. doi:10.1016/0029-554X(62)90007-1.
268
URL http://www.sciencedirect.com/science/article/pii/0029554X62900071
269 270 271
[11] M. M. Bourne, A. DiFulvio, S. D. Clark, S. A. Pozzi, N. Zaitseva, S. Payne, Scalability of Stilbene for Use in Nuclear Nonproliferation Applications, in: Institute of Nuclear Materials Management Annual Meeting Proceedings, Indian Wells, CA, 2015. [12] J. Birks, The Theory and Practice of Scintillation Counting, Pergamon Press, Oxford, 1964. doi:10.1016/B978-0-08-010472-0.
272
50001-X.
273
URL http://www.sciencedirect.com/science/article/pii/B978008010472050001X
14