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Some features and results of thermal neutron background measurements with the [ZnS(Ag)+6LiF] scintillation detector
Author’s Accepted Manuscript Some features and results of thermal neutron background measurements with the 6 [ZnS(Ag)+ LiF] scintillation detector V.V. Kuzminov, V.V. Alekseenko, I.R. Barabanov, R.A. Etezov, A.M. Gangapshev, Yu.M. Gavrilyuk, A.M. Gezhaev, V.V. Kazalov, A.Kh. Khokonov, S.I. Panasenko, S.S. Ratkevich
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S0168-9002(16)31079-8 http://dx.doi.org/10.1016/j.nima.2016.10.038 NIMA59397
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 30 May 2016 Revised date: 20 October 2016 Accepted date: 20 October 2016 Cite this article as: V.V. Kuzminov, V.V. Alekseenko, I.R. Barabanov, R.A. Etezov, A.M. Gangapshev, Yu.M. Gavrilyuk, A.M. Gezhaev, V.V. Kazalov, A.Kh. Khokonov, S.I. Panasenko and S.S. Ratkevich, Some features and results of thermal neutron background measurements with the [ZnS(Ag)+6LiF] scintillation detector, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.10.038 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.
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Some features and results of thermal neutron background measurements with the [ZnS(Ag)+6 LiF] scintillation detector V.V. Kuzminova , V.V. Alekseenkoa , I.R. Barabanova , R.A. Etezova , A.M. Gangapsheva , Yu.M. Gavrilyuka , A.M. Gezhaeva , V.V. Kazalova , A.Kh. Khokonovb , S.I. Panasenkoc , S.S. Ratkevichc,∗ a Institute
for nuclear research, 117312, Moscow, Russia Berbekov Kabardino-Balkarian State University, 360004, Russia c V.N.Karazin Kharkiv National University,61022,Kharkiv, Ukraine
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b Kh.M.
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Abstract Features of a thermal neutron test detector with thin scintillator [ZnS(Ag)+6 LiF]
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are described. Background of the detector and its registration efficiency were
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defined as a result of measurements. The thermal neutron flux at different lo-
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cations, and for different conditions around the Baksan Neutrino Observatory
15
are reported.
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Keywords: thermal neutron detectors, scintillation detector, low-background
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measurements
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1. Introduction
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Neutrons play an important role in a radiation field of surrounding envi-
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ronment in low-background laboratories. The neutron fluence is a critical issue
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of experiments for ββ-decay, the direct dark matter search, and solar neutrino
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experiments.
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Thermal neutrons can be detected through a nuclear reaction with nucleus
24
of 6 Li, contained in the scintillation materials, resulting in alpha and triton pro-
25
duction. An example of such material is a silver-activated zinc sulfide [ZnS(Ag)]
26
scintillator loaded with 6 LiF. The high scintillation yield for α-particles makes
27
this scintillator very efficient for neutron detection. The large area scintillation ∗ Corresponding
author, Tel +380 577075185; +7 8663875149 Email address: [email protected] (S.S. Ratkevich) Preprint submitted to NIM
October 22, 2016
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detector based on thin [ZnS(Ag)+6 LiF]-scintillator plate has been put into oper-
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ation at the Baksan Neutrino Observatory of the Institute for Nuclear Research
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of the Russian Academy of Sciences (BNO) for measurement of thermal neutron
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background flux [1, 2].
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As mentioned above, neutron registration is based on the 6 Li(n, α)3 H+4786
33
keV reaction. The 6 Li cross section for the thermal neutrons is 945 b [3]. The
34
kinetic energy of the reaction products (Eα = 2051 keV, E3 H = 2735 keV)
35
is converted into scintillation light. Spectrometric characteristics of these de-
36
tectors are not well known. One of the aims of our work was to explore these
37
characteristics for continuous monitoring of neutrons in conditions of the ground
38
level laboratory buildings and underground laboratories.
39
2. Test detector
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A schematic view of the test detector with electronics is shown on the Fig. 1.
41
The detector is assembled in a rectangular casing 30 × 30 × 50 cm3 made of
42
galvanized steel 0.7 mm thick. The top and the bottom covers of the casing are
43
detachable. A dividing plate is mounted in the middle of the casing and has
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central 150 mm diameter hole to place an FEU-173 photomultiplier (PMT). A
45
charge sensitive preamplifier (CSP) with ∼ 100 µs self-discharge time is installed
46
on the wall inside of the upper part of the casing. Output pulses from the
47
PMT’s anode resistor (4.8 MΩ) go the CSP and further to input of the digital
48
oscilloscope card (LAn-10M5). The output data are recorded with a personal
49
computer. The sampling frequency is 6.25 MHz. A flat flexible plate with
50
the [ZnS(Ag)+6 LiF] scintillator is placed at the bottom of the lower section.
51
The plate consists of a white sheet of plastic film, the sticky side of which is
52
covered with the scintillator grains having an average thickness of ∼ 0.1 mm [2].
53
The sheet is laminated by the layer of polyethylene terephthalate. An internal
54
side surface of the bottom section is covered with a reflecting Mylar sheet for
55
effective light collection. The pure ZnS(Ag) and 6 LiF have densities of equal
56
to 4.09 g/cm3 [4] and ∼ 2.64 g/cm3 [5], respectively. The mixture is in the
57
proportion 1 : 3, and has a density of ∼ 3 g/cm3 . 2
58
3. Results of measurements
59
Two types of pulses shown in Fig. 2 (a, b) were observed in the measure-
60
ment with the detector at the ground laboratory. The first pulse type (a) has a
61
rise time τf of the preamplifier output pulses in the range of 16 − 25 µs which
62
corresponds to the de-excitation time τs = 8 − 10 µs (τf ≈ 3τs ) [4, 6] of the
63
fine-grained scintillator. The CSP integrates an input current. The integrated
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output pulse has a maximum at the point where the rate of current charging
65
equals the preamplifier discharge rate. The second type of a pulses (b) have a
66
rise time τf = 0.8 µs. Its shape is similar to noise pulses which occur after the
67
scintillator and PMT are exposed to external light. The intensity and ampli-
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tude of the noise pulses in the range of interest fall to zero some hours after
69
irradiation. The remaining pulses of type (b) are not noise, as their intensity is
70
proportional to the intensity of cosmic rays and decreases when moving the de-
71
tector to a deeper underground laboratory. It is known that a photomultiplier’s
72
noise pulses could be created by charged particles from an outer radioactive
73
background, cosmic rays and by charged particles from decays of radioactive
74
isotopes contained in the PMT construction materials [7].
75
To clarify the nature of the type (b) pulses an additional study was done
76
to examine two possibilities. The first possibility was direct generation of the
77
primary electrons from the photocathode or dynodes by cosmic rays. The sec-
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ond one is the appearance of a photoelectron from the photocathode as a result
79
of absorption of Cerenkov radiation created by a charged particle in the glass
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entrance of the PMT. For the second one, the cosmic rays coming from direc-
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tions around the vertical generate Cerenkov radiation directed outward from
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the entrance window of the PMT. This light is returned to the PMT after re-
83
flection from the lower compartment surfaces. To determine which of these two
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possibilities is the real reason, the PMT entrance (photocathode) window was
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covered by the black paper. After that procedure only pulses with a short rise
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time remained. Also, the intensity of such pulses follows the intensity of cosmic
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rays as the detector is moved from the second floor of the laboratory building to
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the ground floor. The short rise time pulses are therefore generated by cosmic
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rays in the photocathode or dynode.
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A series of measurements were taken with the test detector at ground level,
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and at underground locations around the BNO. Several different test conditions
92
were also considered. The locations and conditions are:
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1. Deep Underground Low Background Laboratory (DULB-4900), an un-
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shielded room located in an underground hall with mountain thickness of
95
4900 m water equivalent (w.e.). At this location and the other below item-
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ized places (2-11), two types of spectra have been measured: (a) without
97
any shielding materials, and (b) with the 0.1 × 100 × 100 cm3 Cd sheet
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(absorber) placed under the detector.
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2. DULB-4900 low background compartment with the walls made of 25 cm polyethylene +0.1 cm Cd+15 cm Pb. A spectrum (a) was measured.
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3. The same as location two with a thermal neutron source.
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4. Low background underground laboratory “KAPRIZ” at the 1000 m w.e.
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105
106
107
108
109
110
111
112
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114
115
116
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The spectra (a) and (b) were measured. 5. Low background underground laboratory “NIKA” at the 660 m w.e. The spectra (a) and (b) were measured. 6. “CARPET-2” muon detector set-up at the 5 m w.e. The spectra (a) and (b) were measured. 7. The ground floor of the four-storey laboratory building. The spectrum (a) was measured. 8. The river side of the second floor (room 204) of the four-storey laboratory building. The spectra (a) and (b) were measured. 9. The valley side of the second floor (room 211) of the four-storey laboratory building. The spectrum (a) was measured. 10. Ground building “ELLING” of the “CARPET-2” set-up. The spectra (a) and (b) were measured. 11. The fourth floor (room 404) of the four-storey laboratory building (LAB). The spectrum (a) was measured. 4
118
12. The open soil. The spectra (a) and (b) were measured.
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The items in the above list are following of a cosmic rays absorber thickness
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above the installation in a sequence from 4900 to 0 m w.e.. The depths of each
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location is shown in the third column of the Table 1. A schematic view of a
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longitudinal section of the BNO adit and Andyrchi slope is shown in Fig. 3 which
123
presents the locations of different underground laboratories and the dependence
124
of underground muon flux on the distance from the entrance. A brief description
125
of the locations is given in reference [8].
126
For better understanding of statistical characteristics of the pulses in mea-
127
surements, a 63.12 hour set of data from site eight has been analyzed. The
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total pulse height spectrum is shown in Fig. 4 (spectrum “a”). The distribu-
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tion of rise times from 20% to 80% of the pulse height, normalized per hour, is
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shown in Fig. 5. Two peaks in the distribution of pulse rise times are clearly
131
separated. There is a narrow distribution in the range 0.5-1.6 µs, and a wide
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second distribution in the range 3.2-13 µs. The second range corresponds to
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the pulses produced by the scintillator which are not seen when a light-proof
134
sheet is placed between the scintillator and PMT photocathode. This allows
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the separation of spectrum (a) on the Fig. 4 into two parts - fast (b) and slow
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(c) components.
137
One can see that the fast pulses contribute mainly at the low amplitudes.
138
Detailed analysis of the fast pulse shapes shows that the pulses have different
139
decay shapes. These differences are explained by a small variable contribution
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of the scintillation light to the muon-induced background signal in the PMT.
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The proportion of the two components depends on the number of particles in
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an event, and on the quantity of tracks crossing the scintillator and PMT. The
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thin scintillator has a low sensitivity to the cosmic rays and electrons, so the
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pulse heights are small and lie below the registration threshold.
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A significant number of extraneous pulses can occur during long term low
146
count rate measurements at the underground conditions. The main source of
147
such noise pulses is the switching on and off of laboratory equipment. These
5
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pulses have a shape that is different from signal ones, so they can be excluded
149
from the spectra by using a pulse shape discrimination.
150
Site two is an underground site that is well protected against neutrons,
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and measurements there are used to define the background due to the detector
152
materials themselves. The spectrum from site two, normalized per 100 h, is
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shown in Fig. 6 (spectrum “a”). The corresponding distribution of rise times
154
is shown in the Fig. 7 (curve “a”). A calibration with thermal neutrons from a
155
specially prepared source was carried out in the same low-background conditions
156
(site 3) in order to clarify the nature of these events. The neutron source was
157
made as a set of four α-sources from the set: 226
233
Ra(38 kBq) and triplet (
U+
238
238
Pu +
Pu(48.2 kBq),
239
239
Pu(3.58
158
kBq),
Pu) (40.9 kBq), covered
159
with a beryllium foil. This combined source was placed into polyethylene box
160
with 10 cm wall thickness, and then the box was put under the detector. The
161
amplitude spectrum of the neutron source, normalized to 100 h, is shown in
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Fig. 6 (spectrum “b”). The corresponding distribution of the pulse rise times is
163
shown in Fig. 7 (curve “b”).
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The background spectrum has a longer energy extension than the neutron
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one. At the same time a distribution of a pulse rise time is shifted to the shorter
166
rise-time. Thus, the detector background is created by highly ionizing particles
167
with energies larger than the energy of the neutron reaction products. The
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naturally occurring long lived isotopes
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α-particles, are contained as impurities in the scintillator, and could be possible
170
sources of background. 210
−
Th and
238
U, which decay emitting
Po (T1/2 = 138.4 d, Eα = 5.3 MeV) generated in a
171
decay chain of
172
source. This second isotope originates from radon decay in air, and deposits on
173
the scintillator surface. This background could be suppressed by using special
174
protection against the penetration of radon and its daughters from the gas
175
environment of the scintillator production area. The α-particles born outside of
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the scintillator plate will be absorbed in the covering Lavsan film and will not
177
produce any noticeable effect.
178
Pb (β
210
232
- decay, T1/2 = 21.8 y) could be another background
The range of estimated path lengths of α-particles with the energies of 2051 6
179
keV, 4800 keV and 5300 keV in the scintillator with a density of 3.0 g/cm3 ,
180
are 10.6 µm, 28.6 µm and 32.1 µm respectively [9]. The range of the path
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length of the 2735 keV triton is 56.3 µm. If the main source of detectors own
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background is α-particle from
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than the one from a neutron reaction products due to the difference in energies.
184
The difference may be additionally increased if the relative light yield ZnS(Ag)
185
decreased at lower particle energy according to ionization density rise as it
186
occurs for other inorganic scintillators [4]. The light output for the two reaction
187
products with the 4800 keV sum energy will be less than the light output for the
188
α-particle with the same energy. A shorter rise time of background pulses could
189
be explained by location of the α-source on a surface of the scintillator grains.
190
Trajectories of α-particles will be directed into the grains in this case and the
191
main part of energy will be released inside a grain volume. In Ref. [4] it is
192
noted that disperse scintillators on the basis of ZnS(Ag) possess long afterglow.
193
It could be explained by the increase of a the relative number of long lived
194
excitation traps located on grain surfaces. The particles absorbed in a grain
195
surface layer will produce longer deexcitation time in this case. Vertices of
196
neutron reactions are distributed uniformly in a 6 LiF component volume. The
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reaction products should exit from this materials and fall into the surface layer of
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the ZnS(Ag) grain and produce a scintillation. The ionization density increases
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as the particle energy decreases in accordance with the energy loss dependence
200
[10]. A considerable part of energy will be released in the surface layers of two
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adjacent grains giving longer pulse rise time in comparison with the particle
202
absorbed inside the grain. The above-mentioned features of the distributions
203
corresponding to detectors’ own background pulses can serve as an indirect
204
indication that some number of these pulse are due to collective surface effect
205
of adjacent grains.
210
Po, then the pulse rise time will be greater
206
The detector count rates, per hour, integrated above the third channel of
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the spectra “b” and “c” Fig. 4 are presented in Table 1 for all sites indicated in
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the list.
209
Values of a thermal neutron flux F in the all examined sites could be obtained 7
210
from the neutron count rates. The flux of particles falling on given surface (F )
211
is the ratio of particles number ∆N and time interval (∆t) to this interval:
212
F = ∆N/∆t. Hence, count rate n = ε × F , where ε is the neutron registration
213
efficiency of the scintillation plate. The neutron flux density parameter ϕ is
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used to make the comparison of the results obtained with different geometry
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detectors easy. The definition of ϕ is the ratio of particle flux dFS penetrating
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the volume of an elementary sphere to the area of it’s central cross section
217
dS : ϕ = dFS /dS. The dFS value could be determined using specific neutron
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flux falling on an elementary square scintillator as dFS = 4 × F/S where S is
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the area of the scintillator plane and the coefficient 4 is equal to the ratio of the
220
sphere surface area to the sphere cross section area. As a result ϕ = 4n/(2Sε).
221
It seems impossible to calculate ε-value due to uncertainties in composition
222
and structure of the scintillator layer. This value was obtained experimentally
223
from comparison of n1 count rate of test detector with n1(2) count rate of mod-
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ified detector with additional similar light-proof protected scintillator plate (2)
225
put under the active plate (1). The detector was shielded by 1 mm cadmium
226
foil against thermal neutron flux from upper hemisphere to shape a single-sided
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neutron flux. The count rate of test detector is equal to n1 = ε1 × F and the
228
one for the modified detector is n1(2) = ε1(2) × (F − ε2 × F ) where ε1 is a plate
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(1) absorption efficiency of thermal neutron flux F , ε1(2) is the plate (1) absorp-
230
tion efficiency of flux which had not been absorbed by plate (2). The value of
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efficiency is an integral characteristic of the process of absorption of neutrons
232
coming at different angles and depends on the path passed by neutrons in the
233
scintillator. The angular distribution of neutrons after passing one scintillator
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layer is pulled in the normal direction due to the more effective absorption in
235
plate(2) neutrons with inclined incoming angles. An absorption will be lower
236
for passed neutrons and ε1(2) will be lower than ε1 (ε1(2) ≤ ε1 ). The count rates
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are specified by the expressions n2 = ε2 × F and n2(1) = ε2 (1) × (F − ε1 × F )
238
in a case when the plate (2) is used as the active one.
239
Five unknown variables ε1 , ε1(2) , ε2 , ε2(1) and F are in four obtained equa-
240
tions. One needs to measure additionally a total count rate n[1+2] = n[2+1] in 8
241
the case when both plate are used in active mode to determine precisely all five
242
values. Such measurement is possible if the plates emit scintillation light into
243
both hemispheres in detector with two PMTs. The task could be solved for
244
the reviewed detector if the plate adsorbs neutron not strongly and it is pos-
245
sible to take ε1(2) ≈ ε1 and ε2(1) ≈ ε2 . Then simple algebra gives expressions
246
ε1 = [n1 − n1(2) ]/n2 and ε2 = [n1 − n1(2) ]/n1 .
247
The measurements were done in the site (11) of the list. The result ε1 =
248
0.22±0.01 and ε2 = 0.21±0.01 was obtained. These values were used as normal-
249
ization factors in the calculations of a thermal neutron flux detection efficiency
250
in the case of a stack of two absorbing homogenous plates. This method allows
251
one to exclude by the calculation an uncertainty of the experimental efficiencies
252
connected with the assumptions that ε1 (2) ≈ ε1 and ε2 (1) ≈ ε2 . The corrected
253
efficiency values εc1 = 0.17 ± 0.01 and εc2 = 0.16 ± 0.01 were found as a result.
254
The value εc1 was used for the thermal neutron flux density calculation from the
255
experimental data.
256
A background count rate measured at the site (2) was subtracted from data
257
in a process of determination thermal neutron flux density at other site. The
258
obtained ϕ-values are presented in the last column the Table 1. The limit for
259
thermal neutron flux density in the site (2) was obtained from base measure-
260
ments with a 3 He proportional counter for neutrons [13].
261
Discussion of results
262
The intrinsic background of the scintillator plate was found to be to (2.69 ±
263
0.05) × 10−2 cm−2 h−1 in the site (2). This value is comparable with a value
264
of surface α-activity of the commercial copper M1 and steel samples which
265
is ∼ (0.5 − 1.0) × 10−2 cm−2 h−1 [11]. The surface α-activity of the silicon
266
semiconductor samples could reach ∼ 0.001 cm−2 h−1 . It seems possible to
267
prepare a scintillator plate with similar surface α-activity using specially selected
268
low background [ZnS(Ag)+6 LiF] material and clean technology for the plate
269
preparation. Hence the sensitivity of such scintillator detector (SD) for thermal
270
neutrons may be comparable with sensitivity of the 3 He proportional counter.
271
The present ratio of sensitivities is ∼ 16 as it follows from comparison of the 9
272
data from the sites (2) and (4) of the Table 1. The difference of the rise time
273
distribution of background and neutron pulses also could be useful for plate
274
sensitivity. The results obtained are in good agreement with measurements
275
with the 3 He proportional counter in the Ref. [12].
276
The value of the detector’s own background defines its sensitivity for neu-
277
tron measurements in deep underground conditions as shown in Table 1. The
278
count rate of neutrons in the DULB-4900 (Table 1, site (1a)) is equal to (21.32−
279
16.4)/6.11 = (0.8 ± 0.08) × 10−2 cm−2 h−1 with signal over background of ∼ 0.3.
280
Neutrons are born in surrounding rock mainly due to (α, n)-reactions with light
281
elements. Walls of the “KAPRIZ” (site 4) laboratory are covered with 30 cm low
282
background concrete layer using crushed dunite rock. The concrete considerably
283
reduces the neutron flux from the rock. (The dunite concrete was also used in the
284
construction of the “NIKA”, site five, laboratory.) The comparison of neutron
285
fluxes in the “DULB-4900” and “KAPRIZ” measured with the 3 He proportional
286
counter shows that the concrete reduces the neutron flux by ∼ 5.2 times. One
287
can estimate an expected neutron flux in the “KAPRIZ” using this coefficient
288
and the SD count rate in the site (1a) as (0.15±0.02)×10−2 cm−2 h−1 . The calcu-
289
lation with the data from the Table 1 gives the value (0.05±0.08)×10−2 cm−2 h−1
290
which does not disagree with the estimated one. This last measurement pro-
291
vides a limit for the neutron flux density is 5.9 × 10−6 cm−2 c−1 at 90% C.L. in
292
the sites (1a and 1b).
293
The SD’s own background gives a small part of the total count rate in mea-
294
surements at the surface ground and at shallow underground sites where reac-
295
tions of cosmic rays with the element’s nuclei of the environment are the main
296
source of the neutron.
297
The comparison of the SD count rates with and without the cadmium ab-
298
sorber shows that the absorber decreased the neutron flux by ∼ 1.9 times in a
299
room with a roof, and ∼ 2.4 times in locations without a roof. Thus, the ratio
300
of neutron fluxes from the soil and from the atmosphere at the open place is
301
equal to ∼ 1.4.
302
The SD count rate of short rise times of pulses in the underground conditions 10
303
does not depend practically on a value of the external gamma-ray background
304
level as it seen in the Table 1. This noise component could be born directly in the
305
PMT by high ionizing particles. The pulses could appear at the photocathode
306
in result of direct generation of electrons by α-particles on the surface of the
307
entrance window from the internal α-activity. A count rate of such pulses is
308
also proportional to the cosmic ray muon flux.
309
Conclusions
310
We have studied the characteristics of a thermal neutron detector with
311
[ZnS(Ag)+6LiF] scintillator with rectangular dimensions of 216 × 304 mm2 .
312
Pulse shape analysis done with data from digital oscilloscope have shown that
313
the background from fast rise time pulses is from cosmic rays interacting di-
314
rectly in the photomultiplier and appear in the low energy region of the spec-
315
tra.
316
underground locations around BNO INR RAS with different shielding from
317
cosmic rays. The inherent background of the detector created by α-particles
318
from decays of inner radioactive admixture with surface α-activity at level of
319
(2.69 ± 0.05) × 10−2 h−1 cm−2 was measured. The ratio of the signal to back-
320
ground was ∼ 0.3 at deep underground locations. The neutron pulses have a
321
shorter rise time than the background ones. When the pulse rise time is used for
322
discrimination between neutron and alpha backgrounds, a ∼ 2 times rejection
323
of neutron events was obtained.
Measurements of the thermal neutron flux were made at ground and
324
Acknowledgement
325
The work was carried out in part with the financial support of the Federal
326
Objective Program of the Ministry of Education and Science of the Russian
327
Federation “Research and Development in the 2007-2013 years on the Priority
328
Directions of the Scientific and Technological Complex of the Russia” under the
329
contract No. 16.518.11.7072 and the “Russian Foundation for Basic Research”
330
under the grant No. 14-22-03059.
331
332
We are thankful to Yu.V. Stenkin for providing us with the samples of the scintillator and numerous useful critical comments and discussions.
11
333
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[4] M.N. Medvedev, Scintillation detectors. Moscow: Atomizdat 1977.
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[5] M.E. Globus, B.V. Grinev, Inorganic scintillators (new and traditional ma-
345
346
347
terials). Kharkiv: Akta, 2001. [6] Phosphor
data
-
www.appscintech.com.
6
LiF/ZnS:Ag
Phos-
phor/Scintillator Data Sheet 39- ND - iss1.doc.
348
[7] “PHOTOMULTIPLIER TUBES (principles and applications)”. Produced
349
and distributed by Philips Photonics International Marketing. BP 520, F-
350
19106 BRIVE, France. Philips Export B.V. 1994.
351
[8] V.V. Kuzminov.,
“The Baksan Neutrino Observatory”. // Eur.
352
Phys. J. Plus 127 (2012) 113;
353
http://www.inr.ac.ru/.
354
355
doi:
10.1140/epjp/i2012-12113-0;
[9] O.F. Nemets, Yu.V. Ghofman, Handbook of Nuclear Physics. Kiev: Naukova Dumka. 1975.
356
[10] M.J. Berger, J.S. Coursey, M.A. Zucker and J. Chang, Stopping-Power
357
and Range Tables for Electrons, Protons, and Helium Ions. NIST, Physical
358
Measurement Laboratory, http://www.nist.gov/pml/data/star/.
12
359
[11] E.L. Kovalchuk, V.V. Kuzminov, A.A. Pomansky, “Surface alfa activity
360
of different materials”. Proc. of the Int. Conf. “The Natural Radiation
361
Environment III”, Houston, Texas, April 23-28, 1978, V.1, 1980, P.673.
362
[12] V.V. Alekseenko, I.R.Barabanov, R.A. Etezov, et al., “Results of measure-
363
ments of an environment neutron background at BNO INR RAS objects
364
with the helium proportional counter” // arXiv: 1510.05109 [physics.ins-
365
det].
366
[13] A.Kh. Khokonov, Yu.V. Savoiskii, A.V. Kamarzaev, “Neutron Sensitivity
367
and Detection Efficiency of 3 He- and
368
Nuclei, V.73, No.9, 2010, P.1482; doi: 10.1134/S1063778810090024.
13
10
F3 -Counters” // Physics of Atomic
Table 1: Count rates of the detector at 1 hour for the data integrated above the third channel of the spectra (b) and (c) Fig. 4 and the thermal neutron flux densities.
No. site
Place, conditions
depth, (m w.e.)
1a b 2
DULB-4900 -//- + (Cd) low background compartment -//- + (n-source) KAPRIZ -//- + (Cd) NIKA -//- + (Cd) µ-detector -//- + (Cd) LAB, ground LAB, 204 -//- + (Cd) LAB, 211 ELLING -//- + (Cd) LAB, 404 Open soil -//- + (Cd)
4900
3 4a b 5a b 6a b 7 8a b 9 10 a b 11 12 a b
4900 4900 1000 660 5 1.3 0.7 0.7 0.2 0.2 0
Count rate,h−1 (3-256 channel) Short pulse Long pulse rise time rise time 0.13±0.03 21.3±0.4 0.09±0.03 18.7±0.5 0.10±0.02 16.4±0.3 0.2±0.1 0.13±0.03 0.13±0.03 0.19±0.04 0.14±0.03 8.4±0.5 8.2±0.4 6.5±0.5 16±2 16±1 19.1±0.9 23±1 22±3 22±1 28±3 27±3
14
80±2 16.7±0.3 16.5±0.4 17.8±0.4 16.9±0.3 64±1 43±1 240±3 866±14 482±6 672±5 1415±8 730±16 1439±9 1704±31 702±14
Thermal neutron flux density, (s−1 ×cm−2 ) (2.6±0.4)×10−5 (1.2±0.4)×10−5 ≤ 3.8 × 10−7 (90% C.L.) (3 He prop. counter) (3.4±0.4)×10−4 ≤ 5.9 × 10−6 (90% C.L.) ≤ 5.9 × 10−6 (90% C.L.) (7.5±3.1)×10−6 (3.4±3.1)×10−6 (2.8±0.3)×10−4 (1.4±0.2)×10−4 (1.2±0.1)×10−3 (4.5±0.4)×10−3 (2.5±0.2)×10−3 (3.5±0.3)×10−3 (7.5±0.6)×10−3 (3.8±0.4)×10−3 (7.6±0.6)×10−3 (9.0±0.8)×10−3 (3.7±0.3)×10−3
Figure 1: The schematic view of the test detector with the [ZnS(Ag)+6 LiF] scintillator and its electronics.
15
3.12
a 2.34
amplitude, mV
1.56 0.78 0.0 1.95
b
1.56 1.17 0.78 0.39 0.0
0
20
40
60
80
100
120
140
160
time, µs Figure 2: The two types of test pulses from the detector. “a” - the long pulse rise time, “b” - the short pulse rise time.
16
Figure 3: Schematic view of a section of the Andyrchy slope along the adit (right scale) and dependence of underground muon flux on the laboratory location depth (left scale).
events / channel
3000
a
2000
b c
1000
0 0
20
40
60
80
100
120
channel Figure 4: Pulse amplitude spectra from the detector in the site (8) collected at 63.12 h): “a” – a total spectrum, “b” – a spectrum of pulses with a short front, “c” – a spectrum of pulses with a long front.
17
events / bin / hour
100 10 1 0.1 0.01 0
10
20
30
40
50
60
70 rise time, µs
Figure 5: Front duration distribution of pulses from the spectrum (a) from the Fig. 4 normalized for 1 hour.
events / channel / 100 h
300 250 200 150
b a
100 50 0 0
20
40
60
80
100
120
channel Figure 6: The detector own background pulse amplitude spectrum “a” and the spectrum of a neutron calibration source “b”.
18
events / bin / hour
4
3
2
b
1
a 0
5
10
15
20
25
rise time, µs Figure 7: Distributions of rise times of pulses from the spectrum (a) (curve “a”) and spectrum (b) (curve “b”) from the Fig. 6 normalized for 1 hour.
19
PII: DOI: Reference:
www.elsevier.com/locate/nima
S0168-9002(16)31079-8 http://dx.doi.org/10.1016/j.nima.2016.10.038 NIMA59397
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 30 May 2016 Revised date: 20 October 2016 Accepted date: 20 October 2016 Cite this article as: V.V. Kuzminov, V.V. Alekseenko, I.R. Barabanov, R.A. Etezov, A.M. Gangapshev, Yu.M. Gavrilyuk, A.M. Gezhaev, V.V. Kazalov, A.Kh. Khokonov, S.I. Panasenko and S.S. Ratkevich, Some features and results of thermal neutron background measurements with the [ZnS(Ag)+6LiF] scintillation detector, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.10.038 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.
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Some features and results of thermal neutron background measurements with the [ZnS(Ag)+6 LiF] scintillation detector V.V. Kuzminova , V.V. Alekseenkoa , I.R. Barabanova , R.A. Etezova , A.M. Gangapsheva , Yu.M. Gavrilyuka , A.M. Gezhaeva , V.V. Kazalova , A.Kh. Khokonovb , S.I. Panasenkoc , S.S. Ratkevichc,∗ a Institute
for nuclear research, 117312, Moscow, Russia Berbekov Kabardino-Balkarian State University, 360004, Russia c V.N.Karazin Kharkiv National University,61022,Kharkiv, Ukraine
7 8
b Kh.M.
9
10
11
Abstract Features of a thermal neutron test detector with thin scintillator [ZnS(Ag)+6 LiF]
12
are described. Background of the detector and its registration efficiency were
13
defined as a result of measurements. The thermal neutron flux at different lo-
14
cations, and for different conditions around the Baksan Neutrino Observatory
15
are reported.
16
Keywords: thermal neutron detectors, scintillation detector, low-background
17
measurements
18
1. Introduction
19
Neutrons play an important role in a radiation field of surrounding envi-
20
ronment in low-background laboratories. The neutron fluence is a critical issue
21
of experiments for ββ-decay, the direct dark matter search, and solar neutrino
22
experiments.
23
Thermal neutrons can be detected through a nuclear reaction with nucleus
24
of 6 Li, contained in the scintillation materials, resulting in alpha and triton pro-
25
duction. An example of such material is a silver-activated zinc sulfide [ZnS(Ag)]
26
scintillator loaded with 6 LiF. The high scintillation yield for α-particles makes
27
this scintillator very efficient for neutron detection. The large area scintillation ∗ Corresponding
author, Tel +380 577075185; +7 8663875149 Email address: [email protected] (S.S. Ratkevich) Preprint submitted to NIM
October 22, 2016
28
detector based on thin [ZnS(Ag)+6 LiF]-scintillator plate has been put into oper-
29
ation at the Baksan Neutrino Observatory of the Institute for Nuclear Research
30
of the Russian Academy of Sciences (BNO) for measurement of thermal neutron
31
background flux [1, 2].
32
As mentioned above, neutron registration is based on the 6 Li(n, α)3 H+4786
33
keV reaction. The 6 Li cross section for the thermal neutrons is 945 b [3]. The
34
kinetic energy of the reaction products (Eα = 2051 keV, E3 H = 2735 keV)
35
is converted into scintillation light. Spectrometric characteristics of these de-
36
tectors are not well known. One of the aims of our work was to explore these
37
characteristics for continuous monitoring of neutrons in conditions of the ground
38
level laboratory buildings and underground laboratories.
39
2. Test detector
40
A schematic view of the test detector with electronics is shown on the Fig. 1.
41
The detector is assembled in a rectangular casing 30 × 30 × 50 cm3 made of
42
galvanized steel 0.7 mm thick. The top and the bottom covers of the casing are
43
detachable. A dividing plate is mounted in the middle of the casing and has
44
central 150 mm diameter hole to place an FEU-173 photomultiplier (PMT). A
45
charge sensitive preamplifier (CSP) with ∼ 100 µs self-discharge time is installed
46
on the wall inside of the upper part of the casing. Output pulses from the
47
PMT’s anode resistor (4.8 MΩ) go the CSP and further to input of the digital
48
oscilloscope card (LAn-10M5). The output data are recorded with a personal
49
computer. The sampling frequency is 6.25 MHz. A flat flexible plate with
50
the [ZnS(Ag)+6 LiF] scintillator is placed at the bottom of the lower section.
51
The plate consists of a white sheet of plastic film, the sticky side of which is
52
covered with the scintillator grains having an average thickness of ∼ 0.1 mm [2].
53
The sheet is laminated by the layer of polyethylene terephthalate. An internal
54
side surface of the bottom section is covered with a reflecting Mylar sheet for
55
effective light collection. The pure ZnS(Ag) and 6 LiF have densities of equal
56
to 4.09 g/cm3 [4] and ∼ 2.64 g/cm3 [5], respectively. The mixture is in the
57
proportion 1 : 3, and has a density of ∼ 3 g/cm3 . 2
58
3. Results of measurements
59
Two types of pulses shown in Fig. 2 (a, b) were observed in the measure-
60
ment with the detector at the ground laboratory. The first pulse type (a) has a
61
rise time τf of the preamplifier output pulses in the range of 16 − 25 µs which
62
corresponds to the de-excitation time τs = 8 − 10 µs (τf ≈ 3τs ) [4, 6] of the
63
fine-grained scintillator. The CSP integrates an input current. The integrated
64
output pulse has a maximum at the point where the rate of current charging
65
equals the preamplifier discharge rate. The second type of a pulses (b) have a
66
rise time τf = 0.8 µs. Its shape is similar to noise pulses which occur after the
67
scintillator and PMT are exposed to external light. The intensity and ampli-
68
tude of the noise pulses in the range of interest fall to zero some hours after
69
irradiation. The remaining pulses of type (b) are not noise, as their intensity is
70
proportional to the intensity of cosmic rays and decreases when moving the de-
71
tector to a deeper underground laboratory. It is known that a photomultiplier’s
72
noise pulses could be created by charged particles from an outer radioactive
73
background, cosmic rays and by charged particles from decays of radioactive
74
isotopes contained in the PMT construction materials [7].
75
To clarify the nature of the type (b) pulses an additional study was done
76
to examine two possibilities. The first possibility was direct generation of the
77
primary electrons from the photocathode or dynodes by cosmic rays. The sec-
78
ond one is the appearance of a photoelectron from the photocathode as a result
79
of absorption of Cerenkov radiation created by a charged particle in the glass
80
entrance of the PMT. For the second one, the cosmic rays coming from direc-
81
tions around the vertical generate Cerenkov radiation directed outward from
82
the entrance window of the PMT. This light is returned to the PMT after re-
83
flection from the lower compartment surfaces. To determine which of these two
84
possibilities is the real reason, the PMT entrance (photocathode) window was
85
covered by the black paper. After that procedure only pulses with a short rise
86
time remained. Also, the intensity of such pulses follows the intensity of cosmic
87
rays as the detector is moved from the second floor of the laboratory building to
3
88
the ground floor. The short rise time pulses are therefore generated by cosmic
89
rays in the photocathode or dynode.
90
A series of measurements were taken with the test detector at ground level,
91
and at underground locations around the BNO. Several different test conditions
92
were also considered. The locations and conditions are:
93
1. Deep Underground Low Background Laboratory (DULB-4900), an un-
94
shielded room located in an underground hall with mountain thickness of
95
4900 m water equivalent (w.e.). At this location and the other below item-
96
ized places (2-11), two types of spectra have been measured: (a) without
97
any shielding materials, and (b) with the 0.1 × 100 × 100 cm3 Cd sheet
98
(absorber) placed under the detector.
99
100
2. DULB-4900 low background compartment with the walls made of 25 cm polyethylene +0.1 cm Cd+15 cm Pb. A spectrum (a) was measured.
101
3. The same as location two with a thermal neutron source.
102
4. Low background underground laboratory “KAPRIZ” at the 1000 m w.e.
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
The spectra (a) and (b) were measured. 5. Low background underground laboratory “NIKA” at the 660 m w.e. The spectra (a) and (b) were measured. 6. “CARPET-2” muon detector set-up at the 5 m w.e. The spectra (a) and (b) were measured. 7. The ground floor of the four-storey laboratory building. The spectrum (a) was measured. 8. The river side of the second floor (room 204) of the four-storey laboratory building. The spectra (a) and (b) were measured. 9. The valley side of the second floor (room 211) of the four-storey laboratory building. The spectrum (a) was measured. 10. Ground building “ELLING” of the “CARPET-2” set-up. The spectra (a) and (b) were measured. 11. The fourth floor (room 404) of the four-storey laboratory building (LAB). The spectrum (a) was measured. 4
118
12. The open soil. The spectra (a) and (b) were measured.
119
The items in the above list are following of a cosmic rays absorber thickness
120
above the installation in a sequence from 4900 to 0 m w.e.. The depths of each
121
location is shown in the third column of the Table 1. A schematic view of a
122
longitudinal section of the BNO adit and Andyrchi slope is shown in Fig. 3 which
123
presents the locations of different underground laboratories and the dependence
124
of underground muon flux on the distance from the entrance. A brief description
125
of the locations is given in reference [8].
126
For better understanding of statistical characteristics of the pulses in mea-
127
surements, a 63.12 hour set of data from site eight has been analyzed. The
128
total pulse height spectrum is shown in Fig. 4 (spectrum “a”). The distribu-
129
tion of rise times from 20% to 80% of the pulse height, normalized per hour, is
130
shown in Fig. 5. Two peaks in the distribution of pulse rise times are clearly
131
separated. There is a narrow distribution in the range 0.5-1.6 µs, and a wide
132
second distribution in the range 3.2-13 µs. The second range corresponds to
133
the pulses produced by the scintillator which are not seen when a light-proof
134
sheet is placed between the scintillator and PMT photocathode. This allows
135
the separation of spectrum (a) on the Fig. 4 into two parts - fast (b) and slow
136
(c) components.
137
One can see that the fast pulses contribute mainly at the low amplitudes.
138
Detailed analysis of the fast pulse shapes shows that the pulses have different
139
decay shapes. These differences are explained by a small variable contribution
140
of the scintillation light to the muon-induced background signal in the PMT.
141
The proportion of the two components depends on the number of particles in
142
an event, and on the quantity of tracks crossing the scintillator and PMT. The
143
thin scintillator has a low sensitivity to the cosmic rays and electrons, so the
144
pulse heights are small and lie below the registration threshold.
145
A significant number of extraneous pulses can occur during long term low
146
count rate measurements at the underground conditions. The main source of
147
such noise pulses is the switching on and off of laboratory equipment. These
5
148
pulses have a shape that is different from signal ones, so they can be excluded
149
from the spectra by using a pulse shape discrimination.
150
Site two is an underground site that is well protected against neutrons,
151
and measurements there are used to define the background due to the detector
152
materials themselves. The spectrum from site two, normalized per 100 h, is
153
shown in Fig. 6 (spectrum “a”). The corresponding distribution of rise times
154
is shown in the Fig. 7 (curve “a”). A calibration with thermal neutrons from a
155
specially prepared source was carried out in the same low-background conditions
156
(site 3) in order to clarify the nature of these events. The neutron source was
157
made as a set of four α-sources from the set: 226
233
Ra(38 kBq) and triplet (
U+
238
238
Pu +
Pu(48.2 kBq),
239
239
Pu(3.58
158
kBq),
Pu) (40.9 kBq), covered
159
with a beryllium foil. This combined source was placed into polyethylene box
160
with 10 cm wall thickness, and then the box was put under the detector. The
161
amplitude spectrum of the neutron source, normalized to 100 h, is shown in
162
Fig. 6 (spectrum “b”). The corresponding distribution of the pulse rise times is
163
shown in Fig. 7 (curve “b”).
164
The background spectrum has a longer energy extension than the neutron
165
one. At the same time a distribution of a pulse rise time is shifted to the shorter
166
rise-time. Thus, the detector background is created by highly ionizing particles
167
with energies larger than the energy of the neutron reaction products. The
168
naturally occurring long lived isotopes
169
α-particles, are contained as impurities in the scintillator, and could be possible
170
sources of background. 210
−
Th and
238
U, which decay emitting
Po (T1/2 = 138.4 d, Eα = 5.3 MeV) generated in a
171
decay chain of
172
source. This second isotope originates from radon decay in air, and deposits on
173
the scintillator surface. This background could be suppressed by using special
174
protection against the penetration of radon and its daughters from the gas
175
environment of the scintillator production area. The α-particles born outside of
176
the scintillator plate will be absorbed in the covering Lavsan film and will not
177
produce any noticeable effect.
178
Pb (β
210
232
- decay, T1/2 = 21.8 y) could be another background
The range of estimated path lengths of α-particles with the energies of 2051 6
179
keV, 4800 keV and 5300 keV in the scintillator with a density of 3.0 g/cm3 ,
180
are 10.6 µm, 28.6 µm and 32.1 µm respectively [9]. The range of the path
181
length of the 2735 keV triton is 56.3 µm. If the main source of detectors own
182
background is α-particle from
183
than the one from a neutron reaction products due to the difference in energies.
184
The difference may be additionally increased if the relative light yield ZnS(Ag)
185
decreased at lower particle energy according to ionization density rise as it
186
occurs for other inorganic scintillators [4]. The light output for the two reaction
187
products with the 4800 keV sum energy will be less than the light output for the
188
α-particle with the same energy. A shorter rise time of background pulses could
189
be explained by location of the α-source on a surface of the scintillator grains.
190
Trajectories of α-particles will be directed into the grains in this case and the
191
main part of energy will be released inside a grain volume. In Ref. [4] it is
192
noted that disperse scintillators on the basis of ZnS(Ag) possess long afterglow.
193
It could be explained by the increase of a the relative number of long lived
194
excitation traps located on grain surfaces. The particles absorbed in a grain
195
surface layer will produce longer deexcitation time in this case. Vertices of
196
neutron reactions are distributed uniformly in a 6 LiF component volume. The
197
reaction products should exit from this materials and fall into the surface layer of
198
the ZnS(Ag) grain and produce a scintillation. The ionization density increases
199
as the particle energy decreases in accordance with the energy loss dependence
200
[10]. A considerable part of energy will be released in the surface layers of two
201
adjacent grains giving longer pulse rise time in comparison with the particle
202
absorbed inside the grain. The above-mentioned features of the distributions
203
corresponding to detectors’ own background pulses can serve as an indirect
204
indication that some number of these pulse are due to collective surface effect
205
of adjacent grains.
210
Po, then the pulse rise time will be greater
206
The detector count rates, per hour, integrated above the third channel of
207
the spectra “b” and “c” Fig. 4 are presented in Table 1 for all sites indicated in
208
the list.
209
Values of a thermal neutron flux F in the all examined sites could be obtained 7
210
from the neutron count rates. The flux of particles falling on given surface (F )
211
is the ratio of particles number ∆N and time interval (∆t) to this interval:
212
F = ∆N/∆t. Hence, count rate n = ε × F , where ε is the neutron registration
213
efficiency of the scintillation plate. The neutron flux density parameter ϕ is
214
used to make the comparison of the results obtained with different geometry
215
detectors easy. The definition of ϕ is the ratio of particle flux dFS penetrating
216
the volume of an elementary sphere to the area of it’s central cross section
217
dS : ϕ = dFS /dS. The dFS value could be determined using specific neutron
218
flux falling on an elementary square scintillator as dFS = 4 × F/S where S is
219
the area of the scintillator plane and the coefficient 4 is equal to the ratio of the
220
sphere surface area to the sphere cross section area. As a result ϕ = 4n/(2Sε).
221
It seems impossible to calculate ε-value due to uncertainties in composition
222
and structure of the scintillator layer. This value was obtained experimentally
223
from comparison of n1 count rate of test detector with n1(2) count rate of mod-
224
ified detector with additional similar light-proof protected scintillator plate (2)
225
put under the active plate (1). The detector was shielded by 1 mm cadmium
226
foil against thermal neutron flux from upper hemisphere to shape a single-sided
227
neutron flux. The count rate of test detector is equal to n1 = ε1 × F and the
228
one for the modified detector is n1(2) = ε1(2) × (F − ε2 × F ) where ε1 is a plate
229
(1) absorption efficiency of thermal neutron flux F , ε1(2) is the plate (1) absorp-
230
tion efficiency of flux which had not been absorbed by plate (2). The value of
231
efficiency is an integral characteristic of the process of absorption of neutrons
232
coming at different angles and depends on the path passed by neutrons in the
233
scintillator. The angular distribution of neutrons after passing one scintillator
234
layer is pulled in the normal direction due to the more effective absorption in
235
plate(2) neutrons with inclined incoming angles. An absorption will be lower
236
for passed neutrons and ε1(2) will be lower than ε1 (ε1(2) ≤ ε1 ). The count rates
237
are specified by the expressions n2 = ε2 × F and n2(1) = ε2 (1) × (F − ε1 × F )
238
in a case when the plate (2) is used as the active one.
239
Five unknown variables ε1 , ε1(2) , ε2 , ε2(1) and F are in four obtained equa-
240
tions. One needs to measure additionally a total count rate n[1+2] = n[2+1] in 8
241
the case when both plate are used in active mode to determine precisely all five
242
values. Such measurement is possible if the plates emit scintillation light into
243
both hemispheres in detector with two PMTs. The task could be solved for
244
the reviewed detector if the plate adsorbs neutron not strongly and it is pos-
245
sible to take ε1(2) ≈ ε1 and ε2(1) ≈ ε2 . Then simple algebra gives expressions
246
ε1 = [n1 − n1(2) ]/n2 and ε2 = [n1 − n1(2) ]/n1 .
247
The measurements were done in the site (11) of the list. The result ε1 =
248
0.22±0.01 and ε2 = 0.21±0.01 was obtained. These values were used as normal-
249
ization factors in the calculations of a thermal neutron flux detection efficiency
250
in the case of a stack of two absorbing homogenous plates. This method allows
251
one to exclude by the calculation an uncertainty of the experimental efficiencies
252
connected with the assumptions that ε1 (2) ≈ ε1 and ε2 (1) ≈ ε2 . The corrected
253
efficiency values εc1 = 0.17 ± 0.01 and εc2 = 0.16 ± 0.01 were found as a result.
254
The value εc1 was used for the thermal neutron flux density calculation from the
255
experimental data.
256
A background count rate measured at the site (2) was subtracted from data
257
in a process of determination thermal neutron flux density at other site. The
258
obtained ϕ-values are presented in the last column the Table 1. The limit for
259
thermal neutron flux density in the site (2) was obtained from base measure-
260
ments with a 3 He proportional counter for neutrons [13].
261
Discussion of results
262
The intrinsic background of the scintillator plate was found to be to (2.69 ±
263
0.05) × 10−2 cm−2 h−1 in the site (2). This value is comparable with a value
264
of surface α-activity of the commercial copper M1 and steel samples which
265
is ∼ (0.5 − 1.0) × 10−2 cm−2 h−1 [11]. The surface α-activity of the silicon
266
semiconductor samples could reach ∼ 0.001 cm−2 h−1 . It seems possible to
267
prepare a scintillator plate with similar surface α-activity using specially selected
268
low background [ZnS(Ag)+6 LiF] material and clean technology for the plate
269
preparation. Hence the sensitivity of such scintillator detector (SD) for thermal
270
neutrons may be comparable with sensitivity of the 3 He proportional counter.
271
The present ratio of sensitivities is ∼ 16 as it follows from comparison of the 9
272
data from the sites (2) and (4) of the Table 1. The difference of the rise time
273
distribution of background and neutron pulses also could be useful for plate
274
sensitivity. The results obtained are in good agreement with measurements
275
with the 3 He proportional counter in the Ref. [12].
276
The value of the detector’s own background defines its sensitivity for neu-
277
tron measurements in deep underground conditions as shown in Table 1. The
278
count rate of neutrons in the DULB-4900 (Table 1, site (1a)) is equal to (21.32−
279
16.4)/6.11 = (0.8 ± 0.08) × 10−2 cm−2 h−1 with signal over background of ∼ 0.3.
280
Neutrons are born in surrounding rock mainly due to (α, n)-reactions with light
281
elements. Walls of the “KAPRIZ” (site 4) laboratory are covered with 30 cm low
282
background concrete layer using crushed dunite rock. The concrete considerably
283
reduces the neutron flux from the rock. (The dunite concrete was also used in the
284
construction of the “NIKA”, site five, laboratory.) The comparison of neutron
285
fluxes in the “DULB-4900” and “KAPRIZ” measured with the 3 He proportional
286
counter shows that the concrete reduces the neutron flux by ∼ 5.2 times. One
287
can estimate an expected neutron flux in the “KAPRIZ” using this coefficient
288
and the SD count rate in the site (1a) as (0.15±0.02)×10−2 cm−2 h−1 . The calcu-
289
lation with the data from the Table 1 gives the value (0.05±0.08)×10−2 cm−2 h−1
290
which does not disagree with the estimated one. This last measurement pro-
291
vides a limit for the neutron flux density is 5.9 × 10−6 cm−2 c−1 at 90% C.L. in
292
the sites (1a and 1b).
293
The SD’s own background gives a small part of the total count rate in mea-
294
surements at the surface ground and at shallow underground sites where reac-
295
tions of cosmic rays with the element’s nuclei of the environment are the main
296
source of the neutron.
297
The comparison of the SD count rates with and without the cadmium ab-
298
sorber shows that the absorber decreased the neutron flux by ∼ 1.9 times in a
299
room with a roof, and ∼ 2.4 times in locations without a roof. Thus, the ratio
300
of neutron fluxes from the soil and from the atmosphere at the open place is
301
equal to ∼ 1.4.
302
The SD count rate of short rise times of pulses in the underground conditions 10
303
does not depend practically on a value of the external gamma-ray background
304
level as it seen in the Table 1. This noise component could be born directly in the
305
PMT by high ionizing particles. The pulses could appear at the photocathode
306
in result of direct generation of electrons by α-particles on the surface of the
307
entrance window from the internal α-activity. A count rate of such pulses is
308
also proportional to the cosmic ray muon flux.
309
Conclusions
310
We have studied the characteristics of a thermal neutron detector with
311
[ZnS(Ag)+6LiF] scintillator with rectangular dimensions of 216 × 304 mm2 .
312
Pulse shape analysis done with data from digital oscilloscope have shown that
313
the background from fast rise time pulses is from cosmic rays interacting di-
314
rectly in the photomultiplier and appear in the low energy region of the spec-
315
tra.
316
underground locations around BNO INR RAS with different shielding from
317
cosmic rays. The inherent background of the detector created by α-particles
318
from decays of inner radioactive admixture with surface α-activity at level of
319
(2.69 ± 0.05) × 10−2 h−1 cm−2 was measured. The ratio of the signal to back-
320
ground was ∼ 0.3 at deep underground locations. The neutron pulses have a
321
shorter rise time than the background ones. When the pulse rise time is used for
322
discrimination between neutron and alpha backgrounds, a ∼ 2 times rejection
323
of neutron events was obtained.
Measurements of the thermal neutron flux were made at ground and
324
Acknowledgement
325
The work was carried out in part with the financial support of the Federal
326
Objective Program of the Ministry of Education and Science of the Russian
327
Federation “Research and Development in the 2007-2013 years on the Priority
328
Directions of the Scientific and Technological Complex of the Russia” under the
329
contract No. 16.518.11.7072 and the “Russian Foundation for Basic Research”
330
under the grant No. 14-22-03059.
331
332
We are thankful to Yu.V. Stenkin for providing us with the samples of the scintillator and numerous useful critical comments and discussions.
11
333
[1] V.V. Alekseenko, Yu.M. Gavrilyuk, D.M. Gromushkin, at al., “Correlation
334
of Variations in the Thermal Neutron Flux from the Earth’s Crust with the
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2009, Vol. 45, No. 8, p.709; doi: 10.1134/S1069351309080102.
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[2] V.V. Alekseenko, D.D. Dzhappuev, V.A. Kozyarivsky, at al., “Analysis of
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Variations in the Thermal Neutron Flux at an Altitude of 1700 m above
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mental Methods of Nuclear Physics, Moscow: Energoatomizdat, 1985.
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[4] M.N. Medvedev, Scintillation detectors. Moscow: Atomizdat 1977.
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[5] M.E. Globus, B.V. Grinev, Inorganic scintillators (new and traditional ma-
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terials). Kharkiv: Akta, 2001. [6] Phosphor
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[7] “PHOTOMULTIPLIER TUBES (principles and applications)”. Produced
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[11] E.L. Kovalchuk, V.V. Kuzminov, A.A. Pomansky, “Surface alfa activity
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[12] V.V. Alekseenko, I.R.Barabanov, R.A. Etezov, et al., “Results of measure-
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ments of an environment neutron background at BNO INR RAS objects
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[13] A.Kh. Khokonov, Yu.V. Savoiskii, A.V. Kamarzaev, “Neutron Sensitivity
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368
Nuclei, V.73, No.9, 2010, P.1482; doi: 10.1134/S1063778810090024.
13
10
F3 -Counters” // Physics of Atomic
Table 1: Count rates of the detector at 1 hour for the data integrated above the third channel of the spectra (b) and (c) Fig. 4 and the thermal neutron flux densities.
No. site
Place, conditions
depth, (m w.e.)
1a b 2
DULB-4900 -//- + (Cd) low background compartment -//- + (n-source) KAPRIZ -//- + (Cd) NIKA -//- + (Cd) µ-detector -//- + (Cd) LAB, ground LAB, 204 -//- + (Cd) LAB, 211 ELLING -//- + (Cd) LAB, 404 Open soil -//- + (Cd)
4900
3 4a b 5a b 6a b 7 8a b 9 10 a b 11 12 a b
4900 4900 1000 660 5 1.3 0.7 0.7 0.2 0.2 0
Count rate,h−1 (3-256 channel) Short pulse Long pulse rise time rise time 0.13±0.03 21.3±0.4 0.09±0.03 18.7±0.5 0.10±0.02 16.4±0.3 0.2±0.1 0.13±0.03 0.13±0.03 0.19±0.04 0.14±0.03 8.4±0.5 8.2±0.4 6.5±0.5 16±2 16±1 19.1±0.9 23±1 22±3 22±1 28±3 27±3
14
80±2 16.7±0.3 16.5±0.4 17.8±0.4 16.9±0.3 64±1 43±1 240±3 866±14 482±6 672±5 1415±8 730±16 1439±9 1704±31 702±14
Thermal neutron flux density, (s−1 ×cm−2 ) (2.6±0.4)×10−5 (1.2±0.4)×10−5 ≤ 3.8 × 10−7 (90% C.L.) (3 He prop. counter) (3.4±0.4)×10−4 ≤ 5.9 × 10−6 (90% C.L.) ≤ 5.9 × 10−6 (90% C.L.) (7.5±3.1)×10−6 (3.4±3.1)×10−6 (2.8±0.3)×10−4 (1.4±0.2)×10−4 (1.2±0.1)×10−3 (4.5±0.4)×10−3 (2.5±0.2)×10−3 (3.5±0.3)×10−3 (7.5±0.6)×10−3 (3.8±0.4)×10−3 (7.6±0.6)×10−3 (9.0±0.8)×10−3 (3.7±0.3)×10−3
Figure 1: The schematic view of the test detector with the [ZnS(Ag)+6 LiF] scintillator and its electronics.
15
3.12
a 2.34
amplitude, mV
1.56 0.78 0.0 1.95
b
1.56 1.17 0.78 0.39 0.0
0
20
40
60
80
100
120
140
160
time, µs Figure 2: The two types of test pulses from the detector. “a” - the long pulse rise time, “b” - the short pulse rise time.
16
Figure 3: Schematic view of a section of the Andyrchy slope along the adit (right scale) and dependence of underground muon flux on the laboratory location depth (left scale).
events / channel
3000
a
2000
b c
1000
0 0
20
40
60
80
100
120
channel Figure 4: Pulse amplitude spectra from the detector in the site (8) collected at 63.12 h): “a” – a total spectrum, “b” – a spectrum of pulses with a short front, “c” – a spectrum of pulses with a long front.
17
events / bin / hour
100 10 1 0.1 0.01 0
10
20
30
40
50
60
70 rise time, µs
Figure 5: Front duration distribution of pulses from the spectrum (a) from the Fig. 4 normalized for 1 hour.
events / channel / 100 h
300 250 200 150
b a
100 50 0 0
20
40
60
80
100
120
channel Figure 6: The detector own background pulse amplitude spectrum “a” and the spectrum of a neutron calibration source “b”.
18
events / bin / hour
4
3
2
b
1
a 0
5
10
15
20
25
rise time, µs Figure 7: Distributions of rise times of pulses from the spectrum (a) (curve “a”) and spectrum (b) (curve “b”) from the Fig. 6 normalized for 1 hour.
19