Submitted to Nucl. Instr. & Meth. A
WIS/31/02-July-DPP
Large Area Imaging Detector for Neutron Scattering
Based on Boron-Rich Liquid Scintillator
D. Vartsky1, M.B. Goldberg
Soreq NRC, Yavne 81800, Israel
A. Breskin, R. Chechik
Weizmann Institute of Science, Rehovot 76100, Israel
B. Guerard, J.F. Clergeau
Institut Laue Langevin, Grenoble, France
Abstract
We discuss a new thermal-neutron imaging detector that combines a neutron
converter coupled to a position-sensitive gaseous photodetector. The neutron
converter consists of a thin (0.5 mm) layer of boron-rich liquid scintillator. It is
viewed by an atmospheric -pressure, gas-avalanche photomultiplier with a bi-alkali
photocathode. Scintillation-induced photoelectrons are multiplied by a cascade of Gas
Electron Multipliers (GEM). The multi-GEM supplies the pulse-height, time and
position information for each converted neutron. Such a fast, large -area detector can
operate at high radiation flux.
Key Words: Liquid scintillator, neutron detector, neutron scattering, gaseous
photomultiplier
Pacs: 29.40.Mc, 29.40.Cs, 61.12.-q, 61.12.Ex
__________________________________________________________________
1
Corresponding author: Tel: 973-8-9434589
Fax: 973-8-9434676
E-mail: [email protected]
1. Introduction
Slow neutron scattering is one of the key tools for studying condensed matter. The
advent of high current pulsed-accelerator based spallation sources (SNS, NPS, ESS)
applied to time -resolved high resolution experiments will create a need for new
detectors: la rge -area, real-time imaging devices with faster response and higher
detection efficiency than those presently in use. The requirements for these new
detectors are summarized in Table 1. The currently employed thermal neutron
detectors do not fulfill all these requirements. To overcome these limitations,
intensive R&D efforts are currently being invested by numerous groups [1]. Various
types of slow -neutron detectors are under development, including 3He-filled multiwire
chambers [2] and microstrip chambers [3], new inorganic scintillators [4],
10
B-coated
Gas Electron Multipliers (GEM [5]) detectors [6], high-resolution hybrid detectors
with composite -foil converters and wire chambers [7] or with microstrip multipliers
[8], optically read out GEMs [9] etc.
In this article we present a different concept for an efficient, fast, large -area slow
neutron imaging detector. It is based on a boron-rich organic liquid scintillator
coupled to a position-sensitive gaseous photomultiplier (GPMT [10]).
In principle, the light produced by the liquid scintillator can be recorded by any
position-sensitive
photon
detector,
e.g.,
vacuum-operated
position-sensitive
photomultipliers, Hybrid Photodiodes (HPD) [11], etc. However, at present none of
these can provide a cost-effectiv e light-recording solution that would cover the large
areas, (in excess of 2500 cm2 ) required for the applications under discussion. The
proposed gaseous -imaging photomultiplier, operating at atmospheric pressure with a
visible-light sensitive photocathode , has the potential for responding to the
requirements of large active area, sub-millimeter spatial resolution, fast response and
single -event sensitivity.
In this work we present first results for the newly developed scintillator; preliminary
data for the gaseous visible-photon GPMT is also shown.
2. Detector principle
The neutron detector consists of a thin (< 500 µm) neutron converter made of a
10
B-
rich liquid organic scintillator, coupled to a novel position-sensitive GPMT (Fig. 1).
The light emitted from the scintillator, following the neutron capture reaction
10
B(n,a)7Li , is converted at a semitransparent K-Cs-Sb photocathode coupled to a
gas-operated electron multiplier. The latter is a series of cascaded Gas Electron
Multiplier (GEM) elements [12], sealed to the photocathode within a gas-filled vessel.
The GEM consists of a 50-micron thin, metal-clad insulator foil (e.g. Kapton),
perforated with a dense array of 50-micron diameter holes. Electrons focused into a
hole are multiplied within the hole, under a potential of a few hundred volts, applied
across the GEM foil. Multiplication factors ranging from 103 – 106 are typically
attained for single- to 4-GEMs in cascade, respectively [12, 13]. The multiplied
charge from the last GEM is read out on a position-sensitive electrode; the GPMT
provides the pulse-height, time (with a sub-nanosecond resolution [14]) and spatial
localization of each converted neutron.
3. Scintillator properties
The liquid scintillator based on Tri-Methyl-Borazine, is an improved version of that
originally proposed by Ross and Holsopple [15]. It has been optimized to emit light at
a wavele ngth of 350-400 nm, matching the spectral range of the K-Cs-Sb
photocathode. Table 2 summarizes the properties of the scintillator. Evidently, it is
very fast and its macroscopic cross-section for thermal neutrons is large, allowing for
high probability (~ 90%) of neutron capture in a thin layer of liquid.
The scintillator was tested using an aluminum container for the liquid, sealed with an
indium gasket to a glass window. The window can be coupled with optical grease to a
conventional vacuum photomultiplier or to any position-sensitive light detector. Fig 2
shows the neutron spectrum obtained with two scintillator cocktails: The points
depicts a spectrum obtained with our homemade scintillator, which contain about 22%
boron by weight; the squares are for a commercial scintillator BICRON BC523A that
contains only 5% boron by weight.
It should be noted that, compared to the commercial BICRON BC523A scintillator
our scintillator has a boron content higher by a factor > 4 and its light output is only
14% lower. This permits us to construct an efficient neutron scintillator with a muchreduced thickness, which is very important for reducing the efficiency to gamma rays
and eliminating the localization parallax. Fig 3 shows the stability of the scintillator
over a period of approximately one year. It appears that the light output slowly
increases with time, which might possibly be due to a slow decrease of trace
concentration of dissolved molecular oxygen (known to be a strong quencher) in the
liquid, by adsorpt ion to the container walls.
4. Light readout
The GPMT for the neutron detector is currently under extensive development, and the
results presented here, though very encouraging are rather preliminary. The reader is
referred to more extended articles on that subject [10, 12, 14, 16]. Of major concern
are the high chemical sensitivity of the bialkali photocathode to gas impurities and to
secondary effects induced by the gas -avalanche (e.g. photon- and ion-feedback). The
newly developed GPMT seems to be an adequate multiplier; it can operate at high
gains in noble-gas mixtures [12], which are compatible with the photocathodes, and it
permits the suppression of most of the feedback effects [12, 16]. Though it has been
recently shown that the photocathode can operate in a sealed detector containing
argon and Kapton-made GEMs, so far it has been demonstrated for only limited
periods of time due to micro leaks [16]. Efforts are being made to produce the GEM
elements from more inert materials (glass, ceramic, etc.). Multiplication factors in
excess of 104 have so far been achieved in a two-GEM GPMT sealed at atmospheric
pressure Ar/CH 4 (95/5). The best quantum efficiency values attained in this mode are
of the order of 13% at 350 nm wavelength [16]. Time resolutions of s =1.6 and 0.33
ns, respectively were recorded in a multi-GEM multiplier, for 1 and for 150
photoelectrons [14, 16].
Figure 4 shows an example of a 60 mm diameter sealed GPMT prototype. This
GPMT size is dictated by the dimensions of our present UHV deposit ion-and-sealing
system. Work is in progress to improve the sealing procedure.
5. Summary and conclusions
A new imaging detector for thermal neutrons has been proposed, based on a very thin
liquid scintillator and a gaseous photomultiplier with semitransparent bialkali
photocathode. A boron-rich scintillator (22% boron content by weight) was
developed, with adequate light yield and fast response. The GPMT photon detector is
currently under development, the first prototypes already exhibiting QE above 10% at
400 nm and stability over a few weeks of operation. Sufficiently high gains could be
obtained with Ar/CH 4 (95/5) gas.
It is expected that using the proposed configuration, it will be possible to construct
highly efficient slow -neutron detectors of large dimensions and diverse shapes. As
both the scintillator and the light readout are inherently fast, neutron time-of-flight
resolution will be primarily dictated by liquid scintillator thickness and is expected to
be less than 200 ns for thermal neutrons.
Acknowledgements
The development of the photon detector is being effected in cooperation with Mr. D.
Mörmann and Dr. M. Balcerzyk of the Weizmann Institute; we acknowledge their
contribution to this work. This research is partially supported by the Planning and
Budgeting Committee of the Council for Higher Education in Israel and by the Israel
Science Foundation. A. Breskin is the W. P. Reuther Professor of Research in the
peaceful use of atomic energy.
References
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Detectors: Status and Prospectives, Th. Wilpert, Editor., Hahn Meintner Institut,
Berlin, June 2001, ISSN1433-559X
[2] J. Fried, J.A. Harder, G.J. Mahler, D.S. Makowiecki, J.A. Mead, V. Radeka, N.A.
Schaknowski, G.C. Smith and B. Yu, Nucl. Instr. and Meth. A 478 (2002) 415.
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Manzin, A. Oed and P. Palleau, Nucl. Instr. and Meth. A 471 (2001) 60.
[4] C.W.E. van Eijk, Ref 1, pg. 113.
[5] F. Sauli, Nucl. Instr. & Meth. A386 (1997) 531.
[6] M. Klein, H. Abele, D. Fiolka and Chr.J. Schmidt, Ref. 1, pg. 453.
[7] V. Dangendorf, A. Demian, H. Friedrich, V. Wagner, A. Akkerman, A. Breskin,
R. Chechik and A. Gibrekhterman, Nucl. Instr. and Meth. A 350 (1994) 503 and ref.1,
pg.445.
[8] B. Gebauer, C. Schulz, G. Richter, F.V. Levchanovsky and A. Nikiforov, Nucl.
Instr. and Meth. A 471 (2001) 249.
[9] F.A.F. Fraga, L.M.S. Margato, S.T.G. Fetal, M.M.F.R. Fraga, R. Fereira
Marques, A.J.P.L. Policarpo, B. Guerard, A. Oed, G. Manzini, and T. van Vuure,
Nucl. Instr. and Meth. A478 (2002) 357.
[10] A. Breskin, T. Boutboul, A. Bouzulutskov, R. Chechik, G. Garty, E. Shefer and
B.K. Singh, Nucl. Instr. and Meth. A442 (2000) 58 and references therein.
[11] Delft Electronics Products BV., Holland, Catalogue 2002.
[12] A. Buzulutskov, A. Breskin, R. Chechik, G. Garty, F. Sauli and L. Shekhtman,
Nucl. Instr. And Meth. A442 (2000) 68 and A443 (2000) 164.
[13] Bachmann, A. Bressan, L. Ropelewski, F. Sauli, A. Sharma and D. Moermann,
Nucl. Instr. And Meth. A438 (1999) 376 and references therein.
[14] A. Breskin, A. Buzulutskov, and R. Chechik, Nucl. Instr. And Meth. A438
(2002) 670.
[15] H.H. Ross and H.L. Holsopple, Nucl. Instr. and Meth. 33 (1965) 194.
[16] D. Moermann, M. Balcerzyk, A. Breskin, R. Chechik, B.K. Singh and A.
Buzulutskov. GEM-based gaseous photomultipliers for UV and visible photon
imaging. These proceedings.
Figure captions
Figure 1. A schematic presentation of the novel neutron detector concept: Neutrons
are converted in a thin liquid scintillator and the light is imaged by a gas avalanche
photon detector based on a GEM cascade. The scintillator is separated from the
gaseous detector by a glass window, on which the photocathode is deposited.
Figure 2. Neutron spectra obtained with our scintillator cocktail, containing about
22% boron by weight (dots) and with the commercial scintillator BICRON BC523A
(squares) that contains only 5% boron by weight.
Figure 3. The stability of our scintillator, measured over a period of about one year.
Circles and squares denote for 0.4 and 6 mm thick scintillators, respectively.
Figure 4. A prototype of the sealed visible-light gaseous photomultiplier, 60 mm in
diameter, filled with 1 atmosphere Ar.
Table 1 Requirements from neutron detectors
Parameter
Requirement
Type of operation
Real-time
Spatial resolution (mm)
~1
Detection efficiency (1.8 Å)
> 70%
Efficiency for gamma rays
< 10-7
Detector thickness
As thin as possible
Counting rate/pixel
104 /s
Global counting rate
107 /s
Timing resolution
1-10 µs
Dimensions
At least 50x50 cm2
Table 2 Properties of the boron-rich liquid scintillator
Parameter
Property
Scintillator base
N,N,N, trimethylborazine
Formula
C 3H 12B3N3
Density
0.87 g/cc
Boron content
~ 22%
Macroscopic cross-section
43 cm-1
Light output
400 photons/neutron
Light pulse duration
10 ns
Scintillator cost
~ $1.0/cc
bialkali
photocathod
e
photoelectrons
gas
2D read-out
avalanche
neutron
scintillation photons
10
B liquid
scintillator
window
Figure 1.
GEM-based
electron
multiplier
2000
counts
1500
1000
500
0
0
100
200
channel number
Figure 2.
300
400
Neutron peak position (ch. No.)
150
100
50
0
0
100
200
Time (days)
Figure 3
300
Figure 4