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Thermal neutron small-angle scattering spectrometer (WIT) using a 2D converging slit and annular glass scintillator detectors at KENS
Physica B 156 & 157 (1989) 611-614 North-Holland, Amsterdam
THERMAL NEUTRON SMALL-ANGLE SCATTERING SPECTROMETER (WIT) USING A 2D CONVERGING SLIT AND ANNULAR GLASS SCINTILLATOR DETECTORS AT KENS Nobuo NIIMURA, Mitsuhiro HIRAI’, Akira ISHIDA’, Kazuya AIZAWA*, Kazuyoshi YAMADA and Mitsuaki UEN04 Laboratory of Nuclear Science, Tohoku University, Sendai 982, Japan ‘Kanagawa Institute of Technology, Atsugi 234-02, Japan ‘Faculty of Technology, Tohoku University, Sendai 980, Japan ‘Faculty of Science, Tohoku University, Sendai 980, Japan 4Honda Engineering Co. Ltd., l-10-1 Shinsayama, Sayama, Saitama 350-13, Japan
We have developed a thermal neutron small-angle scattering spectrometer installed at the pulsed thermal neutron source at the KEK neutron facility (KENS) at Tsukuba, Japan. This spectrometer has several features such as a two-dimensional converging Soller slit for thermal neutrons, annular glass scintillator detectors and two beamline monitors also using scintillators.
1. Introduction The small angle neutron scattering spectrometer (SANS) is a special machine for investigating macro-structures, which is now one of the most important instruments for neutron scattering because of its application to wide scientific fields as solid state physics, chemistry, material science, polymer and biology. The demand for this machine is still increasing and most of the SANS spectrometers are installed at cold neutron sources. It is true that cold neutrons are of great advantage to SANS. However, if a good collimated thermal neutron source and a high resolution PSD are available, SANS installed at the thermal neutron source might be a match for the one at the cold neutron source. Especially when this is applied for the TOF method where unmonochromated ‘white’ neutrons are used, a very wide Q range is covered with a simultaneous measurement. By the development of several new instrumental devices such as the two dimensional converging Soller slit for thermal neutrons, the annular glass scintillator detectors and beamline monitors made of scintillators, we have constructed a ther-
mal neutron small-angle scattering spectrometer named WIT. 2. Layout of WIT and two-dimensional multichannel converging Soller slit The layout of WIT is given in fig. 1. The spectrometer is installed at the H2 beam hole which is set in the thermal neutron moderator at KENS. We have constructed a specially designed twodimensional multichannel converging Soller slit. The process of construction is as follows: The Soller slit unit is shown in fig. 2. The thickness is 0.2 mm. The B,C powder of very fine particles (about 38 pm in diameter), which is solved into a binding material, is painted on the surface of the anglewise aluminum. These units are put side by side in 12 lines and piled up in 10 layers. A complete two-dimensional 120-channels converging Soller slit is shown in fig. 3. The neutron beam profile on the center of the focus plane was measured by shifting a 6Li glass scintillator detector (5 X 5 mm* in area) step by step. The results are shown in fig. 4. The FWHM of the profile of the Soller slit was observed and
0921-4526/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
N. Niimura et al. I Thermal neutron small-angle scattering spectrometer
612 ‘LI
GLASS
BEAM
SCINTILLATOR
HONITOR
TRANSHISSION
FOR MEASUREblENr
SAHPLE
_-.---w-v--
CHAHEER
_-_-_---.-_-_-.-e--f
_-
HOOERATOR
SCATTERING
CHAHBER
AL ANNULAR
SHIELD
DETECTORS DETECTORS
FOR
SCATTERING
HED I WI
ANGLE
Fig. 1. The layout of WIT.
2.7mmx2.7mm
L.Ommx~.Omm \
Fig. 2. The Soller slit unit
Fig. 3. A complete
is the same as the estimated value. The S/N ratio is good and no escape streaks are seen beside the peak. A specially designed beam line monitor [l] is set at the exit of the converging slit. It uses ‘Li glass scintillator in powder form with a special particle size (95 pm diameter in average). The area for detection is 6 x 6 cm’. Another monitor of this type is set behind the annular detectors for transmission measurements. The sample chamber is cylindrical, 50cm in diameter. It is connected with a vacuum scattering chamber. Another sample container is put inside the sample chamber when it is necessary
2-dimensional
converging
slit.
613
N. Niimura et al. I Thermal neutron small-angle scattering spectrometer
3. Glass scintillator annular detector and data processing system
0
2
Hor i zontal km) Fig. 4. The neutron focused plane.
4
-4 -2
0
2
4
Vertical (cm)
beam profile on the center
of the
to carry out the measurements under atmospheric pressure. The maximum sample size, which is defined by the cross section of the neutron beam flux at the sample position, is 3 x 3 cm*. The beam window of the sample container is made of a thin aluminum plate, Cl.2mm in thickness. Glass scintillators are very sensitive to -y-rays. While the output pulse height due to y-ray interactions in the scintillator are generally smaller than those due to neutrons, not all y-rays can be distinguished from neutrons by pulse height discrimination. To diminish the number of y-rays incident upon the detector, a lead slit is placed in the sample chamber. The scattering chamber is 1 m in diameter and 2.6 m in length. It is surrounded by a borate resin of 10 cm in thickness for the shielding of neutrons and the inside of the chamber is coated with B,C powder in order to remove the neutron scattering from the wall.
Annular 6Li glass scintillators are used for the detector system, since the small-angle scattering pattern on the area detector is annular about the beam center for the case of the isotropic scattering sample. If an annular detector is provided in SANS, the data handling system including electronics becomes extremely simple. Since a large continuous and variable Q range is required, annular detectors with different radii are preferable. Pieces of 6Li glass scintillators (2 mm in thickness) were stuck on the surface of the annular transparent acrylic resin with optical cement. The ring was wrapped with aluminium foil to reflect scintillation photons escaping the surface of the acrylic resin. As shown in fig. 5, eight photomultipliers (PMTs) were coupled to the rear side of the annular acrylic resin. The thickness of the acrylic resin has been determined as 5 cm so that any position where a scintillation event occurs can be subtended directly by two PMTs, to allow detection of scintillation photons in coincidence. The width of the ring defines the spatial resolution. The resolution of the momentum transfer Q for small angle (=27r+lh, where 4 is a scattering angle and A is a wavelength) is written as follows:
AQ -=Q
A4 4’
A42 = A&,
(1) + A&et 3
(2)
where AA,, and A4det are the fluctuations of the -Li
Glass
Scintillator
Neutron
ultiplier
Acrylic
‘Acid
Resin
Fig. 5. The schematic layout of the annular detector.
614
N. Niimura
et al. I Thermal neutron small-angle
scattering angle which come from the collimator divergence and the width of the ring detector, respectively. The design principle of the annular detector is as follows: i) The minimum radius of the ring detector is larger than the full width of the incident beam to avoid direct irradiation of the beam. ii) In the case of the small ring detectors, A4=,,, is dominant in eq. (2). By considering the resolution matching, A4det is designed to be equal to A4C0,. iii) In the case of the large ring detector, A+,,, is negligible with respect to Ac$~_. So A+de, is defined as AQ/Q which is about 10%. The radius and the widths of 11 annular detectors are tabulated with a Q range covered by the wavelength between 0.4 8, and 6 A in table I. The data processing system for the annular detector is as follows. The signal from the photomultiplier is directed to amplifiers, and their outputs are fed to a pattern discriminator module, which is specially designed to be applied to the annular detector. The function of the pattern discriminator is as follows: A scintillation event occurs and scintillation photons transmit through the acrylic resin to the neighboring photomultipliers. All the possible combinations of the photomultipliers (called a pattern) that accept photons simultaneously are stored in the read-onlymemory (ROM) in the module. This procedure corresponds to a kind of coincidence among any
scattering
spectrometer
Table I The radius, width of the annular covered by the wavelength between
detector and Q range 0.4 8, and 6 A.
Q Radius
(mm)
Width (mm)
49 68 88 109 131 IS4 179 206 23.5 266 299
h=6A
18 18 18 20 20 22 24 26 28 30 32
(8, ‘1 A = 0.4 A 0.195 0.273 0.354 0.444 0.540 0.644 0.759 0.885 1.022 1.173 I.337
0.013 0.018 0.024 0.030 0.036 0.043 0.051 0.059 0.068 0.078 0.089
photomultiplier and this diminishes the electric noise of the photomultipliers. In the end of the down stream of the neutron beam the transmission monitor, which is of the same type as the beam monitor, is settled. The response of the “Li glass scintillator is so prompt that we can measure the intensity of the direct beam without counting loss. Reference [l] M. Hirai, N. Niimura A 259 (1987) 497.
and A. Ishida,
Nucl.
Instr.
Meth.
THERMAL NEUTRON SMALL-ANGLE SCATTERING SPECTROMETER (WIT) USING A 2D CONVERGING SLIT AND ANNULAR GLASS SCINTILLATOR DETECTORS AT KENS Nobuo NIIMURA, Mitsuhiro HIRAI’, Akira ISHIDA’, Kazuya AIZAWA*, Kazuyoshi YAMADA and Mitsuaki UEN04 Laboratory of Nuclear Science, Tohoku University, Sendai 982, Japan ‘Kanagawa Institute of Technology, Atsugi 234-02, Japan ‘Faculty of Technology, Tohoku University, Sendai 980, Japan ‘Faculty of Science, Tohoku University, Sendai 980, Japan 4Honda Engineering Co. Ltd., l-10-1 Shinsayama, Sayama, Saitama 350-13, Japan
We have developed a thermal neutron small-angle scattering spectrometer installed at the pulsed thermal neutron source at the KEK neutron facility (KENS) at Tsukuba, Japan. This spectrometer has several features such as a two-dimensional converging Soller slit for thermal neutrons, annular glass scintillator detectors and two beamline monitors also using scintillators.
1. Introduction The small angle neutron scattering spectrometer (SANS) is a special machine for investigating macro-structures, which is now one of the most important instruments for neutron scattering because of its application to wide scientific fields as solid state physics, chemistry, material science, polymer and biology. The demand for this machine is still increasing and most of the SANS spectrometers are installed at cold neutron sources. It is true that cold neutrons are of great advantage to SANS. However, if a good collimated thermal neutron source and a high resolution PSD are available, SANS installed at the thermal neutron source might be a match for the one at the cold neutron source. Especially when this is applied for the TOF method where unmonochromated ‘white’ neutrons are used, a very wide Q range is covered with a simultaneous measurement. By the development of several new instrumental devices such as the two dimensional converging Soller slit for thermal neutrons, the annular glass scintillator detectors and beamline monitors made of scintillators, we have constructed a ther-
mal neutron small-angle scattering spectrometer named WIT. 2. Layout of WIT and two-dimensional multichannel converging Soller slit The layout of WIT is given in fig. 1. The spectrometer is installed at the H2 beam hole which is set in the thermal neutron moderator at KENS. We have constructed a specially designed twodimensional multichannel converging Soller slit. The process of construction is as follows: The Soller slit unit is shown in fig. 2. The thickness is 0.2 mm. The B,C powder of very fine particles (about 38 pm in diameter), which is solved into a binding material, is painted on the surface of the anglewise aluminum. These units are put side by side in 12 lines and piled up in 10 layers. A complete two-dimensional 120-channels converging Soller slit is shown in fig. 3. The neutron beam profile on the center of the focus plane was measured by shifting a 6Li glass scintillator detector (5 X 5 mm* in area) step by step. The results are shown in fig. 4. The FWHM of the profile of the Soller slit was observed and
0921-4526/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
N. Niimura et al. I Thermal neutron small-angle scattering spectrometer
612 ‘LI
GLASS
BEAM
SCINTILLATOR
HONITOR
TRANSHISSION
FOR MEASUREblENr
SAHPLE
_-.---w-v--
CHAHEER
_-_-_---.-_-_-.-e--f
_-
HOOERATOR
SCATTERING
CHAHBER
AL ANNULAR
SHIELD
DETECTORS DETECTORS
FOR
SCATTERING
HED I WI
ANGLE
Fig. 1. The layout of WIT.
2.7mmx2.7mm
L.Ommx~.Omm \
Fig. 2. The Soller slit unit
Fig. 3. A complete
is the same as the estimated value. The S/N ratio is good and no escape streaks are seen beside the peak. A specially designed beam line monitor [l] is set at the exit of the converging slit. It uses ‘Li glass scintillator in powder form with a special particle size (95 pm diameter in average). The area for detection is 6 x 6 cm’. Another monitor of this type is set behind the annular detectors for transmission measurements. The sample chamber is cylindrical, 50cm in diameter. It is connected with a vacuum scattering chamber. Another sample container is put inside the sample chamber when it is necessary
2-dimensional
converging
slit.
613
N. Niimura et al. I Thermal neutron small-angle scattering spectrometer
3. Glass scintillator annular detector and data processing system
0
2
Hor i zontal km) Fig. 4. The neutron focused plane.
4
-4 -2
0
2
4
Vertical (cm)
beam profile on the center
of the
to carry out the measurements under atmospheric pressure. The maximum sample size, which is defined by the cross section of the neutron beam flux at the sample position, is 3 x 3 cm*. The beam window of the sample container is made of a thin aluminum plate, Cl.2mm in thickness. Glass scintillators are very sensitive to -y-rays. While the output pulse height due to y-ray interactions in the scintillator are generally smaller than those due to neutrons, not all y-rays can be distinguished from neutrons by pulse height discrimination. To diminish the number of y-rays incident upon the detector, a lead slit is placed in the sample chamber. The scattering chamber is 1 m in diameter and 2.6 m in length. It is surrounded by a borate resin of 10 cm in thickness for the shielding of neutrons and the inside of the chamber is coated with B,C powder in order to remove the neutron scattering from the wall.
Annular 6Li glass scintillators are used for the detector system, since the small-angle scattering pattern on the area detector is annular about the beam center for the case of the isotropic scattering sample. If an annular detector is provided in SANS, the data handling system including electronics becomes extremely simple. Since a large continuous and variable Q range is required, annular detectors with different radii are preferable. Pieces of 6Li glass scintillators (2 mm in thickness) were stuck on the surface of the annular transparent acrylic resin with optical cement. The ring was wrapped with aluminium foil to reflect scintillation photons escaping the surface of the acrylic resin. As shown in fig. 5, eight photomultipliers (PMTs) were coupled to the rear side of the annular acrylic resin. The thickness of the acrylic resin has been determined as 5 cm so that any position where a scintillation event occurs can be subtended directly by two PMTs, to allow detection of scintillation photons in coincidence. The width of the ring defines the spatial resolution. The resolution of the momentum transfer Q for small angle (=27r+lh, where 4 is a scattering angle and A is a wavelength) is written as follows:
AQ -=Q
A4 4’
A42 = A&,
(1) + A&et 3
(2)
where AA,, and A4det are the fluctuations of the -Li
Glass
Scintillator
Neutron
ultiplier
Acrylic
‘Acid
Resin
Fig. 5. The schematic layout of the annular detector.
614
N. Niimura
et al. I Thermal neutron small-angle
scattering angle which come from the collimator divergence and the width of the ring detector, respectively. The design principle of the annular detector is as follows: i) The minimum radius of the ring detector is larger than the full width of the incident beam to avoid direct irradiation of the beam. ii) In the case of the small ring detectors, A4=,,, is dominant in eq. (2). By considering the resolution matching, A4det is designed to be equal to A4C0,. iii) In the case of the large ring detector, A+,,, is negligible with respect to Ac$~_. So A+de, is defined as AQ/Q which is about 10%. The radius and the widths of 11 annular detectors are tabulated with a Q range covered by the wavelength between 0.4 8, and 6 A in table I. The data processing system for the annular detector is as follows. The signal from the photomultiplier is directed to amplifiers, and their outputs are fed to a pattern discriminator module, which is specially designed to be applied to the annular detector. The function of the pattern discriminator is as follows: A scintillation event occurs and scintillation photons transmit through the acrylic resin to the neighboring photomultipliers. All the possible combinations of the photomultipliers (called a pattern) that accept photons simultaneously are stored in the read-onlymemory (ROM) in the module. This procedure corresponds to a kind of coincidence among any
scattering
spectrometer
Table I The radius, width of the annular covered by the wavelength between
detector and Q range 0.4 8, and 6 A.
Q Radius
(mm)
Width (mm)
49 68 88 109 131 IS4 179 206 23.5 266 299
h=6A
18 18 18 20 20 22 24 26 28 30 32
(8, ‘1 A = 0.4 A 0.195 0.273 0.354 0.444 0.540 0.644 0.759 0.885 1.022 1.173 I.337
0.013 0.018 0.024 0.030 0.036 0.043 0.051 0.059 0.068 0.078 0.089
photomultiplier and this diminishes the electric noise of the photomultipliers. In the end of the down stream of the neutron beam the transmission monitor, which is of the same type as the beam monitor, is settled. The response of the “Li glass scintillator is so prompt that we can measure the intensity of the direct beam without counting loss. Reference [l] M. Hirai, N. Niimura A 259 (1987) 497.
and A. Ishida,
Nucl.
Instr.
Meth.