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A novel double-crystal analyser system for the PRISMA spectrometer at ISIS
ELSEVIER
Physica B 234-236 (1997) 1061-1063
A novel double-crystal analyser system for the PRISMA spectrometer at ISIS M.J. Bull a'b'*, M.J. Harris b, U. Steigenberger b, M. Hagen c, C. Petrillo d'e, F. Sacchetti d,e a Department of Physics, Birkbeck College, Malet Street, London WC1E 7HX, UK b ISIS Facility, Rutherford Appleton Laboratory, Chilton, Dideot OXll OQX, UK c Department of Physics, Keele University, Keele, Staffordshire ST5 5BG, UK d Dipartimento di Fisica, Universith di Perugia, Via A. Paseoli, 1-06100 Perugia, Italy eISM-CNR, Via E. Fermi 38, 1-00044 Frascati, Italy
Abstract
The crystal analyser spectrometer PRISMA has proven to be particularly successful at surveying extended areas of (~,e) space, efficiently collecting high-quality inelastic and critical scattering data sets from single-crystal samples. In the inelastic mode, mechanical constraints upon the 16 analyser-detector arms limit the final energy to Ef > 12 meV. To overcome this constraint, a novel double-crystal analyser system has recently been installed on the PRISMA spectrometer. Neutrons scattered from the sample are analysed by five groups of paired pyrolytic graphite analysers allowing final energies 2.25 meV
Keywords: Neutron instruments; Spectroscopy; Analysers
At a pulsed neutron source, the inverse geometry crystal-analyser spectrometer is ideally suited to the measurement of dispersion curves of collective excitations in single crystals. Neutrons from a polychromatic thermal neutron beam, after being scattered from a sample, are energy analysed using Bragg reflection from an analyser crystal. The energy transferred between the sample and the neutron can be determined from standard relationships between the total neutron time-of-flight (TOF), known final analysing energy, Ef, and the scattering angle, q~. Since moderated neutrons of all wavelengths are incident upon the sample, both the measured wave vector transfer, r, and the energy transfer, e, are swept along a parabolic measurement trajectory in reciprocal space for a particular crystal orientation. If this characteristic of the spec-
* Correspondence address. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK.
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 0 1 1 - 2
trometer is combined with the use of multiple analyser arms, it then becomes possible to examine an extended region of (r, e) space with a single setting of the instrument [1]. The PRISMA spectrometer at ISIS was constructed to exploit this technique with an array of 16 analyser-detector arms separated by a scattering angle of 2 °. This small ~b-separation was chosen such that the TOF trajectories would be closely packed in ( r , e ) space [2], but limiting Ef > 12 meV if collisions between neighbouring analyser-detector arms were to be avoided. This collision constraint has been overcome with the installation of a double-crystal analyser system [3] on the PRISMA beamline (Fig. 1). Incident neutrons from the 95 K methane moderator are scattered by the sample and energy analysed by five detector arms, each consisting of initial Soller collimation, two pyrolitic graphite analyser crystals, final Soller colli1 t/ mation and a Reuter-Stokes 10 atm ~ 3He gas detector. The analyser arms are separated by 10 ° in ~b, but
1062
M.J. Bull et al./ Physica B 234-236 (1997) 1061-1063
M
s
LMS
A
'-~
= 9.036 rn = 0.600 m
= 0.045 m* = 0.211 m*
>0 E A
lie
0.75
,
~'~'~J ~
0.60
1,.1
0.45 0.30
:~
0.1
' 4
. . . . . . . . . . . . . . 5 6 7 8 9 10
1 11
Final Energy, Ef (meV) 160
Fig. 1. Schematic view of the PRISMA double-crystal analyser system. Incident neutrons (wave vector ki) from the moderator (M), are scattered by the sample (S) through an angle ~ and energy analysed by five detector arms containing two graphite analyser crystals, (F)ront and (R)ear, a detector (D), and two Soller collimators (not shown). OF is the Bragg angle of both analysers for the required final energy, and the rear analyser (R) also has translational freedom (see text). The black areas represent shielding separating the fixed detectors, and starred lengths are defined when LFR is at 90° to LsF.
a sequence of interleaving data sets can be collected for finer reciprocal space coverage. Five separately located diffraction detectors are also available for the alignment of single crystal samples. Ef is set by aligning both analysers to the appropriate Bragg angle with respect to the scattered beam direction, whilst the rear analyser is additionally mounted on a translation unit permitting movement towards or away from the sample to maximise the reflected beam intensity reaching the detectors. Note that during this process, the detectors do not move and remain at fixed positions relative to the analysers. The analyser-detector assembly is surrounded by B4C-resin shielding, and is mounted on a rotating table centered at the sample position to adjust the scattering angle. The resolution ( F W H M ) of the new analyser system (Fig. 2(a)) has been determined from the incoherent scattering of a vanadium standard sample for two collimation arrangements: (i) 30 ~ horizontal collimation between the sample and front analyser, 60 ~ between the rear analyser and detector; (ii) 60' collimation between the sample and front analyser, no collimation before the detector.
~=
]
120
(b)
0 -10
-5
0
5
10
15
20
25
Energy Transfer, co(meV)
Fig. 2. (a) The experimental resolution of the double-crystal analyser system as determined from the incoherent scattering of a vanadium standard sample for the two collimation options described in the text, and (b) the scattering function of amorphous SiO2 measured with the new system.
For (i), A E F / E F = 4 % for all Ef, whilst for (ii), z~xEF/EF ----(2.3 q-0.5Ef)%, both in agreement with calculations [4]. The normal practice is to operate with (ii) due to the larger available flux per detector. In this situation, the resolution at Ef = 12meV is comparable to that of the single-analyser system. Recent experiments have demonstrated that the spectrometer is also well suited to the measurement of collective excitations in amorphous materials and liquids. Since PRISMA is a TOF instrument, it is possible to separate the signal from the (0 0 2) planes o f the analysers from the higher-order signal due to the (0 0 4 ) planes, effectively extending the range of wave-vector transfers covered. For example, Fig. 2(b) shows data from amorphous SiO2 obtained using the (0 0 4) planes of the double-crystal analyser system where Ef --- 20 meV, Xel = 4.39/~-1. The Boson peak believed to be connected to tranverse acoustic
M.J. Bull et al. / Physica B 234-236 (1997) 1061-1063
phonon modes is clearly visible at e = 5 meV and compares well with existing measurements [1]. A more comprehensive survey of the commissioning phase of the new analyser system will be published elsewhere. The financial support of the Italian Istituto della Materia del Consiglio Nazionale delle Richerche for the development of the PRISMA spectrometer is gratefully acknowledged.
1063
References [1] ISIS Annual Report, Rutherford Appleton Laboratory, RALTR-96-050 (1996) & RAL-TR-95-050 (1995). [2] C. Adreani, C.J. Caflile, F. Cilloco, C. Petrillo, F. Sacchetti, G.C. Stifling and C.G. Windsor, Rutherford Appleton Laboratory Technical Report RAL-86-026, 1986. [3] C. Petrillo, F. Sacchetti, U. Steigenberger and M. Hagen, J. Neutron Res. 3 (1996) 93; C. Petrillo, F. Sacchetti, M. Hagen and U. Steigenberger, Rutherford Appleton Laboratory Technical Report RAL-93-091, 1993. [4] U. Steigenberger, private communication.
Physica B 234-236 (1997) 1061-1063
A novel double-crystal analyser system for the PRISMA spectrometer at ISIS M.J. Bull a'b'*, M.J. Harris b, U. Steigenberger b, M. Hagen c, C. Petrillo d'e, F. Sacchetti d,e a Department of Physics, Birkbeck College, Malet Street, London WC1E 7HX, UK b ISIS Facility, Rutherford Appleton Laboratory, Chilton, Dideot OXll OQX, UK c Department of Physics, Keele University, Keele, Staffordshire ST5 5BG, UK d Dipartimento di Fisica, Universith di Perugia, Via A. Paseoli, 1-06100 Perugia, Italy eISM-CNR, Via E. Fermi 38, 1-00044 Frascati, Italy
Abstract
The crystal analyser spectrometer PRISMA has proven to be particularly successful at surveying extended areas of (~,e) space, efficiently collecting high-quality inelastic and critical scattering data sets from single-crystal samples. In the inelastic mode, mechanical constraints upon the 16 analyser-detector arms limit the final energy to Ef > 12 meV. To overcome this constraint, a novel double-crystal analyser system has recently been installed on the PRISMA spectrometer. Neutrons scattered from the sample are analysed by five groups of paired pyrolytic graphite analysers allowing final energies 2.25 meV
Keywords: Neutron instruments; Spectroscopy; Analysers
At a pulsed neutron source, the inverse geometry crystal-analyser spectrometer is ideally suited to the measurement of dispersion curves of collective excitations in single crystals. Neutrons from a polychromatic thermal neutron beam, after being scattered from a sample, are energy analysed using Bragg reflection from an analyser crystal. The energy transferred between the sample and the neutron can be determined from standard relationships between the total neutron time-of-flight (TOF), known final analysing energy, Ef, and the scattering angle, q~. Since moderated neutrons of all wavelengths are incident upon the sample, both the measured wave vector transfer, r, and the energy transfer, e, are swept along a parabolic measurement trajectory in reciprocal space for a particular crystal orientation. If this characteristic of the spec-
* Correspondence address. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK.
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 0 1 1 - 2
trometer is combined with the use of multiple analyser arms, it then becomes possible to examine an extended region of (r, e) space with a single setting of the instrument [1]. The PRISMA spectrometer at ISIS was constructed to exploit this technique with an array of 16 analyser-detector arms separated by a scattering angle of 2 °. This small ~b-separation was chosen such that the TOF trajectories would be closely packed in ( r , e ) space [2], but limiting Ef > 12 meV if collisions between neighbouring analyser-detector arms were to be avoided. This collision constraint has been overcome with the installation of a double-crystal analyser system [3] on the PRISMA beamline (Fig. 1). Incident neutrons from the 95 K methane moderator are scattered by the sample and energy analysed by five detector arms, each consisting of initial Soller collimation, two pyrolitic graphite analyser crystals, final Soller colli1 t/ mation and a Reuter-Stokes 10 atm ~ 3He gas detector. The analyser arms are separated by 10 ° in ~b, but
1062
M.J. Bull et al./ Physica B 234-236 (1997) 1061-1063
M
s
LMS
A
'-~
= 9.036 rn = 0.600 m
= 0.045 m* = 0.211 m*
>0 E A
lie
0.75
,
~'~'~J ~
0.60
1,.1
0.45 0.30
:~
0.1
' 4
. . . . . . . . . . . . . . 5 6 7 8 9 10
1 11
Final Energy, Ef (meV) 160
Fig. 1. Schematic view of the PRISMA double-crystal analyser system. Incident neutrons (wave vector ki) from the moderator (M), are scattered by the sample (S) through an angle ~ and energy analysed by five detector arms containing two graphite analyser crystals, (F)ront and (R)ear, a detector (D), and two Soller collimators (not shown). OF is the Bragg angle of both analysers for the required final energy, and the rear analyser (R) also has translational freedom (see text). The black areas represent shielding separating the fixed detectors, and starred lengths are defined when LFR is at 90° to LsF.
a sequence of interleaving data sets can be collected for finer reciprocal space coverage. Five separately located diffraction detectors are also available for the alignment of single crystal samples. Ef is set by aligning both analysers to the appropriate Bragg angle with respect to the scattered beam direction, whilst the rear analyser is additionally mounted on a translation unit permitting movement towards or away from the sample to maximise the reflected beam intensity reaching the detectors. Note that during this process, the detectors do not move and remain at fixed positions relative to the analysers. The analyser-detector assembly is surrounded by B4C-resin shielding, and is mounted on a rotating table centered at the sample position to adjust the scattering angle. The resolution ( F W H M ) of the new analyser system (Fig. 2(a)) has been determined from the incoherent scattering of a vanadium standard sample for two collimation arrangements: (i) 30 ~ horizontal collimation between the sample and front analyser, 60 ~ between the rear analyser and detector; (ii) 60' collimation between the sample and front analyser, no collimation before the detector.
~=
]
120
(b)
0 -10
-5
0
5
10
15
20
25
Energy Transfer, co(meV)
Fig. 2. (a) The experimental resolution of the double-crystal analyser system as determined from the incoherent scattering of a vanadium standard sample for the two collimation options described in the text, and (b) the scattering function of amorphous SiO2 measured with the new system.
For (i), A E F / E F = 4 % for all Ef, whilst for (ii), z~xEF/EF ----(2.3 q-0.5Ef)%, both in agreement with calculations [4]. The normal practice is to operate with (ii) due to the larger available flux per detector. In this situation, the resolution at Ef = 12meV is comparable to that of the single-analyser system. Recent experiments have demonstrated that the spectrometer is also well suited to the measurement of collective excitations in amorphous materials and liquids. Since PRISMA is a TOF instrument, it is possible to separate the signal from the (0 0 2) planes o f the analysers from the higher-order signal due to the (0 0 4 ) planes, effectively extending the range of wave-vector transfers covered. For example, Fig. 2(b) shows data from amorphous SiO2 obtained using the (0 0 4) planes of the double-crystal analyser system where Ef --- 20 meV, Xel = 4.39/~-1. The Boson peak believed to be connected to tranverse acoustic
M.J. Bull et al. / Physica B 234-236 (1997) 1061-1063
phonon modes is clearly visible at e = 5 meV and compares well with existing measurements [1]. A more comprehensive survey of the commissioning phase of the new analyser system will be published elsewhere. The financial support of the Italian Istituto della Materia del Consiglio Nazionale delle Richerche for the development of the PRISMA spectrometer is gratefully acknowledged.
1063
References [1] ISIS Annual Report, Rutherford Appleton Laboratory, RALTR-96-050 (1996) & RAL-TR-95-050 (1995). [2] C. Adreani, C.J. Caflile, F. Cilloco, C. Petrillo, F. Sacchetti, G.C. Stifling and C.G. Windsor, Rutherford Appleton Laboratory Technical Report RAL-86-026, 1986. [3] C. Petrillo, F. Sacchetti, U. Steigenberger and M. Hagen, J. Neutron Res. 3 (1996) 93; C. Petrillo, F. Sacchetti, M. Hagen and U. Steigenberger, Rutherford Appleton Laboratory Technical Report RAL-93-091, 1993. [4] U. Steigenberger, private communication.