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VESUVIO: a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility
Physica B 276}278 (2000) 200}201
VESUVIO: a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility R. Senesi *, C. Andreani, Z. Bowden, D. Colognesi, E. Degiorgi, A.L. Fielding, J. Mayers, M. Nardone, J. Norris, M. Praitano, N.J. Rhodes, W.G. Stirling, J. Tomkinson, C. Uden INFM-UdR di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133 Roma, Italy Dipartimento di Fisica, Universita% degli Studi di Roma **Tor Vergata++ and INFM-UdR di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133 Roma, Italy ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX UK CNR, Viale dell'Universita% 11, 00145 Roma (IT) and INFM-UdR di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133, Italy Department of Physics, Oliver Lodge Laboratory, University of Liverpool, L69 7ZE UK Dipartimento di Fisica **E. Amaldi++, Universita% degli Studi di Roma Tre and INFM-UdR di Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy R.M.P., V.le E.Ortolani 194, 00125 Dragona, Italy
Abstract The VESUVIO project aims to provide unique prototype instrumentation at the ISIS-pulsed neutron source and to establish a routine experimental and theoretical program in neutron scattering spectroscopy at eV energies. This instrumentation will be speci"cally designed for high momentum, (20 As \(q(150 As \), and energy transfer ( u'1 eV) inelastic neutron scattering studies of microscopic dynamical processes in materials and will represent a unique facility for EU researchers. It will allow to derive single-particle kinetic energies and single-particle momentum distributions, n(p), providing additional and/or complementary information to other neutron inelastic spectroscopic techniques. 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron instruments; Momentum distribution; Incoherent scattering
Inelastic neutron scattering at high-momentum transfer can provide direct information on the atomic momentum distribution, n(p), when the impulse approximation (IA) is valid. The latter is only reached in recoil scattering when the energy and momentum transfer are su$ciently large by utilising large incident neutron energies generally in excess of 1 eV, such as those typically available on spallation neutron sources. The technique
known as deep inelastic neutron scattering (DINS) relies upon the fact that in the IA framework the scattering is no longer a function of the energy and momentum transfer separately and the u and q variables can be coupled through the West variable [1}3], y, de"ned by y"M/q(u!(q/2M)), where M is the mass of the target atom. The scattering law can thus be written, in the case of isotropic systems J(y)"q/MS(q,u)"2p
* Corresponding author. Tel.: #39-6-72594549; fax: #39-62023507. E-mail address: [email protected] (R. Senesi)
pn(p) dp, (1) W where S(q,u) is the usual inelastic structure factor. The measurement of J(y) thus allows to derive single-particle
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 2 4 6 - 6
R. Senesi et al. / Physica B 276}278 (2000) 200}201
Fig. 1. Scattering function J(y) for super#uid He at 1.5 K using U analysers: the open circles are the experimental data: the dashed}dotted line is the result of the "t; the experimental resolution function is also shown for room temperature foil (dashed line) and foil cooled to ¹"77 K (solid line). The improved resolution function with the cold analyser permits a data analysis consistent with non-Gaussian line shapes in the momentum distributions in super#uid and also in normal He [5].
dynamical properties, such as the single-particle momentum distribution and single-particle mean kinetic energy. The VESUVIO project is speci"cally designed to provide a unique instrumentation for deep inelastic neutron scattering and will build on the success and experience of the eVS spectrometer at the ISIS facility. The main objectives of the project are: (a) to realise a novel experimental apparatus and detection techniques for eV neutron spectroscopy; (b) to develop a better neutron scattering formalism for eV neutrons and to train young scientists in the theoretical and experimental aspects of eV neutron scattering. Parts of the objective (a) are: the improvement of the energy resolution by the cooling of the analyser "lters; the construction and optimisation of an azimuthal detector bank for eV neutrons; the construction of a new sample tank and beam tubes and the development of new high count rate electronics and data acquisition systems. The basic element for eV neutron spectroscopy is the nuclear resonance absorption "lter, used to select the "nal energy of the scattered neutrons [4]. The width of
201
the nuclear resonances used for energy analysis contributes to the "nite resolution of the spectrometer and we have already shown experimentally that it can be e!ectively reduced by cooling the "lter [5], as shown in Fig. 1. The eVS spectrometer makes use of U foil analysers. The nuclear resonance centred at E "6.7 eV corres ponds to strong neutron absorptions over a narrow energy range. Although its intrinsic width is small (i.e. full-width at half-maximum (FWHM) "0.04 eV at 6.7 eV), at room temperature it is signi"cantly Doppler broadened by thermal motion (to FWHM"0.11 eV). The VESUVIO project will enable an improvement of the resolution of the analysers to FWHM"0.066 eV by cooling the foils to 77 K, using a closed cycle refrigerator. A prototype tank will be speci"cally developed to provide cold "lter analysers for the entire angular scattering range. It will be shaped to incorporate the new detector assembly and will accommodate the "lter temperature control device. The entrance and exit ports can both be vacuum-tight coupled to the "lter chamber which can be used in both back-scattering and forward-scattering geometries covering in both cases a $303 angular aperture. The detector bank will be realised in a Debye}Scherrer half-cone shape centred around the incoming beam, and it will consist of speci"cally built fast and high-e$ciency cerium-activated lithium glass scintillators. Speci"cally chosen experiments will be performed in order to test the experimental capability of the instrument in the eV region, and these will cover di!erent areas of chemistry and physics (quantum solids and #uids, molecular solids and #uids, hydrogen-bonded single crystals), material science (polymers, catalysts and metal hydrides), and magnetic and electronic excitations. The VESUVIO project is funded under the EC TMR-Access to large-scale facilities (RTD project). References [1] [2] [3] [4]
G.B. West, Phys. Rep. 18 (1975) 263. V.F. Sears, Phys. Rev. B 30 (1984) 44. G. Watson, J. Phys.: Condens. Matter 8 (1996) 5955. J. Mayers, A.C. Evans, Rutherford Appleton Laboratory report RAL-91-048. [5] J. Mayers, C. Andreani, D. Colognesi, J. Phys.: Condens. Matter 9 (1997) 10 639.
VESUVIO: a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility R. Senesi *, C. Andreani, Z. Bowden, D. Colognesi, E. Degiorgi, A.L. Fielding, J. Mayers, M. Nardone, J. Norris, M. Praitano, N.J. Rhodes, W.G. Stirling, J. Tomkinson, C. Uden INFM-UdR di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133 Roma, Italy Dipartimento di Fisica, Universita% degli Studi di Roma **Tor Vergata++ and INFM-UdR di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133 Roma, Italy ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX UK CNR, Viale dell'Universita% 11, 00145 Roma (IT) and INFM-UdR di Roma Tor Vergata, Via della Ricerca Scientixca 1, 00133, Italy Department of Physics, Oliver Lodge Laboratory, University of Liverpool, L69 7ZE UK Dipartimento di Fisica **E. Amaldi++, Universita% degli Studi di Roma Tre and INFM-UdR di Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy R.M.P., V.le E.Ortolani 194, 00125 Dragona, Italy
Abstract The VESUVIO project aims to provide unique prototype instrumentation at the ISIS-pulsed neutron source and to establish a routine experimental and theoretical program in neutron scattering spectroscopy at eV energies. This instrumentation will be speci"cally designed for high momentum, (20 As \(q(150 As \), and energy transfer ( u'1 eV) inelastic neutron scattering studies of microscopic dynamical processes in materials and will represent a unique facility for EU researchers. It will allow to derive single-particle kinetic energies and single-particle momentum distributions, n(p), providing additional and/or complementary information to other neutron inelastic spectroscopic techniques. 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron instruments; Momentum distribution; Incoherent scattering
Inelastic neutron scattering at high-momentum transfer can provide direct information on the atomic momentum distribution, n(p), when the impulse approximation (IA) is valid. The latter is only reached in recoil scattering when the energy and momentum transfer are su$ciently large by utilising large incident neutron energies generally in excess of 1 eV, such as those typically available on spallation neutron sources. The technique
known as deep inelastic neutron scattering (DINS) relies upon the fact that in the IA framework the scattering is no longer a function of the energy and momentum transfer separately and the u and q variables can be coupled through the West variable [1}3], y, de"ned by y"M/q(u!(q/2M)), where M is the mass of the target atom. The scattering law can thus be written, in the case of isotropic systems J(y)"q/MS(q,u)"2p
* Corresponding author. Tel.: #39-6-72594549; fax: #39-62023507. E-mail address: [email protected] (R. Senesi)
pn(p) dp, (1) W where S(q,u) is the usual inelastic structure factor. The measurement of J(y) thus allows to derive single-particle
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 2 4 6 - 6
R. Senesi et al. / Physica B 276}278 (2000) 200}201
Fig. 1. Scattering function J(y) for super#uid He at 1.5 K using U analysers: the open circles are the experimental data: the dashed}dotted line is the result of the "t; the experimental resolution function is also shown for room temperature foil (dashed line) and foil cooled to ¹"77 K (solid line). The improved resolution function with the cold analyser permits a data analysis consistent with non-Gaussian line shapes in the momentum distributions in super#uid and also in normal He [5].
dynamical properties, such as the single-particle momentum distribution and single-particle mean kinetic energy. The VESUVIO project is speci"cally designed to provide a unique instrumentation for deep inelastic neutron scattering and will build on the success and experience of the eVS spectrometer at the ISIS facility. The main objectives of the project are: (a) to realise a novel experimental apparatus and detection techniques for eV neutron spectroscopy; (b) to develop a better neutron scattering formalism for eV neutrons and to train young scientists in the theoretical and experimental aspects of eV neutron scattering. Parts of the objective (a) are: the improvement of the energy resolution by the cooling of the analyser "lters; the construction and optimisation of an azimuthal detector bank for eV neutrons; the construction of a new sample tank and beam tubes and the development of new high count rate electronics and data acquisition systems. The basic element for eV neutron spectroscopy is the nuclear resonance absorption "lter, used to select the "nal energy of the scattered neutrons [4]. The width of
201
the nuclear resonances used for energy analysis contributes to the "nite resolution of the spectrometer and we have already shown experimentally that it can be e!ectively reduced by cooling the "lter [5], as shown in Fig. 1. The eVS spectrometer makes use of U foil analysers. The nuclear resonance centred at E "6.7 eV corres ponds to strong neutron absorptions over a narrow energy range. Although its intrinsic width is small (i.e. full-width at half-maximum (FWHM) "0.04 eV at 6.7 eV), at room temperature it is signi"cantly Doppler broadened by thermal motion (to FWHM"0.11 eV). The VESUVIO project will enable an improvement of the resolution of the analysers to FWHM"0.066 eV by cooling the foils to 77 K, using a closed cycle refrigerator. A prototype tank will be speci"cally developed to provide cold "lter analysers for the entire angular scattering range. It will be shaped to incorporate the new detector assembly and will accommodate the "lter temperature control device. The entrance and exit ports can both be vacuum-tight coupled to the "lter chamber which can be used in both back-scattering and forward-scattering geometries covering in both cases a $303 angular aperture. The detector bank will be realised in a Debye}Scherrer half-cone shape centred around the incoming beam, and it will consist of speci"cally built fast and high-e$ciency cerium-activated lithium glass scintillators. Speci"cally chosen experiments will be performed in order to test the experimental capability of the instrument in the eV region, and these will cover di!erent areas of chemistry and physics (quantum solids and #uids, molecular solids and #uids, hydrogen-bonded single crystals), material science (polymers, catalysts and metal hydrides), and magnetic and electronic excitations. The VESUVIO project is funded under the EC TMR-Access to large-scale facilities (RTD project). References [1] [2] [3] [4]
G.B. West, Phys. Rep. 18 (1975) 263. V.F. Sears, Phys. Rev. B 30 (1984) 44. G. Watson, J. Phys.: Condens. Matter 8 (1996) 5955. J. Mayers, A.C. Evans, Rutherford Appleton Laboratory report RAL-91-048. [5] J. Mayers, C. Andreani, D. Colognesi, J. Phys.: Condens. Matter 9 (1997) 10 639.