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A project to provide enhanced neutron scattering capability at the highest energy transfers


Original research objectives:
ISIS, the world's most intense pulsed source, provides a unique opportunity for the development of new applications of neutron scattering into hitherto uncharted regions of energy and momentum transfer. The exploitation of the intense epithermal neutron flux produced at ISIS has been so far largely unexplored. VESUVIO project aims to set up as a new instrument at ISIS in order to investigate the feasibility of high-energy neutron scattering in the study of single particle momentum distributions in different areas of scientific interest, in both pure and applied science. Studies in this field with eV neutrons provide additional and/or complementary information to other spectroscopic techniques, e.g. deep inelastic scattering of electrons from nuclei and Compton scattering of photons from electrons.

VESUVIO project is based around the following deliverables:
- improved resolution for measurements with eV neutrons
- better detection techniques for eV neutrons
- very high count rate data acquisition systems
* better understanding of the interaction of eV neutrons with condensed matter
(R&D in high energy neutron scattering spectroscopy).

Expected deliverables:
1. To improve analysis and detection techniques for neutrons with eV energies.
VESUVIO was specifically designed for high momentum and energy transfer inelastic neutron scattering studies of microscopic dynamical processes in materials and will represent a unique facility for EC researchers.

1.1 To improve the energy resolution by resonant foil cooling.
At eV energies the only efficient method of energy analysis is via nuclear resonance absorption. Uranium foils have three nuclear resonances centred at E1= 6.7 eV, 20.7 eV and 37 eV, which absorb neutrons strongly over narrow energy ranges. The scattering technique consists of cycling the foil in and out of the scattered neutron beam so that two measurements are taken, one with the foil between sample and detector and one with the foil removed. The difference between these two data sets (i.e. the number of neutrons absorbed by the foil) is the experimental signal and provides a measurement of the intensity of neutrons scattered with final energy E1. The width of the nuclear resonance used for energy analysis contributes to the finite resolution of the spectrometer. Although the intrinsic width of the U resonances is small, at room temperature these widths are significantly Doppler broadened by thermal motion. We proposed to improve the resolution of the analyser by cooling the foil to <50 K in a closed cycle refrigerator to reduce the Doppler broadning.
The INFM/Rome group was the partner responsible for this task.

1.2 To optimise and construct a detector bank for eV neutrons.
Conventional He gas counters are not suitable for use at eV energies since the low neutron absorption cross-section makes their detection efficiency low. Furthermore the dead times in He detectors are long and faster detectors are required to cope with the large instantaneous count rates. We built a high efficiency detector with Li doped scintillator glass, based on the prototype eVS detectors already in operation. The scintillations are detected as light pulses by photo-multiplier tubes. We explored the optimisation of the photo-multiplier tubes, in respect of their location against the Li doped scintillator glass. A detector bank was constructed which can be oriented along any azimuth, but mainly about, 145?-175?. Accruing the improvements from this work into the final design of the detectors. This new detector provides an overall increase in neutron count rate by a factor of at least two.
The INFM/Rome group was the partner responsible for this task.

1.3 To construct a new sample tank.
The new sample tank was shaped to accommodate the new detector assembly and the foil's temperature control device.
The INFM/Rome group was the partner responsible for this task.

1.4 To develop new high count rate electronics and data acquisition systems.
The very high instantaneous neutron count-rates generated in the detectors necessitated the development of new electronics modules, in order to avoid saturation of the data acquisition system. The new modules output a single electronic pulse for a predetermined number of neutrons detected, thereby reducing the total count-rate input to the data acquisition electronics.
The ISIS /RAL was the partner responsible for this task.

2.1 Training of Young Scientists.
The Post-Doctoral research fellows employed were trained in the theoretical and experimental aspects of eV neutron scattering, including: the trial assembly and optimisation of the new detector, the 'best practice' approach to light-tight construction, the experimental methodology of eV neutron experimentation and the theoretical formalism required to interpret results. They performed experiments specifically designed to test the experimental capability of the instrument in the eV region. They received formal training in all safety aspects of their work; including the chemical and radiological hazards as well as the more mundane risks of laboratory work. Throughout their work they were supervised by qualified staff at ISIS. They were also trained in the general managerial and administrative tasks associated with any large scale organisational unit and participated in the writing and presenting of scientific reports.
The ISIS /RAL was the partner responsible for this task,

2.2 Development of a better neutron scattering formalism for eV neutrons.
There is as little theory as there is experimental data available at present in the field of epithermal neutron scattering. A better theoretical understanding is essential before eV neutrons can be utilised to their full potential. The guiding principal of this work was to develop continuity in the scattering formalism. The scattering function should be applicable from the lowest values of neutron momentum transfer up to, and including, the impulsive regime.
All the partners were responsible for this task.
1.1 Energy resolution enhancement.
Cooling of the Uranium Analyser Foil went entirely as expected but the energy resolution of the spectrometer in single difference mode suffered from Lorentzian wing effects. These were difficult to deal with during the data analysis. This led us to use the Double Difference Technique to ameliorate the situation. The double difference technique consists of taking three measurements, with no foil, a foil of thickness t1 and transmission T1 and a foil of thickness t2 and transmission, T2. The "double difference" of the three measurements is.
Defined by:
where T(E) is the foil transmission at energy E, N is the number of atoms/unit volume and t is the foil thickness. The nuclear resonance cross-sections ((E) have a Breit-Wigner form for their intrinsic line shape. Thus,
The double difference technique relies upon the fact that when ((E) is small,

with a similar expression for 1-T2(E). Thus when ((E) is small R2(E)= 0 and the Lorentzian wings of the resolution function in single difference are removed. A factor of almost two-fold improvement to the resolution was achieved.

1.2 To optimise and construct a detector bank for eV neutrons.
A new high efficiency detector was constructed The detector bank is centred around the incoming beam, and consist of Li doped glass scintillator elements arranged within 60( segments that match the 60( segments of the energy analysis filters discussed above. The design developed into a highly segmented arrangement to match the electronics solution being developed under 1.4, below.

1.3 To construct a new sample tank.
The sample compartment-filter exchanger unit is a single stainless steel vacuum vessel made up of two mechanically independent parts: the sample chamber and the filter chamber. This is able to rotate about the incoming neutron beam axis. A special kinematic vacuum seal allows the rotation of the entire filter chamber and foil-support. The foil-support is divided into six equal 60°sectors thus allowing either or both single-difference and double-difference measurements to be performed by rotating the chamber by (60°.

1.4 To develop new high count rate electronics and data acquisition systems.
The saturation problems remaining in the high data rate detector, even after segmentation (see 1.2 above) were overcome through installing a deep FIFO, which quadrupled the available data register list. This was incorporated into a newly designed and copyrighted PCB. The new design is currently the standard device for use in the next generation of data acquisition electronics at ISIS. It will be widely used in the instrumentation suite of the new target station.

2.1 Training of Young Scientists.
The two young scientists directly involved in the VESUVIO project, Emiliano Degiorgi, Andrew Fielding and Roberto Senesi received the training as outlined above. As well as these there were some several visiting young scientists who, through a need to use the spectrometer, were provided with training.

2.2 Development of a better neutron scattering formalism for eV neutrons.
A significant number of contributions to the literature on this subject was made during the course of the project. These are detailed latter, in section 2-result 3.

Highlights of important research results.
A number of papers have been published as a consequence of work performed during the VESUVIO project. These include;
* Calibration of the electron volt spectrometer, a deep inelastic neutron scattering spectrometer at the ISIS pulsed neutron spallation source. A L Fielding and J Mayers Nuc. Inst. Meth. A 480 680-89 (2002).
* Multiple Scattering in Deep Inelastic Neutron Scattering Experiments: Monte Carlo Simulations and Experiments at the ISIS inverse geometry spactrometer. J Mayers, A Fielding and R. Senesi. Nuc. Inst Meth A. A 481 454-463 (2002).
* Double difference method in Deep Inelastic Neutron Scattering on the VESUVIO spectrometer. C. Andreani, D. Colognesi, E. Degiorgi, A. Filabozzi, M. Nardone, E. Pace, A. Pietropaolo, R. Senesi, accepted for publication in Nuclear Instruments and Methods (2003).
* Single particle dynamics in fluid and solid hydrogen sulphide: An inelastic neutron scattering study. C Andreani, E Degiorgi, R Senesi, F Cilloco, D Colognesi, J Mayers, M Nardone, E Pace. J. Chem Phys., 114: (1) 387-398 (2001).


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