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Neutrons in the material world

15 March 2000

Some 15 years ago, an ambitious plan to create a neutron source out of an aging
particle accelerator came to fruition. Today, accelerator, detector and data acquisition
developments continue to play an important role in keeping ISIS at the forefront of
neutron science.

ISIS, the major facility at the Rutherford Appleton Laboratory (RAL) in Oxfordshire,
UK, is the world’s most powerful pulsed spallation neutron source. Since 1984 it has
provided beams of neutrons and muons that have enabled the structure and dynamics
of condensed matter to be probed on a microscopic scale ranging from the subatomic
to the macromolecular, from a proton wavefunction to a protein
structure.

Neutron production

Construction of the source was
approved in 1977, following a proposal by UK scientists who saw an opportunity to
build a world-leading neutron facility replacing the aging NIMROD proton accelerator
at the then Rutherford Laboratory. In contrast with the traditional means of neutron
production by nuclear fission, which involves the production of a continuous stream
of neutrons, ISIS was to be a pulsed neutron source, similar to but much more intense
than the existing IPNS source at Argonne National Laboratory in Illinois,
US.

First, H ions would be accelerated in a pre-injector column to
665 keV, then passed into a linear accelerator consisting of four accelerating RF
cavities, reaching an energy of 70 MeV. At the point of injection into the final
acceleration stage (a 52 m diameter proton synchrotron), the electrons would be
stripped from the H ions by a 0.25 mm
alumina foil, to produce a circulating beam of protons.

At full intensity, 2.5
¥ 1013 protons per pulse would be
accelerated to 800 MeV, before being extracted and sent to a heavy metal target,
producing a burst of neutrons by spallation. This whole process would then be
repeated 50 times per second.

As a result of the low duty cycle of the ISIS
accelerator, the time-averaged heat production in the ISIS target would be a modest
160 kW, but, in the pulse, the neutron brightness would exceed that of the most
advanced steady-state sources. In addition, the structure of the neutron pulse would
be exploited using time-of-flight measurement techniques and white neutron beams,
thereby providing a direct determination of the energy and wavelength of each
neutron detected. The duty cycle of the accelerator would also ensure good
signal-to-noise levels.

The first neutrons were produced in late 1984 and ISIS
was officially inaugurated in October 1985. The facility reached its design specification
of delivering 200 mA pulses to the target station in
1993, and it has run more or less consistently at this level since
then.

Leading edge science

Over the past 15 years, ISIS has
attracted substantial international investment and has developed into a major
international force in condensed matter research. It has seen its complement of
instruments rise from 6 to more than 20 and its user base from 200 to more than
2000. This popularity reflects the fact that the neutron is in many ways the ideal
probe for the study of solids and liquids.

The citation for the 1994 Nobel Prize
for Physics to Brockhouse and Schull for their pioneering work in neutron scattering
put this point succinctly – neutrons simultaneously probe the structure and dynamics
of matter, bringing insight at an atomic and molecular level about where atoms “are”
and what atoms “do”.

Structural and dynamical studies at ISIS have had a
major impact at the cutting-edge of materials development at both the fundamental
and the applied levels. ISIS has been heavily involved in many of the most exciting
stories of recent years, including the physics “Woodstock” of high Tc
superconductors and the discovery of a new form of carbon, C60
(Buckminsterfullerene). On the applied side, work at ISIS underpins the development
of materials such as batteries, detergents, catalysts, pharmaceuticals and
polymers.

More data faster

Over the past 15 years the problems
tackled at ISIS have become ever more diverse and challenging. The desire to collect
more data faster has become irresistible, as areas such as in situand
time-resolved studies on increasingly complex systems become more important. To
handle this trend, detector arrays have expanded enormously from those originally
installed, employing “spin-off” technology borrowed from high-energy physics
techniques.

Next-generation instruments at ISIS, such as the new MAPS
spectrometer and the GEM diffractometer, now include detector arrays with areas as
large as 16 m2 – orders of magnitude larger than those available in 1984
and containing more than 50 million data points per measurement. Much of the
development of these new neutron detectors, both in terms of front-end construction
and signal encoding, has been undertaken at RAL.

In tandem with these large
detector arrays, the data acquisition and storage systems at ISIS must also be state of
the art to handle the huge volumes of data generated. ISIS instruments operate to a
common data acquisition framework based on the RAL-developed electronics. The
ability to develop, build and support such advanced systems in house has again relied
a great deal on experience gained from RAL’s historical and continuing involvement
in other aspects of high-energy and particle physics.

Looking to the
future

While the trend towards massive detector arrays is one way of increasing the
number of neutrons utilized in an experiment, developments in the synchrotron ring
are taking place that will increase the current that can be delivered to the target to
300 mA.

This involves the addition of a second
harmonic to the existing accelerating RF waveform, achieved by the insertion of four
new RF cavities into the existing ring. As well as benefiting all of the instruments
clustered round the existing target station via increased neutron production, this
enhanced current can be shared with a second target station optimized for the
production of longer-wavelength cold neutrons, opening up new research
opportunities in fields such as complex macromolecular assemblies, magnetism, colloid
and surface chemistry, high-resolution diffraction and the biological sciences.
Furthermore, the enhanced current will be essential if the SIRIUS project, which aims
to utilize the spallation source as a method of producing radioactive nuclei for
post-acceleration, is to become a reality.

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