A recent workshop in Frascati highlighted some of the exciting possibilities for future developments in channelling particle beams in ordered structures.
Research on the physical phenomena induced when high-energy beams of charged and neutral particles interact with ordered matter, such as crystal lattices and nanostructures, has seen considerable progress in recent years, from both theoretical and experimental points of view. When charged particles, especially those moving at relativistic speeds, pass through a crystal they feel a strong coherent electric field due to the nuclear charges. Particles can be channelled by the arrangement of atoms in crystals, and specially bent crystals are used in accelerator laboratories to steer high-energy beams.
On 23-26 March this year, INFN’s Laboratori Nazionali di Frascati (LNF) hosted the International Workshop on Relativistic Channelling and Related Coherent Phenomena. Several successful meetings in this field have previously taken place, notably at Maratea in 1986, Protvino (Serpukhov) in 1991 and Aarhus in 1995. This year’s workshop was held at the home of the DAFNE-LIGHT synchrotron radiation facility, which is particularly suitable for experimental work at infrared wavelengths and in X-ray diffraction. The LNF has also been supported by the European Union (EU) as one of the major research infrastructures in Europe to give free access to researchers during the period 2000-2004, and the EU has recently approved a new access to research infrastructure programme at the LNF for 2004-2008.
The main purpose of the workshop was to assess the current state of the art of this fast-growing field and to stimulate research collaboration among the different groups involved, with the aim of prompting the organization and presentation of joint projects in the near future. The success of the workshop can be shown by the number of participants, with around 40 specialists attending from 12 different countries, including Japan, the US and most of the former USSR, and by the high quality of the technical presentations.
Erik Uggerhoj from Aarhus launched a new initiative towards research on strong field effects in ordered matter at multi-TeV energies, entering the territory far above the critical Schwinger field. This could lead to a new multi-TeV electron (positron) beam facility at CERN’s Large Hadron Collider (LHC), providing new opportunities for fixed-target physics and applications. Such research at the LHC would require crystal-assisted extraction of a parasitic beam, a technique based on strong crystal fields. Simulations at the workshop showed that a tiny crystal installed into the collimation system could enhance the efficiency of the LHC collimation by an order of magnitude. The channelling for collimation purposes can then easily be turned into an instrument for beam extraction when needed.
A team from Brookhaven National Laboratory reported on crystal collimation experiments with beams of gold ions at the Relativistic Heavy Ion Collider, which were performed jointly with the Institute for High Energy Physics (IHEP) in Protvino. Channelling efficiencies of about 30% were measured, in good agreement with Monte Carlo simulation. This is a significant step forward, but more work is needed to incorporate a strong crystal field into an accelerator lattice and to benefit from it fully.
Channelling in “bent” crystals is routinely used in experiments at Protvino, where many crystals are installed at six locations around the main ring. Some channelling crystals have been used for extraction for more than 10 years without replacement. Extraction efficiencies of 85% for a 70 GeV beam of 1012 protons have been measured for 2 mm crystals, in excellent agreement with predictions.
In addition to the well known use of bent crystals and focusing crystals demonstrated at IHEP more than a decade ago, crystal undulators are now being introduced to experiments, as Yuri Chesnokov of IHEP reported. Channelling undulators offer sub-millimetre periods and fields of the order of 1000 tesla. Figure 1 shows scanning electron microscope images of an undulator surface that was produced by micromachining a silicon crystal at IHEP. The images, obtained by a team from LNF in collaboration with CNR-IFN (Rome), reveal the undulator’s 50 µm grooves spaced by 200 µm, which produce periodic deformations that propagate in the bulk of the crystal. Samples of this kind, which have been characterized with X-rays and tested with protons, are now ready for the positron beam tests planned for the Beam Test Facility at the LNF, as well as at IHEP and at the Super Proton Synchrotron at CERN.
Positron sources are another application of strong coherent fields. Teams from KEK and Yerevan presented the progress and new ideas in this direction, and a number of talks reported on the theories of coherent radiation and electron-positron pair production in ordered matter.
Nanostructures also offer a new line of research for particle interactions with ordered matter. Several talks were devoted to particle channelling in nanotubes, radiation in periodic nanostructures and the growth of aligned nanostructured arrays. Possibilities for experiments with channelling nanostructures were outlined by teams from LNF and IHEP, where such activities are already underway.
Further reading
The proceedings will be published as a special issue of Nuclear Instruments and Methods B, which is scheduled to appear in December 2004. For further information about the workshop, see www.lnf.infn.it/conference/rc2004.
