These ions are then fed into the second accelerator. This technique is also chemistry-independent, works for essentially all elements, and is fast. Its applicability for the most intense beams of exotic nuclei is still under investigation.

In the ISOL technique, a beam of light projectile nuclei bombards a thick target of a heavy element. The exotic nuclei are produced by a process called spallation, in which the target nucleus is fragmented into pieces, many of which are exotic. These exotic nuclei stop in the hot, thick target and diffuse from the target into an ion source where they are prepared for injection into the second accelerator and reaccelerated. This technique can often produce the highest intensities of certain isotopes and has a long history of technological development, but the extraction process depends on the atomic chemistry and surface properties of the target, is generally not useful for (refractory) elements with low vapor pressure at high temperatures, and is often slow so that short-lived isotopes are not obtained. Typically, considerable research and development are required to establish a useful beam for the first time that a new element is required.

In all three techniques, the exotic nuclei can be stopped to study their radioactive decay or injected into traps for fundamental studies or measurements of their properties such as their mass or charge radius.

Significant technical advances have been made in the development of super-conducting radio-frequency linear accelerators. Improvements in cavity design and material preparation have led to higher field gradients, leading to more efficient acceleration. Independent tuning and phasing of the individual RF modules allow ion acceleration over a wide range of velocities and charge-to-mass ratios. Continuing ion source development has led to the production of large quantities of highly charged heavy ions ideal for energetic heavy-ion drivers. All this technology is also applicable for the reacceleration phase of an exotic-beam facility in which collection efficiency and beam quality are more important than high energy or beam power. Appropriate proton drivers have been available for some time, and the ISOL technique is now well developed.

An essential additional development in facilitating the study of exotic nuclei is advances in experimental instrumentation that now allow measurements to be carried out with beams as weak as a few hundred particles per second or, in special cases, as low as 1 particle per day, whereas traditionally, nuclear structure and astrophysics experiments have usually been carried out with beams on the order of 108 to 1013 particles per second.

Thus, it appears that the technological advances are now available to allow the construction of rare-isotope facilities of enhanced capability that would permit the execution of experiments unimaginable a decade ago.



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