Radioactive beams by fragmentation and ISOL techniques - CERN

emittance.
If a broad range of nuclei far from stability are to be produced, a total energy of
about l GeV in the composite system is needed. Lower total energies will give more
localised populations of the nuclear chart, thus giving higher cross—sections for nuclei
near the stability, however the cross-section far from stability will drop much faster. This
is, for example, the case for the Arenas device at Louvain la Neuvel9), where a 30 MeV
proton beam was used, and for the Oak Ridgem) project.
c) Special problems of the Isol techniques
The ISOL technique needs the extraction of the secondary ions out of the target
material, the ionisation and reacceleration of these ions. Due to different physico-chemical
behaviour each element has different problems. The 20 years of experience of
ISOLDE/CERN show simultaneously that these problems may be solved for most of the
elements, and however for each element a lot of R+D is required to get optimum results.
Besides the chemical behaviour, the diffusion—effusion time from the target to the
ion source is a difficultfproblems. The diffusion out of the target is determined mainly by
the mechanical structure of the target and the temperature, the effusion from the target to
the ion source by the sticking time of the atoms to the walls. Typical total delay times are
of the order of secondsm). This may be a serious handicap for very short lived ions.
The atoms must be ionised, and here other losses will occur. Three possibilities
are studied and/or used actually: production of negative ions for use in tandems.
production of singly charged positive ions, and multiple charged ions. With modern
ECR—sources the last possibility is promising, however a lot of R+D is still necessary to
make this possibility operational for a broad range of elements and of live—times together
with a high power (~lO kW) beam incident on the target near the plasma. Acceleration
will imply further losses, depending on the acceleration mode choosen, efficiency of
stripping if necessary in the acceleration mode chosen, and on the initial ion source
Following the preceeding considerations, it is clear that no general num ber exists
for the total ISOL efficiency. Best numbers which may be expected for a multi-charged
ion source followed by a cyclotron as at Louvain la Neuve and as in the Ganil—Spirale
project may be taken to be 2 x 10%, using 20 % for the total target + ionisation
efficiency, 20 % for the most probable charge state, and 50 % for the acceleration
efficiency. Actual best results are about l order of magnitude below this number.
These problems are avoided in direct use of fragmentation products. where short
life-times of down to l tts are available, and no chemical selectivity exists. OCR Output