The Target Ion Source

The production cave is placed in a well shielded area beneath ground level in the machine building. In addition to the target-ion source system, both high and low-energy "front-ends" are installed in the cave. A view of the target ion source production system is shown in the figure on the left. Importantly, each component can be remotely removed from the cave.

   In the classic ISOL technique a proton or a light-ion beam is accelerated to a high energy and bombards a thick target producing radioactive nuclei by spallation reactions, fragmentation of the target and/or induced fission. Other reaction mechanisms, however, come into play with heavy ions. In particular, projectile fragmentation is the process of most importance. In all cases, the fragments are stopped in the target which is heated to a high temperature (1800K to 2300K) to facilitate the migration of the radioactive atoms to the surface. Usually the target is located a short distance from the ion source and the radioactive atoms effuse via a transfer tube to the plasma region where they are ionised and then accelerated.

   However, given the relatively short range of heavy ions (typically they stop in the production target) one may consider employing very small targets located inside the ion source. Such a configuration would thus eliminate the losses due to sticking of radioactive atoms in the transfer tube - a major source of losses in systems presently in use. As the atoms are ionised and accelerated in a manner identical to that for stable beams, the resulting radioactive beams have good dynamical and optical characteristics when compared with projectile fragmentation, as well as an energy which may be precisely adjusted. The originality of the GANIL project lies in the use of an extended range of heavy-ions produced at high intensities and up to the maximum available energies (e.g. 95 A.MeV, 7.4 eµA for ³⁶Ar¹⁸⁺). Such an approach differs from the proton (or light-ion) beam technique in that the projectile rather than the target is varied in order to produce the different radioactive species.

   For the commissioning of SPIRAL, a solution based on an external carbon target linked to the ECR source by a short transfer tube was chosen. This simple system allows producing gas radioactive ion beams. The carbon target has been chosen due to its excellent release properties, low atomic number and high sublimation temperature. This target guarantees the production of gases with reasonable yields and can be used with high power primary beams. This requirement has necessitated the development of a special target design which can withstand such power loads while conserving fast release properties. From the production point of view, the temperature of the target should be as high as possible and its profile should be as uniform as possible, in order to minimise the delay time between production and release. The temperature profile is related to the properties of the Bragg peak, which is particularly pronounced in the case of heavy ions. Therefore, a special conical design which distributes uniformly the power density over the volume of the target has been developed.

The permanent magnet ECRIS developed for ionising the radioactive atoms released from the production target, NANOGAN III, has performances compatible with the needs of the m/q ratio for the CIME acceleration. The full system has been tested successfully at SIRa prior to the installation in the production cave. The available beam intensities are given here.