Radioactive beams by fragmentation and ISOL techniques - CERN

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intensity. During the last two decades, starting at LBL and continuing with high intensity high energy heavy ion beams at GANH., RIKEN, MSU and more recently at GSI/SIS. the fragmentation of the projectile was successfully used to produce secondary beams] Simultaneously, the ISOL—method (isotope separation on line) was used to produce low energy beams of 60 keV mainly for studies of decay and of ground state properties at ISOLDE/CERN2). The combination of these methods corresponds to the fragmentation of the beam or the target and the reacceleration of the reaction products to energies high enough forstudies of nuclear reactions, this is of the order of 1-100 MeV/nucleon. The first beams using this method were obtained at Louvain—la-Neuve3). Various projects all over the world are very vigourously studied. A quite up-to—date compilation can be found in the proceedings of the Dourdan workshop4). I-Iere we dont want to discuss the relative merits of these projects but compare the fragmentation and the Isol methods for the production of high energy secondary beams. II. BEAM FRAGMENTATION In this section we deduce a simple formula which allows to estimate the intensity of a secondary beam produced by fragmentation and selected by a spectrometer. Three main points will be discussedza) the influence of the primary beam energy on the yield due to limited target thickness and spectrometer transmission, b) the choice of the primary beam, c) the effect of degrading of the the energy of the secondary beam energy on the a) Beam energy The use of the fragmentation of the projectile produces nuclei over a broad range of the nuclear chart. If the aim is the study of nuclei far from stability, a very selective device is needed, in order to have a high rejection of beam particles and of nuclei near stability, produced with several orders of magnitude higher yields. All recent studies have made use of magnetic spectrometers for such a selection. These spectrometers have a limited angular (A9, Arp) and momentum (Ap/p) acceptance.ln the following we will estimate the losses due to such a device. The variance ofthe distributions in the three dimensions (A0. Arp. Ap/phnay be estimated using the Goldhaberjl rule : é€f) Gimg = Ofcrmi -%,%-j OCR Output
where ofmg is the variance of the momentum distribution of the fragment, ofcimi is the fermi·momentum of about 100 MeV/c, and Af and AD the mass of the fragment and the projectile respectively. (2) (4) The variance 0 of the distributions in 0, tp, Ap/p for a given fragment is then where Ei is the incident energy. An other important factor is the limitation of the target thickness due to energyloss differences between the incident and outgoing particle of atomic number Zi and Zi respectively. The energy loss in the target is approximately AE = CA2!/E where c is zi constant. The momentum spread introduced by the target of a thickness t is then We used the fact that the energy per nucleon before and after fragmentation is essentially the same. The maximum useful target thickness tom will be obtained if (Ap/p)img€i ~ omg/p; for thicker targets the loss in transmission due to broadening of the distributions will be stronger than the gain of yield due to a thicker target . The final yield y we will obtain for a given fragment is proportional the target thickness topi, and the angular and the momentum acceptance. If, as is the case very often, the acceptance of the spectrometer is smaller as than the width of the distribution. the yield ys at the exit of the spectrometer will be given by Therefore the yield will increase with the incident energy in MeV/nucleon to the power 7/2. The validity of ths formula can be checked by comparison with codes as LISEO) and Intensity’l. This is shown on fig. l). Deviations observed for energies above several hundreds MeV/nucleon can be attributed to the fact that the distributions are no longer very broad as compared to the acceptance of the spectrometer. Where this happens is dependant on the reaction and the spectrometer acceptance considered. This can be seen by writing explicitly the A-dependance of the yield. Using equation (1). we otain. for example for G9 : OCR Output Ofrag ] ($9i:($(p:()'A :——o
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Radioactive beams by fragmentation and ISOL techniques - CERN

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