Science and Technology Journal of BgNS, VOL. 13, No. 1, JUNE 2009
ISSN 1310-8727
Title:
MAKING PRECISION BEAMS OF THERMALIZED PROJECTILE
FRAGMENTS
Author:
D.J.Morrissey
Abstract:
The versatility of projectile fragmentation reactions to produce very exotic
nuclei at the NSCL is reviewed. Recent results of the production of the most neutron-rich
nuclei by the fragmentation of a 48Ca beam at 140 MeV/nucleon show the production of
several new isotopes including the most neutron-rich aluminium 42Al and silicon, 44Si
nuclides. A simple systematic framework was found to describe the production cross
sections based on thermal evaporation from excited prefragments that allows extrapolation
to other weak reaction products [1,2]. The recent development of a gas-stopper at the
NSCL to deliver rare-ion beams at thermal velocities from fast projectile fragments has
opened a new vista for precision measurements of unstable nuclei. The system can be
applied in a very general way to provide thermalized beams of any isotope produced at the
at the NSCL. For example, direct mass measurements of short-lived nuclei on both sides
of the valley of stability that range from 37Ca and 44S to 70mBr have been completed In a
Penning trap with
10-8[3-5]. The new mass measurements provide information
necessary for the study of super- allowed beta emission of 38Ca, on the r-p process nuclei
66
As and 64Ge, and on the nuclear structure of neutron-rich such as 44S that are difficult to
access with other techniques. A new low energy beam line for atomic physics with lasers
is now under development. The success of the low-energy beam project has spurred the
development of a next generation system to thermalized projectile fragment beams at the
NSCL [6]. The new system will rely on a gas-filled sector-focused cyclotron to confine
the incident ions in the axial an radial directions and, most importantly, segregate the
region of high ionization created by the slowing down process from the region of the
stopped ions. The new ”cyclotron-stopper” avoids the limitation of existing devices
caused by the ionization of the stopping gas in a natural way and will accommodate all
secondary beams from the NSCL facility. The cyclotron-stopper is the first component of
an expansion of the NSCL facility to provide precision rare-ion beams for secondary
nuclear reactions at energies near the Coulomb barrier. A brief overview of this new
project will be presented.
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