Stopped and reaccelerated beams at NSCL (coming summer 2010)

The reacceleration of stopped fast-fragment beams or low-energy ISOL beams to the energy range of 0.3-12 MeV/nucleon allows for experiments such as low-energy Coulomb excitation and transfer reaction studies and also for the precise study of astrophysical reactions. In a project scheduled to be completed by summer 2010, NSCL is coupling a reaccelerator to a gas stopper at its fragmentation facility to provide rare isotope beams of nuclides not available at ISOL facilities in this energy regime. NSCL will implement charge breeding to obtain a compact and cost-efficient reaccelerator. Specifically, an Electron Beam Ion Trap (EBIT) will be used for charge-breeding since it has the potential to rapidly and efficiently output highly charged ions in a single charge state. The system will provide highly charged ions at an energy of 12 keV/nucleon to a compact linear accelerator for further acceleration.

Besides RIKEN (Saitama, Japan), NSCL is the only facility in the world capable of producing low-energy beams created by gas stopping of rare isotopes from fast-fragment beams. NSCL is also the first to use such beams for rare isotope science and during the last three years has completed high-precision Penning trap mass measurements of more than 10 different elements. Along with laser spectroscopy, reacceleration of low-energy beams is the next logical step to maximize the benefit of thermalized beams from projectile fragmentation. Reaccelerated beams in the energy range of 0.3-12 MeV/nucleon open the door to a variety of experiments in the area of nuclear structure studies and nuclear astrophysics.

NSCL is presently constructing a first phase of such a reaccelerator. The working name is ReA3, which will provide rare isotopes from fast-fragment beam in the energy range of 0.3-3 MeV/nucleon. Once completed in 2010, NSCL will begin to pioneer one of the pillars for the FRIB concept seven years before the new facility is scheduled to begin operations. Specifically, by summer 2010 NSCL will provide users with reaccelerated beams of rare isotopes produced by projectile fragmentation.

Choosing the reacceleration scheme

In order to maximize its science reach, a reacceleration scheme for rare isotopes must provide high efficiency for ions of all elements produced, beam-rate capacity adapted to the maximum secondary beam rates, and high beam purity to minimize background and avoid ambiguities in the experimental results. Additionally, to meet specific experimental requirements, the reaccelerated beam should be provided with a variable time structure that ranges from microsecond pulses to continuous beams.

The reacceleration scheme that was chosen for the NSCL was optimized with respect to these properties. It is based on the acceleration of highly charged n+ ions, the so-called n+ scheme. A high-performance charge-breeding scheme based on an EBIT is used together with a modern state-of-the-art linear accelerator using a room-temperature RFQ linear accelerator (linac) combined with a superconducting RF linac.

The scheme provides the best, most-efficient means possible to accelerate rare isotopes. One alternative used at ISAC (at TRIUMF in Vancouver, Canada) is to start with singly charged ions and subsequently strip the ions between acceleration stages. The approach, which works well to bring stable or rare isotope beams up to energies of a few MeV/nucleon, also is complex, costly and associated with significant losses in efficiency for heavier beams due to the use of inefficient stripper foils for increasing the charge state of the ions during the acceleration process. Electron Cyclotron Resonance ion sources have been promoted as charge breeder for reacceleration for many years, but their performance gains are similarly limited.

Electron Beam Ion Sources (EBIS) and EBITs are now widely considered as the best choice because of their superior charge breeding performance. In fact, the first reacceleration with charge breeding was demonstrated at REX-ISOLDE (at CERN in Geneva, Switzerland), where an Electron Beam Ion Source (REX-EBIS) was used.

The reacceleration concept in schematics and images

The series of images and tables that in the navigation bar at left illustrate the NSCL reacceleration concept, which utilizes two gas stoppers, an EBIT charge breeder, and a compact linear accelerator. In brief: the beam from the gas stopper will be sent into an EBIT charge breeder, located on a high-voltage platform. The platform voltage can be raised to a potential of a few tens of keV during breeding and before extraction to match the velocity of the newly created, highly charged ions to the requirements of the downstream RFQ-accelerator. The n+ ions will pass through an achromatic Q/A separator to select a charge state and to suppress unwanted background ions before entering the accelerator. The accelerator is designed to accept beams from the EBIT with a charge-to-mass ratio of approximately 1/4. The beam, extracted from the EBIT with an energy of 48 keVq, will be transferred into an RFQ for bunching and acceleration to 0.6 MeV/nucleon. The beam will then be further accelerated in a superconducting linac to an energy of 3 MeV/nucleon, with the future option to further boost the output energy to 12 MeV/nucleon.

NSCL will be the only fragmentation facility to provide reaccelerated beams. And reaccelerated beams are an integral part of the planned FRIB facility at MSU.

For more information on reacceleration of gas-stopped rare isotope beams at NSCL, contact Georg Bollen at 517-333-6435 or bollen at nscl.msu.edu.