Z. Fragments after FP Slits. High-Resolution Separator. N. Fragments at wedge. Large-Acceptance Separator. Z. Z. Target. Fragments after target. Beam. Fragment Separator. All Experiments. N. N. (up to 5 kW). A1900: single separator. High-Resolution Separator. Beam. Preseparator.
Fragments after FP Slits
Fragments at wedge
Fragments after target
(up to 5 kW)
A1900: single separator
(up to 400 kW)
RIA: two distinct separators
References: H. Geissel et al., Nucl. Inst. and Meth. A 282 (1989) 247
C. Scheidenberger et al., Nucl. Inst. and Meth. B 204 (2003) 119
L. Weissman et al., Nucl. Inst. and Meth. A 522 (2004) 212
B. M. Sherrill, Nucl. Inst. and Meth. B 204 (2003) 765
K. Shepard et al, in: B. Rusnak (Ed.), Proc. of 9th Intl. Conf. on RF Superconductivity, Sante Fe, 1999, LANL, Los Alamos, 2000, p.345
P.N. Ostroumov, Phys. Rev. ST Acc. Beams 5 (2002) 030101.
D.J. Morrissey et al., Nucl. Inst. and Meth. B 204 (2003) 90Design Studies for the RIA Fragment Separators
A.M. Amthor 1,2, D.J. Morrissey 1,3, A. Nettleton 1,2, B.M. Sherrill 1,2, A. Stolz 1, O. Tarasov 1
1National Superconducting Cyclotron Laboratory, 2Department of Physics and Astronomy, Michigan State University, 3Department of Chemistry, Michigan State University
Fragment Separation - e.g. the A1900 at the NSCL
The goal at RIA is to provide very intense secondary beams of a wide variety of isotopes, many previously unavailable for use in experiments. At the RIA primary beam energies, secondary reactions in the target contribute to the overall production rate; hence it is desirable to use thick targets. This results in wide momentum distributions of the fragments. Figure 1 shows a representative example of the increased gain in yield from large separator momentum acceptances with the higher primary beam energies and thick targets to be used at RIA. Also, efficient collection of fission fragments requires larger angular acceptance, because of the energy released in the process.
Note: Isotope yield diagrams are from 86Kr78Ni simulation with primary beam of 140MeV/u
Bρmax = 6Tm
Δp = 5%
Δθ = ±40mr
Δφ = ±50mr
Compensated to 3rd order
Largest acceptance of current facilities
The RIA baseline concept above makes use of two fragment separators.
Maximum stopping efficiency in the 0.5 atm-m He gas cell is achieved using a monochromatic wedge degrader followed by an adjustable thickness homogeneous degrader set so as to leave the peak of the compressed range distribution in the gas cell. In the MOCADI simulation at left a 32.4 atm-m FWHM range distribution of 130Cd is brought to a range distribution with FWHM of 0.93 atm-m, leaving over 40% of fragments within the central 0.5 atm-m of the distribution.
Figure 1: Fragment yield vs. momentum acceptance by primary beam energy for 78Ni produced from 86Kr. At the RIA energy of 400Mev/u the acceptance should be greater than 10%.
RIA Momentum Compensator
The compensated third order system passes approximately 73% of fragments uniformly distributed in a 6-D phase space ellipse with a and b from ±50mr and with δ distributed over a full width of 12%.
H. Weick et al., NIM B 164-165 (2000) 168
The large momentum acceptance of the preseparator produces a beam of the desired fragment with up to a 12% spread in momentum. This corresponds to a similarly large range distribution in He gas, roughly 50 atm-m FWHM. To maximize collection efficiency, the width of the range distribution must be minimized. The momentum compensator performs this function, known as range compression, by dispersing the beam then passing the particles through a monochromatic wedge degrader. The width of the range distribution is thereby reduced to a minimum primarily determined by the range straggling of the ions in the degrader material.
The first order symmetry of the preseparator gives angular magnification equal to one, and any degrader materials present will be profiled to preserve achromaticity. Therefore, the angular and momentum acceptances must be identical to those of the preseparator itself. Aberrations in the preseparator will increase the initial spot size for the momentum compensator, but with d/M 2.5m (satisfied by current designs in first order), a spot size of 2.5mm will give R = 1000, at which point the limited optical resolution contributes little to the final range distribution.