Design Studies for the RIA Fragment Separators

1. 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 High-Energy Area (up to 400 kW) Momentum Compensator Beam Gas-Stopping Cell Preseparator RIA: two distinct separators Wedge Isotope Slits Beam Beam Dump Target 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) 90 Design 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 Motivation RIA 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 86Kr78Ni simulation with primary beam of 140MeV/u Specifications 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. • Preseparator • 100 mr in horizontal and vertical • 12% momentum acceptance • low optical aberrations (< 2 mm) • High-Resolution Separator • 80 mr in horizontal and vertical • 6% momentum acceptance • d/M  2.5m • Fragment separators focus and purify the multi-constituent beams which result from a primary beam striking a production target. The effectiveness of the system is determined by: • Angular acceptance limits ions passed according to lab-frame trajectories from the target. • Momentum acceptance limits ions passed according to deviation from central magnetic rigidity (Bρ = p/q). • Dispersion (d  [x|δ]) defines spatial separation according to momentum deviation. • Resolving power (R = d/Mxo) quantifies the system’s ability to separate fragments—given an initial beam spot size of xo—and depends on the dispersion and the magnification (M  [x|x], the dependence of the final beam spot size on the initial spot size). • Bending strength limits the magnetic rigidity of ions that can be sent down the system’s optic axis. Range Compression 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 • Specifications: • Full angular and momentum acceptance from preseparator • Momentum resolving power R>1000 • d/M = 2.5m RIA Preseparator • The high intensity of beams produced by the RIA linac combined with broad momentum distributions emerging from thick production targets make the design of the fragment separators challenging. • Challenges: • Large angular and momentum acceptances, significant higher order aberrations (5th order or higher) • Power on beam dump, up to 200kW to be collected • Primary beam proximity to desired fragments, very often within momentum acceptance and sometimes with |δBρ|< 1% • Higher energy and greater neutron excess of fragment beams, requires bending strength of 10Tm • Range compressed fragments to be stopped in 0.5 atm-m gas cell, requires aberrations < 2mm 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.