Design Studies for the RIA Fragment Separators

1
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 B?max 6Tm ?p 5 ?? 40mr ?f
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 (lt 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 (B?
  p/q).
• Dispersion (d ? xd) defines spatial
  separation according to momentum deviation.
• Resolving power (R d/Mxo) quantifies the
  systems ability to separate fragmentsgiven an
  initial beam spot size of xoand depends on the
  dispersion and the magnification (M ? xx, 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 systems 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 Rgt1000
• 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 dB?lt 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 lt 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 d 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.