Measuring (n,?) cross sections of the r-process

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Measuring (n,?) cross sections of the r-process

• Lothar Buchmann
• TRIUMF

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Observed Solar-System
Heavy-Element abundances
Solar
r leftovers ( Solar s )
Different processes contribute to the observed
Heavy-Element abundances
s-process
p-process
r-process
log e log10 (Yel/YH)12
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The r-process
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Some simple numbers
Energy For the r-process, typically temperatures
of 1-5 GK are encountered. As there is no Coulomb
barrier, the average energy is simply kT,
i.e. And energy range of the accelerator of
50-600 keV/u is desirable.
Example 132Sn, double magic nucleus, T1/240 s,
t57 s. Simplest assumption, everything is more
or less in equilibrium, Isobars are immediately
removed in the ring.
Next assumption one gets 108 s-1 from the source
into the ring. With the mean lifetime t about
5x109 ions are in the ring in equilibrium.
The ring be 5 m in circumference, the energy be
100 keV/u. Then in 57 seconds 5x108 revolutions
are made corresponding to a particle current of
I5x1017 s-1.
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More simple numbers
Now, let the neutron density be 100/cm3. For 1 m
of sampling, one ends up with 104/cm2. Then the
luminosity L is L5x1021/cm2s-1.
With s100 mb ?Y10-4 s-10.28 h-1, Dragon rates.
What are cross the sections?
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Neutron capture cross sections
Interesting region
13Cn p wave resonance s(res)8 mb
n capture on 238U
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Other Physics

• Precision scattering of neutrons on light nuclei,
  p7Li (scattering lengths).
• 7Lin analog of 7Bep.
• 3. Some n-captures in s-process or
• supernova neutronization (60Fe).

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Possible topology
Accelerator
Possible tunnel
UCN
Storage ring
Recoil separator
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Accelerator questions
1. Energy range The energy range corresponds to
lower ISACI energies except for the very lowest
energies (decalerator?) 2. Mass range is pretty
incompatible with ISACI, unless one is willing to
throw 95 of the ions away. 3. Topography At the
moment, it looks as if ISACI and UNC go opposite
directions. 4. How much would a dedicated
accelerator cost?
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Storage Ring questions
Size can be in principle rather small, However,
determined by charge state (2, 3) and
energies. Cooling Is ion beam cooling
advantageous? Injection Continuous injection
with little losses? Stripping or phasespace and
cooling? Energy sweep To find resonances that
are at best known to a few keV in the cm, it will
be necessary to sweep the beam continuously over
an energy range of 10 to 20 keV/u. This feature
needs to be incorporated into the ring, either
for the interaction and separator region only or
for the entire ring. The reason is, of course,
that the neutron target does not produce any
stopping throughout the target. Detection What
detectors can be mounted at the neutron target?
How will a mass separator be included? Isobar
removal The radioactive beam will decay to the
an isobar. Mostly the charge state will change
and that will remove most of the isobars.
However, as ions are not fully stripped, some may
stay behind.
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Recoil separator
As by neutron capture the mass of the particle
increases by one a recoil mass separator seems
to be the appropriate tool to detect a
signal. The momentum is conserved (besides
gamma-emission) in the collision, so an electric
field needs to be part of the ring behind the
neutron target. A Wien filter that directs the
recoils out of the ring may work. The energies
are rather low for a recoil separator. Is
isobaric separation necessary (i.e., if there is
isobaric beam)? How to achieve Z identification?
Reacceleration? Will the separator follow the
energy sweep?
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Costs
Wikipedia The term handwaving is used in
mathematics and physics to describe arguments
that are not mathematically rigorous.
Beamline and accelerator 5 M Storage Ring 10
M Recoil separator 3 M Civil construction 3 M
Proposal engineering estimates needed!