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

Lothar Buchmann

TRIUMF

Observed Solar-System Heavy-Element abundances

log = log10 (Yel/YH)+12

Solar s-process

p-process

r-process

Different processes contribute to the observed Heavy-Element abundancesr ≈ “leftovers” ( Solar – s )

The r-process

Some simple numbersEnergy: 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, T1/2~40 s, τ=57 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 τ 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 I=5x1017 s-1.

More simple numbersNow, let the neutron density be 100/cm3. For 1 m of sampling, one ends up with 104/cm2. Then the luminosity L is L=5x1021/cm2s-1.

With σ=100 mb Y=10-4 s-1=0.28 h-1, Dragon rates.

What are cross the sections?

Neutron capture cross sections

13C+n p wave resonance σ(res)=8 mb

n capture on 238U

Interesting region

Other Physics

1. Precision scattering of neutrons on light nuclei, p…7Li (scattering lengths).

2. 7Li+n analog of 7Be+p.

3. Some n-captures in s-process or supernova neutronization (60Fe).

Possible topology

UCN

Possible tunnel

Accelerator

Storage ring

Recoil separator

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?

Storage Ring questionsSize: 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.

Recoil separatorAs 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, soan 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?

CostsWikipedia: 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!