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Towards a neutron target and Measuring (n, ɣ) cross sections of the r-process Lothar Buchmann TRIUMF

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Physics of a neutron target 1.Precision scattering of neutrons on light nuclei, p… 7 Li (scattering lengths, potentials, polarization data). 2.n-p precicision radiative capture data Li+n analog of 7 Be+p. 4. Some n-captures in s-process or supernova neutronization (60Fe). 5.The r-process: radiative capture. A neutron target does not slow down the beam, or change its charge state, and is thus ideal for a storage ring. Workshop

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TRIUMF plans TRIUMF plans call for producing large quantities of cold uranium fission products by electro fission. That makes measurements in this region of the r-process something to be considered.

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Observed Solar-System Heavy-Element abundances log = log 10 (Y el /Y H )+12 Solar s-process p-process r-process Different processes contribute to the observed Heavy-Element abundances r leftovers ( Solar – s )

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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. an energy range of the accelerator of keV/u is desirable. Example: 132 Sn, double magic nucleus, T 1/2 ~40 s, τ=57 s. Simplest assumption, everything is more or less in equilibrium, Isobars are immediately removed from the ring. Next assumption: one gets 10 8 s -1 from the source into the ring. With the mean lifetime τ about 5x10 9 ions are in the ring in equilibrium. The ring be 5 m in circumference, the energy be 100 keV/u. Then in 57 seconds 5x10 8 revolutions are made corresponding to a particle current of I=5x10 17 s -1.

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More simple numbers Now, let the neutron density be 1000/cm 3. For 1 m of sampling, one ends up with 10 5 /cm 2. Then the luminosity L is L=5x10 22 /cm 2 s -1. With σ=100 mb Y=10 -3 s -1 =2.8 h -1, Dragon rates or better. What are cross the sections?

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Neutron capture cross sections 13 C+n p wave resonance σ(res)=8 mb n capture on 238 U Interesting region

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Possible topology UCN Possible tunnel Accelerator 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? No principle problem in constructing a mass A=150, q=1 accelerator.

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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 (particle detectors)? 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 describemathematicsphysics arguments that are not mathematically rigorous.rigorous Beamline and accelerator: 5 M Storage Ring: 10 M Recoil separator: 3 M Civil construction: 3 M Proposal engineering estimates needed!

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