Radioactive beams  How to Proceed  Screening effects

Neutron beams

There has been considerable progress over the last 20 years in studying neutron-induced reactions, primarily for understanding and interpreting s-process nucleosynthesis, where the Karlsruhe group (FZK) played a leading role [8]. The experiments rely on different methods of producing the neutrons in the appropriate energy range, either the use of thermal neutrons produced at reactors, which can be utilised to measure the s-wave component of the cross section, or the use of low energy neutron beams for the measurement of possible p-wave or higher energy resonant contributions. Two complementary methods are presently used to provide neutrons at stellar energies. The first method is based on time of flight techniques (TOF) at modern accelerators in connection with neutron sources producing a wide neutron energy range from nearly thermal energies up to energies in the MeV range. The second method is based on neutron energy spectra which simulate a stellar (kT$\approx$25 keV) Maxwell-Boltzmann distribution, typically produced by ⁷Li(p,n)⁷Be or similar light-particle-induced reactions at low energy, high intensity Van de Graaff accelerators. The on-line detection of the reaction products is often limited by high neutron fluxes. Therefore, the activation technique is used frequently, measuring the characteristic decay of the reaction product, in case it is radioactive [26]. Otherwise new on-line detection techniques have to be used for capture $\gamma$-radiation at high neutron fluxes (arrays of BaF₂ detectors less sensitive to neutron radiation, e.g. the Karlsruhe 4$\pi$-detector, or heavily shielded NaI detectors). The recently developed high granularity Ge clover or cluster detectors are highly suitable for future on-line (n,$\gamma$) measurements. Future efforts should include the simulation of stellar neutron spectra for lower temperatures (kT=8 keV rather than at 25 keV) and the further development of detector systems for on-line and off-line investigations. Targets of interest are light and intermediate mass isotopes with their limited amount of available data, the branching point nuclei, requiring the production of long-lived radioactive targets for off-line neutron capture studies (like ⁷⁹Se, ⁸⁵Kr, ⁹⁵Zr, ¹⁰⁷Pd, ^135,137Cs, ¹⁴¹Ce, ¹⁴⁷Pm, ¹⁵¹Sm, ¹⁵⁵Eu, ¹⁶⁹Er, ¹⁷⁰Tm, ¹⁷⁵Yb, ¹⁸⁶Re, ²⁰⁴Tl, ¹⁹³Pt, and remeasurements for ⁹³Zr, and ⁹⁹Tc), and cross sections of thermally-excited low-lying isomeric states (e.g. ¹⁰³Rh, ¹¹⁹Sn, ¹⁶⁹Tm, ^187,188Os, and ¹⁹³Pt). For determining the endpoint of the s-process, neutron capture measurements are needed on the long-lived radioactive ²¹⁰Bi in its ground and isomeric state and the ground state of ²¹⁰Po.