A technique for shell compression measurements of laser fusion targets by neutron activation of a rubidium tracer

At the Nova Laser, the activation of a rubidium tracer incorporated in the shell of ICF targets has become a standard diagnostic technique for measuring the compressed shell areal density 〈ρΔR〉. The isotope 85Rb is activated by 14 MeV implosion neutrons to produce the isomer 84mRb (t1/2=20.5 min) which is used to determine the shell 〈ρΔR〉 while the radioactive isotope 86Rb (t1/2=18.8 d) is used to determine the fraction of target debris collected as well as to assay the amount of rubidium in the target. The 85Rb(n,2n)84mRb cross section at 14.1 MeV was measured (σ=0.514±0.080 b). Details of the detector system and experimental technique are given.