Brian Appelbe, a research fellow in SPC community, has kindly offered to give a seminar titled “Neutron Capture in a Plasma Environment”. As you may know, Brian’s expertise includes HEDP fusion plasmas, and particularly neutron generation. This seminar will discuss his upcoming experiment at the National Ignition Facility to exploit the high neutron fluxes achievable there to develop a platform for neutron capture measurements in stellar-like environments – the full abstract is below. Looking forward to seeing you there!
The slow neutron capture process (“s process”) produces approximately half the elemental abundances of elements between iron and bismuth. It occurs when the neutron capture time scale of an isotope is slower than the β decay time scale and so the s process reaction path closely follows the valley of stability. Understanding the s process and predicting elemental abundances requires an interdisciplinary approach involving nuclear physics data and calculations, stellar models and astronomical observations. The role of nuclear physics is to provide values for neutron capture cross sections and β decay rates which can then be used in stellar models.
In the stellar environments in which the s process occurs, significant populations of nuclear excited states can exist. Models of the s process usually include a “stellar enhancement factor” (SEF) to parameterize the effect of excited states but there is significant uncertainty (up to 100%) about their accuracy. Conventional laboratory experiments, e.g., using linear accelerators or spallation sources, for measuring capture cross sections can only measure the capture cross section of nuclei in the ground state. Direct measurement of the capture cross section of nuclear excited states and, by extension, measurement of SEFs is currently not possible in the laboratory.
The NIF laser is capable of producing plasma environments in which we expect significant populations of nuclear excited states and large neutron fluxes. This can provide a platform for making neutron capture measurements in stellar-like environments, thereby validating stellar models and SEFs. This talk will outline the design of such experiments and the challenges that need to be overcome in order to make accurate cross section measurements.