LA-UR-18-26596

Los Alamos National Lab

*Santa Fe Workshop*

Wed. July 11$^{th}$ 2018

FIRE Collaboration

Fission In R-process Elements

Nuclear physics inputs are critical in determining the resultant nucleosynthesis that occurs in astrophysical environments.

Fission properties in particular are difficult to measure as well as model.

At Los Alamos we have focused on describing:

neutron-induced fission, $\beta$-delayed fission & fission yields.

Our results are based off the FRDM and FRLDM models.

Möller *et al.* ADNDT (2018) • Cote *et al.* ApJ 855 2 (2018) • **Mumpower** *et al.* PPNP 86 (2016)

Schematic fission process

FRLDM Barrier heights

Low barrier ↦ fission ⇑

$r$-process hot spots follow low barriers

Möller *et al.* PRC 91 024310 (2015)

Uses LANL statistical Hauser-Feshbach code

Barrier heights Möller (2015) / FRDM2012 masses

Large region that can fission cycle $r$-process

Kawano *et al.* PRC 94 014612 (2016) • **Mumpower** *et al.* in prep

for $\beta$-delayed neutron emission & fission

Motivation: We want to describe the neutron,$\gamma$ & fission competition during de-excitation

We combine both Quasi-particle Random Phase Approximation (QRPA) and Hauser-Feshbach (HF) theory.

This will allow for the calculation of ground state production probabilities, particle multiplicity and particle spectra.

To do this we make use of the Bohr independence hypothesis of compound nucleus formation

Initial population from the $\beta$-decay strength function from P. Möller's QRPA.

Follow the statistical decay until all excitation energy is exhausted.

Extend the model to describe $\beta$-delayed fission ($\beta$df)

Simplification: one dimensional barrier penetration

Assumes a Hill-Wheeler form for fission transmission

Near the dripline $Q_{beta}$ ⇡ $S_{n}$ ⇣

Multi-chance $\beta$df: *each* daughter may fission

The yields in this decay mode are a convolution of many fission yields!

$\beta$df occupies a large amount of real estate in the NZ-plane

Multi-chance $\beta$df outlined in black

Network calculation of neutron star merger ejecta

$\beta$df alone prevents the production of superheavy elements in nature

Network calculation of neutron star merger ejecta; FRDM2012 inputs

$\beta$df can shape the final pattern near the $A=130$ peak

Network calculation of neutron star merger ejecta; FRDM2012 inputs

Multi-chance $\beta$df contributes at both early and late times

Network calculation of neutron star merger ejecta; FRDM2012 inputs

$\beta$df overtakes (n,f) during the decay back to stability

High thermalization efficiency and large Q-value ↦ influential for the radioactive decay powering the kilonova

The spontaneous fission of $^{254}$Cf __ primary__ contributor to nuclear heating at late-time epochs

The $T_{1/2}\sim 60$ days but yield distribution is not known

Y. Zhu *et al.* arXiv:1806.09724 (2018)

P. Jaffke *et al.* in prep. • Y. Zhu *et al.* arXiv:1806.09724 (2018)

Both near- and middle- IR are impacted by the presence of $^{254}$Cf

Late-time epoch brightness can be used as a proxy for actinide nucleosynthesis

Future JWST will be detectable out to 250 days with the presence of $^{254}$Cf

Y. Zhu *et al.* arXiv:1806.09724 (2018)

My collaborators

J. Barnes, A. J. Couture, W. P Even, C. F. Fryer, E. Holmbeck, P. Jaffke, T. Kawano, O. Korobkin, G. C. McLaughlin, P. Möller, T. Sprouse, R. Surman, N. Vassh, M. Verriere & Y. Zhu

▣ Students ▣ Postdocs

LANL has made recent progress in describing

neutron-induced fission • $\beta$-delayed fission • fission yields

These properties substantially influence nucleosynthetic yields

The production of $^{254}$Cf is important for late-time kilonova observations and is tied to the morphology of the ejecta

Impact of FRLDM yields to be explored in the future

Results at MatthewMumpower.com