Nuclear fission for the $r$-process


Matthew Mumpower

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

Figure by Mumpower

Estimated Barrier Heights

FRLDM Barrier heights

Low barrier ↦ fission ⇑

$r$-process hot spots follow low barriers

Möller et al. PRC 91 024310 (2015)

Neutron-induced fission

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

Combining QRPA + HF

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

Mumpower et al. PRC 94 064317 (2016)

Combining QRPA + HF

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

Follow the statistical decay until all excitation energy is exhausted.

Mumpower et al. PRC 94 064317 (2016)

QRPA+HF extension to $\beta$df

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

Simplification: one dimensional barrier penetration

Assumes a Hill-Wheeler form for fission transmission

Mumpower et al. arXiv:1802.04398 (2018)

Multi-chance $\beta$df

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!

Mumpower et al. arXiv:1802.04398 (2018)

Cumulative $\beta$df probability

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

Multi-chance $\beta$df outlined in black

Mumpower et al. arXiv:1802.04398 (2018)

Application to $r$-process

Network calculation of neutron star merger ejecta

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

Mumpower et al. arXiv:1802.04398 (2018)

Impact on final abundances

Network calculation of neutron star merger ejecta; FRDM2012 inputs

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

Mumpower et al. arXiv:1802.04398 (2018)

Multi-chance $\beta$df contribution

Network calculation of neutron star merger ejecta; FRDM2012 inputs

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

Mumpower et al. arXiv:1802.04398 (2018)

Freeze-out matters

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

Mumpower et al. arXiv:1802.04398 (2018)

Is there an observational signature of fission?

Isolated $^{254}$Cf(Z=98)

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)

Calculated yield

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

Observational Impact

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)

Special thanks to

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 fissionfission 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