Fission in the $r$-process


Matthew Mumpower

Los Alamos National Lab

ARIEL Workshop

Wed. July 18$^{th}$ 2018

FIRE Collaboration
Fission In R-process Elements

Neutron stars

Mass of around 1.1 to 3 M$_\odot$

Density $10^{9}$ to $10^{17}$ kg/m$^{3}$

Magnetic field $10^{10}$ Tesla

Color depends on equation of state...

Ozel & Freire ARA&A 54 401 (2016) • Figure by M. Mumpower

A close binary of neutron stars

GW170817 - named for the day it was discovered

NGC 4993 - about 40 Mpc away

Abbott et al. PRL 119 16 (2017) • Im et al. ApJL 849 1 (2017) • Figure by M. Mumpower

A close binary of neutron stars

Important for GCE

Coalescence time: ~1 million years

Cote et al. ApJ 855 2 (2018) • Figure by M. Mumpower

GW waves are emitted

Chirp mass $\sim 1.188$
implies NS binary

Abbott et al. PRL 119 16 (2017) • Smartt et al. Nature 551 75 (2017) • Figure by M. Mumpower

Neutron stars merge

Rosswog Nature 500 535 (2013) • Figure by M. Mumpower

Many open problems...

Why was GW170817 so bright?

What is the morphology of the remnant?

What is the typical timescale for the merger? (statistics)

What are the properties of the equation of state?

What is the tidal deformability?

What role can neutrinos play?

How much material can be ejected?

What heavy elements can be created?

Figure by M. Mumpower

The Problem

We want to describe the abundances observed in nature

But there is uncertainty in:

The astrophysical conditions

The nuclear physics inputs

Both are required to model the nucleosynthesis

Inputs from nuclear physics

1st order: masses, $\beta$-decay rates, reaction rates & branching ratios

See review paper: Mumpower et al. PPNP 86 (2016)


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

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

Benchmarking: 9% global model uncertainty to measured $P_{1n}$ values
The best in the business!

Mumpower et al. PRC 94 064317 (2016) • Spyrou et al. PRL 117 142701 (2016)
Wu et al. PRL 118, 072701 (2017) • Möller et al. ADNDT (2018) • Mumpower et al. arXiv:1802.04398 (2018)

QRPA+HF applied to $\beta$df

We have extended 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 tidal 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. [accepted ApJL] arXiv:1806.09724 (2018)

Calculated yield

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

Population of $^{254}$Cf(Z=98)

The feeding of $^{254}$Cf is primarily through $\beta$-decay.

Alpha feeding is blocked via $^{258}$Fm spontaneous fission.

Y. Zhu et al. [accepted ApJL] 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. [accepted ApJL] 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