Nuclear fission for the $r$-process
in the era of multi-messenger observations

        LA-UR-19-23312

Matthew Mumpower

ECT$^{*}$ $r$-process Workshop

Tuesday July 2$^{nd}$ 2019


FIRE Collaboration
Fission In R-process Elements

What is the site of heavy element formation?

The answer to this question requires

Knowledge of astrophysical conditions (variations in current simulations)

Knowledge of nuclear physics inputs (1000's of unknown species / properties)

(Both are needed to model the nucleosynthesis)

And precise observations!

In other words, the solution is quite difficult...

Beers & Christlieb ARAA (2005) • Metzger et al. MNRAS (2010) • Barnes et al. ApJ (2016) • Côté et al. ApJ (2018)

Inputs from nuclear physics

1st order: masses, $\beta$-decay rates, capture rates & fission

Dillmann et al. EPJA (2002) • Aprahamian et al. (2018) • See review paper: Mumpower et al. PPNP 86 (2016)

Much will be measured at FRIB

But fission studies will remain relatively inaccessible

∴ Fission theory is critical find any sort of "smoking gun" of heavy element production

Spyrou et al. PRL (2016) • Vilen et al. • PRL (2018) Orford et al. PRL (2018) • Sprouse et al. (2019) • Figure by Mumpower

Nuclear fission in a nutshell

Influence on the $r$-process:

Fission rates and branching determine re-cycling (robustness)

Fragment yields place material at lower mass number; barriers determine hot spots

Large Q-value ⇒ impacts thermalization and therefore possibly observations

Responsible for what is left in the heavy mass region when nucleosynthesis is complete ⇒ "smoking gun"

Meitner & Frisch (1938) • Bohr & Wheeler (1939) • Figure from Verriere & Mumpower in prep. (2019)

Modeling
$\beta$-delayed fission ($\beta$df)

Modeling $\beta$ -delayed fission

We have a model to describe nuclear de-excitation called QRPA+HF

We have recently extended the our QRPA+HF model to describe $\beta$-delayed fission ($\beta$df)

Barrier heights from Möller et al. PRC 91 024310 (2015)

Kodama & Takahashi (1975) • Thielemann et al. (1983) • Shibagaki et al. ApJ 816 2 (2016) • Mumpower et al. ApJ 869 1 (2018)

Multi-chance $\beta$ df

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

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

New fission channel to consider for $r$-process calculations

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

Mumpower et al. ApJ 869 1 (2018) • Mumpower et al. in prep (2019)

($n$,$\gamma$,$f$) competition

Fission can successfully compete with $\gamma$-rays and neutrons

Particle spectra also produced which are of interest for observations

Mumpower et al. PRC (2016) • Mumpower et al. ApJ 869 1 (2018) • Möller et al. ADNDT 125 (2019) • Mumpower et al. in prep (2019)

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. ApJ 869 1 (2018) • Mumpower et al. in prep (2019)

Impact on final abundances

Network calculation of tidal ejecta from a neutron star merger (FRDM2012)

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

This is because of a relatively long fission timescale

Conclusion ⇒ we need a good description of fission yields to understand abundances near $A\sim130$.

Mumpower et al. ApJ 869 1 (2018) • Möller, Mumpower et al. ADNDT (2019) • Mumpower et al. in prep (2019)

A second consequence of $\beta$df

Network calculation of tidal ejecta from a neutron star merger (FRDM2012)

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

Mumpower et al. ApJ 869 1 (2018) • Vassh et al. J. Phys. G (2019)

Fission Hotspots

Both neutron-induced and beta-delayed fission were studied

A range of trajectories and nuclear models show some consistency between predictions

Vassh et al. J. Phys. G (2019)

Long-lived actinides

With careful fission treatments: if actinides are produced, they are usually overproduced versus lanthanides

A sufficient amount of dilution with ligher $r$-process material is required to match the solar isotopic residuals

∴ Fission theory has implications for galactic chemical evolution, etc.

Holmbeck et al. ApJ 870 1 (2019) • Vassh et al. J. Phys. G (2019)

Connecting to
Multi-messenger astrophysics

Observational Impact of Californium

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

This also has implications for merger morphology...

Y. Zhu et al. ApJL 863 2 (2018) • Miller et al. accepted PRD (2019) • Korobkin et al. submitted ApJ (2019)

Merger $\gamma$-rays

Another possible (yet very difficult) option is to attempt to observe the spectra from transients / remnants

For the $r$-process we should search for signatures of actinides...

This involves following potentially complex decay chains...

Korobkin et al. submitted ApJ (2019)

$\gamma$-ray spectrum at 10 kyr

Distinct elements do arise

This depends sensitively on observational timescale

Can we do this with future space missions?

Korobkin et al. submitted ApJ (2019)

Remnant $\gamma$-ray could be observable

Next generation $\gamma$-ray (space-based) have a chance to disentangle merger components

These are tentative numbers from this community

Lunar Occultation Explorer (LOX) at this point seems like our best bet for a composition observable

Korobkin et al. submitted ApJ (2019)

Fission yields
(Coming soon)

Calculated yield (Californium)

Jaffke et al. PRC 67 034608 (2018) • Y. Zhu et al. ApJL 863 2 (2018) • Mumpower et al. in prep. (2019)

Macroscopic-microscopic yields

▣ Experiment ▣ Theory

FRLDM fragment yields have remarkable agreement with known data

Over a range of experiments, evaluations and nuclei!

Verriere & Mumpower in prep. (2019) • Mumpower et al. in prep. (2019)

Special thanks to

My collaborators

A. Aprahamian, J. Clark, E. Holmbeck, P. Jaffke, T. Kawano, O. Korobkin, S. Liddick, G. C. McLaughlin, J. Miller, P. Möller, R. Orford, J. Randrup, G. Savard, A. Sierk, N. Schunck, T. Sprouse, A. Spyrou, I. Stetcu, R. Surman, P. Talou, N. Vassh, M. Verriere, X. Wang, Y. Zhu
& many more...

▣ Students ▣ Postdocs ▣ FIRE PIs

Summary

The $r$-process relies on fission in many ways:

Re-cycling material ▴ Actinide production ▴ Late-time observations

FRIB and other facilities will make a lot of measurements, but fission studies remain relatively inaccessible

Fission theory is crucial to understanding the formation of the heaviest elements (and $A\sim130$)

The FIRE Collaboration will soon provide a suite of new fission properties for the community:

Rates • Branchings • Yields • Q-values • Spectra

Results / Data / Papers @ MatthewMumpower.com

Extra Slides

How we calculate fission yields

We use a discrete random walk over a potential energy surface

This assumes strong disspative dynamics

The ensemble of such random walks produces the fission yield

Verriere & Mumpower in prep. (2019) • Mumpower et al. in prep. (2019)