neutron-rich nucleosynthesis

LA-UR-18-XXXXX

*ANL Seminar*

Monday Nov. 12$^{th}$ 2018

FIRE Collaboration

Fission In R-process Elements

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...

GW170817 - named for the day it was discovered

NGC 4993 - about 40 Mpc away

Important for GCE

Coalescence time: ~1 million years

Chirp mass $\sim 1.188$

implies NS binary

Is GW170817 a typical event?

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?

Kilonova: powered by the radioactive decay of heavy nuclei

Model assumptions: M$_{\textrm{ej}} = 10^{-2}$ M$_\odot$, V$_{\textrm{ej}}=0.3$c

Very low lanthanide fraction

Arcavi *et al.* Nature 551 64 (2017)

Kilonova: powered by the radioactive decay of heavy nuclei

To match data at later times

Some lanthanide production is needed

Tanvir *et al.* ApJL 848 2 (2017)

2 component model: wind & dynamical eject

Red emission: late (weeks); lanthanide dominated

blue emission: early (days); UV dominated

Metzger *et al.* MNRAS 406 4 (2010) • Kasen *et al.* Nature 551 80 (2017) • *Figure by* O. Korobkin

We want to describe the abundances observed in nature

But there is uncertainty in:

The astrophysical conditions (large variations in current simulations)

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

Both are required to model the nucleosynthesis

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

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

The chart of nuclides

Figure by **Mumpower**

All half-lives

Figure by **Mumpower**

Nuclear masses

Figure by **Mumpower**

Neutron capture rates

Figure by **Mumpower**

As of today, to varying degrees of accuracy

Figure by **Mumpower**

FRIB as the $r$-process machine

Figure by **Mumpower**

PRISM: Portable Routines for Integrated nucleoSynthesis Modeling

Sprouse & **Mumpower** in prep (2019)

Large variation in mass model predictions further from stability

Modified from **Mumpower** *et al.* PPNP 86 86-126 (2016)

Masses go into the calculation of all other relevant quantities...

Thus if we change a single mass in our network then we have to recalculate many other quantites!

Hot wind: $S\sim200$, $\tau=80$ ms, $Y_e=0.3$

Uncorrelated mass Monte Carlo study; *not full propagation*

Hot wind: $S\sim200$, $\tau=80$ ms, $Y_e=0.3$

Uncorrelated mass Monte Carlo study; *not full propagation*

Rule of thumb: $\Delta_\text{mass}\sim500$ keV $\Rightarrow$ $\Delta_Y\sim 2-3$ orders of magnitude

**Proposed ways to form the REP**

- Dynamical formation during freeze-out ($R\lesssim1$)

Requires a localized nuclear structure effect (kink) - Via fission fragment yields

Requires dumping heavy products in exactly the right spot

Region of enhanced lanthanide production (important for observations)

Sensitive to nuclear physics inputs and astrophysical conditions

Surman & Engel PRL 79 1809 (1997) • **Mumpower** *et al.* PRC 85 045801 (2012) • **Mumpower** *et al.* ApJ 752 117 (2012)

An example... The Monty Hall problem

A new car is hidden behind one of the doors

The optimal strategy is to switch the initial pick - twice the chance of winning the new car

We **update** our probabilities based off new information

**Reverse engineering**: Update mass prediction based off the match of $r$-process calculation to observations

Bayesian prior (DZ mass prediction) is a straight horizontal line

Our algorithm found a The trend

Orford *et al.* PRL 120 262702 (2018)

The predicted trend matches CPT data!

The downturn between N=102 and N=104 is near the midshell - critical for peak formation - will it also be found?

Orford *et al.* PRL 120 262702 (2018)

The trend is robust for small changes to astrophysical conditions (left)

But if we change the conditions too much (right) we no longer fit the abundances

This result is completely dependent on the N=104 feature which has yet to be measured

The possibility for mass measurements to discrimenate between astrophysical conditions is nearly at hand!

Orford *et al.* PRL 120 262702 (2018) • Vassh *et al.* in prep (2018)

branching ratios & fission

Discovered in 1939 by R.B. Roberts *et al.*

Delayed emission with half-life of precursor

Energetically possible: $Q_\beta$ > $S_n$ Important for neutron-rich nuclei

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

Follow the statistical decay until all excitation energy is exhausted

Möller *et al.* PRC (1997 & 2003) • Kawano *et al.* PRC 94 014612 (2016) • **Mumpower** *et al.* PRC 94 064317 (2016)

Apply energy window method to the entire chart of nuclides

Problem with describing very neutron-rich nuclei

Apply the QRPA+HF method to the entire chart of nuclides

Problem with neutron-rich nuclei goes away

QRPA+HF GT-only $\beta$-strength are within 15% of measured $P_{1n}$ values

Adding FF transitions improves the match to measured data by 3%

Using measured masses improves the match to measured data by 3%

This yields a roughly 9% global model uncertainty to measured $P_{1n}$ values

__The best in the business!__

Can we extend this model to other phenomena ... fission?

Spyrou *et al.* PRL 117 142701 (2016) • **Mumpower** *et al.* PRC 94 064317 (2016) • Wu *et al.* PRL 118, 072701 (2017)

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

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

Assumes a Hill-Wheeler form for fission transmission

**Recall**: 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!

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

$\beta$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

Probe nuclei FRIB may have trouble producing...

Figure by **Mumpower**

A dirty secret: nucleosynthesis simulations have trouble reproducing

(1) the peak height and (2) the position of the A=195 peak (due to the N=126 shell closure)

Final abundances using 20 mass models given the same astro. conditions

The N=126 factory can help us to understand the evolution of shell structure, GT vs FF contributions and will greatly impact $r$-process calculations

**The N=126 shell closure acts as the gatekeeper to fission recycling**

Calculation by **Mumpower**

Is there any precursor to show that actinide nucleosynthesis has occurred in an event?... YES!

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 well constrained

Y. Zhu *et al.* ApJL 863 2 (2018)

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

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 is all directly tied to the strength of the N=126 shell closure!

Y. Zhu *et al.* ApJL 863 2 (2018)

My collaborators

A. Aprahamian, J. Clark, E. Holmbeck, P. Jaffke, T. Kawano, O. Korobkin, S. Liddick, G. C. McLaughlin, P. Möller, R. Orford, J. Randrup, G. Savard, A. Sierk, T. Sprouse, A. Spyrou, R. Surman, N. Vassh, M. Verriere, J. Wu, Y. Zhu

& many more...

▣ Students ▣ Postdocs

The binary neutron star merger event, GW170817, was recently observed using both gravitational waves and electromagnetic signals

It has answered some questions while opening others...

Robust models of nuclear physics are required to understand the abundance and electromagnetic signatures from the heavy element nucleosynthesis that may ensue from such events

Astrophysics can be used as an alternative benchmark for nuclear models, in addition to experimental data / evaluations

An effort to improve nuclear modeing is ongoing at LANL

Collaboration between Argonne & Los Alamos has been very successful with many projects to come!

Results at MatthewMumpower.com

If actinides are produced, they are always overproduced versus lanthanides / rare earth elements

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

Holmbeck *et al.* arXiv:1807.06662 [submitted ApJ] (2018)

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