Los Alamos National Lab

*ICNT: $r$-process workshop*

Monday June 6$^{th}$ 2016

Los Alamos National Lab

*ICNT: $r$-process workshop*

Monday June 6$^{th}$ 2016

**Tuesday:** $r$-process in SN & NS mergers; kilonovae

**Wednesday:** Gravitational waves and observations

**Thursday:** GCE & new fission results

**Friday:** Nuclear structure and recent measurements

*We need to do this more often &... these problems are hard!*

*A focused, cross-discipline approach is warranted*

The impact of individual nuclear properties on $r$-process nucleosynthesis

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

Figure: Experimental reach of future radioactive beam facilities

Interactive data tables can be found online: MatthewMumpower.com

Ultimately... we want to know what is the site of the $r$-process?

What do we currently know on the nuclear physics side?

What is most important nuclear physics for late-times in the $r$-process?

How well can we currently predict final abundance patterns?

What do we need to measure?

How do these measurements help us to move forward?

We heard a lot about this last week...

Entropy or $S$

Timescale or $\tau$

Neutron-richness or $Y_e$

Ejected mass or $M_{ej}$

**A key metric:** Neutron-to-seed ratio ($R$)

Figure by A. Arcones (2011)

Freeze-out is the last phase of the $r$-process when nuclides decay back to stability.

Individual rates and properties become critical to the evolution as nuclei fall out of equilibrium.

In order to accurately predict $r$-process abundances it is imperative to understand how these nuclear inputs evolve with neutron excess.

Two types of evolutions

Hot

- Classical $r$-process with high $T_9$
- Long phase of $(n,\gamma)\rightleftarrows(\gamma,n)$

Cold

- Short or nonexistent $(n,\gamma)\rightleftarrows(\gamma,n)$
- Quasi-equilibrium neutron capture & $\beta$-decay

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

The chart of nuclides

All half-lives

Recently measured beta-decay half-lives

Recently measured beta-decay half-lives

Nuclear masses

Neutron capture rates

As of today, to varying degrees of accuracy

So we must rely on theory... even with FRIB

So we must rely on theory... even with FRIB

PRISM: Portable Routines for Integrated nucleoSynthesis Modeling

Trevor Sprouse (Notre Dame)

Ultimately... we want to know what is the site of the $r$-process?

What do we currently know on the nuclear physics side?

What is most important nuclear physics for late-times in the $r$-process?

How well can we currently predict final abundance patterns?

What do we need to measure?

How do these measurements help us to move forward?

One method is to use Monte Carlo calculations

- Allow the uncertain nuclear input to vary
- Produce an abundance pattern with varied dataset
- Repeat many times to create ensemble
- Combine the ensemble of abundance patterns

masses $\Delta\sim500$ keV (or more!)

$\beta$-decay rates $\Delta\sim2$ to $10$

neutron capture rates $\Delta\sim10$ to $1000$

Towards the neutron-dripline mass model predictions diverge

Tin (Z=50) & Europium (Z=63) isotopic chains

masses $\Delta\sim500$ keV (or more!)

Mumpower et al. PPNP 86 86-126 (2016)

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

Mumpower et al. PPNP 86 86-126 (2016)

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

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

Mumpower et al. PPNP 86 86-126 (2016)

$\beta$-decay rates $\Delta\sim2$ to $10$

Mumpower et al. PPNP 86 86-126 (2016)

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

Rule of thumb: $\Delta_{\beta}\sim10$ $\Rightarrow$ $\Delta_Y\sim 1-2$ orders of magnitude

Mumpower et al. PPNP 86 86-126 (2016)

Evaluated at $T_9=1.0$

Tin (Z=50) & Europium (Z=63) isotopic chains

neutron capture rates $\Delta\sim10$ to $1000$

See Stylianos' talk later in the week

Mumpower et al. PPNP 86 86-126 (2016)

Neutron star merger $r$-process

Reduction in uncertainties from light to dark shading

Rule of thumb: $\Delta_{n,\gamma}\sim100$ $\Rightarrow$ $\Delta_Y\sim 1-2$ orders of magnitude

Liddick et al. PRL to be published (2016)

We seek to understand the formation of this abundance feature

Sensitive to both astrophysical conditions & nuclear physics input

- 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

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

M. Mumpower et al. PRC 85 045801 (2012)

Cold wind: $S\sim300$, $\tau=80$ ms, $Y_e=0.4$

M. Mumpower et al. PRC 85 045801 (2012)

Using the ETFSI-Q mass model

M. Mumpower et al. PRC 85 045801 (2012)

Using the FRDM95 mass model

M. Mumpower et al. PRC 85 045801 (2012)

*A focused, cross-discipline approach...*

Turn the problem around

Try to constrain nuclear physics with obs. & exp.

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

Requires a localized nuclear structure (kink)

Relatively few nuclei to measure

Hints from Jin Wu's $T_{1/2}$ measurements

Very close to making necessary mass measurements (FRIB & other facilities) - Via fission fragment yields

Requires dumping heavy products in exactly the right spot

Extreme $r$-process conditions necessary

Need to make measurements on hundreds of the heaviest nuclei

**Problem:**We can't reach these nuclei, even with FRIB

- Fix the astrophysical conditions (hot, cold or merger)
- Use the Metropolis algorithm: at each step the parameters of the mass model are varied
- Update all nuclear properties self-consistently
- Use abundances, which are well known, (calculate the Likelihood function) to determine if the step was successful

$L\sim$ match red abundances

M. Mumpower et al. arXiv:1603.02600 (2016)

Hot wind $r$-process with default DZ parameters

Success?! But there's a problem!

M. Mumpower et al. arXiv:1603.02600 (2016)

Hot wind $r$-process with new DZ parameters

Success?!... But there's a problem!

M. Mumpower et al. arXiv:1603.02600 (2016)

We need to tell the Metropolis algorithm to match both

Update Likelihood function:

$L\sim$ match abundances + match known masses

M. Mumpower et al. arXiv:1603.02600 (2016)

No combination of DZ parameters can **simultaneously** reproduce the rare earth peak and match the known masses at the same time

The nuclear structure information responsible for the rare earth peak is *missing from the model*

We could move to a nuclear model, but these are more complicated to analyze, with many coupled parameters.

The benefit to DZ is that the abundances are flat to start with.

Let's try to add the missing physics!

M. Mumpower et al. arXiv:1603.02600 (2016)

$M(Z,N) = M_{DZ}(Z,N) + $$a_N$$ exp[-(Z-$$C$$)^2/2$$f$$]$

$a_N$ - Strength of change for given neutron number in MeV

$C$ - Center of the distribution in proton number

$f$ - Rate of fall off back to stability

Now we repeat the Monte Carlo calculations, letting these parameters vary

M. Mumpower et al. arXiv:1603.02600 (2016)

The predicted masses for $Z=60$ (Nd)

Distinguishable predictions given different astrophysical conditions

Hot: local min even-N • wider in N • smaller change to masses

Cold: local min odd-N • tigher in N • larger change to masses

M. Mumpower et al. arXiv:1603.02600 (2016)

Before • During • After peak formation

Great success! Difference is encoded in the astrophysical conditions

M. Mumpower et al. arXiv:1603.02600 (2016)

Similar hot & cold conditions

M. Mumpower et al. arXiv:1603.02600 (2016)

Similar hot & cold conditions

M. Mumpower et al. arXiv:1603.02600 (2016)

What are the consequences of the future measurements?

Either we find the structure... or we don't

If we do: we favor precise conditions for the main $r$-process

If we don't: we favor extreme conditions that __ REQUIRE__ fission recycling... our only option at the moment is mergers

Perhaps nature is more complicated than we think... and we learn something even more profound

*Go make the measurements to find out!*

Accepted to PRL - Liddick *et al.* (2016)

Neutron capture measurements far from stability are challenging

Constrain $^{69}$Ni(n,$\gamma$)$^{70}$Ni by populating $^{70}$Ni via $\beta$-decay of $^{70}$Co

Accepted to PRL - Liddick *et al.* (2016)

Neutron capture measurements far from stability are challenging

Constrain $^{69}$Ni(n,$\gamma$)$^{70}$Ni by populating $^{70}$Ni via $\beta$-decay of $^{70}$Co

Figure: (a) level density (b) $\gamma$SF (c) reduction in rate uncertainty

To be featured as PRL Editors' Suggestion

Accepted to PRL - Liddick *et al.* (2016)

Suppose we apply this method to many neutron-rich nuclei...

Reduction in uncertainties from light to dark shading

Spyrou *et al.* submitted (2016)

Use the same experiment to study neutron emission from $^{70}$Ni

Recently completed fission barrier heights

In the works at Los Alamos: fission fragment yields, neutron-induced fission reaction rates, and $\beta$-delayed fission probabilities

P. Möller & M. Mumpower et al. (2015)

My collaborators

G. C. McLaughlin, R. Surman, A. Aprahamian, M. Beard, I. Bentley, S. Marley, P. Möller, D.-L. Fang, A. W. Steiner, T. Kawano, S. Liddick, A. Spyrou & T. Sprouse

Right now predictions of final $r$-process patterns are shrouded by large uncertainties.

However, targeted experimental campaigns, e.g. in the rare earth region, will help to further constrain nuclear models.

This does not necessarily mean improved predictive power for nuclear models.

We have to learn how to draw conclusions, although models (and data in some cases) are imperfect.

*One thing is certain: We can continue moving forward by working together!*