20 β-delayed neutron emission probabilities across the Z=50 and N=82 shell closure: Implication on the formation of the second r-process peak

Vi Ho Phong

RIKEN Nishina Center

On Behalf of the BRIKEN collaboration

  • Outstanding problems on the formation of  the second (A~130) r-process abundance peak: from astrophysical observations to nucleosynthesis models

  • β-delayed neutrons emission probabilities \(P_{n}\) as important inputs for r-process calculations.

  • New experimental \(P_{n}\) values their impacts on:

    • Benchmarking the theoretical \(\beta\)-decay models

    • Odd-even pattern of the second r-process peak

The second r-process peak: observational clues

Vi H. Phong | AwRI conference | June 15\(^{th}\) 2022

 

  • Te and Sb discrepancy:
    • The second peak is shifted to lower mass compared to solar abundance?
    • Additional contributions to the robust "main" r-process?
    • The second r-process peak is expected to be sensitive to the r-process conditions

robust pattern

star-to-star scatter

C. Sneeden et al. 2008

I. Roederer et al. 2022 

Deficient: Te and Sb

  • Down to which Z the "universality" hold?

Metal-poor stars observation: r-process "universality"

Nucleosynthesis relevant to the second r-process peak

S. Shibagaki el al. 2016, N. Nishimura et al. 2015

  Magneto-hydrodynamic jet (MHDJ) model 

S. Lemaître et al. 2020 and E. Eicher et al.  2015

Neutron star merger model

  • In various scenarios, abundance pattern of the second r-process peak is effectively shaped by the \(\beta\)-decay path to stability during freezeout, which mostly determined by the \(\beta\)-delayed neutrons

\(\beta\)-delayed neutrons of r-process radioactive progenitors

$$P_{xn} = T_{1/2} \int^{Q_{\beta}}_{S_{xn}} S_{\beta}(E_{x}) f(Z,A,Q_{\beta}-E_{x})$$

$$T_{1/2} = \int^{Q_{\beta}}_{0} S_{\beta}(E_{x}) f(Z,A,Q_{\beta}-E_{x})$$

  • Quantified by the β-delayed x neutron emission probability \(P_{xn} =  N_{\beta xn} / N_{\beta}\) , \(P_{n} = P_{1n} + P_{2n}+ P_{3n}+...\)
  • Alter the decay path to stability during freezout
  • Additional source of neutrons for late neutron-capture

A.  Arcones & G. Martinez-Pinedo 2011

Odd-even pattern 

Final nucleous

Precursor

However...

A.  Arcones & G. Martinez-Pinedo 2011

\(Q_{\beta 1n} \)

\(Q_{\beta 2n} \)

Emitter

K. Farouqi et. al. 2011

Large scale survey of \(P_{xn}\): the BRIKEN project

BRIKEN: \(\beta\)-delayed neutron emission probability measurements at RIKEN

RI seperation

RI identification

BRIKEN aims for:

  • Most neutron-rich nuclei: RIBF SRC+ BigRIPS spectrometer
  • Largest detection efficiency: \(^{3}\)He-based neutron detector array + highly segmented implantation detector array: AIDA / WAS3ABi. 

\(P_{xn}\) measurements of neutron-rich nuclei across the Z=50 and N=82 shell closures

r-process path [S. Shibagaki el al. 2016]

Experimental results: feedback to theoretical \(\beta\) decay models

N=84

Z=49 \(\rightarrow\) 50

Crucial benchmarks for the theoretical models for predicting \(P_{xn}\) 

  • Statistical Hauser-Feshbach (HF) models of competition between neutron emission channels and \(\gamma\)
  • Gamow-Teller \(\leftrightarrow\) First-Forbidden competition 
  • Large disagreement between the  between still observed for \(^{136}In\)

P. Moller et. al. 2019 

F. Minato et. al. 2021

I. Borzov et. al. 2020

With HF

Impact on the odd-even pattern of the second r-process peak

Skynet  + Nucnet codes  [J. Lippuner 2015, B. S. Mayer 2007]

Baseline simulation: Y\(_{e}\) = 0.06, S = 12 \(k_{b}\)/b and \(\tau\) = 66 ms

 

 

  • Effect on shaping final odd-even pattern is prominent with and without beta delayed neutrons
  • Removing up to 30 % uncertainties deriving from theoretical models 
  • Significant contribution of  \(\beta\)1n and \(\beta\)2n flows defines the odd-even pattern in the right-wings of the second r-process peak 

Freezout flows and final abundances:

\(^{135}\)In

\(^{134}\)Sn

\(^{136}\)Sn

\(^{131}\)Ag

Further investigation on the odd isotope fraction of Ba

Robust odd-even pattern for low Ye

Improvement of Ba isotopic abundances using the current experimental results.

\(f_{odd,Ba}\) = \((Y_{135Ba}+Y_{137Ba})/Y_{Ba}\)

Metal poor stars \(f_{odd,Ba}\)

C. Wenyuan et al. 2015

Summary

  • New observations of the elements of the second r-process peak calls for new comparison between models and observation.
  • Knowledge on the beta-delayed neutron emission probabilities \(P_{xn}\) is essential for modeling the formation of  second r-process peak during freezeout
  •  \(P_{1n}\) and  \(P_{2n}\) of 20 neutron-rich progenitors of the second r-process peaks have been measured within the BRIKEN project at RIKEN
  • New experimental data provide benchmarks for development of theoretical \(\beta\)-decay models and direct impacts on the odd-even pattern of the second r-process
  • In particular, improving the calculation of odd Ba fractions matching the metal-poor star observations.

Backup slices

Optimizing baseline trajectory

 FRDM+QRPA+HF

Exp.

Data analysis

•Sorting the data produced from 3 independent DAQ

•Merging the data from 3 DAQs based on time-stamp

•Time and position correlation → β decay curves: Tβ − Timplant

\(\beta\)-implant

\(\beta\)1n-implant

\(\beta\)2n-implant

Impact of \(\beta\)-delayed neutrons in various scenarios

Neutron detection efficiency