Status of the A=56, 64, and 68 X-ray burst waiting points Zach Meisel 2016 Ohio University Workshop on Experiments for X-ray Burst Nucleosynthesis

rp-process reaction sequence & waiting points W. Lewin et al. SSRv 1993 →rp-process 56Ni →αp-process X-ray counts/second 64Ge 68Se 72Kr 100Sn H. Schatz et al. PRL 2001 Reaction sequence: 3α→HCNO Z N

Waiting point mechanism 120 20 WP Z N Caused by: - low Q-value (easily establish p,γ–γ,p equilibrium) - long half-life (~seconds) Also impacted by p,γ rate on waiting-point proton-capture daughter.

56Ni waiting-point H. Schatz et al. PRL 2001 Courtesy of W. Ong Z N

56Ni waiting-point: Measured key resonance energies S800 Spectrograph (recoil detection) Coupled Cyclotrons (58Ni beam) GRETINA (in-flight γ) A1900 Fragment Separator CD2 target Production Target (Be) C. Langer et al. Phys. Rev. Lett. (2016) W. Ong et al. Submitted to Phys. Rev. C (2016)

Conclusion: 56Ni stalls the rp-process …but may be bypassed at high ρ,T W. Ong et al. Submitted to Phys. Rev. C (2016)

64Ge waiting-point 64Ge + 2p-capture flow uncertain Calculations by C. Langer 64Ge + 2p-capture flow uncertain 65As mass measured by X.L. Tu et al. Phys. Rev. Lett. 2011 …but relatively large uncertainty (~90keV) remains & systematic bias possibly present Penning trap mass measurement planned M. Eibach & Z. Meisel NSCL approved expt. e15026 (≤10keV uncertainty expected) 66Se mass still needed

68Se waiting-point H. Schatz et al. PRL 2001 Z N

68Se waiting-point: Measured key Q-value Coupled Cyclotrons A1900 Fragment Separator Production Target Radiofrequency Fragment Separator Beta-counting Station M. del Santo, Z. Meisel, et al. Phys. Lett B 2014

Conclusion: 68Se substantially stalls the rp-process M. del Santo, Z. Meisel, et al. Phys. Lett B 2014 Events detected Before expt. After expt. Proton energy [keV] X-ray flux Ep = M69Br - M68Se Events detected *qualitative conclusion agrees with A. Rogers et al. Phys. Rev. Lett. 2011 Proton energy [keV]

Summary of waiting-point nuclei status 56Ni: Flow through 56Ni is weak --> strong waiting point [C. Langer et al. Phys. Rev. Lett. 113, 032502 (2014)] But, a by-pass is possible (need more nuclear data) W. Ong et al. Submitted to Phys. Rev. C (2016) 64Ge: Flow from 64Ge depends on 65As mass [M. Eibach & Z. Meisel NSCL approved expt. e15026] (Uncertainty will remain from 66Se mass) 68Se: 68Se is a strong waiting-point (<13% flow through) [M. del Santo, Z. Meisel, et al. Phys. Lett. B 2014] (Could be stronger, depending on 70Kr mass) 72Kr: Will be investigated similar to 68Se [A. Rogers NSCL approved expt. e12024] 100Sn: Need t1/2 [D. Bazin et al. PRL 2008] Improved half-life would help H. Schatz et al. PRL 2001 *but other reactions also significantly affect the rp-process flow & therefore XRB light-curve + ash composition (R. Cyburt et al. Submitted to Astrophys. J. 2016) Z N

Thanks to:

The current value for ME(65As) may be systematically incorrect There is some evidence which suggests the previous ME(65As) measurement may have been systematically biased to an over-bound value highest frev reference 65As IMS-SR mass measurement TOF spectra: X.L. Tu et al. NIM A 654, 213 (2011 IMS-SR mass measurement fit residual: Residual for 2012 AME ∝ 𝑓 𝑟𝑒𝑣. 67Se 71Kr X.L. Tu et al PRL 106, 112501

65As half-life also points to more negative proton separation energy (i.e. less-bound 65As)

If ME(65As) were less bound, this could resolve the ‘coulomb-shift anomaly’ Systematic overestimating of binding by Tu et al. could cause fp-shell ‘Coulomb-shift anomaly’ X.L. Tu et al. J. Phys G 41 025104 (2014) Coulomb shift: ∆ 𝐸 𝑐 = 𝑀𝐸 > − 𝑀𝐸 < +782.35 𝑘𝑒𝑉, where > is the neutron-deficient species and < is the mirror Charged sphere, 𝐸 𝐶𝑜𝑢𝑙 = 3 5 𝑍(𝑍−1) 𝑒 2 𝑅 ->∆ 𝐸 𝑐 ∝ 𝐸 𝐶𝑜𝑢𝑙,> − 𝐸 𝐶𝑜𝑢𝑙,< ∝ 𝑍 < 𝑅 ∝ 𝑍 < 𝐴 1 3 Reduced for ellipsoid Even 𝑍 > = all protons-paired -->anti-parallel spin-alignment -->spatially symmetric Therefore, Expect parallel linear trends for ∆ 𝑬 𝒄 vs 𝒁 < 𝑨 𝟏 𝟑 Coulomb shift trend in fp-shell: Superimpose linear trend: (expected from charged-sphere approximation) (540 keV) (shift required to achieve linearity) (480 keV) 75Sr-75Rb 67Se-67As 71Kr-71Br (125 keV) 69Br-69Se 71Kr-71Br (250 keV) 65As-65Ge 67Se-67As 63Ge-63Ga 65As-65Ge 59Zn-59Cu 61Ga-61Zn 57Cu-57Ni X.L. Tu et al. J. Phys G 41 025104 (2014)

Rogers et al. 2011 & del Santo et al. 2014 may agree