1. 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

2. 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

3. 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.

4. 56Ni waiting-point
H. Schatz et al. PRL 2001
Courtesy of W. Ong Z N

5. 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)

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

7. 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 …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

8. 68Se waiting-point
H. Schatz et al. PRL 2001 Z N

9. 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

10. 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]

11. Summary of waiting-point nuclei status
56Ni:
Flow through 56Ni is weak --> strong waiting point [C. Langer et al. Phys. Rev. Lett. 113, (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

12. Thanks to:

14. 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,

15. 65As half-life also points to more negative proton separation energy (i.e. less-bound 65As)

16. 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 (2014)
Coulomb shift:
∆ 𝐸 𝑐 = 𝑀𝐸 > − 𝑀𝐸 < 𝑘𝑒𝑉, where > is the neutron-deficient species and < is the mirror
Charged sphere, 𝐸 𝐶𝑜𝑢𝑙 = 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 (2014)

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