Proton-proton correlations observed in two-proton radioactivity of 94 Ag

1. Proton-proton correlations observed in two-proton radioactivity of 94Ag Mukha, I. Mazzocchi, C. et al. Proton-proton correlations observed in two-proton radioactivity of 94Ag. Nature 439, 298-302(19 January 2006) 94Ag is the first nucleus to exhibit both proton and two-proton radioactivity. This is especially unexpected because Silver has an odd number of protons. The relatively large probability of two-proton emission (0.5%) is attributed to a strong prolate deformation of the nucleus, which facilitates simultaneous emission of two protons from either the same or opposite ends of the axis of symmetry. The angular momentum of these protons is conservatively estimated to lie between 6 and 10 units, and have combined energy 1.9 MeV.

2. Proton drip-line from calcium (Ca) to tin (Sn). • One-proton radioactivity was first observed in 1982 • 45Fe, 48Ni, 54Zn, have since been observed to decay by two-proton radioactivity from unbound ground states • 94Ag is bound in ground state against proton emission

3. Lifespan of 94Ag • 94Ag beam produced using 58Ni(40Ca,p3n) reactions and ion source yielding beam intensity of 2 atoms s-1 for (7+) and (21+) Ag isomers (largest spin recorded for any β-decaying nucleus). • (21+) is a long lived meta-stable state owing to its unusually high spin. • 3 decay modes: from β to γ and finally proton-radiation. • T1/2 = 0.39 s • 94Ag decays to an excited state of 92Rh emitting two protons (either simultaneous or sequential). • 92Rh decays to ground state via 2 γ-emissions (frequencies well understood). The path is clear: • Search for coincidence in two-proton emission and characteristic γ-radiation from known 92Rh transitions The path is clear: Search for coincidence in two-proton emission and characteristic γ-radiation from known 92Rh transitions

4. Evidence of 2p decay • Observed energy correlations of protons. • Primarily interested in region between two bars • Sum-energy spectra of two Si detectors in Si1-Si2–γ–γ coincidence with double γ-ray gates on 92Rh (unfilled histogram) and with double γ-gates shifted by 300 keV (filled historgram) • The primary difference being a coincidence peak centered at 1.9 MeV

5. Energy correlations for proton-proton (Ep-p) and proton 92Rh (Ep-Rh) • Low-energy peak corresponds to protons emitted in the same direction and high-energy peak corresponds to protons emitted in opposite directions Sequential or Synchronous? • Sequential proton emission must involve 93Pd as an intermediary. • The energy associated with the transition from 93Pd to 92Rh should reveal itself with a characteristic recoil energy observable in the Ep-Rh plot

6. Energy correlations for proton-proton (Ep-p) and proton 92Rh (Ep-Rh) Sequential • For Ep-p plot, expect uniform intensity across all distribution • For Ep-Rh, two narrow peaks are expected to reflect the population of intermediate states in 93Pd. (This has to do with suppression of transitions from 93Pd yrast states to non-yrast states). • Excited 93Pd states de-excite by fast γ-decay

7. Energy correlations for proton-proton (Ep-p) and proton 92Rh (Ep-Rh) 2p decay • For prolate deformation, two peaks are expected (solid curves represent predictions based on Mukha’s model) if protons emit from opposite ends of elongated nucleus • Relative-energy spectrum for proton-92Rh correlations should be symmetric around the half-decay-energy where the energy of both protons are equal.

8. Epilogue • Observations indicate that 94Ag indeed decays through simultaneous 2p emission. • Standard “simultaneous emission” model inadequate (overstate half-life by 5 orders magnitude) • Mukha proposes strong nuclear deformation to explain relatively high 2p-decay probability • Assuming protons share energy E, L and nucleus has axes ratio 1:1:a, for Lе{6,8,10} results are reproduced for a<0.3 and a>2 • a=0.3 not observed; a=2 super-deformed

9. Epilogue • Observations indicate that 94Ag indeed decays through simultaneous 2p emission. • Standard “simultaneous emission” model inadequate (overstate half-life by 5 orders magnitude) • Mukha proposes strong nuclear deformation to explain relatively high 2p-decay probability • Assuming protons share energy E, L and nucleus has axes ratio 1:1:a, for Lе{6,8,10} results are reproduced for a<0.3 and a>2 • a=0.3 not observed; a=2 super-deformed

10. 94Ag half-life v. deformation ratio a • a ~2 explains Ep-p spectrum nicely (with strong correlation of energies for emission on opposite ends) • Protons are emitted with largest probability in cones centered around the symmetry axis.

11. Conclusion and Summary • 2p-decay is predicted in proton-dripline only for even number protons • For (21+) 94Ag, with an odd number of protons, 2p radioactivity calls for a readjustment in our thinking • (21+) isomer first long-lived nuclear state which exhibits single and double proton decay • Oblate deformation qualitatively explains the data… • But further insight is needed for a quantitative and predictive understanding of 2p decay process

12. Conclusion and Summary • 2p-decay is predicted in proton-dripline only for even number protons • For (21+) 94Ag, with an odd number of protons, 2p radioactivity calls for a readjustment in our thinking • (21+) isomer first long-lived nuclear state which exhibits single and double proton decay • Oblate deformation qualitatively explains the data… • But further insight is needed for a quantitative and predictive understanding of 2p decay process

13. References Mukha, I. Mazzocchi, C. et al. Proton-proton correlations observed in two-proton radioactivity of 94Ag. Nature 439, 298-302 (19 January 2006) Juha Äystö. Nuclear Physics: Odd couple decays. Nature439, 279 (19 January 2006) Plettner, C. et al. On the β-decaying (21+) spin gap isomer in 94Ag. Nuclear Physics733, 20-36 (8 March 2004)