Odd Couple Decays Which Both One- and Two-Proton Decay Modes Juha Äystö Have Been Shown to Exist

19.1 N&V 273 MH 13/1/06 5:21 PM Page 279

NATURE|Vol 439|19 January 2006 NEWS & VIEWS

NUCLEAR PHYSICS rather than sequential emission. As 94Ag was already known to undergo single-proton decay, Mukha and colleagues’ state is the first for Odd couple decays which both one- and two-proton decay modes Juha Äystö have been shown to exist. All previous studies of the two-proton decay The decay of proton-rich nuclei by the emission of a single proton has been of short-lived nuclear resonances from which known about for some time, and is well understood. The latest observation sequential decay is energetically possible have proved that these decays are indeed exclusively of two-proton emission, however, will provoke some head-scratching. sequential. Sequential emission is probably sup- pressed in 94Ag because the daugh- On page 298 of this issue1, Mukha ter state that is populated by the and colleagues report the simultane- Sn 50 first of two sequential decays would ous emission of two protons from a be an excited state of the palladium complex, long-lived state of the 94Ag isotope 93Pd. Rather than emitting silver isotope 94Ag, which has an a second proton, this state would odd number of protons. This type of tend to decay electromagnetically radioactive decay is expected only — by emitting a photon — to its for proton-rich nuclei with an even ground state. But why does the number of protons — so the obser- energetically unfavourable simulta- vation leaves nuclear physicists with neous two-proton decay itself some explaining to do. occur with such high probability? Whether an atomic nucleus is 50 Mukha et al. tackle this question stable or decays depends on the 54Zn using a simple model1 that interplay between two fundamental 48Ni 28 neglects any interaction between forces: the short-range, attractive 45Fe protons, but that is consistent with strong nuclear force and the longer- the observed energy and angular range, repulsive electromagnetic correlations as well as the half-life Coulomb force. Whereas the strong Ca 20 of the 94Ag nucleus. Their model force acts between all the nucleons describes the decay as two inde-

(protons and neutrons) that make number Proton pendent emissions at either end 28 Neutron number up the nucleus, the Coulomb force 20 of a strongly prolate (cigar-like) acts only between protons. This super-deformed nucleus. The means that in nuclei that are Figure 1 | Rich in proton. A part of the nuclide chart displaying proton- authors supply a similar calcula- extremely rich in protons — said to rich nuclei between calcium (Ca) and tin (Sn). The isotopes framed by tion that assumes the emission of a lie beyond the ‘proton drip-line’ the jagged solid line at the top (the ‘proton drip-line’) are predicted to be correlated proton pair with a rela- (Fig. 1) — the strong force can no bound (that is, stable against proton emission). The black squares denote tive spin of zero (a so-called 2He longer bind all protons, and such stable isotopes; proton and neutron numbers 20, 28 and 50 are ‘magic’ model). However, the small num- nucleon numbers for which nuclei are especially stable7. The isotopes nuclei can decay through the emis- with recently discovered two-proton radioactivities are shown by red ber of events, and consequent lim- sion of one or two protons. circles. Three of them, 45Fe, 48Ni and 54Zn (refs 4, 5), are outside the ited accuracy of the results, does In these rare radioactive decays, proton drip-line and decay through two-proton radioactivity from not allow the two modes to be dis- protons tunnel quantum mechani- the unbound ground state. In the case of 94Ag, the ground state is bound tinguished. Further work is also cally out of the nucleus, through the against one-proton and two-proton emission — but the high-spin state needed to independently charac- energy barrier formed by the com- investigated by Mukha et al.1 is unbound for these decay modes. terize the structure and shape of bined effect of the strong and the complex high-spin 94Ag state Coulomb forces. In the lowest allowed energy of the protons was in all cases simultaneous; and thus allow a more detailed interpretation state of a nucleus, it is energetically favourable because of the extra energy required to break of the process. for protons or neutrons to pair up. Single- proton pairs, sequential emission is not ener- Two-proton radioactivity can provide far- proton emission is therefore expected to occur getically possible. A quantum-mechanical reaching insights into problems of low-energy among proton-rich nuclei that have an odd tunnelling model involving three bodies — quantum-mechanical tunnelling in strongly number of protons, whereas two-proton emis- two protons and a remnant core nucleus — interacting systems. A full elucidation of the sion should be characteristic of nuclei with an describes the observationssatisfactorily6, but nature of such decays — whether from the even number of protons. for the process to be further elaborated ground state or from fast-spinning excited Single-proton radioactivity was discovered2 theoretically, more precise information is states — will be a major goal for further ex- several decades ago in the decay of an excited required about the lifetime and decay energy periments at current and future radioactive state of cobalt-53 to the ground state of iron- of the system, and about the energy and angu- ion-beam facilities. ■ 52. Today, about 30 different single-proton lar correlations between the emitted protons. Juha Äystö is in the Department of Physics, radioactivities are known3 for nuclei with The two-proton radioactivity observed by University of Jyväskylä, Jyväskylä 40351, Finland. proton numbers between 50 and 84, and the Mukha and colleagues1 was from 94Ag nuclei, e-mail: [email protected] phenomenon is fairly well understood theor- not in their ground state, but in a metastable, etically. In contrast, two-proton radioactivity long-lived, high-spin state that the authors pro- 1. Mukha, I. et al. Nature 439, 298–302 (2006). 4,5 2. Jackson, K. P. et al. Phys. Lett. B 33, 281–283 (1970). was observed only recently — from the duced by bombarding a nickel-58 target with 3. Woods, P. J. & Davids, C. N. Annu. Rev. Nucl. Part. Sci. 47, ground states of isotopes of iron, nickel and a beam of calcium-40 ions. They also contrived 541 (1997). zinc,45Fe, 48Ni and 54Zn, all of which have an to measure the energy and angular correlations 4. Dossat, C. et al. Phys. Rev. C 72, 054315 (2005). even number of protons — and information of a two-proton decay for the first time; the 5. Blank, B. et al. Phys. Rev. Lett. 94, 232501 (2005). 6. Grigorenko, L. V. & Zhukov, M. V. Phys. Rev. C 68, 054005 on the energy and angular correlations of the proton–proton and proton–nucleus correla- (2003). emitted protons is incomplete. The emission tions derived are characteristic of simultaneous 7. Janssens, R. V. F. Nature 435, 897–898 (2005).