NEWS & VIEWS
NATURE|Vol 439|19 January 2006
rather than sequential emission. As 94Ag was
already known to undergo single-proton decay,
Mukha and colleagues’ state is the first for
which both one- and two-proton decay modes
have been shown to exist.
Juha Äystö
All previous studies of the two-proton decay
of
short-lived nuclear resonances from which
The decay of proton-rich nuclei by the emission of a single proton has been
sequential decay is energetically possible have
known about for some time, and is well understood. The latest observation
proved that these decays are indeed exclusively
of two-proton emission, however, will provoke some head-scratching.
sequential. Sequential emission is probably suppressed in 94Ag because the daugh1
ter state that is populated by the
On page 298 of this issue , Mukha
first of two sequential decays would
and colleagues report the simultaneSn
50
be an excited state of the palladium
ous emission of two protons from a
94Ag
isotope 93Pd. Rather than emitting
complex, long-lived state of the
94
silver isotope Ag, which has an
a second proton, this state would
tend to decay electromagnetically
odd number of protons. This type of
— by emitting a photon — to its
radioactive decay is expected only
ground state. But why does the
for proton-rich nuclei with an even
energetically unfavourable simultanumber of protons — so the obserneous two-proton decay itself
vation leaves nuclear physicists with
occur with such high probability?
some explaining to do.
Mukha et al. tackle this question
Whether an atomic nucleus is
50
54Zn
using a simple model1 that
stable or decays depends on the
48Ni
28
interplay between two fundamental
neglects any interaction between
45Fe
forces: the short-range, attractive
protons, but that is consistent with
strong nuclear force and the longerthe observed energy and angular
range, repulsive electromagnetic
correlations as well as the half-life
Coulomb force. Whereas the strong Ca
of the 94Ag nucleus. Their model
20
force acts between all the nucleons
describes the decay as two inde(protons and neutrons) that make
pendent emissions at either end
28
Neutron number
up the nucleus, the Coulomb force
of a strongly prolate (cigar-like)
20
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 protonauthors supply a similar calculaextremely 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
stable isotopes; proton and neutron numbers 20, 28 and 50 are ‘magic’
longer bind all protons, and such
model).
However, the small numnucleon 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 limsion of one or two protons.
ited accuracy of the results, does
circles. Three of them, 45Fe, 48Ni and 54Zn (refs 4, 5), are outside the
In these rare radioactive decays, proton drip-line and decay through two-proton radioactivity from
not allow the two modes to be disprotons 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 charac1
energy barrier formed by the com- investigated by Mukha et al. 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 enerTwo-proton radioactivity can provide farproton 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 observations satisfactorily6, 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 exseveral 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,
The two-proton radioactivity observed by University of Jyväskylä, Jyväskylä 40351, Finland.
radioactivities are known3 for nuclei with
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).
2. Jackson, K. P. et al. Phys. Lett. B 33, 281–283 (1970).
was observed only recently4,5 — 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).
NUCLEAR PHYSICS
Proton number
Odd couple decays
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