NEWS & VIEWS
Two-proton radioactivity
Caught in the act
The newest form of radioactivity, two-proton decay, has been imaged directly using an optical
time-projection chamber. The protons are emitted in a way that reflects the internal dynamics
of the parent nucleus.
Philip M. Walker and
Ronald C. Johnson
are in the Department of Physics, University of
Surrey, Guildford GU2 7XH, UK.
e-mail: [email protected];
[email protected]
N
uclear physics began with the
discovery of radioactivity in 1896.
Since then, several different forms of
nuclear disintegration have been identified,
starting with the familiar α, β and γ decay.
The newest type, two-proton decay, was
discovered1,2 in 2002. It is the only form of
radioactivity in which an unstable nucleus
emits two of its basic building blocks. Now
the ability to observe both protons as they
are emitted, as reported in Physical Review
Letters3, is revealing the true nature of the
decay process. The two protons do not
emerge simply as a composite particle, a
diproton: they come out separately, although
in a correlated manner. This opens a new
chapter in the study of radioactivity.
The way that an unstable nucleus emits
its constituents gives a key to understanding
the nuclear puzzle. You might think that
the two protons, if they are to be emitted,
would come out one at a time, sequentially.
However, there is a special situation that is
satisfied for iron-45 (45Fe) and a few other
nuclei, for which sequential emission is not
possible. The reason is clear from the diagram
of energy levels (Fig. 1a): as a result of extra
binding from proton–proton pairing in
45
Fe, the nucleus that would be the product
of one-proton emission — manganese-44
(44Mn) with an odd number of protons — is
believed to be a higher-energy state and is
therefore out of reach; the decay of 45Fe must
therefore involve two protons together, or
none. β-decay is also possible, but it turns out
that this accounts for only about 30% of the
disintegrations of 45Fe, and is not a problem
for an investigation of the proton emissions.
Although predicted in 1960, two-proton
radioactivity was not discovered until
relatively recently, when two teams, one
working at GSI in Germany1 and the other
at GANIL in France2, both had success. In
each case, a high-energy heavy-ion beam
Energy
44Mn
+p
45Fe
2-p emission
43Cr
+p+p
Figure 1 Two-proton decay from 45Fe. a, One-proton decay to 44Mn requires energy input and is not a possible
outcome. In two-proton decay to 43Cr, two protons share a kinetic energy of about 1 MeV. b, A 45Fe ion enters the
sensitive region of the chamber from the left, stops in the gas and emits two protons — seen as a V shape.
was used to initiate the formation of 45Fe,
which survives on average for about four
milliseconds. The product nuclei were
stopped inside silicon radiation detectors,
and the emerging protons deposited their
kinetic energy in the silicon, which gave the
detection signal. The teams quickly realized
that stopping the 45Fe ions in a gas was the
way to make further progress. With an
appropriate choice of materials, the gas could
itself serve as a radiation tracking detector,
so that the individual proton trajectories
could be observed. Taking somewhat
different approaches, the two teams have
been in competition to capture such images.
Giovinazzo et al.4 were first to publish
their impressive experimental results, but
there were too few events to come to any
clear conclusions about the proton–proton
correlations. Meanwhile, Miernik et al.3 —
working at the National Superconducting
Cyclotron Laboratory at Michigan State
University, where they could get a better
event rate — have now come up trumps.
The 45Fe nuclei were first made by
bombarding a nickel target with a beam of
58
Ni, which is the lightest available nickel
isotope, accounting for 68% of natural nickel.
The beam energy was nine billion electron
volts, and the intensity was 1011 58Ni nuclei
836
per second. The energy of the collisions was
such that pieces of the beam nuclei were
broken off, and the resulting beam-like
fragments flew through a magnetic separator.
The small number of emerging 45Fe nuclei
(having lost two protons and eleven neutrons
from the original 58Ni) were stopped in a gas
cell. After nine days of beam time, 125 decays
of 45Fe were observed, 70% of them through
two-proton emission.
A novel part of the apparatus was
the stopping cell, which was an optical
time-projection chamber measuring
20 cm × 20 cm × 42 cm. In this, by directing
the released electrons from the ionization
trails at a luminescent foil and taking digital
photographs, it was possible to ‘see’ the two
emerging protons — an example is shown
in Fig. 1b. The faint trail coming in from
the left is from the 45Fe, which stops in the
gas. The average time for its decay is 4 ms,
and during this time interval the gain of the
signal amplifier was increased, so that the
emitted protons appear brighter than the
incoming 45Fe. Furthermore, by analysing the
light output from fast photomultiplier tubes,
the time evolution of the signal could also
be recorded.
This leads to a clear picture of 45Fe
two-proton decay, with the angle between
nature physics | VOL 3 | DECEMBER 2007 | www.nature.com/naturephysics
© 2007 Nature Publishing Group
NEWS & VIEWS
the two protons being the principal focus of
attention. The distribution in this angle is
influenced by correlations between the two
protons, either because of their interaction
with each other and the daughter nucleus
as they fly apart, or because of the structure
of the quantum state of 45Fe before the
decay. In fact the observed lifetime, and the
angular distribution of the 75 events that
could be fully analysed, are each consistent
with calculations5,6 that take into account
both of these sources of correlations. A
remarkable feature of this experiment and
its interpretation is that the combination
of angular and lifetime information gives
a sensitivity to the detailed microscopic
structure of the decaying state that is not
available in single-particle decays. In this
case, a consistent view is emerging of the
way in which the two protons are distributed
among particular quantum orbits in 45Fe
before being emitted.
There are other nuclei that may also
undergo radioactive decay by two-proton
emission, and evidence has been reported
for the 54Zn ground state7, as well as an
isomeric state8 of 94Ag. Of particular interest
is a new result9 for 19Mg, which has been
found to survive, on average, for only six
picoseconds (6 × 10−12 s) before emitting two
protons. This is surely the shortest lifetime
claimed for radioactivity, and it raises
questions as to what is meant by a stable
nucleus. However, as it takes only about
10−21 seconds for a neutron or a proton to
orbit a nucleus, it could be said that 6 × 109
orbits is indeed stable — that’s more times
than the Earth has orbited the Sun since it
was formed.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
Pfützner, M. et al. Eur. Phys. J. A 14, 278–285 (2002).
Blank, B. et al. Phys. Rev. Lett. 89, 102501 (2002).
Miernik, K. et al. Phys. Rev. Lett. 99, 192501 (2007).
Giovinazzo, J. et al. Phys. Rev. Lett. 99, 102501 (2007).
Brown, B. A. & Barker, F. C. Phys. Rev. C 67, 041304R (2003).
Grigorenko, L. V. & Zukhov M. V. Phys. Rev. C 68, 054005 (2003).
Blank, B. et al. Phys. Rev. Lett. 94, 232501 (2005).
Mukha, I. et al. Nature 439, 298–302 (2006).
Mukha, I. et al. Phys. Rev. Lett. 98, 182501 (2007).
Superconductivity
A celebration of pairs
It is fifty years since John Bardeen, Leon Cooper and Bob Schrieffer presented the
microscopic theory of superconductivity. At a wonderful conference in Urbana the ‘good old
days’ were remembered, and the challenges ahead surveyed.
Michael R. Norman
is in the Materials Science Division, Argonne
National Laboratory, Argonne, Illinois 60439, USA.
e-mail: [email protected]
AIP EMILIO SEGRE VISUAL ARCHIVES
W
hy should we be celebrating the
50th anniversary of a theory? For
the Bardeen–Cooper–Schrieffer
(BCS) theory of superconductivity, I see
two very good reasons. First, the work
represents an era where the unity of physics
was fully evident. There is the famous story
of Richard Feynman putting the BCS paper
away in his desk for a few months because
he could not bear to read it — he knew
that they must have got it right. It was a
problem near and dear to his heart, as it was
to a number of other founders of modern
physics. It took 46 years, starting from the
experiments in H. Kamerlingh Onnes’ lab,
to obtain a full microscopic understanding
of superconductivity, and many famous
scientists tried and failed along the way,
Einstein and Heisenberg being notable
among them. Second, all the work that came
after the BCS theory was formulated reminds
us that there is still much more physics to be
done. There are whole classes of materials
where the mechanism of superconductivity
is yet to be understood. These include
heavy-fermion, organic, and copper oxide
superconductors; some of them have been
with us for twenty to thirty years now.
Getting ready to celebrate. In 1972, Leon Cooper (left) and Bob Schrieffer (right) received, together with
John Bardeen, the Nobel Prize in Physics for their theory of superconductivity developed in 1957.
Understanding them is as challenging today
as were the classic superconductors facing
Bardeen, Cooper and Schrieffer back in 1957.
No less than eight Nobel laureates
attended the 50-year celebration at the
nature physics | VOL 3 | DECEMBER 2007 | www.nature.com/naturephysics
© 2007 Nature Publishing Group
University of Illinois at Urbana–Champaign,
the institution where this seminal work was
done, on 10–13 October 2007. And many
more laureates could have been invited,
giving an idea of how influential BCS has
837