Extreme Nuclei
The structure of very exotic nuclei, studied using nuclear reactions
Nuclei that carry a halo around them, nuclei that shimmer into existence for less than 10 20 second and
nuclei that are covalently bound by neutron pairs to form nuclear molecules are all studied by the charged
particle reactions group of Dr Wilton Catford and coworkers at Surrey.
Conceptually, making nuclei from protons and neutrons is like filling two quantum pots with two
different types of particles, and now we are learning just how much the behaviour of one type of particle
depends on the precise level of filling for the other type: the energy spacings of the valence neutron orbitals
are profoundly affected as we take more and more protons from the proton orbitals. With the new
radioactive beam facilities now coming online, to make really big changes to the proton/neutron ratio of
nuclei has moved from the realm of “ thought experiments” to being actually possible in the laboratory.
Caen, in Normandy in France, is where William the Conqueror is buried and was the focus of the
D-Day Landings. More to the point for nuclear physicists, today Caen hosts the GANIL/SPIRAL
Laboratory, which along with GSI/FAIR in Germany is one of the two main European centres for pure
nuclear structure research using beams of radioactive nuclei. These two laboratories form the focus of the
Surrey experimentalists’ present activities and our future plans are centred at FAIR and SPIRAL2.
Experiments at GANIL are able to study in detail the properties of nuclei that have, for example,
many more neutrons than the more usual stable nuclei found on Earth. These exotic nuclei are very short
lived and have to be studied by colliding them with other nuclei within one second or less of their creation.
Two facilities at GANIL called SISSI and SPIRAL allow us to do this. The beams are excellent by
radioactive beam standards but are weak compared to the stable beams formerly used – typically at least a
million times less intense – and we have had to develop new and very efficient experimental techniques.
[Saturn, its moon Tethys, and (bottom) Uranus: NASA HST/IR & Voyager 2 images].
Figure 1: Planets represent nuclei
in this schematic of a nucleon
transfer reaction such as (d,p);
when studied with radioactive
beams these reactions involve
projectiles very much heavier than
the target nuclei – known as
inverse kinematics. Experiments
measure the quantum orbits into
which the transferred neutron is
captured. In our first such
experiment, the projectile was
11
Be which carries a halo
(represented in the picture by
Saturn’s rings).
The Surrey group has been at the forefront worldwide of these new developments, having been the
first to study gamma-ray spectroscopy of nuclei using a large detector array with radioactive beams [W.N.
Catford et al., Nucl. Phys. A616 (1997)303] and one of the first groups to use nucleon transfer reactions to
study the spectroscopy of exotic nuclei [J.S. Winfield et al., Nucl. Phys. A683(2001)48]. Combining our
knowledge from these two types of experiment, we have also constructed and started to exploit the first
dedicated apparatus to exploit the added information from gamma-rays to study nucleon transfer reactions
with radioactive beams. This new device, known as TIARA, was conceived by the Surrey group and
brought into being by the new TIARA collaboration set up by Surrey with other UK and French physicists.
TIARA has been used to study the neutron rich neon isotopes, for example, where the breaking of
the N=20 neutron magic number and the emergence of N=16 as a new magic number for neutron-rich
nuclei has been delineated. This work complemented another recent experiment where we studied the
breaking of the N=8 shell for the very neutron-rich beryllium isotopes. The beryllium studies also allow us
to investigate the different effects of clustering on nuclear stucture when there is a big neutron excess.
Beams of the exotic nuclear species 12Be and 14Be have been used by us to collide with carbon
targets. We can experimentally select grazing collisions in which the projectiles survive largely intact, but
with one or two neutrons “sliced” off by the edge of the target. To analyse these experiments, we can make
use of nuclear reaction theories developed by colleagues in the theory group at Surrey. To take the data, we
typically need to detect neutrons that fly off from the reactions, in coincidence with the beryllium or cluster
fragments. For this, we work together with colleagues from France and Belgium who have developed the
DEMON array – an assembly of over 100 scintillator modules to make one of the world’s leading neutron
detection systems – and our colleagues from the UK in the CHARISSA collaboration (comprising
Birmingham and Surrey physicists) through which we have a world-class particle detection system. With
12
Be, as well as studying the N=8 shell breaking, we have also measured how the nucleus appears to
condense into a nuclear molecule of two 6He nuclei, bound together and rotating rapidly. We are also
tracking down how this clustering develops as we go to beryllium isotopes with even more neutrons.
Figure 2: Dr Wilton Catford inspects
progress during the installation of the
TIARA array at GANIL (France),
located inside the gamma array
EXOGAM and in front of the magnetic
spectrometer VAMOS. The
symmetrically shaped vacuum vessel of
TIARA houses an array of silicon strip
detectors that can measure particles
across 85% of all scattering angles. The
innovative design is also now being
adapted also by other experimental
groups around the world.
Using beams from SISSI at GANIL, we have studied the halo of neutrons that surrounds the 12Be
core to make 14Be. The last two neutrons barely hang on, and in fact they only stay attached if all three of
the particles (core plus two neutrons) are present – a situation that is often described using the analogy of
three intersecting “Borromean” rings that hold each other together, but fall part if any one of the three rings
in removed. (We have done this experiment, and studied the 13Be as it falls apart). Also, the He-like cluster
breakup was studied, and in addition we have seen intriguing evidence that a cluster of four neutrons – or a
“tetraneutron” – can be knocked out of 14Be, leaving a 10Be core. This discovery provoked intense interest
from theorists world-wide (see figure) and is being pursued in further experiments by our collaboration.
Figure 3: An experiment involving the Surrey
reactions group, designed to study properties of the
halo nucleus 14Be, initiated an ongoing debate that
featured in the popular scientific press. With
collaborators at the LPC laboratory in France, tentative
evidence was found for clusters of 4 neutrons that exist
as neutral nuclei – or in other words, a heavy isotope
of Element Zero, or a “tetraneutron”. The technique
itself is new and very promising, but the initial
experimental results are perplexing: the trouble is that
no known theory can explain the existence of a bound
tetraneutron. Further work, both experimental and
theoretical, is continuing around the world, to pursue
these first results.
As well as GANIL, our experiments extend to other European and US laboratories and the
CHARISSA collaboration also has a major research base at Canberra in Australia. The experiments that
we perform at overseas nuclear physics laboratories are supported by our ongoing development of new
detector systems and associated electronics and data acquisition systems. The commissioning and
development of various new silicon strip and scintillator detectors for CHARISSA and TIARA is
performed in the nuclear physics laboratories in Surrey, which have recently been re-equipped as part of a
major SRIF2-supported refurbishment.