The majority of the mass of the universe is made up of atomic nuclei that lie at
the centre of the atom. Nuclei contain positively charged protons and
electrically neutral neutrons. The lightest known nucleus is that of hydrogen
that contains just one proton but no one yet knows how heavy a nucleus can be;
in other words, just how many neutrons and protons can be made to bind together.
The aim of this proposal is to address this question by studying the heaviest
nuclei that can be made in the laboratory. These nuclei are extremely difficult
to create and study. The heaviest man-made elements today (unnamed as yet) have
as many as 116 protons, but have been produced in tiny quantities of a few atoms
only.
Both protons and neutrons are held
together by the strong nuclear force but protons are repelled from each other
because of their electric charge. The nuclear force has an extremely short
range, affecting nearest neighbours only, so that, as more and more protons are
added to a nucleus the electrostatic repulsion will eventually become stronger
than the nuclear binding forces and the nucleus will become unstable. The
neutrons and protons will no longer stick together. This should happen for
nuclei beyond uranium, which has 92 protons but the existence of heavier species
comes about because of the internal structure within the nuclei. Just as noble
gases owe their inert chemical behaviour to a specific arrangement of electrons
that has extra stability, so certain "magic" proton and neutron numbers also
enhance nuclear stability. This project is concerned with a detailed study of
the underlying mechanisms that yield this extra stability and allow "superheavy"
nuclei to exist.
The main focus of this work will be on
nuclei around nobelium with 102 protons, approximately halfway between the
well-studied nuclei around uranium and the frontier of superheavy elements. Here
spectroscopic methods can be used to gain detailed insights into the shape of
the nucleus and the behaviour of the outermost protons and neutrons that leads
to extra stability. These methods use the fact that the principal way that an
excited nucleus loses energy is through gamma decay (emission of a high energy
photon). In heavy, highly charged nuclei this mode is in increasing competition
with an alternative mechanism, internal conversion, in which the excess energy
is given to an atomic electron that is then ejected from the atom. Indeed, to
obtain a complete set of information about such heavy systems it is imperative
to study these competing decays in the same experiment, since either one alone
reveals only a partial picture. This work will involve the design and
construction of a very efficient detector system called SAGE that will detect
both gamma rays and electrons simultaneously.
In order to produce these exotic nuclei,
calcium will be fused with, for example lead. This will be achieved by
accelerating calcium to very high energy, using the cyclotron at the Accelerator
Laboratory in Jyvaskyla, Finland, and then firing the calcium nuclei at a lead
target. The nobelium produced will be isolated and its decays detected in a
sophisticated suite of detection devices.
This program of research will yield the
most sensitive probes of nuclear structure at the high mass frontier to date.
Detailed and systematic spectroscopic studies will be made on a variety of
nuclei around the nobelium nucleus with mass number 254, which in turn, will
lead to a greatly improved understanding of the behaviour of superheavy nuclei.
These experiments will help theorists to refine and test their calculations
that, for many decades, have attempted to predict how heavy nuclei can be, with
widely differing results. The resolution of this problem will help us to
describe the complex many-body system that the nucleus represents.
Schematic of the SAGE spectrometer. The
heavy ion beam enters from the left and strikes the target at the centre of the
Ge detector array. Fusion reaction products recoil into the gas-filled separator
RITU, which transports them to the multi-detector focal plane spectrometer GREAT
where they are identified. The energies of prompt
g
rays are measured using the Ge detectors, while conversion electrons follow
spiral trajectories in the system of solenoidal magnetic fields of SACRED as
they are transported back to the Si detector. The magnetic field axis is offset
(exaggerated) to allow the beam to pass the Si detector to the target.
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