SAGE

09/07/07

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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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