The Physics of EXOGAM
EXOGAM is a European collaboration to build a highly efficient and powerful g-ray spectrometer for nuclear spectroscopy using the exotic radioactive beams from the Systeme de Production d'Ions Radioactifs et d'Acceleration en Ligne (SPIRAL) facility at the Grand Accelerateur National d'Ions Lourds (GANIL).
The field of accelerated radioactive ion beams is an exciting and rapidly developing area of science. The growth is evident in the number of laboratories planning and developing such facilities in Europe, North America and Japan. The majority of the large international community in nuclear physics is looking towards the use of radioactive ion beams to broaden the horizon of our understanding of the physics of the nucleus.
The use of radioactive beams will open a new era in nuclear physics by allowing access to new isotopes and by increasing the production rates of nuclei which can presently only be populated with extremely low cross-sections or not at all. This enables, firstly, wider systematic studies of nuclear phenomena to be made over a much larger range of nuclides, giving a deeper and more general insight into their structure and its basis and unexpected new phenomena may be revealed. Secondly, it allows tests to be made of present nuclear models under extremes of both proton and neutron excess, leading to the elucidation of new physics such as neutron skins and their effects, breakdown and modifications of shell structure, the understanding of the basis of effective interactions, implications of pairing in weakly bound systems and many other unforeseen possibilities. For example, present studies are close to revealing the effects of collective proton-neutron pairing in heavy nuclei, and to accessing a new island of stability for super-heavy nuclei. These and other landmarks are expected to become open to study with radioactive beams. In addition, the improved access to more exotic regions of the nuclear periodic table permits the study of the structure of nuclides important in astrophysical synthesis such as the r- and rp- processes. A study of the structure and stability of isotopes in these reaction chains will add to the understanding of the synthesis processes and also the properties of the environments in which they might occur, see figure for a pictorial representation.
2 PHYSICS CASE.
The availability of radioactive ion beams from the SPIRAL facility will open up a vast range of new nuclear physics. In this section some of the physics areas are discussed. More detailed cases for the use of radioactive beams for nuclear physics are given elsewhere [2.1-2.3]. In pushing back the limitations imposed by the number of species accessible to detailed spectroscopy, current themes and ideas in nuclear structure physics can be extended to new isotopic regions thus leading to a deeper and more general understanding. But more importantly, there are already hints that new physical phenomena might arise in exotic systems, which would significantly extend the breadth and depth of the present field of research.
2.1 Proton-Rich Systems.
The addition of protons to a nucleus rapidly decreases the binding energy of the system through the increase in the repulsive electrostatic interaction between protons, as a consequence nuclei are rapidly reached which are unstable towards the emission of protons. The majority of our current understanding of nuclei and their structure comes from studies of moderately proton-rich species. The production of very proton-rich nuclides, near to the proton dripline, is limited by the use of stable beams because of the extremely small production cross sections for pure neutron emission. These restrictions will be lifted by the use of radioactive beams since it will become feasible to populate such nuclei via the much more prolific charged particle evaporation channels. Not only do such channels correspond to higher cross-sections but charged-particle emission facilitates rather easy high efficiency selection of the nuclei of interest if appropriate ancillary detectors are used in conjunction with the gamma-ray array.
Of the proton-rich species, systems with N = Z are of particular interest. For masses less than 40, N = Z self-conjugate nuclei are strongly bound and b stable, whereas for A ³ 56, the line of b stability dives away towards more neutron-rich systems and N = Z nuclei become progressively less bound as the mass increases. With increasing mass such systems become more and more difficult to access experimentally. Structurally, self-conjugate nuclei are important for the high degree of symmetry they display between the proton and neutron degrees of freedom. Protons and neutrons simultaneously fill identical single-particle orbitals, leading to a large overlap between nucleon wavefunctions. Such systems are therefore subject to very strong correlations in their motion which can amplify many structural phenomena. A rich variety of different types of structure are exhibited; oblate and prolate deformed systems, spherical and superelongated systems occur and the changes in structure from one nuclide to another, and also structural evolution with angular momentum and excitation within a single species, are sudden and dramatic. Such nuclear landscapes provide stringent tests of our nuclear models. Furthermore, the rp process passes through these isotopes and structural properties can influence these reactions chains. There is no definite astrophysical site in which the rp-process is known to occur and so such information is important in order to locate and understand possible explosive nuclear synthetic sites.
The self-conjugate nucleus, 100Sn (N = Z = 50) has recently been shown to be bound in its ground state by experiments carried out at GSI and GANIL [2.4]. Its excited states are expected to be bound up to ~ 4 MeV and higher states might be quasi-bound by centrifugal and Coulomb barriers, which can confine nucleon wavefunctions, even when the proton separation energies are small. It is expected that this nucleus has a rather simple structure associated with a doubly magic system. However, the actual binding energies are very sensitive in such a weakly bound system to residual interactions and correlations. The regions around doubly magic nuclei have traditionally been the source of experimental matrix elements used as input for shell-model descriptions of large regions of nuclei. The study of excited states in 100Sn and neighbouring nuclei, via observation of their electromagnetic decay, will produce important information about the mean field, spin-orbit and residual interactions and nucleon correlations which would be of relevance to the structure of all medium-mass nuclei. Furthermore, the study of the polarising effects of excited valence nucleons, such as those seen in particle-hole states in 16O and 40Ca doubly magic systems, could be extended with the application of radioactive beams to g-ray spectroscopic measurements in 56Ni and 100Sn. Studies of these polarising effects can then be related back to the issue of the onset of stable ground- state deformation.
The N = Z nuclides also provide a unique system for the study of certain valence nucleon interactions which affect nuclear structure in various ways. The valence nucleon interaction can be split up into two forms: a T = 1 force giving rise to the well known p-p and n-n pairing correlations and a T = 0 force which gives rise to collectivity and deformation via configuration mixing. The T = 0 force appears predominately in monopole and quadrupole forms; the latter playing a key role in the development of quadrupole collective effects in deformed regions and quadrupole vibrational degrees of freedom. The T = 0 monopole component is less well known but has been encountered in light N = Z nuclides [2.5]. It is of great interest to study such effects in medium-mass systems, where level densities are high enough for the monopole T = 0 force to give rise to a potentially well-developed pair field. It might be possible for such neutron-proton pairing interactions to lead to the development of a pairing gap in odd-odd systems or effects in the alignments of nucleon pairs at moderate spins and other phenomena. These would be apparent in measurements of the g- decay of energy levels in such nuclei. It is known that the T = 0 effects in N = Z nuclei are large, and the magnitude drops rapidly when moving away from nuclei in the vicinity of N = Z [2.6,2.7]. Self-conjugate nuclei are thus unique candidates for investigating such phenomena.
More subtle aspects of self-conjugate nuclei concern various aspects of isospin. The demise of isospin symmetry has not been properly addressed since it remains a fairly good symmetry throughout the known periodic table. Coulomb effects are small in light nuclei and washed out by a neutron excess in heavier systems, leading to a persistence of isospin symmetry. Along the N = Z line however, a large Coulomb energy can be built up, without the relief of a neutron excess, thus breaking the symmetry in the behaviour of charged protons and chargeless neutrons. Recent estimates of isospin mixing indicate that it has a roughly Z8/3 dependence for self-conjugate nuclei [2.3]. This would correspond to admixtures of T = 0 components of around 5% in the ground-state wavefunction of 100Sn, hence suggesting that the structure of this nucleus may not be as simple as first imagined. No other region allows access to nuclei exhibiting such extreme breakdown in this symmetry. Measurements of the electromagnetic transitions between low-lying states in the heavier N = Z systems could elucidate these expectations with the observation of E1 transitions which are forbidden for self-conjugate nuclei in the case of perfect symmetry.
The spectroscopy of mirror nuclei (N = Z + 1 or Z - 1) provides information on the difference between the proton and neutron nuclear potentials. Spectroscopic studies in mirror nuclei allow a systematic study of the changes between the positions of proton and neutron Fermi surfaces. The interplay between Coulomb and nuclear forces can also be studied via Coulomb energy differences in mirror nuclei. With radioactive beams, it will be possible to study far heavier pairs of mirror nuclei than is currently feasible, and moreover, to study them to high spin. Results of an experiment to investigate the mirror pair 49Mn/49Cr [2.8] have shown that there is a clear correlation between the behaviour of the Coulomb energy differences and the rotational alignment of nucleons.
Nuclear species beyond the proton dripline are unbound towards proton emission. With radioactive ion beams it becomes possible to populate some unbound systems. In principle electromagnetic decays can compete with particle emission, but only realistically in cases where the latter is hindered in some way. For low-lying states with high angular momentum, particle emission probabilities can be reduced by the effects of Coulomb and centripetal barriers and such states might well decay by emitting g-rays as in a normal bound system. The subsequent proton decay of lower- lying states can then be used to identify the g-emitting nuclide. Such techniques, combining an array of gamma detectors with measurements of proton radioactivity using a radioactive beam, will open up previously unstudied regions of the Segre chart, for example, the predicted deformed region centred on very neutron-deficient gadolinium isotopes. In addition, it will allow studies to be made of the relative particle- and g-decay probabilities and the rather sensitive role that structure is expected to play in that competition.
The structure of neutron-rich nuclei presents a major challenge to our understanding of nuclei as a whole. The development of a large excess of one type of particle in nuclei leads to the expectation that the physics of such systems is liable to be fundamentally different from that which we are used to in neutron-deficient and near-stable nuclei. Changes are expected in nucleon-density distributions and effective interactions, which lead to alterations of nuclear structure as discussed below. The nucleus can bind a much greater excess of neutrons than protons, hence the neutron dripline is very far from stability. As a consequence it has only been reached for the very lightest systems. The position of the neutron dripline in heavy systems is an open and hotly debated question. Recent calculations have indicated that a substantial difference between proton and neutron rms radii develops with increasing neutron excess [2.9]. These effects are established in light systems where neutron haloes have been experimentally observed [2.10]. The presence and extent of neutron skins in heavier systems are not fully established and the effects of the development of such a neutron skin in medium- and heavy-mass systems are an open question. If such a change in the density distributions of the two nucleon systems arises, this will be reflected in a concomitant alteration in the geometries of the associated nucleon potentials. The resulting change in the mean fields would influence the single-particle structure near the Fermi surface and therefore have a dramatic effect on the structure of neutron-rich nuclides. Studies of such systems therefore hold the key to addressing the issue of whether a measurable difference does develop between the charge and mass distributions with increasing neutron excess. This would yield information on the relative strengths of the isoscalar and isovector components of the nucleon interaction; e.g., are protons pulled out by the neutron excess thereby reducing charge densities and altering binding energy and stability, or does a neutron skin develop? If so what are its structural consequences?
The spin-orbit interaction was introduced in the early years of nuclear structure to account for the observed sequence of magic numbers in the nuclear system. Its strength obviously has a direct effect on nuclear structure by shifting the ordering and energetic locations of single-particle orbitals. There are predictions that the strength of this interaction is modified by an increasing neutron excess, although a theoretical consensus has not yet emerged [2.9]. Observation of modifications of structure by changes in the spin-orbit interaction would give insight into the origins of the interaction itself; for example, is it generated by a purely two-body force, or as some theorists suspect from studies of light nuclei [2.11], does it have many-body components?
The nuclear pairing interaction has had a profound effect in our appreciation of nuclear structure. In weakly bound systems the nucleon pairing interaction may scatter pairs of particles from bound to continuum states, drastically increasing the pair correlations to the extent that the behaviour of nucleons is altered and structure modified [2.12]. Such phenomena might be observable in the most neutron-rich species near the drip line, evidenced by a possible increase in pair gaps or delay in band-crossing frequencies.
At the simplest level, Coulomb excitation to the low-lying states would be able to quickly map out the basic structural features of whole regions of nuclides. A simple measurement of the positions of the first two or three excited states will enable the development of vibrational collectivity and deformation to be observed. Such effects are influenced to a large degree by the valence single- particle structure, orderings of levels, pairing and major shell gaps. Modifications to fundamental nuclear properties (potential, effective interactions and pairing) would influence the single-particle structure near the Fermi surface, and therefore the basic features exhibited by the structure of low- lying states. A recent example of the use of such methods has employed intermediate-energy Coulomb excitation of a 32Mg beam from fragmentation reactions [2.13]. Measurements made on just the first excited state have produced evidence that the well known N = 20 spherical shell gap is broken down by the onset of strong deformation in heavy neutron-rich isotopes.
The measurement of medium- to high-spin states has been a fruitful area of research in testing the validity of our understanding of nuclear structure. Such states are populated in heavy-ion fusion- evaporation reactions, which, with stable beams, imposes the restriction of studying only moderately proton-rich systems, due to the curvature of the line of stability. The study of nuclear properties at high angular momentum sheds light on the various modes of excitation that nuclei can accommodate and the evolution of such modes as a function of spin and excitation. The study of exotic metastable shapes (for example, octupole, super- and hyper- deformation) in nuclei provides severe trials for nuclear theories; many of the predictions of such states occur at moderate spins in stable and neutron-rich nuclei. Observations of high-K isomers based on yrast many-quasiparticle states can elucidate the effect of extensive blocking of valence orbitals on the nucleon pairing interaction. Again, many predictions of such yrast isomers occur in nuclei only accessible to study with an array deployed at a radioactive beam facility.
High-spin studies also address the general problems of modifications to shell structure and nuclear properties by an increasing neutron excess discussed previously, but now with the added variables of spin and excitation. Do potentials and effective interactions alter at high spin as evidenced by changes in shell gaps, deformation and band-crossing frequencies, or does the presence of appreciable angular momentum alter the development of a neutron skin? Is pairing quenched at high spin or does continuum coupling allow it to persist? Such questions are open at the present time.
Very complete low-spin information exists for some stable nuclei through detailed studies of electromagnetic transitions following neutron capture reactions. Spectroscopy of such systems can be guaranteed to be `complete', in the sense that all states below a particular spin (around 4-5 (h/2p)) and excitation ( ~ few MeV) are certain to be populated. Such extensive data sets have been used to establish the existence of certain symmetries in the nuclear structure of low-lying levels, as described by, for example, the interacting boson models. The point in spin or excitation at which such symmetries are broken, and the role of other nuclear properties in lifting such symmetries, are unknown due to the lack of information extending neutron capture studies to a higher spin and excitation regime. Spectroscopic measurements of such stable systems, at higher spins than those available in neutron capture, would begin to address these areas, but radioactive heavy-ion beams are required.
Nuclear collectivity depends on the availability of specific combinations of nuclear configurations near the Fermi surface. Collective modes appear when pairs of orbits are available which have large matrix elements for the appropriate operator. For example, octupole effects are seen when protons or neutrons differing in orbital angular momentum and total spin by three units are active near the Fermi surface. Other combinations of exotic configurations will become accessible with the advent of radioactive beams. The heavy nuclei offer prospects of investigating even higher multipole modes of excitation. For example, the hexadecapole mode could be studied in heavy Pd (A ~ 116-120) and Os (A > 192) nuclei, where high-K unique-parity proton and neutron orbits, favourable to large e4 shape components, lie close to the Fermi surface. Another exciting possibility is the chance to observe a Dl=5 collective mode by reaching nuclei such as the neutron rich Ra-Th isotopes, where p(p3/2-i13/2) and n(d5/2-j15/2) orbit pairs favour the creation of excitations with sufficient two-quasiparticle components to generate collectivity in this highly exotic mode. It is not feasible to study such modes with stable beams and targets.
There is no definite evidence for the existence of rotationally-stabilised triaxial shapes in nuclei. Such nuclei are expected to occur when the shape-driving effects of valence protons and neutrons are in an opposite sense (i.e., one prolate- and one oblate- driving). This generally occurs when one Fermi surface is at the bottom of a major shell and the other at the top. In heavy nuclei such situations will occur in extremely neutron-deficient or -rich species. For example, the light Hf to Pt nuclei with N ~ 90 would be good candidates with which to pursue stable triaxiality. However, the best examples for studying such states lie just beyond the lightest nuclei that can be populated at high-spin with stable beams and targets. Similarly, oblate ground-state deformation has been observed in very few nuclei (some Pt and Au isotopes). Here oblate-driving proton and neutron orbits are required. Interesting candidates might be found in neutron-deficient Se/Kr nuclei, light Ba isotopes and Au systems. In order to reach candidates for either phenomenon, radioactive beams are necessary.
It has been suggested that nucleon transfer prior to the fusion of two nuclei can lead to large enhancements in sub-barrier fusion cross sections. Transfer reactions involving reaction participants with loosely bound nucleons occur at larger radii than those involving well-bound stable systems, providing further information on sub-barrier effects. Heavy beams with exotic N/Z ratios may also yield enhanced multiple pair transfers, as equilibration of the N/Z ratio favours the onset of neutron and proton currents, whose consequences may lead to new physics. A possible example is related to the pairing degree of freedom and the associated multipair transfer processes. At large inter-nuclear distances proton transfer is inhibited because of the Coulomb barrier, neutron transfer is therefore favoured, especially if one of the participants is neutron rich. If it were possible to transfer a large number of pairs then that could give rise to new modes of collective motion, and a new method of synthesising neutron-rich nuclei.
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