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| ESPERIMENTO PI32, RESPONSABILE: Angela Bonaccorso |
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STRUTTURA E REAZIONI DI NUCLEI ESOTICI
Da alcuni anni un numero sempre crescente di ricercatori teorici italiani ha indirizzato la propria
ricerca allo studio di nuclei esotici. Tali attivita' si sono fin'ora svolte nell'ambito di iniziative specifiche pre-esistenti riguardanti varie tematiche di dinamica, struttura o reazioni nucleari con descrizioni tipo sistema a molti corpi, modello a shell, modelli collettivi, oppure non metodi semiclassici o quantistici per la descrizione di reazioni superficiali di breakup e trasferimento, fusione, scattering elastico con potenziali microscopici, multiframmentazione.
Appare evidente a questo punto la necessita` di coordinare ed omogeneizzare tali sforzi al fine di consolidare e migliorare il contributo italiano a livello internazionale e creare inoltre una solida struttura di supporto per future attivita' sperimentali presso i Laboratori Nazionali. La ricerca proposta consiste nell'evoluzione delle linee gia` avviate descritte nel seguito.
I partecipanti hanno competenze complementari nei campi di struttura (UL,AV) e teoria delle reazioni (AB,MDT,AV). Vantano collaborazioni nazionali (LNS,LNL) ed internazionali comuni (Es: IPN, Orsay; MSU, USA,).
Le loro competenze attuali e coinvolgimento in questo tipo di ricerca riguardano lo sviluppo ed il controllo sull'accuratezza di modelli per reazioni superficiali di tipo trasferimento e breakup per nuclei medio leggeri ed energie dell'ordine delle decine di MeV per nucleone. Lo scopo finale e' l'estrazione di informazioni sulla validita' di concetti usati usati per descrivere la spettroscopia nucleare, quali per esempio il modello a shell e l'effetto dell'interazione di pairing in nuclei con rapporti anomali N/Z ed energie di legame di singola particella estremamente piccole (decimi di MeV), oppure ricchi in protoni. La verifica dell'accuratezza dello stesso nel predire fattori spettroscopici e sequenze di shell, la diminuizione del gap di spin-orbita in presenza di un largo eccesso di neutroni o protoni. Una particolare attenzione e' dedicata a i nuclei con alone di uno e due neutroni ed ai costituenti di base di questi ultimi, i nuclei instabili per emissione di neutrone.
Notevoli sviluppi sono stati ottenuti nello studio del 13Be.
E' stata chiarita la dipendenza dal nucleo proiettile (14Be o 14B) dalla cui frammentazione il 13Be e' ottenuto.
Le reazioni di fusione di nuclei ad alone con nuclei "normali" sono pure usate per comprendere come una descrizione a canali accoppiati della reazione venga modificata dall'accoppiamento a canali di reazione importanti quali il breakup. L'influenza di tali canali sulla diffusione elastica e' inoltre studiata mediate calcoli microscopici di potenziali ottici. Entrambi i potenziali derivanti dal canale di breakup nucleare e breakup Coulombiano sono stati calcolati.
Inoltre per quanto riguarda il breakup Coulombiano e' stato sviluppato un modello a tutti gli ordini di interazione e con tutti i multipoli che viene adesso applicato allo studio del breakup di halo di protoni.
Il fenomeno degli aloni nucleari permette lo studio della materia nucleare a bassa densita` e l'accoppiamento al continuo riveste un ruolo centrale nella comprensione di questi fenomeni. Tuttavia le interazioni efficaci del tipo Skyrme, che si adattano allo studio di nuclei stabili, risultano inadeguate per lo questo caso. Quindi appare necessario ricavare, mediante una conveniente teoria a molti corpi, nuove interazioni efficaci. Si tratta, in altri termini, di ricavare una EOS che riproduce le proprieta` di saturazione della materia nucleare su cui modellare una interazione alla Skyrme con i suoi parametri da estendere ai nuclei finiti. In questo schema correlazionia a bassa densita' inclusi gli effetti di pairing vengono accuratamente riprodotte.
Lo studio dei nuclei esotici presenta pure numerosi risvolti di interesse astrofisico.
Per esempio nonostante l'involucro di una stella di neutroni copra solo l'1% della massa totale, la sua presenza e` fondamentale per l'evoluzione e la dinamica della stella di neutroni (NS). La sua conduttivita` termica determina la relazione tra l'emissione di raggi X e la temperatura interna della NS. Inoltre la sua resistivita` elettrica influenza il campo magnetico osservato nelle NS. Entrambe le proprieta' dipendono dalla struttura e dalla composizione della superficie di una NS insieme all dinamica dei glitches osservati nelle radiopulsar e dei burst di raggi gamma.
Il modello della NS crust e` quello di un reticolo di nuclei
atomici immerso in un plasma formato essenzialmente da elettroni e neutroni. I nuclei sono quindi in prossimita` della drip line.
Le transizioni di fase (ad esempio sferico-deformate) in sistemi lontani dalla stabilta' verranno studiate sulla base di approcci di tipo algebrico. Nuove simmetrie dinamiche vengono evidenziate e saranno studiate. Un'attenzione particolare sara' pure dedicata allo sviluppo del concetto di moto collettivo nei nuclei esotici. In particolare per quanto riguarda nuclei piu' pesanti l'attenzione sara' focalizzata sugli effetti di isospin nelle reazioni a "basse energie" cioe' fino alle energie di Fermi (circa 50AMeV). Cio' include nuovi moti collettivi dovuti all'asimmetria di carica nel canale di ingresso (il Dipolo Dinamico), la dinamica dell' equilibrazione di carica e connessa diffusione di isospin, effetti di isospin sulla transizione di fase liquido-gas, effetti di isospin nella frammentazione di "neck", isoscaling. In altri termini tutte proprieta' legate al termine isovettoriale dell'EOS in condizioni attorno e/o sotto la densita' di saturazione.
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| OBIETTIVI DELL'ESPERIMENTO PI32 |
STRUCTURE AND REACTIONS OF EXOTIC NUCLEI.
Since a few years an increasing number of Italian theoreticians has concentrated his research on the study of exotic nuclei. Such activities have so far been carried out within pre-existing national projects related to a wide spectrum of themes of nuclear dynamics, structure and reactions using many body techniques, shell model, collective modes and semiclassical or fully quantum mechanical approaches to peripheral and central reactions such as transfer and breakup, fusion, elastic scattering via microscopic optical potentials, multifragmentation.
The goal of our project is to start coordinating and homogenizing such efforts to improve our mutual understanding, and to strengthen the Italian contribution on the international scenario. Furthermore our efforts will help creating a solid theoretical structure to support future experimental activities at the INFN national laboratories.
In fact, in the last two decades, the use of radioactive beams of rare isotopes in several laboratories around the world (REX-ISOLDE at CERN, GANIL in France, GSI in Germany, CRC, Louvain la Neuve in Belgium, RIKEN in Japan, DUBNA in Russia, Argonne, MSU, Oak Ridge, Notre Dame in USA , etc.) has provided new research directions and an increasing number of researchers all over the world is converging on such subject. The INFN in Italy is also heavily involved in this field. The facility EXCYT and the large acceptance spectrometer called MAGNEX are being completed at Laboratorio Nazionale del Sud. On the other hand the first step of the SPES project at the Laboratorio Nazionale di Legnaro has been approved in the form of a proton driver. Furthermore the INFN is promoting the new European Radioactive Beam Facility (EURISOL).
Members of our collaboration are actively participating in NuPECC working groups, in particular in the preparation of "The Physics Case" for EURISOL, whose report is available at
http://www.ganil.fr/eurisol/Final\_Report/A-Physics-Case-20-Dec-02.pdf, and in general of the NuPECC Long Range Plan.
The relatively new subject of exotic nuclei is of fundamental importance because while all existing theories for the nuclear interaction and the many body nuclear structure have been based on the study of stable nuclei, very little is known about the way in which standard nuclear models work for the description of unstable nuclei with anomalous N/Z ratio. Important questions to answer are for example: the isospin dependence of the effective nuclear interaction, the modification of the traditional shell sequence with possible vanishing of the shell gaps, the persistence of collective features, the properties of nuclear matter at very low density, the form of the EOS for asymmetric nuclear matter. Similarly in the field of nuclear reactions still open questions are the identifications of the most important reaction channels and the clarification of the associated reaction mechanisms. Many of these features are also related to nuclear reactions of astrophysical interest such as those governing the primordial nucleosyntesis.
The proposed research activity will deal with the following aspects: reaction mechanisms and structure information extraction for nuclei close to the driplines, single particle and collective degrees of freedom, dynamical symmetries at the phase transitions, dynamics of heavy nuclei with anomalus N/Z ratios and isospin degrees of freedom, equation of state.
The partecipants have complementary competences in the fields of structure and reaction theory. They have common national (LNS,LNL) and international collaborations (ie : IPN, Orsay;GANIL, Caen, France, MSU, USA, ect.). Their present abilities and activities in the above research fields are described in the following.
-Reaction Mechanisms (PI )-
In Pisa there is a longstanding tradition for studying
peripheral reactions such as transfer and breakup,
therefore it has been easy and natural to tourn our
attention to the study of halo nuclei.
In recent years we have concentrated on a consistent
treatment of nuclear and Coulomb breakup and
recoil effects treated to all
orders and including interference effects. We have developed a formalism which allows the calculations of energy, momentum and
angular distributions for the core and halo particle and
absolute cross sections. The possibility of calculating
so many observables is almost unique to our model. The
dependence on the final
state interaction used has been clarified and also the
accuracy of the eikonal model cmpared to fully
quantum mechanical theories has been established.
An extension of the method to proton breakup has been
recently presented and we plan to apply it to the
study of reaction of astrophysical interest such as those involving 8B.
Finally a microscopic model for
the calculation of the optical potential in the breakup
channel has been developed. The method originally
used to calculate elastic scattering of halo projectiles
on ligth targets is now being extended to heavy targets
by the inclusion of recoil effects. Also we are extending
our techniques to the calculations of angular correlations.
In the last period we have started to study nuclei unbound
against neutron emission, such as 10Li and 13Be.
They are the constituents of two neutron halo nuclei
(i.e. 11Li and 14Be).
The study of their low lying resonance states is of
fundamental importance for the understanding of
two neutron halo nuclei.
The final goal is to clarify the structure of the core-neutron
interaction. This is by no means a trivial task as such cores are themselves unstable nuclei ( 9Li, 12Be) and therefore cannot be used as target in experimental studies.
We are at present discussing the differences between the
technique of projectile fragmentation and of transfer to the continuum in order to understand whether they would
convey the same structure information.
In 2004 up-to-date we have developped (with G. Blanchon, Ph.D. student) a method to study
projectile fragmentation in which final state interactions
with the core of origin are included. We have applied
it to the study of the low lying resonances of the
unbound 13Be nucleus,obtained from the fragmentation of
14Be and 14B. Important differences can be found in the n-12Be relative energy spectra,depending on the projectile used. Comparison with recent GSI experimental data is in progress.
We have also completed with A. Ibraheem, how was a Ph.D.
student now back in Egypt, the calculation of the optical
potential due to neutron breakup for an exotic projectile
on a heavy target.
Finally with A. Garcia-Camacho (postdoc)we have developed a very accurate numerical implementation
for nuclear and Coulomb breakup to all orders in the interactions and all multipoles in in the Coulomb potential.
Neutron breakup of heavy exotic nuclei has been calculated and a new type of experiments has been proposed. Calculations on proton breakup are in progress.
-Reaction Mechanisms and Structure of Rare Isotopes (PD)-
The Padova group has similar and complementary lines of
research as the Pisa group as far as reaction mechanisms are
concerned. However it has a special interest
for a somehow lower energy domain where fusion and the
coupling to breakup channels are particularly important.
Besides it is active in studying structure problems such as
1. Study of the pairing correlations in low-density
nuclear systems, as in the external part of halo nuclei
2. Microscopic estimate of inelastic excitation to
the low-lying continuum dipole strength via
microscopic continuum RPA calculations
3. Study of isospin symmetry in low- and high-spin
states in medium-mass N=Z nuclei up to 100Sn. Study of the
interplay of T=0 and T=1 pairing.
4. Study of nuclear structure with algebraic models. This line of research
is associated with the use of algebraic models, as the Interacting Boson
Model or its variations, to describe different aspects of nuclear spectra.
Our traditional approach is based on the use of the concept of boson
intrinsic state. In this framework we will study the new symmetries E(5)
and X(5) associated with phase transitions
and individuate mass region far from stability where such critical
points may occur.
5. Study of the role of continuum-countinuum
coupling in the break-up of weakly-bound nuclei.
The research activity of the Padova group has covered several arguments, reaching important results:
1. In collaboration with the experimental group of the Legnaro National Laboratory (LNL), we have analyzed data obtained with the gamma-ray spectrometer Clara combined with the large-acceptance mass spectrometer Prisma. In particular, with the reaction 64Ni+238U, neutron-rich isotopes around mass A=60 have been populated in multinucleon-transfer and deep-inelastic collisions. The data has been interpreted in terms of shell model and algebraic methosd.
The nucleus 58Cr has been observed for the first time up to spin J=8 and it was found that the yrast sequence follows that expected for a nucleus in the critical point of the shape phase transition rom a spherical vibrator ($U(5)$) to a $\gamma$-soft rotor ($O(6)$). This can be described by the E(5) dynamical symmetry and for the first time, in the same physical system, large scale shell-model calculations in the full fp shell have been performed and compared to the E(5) analytical model results and to the Interacting Boson Model. All theoretical results are in excellent agreement with data. The publication is in preparation. To further check the realization of the dynamical symmetry in 58Cr, an experiment has been proposed at LNL to measure the lifetime of the states with a new experimental technique. The experiment will be performed in the second part of this year.
Other neutron-rich nuclei in this mass region have been identified in the reaction and described with large scale shell model calculations performed by the Padova group. For the next year we plan to continue with this line studying other isotopes in this mass region to look for any other shape development as a function of neutron excess.
2. The study of the isospin symmetry in mirror nuclei of mass A=30-40 has been done, using new experimental data obtained at LNL with the expectrometer GASP. In particular, the mirror nuclei with mass A=35 have been been studied at high spin and interpreted by our group in terms of shell model calculations in the sd+fp valence space. Interesingly, an asymetry has been observed in E1 transitions which indicates isospin mixing of the states. An important role of the relativitic electromagnetic spin-orbit coupling has been observed which causes very large Coulomb energy differences.
A review article on Coulomb energy differences is in preparation (M. Bentley and S.M. Lenzi) to be published next year in Prog. Part. and Nucl. Phys. For the next year we plan to extend these studies to larger masses, in particular, to study the T=1 triplet A=58, for which an experiment is approved late this year at Ganil (France). Shell model calculations will be used for these studies.
We have recently studied a model to treat, in a simple way, break-up reactions of light weakly-bound nuclei on heavy targets, based on the assumption of cluster structure of the projectile. This study was motivated on one side by the need to have a simple and reliable tool for analysing experimental data on break-up reactions (in particular in connection with various measurements on 6Li and 7Li at the LNL) and on the other side to better understand the dynamics
of weakly-bound probes. To complement these studies we are now involved in a preliminary evaluation of one- and two-particle transfer form factors with the aim to gain some understanding of transfer phenomena in the
weakly-bound regime.
Collaborazioni in atto:
- E.E.
-Isospin Dynamics in Reactions with Exotic Beams (LNS)-
Two teams are active at the LNS. One is working in the energy range from the Coulomb barrier (Tandem) to the Fermi energies (Superconducting Cyclotron). Our main motivation is to extract physics information on the isovector channel of the nuclear interaction in the medium from dissipative collisions in this energy range using the already available stable exotic ions and in perspective the new radioactive facilties. We have developed very reliable microscopic transport models, in a extended mean field frame, for the simulations of the reaction dynamics in order to check the connection between the tested effective interactions and the experiments, in particular for the isospin degree of freedom. This work is of interest for the understanding of the physics behind the reaction mechanisms and for the selection of observables most sensitive to different features of the nuclear interaction. Moreoever we have a more general theoretical activity on the isospin dynamics in nuclear liquid-gas phase transitions. New instabilities have been evidenced with a different "concentration" between the gas and cluster phases, leading to the Isospin Distillation effects recently observed in experiments. A quantitative analysis can give direct
information on the density dependence of the symmetry term for dilute asymmetric matter, i.e. around and below saturation. We remind the poor knowledge of the isovector part of the nuclear effective interaction at low densities, which is actually of large interest even for structure calculations of drip line nuclei.
The main results obtained in the last year are related to:
1) Isospin transport in low energy dissipative collisions:
the link between diffusion and drift of neutron excess
and the structure of the iso-vector interaction.
2) Isospin in Nuclear fragmentation: determination of
fragment isospin observables, such as isotopic distributions, and correlations with kinematical properties.
-Finite Nuclear Systems in Brueckner Theory(LNS)-
The second team at LNS is interested in relating nuclear properties to elementary interactions between nucleons and to build up an energy density functional starting from a more fundamental level than the present phenomenological energy functionals of non-relativistic mean field or RMF. This can be achieved because of the familiarity of the applicant with the Brueckner theory in infinite nuclear matter including 2-body and 3-body forces. It has been shown that the inclusion of 3-body forces in the Brueckner theory is necessary for obtaining the correct saturation point of nuclear matter and going away from the so-called Coester line. From the results of infinite matter one will construct an energy density functional which can give the same results in nuclear matter and also can be used in finite nuclei. This is quite similar to th energy functional method of atomic physics based on ab initio calculations of the homogeneous electron gas and the local
density approximation (LDA). This nuclear energy functional should be trustable away from the stability region since no adjustment will be made to reproduce the properties of stable nuclei, contrarily to phenomenological energy functionals whose extrapolations can be questionable.
The proposed method is a simpler alternative than direct Brueckner calculations of finite systems. It also allows for studies of excitations of nuclei, within RPA-type of calculations built on top of the mean field ground state. This is again in the same spirit as the time-dependent LDA (TDLDA) method which has proved very successful in atomic cluster physics. The main objectives of the project are:
BHF calculations of asymmetric and polarized matter. Construction of the energy functional.
Calculations of density gradient contributions to the
effective force (surface and spin-orbit).
Ground states of finite nuclei.
Excitations of finite nuclei.
Neutron star crust |
Istituto Nazionale di Fisica Nucleare - Piazza dei Caprettari, 70 - 00186 Roma
tel. +39 066840031 - fax +39 0668307924 - email: presidenza@presid.infn.it
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