Volume 53, Issue 3
September 1999, pages 363-652
pp 363-364 September 1999
pp 365-380 September 1999 Nuclear Structure And Dynamics
The investigation of direct reactions with exotic beams in inverse kinematics gives access to a wide field of nuclear structure studies in the region far off stability. The basic concept and the methods involved are briefly discussed. The present contribution will focus on the investigation of light neutron-rich halo nuclei. Such nuclei reveal a new type of nuclear structure, namely an extended neutron distribution surrounding a nuclear core. An overview on this phenomenon, and on the various methods which gave first evidence and qualitative confirmation of our present picture of halo nuclei, is given.
To obtain more quantitative information on the radial shape of halo nuclei, elastic proton scattering on neutron-rich light nuclei at intermediate energies was recently investigated for the first time. This method is demonstrated to be an effective means for studying the nuclear matter distributions of such nuclei. The results on the nuclear matter radii of 6He and 8He, the deduced nuclear matter density distributions, and the significance of the data on the halo structure is discussed. The present data allow also a sensitive test of theoretical model calculations on the structure of neutron-rich helium isotopes. A few examples are presented.
The investigation of few-nucleon transfer reactions in inverse kinematics may provide new and complementary information on nuclear structure, as well as astrophysical questions. The physics motivation and the experimental concept for such experiments, to be performed due to momentum matching reasons at low incident energies around 5–20 MeV/u at the new generation low energy radioactive beam facilities SPIRAL, PIAFE, etc., is briefly discussed.
pp 381-393 September 1999 Nuclear Structure And Dynamics
Evaporation of alpha particles in heavy ion reactions followed by fusion has proved to be a powerful probe for the properties of emitting nuclei. Detailed experimental data and different model calculations allow us to probe whether the foundation of the statistical model holds for the compound nucleus populated in these reactions. It has been observed that in the case of composite nuclei at moderate energies and angular momenta, evaporation spectra are well explained in terms of the standard statistical model CASCADE code employing optical model transmission coefficients in the description of particle evaporation. However, it has been observed that experimental particle spectra from heavy ion induced fusion reactions are no longer consistent with the predictions of such models. It has been predicted by some authors that in these systems the emission barriers are lower than those expected from optical model transmission coefficients calculated for the respective inverse absorption channels. Some authors claim that these spectra may be well explained in terms of the statistical model incorporating only spin dependent level density and without lowering the emission barriers. The field is not yet free from the controversies. Furthermore, the assumption of the very short formation time in statistical model analysis is one extreme of the general evolution process which in fact is a continuous relaxation process, leading to the composite system from the entrance channel to the equilibrated configuration. Recent dynamical description of heavy ion collisions do not support this assumption in many cases. In symmetric entrance channels and for collisions where centre of mass energy is well above the Coulomb barrier, formation time can be even larger than decay time of the resulting composite system. In such cases realistic approach will be to couple the dynamical evolution of the intrinsic excitation of the composite system to a time dependent statistical model calculation. The above question has been addressed in the light of the alpha particle spectra taken in coincidence with the evaporation residues for the asymmetric 28Si+51V and the symmetric 28Si+27Al systems. The experimental data have been interpreted in the framework of dynamical trajectory model calculations.
pp 395-404 September 1999 Nuclear Structure And Dynamics
The transition of the level density parameter aoff from the low excitation energy value aoff=A/8 MeV−1 to the Fermi gas value aFG ∼ A/15 MeV−1 was discovered a few years ago studying particle spectra evaporated from hot compound systems of A∼ 160. A number of experiments have been recently performed to confirm the earlier findings and extend the investigation to other mass regions and to higher excitation energies. Furthermore, precision coincidence experiments have been done in the lead region in which evaporation residues are tagged by low energy gammarays. Those experiments open the possibility of a detailed study of the level densities in nuclei where the shell effects are important.
pp 405-405 September 1999 Nuclear Structure And Dynamics
With the advent of medium and large gamma detector arrays, it is now possible to look at nuclear structure at high rotational forces. The role of pairing correlations and their eventual breakdown, along with the shell effects have showed us the interesting physics for nuclei at high spins — superdeformation, shape co-existence, yrast traps, alignments and their dramatic effects on nuclear structure and so on. Nuclear structure studies have recently become even more exciting, due to efforts and possibilities to reach nuclei far off from the stability valley. Coupling of gamma ray arrays with ‘filters’, like neutron wall, charged particle detector array, gamma ray total energy and multiplicity castles, conversion electron spectrometers etc gives a great handle to study nuclei produced online with ‘low’ cross-sections. Recently we studied, nuclei in mass region 80 using an array of 8 germanium detectors in conjunction with the recoil mass analyser, HIRA at the Nuclear Science Centre and, most unexpectedly came across the phenomenon of identical bands, with two quasi-particle difference. The discovery of magnetic rotation is another highlight. Our study of light In nucleus, 107In brought us face to face with the ‘dipole’ bands. I plan to discuss some of these aspects. There is also an immensely important development — that of the ‘radioactive ion beams’. The availability of RIB, will probably very dramatically influence our ‘conventional’ concept of nuclear structure. The exotic shapes of these exotic nuclei and some of their expected properties will also be touched upon.
pp 407-407 September 1999 Nuclear Structure And Dynamics
In the present talk I will discuss some ‘rare’ aspects of the E2 band structures and the novel features concerning the dipole bands in this mass region. Reliable and accurate lifetimes have been measured using coincidence recoil distance method. The results of 129,130Ba will be discussed. In contrast to the predictions of the tilted axis cranking model, the dipole bands in Sb-Xe-Ba nuclei can be nicely described as high-K prolate bands. New data from multi-detector arrays has established extended bands structure, their decay to low lying states have been established and the angular correlation supports the predominant, ΔI=1 character. Finally the sensitive measures, i.e. B(M1) rates of the tilted axis model are compared with the high-K formula based on 1-dim cranking model.
pp 409-418 September 1999 Nuclear Structure And Dynamics
The fusion-fission reaction mechanism leading to the massive nucleus formation is studied. We investigate the superheavy nucleus formation in heavy-ion induced reactions by analysing the evaporation residue (ER) production in order to study the fusion dynamics and the decay properties of nuclei close to the stability island at Z=114. We consider the 61Ni+208Pb, 48Ca+238U and 48Ca+244Pu reactions that lead to the Z=110, 112 and 114 superheavy elements respectively.
By using the dinuclear system (DNS) concept of the two interacting nuclei we calculate the quasifission-fusion competition in the entrance channel and the fission-evaporation competition along the de-excitation cascade of the compound nucleus. The dynamics of the entrance channel allows us to determine the beam energy window which is favorable to the fusion, while the dynamic evolution of the compound nucleus on the shell correction to the fission barrier and the dissipative effects influence the fission-evaporation competition in order to obtain the residue nuclei from the superheavy nucleus formation. We also calculate the τn/τtot ratio at each step of the de-excitation cascade of the compound nucleus and we present a systematics of τn/τtot (at first step of the cascade) for many reactions that lead to nuclei with Z=102–114.
pp 419-424 September 1999 Nuclear Structure And Dynamics
The excitation function (EF) data for dissipative processes in 19F + 27Al system in the incident energy interval from 113.5 to 130 MeV are used to obtain the dependence of the charge variance and of the interaction time as a function of the incident energy. Fluctuations are observed in the EFs of both these secondary observables. Their correlation is supported by a mechanism based on stochastic exchange of nucleons.
pp 425-429 September 1999 Nuclear Structure And Dynamics
The lifetimes of the states of −ve parity 2-qp bands of N=40 nuclei 74Se and 76Kr were measured. The transition strengths and quadrupole moments, obtained from the lifetimes, show a large collectivity of such bands in both the nuclei. The alignment frequencies were calculated from Woods-Saxon cranking model. Previously suggested quasi proton nature of band 5 and 6 of 71Se were argued to be based on quasi neutron excitations. The total Routhian surface calculations suggest triaxial shapes with large +ve and −ve values of triaxiality parameter γ after proton and neutron alignments in these bands respectively.
pp 431-435 September 1999 Nuclear Structure And Dynamics
Neutron-rich isotopes of Mo (Z=42) around A ⋍ 100 have been investigated within the formalisms of cranked Nilsson Strutinsky and CHFB, to study several interesting features of nuclear structure in this mass region. The total energy/routhian surfaces have been generated for the isotopes of Mo ranging from A ⋍ 96 − 112, as a function of deformation (β2 and γ) for ground state and higher angular momentum states. Results of calculations using two different formalisms have been compared and combined to have a better understanding of the underlying mechanism of shape evolution.
pp 437-441 September 1999 Nuclear Structure And Dynamics
The characteristics of the dissipative processes in the collisions of light heavy ion systems at incident energies below 10 MeV/nucleon have been studied. The correlations between different experimental observables show similar trends as those known at much heavier systems and semiempirical relationships are established starting from assumptions on the nature of the microscopic mechanisms. The charge equilibration process in light systems is also studied.
pp 443-446 September 1999 Nuclear Structure And Dynamics
For studying cluster radioactivity in the actinide region as well as trans-tin region two types of models are used: the pre-cluster formation model and the unified fission model. In the case of the actinide region, the cluster-like shapes are preferred for very high asymmetry while fissioning shapes are more suitable for less asymmetry and symmetry (the line of demarcation being around Ac=31). In this work this line of demarcation is studied in the case of the trans-tin region. The results of this study show that the transition from cluster mode to fission mode takes place at Ac=16.
pp 447-451 September 1999 Nuclear Structure And Dynamics
High-spin states in 135,136Ba, 136La, 137Ce, 139Nd were populated following (HI, χn) reactions and subsequent radiation was studied using in-beam γ-ray spectroscopy methods. Level schemes with new states belonging to the above mentioned nuclei are given.
These nuclei situated near N=80 have been analysed within the framework of the interacting-boson model (IBM), applied to the description of even-even, odd-even and odd-odd nuclei to calculate excitation energies and electromagnetic properties for the above mentioned nuclei.
pp 453-456 September 1999 Nuclear Structure And Dynamics
The incomplete fusion, onset of multifragmentation and vaporization is studied in Ca-Ca collisions at bombarding energies between 20–1000 A MeV and at impact parameters between b=0 to bmax using quantum molecular dynamics model. We find incomplete fusion events at E/A=20 MeV. The light mass fragment production at a given incident energy does not show any rise and fall with a change in the impact parameter. Whereas, the IMF production at higher energies (≥ 150 A MeV) has a clear rise and fall.
pp 457-461 September 1999 Nuclear Structure And Dynamics
High spin states of 72,73,74Se nuclei are discussed using calculations from the cranked Nilsson Strutinsky method with tuning to fixed spins. The low spin anomaly in the yrast bands of these nuclei is interpreted in a rotational co-existence picture. High K rotational isomers are proposed for Iπ=4+ in 72Se and 6+ in 74Se.
pp 463-468 September 1999 Nuclear Structure And Dynamics
Nearly-spherical nuclei in three mass regions have recently been observed to exhibit rotational-like features. We have identified almost 80 such bands; largest number (43) lie in the lead region. Most of these bands are assigned oblate multi-quasiparticle configurations. Their interpretation in terms of ‘magnetic rotation’ does not allow for signature splitting in these bands. We have however found signature splitting as well as signature inversion in many bands. We apply the two-quasiparticle plus rotor model to understand the occurrence of signature splitting vis-a-vis the role of ‘shears mechanism’ in these bands.
pp 469-472 September 1999 Nuclear Structure And Dynamics
We examine the possible role of electron-capture on the thermally populated first 2+ excited state of 44Ti in hot astrophysical environments pertaining to post explosive nucleosynthesis supernova debris. We find in a simple schematic model that the astrophysical weak interaction rate for electron-capture decay of 44Ti can depend considerably on temperature and hence on time. We propose a time varying decay rate for the evolving supernova debris and demonstrate its consequence for the 44Ti mass yield of the supernova Cas A, observed through the measured 1.157 MeV γ-ray flux from the electron-capture decay of 44Ti.
pp 473-481 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
In this talk I will discuss transfer reactions between heavy ions with special emphasis to multinucleon transfer. I will use a semi-classical model that incorporates, in an independent description, both inelastic excitation to collective states and one-particle transfer channels. The importance of evaporation in determining the isotope distribution of the final yields of the reaction will be discussed.
pp 483-483 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Considerable interest has been devoted to fusion reactions between light heavy ions specially between weakly bound ones, due to the anomalous decrease of the fusion cross sections when compared to the total reaction cross section in the energy region around the barrier [1–4].
While the exact nature of the process responsible for the fusion cross section limitation at barrier energies is still unclear, this study shows an inhibition of the yield as the system mass decreases, resulting from the progressive increase of the barrier height and decrease of the effective barrier radius . Furthermore, extensive efforts have been made recently in the study of energy-damped binary yields from light heavy-ion collisions [2,4]. Based on the substantial amount of data accumulated so far, it is now generally accepted and supported by the transition state model , that the observed yields arise mostly from a fusion-fission process. Data on complete fusion, fusion-fission and ‘elastic fission’ for the 9Be, 10,11B+10,11B; 16,17,18O + 10,11B; 19F+12C; 6,7Li+9Be, 12C reactions among others, are presented. For the loosely bound nuclei it was found that the severe fusion cross section limitation is due to a low survival probability of the weakly bound nuclei until the instant of the collision .
pp 485-494 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Systematic studies of heavy-ion induced fission reactions at near-barrier energies carried out in the last decade have brought out many interesting aspects of fission process in general. The recent experimental findings which show dependence of fission fragment angular distributions on entrance channel, shape, size and spin of the interacting nuclei and shell closure of the intermediate compound nucleus are summarised in the present paper.
pp 495-504 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Through precision measurements of fusion cross sections at energies close to the Coulomb barrier and through the application of the method of “experimental barrier distributions” which these permit, many recent advances have been made in our understanding of the dynamical processes occurring during a heavy-ion collision. It is now clear that the target and projectile reach one another in superpositions of states which correspond to different orientations for rotational nuclei or to different induced deformations for vibrational nuclei. The creation of a neck of neutron matter has also long been postulated and by studying the isotopic dependence of the fusion reaction, some recent results in the 10Ca+90,96Zr systems appear to confirm this result. For large Z1Z2 a type of extra-push effect can arise from the same inelastic entrance-channel effects which enhance the fusion of lighter systems, though this will be absent in cases where the enhancement arises from neutron transfers.
The existence of different barriers will of course influence all other reaction channels. Fusion simply allows one to visualise the barriers most easily, since for this process, the total cross section is an incoherent sum of the contributions from all relevant eigenchannels. Some effects in other channels have already been observed. Other possible effects will be discussed. These include; the exploitation of the lowest-energy barrier to produce exotic evaporation residues and strongly deformed high-spin states at low excitation energy.
pp 505-512 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Preliminary data on the fusion of 36S+96Zr are reported; the excitation function near the barrier is intermediate between those of 40Ca+90,96Zr. The peculiar role of the strong 3− octupole vibration of 96Zr is pointed out, in addition to the couplings to neutron transfer channels with positive Q-values. Recent data on 40Ca+124Sn are also shown; for that system the fusion barrier distribution is wide without separated peaks, similar to the case of 40Ca+96Zr. Simplified coupled-channel calculations have been performed, including surface vibrations and sequential neutron pick-up channels, with form factors that fit the single- and multi-nucleon transfer data for 40Ca+124Sn. A good agreement with the data is found.
pp 513-520 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
The availability of precisely measured fusion excitation functions have allowed the determination of experimental fusion barrier distributions. This concept is utilised in 9Be+208Pb reaction, to reliably predict the expected complete fusion cross-sections. However, the measured cross-sections are found to be only 68% of those predicted. The large cross-sections observed for incomplete fusion products support the interpretation that this suppression of fusion is caused by 9Be breaking up into charged fragments before reaching the fusion barrier.
pp 521-528 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Study of heavy ion induced fusion-fission reactions at near and below barrier energies has attracted a great deal of attention in recent years, due to the observations of anomalous features in the fragment angular distributions for many target-projectile systems. Additionally there are also measurements of the fragment spin distributions and time-scales of the fusion-fission reactions, which have provided important information on the dynamics of these processes. In the present paper, the emphasis would be to highlight some of the recent experimental findings and their implications on the dynamics of the fusion-fission reactions in heavy ion collisions at near and above barrier energies.
pp 529-533 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Large enhancements have been observed in the sub-barrier fusion cross sections for Ti+Ni systems in our previous studies. Coupled channel calculations incorporating couplings to 2+ and 3− states failed to explain these enhancements completely. A possibilty of transfer channels contributing to the residual enhancements had been suggested. In order to investigate the role of relevant transfer channels, measurements of one- and two-nucleon transfer were carried out for 46,48Ti+61Ni systems. The present paper gives the results of these studies.
pp 535-539 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
The ground state and excited state transfer yields for the 2-neutron pickup channel in the 28Si+68Zn system have been measured explicitly. The recoil mass separator at the nuclear Science Centre, New Delhi was used for the measurement. A NaI(T1) detector was used for detecting the deexcitation γ’s from the transfer products. The kinematic coincidence technique was employed for the transfer measurement. Simplified coupled channels calculations show that out of all transfer channels the major contribution to the sub-barrier enhancement comes from the ground state 2 neutron pickup channel with a ground state Q-value of+1.83 MeV.
pp 541-544 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
The inclusive and exclusive measurements were carried out for 7Li projectile breakup on 27Al target at 48 MeV. In the inclusive data we have observed a broad peak around the beam velocity for alphas and tritons. The exclusive data for alpha-triton coincidences show good agreement with the post-form DWBA theory of breakup reactions.
pp 545-548 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
A new asymmetric parabolic effective fusion barrier model for heavy ion fusion is developed.
pp 549-552 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
In this paper, we report our measurements of back-angle oxygen and carbon particle yields from 16O+89Y, 12C+93Nb reactions forming the same compound nucleus 105Ag at the same excitation energy and spin distribution. We find anomalously large oxygen yield and entrance channel dependence at high excitation energies from 16O+89Y reaction implying formation of a dinuclear orbiting complex. Possible connection between nuclear orbiting and fast fission is also discussed.
pp 553-557 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Measurement of elastic and quasielastic reaction cross sections were done in 16O + 118Sn system at two different energies above the barrier. Attempts are being made to understand the results in the framework of coupled reaction channel model.
pp 559-562 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
α angular correlations were calculated using the coupled channels formalism for 16,18O break-up reactions. Prominent Z dependency was observed in both the cases. 16O break-up shows distinctively different behavior with different target materials.
pp 563-566 September 1999 Nuclear Reactions At Near And Sub-Barrier Energies
Time of flight and energy of fission fragments were measured using pulsed beam. Fission fragment mass and energy integrated angular distributions were extracted. Fission fragment anisotropy was explained in the framework of saddle point model.
pp 567-575 September 1999 Nuclei Act Extreme Conditions
The discovery of neutron rich isotopes of the lightest elements on the neutron drip line exhibiting a halo structure has opened up new vistas in research activities. The novel structural features associated with the halo phenomena have been the subject for extensive theoretical and experimental investigations in recent times. In this talk, I propose to present a broad overview of the recent developments in this field, bringing out the striking features which show that a large number oflight nuclei near the neutron drip line are characterized by a clear separation between a ‘normal’ core nucleus and a loosely bound low density veil of neutrons. Specifically, the two neutron halos offer a natural premises, from a theoretical standpoint, to employ three body techniques for studying their detailed structural properties. A considerable part of the talk will be devoted to report and highlight the results on a number of light halo nuclei such as 11Li, 11Be, 19B and 22C on which we have been carrying out investigations employing a simple but realistic three body model. These three body systems which have been termed as ‘Borromean’ (i.e while three body systems are bound, the corresponding binary subsystems on the other hand are unbound) are characterized by large spacial extension and very low separation energy of the neutron. They are, therefore, ideally suited for exploring the possibility of the existence of Efimov states in two neutron halo nuclei. We have recently carried out the three body analyses to predict the possibility of the occurrence of such states on which experimental work at various laboratories is underway.
pp 577-584 September 1999 Nuclei Act Extreme Conditions
The rare nuclear processes of cluster radioactivity, cold fission and cold fusion, studied on the basis of the quantum mechanical fragmentation theory (QMFT), are reviewed. This theory was advanced as early as in 1974–75 by the author and collaborators, first at Frankfurt and then developed both at Frankfurt and Chandigarh. The QMFT predicted all the three phenomena to occur most probable as cold processes, prior to their experimental observations. The success of experiments with respect to the QMFT and some new results are presented as predictions of this theory for future experiments.
pp 585-594 September 1999 Nuclei Act Extreme Conditions
S-factors for direct capture reactions can be found at astrophysical energies from asymptotic normalization coefficients which provide the normalization of the tail of the overlap function. For example the overlap for 8B → 7Be+p defines the S-factor for 7Be (p, γ)8B. Peripheral transfer reactions offer a technique to determine these asymptotic normalization coefficients. As a test of the technique, the 16O(3He, d)17F reaction has been used to determine asymptotic normalization coefficients for transitions to the ground and first excited states of 17F. The S-factors for 16O(p, γ)17F calculated from these 17F → 16O+p asymptotic normalization coefficients are found to be in very good agreement with recent measurements. Following the same technique, the 10B(7Be, 8B)9Be and 14N(7Be, 8B)13C reactions have been used to measure the asymptotic normalization coefficient for 7Be(p, γ)8B. This result provides an indirect determination of S17(0).
pp 595-606 September 1999 Nuclei Act Extreme Conditions
The calculated rate of events in some of the existing solar neutrino detectors is directly proportional to the rate of the 7Be(p, γ)8B reaction measured in the laboratory at low energies. However, the low-energy cross sections of this reaction are quite uncertain as various measurements differ from each other by 30–40%. The Coulomb dissociation process which reverses the radiative capture by the dissociation of 8B in the Coulomb field of a target, provides an alternate way of accessing this reaction. While this method has several advantages (like large breakup cross sections and flexibility in the kinematics), the difficulties arise from the possible interference by the nuclear interactions, uncertainties in the contributions of the various multipoles and the higher order effects, which should be considered carefully. We review the progress made so far in the experimental measurements and theoretical analysis of the breakup of 8B and discuss the current status of the low-energy cross sections (or the astrophysical S-factor) of the 7Be(p, γ)8B reaction extracted therefrom. The future directions of the experimental and theoretical investigations are also suggested.
pp 607-607 September 1999 Nuclei Act Extreme Conditions
This paper will review the dramatic increase in our knowledge of one and two proton unbound nuclei  such as recoil decay tagging  are revealing unique insights into the structure of nuclei beyond the proton drip-line. These studies of excited states provide complementary information to proton radioactivity studies, particularly regarding the role of deformation . Radioactive beams are being used to study two-proton unbound resonances and to study explosive nuclear astrophysical reactions in the region of the proton drip-line.
pp 609-618 September 1999 Nuclei Act Extreme Conditions
T Radon H Geissel F Attallah K Beckert F Bosch A Dolinskiy H Eickhoff M Falch B Franczak B Franzke Y Fujita M Hausmann M Hellström F Herfurth Th Kerscher O Klepper H-J Kluge C Kozhuharov Yu Litvinov KEG Löbner G Münzenberg F Nolden Yu Novikov Z Patyk W Quint H Reich C Scheidenberger B Schlitt J Stadlmann M Steck K Sümmerer L Vermeeren M Winkler Th Winkler H Wollnik
Two experimental methods of measuring masses of exotic nuclei in the storage ring ESR are presented. Bismuth and nickel fragments were produced via projectile fragmentation, separated and investigated with the combination of the fragment separator FRS and the ESR: (i) Direct mass measurements of relativistic projectile fragments were performed using Schottky mass spectrometry (SMS), i.e., exotic nuclei were stored and cooled in the ESR. Applying electron cooling, the relative velocity spread of circulating low intensity beams can be reduced below 10−6. Under this condition a mass resolving power of up to m/Δm=6.5·105 (FWHM) was achieved in a recent measurement. Previously unknown masses of more than 100 neutron-deficient isotopes have been measured in the range of 60≤Z≤84. Using known Qα values the area of known masses could be extended to more exotic nuclei and to higher proton numbers. The results are compared with mass models and extrapolations of experimental values. In a second experiment with 209Bi projectiles the area of the measured masses was extended to lower proton numbers. Due to various improvements at the ESR the precision of the measurements could be raised. (ii) Exotic nuclei with half-lives shorter than the time needed for SMS (present limit: T1/2 ≈ 5 sec) can be investigated by time-of-flight measurements whereby the ESR is operated in the isochronous mode. This novel experimental technique has been successfully applied in first measurements with nickel and neon fragments where a mass resolving power of m/Δm=1.5·105 (FWHM) was achieved.
pp 619-630 September 1999 Nuclei Act Extreme Conditions
The dinuclear system concept of complete fusion of nuclei has been applied to the analysis of superheavy elements synthesis. The optimal excitation energy of compound nuclei and production cross sections in the cold synthesis of heavy elements with charge Z=102–112 have been calculated. The possibility of synthesizing the element with magic number Z=114 in cold and hot fusion reactions has been considered.
pp 631-631 September 1999 Nuclei Act Extreme Conditions
The proton drip line defines one of the fundamental limits to nuclear stability. Nuclei lying beyond this line are energetically unbound to the emission of a constituent proton from their ground states.
For near-spherical nuclei in the region of the drip line between Z=69 (Tm) and Z=81 (T1), proton decay transition rates have been shown to be well reproduced by WKB calculations using spectroscopic factors derived from a low-seniority shell model calculation . Another approach using spectroscopic factors obtained from the independent quasiparticle approximation has also proved successful in this region . These interpretations have allowed the extraction of nuclear structure information from nuclei well beyond the proton drip line.
The rare-earth proton emitters 141Ho and 131Eu have recently been observed , and their decay rates can only be explained by assuming large deformation for these nuclei. In addition to providing information on the wavefunctions and deformations of these nuclei, these results offer the opportunity to study the phenomenon of quantum mechanical tunneling through a deformed potential barrier.
pp 633-633 September 1999 Nuclei Act Extreme Conditions
We shall report on the recently observed dependence of the lifetime of the first excited state in 125Te on the ionic charge state. Then we shall give an interpretation of the dependence of the half-life in terms of a new type of nuclear internal conversion without emission of the electron into the continuum of electron energies. We have named this process internal conversion between bound atomic states or BIC. The resonant character of the BIC will be established and the main parameters governing the decay process will be discussed [1–3].
Finally the results of a recent experiment performed at the GANIL accelerator attempting to measure directly the value of the internal conversion coefficient associated with BIC in 125Te ions with charge states ranging between 44+ and 48+ will be given.
In conclusion we shall discuss the relation between the BIC and nuclear excitation by electron transition, NEET, in the excitation of some nuclear isomeric states.
pp 635-635 September 1999 Nuclei Act Extreme Conditions
Cluster radioactivity is a process by which nuclei equal and heavier than the α-particle is emitted spontaneously. The clusters usually emitted in this process are the α-particle, carbon, oxygen, neon, magnesium, silicon etc. When the mass of the cluster becomes comparable with the mass of the daughter, symmetric fission takes place. Thus the cluster radioactivity is an intermediate process between the well known α-decay and the spontaneous fission. In earlier years such cluster radioactivity was found mostly in actinide nuclei like radium, uranium etc. Very recently it has been predicted that such decays are possible in a new region around 111Ba. There has been an exciting experimental detection of the emission of 12C from 111Ba leading to 102Sn, which is attracting a lot of attention recently.
To study the phenomenon of cluster radioactivity there are various theoretical models in vogue. The existing models generally fall under two categories: the unified fission model (UFM) and the preformed cluster model (PCM). The physics of the UFM and the PCM are completely different. The UFM considers cluster radioactivity simply as a barrier penetration phenomenon in between the fission and the α-decay without worrying about the cluster being or not being preformed in the parent nucleus. In the PCM clusters are assumed to be preborn in a parent nucleus before they could penetrate the potential barrier with a given Q-value. The basic assumption of the UFM is that heavy clusters as well as the α-particle have equal probability of being preformed. In PCM, clusters of different sizes have different probabilities of their being preformed in the parent nucleus.
We have developed three fission models during the last decade using the cubic potential for the pre-scission region. The use of these models in the study of cluster radioactivity in both the actinide and barium regions will be discussed in this talk in comparison with the other existing theories.
pp 637-646 September 1999 Nuclei Act Extreme Conditions
Nuclear fission from excitation energies around 11 MeV was studied at GSI, Darmstadt for 76 neutron-deficient actinides and pre-actinides by use of relativistic secondary beams. The characteristics of multimodal fission of nuclei around 226Th are systematically investigated and related to the influence of shell effects on the potential-energy and on the level density between saddle point and scission. A systematic view on the large number of elemental yields measured gave rise to a new interpretation of the enhanced production of even elements in nuclear fission and allowed for a new understanding of pair breaking in fission.
pp 647-652 September 1999 Nuclei Act Extreme Conditions
Thermal fluctuations in angular momentum due to excitation is investigated. Shape changes or structural rearrangement are observed as a consequence of fluctuation in second moment of spin. The uncertainty in angular momentum is considerably enhanced due to thermal fluctuation and is strongly dependent on spin and structural changes.
Volume 93 | Issue 5
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