Comment on isospin mixing in nuclei

The central collisions of 70 GeV/c protons with Ag, Br and Pb nuclei have been studied, with the help of BR-2 photoemulsion and emulsion of the same constitution loaded with Pb nuclei. It is shown that the average multiplicity of produced particles (s-particles) increases slowly with increasing the atomic weight of the target nucleus. The ratio of the average multiplicity for these particles to the average charged particle multiplicity for protonproton interactions equals 3.2 forp-Ag, Br and 3.5 forp-Pb. The average multiplicities of particles from nuclear disintegration with energies for protons from 26 up to 400 MeV (g-particles) are 14.2±0.8 forp-Ag, Br and 23.0±1.0 forp-Pb corresponding to the emission of about half of all nucleons from the nucleus at the first rapid stage of interaction. The difference of thes-particle pseudorapidity distributions forp-Ag, Br, Pb andp-p interactions at pseudorapidities larger than 4.2 can be explained by successive interactions of the incident proton with nucleus nucleons.     

Escape of charged particles at stopping annihilation of antiprotons in the heavy nuclei of nuclear photoemulsion

“Rare” annihilation channels for antiprotons stopping on heavy (Ag, Br) nuclei of photoemulsion, have been sought; 4872 stops of antiprotons on photoemulsion nuclei are analysed. Events of formation and decay of the hyperfragment _Λ⁴H, escape of ⁸He and ⁸Li nuclei, one-prong stars with the mean range 79.5±5.1 μm of secondary slow “b” particles are found among the annihilation stars at capture on nuclei (Ag, Br). The lower limits for the production probability of _Λ⁴H and ⁸He, ⁸Li nuclei per antiproton stopping in the nuclei (Ag, Br) are

W_Λ⁴H2×10⁻⁴ and W_⁸He,⁸Li=(1.3±0.6)×10⁻³.

The branching ratio for the production of one-prong stars with the secondary “b” particles is at least (1.3±0.6)×10⁻³. Possible mechanisms for a production of these events in annihilation processes are considered.     

Experimental estimate of the diffractive-dissociation phenomenon following neutrino interaction with photoemulsion nuclei

The first experimental estimate of the diffractive-dissociation (DD) cross section for muon-neutrino interaction with nuclei is presented. Eleven events that satisfy the DD criteria (x _B<0.1, Δη>2) are found among 207 events induced by neutrino interactions with Ag and Br nuclei of nuclear photoemulsion and borrowed from the database of the E-564 experiment (FNAL). The ratio of the DD cross section to the total deep-inelastic cross section for the charged current is 0.29±0.09. Four of the 11 events proved to be candidates for events of coherent DD.     

Production and decay of heavy proton drip-line nuclei

Recent work at the NSCL using unusual isotopic beams to study nuclei along the proton drip-line up to A≈95 is presented. The most recent results were obtained by the fragmentation of a ¹⁰⁶Cd beam. Following identification, measurements of the decay properties of such exotic nuclei in specialized measurements of beta-decay half-lives of the stopped ions were undertaken. The results of the studies that focused on the waiting point nuclei ⁶⁵As and ⁶⁹Br in the astrophysical rp-process are also presented. Very recent measurements of the masses of some of these exotic nuclei with a direct time of flight technique are briefly described.     

Evidence for a nuclear phase transition in target nuclei after relativistic nuclear interactions

The degree of excitation of the emulsion target nuclei due to nuclear interactions of oxygen and sulfur projectiles at 200 GeV/nucleon incident energy has been investigated. Using the plausible assumption that the numberN _b of slow particles emitted from the struck target nucleus can be interpreted as a measure of the temperatureT of the residual nucleus, we have found that there exists a critical temperatureT _c of the excited target nucleus. For Ag and Br target nuclei this temperature corresponds to <N _b>≌12 and it is attained when the impact parameters are less than about 4 fm.     

Fragmentation and particle production in interactions of 10.6 GeV/N gold nuclei with hydrogen, light and heavy targets

We have investigated the interactions of 10.6 GeV/n Au in nuclear emulsion. Two methods of separating interactions into those with hydrogen, light (C,N,O) and heavy (Ag,Br) target nuclei were used, giving almost identical results, which strengthened our confidence in the correctness of these methods. We also measured the angular distributions of singly and multiply charged relativistic particles emitted from the interaction vertices and the charges of the multiply charged projectile fragments. The fragmentation of the projectile Au nuclei and of the target nuclei were analyzed. The multiparticle production was studied as a function of the mass of the target nucleus. The multiplicity and the pseudorapidity distributions of relativistic singly charged particles were compared with the predictions of the RQMD Model. Received: 22 April 1998 / Published online: 8 September 1998     

Symmetry in exotic nuclei

Symmetries have played an important role in the elucidation of the structure of nuclei and will continue to do so for exotic nuclei. As an example, an application of pseudo-SU(4) symmetry is discussed. It can be used as a starting point for a boson model that includes T = 0 as well as T = 1 bosons (IBM-4); applications are presented for N = Z nuclei from ⁵⁸Cu to ⁷⁰Br. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: isacker@ganil.fr     

Nuclear and Coulomb interactions in coherent fragmentation of relativistic nuclei in a photoemulsion

The role of the nuclear and Coulomb interactions in coherent fragmentation of relativistic nuclei is discussed with the cluster-model analysis of the experimental data on the fragmentation of ⁷Li (P = 3A GeV/c) via the ³H + ⁴He channel in photoemulsion used as an example. The calculated electromagnetic contribution of ～10% to the cross section is not in conflict with the photoemulsion data and the upper estimate of ～40% derived from the earlier measurements of total cross sections for fragmentation of light nuclei using the counter technique. The observed irregularities in the differential cross section for ⁷Li fragmentation with a separated Coulomb peak at a very small momentum transfer Q are ascribed to the overlap of nuclear diffraction patterns arising from light (C, N, O) and heavy (Ag, Br) photoemulsion nuclei. The predicted diffraction cross sections in the inelastic channel drastically differ from the usual shape of nuclear diffraction in an elastic channel. For pure targets, they have a shape of oscillations with a few peaks of comparable intensity and show strong dependence on the form of the surface nuclear density and radii of the intranuclear cluster and target nucleus. The probability for two-body clustering in ⁷Li is estimated at about 0.7.     

Are nuclei withN∼43 deformed?

The level schemes of⁷⁵Se and⁷⁹Kr have been established through gamma ray and conversion electron spectroscopy following⁷⁵As (p, nγ ^?e) and⁷⁹Br (p, nγe^?) reactions. The data on these nuclei and the nuclei in the neighbourhood of this mass region are discussed and evidence is presented to show that the nuclei withN～43 are deformed.     

Fission studies of highly excited nuclei

The binary fission process in Bi, Au, W, Sb, Ag and Ni induced by 1 GeV protons has been investigated. The energies, time-of-flight and angular correlations of coincident fragments were measured by using a double arm spectrometer permitting a direct mass determination. Mass distributions are found to change their shape for nuclei lighterW. The interaction of protons with nuclei followed by fission is described in the framework of the cascade-evaporation model and the statistical approach. The observed mass distributions are in a good agreement with the results of the calculation. The experimental results present evidence for existence of an instability region with respect to the mass asymmetry coordinate in nuclear fission.     

Isomer ratios for products of photonuclear reactions with middle-weight nuclei

The isomer ratios for nuclei ^104m,g Ag, ^110m,g In, ^108m,g In as products of the photonuclear reactions ¹⁰⁷Ag(γ, 3n)^104m,g Ag, ¹¹³In(γ, 3n)^110m,g In, ¹⁰⁹Ag(γ, 5n)^104m,g Ag, ¹¹⁵In)(γ, 5n)^110m,g In, ¹¹⁵In(γ, 7n)^108m,g In are obtained. The Bremsstrahlung of electrons with energies within 32–84 MeV from the LU-40 linear accelerator was used for the irradiation of targets. The energy resolution and mean current of the electron beam were ～1% and 5 μA, respectively. The induced activity was used to obtain experimental isomer ratios. An HPGe spectrometer with an energy resolution of 1.9 keV for ⁶⁰Co γ-line 1332 keV was used to acquire the instrumental γ-ray spectra of the activation products. A set of serial measurements with various acquisition times was performed. The cooling times varied from 5 seconds to several hours for Ag and up to tens of hours for In specimens. The obtained experimental isomer ratios are compared with the results from theoretical calculations.     

Inelastic interactions of fast nucleons with nuclei and fission of nuclei

The cross section for the fission of actinide nuclei that is induced by fast neutrons is considered as a fraction of the cross section for inelastic nucleon interaction with nuclei. In turn, inelastic nucleon interaction with a nucleus is treated as scattering on intranuclear nucleons. It is shown that this interaction model describes satisfactorily the cross section for the inelastic interaction of 60- to 2200-MeV nucleons for a broad set of nuclei and that the energy dependence of the cross section for the fission of actinide nuclei that is induced by 400- to 1000–MeV protons replicates the energy dependence of the cross section for inelastic interactions with respective nuclei. From the model used, it follows that the cross sections for proton-nucleus interactions exceed cross sections for respective neutron-nucleus interactions in the energy range extending up to 550 MeV; at higher energies neutron cross sections are larger than proton cross sections.     

Flow effects in 84Kr induced collisions in emulsion at 0.95 GeV per nucleon

Using emulsion detector the collective flow signals in inelastic interactions of ⁸⁴Kr nuclei with Ag(Br) at 950 MeV/nucleon are studied. A transverse momentum analysis is performed to determine the reaction plane. The bounce-off of spectator fragments is observed. In azimuthal distributions relative to the reaction plane squeeze-out and side-splash of participants are seen. Received: 27 September 1999     

Collective bounce-off phenomenon in139La+Ag(Br) reaction at 1.1 A GeV energy

Out of a total statistics of 896¹³⁹La+Ag(Br) interactions, 128 interactions having multiplicity of target fragments (Z⩾1)⩾8 and projectile fragments (Z⩾2)⩾4 have been selected. They correspond to quasi-peripheral interactions. Azimuthal angle correlation between sources of target fragments (TFs) and projectile fragments (PFs) shows the existence of bounce-off effect. Using data of La+Ag(Br) and⁸⁴Kr+Ag(Br) reactions it is shown that individual helium [Z=2, PFs] and heavier fragment [Z⩾3, PFs] show different emission characteristics. Further, a two prong correlation functionT(Φ _ij ) plotted for heavier fragments and helium fragments separately, indicates the possibility of existence of different physical conditions. This observation is supported by the different momentum widths of helium fragments and heavier fragments. From the momentum width data of Kr+Ag(Br) reactions normalized density comes out to be ≈4.7. Using quasi-elastic kinematics for the bounce-off nuclei, the excitation energy has been computed from the experimental data of flow angles. The strength of bounce-off seems to decrease with the increase of excitation energy or temperature.     

Mass distributions of fission fragments emitted by highly excited nuclei

The mass distributions of fission fragments emitted by highly excited nuclei have been investigated for the determination of some peculiarities of fission process. It has been shown that the fragment mass distributions obtained by the time-of-flight method change their character in the region of Ag, as expected in the liquid drop model (LDM). The cascade-evaporation model and the statistical method of Fong describe the experimental data with a good accuracy.     

Cluster structures in light nuclei

Clustering in light stable and unstable nuclei is discussed. After a brief review of the clustering in stable nuclei, we make a new prediction of the existence of the alpha cluster condensed states in ¹²C and ¹⁶O. Discussions of clustering in light unstable nuclei are made in the cases of Be and B isotopes up to the neutron dripline. Received: 1 May 2001 / Accepted: 4 December 2001     

Theoretical half-lives for the heaviest nuclei

Spontaneous-fission and alpha-decay half-lives are calculated for even-even nuclei with the atomic numberZ=104–110. The results reproduce rather well the existing experimental data for these nuclei and predict rather large total lifetimes for nuclei even heavier than observed up to now. The latter is mainly due to a deformed shell at the neutron numberN=162–164, obtained in calculations.     

Excited states of deformable nonaxial odd nuclei

Rotationally single-particle and vibrational excitations of deformable nonaxial odd nuclei are investigated with allowance for the interaction of collective and single-particle states. The ratios of excitation energies, of reduced probabilities of E2 transitions, and of quadrupole moments for deformed nonaxial odd nuclei are calculated up to high-spin states.     

Competition between strong and de-coupling in odd-odd mass nuclei

Competition of strong and decoupling is studied systematically for odd-odd mass nuclei as a function of the position of the Fermi surfacesε _F (π) andε _F (υ) and the deformationsβ andγ. As an example we use⁸⁰Br. For a prolate deformation the odd proton likes to decouple while the odd neutron couples strongly. This favours a ground state angular momentum of 5 to 6. The strong coupling is described by the admixture of ad _5/2 nucleon. Only for the neutrons the admixture is so appreciably that it affects the spectrum and improves the agreement between theory and experiment for⁸⁰Br.     

Nuclear fragmentation cross-sections of 400 A MeV 36Ar and 40Ar in collisions with light and heavy target nuclei

We have measured fragmentation cross-sections of Ar projectile nuclei at beam energy of 400 A MeV using experimental set-ups with plastic nuclear track detectors and different targets. In this paper total charge changing cross-sections and elemental fragmentation cross-sections for the production of fragments with charges ZF > or = 7 in interactions with H, C, Al, Cu, Ag and Pb target nuclei are presented. The dependence of the cross-sections on the fragment charge number and target charge number are discussed. The experimental results are compared to predictions of semi empirical cross-section models.