Electromagnetic dissociation of relativistic 8B nuclei in nuclear track emulsion

Experimental data on fragmentation channels in peripheral interactions of ⁸B nuclei in nuclear track emulsions are presented. A detailed analysis made it possible to justify selections of events of the electromagnetic-dissociation process ⁸B → ⁷Be + p and to estimate its cross section. Events of ¹⁰C peripheral dissociation that were observed in the same exposure are described.     

Fragmentation of relativistic nuclei in peripheral interactions in nuclear track emulsion

The technique of nuclear track emulsions is used to explore the fragmentation of light relativistic nuclei down to the most peripheral interactions: nuclear “white” stars. A complete pattern of the relativistic dissociation of a 8B nucleus with target fragment accompaniment is presented. Relativistic dissociation ⁹Be → 2α is explored using significant statistics, and a relative contribution of ⁸Be decays from 0+ and 2+ states is established. Target fragment accompaniments are shown for relativistic fragmentation ¹⁴N → 3He +H and ²²Ne → 5He. The leading role of the electromagnetic dissociation on heavy nuclei with respect to breakups on target protons is demonstrated in all these cases. It is possible to conclude that the peripheral dissociation of relativistic nuclei in nuclear track emulsion is a unique tool to study many-body systems composed of the lightest nuclei and nucleons in the energy scale relevant for nuclear astrophysics. The text was submitted by the authors in English.     

Peripheral fragmentation of 8B nuclei in nuclear emulsion at an energy of 1.2 GeV per nucleon

The results obtained by studying the charge topology of fragments produced in the peripheral dissociation of relativistic ⁸B nuclei in emulsion are presented. Fifty-five events of the peripheral dissociation of a ⁸B nucleus in events where there was no production of target-nucleus fragments and mesons (“white stars”) were selected. A leading contribution of the ⁸B → ⁷Be + p mode, which has the lowest energy threshold, was revealed on the basis of these events. Information about the branching ratios for dissociation modes characterized by a higher multiplicity was obtained. The dissociation of the ⁷Be core in ⁸B bears resemblance to the dissociation of a free ⁷Be nucleus. The transverse-momentum distributions of fragments originating from the ⁸B → ⁷Be + p dissociation mode were obtained. For these distributions, a small mean value of 〈P*_T〉 = 52 ± 5 MeV/c in the c.m. frame suggests a low binding energy of the outer proton in the ⁸B nucleus. An indication of a strong azimuthal correlation of the fragments ⁷Be and p was found.     

Peripheral interactions of relativistic 14N nuclei with emulsion nuclei

The results of investigation of the dissociation of the 2.86-A-GeV/c ¹⁴N nucleus in an emulsion are presented. The cross sections for various fragmentation channels are given. The invariant approach to analysis of fragmentation is used. The momentum and correlation characteristics of the α particles for the ¹⁴N → 3α + X channel in the laboratory system and c.m.s. of three α particles are examined. The results obtained for the ¹⁴N nucleus are compared with similar data for the ¹²C and ¹⁶O nuclei.     

Fragmentation of relativistic gold nuclei in nuclear emulsion

Basic data are presented on the parameters of fragmentation, multiplicities and angular spectra of low energetic targed associated particles in the interction of ¹⁹⁷Au nuclei of 10.7A GeV.     

Exposure of nuclear track emulsion to a mixed beam of relativistic 12N, 10C, and 7Be nuclei

A nuclear track emulsion was exposed to a mixed beam of relativistic ¹²N, ¹⁰C, and ⁷Be nuclei having a momentum of 2 GeV/c per nucleon. The beam was formed upon charge exchange processes involving ¹²C primary nuclei and their fragmentation. An analysis indicates that ¹⁰C nuclei are dominant in the beam and that ¹²N nuclei are present in it. The charge topology of relativistic fragments in the coherent dissociation of these nuclei is presented.     

Exposure of nuclear track emulsion to 8He nuclei at the ACCULINNA separator

Nuclear track emulsion is exposed to a beam of radioactive ⁸He nuclei with an energy of 60 MeV and enrichment of about 80% at the ACCULINNA separator. Measurements of 278 decays of the ⁸He nuclei stopped in the emulsion allow the potential of the α spectrometry to be estimated and the thermal drift of ⁸He atoms in matter to be observed for the first time.     

Secondary nuclear fragment beams for investigations of relativistic fragmentation of light radioactive nuclei using nuclear photoemulsion at Nuclotron

Slowly extracted relativistic beams of light nuclei and a beam transportation line net system constitute a good base for secondary nuclear beams forming at the LHE accelerator facility. A recent years activity in the field at the Laboratory is connected with a project on study light nuclei structure by means the emulsion technique [1,2]. The paper shortly summarizes results of the work.     

Some special features of the fragmentation of a relativistic nucleus 11B in photoemulsion

Events where two doubly charged fragments are directly formed from a fragmenting nucleus ¹¹B of momentum 2.75 GeV/c per nucleon are separated, the cascade production of such fragments via the fragmentation channel ⁸Be → 2α being avoided. Where possible, the mass numbers of these doubly charged fragments are determined by using a signal from Coulomb scattering in photoemulsion. It is found that the measured fraction of the isotope ⁶He is (9.6 ± 1.5)%, while its calculated probability is about 12%. The transverse-momentum distributions obtained experimentally for the isotopes ³He and ⁴He are compatible with a Rayleigh distribution characterized by constant values of 110.4 ± 6.0 and 127.5 ± 6.0 MeV/c, respectively. These features of the distributions agree with a purely statistical mechanism of the fragmentation of ¹¹B nuclei.     

Recent applications of nuclear track emulsion technique

A survey of recent results obtained using the nuclear track emulsion (NTE) technique in low energy applications is given. NTE irradiation with 60 MeV ⁸He nuclei provides identification of their decays at stopping, evaluation of the possibility of α range spectrometry, and observation of drift of thermalized ⁸Не atoms. Correlations of α particles studied in ¹²С → 3α splitting induced by 14.1 MeV neutrons indicate the presence of a superposition of 0⁺ and 2⁺ states of the ⁸Be nucleus in the ground state of ¹²С. Angular correlations of fragments are studied in boron-enriched NTE, and the prospects of NTE application in radioactivity and nuclear fission research are discussed. It is proposed to use an automated microscope to search for collinear tripartition of heavy nuclei implanted in NTE. Surface irradiation of NTE by a ²⁵²Cf source is started. Planar events containing fragment pairs and long range α particles, as well as fragment triples, are studied. NTE samples are calibrated using Kr and Xe ions with an energy of 1.2 and 3 A MeV.     

Dissociation of 10C nuclei in a track nuclear emulsion at an energy of 1.2 GeV per nucleon

The charge topology in the fragmentation of ¹⁰C nuclei in a track nuclear emulsion at an energy of 1.2 GeV per nucleon is studied. In the coherent dissociation of ¹⁰C nuclei, about 82% of events are associated with the channel ¹⁰C → 2α+ 2p. The angular distributions and correlations of product fragments are presented for this channel. It is found that among ¹⁰C → 2α+ 2p events, about 30% are associated with the process in which dissociation through the ground state of the unstable ⁹Be_g.s. nucleus is followed by ⁸Be_g.s. + p decays.     

Multiparticle production of doubly charged fragments in the fragmentation of relativistic nuclei

Features of angular distributions of events involving two or more doubly charged fragments of relativistic nuclei ²²Ne, ²⁴Mg, ¹⁴N, ¹¹B, and ¹⁰B in photoemulsions are studied. It is found that, in all cases, with the exception of the case of the intermediate-state decay ⁸Be → 2α, the fragments in these events are independent of one another. The inclusive angular distributions of fragments of relativistic nuclei ²²Ne for events in which the number of particles ranges between one and five are identical. Thus, the emission angle of each fragment of a relativistic nucleus does not depend either on other fragments or on the presence or absence of product particles and target-nucleus fragments in an event.     

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.     

Topology of “white stars” in the relativistic fragmentation of light nuclei

Experimental observations of the multifragmentation of relativistic light nuclei by means of emulsions are surveyed. Events that belong to the type of “white stars” and in which the dissociation of relativistic nuclei is not accompanied by the production of mesons and target-nucleus fragments are considered. An almost complete suppression of the binary splitting of nuclei to fragments of charge in excess of two, Z > 2, is a feature peculiar to charge topology in the dissociation of Ne, Mg, Si, and S nuclei. An increase in the degree of nuclear fragmentation manifests itself in the growth of the multiplicity of singly and doubly charged fragments (Z = 1, 2) as the charge of the unexcited fragmenting-nucleus part (which is the main part) decreases. Features of the production of systems formed by extremely light nuclei α, d, and t are studied in the dissociation of the stable isotopes of Li, Be, B, C, N, and O to charged fragments. Manifestations of ³He clustering can be observed in “white stars” in the dissociation of neutron-deficient isotopes of Be, B, C, and N.     

Interactions of relativistic nitrogen nuclei in an emulsion at 2.1 GeV/nucleon

An analysis of multiple production induced by 2.1 GeV/n ¹⁴N ions in nuclear emulsions is presented. Multiplicity and angular distributions of charged secondaries and correlations among them are discussed. The presented data are compared with relevant values from proton-nucleus interactions. The possible appearence of collective phenomena in nucleus-nucleus interactions is discussed.     

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 of ^16O nuclei in nuclear emulsion

We present the topology of 16O fragmentation at 60A GeV in reactions with emulsion nuclei.We discuss the multiplicity distributions of projectile fragments.charged secondaries and their dependence on the target residues.     

Interactions of 10.6 GeV/nucleon gold nuclei in nuclear emulsion

Stacks of nuclear emulsion were exposed at the Brookhaven AGS to a 10.6 GeV/n beam of gold nuclei in the BNL 868 experiment. A preliminary analysis of the main features of the interactions of these nuclei is reported here and compared with those observed previously at lower energies,E<1 GeV/n. The multiplicity and pseudo-rapidity distributions of the singly charged particles emitted both in central and in minimum bias samples of interactions have been analyzed in order to look for any non random effects indicative of the nuclear state during the interactions. The fragmentation characteristics of the gold projectiles appear to be different at 10.6 GeV/n from those at ≦1 GeV/n incident energy. Fission of the gold projectiles, which was relatively frequent at low energy, is almost entirely suppressed. Some of alpha particles from the projectile are emitted with very large transverse momenta. In gold-emulsion interactions the residual target nuclei are significantly smaller than in oxygen and sulfur interactions in emulsion.

     

Some relativistic aspects of nuclear dynamics at the electrodisintegration of nuclei

An approach aimed to extend the applicability range of the nonrelativistic microscopic calculations of electronuclear response functions is reviewed. In the quasielastic peak region these calculations agree with experiment at momentum transfers up to about 0.4 GeV/c, while at higher momentum transfers being beyond 1 GeV/c a disagreement is seen. In view of this, a reference frame where dynamic relativistic corrections are small was employed to calculate the response functions and the results were transformed exactly to the laboratory reference frame. This proved to remove the major part of the disagreement with experiment. All leading-order relativistic corrections to the transition charge operator and to the one-body part of the transition current operator were taken into account in the calculations. Furthermore, a particular model to determine the kinematical inputs of the nonrelativistic calculations was introduced. This model provides the correct relativistic relationship between the reaction final-state energy and the momenta of the knocked-out nucleon and the residual system. The above-mentioned choice of a reference frame in conjunction with this model has led to an even better agreement with experiment.     

Production of light nuclei,hypernuclei and their antiparticles in relativistic nuclear collisions

We present, using the statistical model, an analysis of the production of light nuclei, hypernuclei and their antiparticles in central collisions of heavy nuclei. Based on these studies we provide predictions for the production yields of multiply-strange light nuclei.