Multifragmentation of gold nuclei by light relativistic ions: Thermal breakup versus dynamical disintegration

The multiple emission of intermediate-mass fragments (IMF) is studied for collisions of p, ⁴He, and ¹²C on Au with the 4π FASA setup. The mean multiplicities of IMF saturate at a value of around 2 for incident energies above 6 GeV. An attempt at describing the observed IMF multiplicities in the two-stage scenario, a fast cascade followed by a statistical multifragmentation, fails. Agreement with the measured IMF multiplicities is obtained by introducing an intermediate expansion phase and modifying empirically the excitation energies and masses of remnants. The angular distributions and energy spectra from p-induced collisions are in agreement with the scenario of “thermal” multifragmentation of a hot and expanded target spectator. In the case of ¹²C + Au(22.4 GeV) and ⁴He (14.6 GeV) + Au collisions, deviations from a pure thermal breakup are seen in the fragment energy spectra, which are harder than those both from model calculations and from the measured ones for p-induced collisions. This difference is attributed to a collective flow with the expansion velocity at the surface of about 0.1c (for ¹²C + Au collisions).     

Properties of hot nuclei originating from collisions of light relativistic ions with heavy targets

Nuclear multifragmentation is the main channel of decay of hot nuclei at temperatures of 5 to 6 MeV. Investigation of this process permits experimentally determining the critical temperature for the liquid-gas nuclear phase transition. It was found that T _c = 17±2 MeV. Characteristic times of the process are measured by analyzing correlation functions for fragment pairs. The mean fragment-emission time was found to be 1 to 2 × 10^?22 s.     

Multifragmentation study on 30 AMeV32S+58Ni

In a multidetector experiment on 26 or 30 AMeV³²S+⁵⁸Ni, up to four coincident heavier or intermediate-mass fragments were observed. One of these occasionally has the characteristics of a projectile-like fragment, up to three may be attributed to the decay of the heavy reaction product. Taking the velocity of the fragments as a measure of the heavy-product excitation energy, one finds evaporation, fission and multifragmentation to follow one another with rising excitation. Model simulations of sequential decay with up to two binary fissions and, alternatively, of simultaneous statistical multifragmentation were performed for comparison with experimental distributions of mass, velocity and (for events with three slow intermediate-mass fragments) relative azimuthal angle. Though in the three-fragment events indications of simultaneous multifragmentation are present, the sequential binary decay predominates. Evaporated protons and α particles detected in coincidence have a mean multiplicity growing with excitation energy, while the temperature governing the spectra has a plateau with a value of about 5.5 MeV.     

Emission time of the intermediate mass fragments in collisions of relativistic deuterons with the gold target

The relative velocity correlation function of pairs of intermediate mass fragments has been studied for d + Au collisions at 4.4 GeV. Experimental correlation functions are compared to that obtained by multi-body Coulomb trajectory calculations under the assumption of various decay times of the fragmenting system. The combined approach with the empirically modified intranuclear cascade code followed by the statistical multifragmentation model was used to generate the starting conditions for these calculations. The fragment emission time is found to be less than 40 fm c ⁻¹.     

Neutron production in the interaction of 2-GeV protons with nuclei

The double-differential cross sections for neutron production in the interactions of 2-GeV protons with Be, Al, Cu, Cd, and Pb nuclei were measured by the time-of-flight method in the region of angles larger than 30°. The respective experimental data are analyzed within the phenomenological model of four moving sources, including those associated with neutron emission in primary nucleon-nucleon collisions, the decay of a hot source (fireball), the multifragmentation process, and the deexcitation of nuclear fragments via neutron evaporation. Temperature-parameter values are universal for all sources and are virtually independent of the target nucleus and of the projectile energy in the region above 0.5 GeV. It is found that, for all of the above reactions, the relative contribution to the mean neutron multiplicity from the decay of a hot source and multifragmentation is about 41%.     

Liquid-fog and liquid-gas phase transitions in hot nuclei

Thermal multifragmentation of hot nuclei is interpreted as the nuclear liquid-fog phase transition inside the spinodal region. The exclusive data for p(8.1 GeV) + Au collisions are analyzed within the framework of the statistical model SMM. It is found that the partition of hot nuclei is specified after expansion to a volume equal to V _t = (2.6 ± 0.3)V ₀. The freeze-out volume is found to be twice as large: V _f = (5 ± 1)V ₀. The similarity between multifragmentation and ordinary fission is discussed. The text was submitted by the authors in English.     

Thermal hard-photons probing multifragmentation in nuclear collisions around the fermi energy

Hard-photon (E _γ>30 MeV) emission originating from photon-neutron bremsstrahlung collisions is investigated in four different heavy-ion reactions at intermediate bombarding energies (³⁶Ar+¹⁹⁷Au, ¹⁰⁷Ag, ⁵⁸Ni, ¹²C at 60 A MeV) coupling the TAPS photon spectrometer with two charged-particle multidetectors covering more than 80% of the solid angle. The hard-photon spectra of the three heavier targets result from the combination of two distinct exponential distributions with different slope parameters, a results which deviates from the behaviour expected for hard-photon production just in first-chance proton-neutron collisions. The thermal origin of the steeper bremsstrahlung component is confirmed by the characteristics of its slope and angular distribution. Such thermal hard-photons convey undisturbed information of the thermodynamical state of hot and excited nuclear systems undergoing multifragmentation.     

Baryogenesis at the weak phase transition

The weak phase transition of the hot big bang can produce quarks, leptons and weak bosons which are out of thermal equilibrium. In a simple extension of the standard model it is shown that the reactions following top quark decays can generate the cosmological baryon asymmetry. The top quark mass must be close to 80 GeV and the Higgs boson must be lighter than 1 GeV. This baryogenesis mechanism can be directly tested at e⁺e⁻ and hadron collider by searching for spectacular events containing six or more bottom quarks and a violation of baryon number at the decay vertex of a long lived neutral particle.     

Multiparticle emission in the interactions of hadrons with complex nuclei at incident momenta from 2 to 4 GeV/c

The charged secondary particles emitted in inelastic reactions of 4 GeV/cπ⁺ and π^? with the nuclei Be, Cu and Ta are thoroughly studied in their multiplicities, particle correlations and the distributions of kinematical variables, with the 1 m hydrogen bubble chamber into which the three nuclear target plates were installed. The numbers of charged secondaries emitted in the π^?(2.3 GeV/c), π⁺(2 GeV/c) and p(4 GeV/c) reactions are also investigated. From these data, characteristic features of the multiplication process of secondaries in nuclear matter are clarified. The experimental results are compared with the calculation of an intra-nuclear cascade model which takes into account the process of pion absorption by two-nucleon clusters in the nucleus. It is found that the experimental results on the properties of charged secondaries are consistently explained by the model. The model can also give an account of the experimental finding that fairly high momentum protons are emitted from the heavier nuclei into the backward hemisphere in the lab system.     

Collective flow in nuclear fragmentation induced by 4.4 GeV deuteron on gold target

Nuclear multifragmentation in d (4.4 GeV) + Au collision was studied with the 4π setup FASA installed at the external beam of the Dubna Nuclotron. Data obtained are analyzed within the statistical model of multifragmentation. It is found that the kinetic energy spectra of intermediate mass fragments deviate from the predicted ones. It is explained by the collective flow caused by the thermal expansion of fragmenting nucleus.     

Heavy fragments produced in proton-nucleus and nucleus-nucleus collisions at relativistic energies

Mesons, nucleons and light and heavy nuclei are observed as a result of a proton-nucleus or a nucleus-nucleus collision at relativistic energies (E_p≥0.5 GeV/u). This review concentrates on the properties of the heavy fragment nuclei (A ≥10. The experimental results for the distributions in mass, charge, angle and kinetic energy are presented. The data are discussed in the framework of models for spallation, fission and multifragmentation.     

Multifractal critical thermodynamics in nucleus-nucleus collisions

Critical points approach in the frames of multifractal thermodynamics is suggested to interpret the experimental data on nuclear multifragmentation which come from interactions in nuclear emulsion (in which ¹⁹⁷ ₇₉Au₁₁₈ nuclei of energy ∼1 GeV/nucleon break up into fragments) and from the charge distributions of projectile fragments in sulphur (³²S) fragmentation at 200 GeV/nucleon. It is also shown that multifragmentation after macro-solids collisions exhibits properties analogous to those observed in the nuclear multifragmentation experiments. Received: 19 October 1999     

Reducibility and thermal scaling in nuclear multifragmentation

Recent studies have revealed the existence of a number of reducibility and thermal scaling properties in nuclear multifragmentation. The probability of emitting n-fragments is found to be reducible to the probability of emitting a single fragment through the binomial expression. The resulting one fragment probability shows thermal scaling by producing linear Arrhenius plots. Similarly, the charge distributions associated with n-fragment emission are reducible to the one-fragment charge distribution. Thermal scaling is also observed. The reducibility equation contains a constant whose value, zero or positive, can be related to a univariant (two phase) or bivariant (one-phase) regime. The light fragment particle-particle angular correlations also show reducibility to the single-particle angular distributions as well as thermal scaling. A mass scaling associated with the angular correlations suggests emission from several small sources (A ≈ 20). The limits of applicability of scaling and reducibility are discussed as well as their implications for the mechanism of multifragmentation.     

Heavy-nucleus multifragmentation induced by coherent bremsstrahlung photons with endpoint energy <Emphasis Type="Italic">E</Emphasis><Stack><Subscript>γ</Subscript><Superscript>max</Superscript></Stack>=4.1 GeV

New experimental data are presented that were obtained by studying the multifragmentation of ¹⁹⁷Au, ²⁰⁹Bi, ²³⁸U, and ²⁴³Am nuclei in a coherent bremsstrahlung-photon beam with an endpoint energy of 4.1 GeV from the Erevan Synchrotron. For the first time, four or more fragments are observed. For all the nuclei studied here, the respective production cross sections are about (1–3)×10^?3 of the total inelastic photon-nucleus cross section. The fragment yields are isotropic.     

Medium effects in the production and decay of vector mesons in pion-nucleus reactions

The ω-, ϱ-, and φ-resonance production and their dileptonic decay in π⁻ A reactions at 1.1–1.7 GeV/c are calculated within the intranuclear cascade (INC) approach. The invariant mass distribution of the dilepton pair for each resonance can be decomposed in two components which correspond to their decay “inside” the target nucleus and in the vacuum, respectively. The first components are strongly distorted by the nuclear medium due to resonance-nucleon scattering and a possible mass shift at finite baryon density. These medium modifications are compared to background sources in the dilepton spectrum from πN bremsstrahlung as well as the Dalitz decays of ω and η mesons produced in the reaction. Detailed predictions for π⁻Pb reactions at 1.3 and 1.7 GeV/c are made within several momentum bins for the lepton pair.     

Study of the charge-exchange reactions\pi ^ - p \to (\pi ^0 ,\eta ,\eta \prime )n at 63 GeV

Muon electron pairs were detected in an Al multiplate spark chamber, exposed to a neutrino beam from the CERN PS. The leptons were not accompanied by other particles, except occasionally by protons. The background came mainly from muon associated π⁰ production, with one decay gamma lost. It was determined empirically, together with the small contribution from υ _e reactions. For electron energies above 2 GeV the background is 5.7±1.5 events, whereas 18 (μe)-candidates have been observed. Hence the effect is established, with a rate of about 10^?4 as compared to the muonic reactions above 3 GeV. Charm creation as the origin of this (μe)-production process is excluded; heavy neutral lepton production does not fit the kinematics observed. Instead the events are compatible with the two-body decay of an object with variable invariant mass of order 1 GeV, possibly resulting from axion interactions.     

Multifragmentation of Gold nuclei in the interactions with photoemulsion nuclei at 10.7 GeV/nucleon

Recent results from the EMU-01/12 collaboration are presented for 10.7 GeV/nucleon gold nuclei interactions in emulsion. The distributions of “bound” charge (Z _bound , Z _b3 ), multiplicity distributions, fragment correlations and fluctuations are discussed. The data are compared to similar results obtained on the ALADIN setup at 600 MeV/nucleon. It is shown that multifragmentation of gold nuclei at high and intermediate energies has common features. It is also obtained that the IMFs have reduced multiplicity at high energies. The data are analyzed within the scope of the statistical model of multifragmentation. This model requires the following predetermined model ingredients: mass, charge and excitation energy of nuclear residuals. The simple estimation method of these characteristics is proposed in the framework of the Glauber approach. It is shown that the multifragmentation model reproduces qualitatively the present data. A dramatic discrepancy between the predicted and experimental yield of two charged fragments is found. The evolution of transverse momentum of fragments as a function of Z _bound is presented. It is shown that the model greatly underpredicts the transverse momentum of fragments. It is interpreted as evidence of a strong radial flow of spectator fragments.     

Universality of multifragmentation of residual nuclei produced in high-energies nucleus-nucleus interactions

Experimental data on the multifragmentation of residual nuclei produced in the krypton interactions with photoemulsion nuclei at 0.9 GeV per projectile nucleon are presented and compared with similar data on fragmentation from experiments where gold nuclei of energy 10.7 GeV per nucleon appear as projectiles. It is shown for the first time that there exist two modes of nuclear multifragmentation, those where less (first mode) or more (second mode) than half of nucleons are knocked out of the incident nucleus. Residual nuclei that have close masses and which are produced in various reactions accompanied by the knock-on of more than half of nucleons of the initial nucleus fragment in nearly the same way. In addition, evidence for a radial flux of spectator fragments is obtained for the first time in the decay of residual nuclei of krypton projectiles.     

Source velocity at relativistic beams of <Superscript>4</Superscript>He

The source velocities (β = ν/c) extracted from rapidity plots of the fragment invariant probability distribution in terms of the longitudinal versus transversal velocity components has been studied for ⁴He + Au collisions at 4 and 14.6 GeV. It was found transition from broad range source velocities distribution in case of ⁴He(4 GeV) + Au to fixed source velocity in case of ⁴He(14.6 GeV) + Au.