Characteristics of alpha projectile fragments emission in interaction of nuclei with emulsion

The properties of the relativistic alpha fragments produced in interactions of ⁸⁴Kr at around 1 A GeV in nuclear emulsion are investigated. The experimental results are compared with the similar results obtained from various projectiles with emulsion interactions at different energies. The total, partial nuclear cross-sections and production rates of alpha fragmentation channels in relativistic nucleus-nucleus collisions and their dependence on the mass number and initial energy of the incident projectile nucleus are investigated. The yields of multiple alpha fragments emitted from the interactions of projectile nuclei with the nuclei of light, medium and heavy target groups of emulsion-detector are discussed and they indicate that the projectile-breakup mechanism seems to be free from the target mass number. It is found that the multiplicity distributions of alpha fragments are well described by the Koba-Nielsen-Olesen (KNO) scaling presentation. The mean multiplicities of the freshly produced or newly created charged secondary particles, normally known as shower and secondary particles associated with target in the events where the emission of alpha fragments were accompanied by heavy projectile fragments having Z ≥ 3 seem to be constant as the alpha fragments multiplicity increases, and exhibit a behavior independent of the alpha fragments multiplicity.     

Two source emission behavior of projectile fragments alpha in 84Kr interactions at around 1 GeV per nucleon

The emission of projectile fragments alpha has been studied in ⁸⁴Kr interactions with nuclei of the nuclear emulsion detector composition at relativistic energy below 2 GeV per nucleon. The angular distribution of projectile fragments alpha in terms of transverse momentum could not be explained by a straight and clean-cut collision geometry hypothesis of Participant — Spectator (PS) Model. Therefore, it is assumed that projectile fragments alpha were produced from two separate sources that belong to the projectile spectator region differing drastically in their temperatures. It has been clearly observed that the emission of projectile fragments alpha are from two different sources. The contribution of projectile fragments alpha from contact layer or hot source is a few percent of the total emission of projectile fragments alphas. Most of the projectile fragments alphas are emitted from the cold source.     

Fragmentation of12C nuclei in emulsion at 50 GeV/c

A total of 852¹²C-emulsion nucleus interactions at 4.2 GeV/c per incident nucleon was investigated. At least one charged projectile fragment was observed in 733 events, in which the multiplicity and angular distributions ofZ=1,2, and ≧3 projectile fragments were studied. Five events were observed in which¹²C projectile nuclei were fragmented into twoZ=3 fragments. Thus the cross section of this process is about 6×10^?3 of the inelastic cross section. The angular distribution of projectile fragments becomes narrower as the fragment charge increases. At all values of fragment charges, a pronounced peak in the angular distribution can be observed at zero emission angle. In this paper, only the projectile-fragmentation events possessing no heavily ionizing particle (n _h =0 events) have been investigated. Our sample contains 84 of these events, i.e., about 10% of the total inelastic events. The number of events withZ _max, the charge of the emitted principal fragment, equal to 1, 2, 3, 4, and 5 are 11, 52, 13, 4, and 4, respectively. Of these 84 events, 36 interactions have a total charge of emitted projectile fragmentsZ ^* equal to 6, i.e., as much as the beam chargeZ _p . Of the 36 events, 17 produce no charged pions and of the 17 events, 10 only represent the dissociation of¹²C→3α, i.e., 1.2% of the total inelastic interactions. The number of events withZ ^*=5, 4, 3, 2, and 1 are 27, 14, 4, 2, and 1, respectively. The average number of produced charged pions per one interacting projectile nucleon was estimated to be 1.2±0.1. This value agrees with the corresponding one in elementary interaction at the same energy per nucleon, a result pertaining to the incoherent production model in collision of two nuclei. In this class of events,n _h =0, the number of stars in which H, He, Li, Be, and B isotopes were detected are 59, 58, 13, 4, and 4, respectively. The projected angular distributions ofZ=1 and 2 projectile fragments are Gaussian shaped, narrow, consistent with isotropy, and depend on the fragment. These distributions are consistent with quantum mechanical calculations using the sudden approximation and shell-model functions. From the angular measurements ofα-particle tracks in the dissociation¹²C→3α events, the distribution ofα-particle transverse momentum inside the carbon projectile nucleus was deduced. It seems that the dissociation of¹²C→3α happens via an intermediate⁸Be state.     

Mechanism of light fragment production in the interaction of 8.8 GeV 4He with 208Pb

The events with at least three fragments with Z>2 from the interaction of 8.8 GeV ⁴He with ²⁰⁸Pb nuclei have been examined using a CR-39 plastic track detector. The analysis shows that the majority of these events is produced in central collisions. But, it is also observed that no negligible part of them originates from peripheral collisions and has one heavy and two light fragments (2<Z_L<20). This indicates that various mechanisms contribute to the production of light fragments in high energy nuclear interactions.     

Calibration of CR39 detectors with new system for Fe26+ ion beam and measurement of total charge changing cross-section in Al target

In the present study, CR39 track etch detector was calibrated with a new system and the total charge changing cross-section of 300 A MeV Fe²⁶⁺ ion beam in aluminum target was measured. The CR39 nuclear track detectors were used to identify the incident charged particles and their fragments using an optical microscope DM6000 M and automated image analyzer system installed with Leica QWin Plus software. The CR39 detectors before and after the target were calibrated and found to have the same charge response; the charge resolution in both of the detectors were 0.19e and 0.20e, respectively. The calibration points were fitted with a polynomial of degree one and all the points are within the limits of the experimental errors. The response functions were also obtained and fitted with a polynomial of degree three which are quite good throughout Z/β = 4.6 to 41.4. The value of the total charge changing cross-section is σ_tot = (1663 ± 236) mb. The total charge changing cross-section was compared with the experimental results of others and also fitted by the Bradt-Peters geometrical cross-section.     

Production of Helium Projectile Fragments in 16O-Emulsion Collisions at 60 A GeV

Production of helium projectile fragments in ¹⁶O-emulsion interactions at 60 A GeV is investigated. The total charge changing and partial production cross-sections are measured experimentally on the basis of helium multiplicity. The multiplicity distribution of helium projectile fragments obeys a KNO scaling. In the peripheral collision of ¹⁶O at 60 A GeV in nuclear emulsion, the production of target fragments depends on the multiplicity of helium projectile fragments linearly. The averge multiplicity of target fragments decreases with the increasing of the number of helium projectile fragments which can be well expliained by the model of participant-spectator (the nuclear geometric model) of nucleus-nucleus interactions.     

Multiplicities in84Kr interactions in emulsion at 800–950 MeV/nucleon

Inelastic interactions induced by⁸⁴Kr nuclei at 800–950 MeV/nucleon have been studied using a high angular resolution emulsion detector. Data on multiplicities of the produced particles, projectile and target fragments are shown. Comparisons with cascade calculations have been performed.We thank to Swedish Research Council for Natural Sciences for the financial support. Scanning and measurements performed by E.G. Broomé and V. Kopljar are acknowledged.     

Study of large fragments of uranium projectiles of energy 0.927 GeV/n in Al target

Summary A stack consisting of CR-39(DOP) detector with 2.6 cm Al target was exposed to a²³⁸U beam of energy 0.927 GeV/n from LBL BEVALAC to study the large-fragmentation phenomena during U+Al interaction. The diameter distribution of the etch pits formed in the plastic detectors for the first three projectile fragments of chargeZ _F≥89 yields the partial cross-section data which is comparable to the data obtained from the cone length distribution. The present result on the partial cross-section is significantly higher than those expected from the abrasion-ablation model of Wilson, Townsed and Badavi (1987). The authors of this paper have agreed not to receive the proofs for correction.     

Analysis of autoionization resonances in the Hg 6s 2-photoionization by measurements of photoelectron polarization

It is shown that nuclear target fragmentation in proton and heavy ion induced reactions, in particular the following experimental facts concerning the mass-yield distribution can be understood in terms of a semiclassical model:(i) its independence on the mass of the projectile at approximately the same incident energies,(ii) its trend of approaching a limit at higher bombarding energies,(iii) its “U-formed” shape at sufficiently high bombarding energies. Standard methods in statistical theory of chemical equilibrium are used to calculate the mass-yield distribution for medium and heavy target nuclei in high-energy nuclear reactions where the Coulomb interaction between the fragments is taken into account selfconsistently. The result shows: The fact that the decaying rest target nucleus and its fragments are bounded objects of finite size and finite charge have significant influences, especially on the form of the mass-yield distributions.     

An experimental study on cross-section ratio of coherent to incoherent scattering for 145 keV incident gamma photons

The coherent (Rayleigh) to incoherent (Compton) scattering cross-section ratio of elements, in the range 6 ≤ Z ≤ 82, are determined experimentally for 145 keV incident gamma photons. An HPGe (High purity germanium) semiconductor detector is employed, at scattering angle of 50°, 70° and 90°, to record the spectra originating from interactions of incident gamma photons with the target under investigation. The intensity ratio of Rayleigh to Compton scattered peaks observed in the recorded spectra, and corrected for photo-peak efficiency of gamma detector and absorption of photons in the target and air, along with the other required parameters provides the differential cross-section ratio. The measured values of cross-section ratio are found to agree with theoretical predictions (corresponding to 4.939, 6.704 and 8.264 Å⁻¹ photon momentum transfer) based upon non-relativistic form factor, relativistic form factor, modified form factor and S-matrix theory.     

Search for projectile fragments with fractional charge in relativistic heavy ion collisions

We measured the charge of about 35 000 projectile fragments with Z5e produced by 14.5 GeV/nucleon and 200 GeV/nucleon ¹⁶O beams in a Pb target using CR39 plastic nuclear track detectors. A minimum track length of 3 mm in the detector without nuclear interaction was required. No evidence for fragments carrying a fractional charge was found.     

Interaction mean free paths of He fragments from ultrahigh-energy nucleus-nucleas collisions in emulsion

The interaction mean free paths of He fragments produced by the collisions of ³²S and ¹⁶O at 200 GeV/n in nuclear emulsion have been investigated. 906 He fragments giving rise to 374 secondary interactions have been used in the present analysis. Within the present statistics, we do not observe any anomalous behavior in the mean free paths of He fragments. The interaction mean free paths are found to be independent of the projectile mass and the impact parameter of collision, at the highest available beam energies.     

Study of number of black prongs in two generations of nuclear interactions in photoemulsion irradiated by 72 GeV Ne

This work is a study of new aspects in relativistic nucleus–nucleus interactions using the nuclear emulsion technique and it is aimed at development of a method of search of these interactions. Layers of nuclear photographic emulsion type-BR2 with sizes were irradiated with 72 GeV ²²Ne beam parallel to emulsion surface. The scanning length was 110 mm. The new idea is the counting of black prongs in nuclear–nuclear interactions due to primary projectile ²²Ne ions and due to their α- and Z3 projectile fragments, i.e. their (Z2)-secondary fragments. The ratio of their black prong multiplicity distributions is measured. Black prongs are low-energy Z=1 particles in case of protons with energies below 30 MeV emitted from the residual nuclei of these high-energy interactions. Two divisions of events were used. The first one was over more peripheral events, more central and exceedingly central collisions. The second division was over collisions creating fragments Z2 and without their creation. The ratios between numbers of events in such different subsets, as well the ratios between of different mean values of N_b multiplicities averaged inside these subsets were studied. These results are discussed, also in the context of other thick target experiment, studied with radiochemistry or neutron counting techniques. In addition, different versions of the mean free nuclear ranges of Ne-ions and their heavy secondary fragments have been measured and calculated. We have scheduled the further strategy of search of new phenomena in similar multiplicity distributions at greater statistics of future experiments.     

‘Slow’ residues observed in the reaction20Ne+208Pb at E/A=8.6, 11.4 and 15.0 MeV/u

Velocity distributions of heavy residuesA _Res>A _tar,Z _Res>Z _tar identified by means ofα spectroscopy, have been investigated at the velocity filter SHIP in reactions²⁰Ne+²⁰⁸Pb at projectile energies E/A=8.6, 11.4 and 15.0 MeV/u. Besides products from complete or nearly complete fusion, characterized by velocity distributions peaking atν/ν _CN?0.8–1.0, heavy residues with mean velocities of about half of the compound nucleus velocity were observed. The Z-distribution of this component was found to peak atZ=87. It is interpreted as residues from fusion of target nuclei with projectile fragments produced by nearly symmetric break-up. The experimental results were compared with predictions of theoretical models: cross sections for incomplete fusion were calculated using the sum-rule model of Wilcynski et al., while residue cross sections were calculated using the evaporation code HIVAP. A fair agreement between experimental and calculated mass distributions of heavy residues and transferred projectile fragments is achieved if an energy dissipation of ?23% (at E/A=8.6 MeV/u) and ?8% (at E/A=11.4 MeV/u) of the incident projectile energy is introduced. The observed peak of theZ-distribution atZ=87 is predominantly effected due to a higher fission probability of products withZ>87 during the deexcitation process and experimental limitations in the identification of products withZ≦86 by means of a spectroscopy, which cause a decrease of the observed production rates towards lowerZ.     

Nuclear fragmentation degree in the production of light projectile fragments

The projectile-target multiplicity correlations at the lowest (3.7 A GeV) and highest (200 A GeV) available energies in Sulfur-emulsion interactions are investigated in the framework of the participant-spectator model. We examined the light projectile spectator (singly, N_z=1 and doubly, N_z=2 charged projectile fragments). The measured average multiplicities of N_z=1 and N_z=2 at both incident energies showed a decrease with increasing the fragment charge, as well as the average values within experimental errors showed independence on the incident energy. The multiplicity measurements of different emitted secondary charged particles (produced particles, shower tracks, and target fragments, gray and black tracks) in the interactions of ³²S-Em at 3.7 A and 200 A GeV were carried out. However, the correlation between the mean multiplicities of secondary charged particles and both of the singly and doubly charged projectile fragments were investigated to shed some light on the interface of spectator and participant regions, and on the possible influences of the phenomena from the two regions on the collision dynamics.     

INTERACTION MEAN FREE PATH OF He PROJECTILE FRAGMENTS FROM 16O-EM COLLISION AT 60 A GeV

The interaction mean free path of He projectile fragments, produced by the collisions of ^{16O at 60 A GeV in a nuclear emulsion, has been investigated. In the present analysis, 1555 He projectile fragments, giving rise to 320 secondary interactions, have been used. At a level of 3% a very weak signal of anomalons is observed, which comes mainly from the 3×He channel.}     

Mechanism of negative ion formation in low velocity collisions at surfaces

Theoretical calculations using the coupled angular modes (CAM) method have been used to interpret experimental findings related to the scattered oxygen negative ion fraction (O⁻) in collisions of low energy O⁺ with an Al(1 1 0) surface. The increasing O⁻ ion fraction observed experimentally at low O⁺ velocities can be explained in terms of the distance of closest approach Z_c to the Al surface and the specific charge state of the projectile at this distance. Both Z_c and the charge state influence the charge transfer process between the projectile and the surface at low collision energies.     

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.     

Nuclear multifragmentation: Basic concepts

We present a brief overview of nuclear multifragmentation reaction. Basic formalism of canonical thermodynamical model based on equilibrium statistical mechanics is described. This model is used to calculate basic observables of nuclear multifragmentation like mass distribution, fragment multiplicity, isotopic distribution and isoscaling. Extension of canonical thermodynamical model to a projectile fragmentation model is outlined. Application of the projectile fragmentation model for calculating average number of intermediate mass fragments and the average size of the largest cluster at different Z _bound, differential charge distribution and cross-section of neutron-rich nuclei of different projectile fragmentation reactions at different energies are described. Application of nuclear multifragmentation reaction in basic research as well as in other domains is outlined.     

Production of fragments with Z≥ 8 in interaction of 12.7 GeV 4He+U, Pb, Au and Ag: track detector study

The processes of production of fragments with Z≥ 8 in the interaction of 12.7 GeV ⁴He with U, Pb, Au and Ag have been analyzed using the polycarbonate track detector Makrofol. The sandwich technique was used which enables direct observation of multiply charged fragment emission by a single nucleus. The decay channels ending with one, two, or more (three, four) fragments were detected. A classification scheme based on the multiplicity of heavy fragments with Z > 20 was used in order to identify the events belonging to the different reaction channels. The cross sections, excitation energies and multiplicities of intermediate-mass fragments 8 ≤Z≤ 20 have been determined for the various reaction mechanisms, and their variation as a function of the target mass has been investigated. Received: 28 November 1997 / Revised version: 24 April 1998