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.     

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.     

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.     

The breakup of 16O into He: An event-by-event study of nucleus-nucleus collisions at 75, 175 and 2000 MeV/A

We present an event-by-event study of the breakup of the ¹⁶O in ¹⁶O + emulsion nucleus interactions at 75, 175 and 2000 MeV/A. The events are categorized according to their multiplicity of projectile He nuclei. The multiplicity depends on the degree of target destruction. Although the fragmentation model describes the gross features of inclusive He spectra, an event-by-event study reveals deviations from the model. The momenta of the He nuclei, emitted from the projectile, depend on helium multiplicity and the breakup properties of the target nucleus. The probability that the ¹⁶O projectile breaks up into multiple He fragments is larger at 75 MeV/A than at 2000 MeV/A. At 75 MeV/A the mean velocity of projectile He is on the average 0.06c below the projectile velocity. This recoil velocity depends on the target nucleus destruction also for the most peripheral collisions.     

Inner-Shell X-ray production in adiabatic iodine-Z 2 collisions,withZ 1+Z 2≧100

K-, L- andM-x ray production cross sections for the molecular excitation regime in heavy ion-atom collisions (Z ₁+Z ₂≧100) are reported for the projectile velocity range 1.3–3.7 a.u. Outer-shell vacancy configurations of the collision partners having experienced inner shell vacancy creating collisions are deduced from the x-ray spectra. The influence of outer shell vacancy occupation on fluorescence yields is discussed and vacancy production cross sections are derived from x-ray production cross sections.     

Fast helium production in interactions of 3.7 A GeV <Superscript>24</Superscript>Mg with emulsion nuclei

We have studied the properties of the relativistic helium fragments emitted from the projectile in the interactions of ²⁴Mg ions accelerated at an energy of 3.7 A GeV with emulsion nuclei. The total, partial nuclear cross-sections and production rates of helium fragmentation channels in relativistic nucleus-nucleus collisions and their dependence on the mass and energy of the incident projectile nucleus are investigated. The yields of multiple helium projectile fragments disrupted from the interactions of ²⁴Mg projectile nuclei with hydrogen H, light CNO and heavy AgBr groups of target emulsion nuclei are discussed and they indicate that the breakup mechanism of the projectile seems to be independent of the target mass. Limiting fragmentation behavior of fast-moving helium fragments is observed in both the projectile and target nuclei. The multiplicity distributions of helium projectile fragments emitted in the interactions of ²⁴Mg projectile nuclei with the different target nuclei of the emulsion are well described by the KNO scaling presentation. The mean multiplicities of the different charged secondary particles, normally defined shower, grey and black ( n_s, n_g and n_b) emitted in the interactions of 3.7 A GeV ²⁴Mg with the different groups of emulsion nuclei at different ranges of projectile fragments are decreasing when the number of He fragments stripped from projectile increases. These values of n_i ( i = s, g, b and h particles) in the events where the emission of fast helium fragments were accompanied by heavy fragments having Z 3 seem to be constant as the He multiplicity increases, and exhibit a behavior independent of the He multiplicity.     

Study of emission characteristics of the projectile fragments produced in the interaction of 84Kr36 with nuclear emulsion detector at 1 GeV

This article is focused on the characteristics of the projectile fragments of charge 1 ≤ Z ≤ 10 produced in the interaction of the ⁸⁴Kr₃₆ with nuclear emulsion detector at 1 GeV per nucleon. We have studied the average charge distribution and multiplicity distribution of the projectile fragments having charge 1 ≤ Z ≤ 10. We have also studied the emission behavior of various projectile fragments produced from the interaction with different target groups of nuclear emulsion detector. From this study we have observed that the emission of projectile fragments is strongly dependent on the interaction with different type of the target groups of nuclear emulsion detector as well as on the mass of the projectile beam. The results are compared with other experimental observations carried out at relativistic energy and found to be consistent.     

Structures and angular distributions of slow and relativistic charged particles produced in 7Li-emulsion interactions at 3 GeV/c per projectile nucleon

Experimental results on the multiplicity distributions of various particles produced in the interactions of ⁷Li with emulsion nuclei at a momentum of 3 GeV/c per projectile nucleon are reported. A comparison with data on collisions induced by other nuclei at a nearly identical momentum per nucleon is presented in order to reveal the dependence on the projectile mass. The internal structure of ⁷Li is explored by studying the projectile fragment. The mean multiplicity of shower particles, 〈n _s〉, induced by ⁷Li is found to be less than that in the case of ⁶Li projectiles. The angular distributions of target fragments and relativistic charged secondaries are investigated. No shock-wave phenomena are observed. Forward-to-backward ratios are calculated for each case. The probability distributions for relativistic secondaries produced per unit rapidity are studied in detail, along with the rapidity densities and their dependence on the projectile and the target mass. A comparison of the angular spectra of shower particles produced in central and peripheral collisions supports the limiting-fragmentation hypothesis. The collisions in question seem to become more central with increasing shower-particle multiplicity.     

K-shell ionization cross sections in nearly symmetric collisions by S and Cl impact

K-shell ionization cross sections in collisions of S and Cl ions with target atoms with atomic numberZ ₂=11–22 have been measured in the energy range from 3.9 to 48 MeV. The data are interpreted in terms of the statistical model of electron diffusion to the continuum. The distribution ofK-vacancies between the projectile and the target atoms are discussed in the framework of the vacancy sharing process.     

Strange baryons production and projectile fragmentation in p−A collisions at the AGS

Experiment 910 (E910) has studied proton-nucleus collisions at 12.3 and 17.5 GeV incident energies using a variety of targets. Centrality selection is provided via “grey” track multiplicity which can be statistically related to ν, the number of scatterings of the proton in the target nucleus. Previously published semi-inclusive measurements of Ν production are discussed and preliminary measurements of “leading” Ν production in 17.5 GeV p-Au collisions are presented as a function of ν. The data indicate that the probability for the projectile to fragment into a strange baryon increases significantly with increasing ν for ν≤3; an extrapolation of the E910 data to nucleus-nucleus collisions can reproduce most of the enhancement in Ν production observed in Pb-Pb collisions at 40, 80 and 160 A. GeV. Preliminary measurements of Ξ^? production in 17.5 GeV p-Au collisions show a rapid increase in yield with increasing ν. The data are discussed in the context of the Van Hove fragmentation model. Preliminary proton and pion spectra are presented for 12.3 GeV p-Be collisions. The data show a rapid degradation of outgoing proton momenta with increasing N _grey while only modest changes are observed in the pion fragmentation spectra.     

Systematic experimental survey on projectile fragmentation and fission induced in collisions of 238U at 1 A GeV with lead

Projectile fragmentation and fission, induced in collisions of ²³⁸U at 1 A GeV with lead, have systematically been studied. A complete survey on the isotopic production cross sections of all elements between vanadium (Z = 23) and rhenium (Z = 75) down to a cross section of 0.1 mb is given. About 600 isotopes produced in fragmentation and about 600 isotopes produced in fission were identified in the GSI fragment separator FRS from magnetic rigidities, time-of-flight values, and the energy loss in an ionisation chamber. In addition, the velocity distributions of all these reaction products have been mapped, and the products are unambiguously attributed to the different reaction mechanisms due to their kinematical properties. The results are compared with empirical systematics and previous data. The velocity of the fragments obtained in the fission process by the Coulomb repulsion allows one to reconstruct the TKE-value of the break-up and to identify the atomic number of the fissioning nucleus in hot fission. The mean velocities of light projectile fragments were found to be higher than the beam velocity.     

Helium production from 84Kr-- and 197Au--emulsion interactions at high energies

The properties of the relativistic helium fragments produced in interactions of 84 Kr at 1.8 A GeV and 197 Au at 10.7 A GeV in emulsion are investigated. The experimental results are compared with those obtained from various projectiles with emulsion collisions at different energies. It is found that the multiplicity distribution of helium projectile fragments (HPFs) is well described by the Koba-Nielsen-Olesen (KNO) scaling presentation. The second Mueller moment f 2 of the HPF multiplicity distribution is independent of the projectile energy for the same projectile, but it is dependent on the projectile mass number. The value of f 2 increases with the increase of projectile mass number A p . The negative value of f 2 , when A p < 69, means that the emission of HPFs is anticorrelated, but positive value of f 2 , when A p > 69, refers to that the emission of HPFs is correlated. The non-zero f 2 moment in this experiment implies the strong correlation existing between the HPFs.     

Fragmentation of target spectators in ultrarelativistic heavy ion collisions

We demonstrate that the assumption that target spectators fragment isotropically in a gently moving coordinate system is in agreement with pseudorapidity distributions, measured in central ultrarelativistic heavy ion collisions of projectile energy from 14.5A GeV to 200A GeV. Target spectator remnants might be excluded from measurement by introducing a low energy cutoff for the accepted particles at approximately 200 MeV. An approximate scaling is presented for the pseudorapidity distribution of the target spectator fragments. Theoretical tools used for studying intermediate energy heavy ion collisions seem to be applicable in describing target spectator fragmentation in ultrarelativistic heavy ion collisions.     

Nuclear effects in 800 GeV proton interactions

We discuss here the nuclear effects in multiparticle production processes in 800 GeV proton-emulsion nuclei interactions. The multiplicity of medium energy particles (N _g ) was used as a measure of the average number of projectile collisions (<v(N _g )>). To study nuclear effects, we determined <v(N _g )> and ratio of normalized rapidity density of all produced charged particles in hadron-nucleus to hadron-hadron interaction,R(N _η ). It was found that the average multiplicity of shower particles increases with <v(N _g )>. From theR(N _η ) distribution, we find; a strong maxima in the target fragmentation region, a plateau in the central region and depletion of particle density in the projectile fragmentation region.     

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.     

Experimental determination of the production mechanism for noncharacteristic radiation in swift heavy-ion collisions

Absolute differential cross sections of non-characteristic radiation emitted in energetic heavy-ion collisions (E _P =3.92–48 MeV,Z=11–22) can only be explained in the framework of quasimolecular radiation. All other processes, particularly bremsstrahlung mechanisms are found to be negligible. For high projectile and photon energies the one-step quasimolecular radiation is dominant in solid targets, whereas the two-step process contributes mainly at low projectile and photon energies.     

Study of the 40Ar + 68Zn reaction at 27.6 MeVNucleon

The various reaction mechanisms observed in the ⁴⁰Ar(1102 MeV) + ⁶⁸Zn reaction have been studied. Projectile-like fragments appear to result, in addition to deep-inelastic collisions, from projectile breakup, i.e. a process bearing a close similarity to high-energy fragmentation, and from direct transfer of nucleons. The origin of light fragments (4 ? Z ? 10) can be explained in terms of three distinct sources; one moving with the projectile velocity, a second having a velocity corresponding to full transfer of the projectile linear momentum to the projectile-plus-target system, and a third source moving with a velocity intermediate between the first two and corresponding to a very excited and non-equilibrated localized region of the nuclear system. The heavy fragments result essentially from an incomplete fusion process. Some features of the various types of fragments have been calculated. Comparisons with existing models are presented.     

Quasimolecular KX-ray excitation by bombarding As,Zr, Nb,Mo and Rh target with Nb ions

By bombarding various targets with 65 MeV and 40 MeV Nb ions continuous X-ray distributions have been obtained, which range up to the KX-ray energies of quasiatoms woth Z=Z₁+Z₂. The hogh-energy parts of these continua are interpreted as KX-radiation of quasimolecules transiently formed during the adiabatic heavy-ion atomic collisions.     

Inclusive particle spectra in p(π+)n interactions at 195 GeV/c

We present results on single-particle inclusive distributions in p(π⁺)n reactions from a Fermilab experiment using the hybrid 30″ bubble chamber and PWC facility. Distributions in rapidity, y, Feynman x and p²_T for π⁺ and π^? are presented and compared with other experiments. The effects of different targets and projectiles (neutrons, protons and pions) on the distributions are demonstrated and discussed in terms of projectile and target fragmentation. The average transverse momentum 〈P_T〉 is studied as a function of x, y and multiplicity.