Fragmentation of <Superscript>7</Superscript>Li relativistic nuclei on a proton into the <Superscript>3</Superscript>H + <Superscript>4</Superscript>He channel

In a track nuclear photoemulsion exposed to a beam of ⁷Li nuclei accelerated to a momentum of 3 GeV/c per nucleon at the synchrophasotron of the Joint Institute for Nuclear Research (JINR, Dubna), 13 events in which ⁷Li nuclei interacting with protons break up into ³H and ⁴He fragments were detected among 3730 inelastic-interaction events. For this fragmentation channel, the cross section was found to be 8 ± 2 mb. The average value of the fragment total transverse momentum was 214 ± 5 MeV/c. This value exceeds markedly the average value of the transverse-momentum transfer in the coherent dissociation of ⁷Li nuclei on track-emulsion nuclei (166±5MeV/c). The recoil-proton transverse momentum was on average 98% of the total proton momentum. The longitudinal-momentum distribution of protons was characterized by a variance of 16 MeV/c and a mean value of 37 ± 2MeV/c.     

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.     

Core-breakup reactions of the halo nuclei 11Be and 11Li: momentum distributions and shadow effects

The halo nuclei ¹¹Be and ¹¹Li have been studied in core-breakup reactions where the halo neutrons are expected to be released without a major distortion due to the reaction. The widths of the halo-neutron momentum distributions have been extracted in coincidence with He fragments, Γ = 32 ± 4 MeV/c, and Li fragments, Γ = 42 ± 4 MeV/c for ¹¹Be and with He fragments, Γ = 42 ± 6 MeV/c for ¹¹Li. The ¹¹Be breakup gives a very low neutron multiplicity of 0.38±0.09 which is a manifestation of the shadowing of the neutron in the core-breakup reaction. This value can be understood from a simple theoretical calculation, which also accounts for the observed transverse momentum widths at small angles.     

Momentum distributions of <Superscript>4</Superscript>He nuclei from the <Superscript>6</Superscript>He and <Superscript>6</Superscript>Li breakup

Experimental data on the momentum distributions of ⁴He nuclei originating from ⁶He and ⁶Li breakup on various targets are presented over a wide beam energy range. The experiment with ⁶He was performed at the DRIBs accelerator complex for radioactive beams at the Joint Institute for Nuclear Research (JINR, Dubna). The intensity of the ⁶He beam used was 5 × 10⁶ particles per second and its energy was 10 MeV per nucleon. The momentum distributions of breakup products were measured by means of the MSP-144 magnetic spectrometer. The distribution width was shown to be virtually independent of the target mass. A small value of this width, σ ～ 28 MeV/c, confirms the presence of a halo in ⁶He. The measurements performed with ⁶Li beams of energy 18 and 46 MeV per nucleon at the U-400M accelerator yielded a width value of σ ～ 50 MeV/c for the momentum distributions of ⁴He nuclei, which is intermediate between that for ⁶He and those for stable nuclei. A compilation of the widths of the momentum distributions of fragments originating from the breakup of various nuclei is presented versus the binding energy of one or two neutrons in these nuclei, the target mass and the projectile energy.     

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.     

Yields of hydrogen and helium isotopes in fragmentation of carbon ions with the energy of 0.3 GeV/nucleon

Data on the yields of hydrogen isotopes (p, d, t) and helium isotopes (from ³He to ⁸He) at the angle of 3.5° in fragmentation of the carbon ions with the energy of 0.3 GeV/nucleon on the beryllium target are reported. The momentum spectra of the fragments in the rest frame of the fragmenting nucleus are measured in a considerably larger momentum range than in the previous experiments.     

Mass Measurements in Nuclear Reactions

The present status of mass measurements from reactions producing nuclei at the driplines, including those unstable to nucleon or cluster emission, is discussed. The results of recent heavy ion and π-meson induced experiments on the study of the superheavy hydrogen isotopes (⁴H, ⁵H, ⁶H), helium (⁹He, ¹⁰He), lithium (¹⁰Li, ¹¹Li) and beryllium (¹³Be) are given. The possibilities of mass measurements in radioactive ion beam experiments are also considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Contributions of excited 6Li and 7Li nuclei to the production of 4He+2H and 4He+3H systems in 16O p collisions at a momentum of 3.25 GeV/c per nucleon

New experimental data on the cross sections for the yield of excited ⁶Li* and ⁷Li* nuclei and on their contributions to the production of ⁴He + ²H and ⁴He+³H light dinuclear systems in ¹⁶O _p collisions at a momentumof 3.25 A GeV/c per nucleon are presented.     

Nuclear-matter distributions of halo nuclei from elastic proton scattering in inverse kinematics

Proton-nucleus elastic scattering at intermediate energies, a well-established method for probing nuclear-matter density distributions of stable nuclei, was applied for the first time to exotic nuclei. This method is demonstrated to be an effective means for obtaining accurate and detailed information on the size and radial shape of halo nuclei. Absolute differential cross-sections for small-angle scattering were measured at energies near 700 MeV/u for the neutron-rich helium isotopes ⁶He and ⁸He, and more recently for the lithium isotopes ⁶Li, ⁸Li, ⁹Li and ¹¹Li, using He and Li beams provided by the fragment separator FRS at GSI Darmstadt. Experiments were performed in inverse kinematics using the hydrogen-filled ionization chamber IKAR which served simultaneously as target and recoil-proton detector. For deducing nuclear-matter distributions, differential cross-sections calculated with the aid of the Glauber multiple-scattering theory, using various parametrizations for the nucleon density distributions as input, were fitted to the experimental cross-sections. The results on nuclear-matter radii and matter distributions are presented, and the significance of the data for a halo structure is discussed. Nuclear-matter distributions obtained for ⁶He and ⁸He conform with the concept that both nuclei compose of α-particle like cores and significant neutron halos. The matter distribution in ¹¹Li exhibits, as expected from previous reaction cross-section studies with nuclear targets, the by far most extended halo component of all nuclei being investigated. In addition the present data allow a quantitative comparison of the structure of the He and Li isobares of either the mass number A = 6 or A = 8. The measured differential cross-sections have also been used for probing density distributions as predicted from various microscopic calculations. A few examples are presented. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: p.egelhof@gsi.de     

Transverse momentum distributions of a 9Li fragment in the (11Li, 9Li) reaction and neutron correlations

The transverse momentum distribution of a ⁹Li fragment from the (¹¹Li, ⁹Li) reaction at an incident energy of 800 MeV/nucleon is calculated within the Glauber theory and compared with the longitudinal one. The both distributions are not very much different at high energy. A simple hybrid model wave function[1] comprising both aspects of shell model and di-neutron cluster model gives a reasonable fit to the measured distribution.     

Momentum distributions in A = 3 and 4 nuclei

We develop a general Monte Carlo method to study momentum distributions of nucleons and nucleon clusters in nuclei. The method is used to calculate the momentum distributions of protons and neutrons in A = 3 and 4, d + p amplitudes in ³He, and t + p and d + d amplitudes in ⁴He nuclei, with improved variational wave functions. The nucleon and d + p momentum distributions in ³He are also calculated from a five-channel Faddeev wave function. The calculations are based on realistic hamiltonians that include the three-nucleon interaction, and give reasonable binding energies and densities for light nuclei and nuclear matter. The calculated proton and d + p momentum distributions in ³He at low k are in good agreement with the results obtained by the Saclay analysis of the electron scattering data in the plane-wave impulse approximation; however, at higher values of k, the calculated momentum distributions are larger. The calculated values of the asymptotic D- to S-wave ratio of the d+n and d+d amplitudes are also in agreement with the values obtained from (d, t) and (d, α) reactions. The number of deuterons is found to be 1.38 and ∼2.4 in A = 3 and 4 nuclei, while the number of tritons in ⁴He is ∼1.6. This large value of the triton number reflects the large contribution (more than 90% at small k) of the t+p state to the total proton momentum distribution in ⁴He.     

Kaon production in\bar pp interactions at c.m. energies from 200 to 900 GeV

A detailed analysis ofK _s ⁰ production in \(\bar pp\) |<2.5 the average transverse momentum is found to be 0.53±0.07 GeV/c at 200 GeV and 0.62±0.08 GeV/c at 900 GeV, which is an increase with respect to data at c.m. energies below 60 GeV. TheK _s ⁰ production cross sections in inelastic collisions are 29±4 mb at 200 GeV and 63±6 mb at 900 GeV, showing an increase compared to lower energy data. The central kaon density is found to increase as a logarithmic function of energy. At 900 GeV, where statistics are sufficient to allow one to draw conclusions, the average transverse momentum is higher in events with large charged multiplicity than in events with low multiplicity.     

Production of baryon resonances in collisions of 4He nuclei with carbon nuclei at a momentum of 4.2 GeV/c per nucleon

Under conditions of 4π geometry, Δ⁰-and Δ⁺⁺-resonance production in collisions of ⁴He nuclei with carbon nuclei at a momentum of 4.2 GeV/c per nucleon was investigated for the first time. The resonance masses and widths are estimated by analyzing the experimental and background invariant-mass distributions of (pπ ^±) pairs. Contributions to pion production from Δ-resonance decays, as well as from direct pion production, are determined. The results are compared with corresponding data obtained for CC collisions at the same primary momentum of 4.2 GeV/c per nucleon. By and large, the results obtained here comply with the existence of a collective mechanism of delta-isobar excitation in nuclei.     

Multi-nucleon transfer reactions and the formation of light charged particles in the systemnatAg+40Ar (285 MeV)

In the reaction^natAg+⁴⁰Ar (285 MeV) there have been measured the energy spectra of the isotopes of elements from H to Cl at an emission angle of 40°, the energy spectra and angular distributions of¹H,²H,³H, and⁴He, and the angular distributions of Li, Be, B, and C. The contribution from multinucleon transfer reactions to the formation of light charged particles is discussed.     

A dependence of the cross sections for the cumulative production of pions and protons in proton-nucleus and nucleus-nucleus interactions

The behavior of the inclusive cross sections for the cumulative production of π ^± mesons and protons in pA, DA, and ⁴HeA interactions is studied versus the atomic mass number of fragmenting nuclei. The primary-beam momenta were 4.5 GeV/c per nucleon. Secondary pions and protons were recorded with a fixed momentum of 0.5 GeV/c at an angle of 120°. Specifically, the experiment explored the fragmentation of D, He, ⁶Li, ⁷Li, C, Si, ⁵⁸Ni, ⁶⁴Ni, ⁶⁴Zn, ¹¹⁴Sn, ¹²⁴Sn, and Pb nuclei. The energy spectra of π ⁺ and π ^? mesons and protons with momenta in the range 0.3–0.7 GeV/c (the emission angle being 120°) were measured in an 8.9-GeV/c proton beam for Ni, Zn, and Sn isotopes. The special features in the behavior of the cross sections are found and discussed, and a comparison is drawn with the results of other studies.     

Fragmentation channels of relativistic 7Be nuclei in peripheral interactions

At the JINR Nuclotron, ⁷Li nuclei are accelerated. The charge-exchange reaction involving these nuclei at an external target provides a secondary 1.23-A-GeV ⁷Be beam. This beam is used to irradiate emulsion chambers. The mean free path λ_inel(⁷Be) = 14.0 ± 0.8 cm for inelastic ⁷Be interactions in an emulsion coincides within the errors with those for ⁶Li and ⁷Li nuclei. More than 10% of the ⁷Be events are associated with the peripheral interactions in which the total charge of the relativistic fragments is equal to the charge of the ⁷Be nucleus and in which charged mesons are not produced. An unusual ratio of the helium isotopes is revealed in the composition of the doubly charged fragments of the ⁷Be nuclei: the number of the ³He fragments is twice as large as that of the ⁴He fragments. Each of 50% of peripheral interaction events includes two doubly charged fragments. The channels of the ⁷Be fragmentation into charged fragments are presented. In 50% of events, the ⁷Be fragmentation occurs only into charged fragments without the emission of neutrons. The ⁴He + ³He channel dominates, whereas each of the ⁴He + d + p and ⁶Li + p channels constitutes 10%. Two events without neutron emission are observed in the ³He + t + p and ³He + d + d three-body channels. The mean free path for the coherent dissociation of relativistic ⁷Be nuclei into ⁴He + ³He is equal to 7 ± 1 m. The main features of the fragmentation of relativistic ⁷Be nuclei in such peripheral interactions are explained by the ³He + ⁴He two-cluster structure of the ⁷Be nucleus.     

Breakup temperature of target spectators in 197Au + 197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction ¹⁹⁷Au + ¹⁹⁷Au at 1000 MeV per nucleon. Pairs of ^3,4He and ^6,7Li isotopes and pairs of ^3,4He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decayat relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at τ_lab = 150°, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage.     

Further investigation of the γ-transitions in Λ4H and Λ4 hypernuclei

The γ-spectra induced by stopped K^? mesons in ⁶Li and ⁷Li targets in coincidence with the accompanying charged pions and π⁰ mesons. The γ-line at (1.04 ± 0.04) MeV observed with charged pions was identified as a γ-transition in _Λ⁴H, whereas the γ-line at (1.15 ± 0.04)_MeV observed with π⁰ mesons was ascribed to a γ-transition in _Λ⁴He. Using these new values for the excitation energies of the 1⁺ levels of _Λ⁴H and _Λ⁴He hypernuclei, the phenomenological ΛN potential for the s-state interaction was recalculated.     

ΛO production in π−p interactions at 20 GeV/c

Lambda production in 20 GeV/c π^? interactions in hydrogen is studied. The distributions of the Λ^O and pion c.m. production angle, transverse momentum and c.m. longitudinal momentum as functions of the charge multiplicity are discussed. The effective-mass distributions are also studied and it is found that Y^1±(1385) production occurs in 18% of the interactions.