Projectile Rest Frame Research Articles

Nuclear Fragments in 12C + 9Be Interactions at an Energy of 2 GeV per Nucleon

Differential cross sections for the production of nuclear fragments at an angle of 3.5° in the fragmentation of 2-GeV/nucleon carbon ions on a beryllium target were measured in the FRAGM experiment performed at the ITEP-TWA heavy-ion accelerator-accumulator complex. The predictions of four Monte Carlo models of ion-ion interactions (BC, INCL, LAQGSM, and QMD) were tested on the basis of a comparison with the momentum spectra of fragments. Successes and drawbacks of these models are discussed. The energy spectra of protons and 3He nuclei in the cumulative region in the projectile rest frame are well described by an exponential dependence, which make it possible to estimate temperature parameters of their emission source.

Measurement of Auger electrons emitted through Coster–Kronig transitions under irradiation of fast [formula omitted] ions

We measured the yield of Auger electrons emitted through Coster–Kronig transitions from Rydberg states 1s2p(3P)nl (n=7,8) and 1s22p(2P)nl (n=5,6,7) of emergent atomic ions Cq+ under irradiation of 3.5-MeV/atom C+ and C2+ ions on thin C foil targets. The Auger electron yields are suppressed for C2+ irradiation compared with C+ irradiation and the relative yield becomes larger as n increases. Thus, amount of scattered electrons having lower relative energy in the projectile rest frame becomes larger. The results obtained in this study support the influence of projectile velocity on the cluster effect of secondary electron yields.

One- and two-neutron removal cross sections of O24

Cross sections of one- and two-neutron removal reactions of 24O, leading to the 23O(½+) ground state and to bound final states of 22O, have been measured at the National Superconducting Cyclotron Laboratory. The experiment was conducted using the S800 spectrograph and a 24O beam energy of 92.3 MeV/u. The measured 23O ground state and 22O inclusive cross section values, of 74(11) mb and 146(33) mb, respectively, are in good agreement with calculations using eikonal reaction dynamics and shell-model nuclear structure overlaps. The widths at half maximum of the associated parallel momentum distributions of these cross sections, deduced from Gaussian fits, are 115(13) MeV/c for 23O and 309(36) MeV/c for 22O in the projectile rest frame. The data and calculations strongly support the shell-model description of 24O as a spherical, doubly-magic structure.

Small-angle fragmentation of carbon ions at 0.6 GeV/n: a comparison with models of ion-ion interactions

Momentum distributions of hydrogen and helium isotopes from 12C fragmentation at 3.5° were measured at 0.6 GeV/nucleon in the FRAGM experiment at ITEP TWA heavy ion accelerator. The fragments were selected by correlated time of flight and dE/dx measurements with a magnetic spectrometer with scintillation counters. The main attention was drawn to the high momentum region where the fragment velocity exceeds the velocity of the projectile nucleus. The momentum spectra of fragments span the region of the fragmentation peak as well as the cumulative region. The differential cross sections cover six orders of magnitude. The distributions measured are compared to the predictions of three ion-ion interaction models: BC, QMD and LAQGSM03.03. The kinetic energy spectra of fragments in the projectile rest frame have an exponential shape with two temperatures, being defined by their slope parameters.

Nuclear fragmentation study at ITEP heavy ion facility

In an experiment performed at ITEP TWA heavy ion accelerator, the yields of hydrogen (p,d,t) and helium (from 3He to 8He) isotopes at 3.5° from fragmentation of 12C at T0 = 0.2 − 3.2 GeV/nucleon on a Be target have been measured. Momentum spectra of the fragments in the projectile rest frame have been obtained in larger momentum intervals than in the previous experiments with heavy ion beams. The main attention was given to the region of high momentum where fragment velocity exceeds the velocity of the projectile nucleus. The obtained data cover about 6 orders of the differential cross section magnitude. It made possible the observation of a transition from the Gaussian shape of the longitudinal momentum spectra in projectile rest frame, expected for the evaporation mechanism, to the exponential shape, typical for the cumulative (pre-equilibrium) processes. The Feynman x distributions for protons are analyzed in the framework of quark-gluon string model. The probabilities of existence of six-and nine-quark clusters are estimated and compared with the results on two- (three-) nucleon short range correlations in nuclei measured at Jefferson Laboratory.

One-Neutron Removal Measurement RevealsO24as a New Doubly Magic Nucleus

The first measurement of the momentum distribution for one-neutron removal from (24)O at 920A MeV performed at GSI, Darmstadt is reported. The observed distribution has a width (FWHM) of 99 +/- 4 MeV/c in the projectile rest frame and a one-neutron removal cross section of 63 +/- 7 mb. The results are well explained with a nearly pure 2s_{1/2} neutron spectroscopic factor of 1.74 +/- 0.19 within the eikonal model. This large s-wave probability shows a spherical shell closure thereby confirming earlier suggestions that (24)O is a new doubly magic nucleus.

Spallation of 32S ions at ultra-relativistic energies in nuclear emulsion

Partial production cross sections of electromagnetic breakup of 32S projectiles at two widely differing energies were measured using an emulsion target. The electromagnetic dissociation represents about 6% and 17% of the total number of nuclear events at Elab/A = 3.7 and 200 GeV, respectively. The experimental electromagnetic cross sections σexpEMD show an energy dependence that is well reproduced by the Weizsacker–Williams approximation. The value of σexpEMD at 200A GeV (with γ >> 1) shows a good agreement with that predicted by the combined model of Pshenichnov et al. Examination of the reactions (γ, p) and (γ, He) reveals that the ratio of 1 p/1 He cross sections is close to unity at Dubna energy (3.7A GeV), while it rapidly increases to 3.0 ± 0.6 at CERN energy (200A GeV). In most of the observed dissociation modes, the total kinetic energy of the fragments in the projectile rest frame is lower than 50 MeV. Nevertheless, it is possible that the contribution of excitation modes of the different multipolarities is determining in this region. PACS No.: 25.70

Recent results on fast intermediate velocity electron production induced by 19+ 45 A MeV 58Ni highly charged ions on thin solid targets

In order to study the emission of energetic electrons induced by the impact of swift heavy ions on thin solid targets, we carried out a series of experiments at the superconducting cyclotron of the Catania Laboratori Nazionali del Sud. We report results on a recent experiment where electron–electron coincidences were measured in a forward ring by bombarding a thin carbon target of 7.4 μg/cm2 with 19+ 45 AMeV 58Ni beam. The velocity1–velocity2 bidimensional plot is dominated by events in which the two detected electrons have a velocity close to the beam velocity 9.03 cm/ns (convoy electrons). The remaining small fraction of coincidences has still a convoy electron and a second electron having either a velocity almost twice the beam velocity 16.5 cm/ns (binary encounter, BE electrons) or a velocity of about 12.7 cm/ns intermediate between BE and convoy velocities (IV electrons). We interprete this last intermediate component as due to in-flight de-excitation of highly excited n+58Ni ions by Auger electrons. Although less distinct, we observe also an intense peak close to the convoy velocity peak, centered at ≈9.7 cm/ns, corresponding to electrons emitted with an energy of only about ≈170 eV in the projectile rest frame of reference.

Momentum distribution of 15B fragments from the breakup of 17B.

The two-neutron removal cross section (sigma(-2n)) and the longitudinal momentum distribution of 15B fragments from the breakup of 17B on 9Be were measured at 70A MeV. The distribution in the projectile rest frame is characterized by a FWHM of 80+/-10 MeV/c for 15B. The sigma(-2n) is found to be 0.22+/-0.05 b. A Glauber-type analysis of the data provides clear evidence of a two-neutron halo structure in 17B.

Superelastic scattering of electrons from metastable He-likeC4+andO6+ions

Superelastic scattering, a collision process in which the scattered electron gains energy, has been investigated for metastable C 4 + (1 s2s 3 S) and O 6 + (1s2s 3 S) He-like ions colliding with quasifree H 2 -target electrons. The measured superelastic-scattering cross sections for 1s2s 3 S→1s 2 1 S deexcitation show a strong dependence on the collision energy and the atomic number of the He-like ion. R-matrix calculations for the time-reversed equivalent process of electron-impact excitation are compared with the measured cross sections through the principle of detailed balance. In these calculations, the impulse approximation is utilized in conjunction with the Compton profile of the target electrons for the collisions of ions with quasifree electrons studied here. Calculations were performed taking into account the anisotropy of the cross sections as a function of the scattering angle, which equals 180° in the projectile rest frame for the reported measurements, Comparison between theory and experiment was used to infer the metastable 1s2s s 3 S fraction in the incident He-like beam.

Projectile velocity and target temperature dependence of charge-state distributions of multicharged ions scattered during grazing interactions with a Au(110) surface.

We have made systematic measurements of scattered projectile charge-state distributions for multicharged He, N, Ne, Na, and Ar ions grazingly incident on a Au(110) single-crystal target. For projectiles whose neutral binding energies Lie below the valence band (N, Ar, He, and Ne), observed 1+ charge fractions were small at low velocities and steeply increased above well-defined threshold velocities. However, for Na projectiles with neutral binding energy above the Fermi level, a large (about 84%) 1+ charge fraction was found. The velocity dependence of the Na neutral fraction shows a ''kinematic resonance'' due to the virtual population of electronic states above the Fermi level in the projectile rest frame. The measured sample temperature dependence of the scattered 1+ charge fraction for Ne9+ projectiles incident along the [110] surface channeling direction reveals a significant decrease in projectile neutralization once the (2X1)-(1X1) phase transition temperature of the Au(110) surface at 650 K has been reached.

Two-electron excitation to Rydberg levels in fast I 6+ on hydrogen collisions

The emission of electrons in the forward direction in collisions of 0.3 MeV/u I 6+ with H 2 has been studied, and strong autoionization peaks are observed on the shoulder of the cusp peak. The energies of these autoionization lines in the projectile rest frame are determined by high-resolution electron spectroscopy. Using the electron projectile final charge state coincidence technique, we probe different collision mechanisms, which create continuum electrons that are slow in the projectile rest frame. We conclude that the observed autoionization lines are due to two electron excitation to projectile Rydberg levels.

Dominant two-step process in nuclear fragmentation at high energies.

Definitive measurements of negative {l angle}{ital p}{sub {ital L}}{r angle} in the projectile rest frame are carried out for isotopes produced by fragmentation of relativistic {sup 16}O and {sup 12}C nucleus projectiles. They are analyzed for the evidence of the dominant two-step diffractive excitation processes during the fragmentation. Average energies in the range of 100 MeV or less are explored and the decays of the excited states with lifetimes longer than 10{sup {minus}21} s are investigated.

Orbital dispersion and wavefunction mapping in inclusive break-up experiments

Inclusive break-up cross sections of the reactions12C (6Li,a)X and208Pb (6Li,a)X at 26 MeV/amu, observed for the emission of break-up fragment into the forward hemisphere, are analysed in view of the information about the internal momentum distribution of the projectile. The longitudinal momentum distribution of the fragments in the projectile rest-frame is found to reflect the internal momentum distribution of the clusters whereas the transverse distribution is affected by the projectile-target interaction.

Transfer excitation processes in ion-atom collisions at high energies

A theory of ion-atom and ion-molecular collisions at high energies is formulated in which the projectile ions undergo simultaneous electron transfer and excitation (TE). A unified description of both resonant and nonresonant processes is presented. The resonant mode (RTE) is mediated by one of the target electrons, which excites the projectile and at the same time is captured by the projectile ion, thus forming a doubly excited intermediate state of the projectile ion. In the impulse approximation, this is related to the dielectronic recombination process in electron-ion collisions. The same doubly excited states can also be formed by one of the target electrons and the target core, interacting with the projectile ion in which the target core does the exciting and the electron is simply transferred. This is a nonresonant process (NTE), although the projectile is again left in a doubly excited state. We propose here still another mechanism for the formation of doubly excited states, UTE. The new process involves two target electrons in a nonresonant way, just as in the NTE, such that the electron that excites the projectile is left in the continuum, as viewed in the projectile rest frame. Some of the experimental evidence for thesemore » reactions is discussed.« less

Inclusive neutron spectra at 0 degrees from the reactions Pb(Ne,n)X and NaF(Ne,n)X at 390 and 790 MeV per nucleon.

Inclusive neutron spectra at 0\ifmmode^\circ\else\textdegree\fi{} were measured from both 390- and 790-MeV/u Ne ions on targets of Pb and NaF. A striking peak at a neutron energy slightly below the beam energy per nucleon has a width (in the projectile rest frame) of \ensuremath{\sim}58 MeV/c, which is essentially the same for these two energies and these two targets. We suggest that this narrow peak is projectile evaporation and that it is superimposed on the broader fragmentation process predicted by statistical models. The high-momentum region of the cross section may reflect collective behavior.

Evidence for two different reaction mechanisms in relativistic heavy-ion collisions

An exclusive-type study of the transverse momentum distribution of alpha particles emerging as relativistic projectile fragments from collisions of a 1.9 GeV/nucleon 56Fe beam in nuclear emulsion reveals that the events fall into either one or the other of two distinct classes with sharply differing characteristics. Each of these classes is characterised by a Maxwellian distribution in the projectile rest frame, but the associated temperatures differ by a factor of about four (namely approximately=10 MeV and approximately=40 MeV; the same 40 MeV temperature is observed among target-related alpha particles emitted from 12C collisions in nuclear emulsion at 0.87 GeV/nucleon and 2.1 GeV/nucleon). The authors show that the 'hot' (T approximately=40 MeV) events are not simulated by 'projectile bounce' effects, and that they occur among events of high multiplicity as well as among those of low multiplicity. They are regarded as direct evidence for a highly collective mechanism.

Momentum Distributions of Isotopes Produced by Fragmentation of RelativisticC12andO16Projectiles

The fragment momentum distributions in the projectile rest frame are, typically, Gaussian shaped, narrow, consistent with isotropy, depend on fragment and projectile, and have no significant correlation with target mass or beam energy. The nuclear temperature is inferred from the momentum distributions of the fragments and is approximately equal to the projectile nuclear binding energy, indicative of small energy transfer between target and fragment.