Relativistic Heavy Ions Research Articles

Energy deposition by relativistic heavy ions in thin argon absorbers

Experimental data on energy deposition for argon, krypton and xenon ions in the energy range from 100 to 950 MeV/u passing through an ionization chamber filled with an Ar (90%) + CH 4 (10%) gas mixture of thickness equivalent to 60 mg/cm 2 of argon are reported. Measured values are compared with the proposed model of energy deposition and with the model of Badhwar and Adams et al. The observed data support the picture of the escape of high energy δ rays from the active detector volume.

Electron and positron emission angle distributions in pair production by relativistic heavy ions

Abstract The cross section for direct electron-positron pair production by relativistic heavy ions, differential in the emission angle of one of the particles, is calculated using lowest order QED theory. A general method is given for computing all the terms in the exact lowest order expression, based on previous work. Some numerical results are given for selected incident particle energies and the results are compared with the corresponding calculation using the Weizsacker-Williams method.

Risk from relativistic heavy ions on manned space missions.

The risk from exposure to radiation posed to space travelers outside the magnetic shielding provided by the geomagnetosphere will come from two sources: the slowly varying but low intensity high-energy galactic cosmic rays and the more intense predominantly low-energy protons from large solar particle events associated with magnetic disturbances originating sporadically on or within the solar surface during the active period of the 11-year solar cycle. The energy spectra of the protons in solar particle events are quite soft, with large numbers of low-energy protons and a rather steep decrease of the energy spectra with increasing energy. This allows for the possibility to provide, within the space vehicle or habitat, a well-shielded area sometimes called a "storm shelter" or "safe haven" where the travelers could gather during the largest particle events. Intensity risetimes on the order of half an hour or more and overall event durations of 1 to 2 days would make actively seeking a well-shielded shelter for the duration a distinct possibility. The high-energy and penetrating nature and relative constancy of the galactic cosmic rays, on the other hand, do not allow the use of highly shielded areas as a means of protection against them. The first question to answer becomes: what is the risk to human health from the galactic cosmic rays? We need to have a good idea of the answer to this question before we can address the problem of how to best protect human health or, indeed, whether any specific measures need to be taken.

The GSI projectile fragment separator (FRS): a versatile magnetic system for relativistic heavy ions

The projectile fragment separator FRS designed for research and applied studies with relativistic heavy ions was installed at GSI as a part of the new high-energy SIS/ESR accelerator facility. This high-resolution forward spectrometer has been successfully used in first atomic and nuclear physics experiments using neon, argon, krypton, xenon, and gold beams in the energy range from 500 to 2000 MeV/u. For the first time relativistic xenon and gold fragments have been isotopically separated. In this contribution we describe first experiments characterizing the performance of this spectrometer.

Cross section for production of low-energy electron-positron pairs by relativistic heavy ions.

Starting with the lowest-order unscreened QED matrix element for electron-positron pair production by heavy charged particles, the paper calculates the cross section for this process differential in all independent variables and valid for all pair energies small compared to the incident particle energy. Integration over the possible emission angles of one of the pair members gives an expression that is valid for low-energy pairs that can be compared with previous work based on the Weizsaecker-Williams method. Integration over the possible angles of the other pair member then yields an expression identical to one derived by Racah. The high energy-transfer limit of the expression for the cross section integrated over electron and positron angles is found to be identical to that of Kelner in the unscreened case.

Multiple electron-positron pair production in relativistic heavy-ion collisions: A strong-field effect.

First-order calculations of ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ pair production in relativistic heavy-ion collisions exceed unitarity for heavy nuclei and \ensuremath{\gamma} values of the order of \ensuremath{\gamma}g1000, at impact parameters of the order of the Compton wavelength of the electron. It is shown how unitarity can be restored by using the sudden approximation, which is very appropriate for this kind of collision. It is found that the probability of creating N ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ pairs in a single collision is given by a Poisson distribution. Such strong-field effects will occur and can in principle be studied with relativistic heavy ions in the planned colliders like the relativistic heavy-ion collider at Brookhaven National Laboratory or the large hadron collider at CERN.

Dynamical scaling in multifragmentation reactions.

The scaling behavior of the observed fragment yields in the reactions induced by high-energy proton and relativistic heavy ions can be explained in terms of a rate equation. In the mean field approximation we have been able to show that the solution of the rate equation after a large time develops a scaling form that can then be compared with experiments.

Relation between scintillation and ionization produced by relativistic heavy ions in liquid argon.

The scintillation intensity S and the collected charge Q in liquid argon have been compared with each other for projectiles of relativistic protons and He, La, and Au ions. According to an elementary theory the quantity S/S/sub 0/ is a linear function of the quantity Q/Q/sub infinity/, where S/sub 0/ is the scintillation intensity at zero electric field and Q/sub infinity/ is the charge to be collected at an infinite field. The experimental results for protons and He ions lie between the line and the data for 1-MeV electrons. The results for La ions lie along the line. The results for Au ions show systematic deviations from the line and can be explained by quenching of scintillation.

Collective spin-isospin vibrations and δ-excitation in heavy-ion collisions

A coherent mechanism for the excitation of spin-isospin modes in heavy ion reactions is presented and discussed in two kinematic regimes. The spin-isospin response of nuclei in the particle-hole and in the Δ-hole channels is first studied in charge exchange reactions induced by relativistic heavy ions. Recent data on the reaction 12C[ 12C, 12N(1 +; GS)] at 900 and 1100 MeV per nucleon are particularly suited to study these modes and can be well described by the exchange of effective phonons between the projectile and the target; these effective phonons are built up from the summed interactions between the individual nucleons of the colliding ions. This mechanism in the Δ-hole channel is then used to derive an estimate of the cross section for subthreshold pion production in the 12C[ 12C, 12C(1 +; T = 1; 15.11 MeV)] reaction at 95 MeV per nucleon.

Coulomb and photo cross sections for nucleon emission by relativistic heavy ions and application to 40Ar on 89Y

The Coulomb-induced emission of nucleons by relativistic heavy ions is studied with respect to partial cross sections related to the various multipole orders of the electromagnetic field. Since the partial Coulomb cross sections can be factorized into the numbers of virtual photons and the photo cross sections, we first calculate the photo cross sections using the oscillator shell model and 1-particle-1-hole configurations. Then the Coulomb cross sections are obtained by folding the photo cross sections with the numbers of virtual photons. We apply the calculations to the Coulomb scattering of 40Ar ions on 89Y targets, where experiments at E lab = 1.8 GeV/ nucleon were carried out by Mercier et al. The contributions of the various multipole orders are discussed as functions of the projectile energy and the energy range of the emitted nucleons. Also methods for the separation of the most contributing multipole orders are developed.

Improved calculation of total cross section for pair production by relativistic heavy ions.

A calculation of the total cross section for direct electron-positron pair production by heavy ions is described. It combines the use of the Weizs\"acker-Williams method for low-energy transfers and existing calculations for high-energy transfers. Higher-order corrections to the total cross section are calculated based on the Weizs\"acker-Williams method and existing results for pair production by photons.

Electromagnetic excitation of one- and multiphonon giant resonance states with relativistic heavy ions

Electromagnetic excitation of one- and multiphonon giant resonance states with relativistic heavy ions

Characteristics of CR-39 doped with chlorinated compounds

Characteristics of CR-39 plastic containing chlorinated compounds such as HCB and DCD have been studied using relativistic heavy ions from the LBL Bevalac. These results are compared with those of pure CR-39, focusing in particular on etching properties and on whether a penetrating etch hole is produced along each particle path by a long duration etch. The relation between the reduced track etch rate and the etchant concentration for CR-39 containing HCB is quite different from that of pure CR-39. Dilute etchants gives much higher track sensitivities than do more concentrated solutions in a wide range of REL. Since type HCB/DCD CR-39 loss its sensitivity in the interior of the bulk material for low REL produced by 2.1 GeV/ n Ne ions, penetrating etch holes are not produced along the particle paths after a heavy etching.

Improvements of CR-39 for massive magnetic monopoles search

We have been carrying on massive magnetic monopole search using CR-39 track detector array. For this experiment, we improved the characteristics of CR-39 plastic. A number of different types of CR-39 have been studied by using relativistic heavy ions. These results are compared each other, focussing in particular on the sensitivity for penetrating holes which are produced along each particle path and on the stability for a long exposure. The results for CR-39 doped with a small amount of antioxidants (Naugard 445) indicate significant improvement in all regards.

Atomic collisions with relativistic heavy ions. IX. Ultrarelativistic collisions.

The behavior of projectile electron capture and ionization cross sections at ultrarelativistic energies (g10 GeV/amu) is discussed. Three mechanisms contribute to electron capture at these energies: radiative and nonradiative capture and the capture of the electron following electron-positron pair production. The radiative and nonradiative cross sections both fall off as ${\ensuremath{\gamma}}^{\mathrm{\ensuremath{-}}1}$ at high energies, (\ensuremath{\gamma}-1)${\mathrm{Mc}}^{2}$ being the ion energy. The vacuum capture cross sections increase as the ln\ensuremath{\gamma}. Projectile 1s ionization cross sections also increase as ln\ensuremath{\gamma} at high energies if target screening is negligible. However, when the projectile atomic number is smaller than the target atomic number, the transverse interaction giving the ln\ensuremath{\gamma} term is screened and reduced. Therefore, the total 1s ionization cross sections are dominated by Coulomb ionization, and are nearly constant with increasing projectile energy. Tables of reduced capture and ionization cross sections are given, and are applied to calculating the charge states of ions in matter and the lifetime of ions in storage rings.

Atomic collisions with relativistic heavy ions. VIII. Charge-state studies of relativistic uranium ions.

Charge-state distributions were measured for incident 105-, 220-, 430-, and 955-MeV/amu U ions on thin and thick solid targets. Initial bare, one-, two-, three-, nine-, and 24-electron ions were used, so that single and multiple ionization of K-, L-, and M-shell electrons and electron capture into bare ions and into ions having initially full or partially full K and L shells could be observed. Multiple-ionization effects in two-electron U ions and in ions having many electrons in the K, L, and M shells are calculated by using the semiclassical approximation and assuming binomial statistics. Binding effects on K-shell ionization are compared with previous measurements using relativistic Xe ions, and with the Glauber theory. For 430- and 955-MeV/amu U ions, good agreement is obtained between measured capture cross sections and calculated radiative and nonradiative (eikonal-approximation) cross sections. Some disagreement is found at lower energies. An approximate model is proposed for multiple-capture cross sections. Finally, the measured equilibrium charge states are compared to ground-state models and to models including excited-state effects. Excited-state effects are less important for relativistic U ions than for any other ion in matter. Differences between the excited- and ground-state models are found to be too small to be observed.

High energy heavy ions: techniques and applications

Pioneering work at the Bevalac has given significant insight into the field or relativistic heavy ions, both in the development of techniques for acceleration and delivery of these beams as well as in many novel areas of applications. This paper will outline our experiences at the Bevalac; ion sources, low velocity acceleration, matching to the synchrotron booster, and beam delivery. Applications discussed will include the observation of new effects in central nuclear collisions, production of beams of exotic short-lived (down to 1 μs) isotopes through peripheral nuclear collisions, atomic physics with hydrogen-like uranium ions, effects of heavy “cosmic rays” on satellite equipment, and an ongoing cancer radiotherapy program with heavy ions.

Atomic collisions with relativistic heavy ions. III. Electron capture.

Cross sections for the electron capture by 82- , 140- , and 200-MeV/amu ${\mathrm{Xe}}^{54+}$, ${\mathrm{Xe}}^{53+}$, and ${\mathrm{Xe}}^{52+}$ ions incident on thin solid targets from Be to Au were measured. The measurements are compared with calculations of radiative and nonradiative capture. The nonradiative capture calculations are based on the relativistic eikonal approximation which includes transitions from K, L, and M shells of the target to nearly all projectile shells. In high-Z targets, nonradiative capture into excited states of the projectile is dominant, as predicted by the eikonal calculations and confirmed by comparing measured cross sections for bare ${\mathrm{Xe}}^{54+}$ and ${\mathrm{Xe}}^{52+}$ (1${s}^{2}$) projectiles. A simple formula for estimating nonradiative-electron-capture cross sections for relativistic-ion accelerator design and other applications is derived. Double and triple electron capture are also observed in the low-velocity, high-Z collisions.

Nuclear fragmentation processes in the 20Ne + 27Al system at 30 MeV/ A

Inclusive energy spectra and angular distributions of Z ≦ 9 fragments from the Ne + AI reaction have been measured at 30 MeV/ A. The spectra present three components. The high-energy component has a reduced momentum width σ 0 of approximately 65 MeV/ c, which is lower than the values observed in reactions induced by relativistic heavy ions. Conversely, for the low-energy component, the results are rather similar to high-energy heavy-ion data.

Relativistic heavy ions at bnl—ongoing projects and plans for the future

The AGS at Brookhaven National Laboratory is now being prepared to accelerate heavy ions up to mass 32 to energies of ∼15 GeV/ A. Negative ions are to be made in a sputter ion source, accelerated in the BNL tandems to energies of ∼ 7 MeV/ A, stripped of all electrons, and transported the 2100 ft to the AGS. Some considerations for the injection and acceleration of tandem beams directly into the AGS are described as are future plans for the acceleration of heavier mass ions in the AGS and ultimately the acceleration of ions in a two ring collider (RHIC) to 100 GeV/A. The physics programs for which these accelerators are designed are briefly discussed.