Collisions Ions Answer Key
Rojo Scherer
The last decades of heavy ion collisions have been marked by significant progress in understanding the transverse structure and dynamics in heavy ion collisions. Spurred by the observations of highly distorted correlation structures in heavy ion collisions the community has developed a detailed picture encapsulated in sophisticated dynamical models to capture these details. These models have led to significant improvements in our understanding of the emergent properties of high density, high temperature QCD, including transport properties.
While many of the questions are fascinating in their own rights, failure to answer some will make it difficult to draw conclusions about other aspects of Heavy Ion collisions, including the nature of net baryon fluctuations and the extent to which hydrodynamic noise influences the correlation functions used to determine the transport properties of the QGP. In this workshop, we will discuss these open questions and the opportunities that may be available to answer these questions in a physics program that could be conducted during the final phases of RHIC operations.
Presentations will be mainly by invitation. We expect to be able to accommodate a limited number of contributed talks. If you are interested in presenting a contributed talk, please contact Paul Sorensen directly atps...@bnl.gov.
Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for the U.S Department of Energy's Office of Science by Brookhaven Science Associates.
Classically, electrons collide with other electrons and massive (by comparison), stationary positive ions as they conduct down a wire when an electric field is applied. Is there a good mechanism according to classical theory for the collisions between the electrons and positive ions? Is said mechanism electron scattering? If so, why would these oppositely charged particles only ever approach one another but never actually touch (which I assume is how they behave)?
The collisions between the electrons and the ions in the wire is not electron scattering. Electron scattering is when the electrons bounce off from a crystal lattice of atoms, displaying wave characteristics. That is a completely different thing right there.
First of all, even within a lattice, the electron has a lot more free space to go undisturbed. But since we are talking about electrons from every one of those atoms, then surely some time or the other, the electron would get close to some nucleus and lose a bit of their kinetic energy. So, the electron's speed goes down and the direction is changes due to the attraction from the nucleus. I suppose in many situations, there are as many electrons that travel through the lattice undisturbed as there are those electrons that are disturbed in their trajectories and hence slowed down - or even absorbed.
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Is there a saddle-point mechanism for ionization in intermediate-energy ion-atom collisions? Since Olson [Phys. Rev. A 33, 4397 (1986)] proposed the idea that the electrons stranded in the potential saddle between the two Coulomb centers dominate the ejected-electron spectra, multiple experimental and theoretical attempts have been made to answer this question. However, the topic has remained controversial. Here we provide a theoretical analysis of this question which can contribute significantly to a definitive answer, at least for intermediate and large projectile energies. To this end we calculate the energy and angular distribution of electrons emitted in proton-helium collisions. We use the two-center four-body wave-packet convergent close-coupling method based on the correlated two-electron structure for the helium target. The doubly differential cross sections obtained at 52 and 103 keV show no sign of a hump near one-half the relative velocity of the collision, which is expected according to the saddle-point ionization theory. At the same time, the results are in excellent agreement with measurements by Meckbach et al. [J. Phys. B 24, 3763 (1991)]. Two mechanisms for the production of electrons are clearly identified: direct ionization (direct knockout) and electron capture to the continuum (ECC) of the projectile. The electron speed (equivalently, energy) where direct ionization peaks is found to be practically independent of the ejection angle. However, the ECC peak is shown to shift towards one-half the relative velocity with increasing electron angle. It is concluded that the signatures of the suggested saddle-point mechanism may actually be due to a shift of the well-known ECC peak when electrons are emitted into angles away from the forward direction. Thus, the answer to the question is in the negative.
Doubly differential cross section for p-He ionization at 52 keV. The WP-CCC results are compared with data from Meckbach et al. [23]. The DI and ECC components are also shown. The key in the bottom right panel applies to all panels.
The ionization of a hydrogenlike heavy ion by the impact of a charged projectile under simultaneous irradiation by a short laser pulse is investigated within the nonperturbative approach, based on numerical solutions of the time-dependent Dirac equation. Emphasis is placed on the question of whether the laser- and impact-ionization channels interfere with each other and how this interference affects the ionization probability. To answer this question we perform detailed calculations for the laser-assisted collisions between hydrogenlike Pb81+ and α particles. The results of the calculations clearly indicate that for the experimentally relevant set of (collision and laser) parameters, the interference contribution can reach 10% and can be easily controlled by varying the laser frequency.
Probability wα of the ground-state ionization of hydrogenlike lead by an α particle impact: (a) wα, calculated for the center-of -mass energy Ec.m.=10 MeV, as a function of impact parameter ρ and (b) energy dependence of the ionization probability for the head-on collision, ρ=0.
Less than 24 hours after the end of the Large Hadron Collider's first high-energy run of proton collisions, beams of lead ions are already circulating in the LHC. If you're like us, you've probably spent the last two years hearing about the importance of proton collisions and learning about how protons are accelerated and collided at the LHC. So now you might be wondering: What are lead ions? Is accelerating them any different than protons? And why dedicate one of the LHC's precious months of operation to lead-ion collisions?
Symmetry is here to help. In this article we'll answer the first two questions and briefly touch on the third. A follow-up article next week will delve more deeply into the physics research that scientists on the ALICE, ATLAS and CMS experiments will do with lead-ion collision data.
Lead ions start as lead atoms, which in nature have an atomic nucleus containing 82 protons and between 122 and 126 neutrons, surrounded by a cloud of 82 electrons. An atom of lead becomes an ion of lead when some or all of its electrons are stripped away, leaving the remaining portion of the atom positively charged. The LHC acceleration process gradually strips away all of the lead atoms' electrons, leaving a beam composed only of lead nuclei.
The LHC only accelerates one type, or isotope, of lead that contains 126 neutrons. Since protons and neutrons have approximately the same mass, an LHC lead ion weighs roughly 208 times more than a proton. It's no wonder that physicists refer to lead ions as "heavy ions."
The first accelerator to collide heavy ions was the appropriately named Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York, which set the current world record for highest-energy heavy-ion collisions by smashing gold ions at energies of 200 GeV per nucleon. (Physicists prefer to quote heavy-ion beam energies "per nucleon," where protons and neutrons are both nucleons, as this allows for an easier comparison with the energies of beams of protons and other types of ions.)
The LHC's lead-ion beams start with a piece of pure lead 2 centimeters long that weighs 500 milligrams. The lead "sample" is heated to about 500 degrees Celsius to vaporize a small number of atoms. An electrical current is used to remove a few of the electrons from each atom, and then the newly created ions begin the ride of their life.
The ions first travel through a linear accelerator called Linac3, picking up a small amount of energy (.0045 GeV per nucleon) before having more electrons removed. Next, the ions are accumulated and accelerated to .072 GeV per nucleon in the Low Energy Ion Ring, or LEIR. These first three stages in the process - vaporization and acceleration in Linac3 and LEIR - are unique to ions, but as soon as they leave the LEIR they travel the same path as their lighter cousins the protons.
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