Atom cascaded

Fig. 1. Atomic cascade in antiprotonic hydrogen. The hadronic interaction is observed by a level shift and a broadening of the low-lying atomic levels as compared to the calculated binding energies and radiative decay widths assuming a pure electromagnetic interaction...

Exotic atoms are produced by stopping a beam of negatively charged particles like muons, pions, or antiprotons in a target, where they are captured in the Coulomb potential of the atoms at high principal quantum numbers n. These systems deexcite mainly by fast Auger emission of electrons in the upper part of the atomic cascade and more and more by X-radiation for lower-lying states. 

The experiments aiming at the determination of the charged pion mass use X-ray transitions from the intermediate part of the atomic cascade, where the influence of the hadronic interaction can be neglected. The two most recent experiments both measured the (5from different elements. In one case, a solid state Mg target was taken [15,16], whereas the most recent experiment used an N2 gas target [12,17]. 

Figure IS. Flow chart for hot atom cascading faUoff simulation dashed boxes represent input data (95 ...

Muons bound in an atomic orbit in transuranium elements can also cause nuclear fission, but that mechanism is quite involved (Measday 2001). There are two types of fission events, prompt and delayed. The prompt events are induced by energy from the atomic cascade of the muon, whereas the delayed events are from muon capture via the weak interaction. 

As the X-ray transition energies are proportional to the reduced mass of the system and maybe split due to spin-orbit and spin-spin interactions, studies in the intermediate region of the atomic cascade are used to measure the mass, charge, and the magnetic moment of certain negative particles. Such an example will be shown later for the case of the antiproton. 

Physical information can be obtained, e.g., by measuring the shift and broadening of the energy levels of the hadron in the atom due to nuclear interactions as compared to a purely electromagnetic case. The shifts and widths can be measured for one or two levels only in each kind of hadronic atom as the strong nuclear absorption - which causes the level broadening -terminates the atomic cascade at a certain principal quantum number This lowest n sensitively depends on the mass of the particle and on the atomic number of the nucleus min = 1 for pionic oxygen and 8 for antiprotonic nickel. 

The polarizers used in this experiment were of some interest, being of the pile-of-plates variety, with each polarizer consisting of two sets of seven plates symmetrically arranged so as to cancel out transverse ray displacements. A typical time correlation spectrum for this type of experiment using an atomic cascade is shown in Figure 10. 

Figure 13. Interfering decay channels in a calcium atomic cascade in the presence of an applied magnetic field. ...

The steady state theory begins with a fictionalized representation of the hot atom cascade in which the entire collection of atoms is assumed to be present initially. This mathematically convenient model is also rigorously applicable, provided that the recoil atoms are mutually non-interacting and that they are produced with very small total concentration and with spatial uniformity throughout the host reservoir. 

A. Aspect, C. Imbert, and G. Roger, Absolute Measurement of an Atomic Cascade Rate Using a Two Photon Coincidence Technique. Application to the 4p Sq - 4s4p P - 4s21Sq Cascade of Calcium excited by a Two Photon Absorption, Opt. Comm. 34 46 (1980). 

J.S. Bell, Atomic-cascade Photons and Quantum-Mechanical Nonloca-... 

Ar, Cs, Ga or other elements with energies between 0.5 and 10 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection of atoms, ions and multi-atomic clusters (figure Bl.25.8). In SIMS, secondary ions (positive or negative) are detected directly with a mass spectrometer. 

The momentum of a fast-moving atom or ion is di.ssipated by collision with the closely packed molecules of the liquid target. As each collision occurs, some of the initial momentum is transferred to substrate molecules, causing them in turn to move faster and strike other molecules. The result is a cascade effect that ejects some of the substrate molecules from the surface of the liquid (Figure 4.2). The process can be likened to throwing a heavy. stone into a pool of water — some... 

A typical cascade process. A fast atom or ion collides with surface molecules, sharing its momentum and causing the struck molecules to move faster. The resulting fast-moving particles then strike others, setting up a cascade of collisions until all the initial momentum has been redistributed. The dots ( ) indicate collision points, tons or atoms (o) leave the surface. 

In a cascade process, one incident electron (e ) collides with a neutral atom ((S)) to produce a second electron and an ion ( ). Now there are two electrons and one ion. These two electrons collide with another neutral atom to produce four electrons and three ions. This process continues rapidly and — after about 20 successive sets of collisions — there are millions of electrons and ions. (The mean free path between collisions is very small at atmospheric pressures.) A typical atmospheric-pressure plasma will contain 10 each of electrons and ions per milliliter. Some ions and electrons are lost by recombination to reform neutral atoms, with emission of light. 

If the temperature were raised, more molecules would attain the excited state, but even at 50,000°C there would be only one excited-state atom for every two ground-state atoms, and stimulated emission would not produce a large cascade effect. To reach the excess of stimulated emissions needed to build a large cascade (lasing), the population of excited-state molecules must exceed that of the ground state, preferably at normal ambient temperatures. This situation of an excess of excited-state over ground-state molecules is called a population inversion in order to contrast it with normal ground-state conditions. 

Interaction of an excited-state atom (A ) with a photon stimulates the emission of another photon so that two coherent photons leave the interaction site. Each of these two photons interacts with two other excited-state molecules and stimulates emission of two more photons, giving four photons in ail. A cascade builds, amplifying the first event. Within a few nanoseconds, a laser beam develops. Note that the cascade is unusual in that all of the photons travel coherently in the same direction consequently, very small divergence from parallelism is found in laser beams. 

The two electrons emerging from the collision are again speeded until each produces another electron by collisional ionization of another atom of argon. The process continues so the first incident electron becomes two, the two become four, and so on. This cascade increases the number of electrons and ions in the gas to form a plasma within a few milliseconds. 

Fig. 1. The ballistic interactions of an energetic ion with a sohd. Depicted are sputtering events at the surface, single-ion /single-atom recoil events, the development of a collision cascade involving a large number of displaced atoms, and the final position of the incident ion. ° = normal atom ...

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

CoUision cascades (see Fig. 1) lead to a distribution of vacancies, interstitial atoms, and other types of lattice disorder in the region around the ion... 

The defects generated in ion—soHd interactions influence the kinetic processes that occur both inside and outside the cascade volume. At times long after the cascade lifetime (t > 10 s), the remaining vacancy—interstitial pairs can contribute to atomic diffusion processes. This process, commonly called radiation enhanced diffusion (RED), can be described by rate equations and an analytical approach (27). Within the cascade itself, under conditions of high defect densities, local energy depositions exceed 1 eV/atom and local kinetic processes can be described on the basis of ahquid-like diffusion formalism (28,29). 

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