Cluster emission

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Cluster emission is an exotic decay that has some commonalities with a-decay. In a-decay, two protons and two neutrons that are moving in separate orbits within the nucleus come together and leak out of the nucleus as a single particle. Cluster emission occurs when other groups of nucleons form a single particle and leak out. Several of the observed decays are shown in Table 10. The emitted clusters include C, Ne, Mg, and Si. The... 

Molecular cluster ions are highly useful because they reveal which elements are in contact in the sample. Of course, this presupposes that such clusters are emitted intact and are not the result of recombination processes above the surface. Oechsner [12] collected evidence that direct cluster emission processes predominate in cases where relatively strong bonds exist between neighbor atoms. Direct emission be-... 

Cluster emission, 27 305 Cluster glass transitions, 74 469 Clustering techniques, 6 16-17 Cluster sampling, 26 1018 C-Methylcalix[4]resorcinarene, 74 165 CMOS image sensors, fabrication... 

Fig. 10 (a) Fluorescence image of methanol-fixed NIH3T3 cells loaded with peptide encapsulated silver clusters for 1 h at room temperature, (b) Time profile of the time series images of cell stained with silver nitrate showing the fast silver cluster emission centered in the nucleus at short times with a maximum at 320 nm. Note that black indicates an intermediate intensity level in this color scheme [57]... 

It was relatively recently that heavy cluster emission was observed at a level enormously lower than these estimates. Even so, an additional twist in the process was discovered when the radiation from a 223Ra source was measured directly in a silicon surface barrier telescope. The emission of 14C was observed at the rate of 10-9 times the a-emission rate, and 12C was not observed. Thus, the very large neutron excess of the heavy elements favors the emission of neutron-rich light products. The fact that the emission probability is so much smaller than the simple barrier penetration estimate can be attributed to the very small probability... 

In the course of the fusion of the projectile and target nuclei, it is possible that one of the partners will emit a single nucleon or a nucleonic cluster prior to the formation of a completely fused system. Such processes are referred to as preequilibrium emission (in the case of nucleon emission) or incomplete fusion (in the case of cluster emission). As the projectile energy increases, these emission processes become more important and they generally dominate over fusion at projectile energies... 

A second type of cluster emission involves molecular species which can be as simple as carbon monoxide or as complicated as the dodecanucleotide mentioned above. In the first case, the CO bond strength is 11 eV, but the interaction with the surface is only about 1 eV. Calculations indicate that this energy difference is sufficient to allow ejection of CO molecules, although 15 percent of them can be dissociated by the ion beam or by energetic metal atoms (6). For such molecular systems it is easy to infer the original atomic configurations of the molecule and to determine the... 

Figure 2.4. Cluster contour maps superposed on optical images (see Jones Forman 1999 for details). The clusters illustrate the morphological classes described in the text and listed in Table 2.2. The last class, G , where the cluster emission is dominated by that from individual galaxies, is not shown. 

Molecular cluster ions are very useful because they reveal which elements are in contact in the sample. Of course, this presupposes that such clusters are emitted intact and are not the result of recombination processes above the surface. Oechs-ner [13] collected evidence that direct cluster emission processes predominate in the case when relatively strong bonds exist between neighbor atoms. Direct emission becomes even more likely if the two atoms differ significantly in mass, and when the heavier atom receives momentum from the sputter cascade in the solid. Thus, there is little doubt that clusters of the type ZrO+, FeCl3-, MoS+, CH3+, PdCO+, or Rh2NO+ (which we will encounter in the applications later) stem from direct emission processes and reflect bonds present in the sample [2, 4], Some evidence exists, however, that atomic recombination may play a role in the SIMS of metals, and in alloys where the two constituents have comparable mass... 

One should also notice that the fluorescence of the Si <- S0 cluster emission has been recorded, which is good evidence that the reaction does not occur in the first excited state. 

Figures 5-2 and 5-3. First, the dispersed emission from the cluster

Figure 5-7. Dispersed emission spectra of aniline(N2)i clusters following excitation to several vibrational states of St. Relative energy is the shift, in wavenumbers, from the excited transition. The top spectrum (TJ excitation) shows an inset trace for an expanded scale about the 0° intense feature 10b, 0 J, and JJ emission can be observed. Note that the relaxed cluster emission from 0 J (following IVR) is broad as expected (compare with 6aJ + 55 cm -1 and 6aJ excitation).

The rather low energies of the LMCT emission may be associated with the electronic interaction in the cluster moieties. It has been suggested that these clusters can serve as molecular models for the luminescence of semiconductors such as ZnO and CdS. Theoretical work has confirmed the conclusion that the cluster emission indeed originates from LMCT states. However, despite structural similarities the clusters cannot be considered to be real molecular models of the semiconductors [117]. 

By now this process has been verified experimentally by research groups in Oxford, Moscow, Berkeley, Milan and other places. Accordingly, one has to revise what is learned in school there are not only 3 types of radioactivity a-, j8-, y-radioactivity), but many more. Atomic nuclei can also decay through spontaneous cluster emission (that is the spitting out" of smaller nuclei like carbon, oxygen,. ..). Figure 8.10 depicts some nice examples of these processes. 

The existence of cluster substructure in nuclei is supported also by the observation of cluster decay. Besides a particles, the heavy nuclei can emit Ne, Mg, Si clusters, too. The partial half-lives for these decays depend on the penetrability of the Coulomb barrier in a way very similar to a decay (Mikheev and Tretyakova 1990 see Geiger-NuttaU-type relations in O Sect. 2.4.1.1). The heavy cluster decay is a very rare phenomenon. For example, in the decay of Ra there were 65 x 10 a particles observed, while only 14 cluster emissions during the same time (Rose and Jones 1984). For the detection of rare clusters solid state track detectors are very suitable. 

Arezki, B., Delcorte, A., Garrison, B.J, Bertrand, P. (2006) Understanding gold-thiolate cluster emission from self-assembled monolayers upon kUoelectronvolt ion bombardment. /. Phys. Chem. B, 110, 6832-6840. 

Figure 32. Cluster emission device - proteins are insulators (band gap of 2 eV), electric breakdown at -500 V/pm, nano-cluster-enhanced breakdown at about 3-200 V.

X-ray images of the extended cluster emission obtained by Einstein and ROSAT often show that the spatial distribution of the intracluster gas is centrally peaked. Such observations indicate a strong condensation of matter in the center of the extended emission, since the X-ray emissivity is proportional to the square of the gas density. These strong central peaks are usually interpreted in terms of a cooling flow in which the cooling of the dense gas at the center via X-ray emission produces a slow infall of outer material. As material moves into the center of the intracluster gas, inhomogeneities in the infalling matter should cause condensations of cool matter to fall out of the flow. Condensation rates of 10-100 solar masses per year are implied by the X-ray observations. 

Since the intracluster medium will become polluted by heavy elements because of the explosion of massive stars in the member galaxies, the amount of heavy elements in the cluster gas is a clue to the efficiency of this process, and an indirect clue to the heating mechanism. Recent observations with ROSAT and ASCA suggest that many observed clusters have lower than solar iron abundance, suggesting that chemical pollution by supernovae is not so important. However, spatially resolved X-ray spectra of galaxy cluster emission by ROSAT show that the central... 

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