Simultaneous emission

All reactor-produced plutonium contains a mixture of several plutonium isotopes. The continuous decay of 241pu (14.8 year half-life) is the source of 241/. jhis isotope decays by alpha emission with the simultaneous emission of 60 kilovolt gamma rays in 80% abundance. In order to minimize personnel exposure, this element is removed from the metal prior to fabrication. 

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

Two-photon emission is a process in which electronic transition between quantum levels occurs through the simultaneous emission of two photons. [27] presents more details about two-photon emission. Since metal clusters emit in the near-infrared region, they can be made useful for two-photon imaging with infrared excitation. Two-photon emission of Au25 clusters is observed at 830 nm by exciting at... 

Fig. 3.2. Scheme of simultaneous emission of three luminescence centers in the same spectral range with different decays, which may be resolved by time-resolved spectroscopy... 

The rectifier, or diode, is an electronic device that allows current to flow in only one direction. There is low resistance to current flow in one direction, called the forward bias, and a high resistance to current flow in the opposite direction, known as the reverse bias. The operation of a pn rectifying junction is shown in Figure 6.17. If initially there is no electric field across the junction, no net current flows across the junction under thermal equilibrium conditions (Figure 6.17a). Holes are the dominant carriers on the / -side, and electrons predominate on the n-side. This is a dynamic equilibrium Holes and conduction electrons are being formed due to thermal agitation. When a hole and an electron meet at the interface, they recombine with the simultaneous emission of radiation photons. This causes a small flow of holes from the jp-region... 

Several authors have reported that in polar solvents the overall phosphorescence decay of some phenyl alkyl ketones has a long- and a short-lived component they attribute this to simultaneous emission from 3(77,77 ) and 3(77,77 ) states that are not in equilibrium with each other. This interpretation assumes that phosphorescence, a spin-forbidden process, occurs more rapidly than internal conversion from T2 to 7 and therefore seems improbable. It is more likely that one of the phosphorescent species is a photochemical product of the original ketone.13,14... 

Let us consider first the decay of a strongly excited local mode due to simultaneous emission of k > 2 phonons. The Hamiltonian of the system under consideration is... 

Electron in the selftrapping dominated region is trapped by the phonons-1 but due to the interactions mediated by phonons-2 the electron can fluctuate to the higher level. Due to the reflection symmetry of the phonons-2 continuum oscillations of the electrons at simultaneous emission and absorption of phonons-1 occurs. These oscillations couple the levels and so the electrons into pairs. This mechanism was described in a recent paper [10] for a lattice model. 

Figure 2 Vibrational energy relaxation (VER) mechanisms in polyatomic molecules, (a) A polyatomic molecule loses energy to the bath (phonons). The bath has a characteristic maximum fundamental frequency D. (b) An excited vibration 2 < D decays by exciting a phonon of frequency ph = 2. (c) An excited vibration >d decays via simultaneous emission of several phonons (multiphonon emission), (d) An excited vibration 2 decays via a ladder process, exciting lower energy vibration a> and a small number of phonons, (e) Intramolecular vibrational relaxation (IVR) where 2 simultaneously excites many lower energy vibrations, (f) A vibrational cascade consisting of many steps down the vibrational ladder. The lowest energy doorway vibration decays directly by exciting phonons. (From Ref. 96.)...

Equation (5), VER involves a higher-order anharmonic coupling matrix element, which gives rise to decay via simultaneous emission of several phonons nftjph (multiphonon emission). In the ACN case, three phonons must be emitted simultaneously via quartic anharmonic coupling (or four phonons via fifth-order coupling, etc.). 

Upon flash photolysis of H3SiCl (D3SiCl), H3SiBr and H3SiI (D3SiI), systems of absorption bands appear near 20000 cm"1 these are caused by HSiCI (DSiCl), HSiBr and HSil (DSil). Simultaneously, emission spectra with similar structure... 

The recombination of an electron-hole pair, or the capture of a carrier into a deep trap, releases much more energy than can be taken up by a single phonon. Multiphonon recombination is represented by transition A in Fig. 8.3. The probability of the simultaneous emission of n phonons is (Stoneham 1977),... 

Nomadiative relaxation between the 4f states of lanthanide ions can occur by the simultaneous emission of several phonons. The multiphonon emission rate decreases exponentially with the energy gap AA to the next-lower level ... 

A very rare alternative to y-ray emission is the simultaneous emission of an electron and a positron. This possibility only exists if the excitation energy is greater than the energy necessary for the generation of an electron and a positron (pair formation), which amounts to 1.02 MeV. 

More frequently, p emission occurs after decay in a two-stage process P decay leads to an excited state of the daughter nuclide, and from this excited state the proton can easily surmount the energy barrier. This two-stage process is called jff -delayed proton emission. It is observed for several P emitters from to " Ti with N = Z - 3, with half-lives in the range of 1 ms to 0.5 s. Simultaneous emission of two protons has been observed for a few proton-rich nuclides, e.g. Ne ti/2 10 ° s). Proton decay from the isomeric state is observed in case of Co (probability 1.5%, q/2 0.25 s). 

Figure 9.4. Recoil effect due to the simultaneous emission of an a or particle, respectively, and a y-ray photon.

The rate constant for ET can mathematically be regarded as the optical spectrum of a localized electron in the limit where the photon energy to be absorbed or emitted approaches zero. Erom the theory of radiative transitions [10, 12] and r / -b 1) = / for a positive integer /, we see that the factor multiplied to on the right-hand side of Eq. 27 represents the thermally renormalized value of the Franck-Condon factor [i.e., the squared overlap integral between the lowest phonon state in Vy(Q) and the ( AG /te)-th one in piQ)] for ET. The renormalization manifests itself in the Debye-Waller factor exp[—,vcoth( / (y/2)], smaller than e which appears also in neutron or X-ray scattering 12a]. Therefore, yen in Eq- 27 represents the effective matrix element for electron tunneling from the lowest phonon state in the reactant well with simultaneous emission of i AG /liw) phonons. 

Stokes, G. G., On the simultaneous emission and absorption of rays of the same definite refrangibiUty being a translation of a portion of a paper by M. Leon Foucault, and of a paper by Prof. Kirchhoff. Phil. Mag. [4] 19, 193-197 (1860). 

Positrons are generated by the decay of certain unstable isotopes, one of the most commonly used being f Na which decays to, p Aewith the simultaneous emission of e and y-ray. The emitted positrons subsequently thermalize and annihilate with electrons of the material producing y-rays. Positrons can either annihilate directly with electrons or may capture an electron forming a meta-stable intermediate e e pair called positroniiim. 

The most common neutron detectors are of the proportional gas type. Since neutrons themselves have no charge and are non-ionizing, they are harder to detect than X-rays. Detection relies on the absorption of the neutron by an atomic nucleus with the simultaneous emission of a y-ray photon, often referred to as an (n,y) reaction. Since the absorbing material must absorb neutrons and be capable of existing in gaseous form, the choice of substances is limited. The most common is He gas, which relies on the reaction ... 

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