Angular correlation measurements

With position sensitive detectors the deviations from antiparallel emission of two annihilation photons can be observed. The small angles on the order of millirad are translated into two dimensional electron (or positronium) momentum distributions. The resolution is sufficient to distinguish positron from para-positronium annihilations and to determine the velocity distribution of positronium. 

Early angular correlation (AC) experiments on materials with closed and open porosity demonstrated that positronium indeed emerges from the 

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See lext. XD = X-ray diffraction 1R = infrared spectrum R = Raman spectrum UV = ultraviolet spectrum H-NMR = ]HNMR spectrum C-NMR = 13CNMR spectrum F-NMR =, 9FNMR spectrum MS = mass spectrum PES — photoelectron spectrum E - electric polarization and dielectric loss measurements D = dipole moment measurements TDPAC = time differential perturbed angular correlation measurements GC = gas chromatography TA = thermal analysis M = molecular weight A = electrical conductance. c Isolated as the THF adduct M(dik)Cl3-C4HgO. 

An Investigation of the 3 decay of the 0.59s 145Cs was made at the TRISTAN on-line mass separation facility. The level scheme for 145Ba has been constructed. The proposed spin and parity assignments are based upon transition multipolarities and yy angular correlation measurements. 

An intrafullerene dynamics of Ce atom in the Cs2 cage was also studied by time-differential perturbed angular correlation measurements (Sato et al., 1998). The observed angular correlation shows the presence of two different chemical species of Ce Cs2- The data at low temperatures reveal that Ce stays at a certain site for one of the species, whereas for the other the atom has an intramolecular dynamic motion. 

State and scattering parameters and their relation to angular-correlation measurements... 

Perturbed angular correlation measurements The principle relies on the fact that hyperfine interactions occur between nuclearmoments and the electromagnetic... 

The cascade gamma transitions fit well into the level schemes deduced from the heavy particle reactions. A p—y) angular correlation measurement suggests / = I or I for the 5-3 MeV doublet in Reduced widths have been evaluated for the virtual levels [73] and for some of the bound levels of... 

The 1.368 MeV level is known to have J = 2 from a-y angular correlation measurement in the Na (pay) reaction. This agrees with the 0-2-4 type of gamma-gamma correlation pattern found for the successive transitions resulting from the decay of Na to the level at 4.122 MeV. Even parity is established for the levels of this cascade by a polarisation-correlation experiment. The radiative transitions are both 2 as expected from the spin values this follows from measurements of the pair internal conversion coefficient (Sect. 21). There is no evidence of a crossover transition from the 4.122 MeV level to the ground state (Beghian et aL. )... 

The reactions of the Co ion (containing unpaired electrons) with positronium and various free radicals (e.g., diphenylpicrylhydrazyl or 2,2,6,6-tetramethyl-4-oxopiperidin-l-oxyl) were investigated as a function of the composition of, for example, propanol-water mixtures [Go 68b]. Angular correlation measurements indicated that the Co ion interacts with positronium in an ortho-para conversion reaction, the rate of which was determined on the basis of the lifetime spectra. 

The limited application of positron beams is explained by the fact that Doppler-broadening spectroscopy is the only detection method that works with beams without any difficulty. Although angular correlation measurements are possible with positron beams, it requires a very high efficiency beamline. Only very few beams fulfill the requirements of ACAR measurements, but their results are very impressive. The combination of low-energy positron beams and 2D-ACAR measurements is the only possibility to gain direct information on the momentum distribution of electrons at the surface of metals and semiconductors (Chen et al. 1987). 

This example shows that angular correlation measurements make it possible to study several details of the excitation mechanism (i) the type of coupling which leads to the excitation, (ii) the decoupling of electronic and nuclear motion at large distances, and (iii) the transfer of angular momentum. Even more information can be obtained in special cases like He excitation in He" -I- He collisions. 

Positron annihilation spectroscopy can provide essential information about the deterioration in the mechanical properties of RPV steels (microstructural defects and precipitates) during their irradiation, which is known as neutron embrittlement. Currently, there are three main techniques based on annihilation phenomena positron lifetime spectroscopy, Doppler-broadening spectroscopy and angular correlation measurements. 

Figure 4.37 a Typical ID ACAR spectrum, and b scheme of an angular correlation measurement system... 

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