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Optical Tests of Foundations of Quantum Theory
Published in Yanhua Shih, An Introduction to Quantum Optics, 2020
A more practical example of the EPR-Bohm state concerns the polarization states of photon pairs, such as the spin-zero state of a high energy photon pair disintegrated from the annihilation of positronium. Suppose initially we have a positron and an electron in the spin-zero state with antiparallel spins. The positronium cannot exist very long: it disintegrates into two γ-ray photons within ~ 10−10 second of its lifetime. The spin zero state is symmetric under all rotations. Therefore, the photon pair may be disintegrated into any direction in space with equal probability. The conservation of linear momentum, however, guarantees that if one of the photon is observed in a certain direction, its twin must be found in the opposite direction (with finite uncertainty Δ(p1 + p1) ≠ 0). The conservation of angular momentum will decide the polarization state of the photon pair. As shown in Fig. 14.0.1, in order to keep spin-zero, if photon 1 is right-hand circular polarized (RHC), photon 2 must be also right-hand circular polarized. The same argument shows that if photon 1 is left-hand circular (LHC) polarized, then photon 2 has to be left-hand circular polarized too. Therefore, the positronium may decay into two RHC photons or two LHC photons with equal probability.
The Use of Thermal Analysis in Polymer Characterization
Published in Nicholas P. Cheremisinoff, Elastomer Technology Handbook, 2020
In recent years a new instrument has been developed for making measurements of free volume in polymers. The free-volume microprobe (FVM) has been reported by Mayo et al.219 The microprobe uses positrons emitted during the radioactive decay of 22Na. A 1.28-MeV 7 ray accompanies the “birth” of the positron and this is detected by a scintillator mounted in front of a photomultiplier tube. The positron loses energy through collisions until it interacts with an electron to form the short-lived pseudo-atom, positronium. Mutual annihilation of the positron/electron pair results in a characteristic 0.511-MeV 7 ray, which is detected by a second photomultiplier. The interval between the birth event and the death event is recorded as the lifetime of the positron. Positronium exists in two forms, para and ortho, with the ortho form having a longer lifetime. The measurement principle relates the decay time of ortho-positronium to the polymer free volume; a longer lifetime indicates a larger free volume.
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Published in Harald Paganetti, Proton Therapy Physics, 2018
In this radioactive transformation, one proton is transformed into one neutron, resulting in the emission of a positron and a neutrino with a continuous energy spectrum. While the neutrino escapes without interaction, the positron is slowed down typically within few millimeters in the medium, losing energy in Coulomb inelastic collisions with the atomic electrons and suffering several angular deflections. Once almost at rest, the positron either annihilates as a free particle with an electron of the medium into two photons or it captures an electron to form an unstable bound state e-−e+, so-called positronium. Although the two different atomic states of the positronium can lead to different annihilation processes into two (para-positronium) or three (ortho-positronium) photons, the three γ-emissions are in practice negligible. Thus, all detectable radiation can be attributed to the two annihilation γ-quanta which, according to momentum and energy conservation laws, are emitted in opposite directions and carry an energy of 511 keV each, equal to the positron and electron rest mass. Deviations from perfect colinearity can occur because of the residual energy of the electron–positron system, resulting into emission angles that approximately follow a Gaussian distribution centered at 180° with ~0.3° full-width at half-maximum (FWHM) for typical residual energy values of about 10 eV.
Electronic ionisation-induced annealing of pre-existing defects in Al2O3 and CaF2 single crystals
Published in Philosophical Magazine, 2022
M. Izerrouken, R. Hazem, S. Kuzeci, C. Tav, U. Yahsi, S. Limam
Figure 2 shows the optical absorption spectra obtained before and after irradiation by neutrons and Xe ions irradiation to a fluence of 1 × 1012 ions·cm−2. CaF2 sample before irradiation shows no visible absorption band in the region 200– 800 nm, indicating the presence of no point defects. After irradiation, neutrons and Xe ions induced similar absorption bands centred at about 337, 400, and 550 nm indicated by the arrow in the figure assigned respectively to F-2-centre [12], F-centre and Ca colloids [13–16]. We note that the band associated with Ca colloids is wider in the case of Xe ions irradiation, indicating the formation of large Ca colloids compared to the neutrons irradiation. These results indicate that F-centre, F-centres aggregates, and Ca colloids are generated by both neutrons and Xe ions irradiation in CaF2. The S parameter versus positron energy/depth obtained using DBS for un-irradiated sample, neutrons irradiated sample, and Xe ions irradiated sample with a fluence of 5 × 1012 ions·cm−2 is reported in Figure 3. When positron enters the material, it annihilates either from the surface state, positronium bound state, free state, and trapped states at different types of defects. Thus the overall S parameter is the contribution of all different annihilation sites. We also note that positronium Ps may be formed at the outer surface of a sample or in large voids in semiconductors and insulating materials. As can be seen from Figure 3, all the S(E) curves show a first rising part at low energy (<2 keV) due to the surface effects. Then it increases again to reach a constant value above 11 keV in the case of an un-irradiated sample mainly attributed to the positronium annihilation since the sample contains no defects according to the optical measurements. Indeed, positronium formation is observed in CaF2 crystal [26, 27]. In the irradiated samples, the S parameter decreases and remains almost constant above 11 keV. As demonstrated by optical measurements, it is assigned to the annihilation of the positron in the F-centres cluster and Ca colloids present in the samples. This is in good accordance with Vlceket al. [28] investigation where the authors attributed the defects trapping positron in CaF2 to Fluorine aggregates (at least 4F-vacancies), calcium vacancy, Fluorine interstitial (H-centre), and fluorine bubbles (agglomeration of H-centres). These defects, with a high specific trapping rate, significantly influence the S parameter. Its value depends on the size of the defect clusters. In addition, the presence of such defects in the CaF2 structure quenches the positronium formation [29].