Positronium formation process in organic liquids

Abstract

The formation of positronium in benzene solutions containing a variety of halogenated compounds, such as CC14, CH2C12, C2HC13, 1,1,1-C2H3C13, o-C6H4CH3Br, o-C6H4Cl2, o-C6H4CH3C1, C6H5C1, and other mono-and dihalobenzene, was studied in the presence and absence of C6F6 additives. The observed /2 values, which are indicative of the number of thermalized positronium atoms formed, showed in each system a good correlation with dissociative electron attachment parameters of the solute species. C8F8 additives increased the number of Ps atoms reaching thermal energies. The experimental results are discussed in terms of a reaction scheme in which Ps is formed as a result of electron abstraction from the surrounding molecules by energetic positrons. These Ps atoms can either react rapidly with solute species if their excess kinetic energy favors such a reaction, or with species generated in the reactions of free electrons produced in the positron spur with solute molecules via dissociative electron attachment. The number of these reactive species is mainly determined by the number of free electrons having sufficient energies to undergo dissociative electron attachment. A compound such as C6F6, which has a high cross section for nondissociative electron attachment, competes for free electrons with the solute, and protects the latter from radiolysis. Thus in the presence of CeF8 fewer reactive species are formed in the spur with which the Ps can rapidly react and more Ps atoms will reach thermal energies. An alternative to this reaction scheme is the previously reported spur reaction model which assumes positronium formation via free positron and free electron combination in the positron spur. The results will be also discussed within the framework of this latter model.

Characterization of positronium properties in doped liquid scintillators

“…I, the o-Ps formation fraction decreases as the Gd concentration increases, whereas the lifetime is almost constant. It can be noted that when a very small fraction of doper is added (0.01%) the formation fraction increases slightly with respect to the case of pure LAB: this is a known effect as explained in Ref [15]. The trends of the o-Ps lifetime and formation fraction as a function of the doper concentration are shown in Fig.…”

“…It can be noted that when a very small fraction of doper is added (0.01%) the formation fraction increases slightly with respect to the case of pure LAB: this is a known effect as explained in Ref. [17].…”

Supermolecular Structure, Free Volume, and Glass Transition of Needleless Electrospun Polymer Nanofibers

“…Over the years, abundant studies on free volume have been reported in molecular media like liquids, , polymers, ,,− polymer blends, − composites, , and to some extent in fibers , using positronium annihilation lifetime spectroscopy (PALS). Such studies include reports on free volume changes with the change in topology of polymers due to crystallinity, solvent casting, solvent evaporation rate, tensile stretching, and electric field effects .…”

“…Since the polymer chain orientation is closely associated with free volume, 26,27 the spatial distribution of molecular chains within the nanofiber needs further understanding in terms of associated free volume and its distribution. 26,28 Over the years, abundant studies on free volume have been reported in molecular media like liquids, 29,30 polymers, 27,28,31−34 polymer blends, 35−37 composites, 38,39 and to some extent in fibers 40,41 using positronium annihilation lifetime spectroscopy (PALS). Such studies include reports on free volume changes with the change in topology of polymers due to crystallinity, 28 solvent casting, 31 solvent evaporation rate, 42 tensile stretching, 27 and electric field effects.…”

Effect of halogenated compounds on positronium formation in polycarbonate and polysulfone matrices

“…The e †ect of various additives on Ps formation has been extensively studied for molecular liquids.4,12h19 One of the most important Ðndings is that the addition of an electron acceptor, such as nitrobenzene derivatives and chlorinated hydrocarbons, causes a marked reduction in the Ps intensity (inhibition of Ps formation).13h16 Interestingly, certain electron acceptors such as hexaÑuorobenzene were found (C 6 F 6 ) to increase the Ps yield (enhancement of Ps formation).17 Addition of these compounds to a solution containing a Ps inhibitor was found to recover the Ps yield reduced by the inhibitor (anti-inhibition of Ps formation).18,19 These phenomena of inhibition, enhancement and anti-inhibition of Ps formation have been successfully explained on the basis of the spur reaction model, although an attempt was made to interpret them with the help of the Ore model and reactions of hot Ps atoms. 13 Recently, we observed that the o-Ps yield in amorphous polymers was considerably reduced by addition of a nitrocompound, 2,2@-dinitrobiphenyl (DNB). 20…”