ИСТИНА

Positrons (e+) as well as positronium atoms (Ps) are convenient probes of the local nanoscale structure in a condensed phase and the radiolytic processes occurring therein [1]. When a fast positron is injected into a molecular medium it loses most of its energy through ionizations. This energy is locally converted into heat. For understanding the Ps formation process we must pay attention to the terminal part of the e+ track (e+ blob) because Ps formation takes place therein. The local temperature of the blob can increase by several tens of degrees, and therefore change temporarily the local properties and affect the Ps formation. The effect of local melting of ice close to its bulk melting point is discussed [2]. The recently developed Gamma-induced Positron Spectroscopy (GiPS) [3] sheds light on Ps formation at short times. It is shown that in aqueous solutions a positronium atom is initially formed in a quasi-free state, and, after 50-100 ps, becomes localized in a nanobubble. Moreover, combination of these data with conventional LT studies of NaNO3 aqueous solutions shows that “hot e+” may be trapped by an electron scavenger and the hydrated electron is not involved in the Ps formation [4]. This question - why a hydrated electron is not actually a precursor of a Ps atom, although this process is energetically possible - remains still open. Apparently, the inefficiency of the Ps bubble state formation from its hydrated precursors (e- and e+) is of a kinetic origin. Namely, this process is connected with the need for spatial reorganization (reorientation) of a large number of molecules in the medium, which form the hydration shells of е-aq and е+aq. Only after the destruction of these shells the Ps bubble may be formed. Because of the relatively small energy gain (per each reorientating molecule), this process turns out to be very slow compared with the lifetime of the hydrated e+ (~0.5 ns). [1] S.V. Stepanov, V.M. Byakov, D.S. Zvezhinskiy et al., Advances in Physical Chemistry, vol. 2012, Article ID 431962 (2012) [2] S.V. Stepanov, D.S. Zvezhinskiy, V.M. Byakov, G. Duplatre, P.S. Stepanov Acta Physica Polonica A. 125(3), 691 (2014) [3] D.S. Zvezhinskiy, M. Butterling, A. Wagner, R. Krause-Rehberg, S.V. Stepanov J. Phys. Conf. Ser., 443, 012057 (2013) [4] S.V. Stepanov, G. Duplatre, V.M. Byakov, D.S. Zvezhinskiy, V.S. Subrahmanyam Acta Physica Polonica A. 125(3), 770 (2014)