Temperature and phase dependence of positron lifetimes in dl‐camphor

Abstract

It i s known that in molecularly simple materials, the formation and quenching of the positronium atom (Psktakes place preferablyatthe sites of low electron density (1). The effects of temperature, density, viscosity, and phase changes on the value ( t ) and the intensity (I ) of the delayed component in the lifetime spectra of positrons annihilating in such materials a r e often interpreted on the basis of the free volume model (2 to 5). According to this model, 'c2 and I depend upon the 2 total free volume of the medium and its distribution into sites. These studies have also been extended to liquid crystals and the sensitivity of 'C and I to phase changes experimentally established, by Cole and Walker (6 to 8) and by Nicholas and Ache Experimental set-up The experimental set-up has been described in an earlier 0 paper (10). The same set-up was used to study dl-camphor from -180 to +20 C.Since camphor has a very high vapour pressure and the pellets tend to evaporate in vacuum, the measurements were taken under dry argon atmosphere. The temperature of the sample was kept constant within 0.25 0 C and several cycles of temper- 1) Present address:

Positron annihilation studies in non-metallic solids: D-camphor and silver iodide

“…1 shows the variation of the LS parameters. Comparing with previous results on dl-camphor [3], the values of z, and of zz a t 294 K both appear higher. For the latter parameter, this is certainly ascribable to the fact that the analysis of the LS spectra was made on the basis of two components, and not three as in the present work, so that the value of the shorter lifetime also included the very short p-Ps lifetime (0.125 ns).…”

“…However, the progressive increase in t3 up t o 374 K while I3 remains constant is not consistent with this model. Also, quantitative examination of these parameters a t the transition temperature does not yield a constant "free volume parameter" as predicted in view of the very small variation in vm [3]. By comparison with, e.g., the variation of t3 with the free volume in such compounds as hexane [MI, the increase in z3 from 1.66 to 2.1 ns between 295 and 380 K appears much too large to be correlated with the small change in free volume implied by the values of w , .…”

“…Camphor presents a phase transition from a crystalline, ordered, tetragonal phase t o a plastic, rhombic phase a t 246 K, then another t o a plastic, cubic phase a t 377.5 K before melting a t 452 K [19 t o 211. The first transition has been examined in the dl variety, using LS [3]. Investigating the plastic-plastic transition could give interesting complementary information on P s formation and on the properties of plastic phases [4, 51.…”