Ole E. Mogensen

Positron Lifetimes in Pure and Doped Ice and in Water

Positron lifetime spectra were measured in mono‐ and polycrystalline light ice, polycrystalline heavy ice, doped light ice, as well as in light and heavy water. All spectra were resolved into three components. At temperatures between −196° and −100°C the lifetimes and relative intensities are virtually constant, being 0.12 nsec and 20%, 0.44 nsec and 28%, 0.68 nsec and 52%, and attributed to annihilation of para‐Ps, free positrons, and ortho‐Ps, respectively. Above −100°C the longest lifetime increases with temperature to 1.1 nsec at 0°C. Its relative intensity is unchanged, while the parameters for the other components show a complex behavior. The spectra for mono‐ and polycrystalline light ice and for polycrystalline heavy ice are identical. For water long lifetime components attributed to ortho‐Ps are 1.86 nsec, 27% for H2O and 2.01 nsec, 22% for D2O. Theoretical explanations are suggested. Fast frozen solutions of HF with concentrations larger than approximately 10−5 mole fraction have a lifetime comp...

Positron spur reactions with excess electrons and anions in liquid organic mixtures of electron acceptors

Abstract By means of the positron lifetime technique we have measured positronium (Ps) yields in mixtures of nonpolar liquids with various electron scavengers which bind the electron fairly weakly (1–2 eV) in stable anions. The results are discussed with reference to recent excess electron works, and new experiments on anions and excess electrons are proposed. The minimum of the Ps yield versus CS2 concentration curves caused by partly delocalization of electrons on several scavenger molecules, which was observed previously in saturated aliphatic hydrocarbons occurred also in the saturated cyclic hydrocarbon cyclohexane, but did not appear in the aromatic benzene. This might be explained by the weak electron acceptor property of aromatics. In the Ps yield versus SF6 concentration curve in hexane a similar minimum appeared as in the CS2 case, probably by the same reason. By adding 0.8 M CS2 to the system the minimum was shifted towards the lower concentrations, which might be explained by some overlap of the energy levels on CS2 and SF6. Antiinhibition effect of C6F6 and CS2 was studied in 0.05 M CCl4/hexane solutions. In the CS2 case first a small minimum appeared in the Ps yield versus CS2 concentration curve. Alcohol clusters did not show antiinhibition in 0.05 M CCl4/neopentane system. Weak electron scavengers (C6F6, naphthalene, biphenyl, benzene) which generally act as antiinhibitors were added to pure non-polar solvents (hexane, isooctane, cyclohexane) and caused Ps enhancement. This can be attributed to “antirecombination” effect with a similar mechanism as that of the antiinhibition, but some contribution from positive ion scavenging cannot be excluded either. The increase of the Ps yield in hexane as a function of C6F6 or naphthalene concentration was much steeper if 0.8 M CS2 was added to the system, indicating that antirecombination and antiinhibition took place simultaneously. On addition of hexane, isooctane, CS2 or 0.8 M CS2/hexane to pure C6F6 the Ps yield drastically decreased. This correlates well with the drastic decrease of the electron mobility by adding similar additives to C6F6 observed recently.

A Positron Annihilation Investigation of Defects in Neutron Irradiated Copper

Abstract The response of positron annihilation parameters to the defect structures produced by neutron irradiation in copper, and their annealing behaviour, have been studied on specimens irradiated at two different temperatures, 50°C and 250°C. Both lifetime and angular correlation measurements were made while some aspects of the irradiation damage substructure were also covered by transmission electron microscopy. Marked changes in the positron parameters were found after the two irradiations. After the 50°C irradiation the dominant TEM defect was a high concentration of small loops, after the 250°C irradiation it was a population of voids. The voids gave rise to a long-lived component (420 ± 75 psec) that disappeared during isochronal annealing to between 450T and 550T in agreement with TEM observations. Another trapped positron component (180 ± 7 psec) appearing after both irradiations annealed out between 275 ± 25°C and 475 ± 25°C. Although we could not entirely exclude that the defects giving rise t...

Inhibition of Ps formation in benzene and cyclohexane by CH3Cl and CH3Br. Correlation to radiation chemistry results

Abstract Positron-annihilation lifetime spectra have been measured for mixtures of CH 3 Cl and CH 3 Br in cyclohexane and of CH 3 Cl in benzene. The ortho-positronium (Ps) yield decreased monotonically from 38% and 43% in cyclohexane and benzene respectively to 11% in pure CH 3 Cl and 6% in pure CH 3 Br. The strength of the inhibition of Ps formation by CH 3 Br was ten times that of CH 3 Cl in cyclohexane, because the CH 3 Br − anion debrominates rapidly, while CH 3 Cl − is long-lived (= 30 ns) compared to the maximum time of Ps formation of 400–500 ps. as shown in radiation chemistry. The positron can pick off the electron from the CH 3 X − anions to form Ps. while it forms a bound state with the halides. X − . CH 3 Cl was a roughly three times weaker Ps inhibitor in benzene than in cyclohexane, which shows that CH 3 Cl − does not dechlorinate in times comparable to or shorter than 400–500 ps in benzene. An improved model for the explanation of Ps formation in mixtures, where the Ps yield versus electron scavenger concentration has a minimum, is proposed and discussed.

Solvated positron chemistry. Positron reactions with pseudo-halide ions in water

Abstract The hydrated positron e + aq reactions with SCN − , OCN − , CN − , S 2− were studied by means of the angular correlation technique. The positron forms bound states with SCN − , CN − , and S 2− but not with OCN − . Apparently, the e + aq reaction with SH − results in a positron bound state with S 2− . It was difficult to determine the shapes of the bound-state angular correlation curves. Only in the SCN − case could a very rough estimate of the rate constant be obtained. Estimates of the binding energies relative to those of the corresponding halide ion states could be determined for SCN − and S 2− .

Temperature dependence of the inhibition of positronium by chlorine-substituted hydrocarbons in non-polar liquids

Abstract Positron annihilation lifetime spectra were measured for solutions of 1,2,3,5-C 6 H 2 Cl 4 in hexane, toluene, m -xylene and mesitylene, CCl 4 in hexane and toluene, and C 2 HCl 3 in n -hexane for concentrations below 1 M and at various temperatures between −30°C and 67°C. The Ps inhibition by C 6 H 2 Cl 4 was roughly 14 times stronger at −30°C than at 67°C in toluene, m -xylene, and mesitylene, while that of CCl 4 displayed hardly any temperature dependence in hexane and toluene. Previously, the Ps inhibition by C 6 H 2 Cl 4 in various liquids at 20°C was explained by a competition between electron pick off by the positron from C 6 H 2 Cl 4 − formed by electron scavenging in the positron spur, and dehalogenation of the same anion at picosecond times after the spur formation. Three effects might reasonably explain the strong temperature dependence of the Ps inhibition of C 6 H 2 Cl 4 : (1) The dechlorination time of C 6 H 2 Cl 4 − might be much shorter at low than at high temperatures. (2) The positron mobility might decrease very strongly with decreasing temperature. (3) Positrons possibly trapped on C 6 H 2 Cl 4 might thermally detach at high but not at low temperatures.

Solvated positron chemistry competitive positron reactions with halide ions in water

Abstract It is shown by means of the angular correlation technique that the binding of positrons to halides is strongly influenced by solvation effects. For aqueous solutions we find increasing values for the binding energies between the halide and the positron with increasing mass of the halide. This is contrary to the calculations of Cade and Farazdel for the vacuum case.

Comment reply to comment on the positron reactions with pseudo-halides in water

Abstract The discrepancies between the Riso and Strasbourg angular correlation curves for the positron bound states with SCN− and CN− are explained. The Riso interpretation (i.e. hydrated positron reactions at low (= 0.01 M), and encounter pair effect at high (= 1 M), X− concentrations) of the positron reaction with halides and pseudo-halides in water and the partial Ps inhibition by the same ions are discussed.

Positronium formation and hydrated positron reactions in H2O, D2O, 1.74 M PPS/H2O and 1.74 M PPS/D2O solutions of Cl−, Br− and I−

Abstract Angular correlation of annihilation photons were measured for H2O, D2O, 1.74 M PPS/H2O and 1.74 M PPS/D2O solutions of Cl−, Br− and I−. The three components of the angular correlation spectra for D2O and H2O were nearly identical in shape. The positronium (Ps) yields for the H2O and D2O solutions, extracted from the angular correlation spectra, were somewhat different from those extracted previously from the lifetime spectra. Positrons form bound states [X−, e+] with Cl−, Br− and I−, but not with acetate, oxalate, SO2−4, OH− and F− ions, in water. The percentage of positron bound state formation before annihilation (lifetime 400 ps) was determined for the three halides in the four solvents. Simple kinetic equations (“trapping model”) with time dependent rate constant, solved analytically, could explain the [X−, e+] formation in H2O fairly well for concentrations below 0.03 M X−, if a diffusion controlled reaction with positron diffusion constant D = 5 × 10−5 cm2/s and reaction radius R = 1 nm were assumed. The three halides gave roughly identical [X−, e+] formation below 0.03 M X−. The difference between the four solutions could be explained partly only in terms of viscosity change for the model used. The effective rate constant decreased strongly for increasing X− concentration above 0.03 M X−. Ion association may influence the e+aq reaction. The high concentration results are very difficult to explain.