Hans J. Ache

Positronium Chemistry in Aqueous KMnO4 Solutions

The reaction of positronium was studied in aqueous KMnO4 solutions at various concentrations and pH. Lifetime measurements show a distinct decrease of τ2 and increase of I2 with increasing temperatures, as expected for chemical reactions in which thermalized positronium is involved. No dependence on the pH of the solution could be observed. The activation energy calculated from the temperature dependence of τ2 is rather small and about 0.1–0.2 eV. The free positron annihilation lifetime was found to be approximately 0.55 nsec, which is in good agreement with the value obtained by using Diracs formula, 0.5 nsec for water.

Phase and Temperature Dependence of Positron Annihilation in Liquid Crystals

Positron lifetime measurements were carried out in a series of liquid crystalline compounds. The resulting lifetime spectra were decomposed into two components. The short‐lived component (lifetime τ1 and intensity I1) was attributed to free‐positron and parapositronium annihilation whereas the long‐lived component τ2, I2 was considered to be the result of orthopositronium annihilation. Changes of τ2 and I2 during mesomorphic phase transitions are generally very small and can be related to the transition entropies involved, supporting the contention that the positron annihilation process is affected by variations in the structural regularities of the environment. Various mechanisms which could account for the observed changes of the positron lifetimes and their intensities at the phase‐transition points, are discussed.

On the preparation of 80Br- or 82Br-biomolecules via excitation labelling methods

The direct decay induced 82Br (or 80Br) labelling by exposing the solid substrate molecules, such as deoxyuridine, L-tyrosine, guanosine, deoxycytodine, phenylalanine and acetic acid, to gaseous CF382mBr (or CF380mBr) was studied. The radiochemical yields of the brominated products are relatively small and range from 1% in the case of bromo deoxyuridine to 11% for bromoacetic acid. The modification of this technique by adding Cl2 gas to the reaction mixture improves the yields in several cases drastically (up to 80% for bromo-guanosine and bromo-L-tyrosine). Similar improvement can be achieved by exposing crystalline KBrO3 for some time to CF382mBr (or CF380mBr) and dissolving subsequently the KBrO3 in an acidic solution of the substrate.

On positronium formation in binary mixtures of organic compounds

The intensities of the long‐lived component I2 in the positron lifetime spectra were measured for a variety of mono‐ and dihalogenated benzenes, benzylalcohol, benzaldehyde, benzonitrile, and acetophenone in benzene solution. The results indicate that I2 is not an additive property, e.g., small amounts of bromobenzene (less than 0.1 mole fraction) added to benzene reduce I2 to the value observed in the pure compound. The capability of the various systems to effectively reduce I2 values as well as the minimum I2 values observed in these systems have been evaluated. The results are discussed in terms of the existing models of Ps formation and interactions in binary mixtures.

Positronium formation in aqueous micellar solutions of sodium dodecylsulfate

Positron annihilation lifetime measurements on aqueous solutions of sodium dodecylsulfate (NaLS) were performed over a wide range of temperatures and surfactant concentrations. The sensitivity of the positronium formation process toward changes in physical properties accompanying micellization is demonstrated. The critical micelle concentration (CMC) is associated with a sharp decrease in positronium formation. The CMC of 8.4±0.5 mM at 20 °C in this system is in agreement with literature values. At higher temperatures the CMC is shifted to slightly higher surfactant concentrations (10.4±0.7 mM at 90 °C). The formation probability for thermal positronium as expressed by I2, the intensity of the long‐lived component in the lifetime spectra, gradually decreases above the CMC with increasing surfactant concentration reaching a limiting value. A model is suggested involving the reactions of ’’hot’’ positrons or positronium atoms with micellar aggregates leading to the observed reduction of thermalized positron...

The preparation of Cyanide-11C for use in the synthesis of organic radiopharmaceuticals II☆

Abstract Three methods are described for the preparation of carbon-11 containing cyanide. Two methods involve direct recoil synthesis, 11 C + N 2 + H 2 → H 11 CN and 11 C + NaCN → Na 11 CN, and a third method involves a recoil plus chemical synthesis, 11 CO 2 + K + NH 3 → K 11 CN. The methods are compared for their usefulness in the preparation of radiopharmaceuticals containing carbon-11.

Radioisotopic labelling by surface catalysis—I. Preparation and quality control of 131I-4-iodoantipyrine☆

Abstract A new method of radioiodination, apparently involving halogen exchange by surface catalysis, has been developed for the synthesis of 131I-4-iodoantipyrine (131I-4-IAP) in high specific activity. On a silica gel surface, for instance, radioiodide reacts with antipyrine (AP) in hydrochloric acid at pH 1.0 to form 131I-4-IAP in a radiochemical yield of 82.2 ± 1.8%. Subsequent separation of the 131I-4-IAP from the AP by thin layer chromatography (TLC) has afforded greater than 99.5% radiochemically pure 131I-4-IAP. Quality control procedures utilizing TLC and high performance liquid chromatography have been developed.

Radiation Damage Dependence of Positron Lifetimes in Sodium Halide Crystals

Position lifetime spectra in NaCl and NaF single crystals were measured and analyzed as a function of the crystal defect concentration introduced by proton and gamma irradiation. In the irradiated sodium chloride samples a long‐lived component was observed with a lifetime (τB) of 8.2 × 10−10 sec and an intensity (IB) of up to 28%. Thermal annealing of the sample caused a drastic reduction of the intensity of this component (IB) to about 10%. Similar observations were made in the positron lifetime spectra in the NaF system.

On the preparation of 80Br-biomolecules II: 80Br for I exchange in Iodo-Biomolecules

Abstract The radiochemical yields of 80Br-5-bromodeoxyuridine and 80Br-bromoacetic acid obtained by employing various modifications of the direct decay induced 80Br labelling method which exposes substrate molecules to gaseous CF380mBr are reported. The results indicate a drastic improvement of the amount of 80Br incorporated into these products if the labelling is accoplished by applying the “CF380mBr-KBrO3” gas exposure technique to induce a 80Br-for-I exchange in the corresponding (inactive) iodo derivatives.

On the preparation of 80Br-biomolecules—III: The effect of various experimental parameters on radiochemical yields

Abstract The effect of several experimental parameters on the efficiency of the 80 Br incorporation via the “CF 3 80 m Br-KBrO 3 ” gas exposure resulting in carrier free radiobrominated compounds was investigated by using two model compounds, L -tyrosine and guanosine. The 80 Br labelling proceeds very rapidly in the case of the L -tyrosine and is fairly independent of “labelling time” and substrate concentration as long as a certain minimum amount of L -tyrosine is used. This is in contrast to the guanosine system where rapid secondary reactions reduce the initially high yields of 80 Br-8-bromoguanosine at extended “labelling times” and where larger amounts of substrate are needed to produce optimum yields.