Measurement of 14C activity by liquid scintillation counting.

Measurement of (14)C activity in various types of samples has been performed in the Zagreb Radiocarbon Laboratory since 1968 by proportional counting technique and since 2001 by liquid scintillation counting (LSC) technique by using LSC Quantulus 1220. We either prepare benzene or absorb CO(2) obtained from the sample in a scintillation cocktail. Various measurement protocols were developed for different types of vials. Examples of environmental (14)C activities in atmospheric CO(2) and in biological samples are presented.

The frequency distribution of the number of ion pairs in irradiated tissue.

The statistical distribution of the number of ion pairs per ionizing event in a small volume simulating a tissue sphere was obtained by applying the Expectation-Maximization (EM) algorithm to experimental spectra measured by exposing a Rossi-type spherical proportional counter to gamma radiation. The normalized experimental spectrum, r(x), which is the distribution of the number of ion pairs per event from both the primary track and the subsequent electron multiplication, can be represented as Sum(n) p(n) x f(n,x), where the f(n,x)'s for n = 1, 2, 3, ..., n are the normalized spectra for exactly 1, 2, 3, ..., n primary ion pairs and are calculated by convoluting the single-electron spectrum. The coefficients pn represent the mixing proportions of the spectra corresponding to 1, 2, 3, ..., n ion pairs in forming the experimental spectrum. The single-electron spectrum used in our calculations is the distribution of the number of ion pairs due to the multiplication process, and it is represented in analytical form by the Gamma distribution f(1,x) = a x x(b) x e(-cx), where x is energy, usually in eV, and a, b and c are constants. The EM algorithm is an iterative procedure for computing the maximum likelihood or maximum a posteriori estimates of the mixing proportions p(n), which we also refer to as the primary distribution of ion pairs in a microscopic spherical tissue-equivalent volume. The experimental and primary spectra are presented for simulated tissue spheres ranging from 0.25 to 8 microm in diameter exposed to 60Co gamma radiation.

Theoretical study of W values in hydrocarbon gases.

The recent measurements by D. Srdoc, B. Obelic, and I. Krajcar Bronic (J. Phys. B, 20, 4473-4484, 1987) and by I. Krajcar Bronic, D. Srdoc, and B. Obelic (Radiat. Res., 115, 213-222, 1988) revealed nonsmooth, oscillatory variation of the W value for low-energy electrons as a function of the number of carbon atoms in normal alkanes. This behavior is contrary to what has been expected generally and prompted us to undertake the present study. Calculations were carried out using the continuous-slowing-down approximation for the degradation spectrum, the binary-encounter theory for cross sections, and relevant molecular properties, to explain the behavior of the W value. The main contributor to the oscillatory variation is the branching ratio for neutral dissociation to ionization of super-excited states. We also present an interpretation of the trend of the W value in the series C2H2, C2H4, and C2H6.

The W values for photons and electrons in mixtures of argon and alkanes.

The mean energy required to form an ion pair for 5.89-keV 55Fe and 1.49-keV Al photons in mixtures of argon-methane and argon-butane gas was measured using the proportional-counter technique. The measured W values in the mixtures are lower than what is expected for regular mixtures, i.e., mixtures without an appreciable intermolecular energy transfer between the mixture components. The additional ionization yield through the nonmetastable Penning processes reaches a maximum abruptly at very low partial pressure of polyatomic admixtures (alkanes), decreasing steadily with higher admixture partial pressure.