Dušan Srdoč

Experimental Technique of Measurement of Microscopic Energy Distribution in Irradiated Matter Using Rossi Counters

An experimental method for measuring the distribution of energies deposited in small tissue-equivalent volumes is described. The energy deposited in a cavity is measured by means of a proportional ...

The Mean Energy Required to Form an Ion Pair for Low-Energy Photons and Electrons in Polyatomic Gases

The mean energy required to form an ion pair (W) is measured for photons having energy 0.277, 1.49, and 5.89 keV in several polyatomic gases and two tissue-equivalent gas mixtures. An increase of W values with decreasing energy is observed. Generally, a continuous decrease of W with an increasing number of atoms in a series of organic compounds was reported (ICRU Report No. 31), while our measurements show an oscillation of W values in alkanes; the W value for an alkane with an even number of carbon atoms in the molecule (ethane and butane) is lower than that for the next higher alkane with an odd number of C atoms (propane and pentane, respectively). The W value for low-energy electrons was calculated from experimental photon W values. A good agreement between the calculated and the experimental W values in the low-energy region, and the high-energy W values is obtained. Our results fill the gap between the very low-energy region (below 300 eV) and the high-energy (above 10 keV) region.

Microdosimetry of Monoenergetic Neutrons

Tissue spheres 0.25, 0.5, 1.0, 2.0, 4.0 and 8.0 micron in diameter were simulated using a wall-less spherical counter filled with propane-based tissue-equivalent gas. Microdosimetric spectra corresponding to these site sizes were measured for five neutron energies (0.22, 0.44, 1.5, 6 and 14 MeV) and the related mean values /yF and /yD were calculated for several site sizes and neutron energies. An elaborate calibration technique combining soft X rays, an 55Fe photon source and a collimated 244Cm alpha-particle source was used throughout the measurements. The spectra and their mean values are compared with theoretically calculated values for ICRU standard tissue. The agreement between the calculated and the measured data is good in spite of a systematic discrepancy, which could be attributed, in part, to the difference in elemental composition between the tissue-equivalent gas and plastic used in the counter and the ICRU standard tissue used in the calculations.

Radiocarbon and uranium-series dating of the Plitvice lakes travertines.

Radiocarbon and uranium-series ages of the calcareous deposits of the Plitvice Lakes show that travertines were deposited during three warm, humid, interglacial oxygen isotope stages. According to our measurements, only calcite crystals or crystal aggregates represent reliable material for both 230 Th/ 234 U and 234 U/ 238 /U dating. Compact old travertine in the form of sandstone is less reliable; it can be dated by both methods provided that its detrital contamination is not significant, demonstrated by very low 14 C activity ( 230 Th/ 232 Th ratio. Old porous travertine contaminated with recent carbonates and Th-bearing clay (pMC > 5, 230 Th/ 232 Th 14 C dating corroborated by sedimentological and palynologic studies as well as by both 230 Th/ 234 U and 234 U/ 238 U disequilibrium methods. The intensive growth of travertine barriers coincided with significant climate warming in the Holocene. Stage 5 deposition is confirmed by the 230 Th/ 234 U dating of crystalline calcite aggregates embedded in the travertine matrix and by concordant 230 Th/ 234 U and 234 U/ 238 U ages, assuming that the 234 U/ 238 U activity ratio of 1.88 observed in modern streams and in Holocene deposits can be extended to past epochs. The travertine deposition period was very short, peaking ca. 120 ± 10 ka bp. Stage 11 deposition is indicated by 234 U/ 238 U dating only, the period being within the 234 U decay range, but not that of 230 Th. Stage 11 travertine was deposited ca. 420 ± 50 ka bp. We did not find travertine samples with U-series ages indicating a growth period during relatively warm Stages 7 and 9; due to the scarcity of old travertine outcrops, these and possibly other stages cannot be excluded on the basis of presented data. All of these isotopic dating results concur with the field relation of the travertine complex of the Plitvice Lakes.

Anthropogenic Influence on the 14C Activity and Other Constituents of Recent Lake Sediments: A Case Study

Anthropogenic activities that introduce an excess of nutrients and other pollutants into rivers and lakes are causing significant changes in their aquatic environment. Excessive nutrients greatly accelerate eutrophication, and lake marl formed during eutrophication differs from that formed in oligotrophic water. We analyzed recent sediment cores from Prone and Kozjak lakes located in Plitvice National Park, central Croatia. Analyses consisted of 14C activity of calcareous lake marl, the ratio of stable isotopes (b13C, 5180), organic compounds in the sediment and the distribution of diatoms. Previous 14C activity measurements helped to determine the sedimentation rate and thus the time period of increased input of nutrients into lakes. We determined the increased 14C activity in lake sediments caused by nuclear bomb effect in recent depth profiles. We attributed the sudden increase in diatom species, Cyclotella operculata unipuctata andAchnanthes clevei rostrata, in the uppermost 5-cm layer, to eutrophication of the lake water. We performed a molecular characterization of hydrocarbons isolated from the sediments, and applied computer-assisted high-resolution gas chromatography/mass spectrometry to estimate contributions of biogenic, fossil and pyrolytic hydrocarbons.

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.

A Comparison of Calculated and Measured W Values in Tissue-Equivalent Gas Mixtures

We calculated the mean energy required to produce an ion pair (W) in methane-, propane- and butane-based tissue-equivalent (TE) gas mixtures from W values in pure constituent gases according to various models for energy partition among gas components. We found an agreement between the experimental and calculated W values in the methane-based TE gas regardless of the model concept. In contrast, only those models which take into account differences in the stopping powers, total ionization cross sections and model constants of gas components give acceptable results for the propane-based TE gas. The calculated W value for high-energy electrons in the isobutane-based TE gas mixture is 25.2 eV for high-energy electrons and 28.0 eV for approximately 5 MeV alpha particles.

Rudjer Bošković Institute radiocarbon measurements XIV

In this report we present dating of several large series of samples mostly collected in Croatia: (i) Iron Age and Medieval samples excavated in Zagreb, (ii) speleothems from a karst site in C Croatia, (iii) shallow lake sediments from the Plitvice Lakes, (iv) tufa from the Krka River, (v) sea sediments from Adriatic sea, and (vi) tree rings from Hungary, Croatia and Slovenia. Sample preparation and proportional counter technique are essentially the same as reported earlier (Srdoc et al. 1971). Processing of data has been computerized (Obelic 1989).

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.