Microdosimetric Distributions Research Articles

Photon Induced Microdosimetric Distributions in Nanometre and Micrometre Sites

Photon Induced Microdosimetric Distributions in Nanometre and Micrometre Sites

Microdosimetric Distributions for Photons

Experiments and calculations are compared on d(y) distributions produced by heavy filtered, narrow X rays of average energies between 12 and 250 keV. The measurements were performed with a walled tissue-equivalent proportional counter (KFA counter). Numerical simulations of microdosimetric distributions were done with the electron track structure code MOCA-8. Electrons produced by photoelectric and Compton effect were treated separately in calculations in order to distinguish the contributions of both types of events in the resulting dose distributions. Transmission and energy degradation of the primary photon spectrum in the counter walls were assessed using a photon transport code. A good agreement between the measured and calculated microdosimetric distributions was achieved. In the energy range considered the shape of photon induced microdosimetric distributions depends not directly on the type of physical interactions in the counter but on the relation between the secondary electron range and the size of sensitive site.

Microdosimetric Distributions for Photons

Experiments and calculations are compared on d(y) distributions produced by heavy filtered, narrow X rays of average energies between 12 and 250 keV. The measurements were performed with a walled tissue-equivalent proportional counter (KFA counter). Numerical simulations of microdosimetric distributions were done with the electron track structure code MOCA-8. Electrons produced by photoelectric and Compton effect were treated separately in calculations in order to distinguish the contributions of both types of events in the resulting dose distributions. Transmission and energy degradation of the primary photon spectrum in the counter walls were assessed using a photon transport code. A good agreement between the measured and calculated microdosimetric distributions was achieved. In the energy range considered the shape of photon induced microdosimetric distributions depends not directly on the type of physical interactions in the counter but on the relation between the secondary electron range and the size of sensitive site.

Estimation of the quality factor on the basis of multi-event microdosimetric distributions.

The measurement of microdosimetric distributions for the purpose of estimating the quality factor, Q, may be encumbered in pulsed radiation fields--as produced, for instance, by accelerators with low duty cycle--because of a signal pile-up. We propose a method of estimating Q from the first several moments of multi-event distributions. In addition to overcoming the high dose-rate problems, the measurement of such distributions can be performed in significantly smaller volumes than conventional microdosimetry, thus raising the possibility of reducing the site diameter (presently 1 micron) for which y in the function Q(y) is specified.

The effects of geometrical factors on microdosimetric probability distributions of energy deposition

It is shown that energy deposited by external radiation in a simulated critical site, analogous to that of an integrated circuit, can be significantly altered by changing its shape as well as the magnitude of the volume, area or thickness. By a judicious choice of the shape of a sensitive site, the relative numbers of critical events can be reduced significantly without an increase in the volume of the device. Probability distributions for energy deposition (microdosimetric spectra) in a unit-density plastic volume have been calculated for exposures to monoenergetic neutrons with energies of 1 and 14 MeV to illustrate the dependency on shape as well as volume of both the distributions of events and the mean energies deposited.

Dose Mean Lineal Energy for Fast Neutrons in Small Spherical Targets

Abstract The calculation of microdosimetric distributions for neutrons, has so far been performed for target sizes in the micrometre region, where the LET approximation is valid. In small spherical targets straggling and d ray effects strongly influence the energy deposition. The dose mean lineal energy -D can, however, be calculated using Kellerer's proximity function approach from a computer simulated ion track event pattern. In such tracks, different structural components can be distinguished. The calculation was performed for the main constituents of secondary charged particle spectra produced by 14.1 MeV neutrons in tissue. The influence of d rays on the energy deposition in small targets of 1 to 100 nm diameter is shown and the deviation from the LET approximation is discussed.

An Analytic Model for Ionisation Distributions Produced in Nanometre Volumes by Recoil Protons

Abstract A concise mathematical model is presented for the fast computation of microdosimetric distributions for the number of ionisations produced in sub-cellular volumes by energetic protons of 0.3 to 20 MeV. The model is based on the results of extensive Monte Carlo computer simulations of charged particle tracks and can be used to take into account, with only a small computing effort, straggling and delta ray transport effects in the nanometre dosimetry of neutrons.

CIRCEG, A Portable Device for Photon-Neutron Dosimetry

A portable photon-neutron dosemeter has been developed for working area and environmental monitoring. The electronic device called CIRCEG consists of a 256-channel pulse height analyser and a microprocessor system. Associated with a tissue equivalent proportional counter it gives the absorbed dose and the dose equivalent for photons and neutrons in a single measurement. Moreover, it computes the quality factor and displays the microdosimetric distributions in lineal energy (y). The materials, methods and experimental results are described. The range of dose equivalent rates due to neutrons is from 1 µSv.h-1 to 0.1 Sv.h-1.

CIRCEG, A Portable Device for Photon-Neutron Dosimetry

Abstract A portable photon-neutron dosemeter has been developed for working area and environmental monitoring. The electronic device called CIRCEG consists of a 256-channel pulse height analyser and a microprocessor system. Associated with a tissue equivalent proportional counter it gives the absorbed dose and the dose equivalent for photons and neutrons in a single measurement. Moreover, it computes the quality factor and displays the microdosimetric distributions in lineal energy (y). The materials, methods and experimental results are described. The range of dose equivalent rates due to neutrons is from 1 µSv.h-1 to 0.1 Sv.h-1.

Microdosimetric event size distributions in small sites in water vapour irradiated by protons

The author describes a calculation of the statistical distribution of energy deposition events in small spherical sites in water (with diameters of 1 mu m or smaller in unit density material) irradiated by protons. The calculation is based on the use of a semi-empirical differential ionisation cross section for water vapour given by Wilson and Miller, and takes into account proton energy loss straggling and the transport of energy by secondary electrons. Results are given for irradiations by monoenergetic protons with energies from 1 to 20 MeV, and for irradiations with protons set in motion by interactions of 14 and 20 MeV neutrons with hydrogen.

125Iodine decay in DNA: a discussion of its effectiveness for the breaking of DNA strands.

125I incorporated in DNA is known to be exceptionally toxic. Values of D0 range from about 40 to about 90 decays for survival of mammalian cells. The effectiveness of 125I in DNA with respect to the induction of breaks of the DNA strands, however, appears to be comparatively low. The numbers of strand breaks per energy deposited in subnuclear cellular structures such as DNA is smaller for a disintegration of 125I than for gamma-rays. The difference in effectiveness diminishes with increasing mass of the considered sensitive volume. The apparent inefficiency of 125I-decay may, on one hand, result from a waste of local energy deposition. On the other hand, it may be caused by a multitude of local strand breaks (clusters) induced by 125I-decay which are measured as one break only by the conventionally applied techniques of strand break measurement. The apparent inefficiency of 125I may be evidence furthermore for the importance of considering not only the DNA as the sensitive target but with equal pertinence the gross sensitive volume, i.e. the whole cell nucleus. Further, for drawing meaningful comparisons, it may be necessary to take into consideration the microdosimetric event size distributions for the critical targets.

Evaluation of a Specific Quality Function for Mutation Induction in Human Fibroblasts

In a recent paper we put forward arguments that the quality factor should be based on a microdosimetry quantity, lineal energy, rather than LET. To that end, a formalism was described involving the concept of the Specific Quality Function (SQF), which is obtained, for a particular endpoint, by correlating biological data with microdosimetric distributions in lineal energy. Such SQFs are used towards setting, by consensus, the quality factor Q(y). In this paper an SQF is obtained from published data on mutation induction in human fibroblasts. A procedure for smoothing the SQFs is discussed.

Combined Primary and Secondary Particle Transport Calculations of Microdosimetric Distributions in Tissues and Tissue Substitutes

Combined Primary and Secondary Particle Transport Calculations of Microdosimetric Distributions in Tissues and Tissue Substitutes

Ionisation Distributions from Proton and Other Tracks in Water Vapour

The low pressure cloud chamber at Harwell can now be operated with pure water vapour. Photographs have been obtained of the tracks produced by the interaction of low energy protons (85, 230 and 390 keV), low energy X rays (K X rays from carbon and aluminium), alpha particles (5.3 MeV) and a number of proton track ends. W values and a variety of microdosimetric distributions have been derived from this data (e.g. radial ion distributions, distributions of inter-droplet distances and lineal energy distribution). Comparisons with data derived from Monte Carlo distributions are in progress, as is the analysis of the ends of proton tracks.

Combined Primary and Secondary Particle Transport Calculations of Microdosimetric Distributions in Tissues and Tissue Substitutes

Primary radiation transport cannot be neglected, either in evaluating event size spectra registered by microdosimetric proportional counters or when estimating cellular responses to incident radiation in an extended tissue material. A simple numerical method is proposed that combines discrete ordinates radiation transport calculations with the analytical approach for charged particle slowing down and energy transfer to small volumes. Resulting dose distributions in lineal energy are compared with experimental even size spectra. The method has been applied for several purposes. Conclusions are drawn concerning practical problems of radiation metrology.

On the Microdosimetric Definition of Quality Factors

A formulation of the concept of quality factor based on the microdosimetric quantity lineal energy, y, is described. Toward this end, functions--denoted Specific Quality Functions (SQF)--are defined such that (a) they can be determined directly from observation and (b) a number of them can be used toward setting, by consensus, the values of a microdosimetrically-based quality factor. The determination of SQFs is exemplified by correlating data on the yields of dicentric aberrations in human lymphocytes with measured and calculated microdosimetric distributions. The implications of this approach to problems of radiation protection are discussed.

Microdosimetric distributions for nanometer-size targets in water irradiated with 60Co gamma rays: frequency distributions of total number of effective primary events produced in a target.

Microdosimetric distributions for nanometer-size targets in water irradiated with 60Co gamma rays: frequency distributions of total number of effective primary events produced in a target.

Microdosimetric Characteristics of High Energy Neutron Beams and Assessment of Quantities Relevant for Radiation Protection

Microdosimetric measurements were performed in fast neutron beams used for therapy. The energy deposition spectra were measured in a Plexiglas phantom with TE proportional counters for collimated d(50)+Be and p(65)+Be neutron beams. For p(65)+Be neutrons, the spectra were determined on the beam axis at different depths; spectra were also compared, at 15 mm depth, on the beam axis, in the penumbra region, and outside the geometrical limits of the beam. Values of 94.7 and 78.3 keV. mu m/sup -1/ were obtained for the dose mean lineal energy for d(50)+Be and p(65)+Be neutrons respectively. Quantities relevant for radiation protection such as mean quality factors, Q macr , are calculated from the experimental microdosimetric distributions; different methods of approximation are compared. Finally, present Q macr -values are compared with those published for monoenergetic neutrons, and the recommended ICRP values.

Microdosimetric Distributions for Nanometer-Size Targets in Water Irradiated with 60 Co Gamma Rays: Frequency Distributions of Effective Primary Events by Individual Tracks of Electrons

A calculation method of frequency distributions for the microdosimetric quantity j (number of effective primary events per track per target), fj , is presented. fj distributions are calculated for nanometer-size targets in water irradiated with 60Co gamma rays, using degradation spectra of total electrons in water, phi t(E). Further, fractional total electron differential fluences effective to produce j effective primary events, phi t(E, j), are calculated. It is found that the frequency for occurrence of no effective primary event tracks (tracks traversing the target without any effective primary event inside the target) is very large for nanometer-size targets, and that the delta-ray effect on the fj distribution becomes more significant with the decrease of the target size and especially j effective primary events (j greater than or equal to 1) for very small targets are due mostly to delta rays.

On the Determination of Microdosimetric Parameters in Time-Varying Radiation Fields: The Variance-Covariance Method

Microdosimetric measurements in their usual form are difficult in fields of high intensity where events, i.e., passages of charged particles, cannot be individually resolved. The variance method can then be utilized to obtain the dose averages of the microdosimetric distributions from repeated measurements over finite time intervals. An essential condition for the method is that the dose per measurement interval is constant, but this condition cannot be fulfilled in accelerator-produced fields that consist of variable radiation pulses. A modified method is therefore proposed that does not require equal doses per measurement interval. It utilizes two detectors that operate in phase, and it derives the microdosimetric parameters from the difference between the variance of the signal from one detector and the covariance of the concomitant signals from the two detectors.