Low-pressure Cloud Chamber Research Articles

Experimental Calculations of Droplet Diffusion in a Low-Pressure Cloud Chamber

A low-pressure cloud chamber was used for several years to display the tracks created by the passage of ionizing particles through vapors of interest. The spatial distributions of the ions that were formed were of special interest, but the accuracy with which these distributions could be determined was reduced by the presence of diffusion. This meant that the droplets, when photographed, had moved significantly away from the point of creation of the parent ion. In the present investigation photographs obtained by previous workers have been analyzed in an attempt to quantify the extent to which the droplets had diffused. The results suggest that the diffusion, when converted to standard density (1000 kg/m3), was independent of the pressure inside the cloud chamber and the mixture used. It could be represented by a one-dimensional root-mean-square diffusion distance whose value was calculated to be 2.42 +/- 0.04 nm. Values for the diffusion of thermalized electrons (< approximately 4 eV) before capture to form negative ions were also calculated. They appeared to lie in the range 3.5-5.0 nm, and were again independent of the pressure and nature of the mixture. The magnitude of the diffusion was large enough to mask any measurable prediffusion structure for a distance in the region of 10 nm radially around the track path of the alpha-particle and proton tracks analyzed.

Track Studies in Water Vapor Using a Low-Pressure Cloud Chamber: II. Microdosimetric Measurements

A low-pressure cloud chamber has been adapted to operate with pure water vapor. Photographs were obtained of tracks arising from the passage of ionizing radiation. The sources used were low-energy X rays, 242Cm alpha particles, and low-energy protons. Distributions of lineal energy, radial distances around an ion track, and interdroplet distances were measured and compared with the predictions of Monte Carlo calculations. After allowing for diffusion and the limitations of the geometry of the system, the measured and calculated distributions were found to be in good agreement.

Track Studies in Water Vapor Using a Low-Pressure Cloud Chamber: I. Macroscopic Measurements

Techniques have been developed to operate a low-pressure cloud chamber with pure water vapor. Photographs have been obtained of the tracks arising in this medium from the passage of ionizing radiation. The sources used were low-energy X rays, 242Cm alpha particles, and low-energy protons. Track lengths of the electrons were similar to those found previously in tissue-equivalent gas. W values of 35.6 +/- 0.4 and 32.6 +/- 0.6 eV per ion pair for carbon and aluminum X rays also compare closely with those in tissue-equivalent gas, but are somewhat higher than the predictions of Monte Carlo calculations. Differential w values were obtained: for alpha particles of energy 5.3 MeV the value was 33.0 +/- 3.0 eV per ion pair; for protons of energy 390, 230, and 85 keV the values were 30.6 +/- 1.9, 31.9 +/- 2.0, and 33.6 +/- 3.4 eV per ion pair. The energy losses of protons in water vapor were measured in a second (dummy) chamber used for energy calibration. Results support Janni's values of stopping power for protons in the energy range 40-480 keV.

Comparison of Particle Tracks Calculated by Monte Carlo Computer Codes with Experimental Tracks Observed with the Harwell Low Pressure Cloud Chamber

Comparison of Particle Tracks Calculated by Monte Carlo Computer Codes with Experimental Tracks Observed with the Harwell Low Pressure Cloud Chamber

Comparison of Particle Tracks Calculated by Monte Carlo Computer Codes with Experimental Tracks Observed with the Harwell Low Pressure Cloud Chamber

Abstract Photographs of the droplets associated with the ionisations caused by charged particle tracks in the Harwell low pressure cloud chamber have been analysed. The radiation types these represent are alpha particles, protons and low energy X rays (carbon and aluminium) in either tissue-equivalent gas or water vapour. The tracks were used to test the validity of two Monte Carlo codes developed by Wilson and Paretzke, namely MOCA14 for the generation of proton and alpha particle tracks, and MOCA8 for the generation of electron tracks. The comparisons showed that the code MOCA14 would appear to be valid for protons with energies greater than about 390 keV, and for alpha particles with energies greater than 1.6 MeV. No disagreement was found between the low energy X ray tracks from the cloud chamber and the tracks calculated from MOCA8, although this comparison was severely limited by droplet diffusion.

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.

Droplet growth and diffusion in a low-pressure cloud chamber

A low-pressure cloud chamber which can make visible individual ions in the tracks of ionizing particles is available at Harwell. The limitation on the spatial accuracy of the information obtained, is the diffusion of droplets during their growth. The initial stages of droplet growth are particularly important since most of this diffusion takes place while the ‘droplet’ contains only a few molecules. This paper discusses the early stages of droplet growth from ions in a low-pressure cloud chamber and derives values for droplet diffusion during growth. The growth rates for the first few attachments to an ion are derived from chemical reaction data. Later growth rates are predicted using conventional droplet growth-rate equations. The effects of droplet heating are included as these are significant in a low-pressure chamber. The values derived for diffusion are compared with estimates based on comparing the expected shape of particle tracks with their observed shape in the chamber.

Microdosimetric Properties of Alpha-Particle Tracks Measured in a Low-Pressure Cloud Chamber

A cloud chamber which can make visible individual ions in the tracks of ..cap alpha.. particles is available at Harwell. Work on the theory of droplet formation and growth has enabled a tissue-equivalent gas of lower density than previously possible to be used in the chamber. This has enabled even the closest ions in ..cap alpha..-particle tracks to be individually resolved. Extensive spatial analysis has been carried out on ..cap alpha..-particle track segments at 5.0, 3.4, and 1.1 MeV. Radial distributions of ionization density with distance from the track have been derived, Rossi Y distributions have been calculated for target sizes of 10 and 50 nm (at 1000 kg m/sup -3/) and, finally, interdroplet distance distributions have been derived. These results may be compared to Monte Carlo calculations of track structure and may also provide useful data for theories of radiation damage.

Microdosimetric Properties of Electron Tracks Measured in a Low-Pressure Cloud Chamber

A cloud chamber which can make visible individual ions in the tracks of low-energy electrons and ..cap alpha.. particles is available at Harwell. Work on the theory of droplet formation and growth has enabled a tissue-equivalent gas with a density of 8.8 X 10/sup -3/ kg m/sup -3/ to be used in the chamber. This is half the density of previous mixtures. This has enabled ions in particle tracks as close as a distance equivalent to 0.5 nm in water at 1000 kg m/sup -3/ to be individually resolved. Extensive spatial analysis was carried out on electrons from aluminum X rays, and the results were compared with those previously obtained in an earlier gas mixture. The accuracy of these results is limited mainly by diffusion of droplets during growth (previously calculated to be equivalent to (2.8 +/- 1.5) nm in water at 1000 kg m/sup -3/). Track lengths and mean LET values were derived and shown to be in agreement with other work. In order to provide more detailed imformation on track structure, Rossi ''Y'' distrbutions were calculated from target sizes of 10 and 50 nm in water. Interdroplet distance distributions were also derived. These distributions were compared with Montemore » Carlo calculations and the agreement was found to be within 10%. This increases confidence in both the cloud-chamber analysis and the Monte Carlo calculations.« less

Analysis of Ionization Distributions in a Low-Pressure Cloud Chamber

A low-pressure cloud chamber has been constructed which can resolve individual ions in the tracks of low-energy electrons. Droplets formed by photoelectrons and Auger electrons from characteristic aluminium X rays have been photographed and their three-dimensional coordinates reconstructed. The accuracy of these coordinates is limited mainly by diffusion in the chamber which is estimated to be equivalent to 4.0 + 1.5 nm in water at unit density. This paper describes a method of computing and analyzing individual electron tracks from the droplet coordinates. Track-length data have been determined for each type of track. Rossi Y distributions have been calculated for target sizes equivalent to 6, 12, and 24 nm in water at unit density. Histograms of interdroplet distances (T distributions) have been calculated and compared to similar distributions derived from results of Monte Carlo calculations. The cloud chamber has been filled with an approximately tissue-equivalent mixture. Results should be useful for checking the physical basis of target theories of radiation action.

A low-pressure cloud chamber to study the spatial distribution of ionizations

To further the understanding of the biological effects of radiation a knowledge of the spatial distribution of ionizations in small volumes is required. A cloud chamber capable of resolving the droplets formed on individual ions in the tracks of low-energy electrons has been constructed. It is made to high-vacuum specifications and contains a mixture of permanent gases and vapours, unsaturated before expansion, at a total pressure of 10 kPa. Condensation efficiencies close to 100% are obtained without significant background from condensation on uncharged particles and molecular aggregates. This paper describes the chamber, associated equipment and method of operation and discusses the performance of the system.

3026. A low-pressure cloud chamber to study the spatial distribution of ionizations

3026. A low-pressure cloud chamber to study the spatial distribution of ionizations

Die Ergebnisse der Ersten Kondensationsversuche in der Unterdruckapparatur

A description is presented of the newly adaptedFindeisen low-pressure cloud chamber and of the first measurements made to determine the influence of temperature and expansion rate on the start and duration of fog. The relations obtained are explained by physical and chemical properties of condensation nuclei and of their surfaces. A deviation from the obtained dependence of the start of fog on air temperature, supersaturation and expansion rate is explained by means of the simplifiedFindeisen formula for computing maximal supersaturation in water clouds. An attempt is made to calculate the radii of droplets from coloured rings brought about by diffraction of light on fog droplets.

Determination of the Ranges and Straggling of Low-Energy Alpha Particles in a Cloud Chamber

Ranges of 105 alpha particles with energies of 0.48, 0.545, and 0.615 Mev as fixed by a velocity selector have been measured by the use of a low-pressure cloud chamber. From the results of these measurements the average ranges ${R}_{\mathrm{Av}}$ and the straggling coefficients in air at 15\ifmmode^\circ\else\textdegree\fi{} and 760 mm were calculated for these three energies. The values of ${R}_{\mathrm{Av}}$ were found to be respectively 0.299, 0.327 and 0.354 cm; and the values of the standard deviation were respectively 0.011, 0.0135, and 0.010 cm. After a correction to take into account the difference in definition, the ${R}_{\mathrm{Av}}'\mathrm{s}$ are two to three percent higher than those following from a range-energy curve given by Bethe.

An Independent Determination of the Binding Energy of the Deuteron

This paper is an account of an experiment designed to determine the binding energy of the deuteron by a method which is relatively insensitive to uncertainties in the energy-range relation for protons of low energy. The protons, produced by the disintegration of Th C\ensuremath{''} $\ensuremath{\gamma}$-radiation, were observed in a low pressure cloud chamber in a strong magnetic field. The curvatures of the tracks allowed the calculation of the corresponding kinetic energies. The final value of the binding energy as given by this experiment is ${\overline{V}}_{B}=(2.17\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}{10}^{6} \mathrm{electron}\mathrm{volts},$ which is in reasonable agreement with previous determinations by others. The uncertainty 0.05 Mev is the maximum uncertainty to be expected in this value of the binding energy.