Dictionary
Mario P. Iturralde in Dictionary and Handbook of Nuclear Medicine and Clinical Imaging, 1990
Cloud chamber. An early type of nuclear radiation detector, by which the track of an ionizing particle may be observed and recorded photographically. Its operation depends on the formation of a supersaturated region in a gas containing a condensible vapor. The saturated vapor pressure near a drop increases with the curvature of the surface, and thus a vapor pressure which is saturated for a large drop or flat surface is below saturation for a small drop. For a given supersaturation, therefore, there exists a critical drop size below which the drop will evaporate and above which it will grow. The nuclei upon which drops will tend to form and grow include dust particles and positive and negative ions; the latter allow growth because their electrostatic charge reduces the saturation vapor pressure in their vicinity. The result is that the track of an ionizing particle appears as a line of small droplets. There are two types of cloud chamber, expansion and diffusion.
Detectors, Relative Dosimetry, and Microdosimetry
Harald Paganetti in Proton Therapy Physics, 2018
Some track structure measurement devices have been used in proton beams that are mentioned, of which a more extended review can be found in Ref. [62]. In cloud chamber microdosimetry, a low-pressure supercooled gas is used in which the individual ionizations of the proton and its secondary particles create a 3D pattern of droplets that can be resolved by stereoscopic photography providing extremely high-resolution detail on the location of individual ionizations within the gas. In an optical ionization chamber, electrons in the particle track are made to oscillate rapidly by the application of an external, short-duration, high-voltage electric field. The excited electrons produce additional ionization and electronic excitation of the gas molecules in their immediate vicinity, leading to fluorescent light emission from the gas allowing the location of the electrons to be determined with a resolution of 10 nm. In three-dimensional optical random access memories, the energy deposited along the proton track changes a bistable photochromic material from the stable nonfluorescent form to a quasistable fluorescent form. The location of the fluorescent sites can be read out by confocal laser microscopy.
W
Anton Sebastian in A Dictionary of the History of Medicine, 2018
Wilson, Charles Thompson Rees (1869–1959) Scottish pioneer of atomic and nuclear physics from Glencorse near Edinburgh. He developed the cloud chamber for studying atomic particles in 1897. He was professor of natural philosophy at Cambridge from 1925 to 1934. He shared the Nobel Prize for Physics in 1927 for his work on ionization of water.
Track to the future: historical perspective on the importance of radiation track structure and DNA as a radiobiological target
Published in International Journal of Radiation Biology, 2018
Even early on the importance of the nanometer scale was recognized. The track structure descriptions were improved by incorporating non-random fluctuations and clustering of ionization events along one-dimensional paths based on cloud-chamber measurements. The concept of clusters of ionization being a critical feature of radiation damage was applied to microbial data. Howard-Flanders (1958) and Brustad (1962) were able to model data assuming a lethal lesion required a minimum of 1–10 ionizations within of 3–10 nm. This approach was subsequently applied to mammalian data, Barendsen (1964) found that the low-LET response fitted best assuming 10 or more ionizations in 7 nm, while the high-LET required 15 or more ionizations in 10 nm. This approach was extended by Goodhead et al. (1980), by assuming there were two types of lesions. One lesion requiring 3–9 ionizations in 3 nm (with a low probability of effect) which dominates for low-LET radiation and the other lesion requiring 10 or more ionizations in 3 nm (with a high probability of effect) corresponding to a high-LET lesion. Although the approach made no assumptions about the nature of the target, the analyses pointed to the target dimensions being of the order of a few nanometers.
Advancements in the use of Auger electrons in science and medicine during the period 2015–2019
Published in International Journal of Radiation Biology, 2023
Atomic vacancies that lead to Auger and ICD processes are created by several mechanisms. One mechanism that creates an inner atomic shell vacancy is the photoelectric effect. The Auger electrons that are emitted following the photoelectric effect were observed by Pierre Auger when he irradiated a cloud chamber with X-rays (Auger 1923). Radionuclides undergoing internal conversion (IC) transitions also create inner atomic shell vacancies, as do radionuclides that decay by electron capture (EC). The shower of low energy electrons that follow was seen by Meitner when she was conducting experiments on radioactive decay (Meitner 1923). The stochastic nature of the atomic and molecular electronic relaxation process results in different yields and energies of electrons for each initial vacancy created. Most of these electrons have very low energies (∼20–500 eV) which have extremely short ranges in water (∼1–10 nm). Biological molecules near the Auger cascade are impacted by the direct effects of electron irradiation as well as indirect effects caused by radical species that are produced during the radiolysis of water by these electrons (Figure 2) (Wright et al. 1990). Other physical mechanisms such as Coulomb explosion, caused by extremely rapid charge neutralization of highly ionized atoms, can cause damage to the molecule in which electronic vacancies are created (Pomplun and Sutmann 2004).
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