High-LET

It is clear from the present discussion that LET alone does not completely determine track structure. Track length (or particle velocity) is also a contributing factor. For example, at the same intermediate LET, high-energy heavy-ion tracks would be cylindrical while those for the KeV electrons would be better described as either spheroidal or as partially overlapped spurs (Samuel and Magee, 1953). 

FIGURE 3.8 A typical MeV proton track in water. See text for details. Reproduced from Mozumder (1969), by permission of John Wiley Sons, Inc.  

Chapter 3 Structure of Charged Particle Tracks in Condensed Media 

Charge exchange is important all along the high-LET tracks. The effective ionic charge is determined by cross sections of electron capture and loss, which depend predominantly on the ionic velocity. Electron loss may be simply described by an ionization of the incident ion in its own reference frame due to the impact of medium electrons and nuclei. Following Bohr (1948), Mozumder et al. (1968) wrote the cross section for this process as1 

the contributions of both the nuclei and the electrons are considered. The maximum energy transfer for electrons is (l/4)mv2, but since their contribution to the loss cross section is small, no great error is committed in taking the maximum energy transfer as 2mv2 for all ionizations represented in Eq. (3.4). Electron capture is a three-body process best visualized as ionization of a molecule of the medium with the ejected electron having a speed v at least equal to that of the incident ion, followed by capture of that electron in an orbit around the impinging ion. Mozumder et al. (1968) modified an earlier formula of Bohr (1948) and wrote the capture cross section as 

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High-LET—Energy transfer characteristic of heavy charged particles such as protons and alpha particles where the distance between ionizing events is small on the scale of a cellular nucleus. 

Goldman, M., An overview of high LET radiation effects in cells, in The Health Effects of Plutonium and Radium (W. S. S. Jee, ed.), pp. 751-766, J. W. Press, Salt Lake City, Utah (1976). 

In theory, one assumes the formation of radicals before the chemical stage begins (see Sect. 2.2.3). These radicals interact with each other to give molecular products, or they may diffuse away to be picked up by a scavenger in a homogeneous reaction to give radical yields. The overlap of the reactive radicals is more on the track of a high-LET particle. Therefore, the molecular yields should increase and the radical yields should decrease with LET. This trend is often observed, and it lends support to the diffusion-kinetic model of radiation-chemical reactions. 

The Samuel-Magee model can be extended to a-particle tracks, considered as cylindrical columns formed by excessive spur overlap due to high LET. To a good approximation, the length I of the cylinder remains constant while its radius grows by diffusion. In this geometry, the normalized radical distribution is given by... 

First, we want to make a comment about possible local temperature rise due to energy absorption. An early theory of radiation effect s was based on the point-heat hypothesis (Dessauer, 1923). Later analysis showed that the temperature rise would be too feeble and too transient for low-LET radiation to cause any real change (see Mozumder, 1969). There is no experimental evidence for temperature rise for low-LET radiations. The case of high-LET radiations is still open, though. 

An early theory of ionic recombination in liquids was developed by Jaffe (1913) for application at a relatively high LET. However, in Jaffe s theory, coulombic interactions are ignored and the positive and negative ions are assigned the same mobilities and distribution functions. Therefore, its use in a... 

Syrian hamster, Cricetus sp. 0.12-2 Gy, single acute exposure Human, Homo sapiens Genes modifying cytoskeletal development adversely affected at all doses within 3 h by both high LET (neutrons) and low LET (gamma rays, X-rays) radiations 11... 

There was no procedure to evacuate nonessential personnel from the area in the case of a high LET alarm or a power failure. Because the contractors and some other workers in the area do not have radios to monitor unit communications, they would not know to leave the area unless the evacuation alarm was sounded. (Emergency planning and response)... 

It may be, he said sourly, but be careful If there is a fourth time—Heaven forbid —the stakes will be very high. Let me see that wound of yours. ... 

Gervais B, Beuve M, Olivera GH, Galassi ME (2006) Numerical simulation of multiple ionization and high LET effects in liquid water radiolysis, Radiation Physics and Chemistry 75 493-513... 

Although HO2 and its conjugate base are only significant primary radicals for high LET radiation, they are important secondary radicals in oxygenated solution where they are formed in reactions (55)-(58) ... 

For a given medium such as water, its density and the velocity of the incident heavy ion contribute to the formation of the columnar track structure. The spacing of the primary energy loss events must be within the delocalization of the secondary electrons for elfects attributed to high LET to occur. For this reason, typical LET effects will not be... 

The measured H atom G-value is about 0.25 at MZ jE = 1, while the equivalent yield of hydrated electrons is found at MZ jE = 10. The persistence of the hydrated electron to higher MZ jE values suggests that it does not decrease to zero at an infinite value of MZ jE. Most H atoms are produced in conjunction with OH radicals in the core of the heavy ion track. The recombination rate constant is high so there is a small probability that H atoms will escape the track at high LET (MZ jE). H atoms can be formed by hydrated electron reactions and their yield cannot decrease to zero if hydrated electron yields do not. However, hydrated electron yields are low at high MZ /E values so the H atom yield can be considered negligible in this region. 

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