with very energetic solar flares or fast coronal mass ejections. Such events, which accelerate particles, tend to cluster in the more active phases of the approximately 11-year solar cycle.

Somatic effects of radiation:

Effects that occur in an irradiated individual due to damage produced in various tissues of the body, as opposed to genetic effects, which occur in the offspring of an irradiated individual owing to damage in germ cells. Potentially important somatic effects of radiation include induction of cancer and damage to the central nervous system. Cataract formation is also possible.

Specific energy:

The actual energy per unit mass deposited per unit volume in a given event. This is a stochastic quantity as opposed to the average value over a large number of instances (i.e., the absorbed dose).


Random events leading to effects whose probability of occurrence in an exposed population of cells or individuals (rather than severity in an affected cell or individual) is a direct function of dose; these effects are commonly regarded as having no threshold. Hereditary effects are regarded as being stochastic; some somatic effects, especially carcinogenesis, are regarded as being stochastic.

Target theory (hit theory):

A theory explaining some biological effects of radiation on the basis that ionization, which occurs in a discrete volume (the target) within a cell, directly causes a lesion that later results in a physiological response to the damage at that location; one, two, or more hits (ionizing events within the target) may be necessary to elicit the response.

Threshold hypothesis:

The assumption that no radiation injury occurs below a specified dose.

Transport calculation:

Calculation of particle distributions and energy behind a specific shield, derived from the basic nuclear cross sections for interactions and fragmentation in shielding.

Whole-body external dose:

The dose of radiation from sources outside the body that irradiate the entire body. The dose from cosmic radiation is an example of a whole-body external dose.

x radiation:

Also x rays; penetrating electromagnetic radiation, usually produced by bombarding a metallic target with fast electrons in a high vacuum.

Xeroderma pigmentosum:

An inherited disease in which individuals are highly susceptible to cancer induced by exposure to solar radiation. Xeroderma pigmentosum cells have a defect in the ability to repair ultraviolet damage to their DNA, a defect that apparently accounts for the susceptibility.

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