The numbers of nuclei or electrons in different energy levels. At thermal equilibrium, the relative populations of the energy levels will be given by the Boltzmann distribution.

See also Boltzmann Distribution.

(rad) One rad is the absorbed radiation dose of 100 ergs of energy by one gram or 0.01 joules of energy by one kilogram of matter. In the SI system of units, the rad is replaced by the gray (Gy). 1 Gy = 100 rad. See also Absorbed Dose Rate.

A computed tomography (CT) of the abdomen images the region from the thoracic diaphragm to the pelvic groin. The computed tomography technique uses x-rays to differentiate tissues by their different radiation absorption rates.
Oral contrast material can be given to opacify the bowel before scanning. An i.v. injection of a contrast agent (x-ray dye) improves the visualization of organs like liver, spleen, pancreas and kidneys and provides additional information about the blood supply.
Spiral- or helical CT, including improvements in detector technology support faster image acquisition with higher quality. Advanced CT systems can usually obtain a CT scan of the whole abdomen during a single breath hold. This speed increases the detection of small lesions (caused by differences in breathing on consecutive scans) and is beneficial especially in pediatric, elderly or critically-ill patients.
Changes in patient weight require variations in x-ray tube potential to maintain constant detector energy fluence. An increased x-ray tube potential improves the contrast to noise resolution (CNR).

An abdominal CT is typically used to help diagnose the cause of abdominal pain and diseases such as:
Other indications for CT scanning of the abdomen/pelvis include planning radiation treatments, guide biopsies and other minimally invasive procedures. Advanced techniques include for example 3D CT angiography, multiphasic contrast-enhanced imaging, virtual cystoscopy, virtual colonoscopy, CT urography and CT densitometry.

See also Contrast Enhanced Computed Tomography.

The acute dose is the amount of energy accommodated in human tissue or in biological systems.

See also Acute Exposure, Acute Radiation Syndrome, Biodosimetry and Annual Limit On Intake.

Alpha particles consist of two neutrons and two protons (nucleus of He), have a positive charge of 2 and a velocity in air of approximately one-twentieth the speed of light.
Discovered by Ernest Rutherford in 1899 (Rutherford-Bohr planetary atom model) alpha particles became emitted by very large atoms in an unstable energy state (high atomic number mostly over 82, with a too low neutrons//protons ratio (e.g. <= 1.5)).
Through their relative slow travel speed, they get stopped by e.g. a thin sheet of paper or the outer layers of human skin or travel only inches; once stopped they pick up free electrons and become helium.
These alpha particles are only dangerous to humans when the alpha-emitting material is inhaled or ingested (it causes damage that may lead to cancer) or comes into contact with the lens of the eye, caused by their less penetrating properties.