RADIATION DETECTION INSTRUMENTS

DETECTOR FUNDAMENTALS

a. Nuclear radiation cannot be detected by any of the human senses; therefore, detection instruments must be used to measure radiation intensity. If such instruments were not available, it would be possible for personnel to be exposed to sizable, even lethal, doses of radiation without warning at the time of exposure. Only with the knowledge provided by radiation detection instruments can personnel be adequately warned against exposure. |

b. Instruments for detecting and measuring radiation are called radiac instruments. The word "radiac" was coined from the first letters of words describing what the instruments are intended for--radioactivity detection identification, and computation.

c. Radiac instruments measure radiation by the detection and evaluation of an event produced by radioactive decay. There are several different methods, based upon the various phenomena associated with nuclear radiation, which can be used to detect and measure radiation. The major methods are:

(1) Electrical collection of ions. When gases are ionized by radiation, the ions formed move about in a haphazard manner and eventually the negative and positive ions will be neutralized. However, if an electrical field is established in a confined volume of gas by two oppositely charged surfaces (electrodes), the negative ions will move toward the positive electrode while the positive ions will move toward the negative electrode. Upon hitting the electrodes, the ions will be neutralized and will reduce the charge on the electrode. The amount of reduction can be measured and will give an indication of the amount of radiation present.

(2) Scintillation. Certain crystalline materials, such as zinc sulfide, and sodium iodide, have the property of emitting flashes of light (scintillation) when struck by ionizing radiation. The intensity of the light emitted by the scintillating crystal is proportional to the energy of the ionizing radiation.

(3) Semiconductors. When ionizing particles strike certain semiconductors, hole-electron pairs (ions) are either created or destroyed, resulting in a pulse of current proportional to the intensity of the radiation field.

(4) Photographic. Ionizing radiation causes chemical changes to photographic film similar to the effects of ordinary light. Varying quantities of ionizing radiation result in a corresponding change in the optical density of the developed film, providing a reliable means of detecting and measuring radiation.

X-ray Schools | X-ray and Radiation Safety
For Informational Purposes Only - Based On US Army Radiation Safety Training