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
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.
1-20. SELECTION CRITERIA
a. Numerous radiation detection instruments exist which utilize one or more of
the operating principles discussed above. The selection of instruments to use in
radiation environments is based on the kind of information sought and type of radiation
b. In most cases involving radiation protection surveys, the instrument operator
is seeking information concerning the amount of radiation per unit time (dose rate) or