Table 5-1. Radiation Protection Standards
Whole body, head and trunk,
5(N-18); N greater
blood-forming organs, gonads,
than 18 years
lens of the eye
Skin and thyroid
Hands and forearms, feet
0.5 (whole body)
Exposure limits. No individual shall ever knowingly expose himself or cause others to be exposed to levels of
radiation greater than those specified in table 5-1, except in case of extreme emergency.
Radiation Detectors. The following paragraphs describe various radiation detectors.
General. Radiation detectors are used to measure the accumulated dosage of Roentgen Equivalent Mans
(rem) of occupationally exposed individuals and to determine the level of radiation of an object or area.
Types. The four most common types of radiation detectors are described in the following paragraphs.
Ionization chamber. The radiation detectors of most ionization clamber type survey instruments are
either cylindrical or rectangular in shape with a wire electrode (anode) in the center which runs the length of the chamber.
This wire is electrically insulated from the chamber wall (cathode) and an electrical field or charge is established between
the wire and the chamber wall. The chamber is filled with air at atmospheric pressure. As X-or gamma radiation
penetrates the chamber, ions are produced as a result of interactions (ionization) in the gas in the chamber. Under the
influence of the electrical field, the negative ions will move toward the positive electrode (anode), while the positive ions
will move toward the negative electrode (cathode). Upon hitting the electrodes, the ions neutralize part of the charge on
the electrodes. The extent of the neutralization can be measured and will given an indication of the amount of radiation
to which the chamber has been exposed. The relative response of four survey instruments is shown in Table 5-2. The
relative response is defined as the indicated exposure rate divided by the true exposure rate. The Victoreen Model 440
and the Heat Pipe Model VR-10 survey meters are now standard instruments for industrial radiography. The Victoreen
Model 592B, although still in use, is being phased out, due to its poor energy response.
1 AN/PDR-27 type instrument. It can be concluded from Table 5-2 that the AN/PDR-27 GeigerMueller
type survey instrument does not accurately measure X-ray exposure rates. The use of the external probe with the beta
shield off results in exposure rates that are higher than the true rates. However, the maximum exposure rate that can be
measured using the external probe is 5.0 mR/hr. exposure rates from low energy X-rays cannot be accurately measured
since the internal GM tube is shielded by a container which blocks low energy electro magnetic radiation. At 32 kev, for
example, this instrument measures only 1% of the true exposure rate. The AN/PDR-27 or other GeigerMueller type
survey meters SHALL NOT be used in conjunction with industral X-ray operations.
2 Victoreen Model 529B instruments. The Victoreen Model 592B survey meter is limited in its ability to
detect low energy X or gamma rays. At 33 key, for example, this instrument measures only 23% of the true exposure
rate. Thus, this instrument could, in some cases, result in an improper evaluation of the radiation