PROTECTIVE SHIELDING

Protective shielding is one method of defense against nuclear radiation. The tremendous penetrating power of gamma rays makes it difficult to provide enough shielding to protect personnel from gamma rays. However, the structure of the ships provides some protection.

The main materials likely to provide shielding aboard ship are steel plating, piping, machinery, water, fuel oil, and perhaps wood. Shielding materials at shore facilities also include concrete and earth.

The amount of shielding required to stop gamma rays is measured in half-value layer thicknesses or "half-thicknesses," for short. A half-thickness is defined as the amount of material necessary to cut down the amount of radiation to one-half of its original value. The half-thickness for each material is different. For example, a concrete shield about 6 inches thick or an earth shield about 7 1/2 inches thick will cut the gamma radiation in half. Suppose that you are in a place where the gamma radiation exposure is 400 roentgens. If you are behind a half-value layer thickness at the time, you will receive a dose of 200 roentgens. Now suppose you are standing behind two shields, each of which is a halfthickness. The 400 roentgens of gamma radiation is reduced to 200 roentgens by the first half-thickness and then to 100 roentgens by the second half-thickness. With each additional halfthickness shield, you reduce the remaining gamma radiation by half. Remember that these thicknesses do not stop gamma radiation completely; instead, each cuts it in half. In a nuclear attack, one half-thickness of steel or concrete might be enough shield to keep you from getting a lethal dose of gamma radiation.

The half-thicknesses of some materials are estimated below. Note that initial radiation is more penetrating than residual radiation and requires a larger thickness to reduce the radiation to one-half of its original value. These materials are listed in the order of their effectiveness as shields against gamma radiation.

RADIAC INSTRUMENTS

Nuclear radiation cannot be detected by any oft he five senses. Therefore, special instruments and devices have been developed to do this job. From the military standpoint, we not only need to detect radioactivity, we need to know where the radiation is and what the intensity is. Radiation detection, indication, and computation (radiac) instruments serve both of these needs. Radiac instruments are designed to perform the following functions:

Detect alpha, beta, gamma, and neutron radiation

Measure the intensity of radiation

Measure radiation dose

Determine the extent of contamination

. Provide information for calculating the

length of time that the contamination will exist in the area . Determine the effectiveness of decontamination measures

The following information will cover specific radiac instruments that you may use. The Naval Ships' Technical Manual, chapter 070, also has information on radiacs.