RADIATION SHIELDING MATERIAL THICKNESS IN INCHES
Figure 13.1 (Principles of Protection p. 420)
Note: The initial gamma radiation HVL varies only slightly from that of fallout gamma and is therefore treated the same in this table
Example: For shielding against gamma radiation, a protection factor of 1,024 would be provided by 8.0 inches of lead, 12.0 inches of steel, 39.0 inches of concrete,, 55.0 inches of earth, or 118 inches of wood.
PRINCIPLES OF PROTECTION, The US Handbook of NBC Weapon Fundamentals and Shelter Engineering Design Standards, Walton McCarthy, 727 pp. 2013, 6th edition, Brown Books.
What is the protection for of 48 inches of earth and 12 inches of concrete from fallout?
For the earth cover protection, go to the Earth-Gamma column. The nearest values to “48 inches” are 44 and 49.5. Round down to 44 as this is a more conservative. 44 inches of earth has an HVL of 8.
For the concrete cover protection, go to the Concrete-Gamma column. The nearest values to “12 inches” are 11.7 and 15.6. Round down to 15.6 inches of concrete cover. 11.7 inches of concrete has an HVL of 3.
Combine the 8 earth and 3 concrete HVL’s. An HVL of 11 has a PF of 2,048. The protection of 44 earth inches and 11.7 concrete inches is 1/2048th of the unprotected dose
The source of neutron radiation comes directly from the fireball and has a life of less than sixty seconds. During this time period, neutron radiation induces extremely high levels of radiation doses. The source of gamma radiation is the fireball and fallout. (See Figure 4.1.) The quantity of fallout varies depending on the type of burst, size of weapon, type of soil at ground zero, distance from ground zero, weather, and so on. However, to allow some general reference point as to the quantity of fallout, an analogy can be made with respect to common lawn fertilizer. A typical 40 bag of fertilizer would be spread over an area of approximately 5000 ft2. The quantity of fallout deposited directly downwind close to ground zero would be analogous to spreading this bag of fertilizer over an area of approximately 1200 ft2 or an area measuring 35 ft. x 35 ft.
A barrier of earth, concrete, lead, or steel is placed between the source of radiation and the protected target. Each material has a given ability to reduce the quantity of radiation which passes through it. This ability to shield from ionizing radiation is based primarily on the density and moisture content of the material.
Geometry shielding is a method where a mechanical design reduces the radiation entering the shelter. Radiation does not travel in a perfectly straight line and does, in fact, bend or scatter around corners. As it does, it is attenuated (reduced). This is the reason for 90° angled entranceways or offset entranceways for blast shelters. Another aspect of geometry shielding is the Inverse Square Law. Radiation decreases in intensity with distance. The further from the radiation source, the lower the radiation intensity or dose will be. However, the reduction is very small. At one hundred feet away from the fallout, the reduction in radiation is approximately one-half. At six hundred feet from the fallout radiation source, the reduction is approximately one-tenth.1 Taking refuge in the second story of a house to gain distance from the fallout on the ground will provide little if any reduction in radiation dose. In fact, the dose may be greater due to the presence of fallout on the roof of the house.