Before a discussion of the functional requirements of reactor containment it is important to put radioactivity and its effects into perspective within our present environment.

5.1.1 Definition of Terms

The following are a set of definitions useful in any discussion of radio­activity :

a. Curie (Ci). A measure of radioactivity. A curie of any radioactive nuclide undergoes 37-109 transformations per second.

b. Roentgen (r). Named for William Roentgen, the discoverer of x rays, a roentgen is the quantity of x or у radiation which will produce one elec­trostatic unit of charge in 1 cc of air at STP. It corresponds to an energy of 83 ergs.

c. Radiation absorbed dose (rad). A quantity of radiation that delivers 100 ergs of energy to 1 gm of substance. In this case, body tissue is the substance of interest.

d. Roentgen equivalent, man (rem). This is the biological unit: the quan­tity of radiation equivalent in biological damage to 1 rad of standard x rays. This unit will be used most often in this chapter.

e. Relative Biological Effectiveness (RBE). This is the connection be­tween the biological unit of the rem and the radiative unit of the rad.+

RBE = 1 rem/1 rad (5.1)

f. Linear Energy Transfer (LET). The average amount of energy lost per unit of particle spur-track length. The linear energy transfer of course, depends on the particle, its energy, and on the material involved. Table 5.1 shows LET values for body tissue (/).

The biological effect of radiation on body tissue, the RBE, is therefore related to the LET value for the particle and for the energy of that particle.