**1. Introduction**

The content of this chapter includes a brief history of gamma radiation, units of radiation measurement, ecological importance, tables including the half life of gamma emitting nuclides, comparative sensitivity of living organisms to gamma radiation, biological magnification of radioactive and nuclear materials, and brief descriptions of case studies of Woodwell (1962), Stalter and Kincaid 2009), and nuclear power plant disasters (Three Mile Island, USA, 1980, Chernobyl 1986, Japan 2011).

Gamma radiation is somewhat similar to x-rays in that both pass through living materials easily. Also referred to as "photons" they travel at the speed of light. Gamma rays have sufficient energy to ionize matter and therefore can damage living cells. The damage produced in the cell or tissue is proportional to the number of ionizing paths produced in the absorbing material. Isotopes of elements that are emitters are radionuclides important in fission products from nuclear testing, nuclear power plant disasters or waste.

The injurious affect of gamma rays depends on (1) their number (2) their energy and (3) their distance from the source of radiation. Radiation intensity decreases exponentially with increasing distance. Radiation damage on vascular plant species was demonstrated by Woodwell (1962) who subjected a mature pine oak forest at Brookhaven National Laboratory to gamma radiation from a cesium 137 source (Figure 1).

Fig. 1. Radiation dose and damage to a pine-oak forest, Brookhaven National Laboratory, 1961. Zones delineated by vertical lines (Woodwell 1962, Stalter and Kincaid 2009).

mouse to ionizing radiation. McCormick and Golley (1966) presented data on irradiation of natural vegetation in the southeastern United States while Monk (1966) published a similar study on the effects of short-term gamma radiation on an old field. Witherspoon (1965, 1969) examined radiation damage to a forest surrounding an unshielded fast reactor in l965, and followed this study with a report in 1969 on radiosensitivity of forest tree species to acute fast neutron radiation. Odum and Pigeon (1970) researched the effect of irradiation and

Three units, the gigabecquerel (GBq), gray (GY), and roentgen (R) are used to measure radiation. The GBq measures the number of gamma rays emitted from a source of radiation and is a unit of radioactivity that is defined as 1.37 x 10-12 atomic decays each second. The weight of the material comprising a GBq varies. One gram of radium is 37 GBq while 10-7th of a gram of newly formed radio-sodium is also 37 GBq since both release 3.7 x 10-10 disintegrations/second (Odum 1971). In dealing with biological systems, smaller units are generally used such as the millicurie microcurie and picocurie which are 10-3, 10-6 and 10-12

A second measurement of radiation is the GY. The absorbed dose of 1 GY means the absorption of 1 joule of radiation energy per kg of tissue. The third, the roentgen is nearly the same as the GY, and is used as a unit of measurement for exposure to gamma and x rays. Both are units of the total dose of radiation received by an organism. The dose rate is the

There are different kinds of atoms of each element; these are referred to as isotopes. Some isotopes are radioactive, some not. Radioactive isotopes are unstable. These decay into other isotopes releasing radiation. Each radioactive isotope, radionuclide, have a specific rate of

Radionuclides fall into well defined groups (Tables 1 and 2). Naturally occurring nuclides are included in Table 1 while those from fallout produced by fission or uranium and other elements are found in Table 2. Fission isotopes are produced from nuclear explosions which have for the most part been eliminated and from "controlled" reactions that produce nuclear power. While most of the aforementioned nuclides are not essential for the growth of organisms, they may be incorporated in biogeochemical cycles and become concentrated in food chains, especially strontium and cesium. Thus Woodwell (1962) used cesium as a gamma radiation emitter in his well published study of an irradiated pine oak forest at Brookhaven National Laboratory, Long Island, New York. More will be said about this

There is a wide range of sensitivity of organisms to radioactivity. Mammals are most sensitive while bacteria are most resistant especially as spores. Moreover there is a wide range of tolerance to radiation during the life cycle of an organism. Radiation sickness in

ecology of a tropical rain forest in Puerto Rico.

amount of radiation received per unit time.

disintegration, its half life.

research later in this paper.

**4. Ecological importance of radionuclides** 

**5. Sensitivity of organisms to radioactivity** 

**3. Units of measurement** 

respectively.

Gamma rays are external emitters that penetrate biological materials easily and produce their insidious effects without being taken internally. Alpha and beta particles are internal emitters; their damage to organisms is greatest when taken internally. Odum (1971) summarizes this concept best, "the alpha beta gamma series is one of increasing penetration but decreasing concentration of ionization and local damage." Alpha and beta radiation, unlike gamma radiation, are corpuscular in nature. While alpha particles travel but a few centimeters, and can be stopped by a layer of dead skin, they are dangerous because they produce a large amount of local ionization which can cause mutations disrupting cell processes. Beta particles are high speed electrons. While much smaller than alpha particles, they are able to travel up to a couple of centimeters in living tissue, giving up their energy over a large path. Beta particles, like alpha particles can damage tissue, and like alpha particles, can cause mutations that affect the functioning of cells.
