Chemistry Lecture No.4

Chemistry Lecture No.4______By : Asst. Led Tariq-H-Al-mgheer
The Sl unit of absorbed dose is the gray, whose symbol is Gy. A gray is defined as 1 J of energy absorbed per 1 kg of tissue. One hundred rads are equivalent to one gray
The energy absorbed by living tissue is not the only factor that contributes to the biological hazards of radiation. For this reason, a dose of 1 rad from one source is not necessarily equal to a dose of 1 rad from another. The rem is the unit that takes all these differences into account. REM
One rem is 1 rad multiplied by a factor called the relative biological equivalent (RBE). Thus, Number of rem = RBE X number of rad
The factor RBE takes into account the differences in biological damage caused by different kinds of ionizing radiation of the same energy. For example, the RBE of an alpha particle is 10 times that of a beta particle. This means that alpha particles released in tissue cause 10 times more damage than beta particles of the same energy. The rem is a more accurate and comparable measure of biological damage caused by different ionizing radiation. For this reason, it is commonly used to describe allowable doses of radioactivity. LDso VALUES
The ionizing radiation dosage at which injury to humans begins is unknown. The best we can do is to express the short-term exposure to ionizing radiation that is fatal to various forms of life, This is expressed as the 30-day LD5o value, defined as the dose, in rem, that is fatal to 50 percent of the population within 30 days.
The LD5o value for humans is estimated to be 500 rem. Thus, a person suddenly exposed to 500 rem of ionizing radiation would have a 50 percent chance of remaining alive after 30 days. Even after 30 days, the possibility of illness and death would still be great. The LD5o values of most mammals are in the range 250 to 1000 rem. Simpler forms of life such as insects can tolerate much more radiation. For example, some insects can tolerate more than 50,000 rem, and some microorganisms can tolerate even more.
We cannot escape ionizing ndiation. There are small amounts of naturally occurring radioactive substances in the 3"il, in the food we eat, and in the water we drink. Streams of particles called cosmic rays enter our atmosphere from the sun and bombard us These are all part of natural background radiation. The exposure we receive from these natural sources is very small. Each person receives about 100 millirem annually from natural background radiation.
The use of radioactive substances in medicine, industry, and nuclear power plants has increased our risk of exposure to radiation. Many of these radioactive substances are not found naturally but are made for a specific use. TRANSMUTATION
Changing one element into another, either in nature or in the laboratory, is called transmutation. Scientists do this by means of bombardment reactions.