After reading this essay you will learn about:- 1. Types of Radiation 2. Effects of Radiation 3. Protection.
Essay on the Types of Radiation:
i. Atomic Radiation:
Atoms of certain elements emit spontaneously very minute particles and electromagnetic radiations. These tiny particles may carry electric charge as in the case of alpha or beta rays; they may be neutral as in the case of neutrons. The electromagnetic radiations emitted are X-rays (from atom) and gamma rays (from nucleus) and they do not carry any electric charges.
Radioactivity is the result of spontaneous disintegration of an atom during which it emits alpha, beta or gamma particles. Simultaneously, the atom changes into another element which is itself radioactive, and this in turn disintegrates to become something else and the process continues. The sequence of changes is known as the transformation series.
The disintegration decay activity of a radioactive element is entirely random. A radioactive element has a definite rate of decay known as the half-life period. Half- life is defined as the time taken for half the atoms in any given sample of the substance to decay.
An example of decay of uranimum238 is shown below:
Alpha rays are helium nuclei, that is, helium atoms which have lost their two orbital electrons, and hence they have a net positive charge. They are all ejected with approximately the same velocity ranging from 1.4 x 109 to 1.7 x 109 cms per second. They have a range of several centimeters in air, but most are stopped by a very thin sheet of aluminum foil, about 0.1 mm thick.
Beta rays are streams of high energy electrons similar to cathode rays. They are emitted with variable velocities, approaching that of light (3 x 108 m/s). They are far more penetrating than the alpha rays and are stopped by a 1 cm thick aluminum sheet.
Gamma rays consist of electromagnetic radiations of very short wavelength and occupy a band among the X-rays which have the shortest known wavelengths. They have the same velocity as that of light. The highest energy gamma rays are very penetrating and approach absorption (or attenuation) only after traversing few centimeters of lead.
Alpha radiation consists of heavy, doubly positive charged particles emitted by atoms of elements such as uranium and radium.
Beta radiation consists of electrons. These electrons are much lighter than alpha particles and carry unit negative charge.
Gamma rays are a form of electromagnetic radiation similar to X-rays, light and radio waves.
Neutrons are uncharged heavy particles contained in the nucleus of every atom heavier than hydrogen.
Alpha radiation may just penetrate the surface of the skin:
It can be stopped completely by a sheet of paper.
Beta particles however need a few millimeters of aluminium sheet for stopping them. Gamma rays are more penetrating but can be shielded by lead slabs, thick concrete or water. X-rays are less penetrating than gamma rays. Neutrons are shielded by concrete and hydrogen-containing substances like wax, water etc.
ii. Natural Background Radiation:
Many are unaware of the existence of natural background radiation. The source of these are extra-terrestrial and terrestrial. The extra-terrestrial natural radiation comes from outer space and hence these are called cosmic rays. The terrestrial natural radiation originates from the radioactive substances in the earth’s crust.
These radiations irradiate the human body from outside:
This exposure is known as External Exposure. ‘Internal Exposure’ arises from both these sources from naturally occurring nuclides which are taken up into the body through normal physiological pathways. A man has about 5,000 Bq* radioactivity in his body mainly due to 40K.
a. External Exposure:
The external exposure of man resulting from cosmic ray component varies slightly with the geomagnetic latitude and, to a great extent, it increases with the altitude above sea-level, the dose rate approximately doubling for each 1.5 km above sea level for the first several kilometers. At sea level, the exposure is 10 per cent lower at the equator compared to those at middle latitudes.
The terrestrial component of the natural background is dependent upon the composition of the soils and rocks. On average, population receives an external exposure, from the terrestrial component, of 350 (± 100) µSv per year. These figures are for outdoor exposures. Indoor exposures are generally higher, depending upon the type of houses one lives in e.g. granite houses could give an order of magnitude higher exposure.
In India, external dose measurements in over 200 stations from different parts of the country indicate an annual average external exposure of 690 (± 200) µSv/yr.
b. Internal Exposure:
Internal exposure results from radio-nuclides entering the body through ingestion or inhalation.
These radio-nuclides are either cosmogenic (i.e. produced by the interaction of cosmic rays with atoms in the upper atmosphere) or primordial, in the sense that they existed in the earth’s crust throughout its history. Internal exposure from cosmogenic nuclides (such as 4H, 7Be, 14C, 22Na) is very small. The short-lived 222Rn and its decay products contribute a major fraction of internal dose.
Other nuclides like 40K the decay products of Thorium (220Ra) and 210Po contribute to a lesser extent. The internal exposure due to these primeordial nuclides is about 1,300 µSv per year. Total annual average exposure of man is 2,000 µSv/yr. These exposures are given in Table 22.1.
c. Other Exposures:
In addition to natural background radiation, man is exposed to sources of radiation that he himself has created; for example, X-rays and other types of radiation used for medical purposes: fallout from testing of nuclear weapons, occupational exposure from nuclear and other industries using ionising radiation and radioactive materials released in the course of nuclear power production.
Table 22.1 shows the contribution of the various sources to the average annual dose to the population (a typical case). It is obvious that the contribution due to nuclear discharges is negligible (0.1%).
It may be interesting to compare the radiation exposures from various sources. An air travel in a jet plane gives an exposure of 8.5 µSv/hr while in a supersonic it is 16µSv/hr. One chest X-ray may give about 200µSv.
The radiation exposure near a nuclear power plant is only few tens micro sievert per year which is negligibly small and lies within the statistical variations in natural background radiation levels. But natural disaster like earthquake, tsunami, or several cyclone may lead to nuclear plant damage and explosion as seen March 2011 at Fukshima Daiichi, Japan.
Comparison of Risks:
Every human activity like smoking, rock climbing, driving and many such activities involve certain amount of risk. Table 22.3 compares the probability of death per year in some of the human activities. It is very clear that work with radiation is one of the safest human activities involving least risk.
Essay on the Effects of Radiation:
More is known today about effects of exposure to radiation than about any other physical and chemical agent in our environment. The health effects of radiation exposure are not unique. The main effects of radiation are cancer induction and genetic effects. In fact, out of the natural cancer incidence or fatality, only about 10 per cent can be accounted by the exposure to natural background radiation.
Thus the effects that can be attributed to low level radiation exposure are also caused by large number of other environmental agents. Effects of radiation exposure to low level radiation should not be disregarded; but it should be recognised that they are very small compared to risks of similar effects from other known and unknown agents.
At the radiation exposure standards set by nuclear industry for occupational workers as well as members of public, no discernible effects can arise in population groups. Since the disaster caused by Nagasaki and Hiroshima atom bombs in August 1945, people all over the world became aware of the harmful effect of radioactive substances which emits radiations
Ionising Radiation and Effects on Man:
Like radioactive material exposure, ionising radiation also have deleterious effect on living being specially mankind. The degree of effect however depends on dose and duration of exposure (Table 22.4).
Essay on Protection against Radiation:
The protection against radiation hazards was not considered seriously as a major environmental threat for human health. However, the realisation of deleterious effects of radiation hazards in recent years necessitates the need for protection against radiation hazards.
Global banning of nuclear weapons, safe disposal of nuclear plant wastes and proper dumping of radioactive materials used in therapeutic purpose etc. are the major activities undertaken currently as a measure of radiation hazard protection. In addition, the workers handling radioactive materials require to undertake appropriate protection measures during working phase at the nuclear plants premises.
International Atomic Energy Commission set up a standard guideline for the said purpose.
The International Commission in Radiological Protection (ICRP) makes recommendations to protect both workers in professions where radioactivity is a particular hazard, and the general public.
They do not as yet set exact limits in all cases, but set forth the following general principles:
(a) no practices (which produce increased radiation) shall be adopted unless their introducing produces a positive benefit.
(b) all exposures shall be as low as reasonably achievable (ALARA), economic and social factors being taken into account.
(c) the dose to which an individual is exposed shall not exceed the limits recommended for the appropriate circumstances by the commission.
ICRP’s general guidelines are that radiation workers should not be exposed to more than 50 m Gy (5 rads) a year, and the maximum exposure for the general public should be one tenth of this (5 m Gy or 0.5 rads).
Nuclear plants are located, designed, constructed and operated to conform to very stringent safety standards to ensure that the risk associated with atomic energy is extremely low. An exclusion zone of 1.6 km radius around the reactor is established which is free of habitation.
An area in the annulus between the exclusion zone boundary and the 5 km radius is provided as sterilized zone. Off-site emergency plans are also worked out for an area of about 16 km radius to meet any contingency in the unlikely event of an accident.