Is there a safe level of radiation exposure? The Petkau effect

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1 Page 1 of 8 Is there a safe level of radiation exposure? The Petkau effect Dr GOURI GOUTAM BORTHAKUR Department of Physics, Jorhat Institute of Science and Technology Jorhat , Assam Mail borthakur.gg@gmail.com Summary : Exposure of human body to ionizing electromagnetic radiation was thought to be severe from high-level exposures only, But in 1972, Dr. Abram Petkau showed that pow level exposure for prolonged period have the same effects, and hence there is so safe lower limit to ionizing radiation exposure. Introduction : Radiation or electromagnetic radiation is broadly classified into two classes- ioninsing radiation and nonionising radiation. This classification actually stems from the effect of human body when it is exposed to the radiation. The low frequency or long wavelength radiations, such as radio or microwaves, carry low energy, and hence they cant ionize the chemicals in the cells of the body, on passing through them. Such waves are called nonionising radiation.the other types, that is the high frequency or short wavelength radiations, such as alpha, beta, gamma, or X rays- can ionize the chemicals, in the cells of the body,on passing through them.these ionized chemicals become reactive, in unusual ways and produce new chemicals which are either toxic, or harmful. Thus their incarnation can alter the metabolism of bidy in certain ways that can be

2 Page 2 of 8 carcinogenic, pathetic, or lethal. Each of us is exposed every day to certain quantities of background radiation that are naturally produced. Some of this comes from cosmic radiation from outer space. Two forms of this radiation are speeding protons and neutrons, which enter the earth's atmosphere and collide with the air we breathe. Carbon 14, which can cause long-term biological damage including genetic mutations, is created when a cosmic neutron collides with nitrogen in the atmosphere. People who live in higher altitudes generally receive more cosmic radiation than those in the lowlands because there is less protective atmosphere for shielding. Radiation exposures also result from the natural radioactivity in many of the earth's minerals. There are some extreme examples: thorium-bearing sands in Kerala, India, and soils in Brazil measure as much as twenty times above average background levels. Isotopes present in the body, such as potassium 40 and radium 226, also contribute to background levels. But background is something that cant be erased! Notorious Human-made radiation can add to that toll. Fallout from nuclear weapons testing, atomic reactor emissions, the mining and milling of uranium, the creation and storage of nuclear wastes, the transportation and use of radioactive materials in industry, and the exposure of millions of people to medical X rays all have their costs in terms of human health. Types of Ionising Radiation Radiation is ionizing when it has enough energy to remove one or more electrons from an atom with which it comes in contact. When this occurs, the ionized atom is made chemically reactive and capable of damaging living tissue. Nonionizing radiation--as in the form of microwaves--falls on the other end of the electromagnetic spectrum and does not have sufficient

3 Page 3 of 8 energy to physically displace electrons of atoms. It can also, however, be damaging to human health. There are essentially five types of ionizing radiation with which we are concerned here: 1. Alpha radiation this is stream of He nucleus emitted form a heavey nucleus. On the average, a thin sheet of paper or two inches of air can usually stop an alpha particle. But, when alphaemitting elements are inhaled or ingested into the body, the high-energy particles they emit can rip into the cells of sensitive internal soft tissues, creating serious damage. 2. Beta radiation is composed of streams of electrons that often travel at close to the speed of light. In some cases beta particles are emitted from a nucleus when a neutron breaks down into a proton and electron. The proton stays in the atom's core while the electron shoots out. Because they move faster than alpha particles, and weigh much less, beta particles are far more penetrating than alpha particles. Beta emissions to the skin can lead to skin cancer. And like elements that emit alpha particles, beta-emitters can be very dangerous when inhaled or ingested into the body. 3. Neutron emissions occur when the nucleus of an atom is struck by a particle that causes the unsticking of the "binding energy" in the atom's core. The resulting disequilibrium causes neutron particles to be shot out in a way that makes them capable of penetrating solid steel walls. Several feet of water or concrete are required to stop most of them. Because of their tremendous penetrating ability, neutrons can be very damaging to the human body, so forms the principle of a nutron bomb. When neutrons strike atoms of elements that are not fissionable, they can render them radioactive by changing their atomic structure. For example, in a building near a neutron bomb explosion, the neutrons can change stable cobalt in the steel girders to cobalt 60, an emitter of highly penetrating gamma radiation.

4 Page 4 of 8 4. Gamma radiation is a form of electromagnetic or wave energy similar in some respects to X rays, radio waves, and light. Like X rays, gamma radiation is highly energetic and can penetrate matter much more easily than alpha or beta particles. Gamma rays are usually emitted from the nucleus when it undergoes transformations. An inch of lead or iron, eight inches of heavy concrete, or three feet of sod may be required to stop most of the gamma rays from an intense source. 5. X rays are produced whenever high-energy electrons are accelerated or decelerated as they penetrate matter. X rays are produced by machine when electrons are accelerated to extremely high speed and are then crashed into a solid target. They are also produced in nuclear fission when electrons are accelerated out of the fissioning nucleus and are then slowed down by air and other materials. The energy released in the collision is a form of electromagnetic radiation, and is comparable in penetrating power to gamma rays. Because X rays can expose film after passing through some substances--such as human flesh and some building materials--they have been widely used in medicine and some industrial processes. It is believed by many that because they are directly applied to the human body, medical X rays are at present the single greatest source of external exposure to human-made radiation. But unlike radioactive products that can escape into the environment and concentrate in the food chain, X-ray exposure can be controlled more easily than the fallout from a nuclear bomb or power plant. Effect of radiation on cells : Radiation attacks the human body at its most basic level--the cell structure. Cells carry out the vital functions necessary to sustain and develop all living creatures. Over ten trillion cells make up the human body. The cell takes in food, gets rid of wastes,

5 Page 5 of 8 produces protein vital to life, and reproduces itself. Just as all living things are made up of cells, so every new cell is produced from another cell. The nature of the cell is determined by the genetic material in its nucleus. Enormously complex, and not fully understood as yet, the genetic "coding" in each nucleus is carried by a complex protein called DNA--deoxyribonucleic acid. This DNA is tightly coiled in the forty-six chromosomes, which are stored in the cell nucleus. Surrounding the nucleus is the cytoplasm, the "factory" that carries out the directions of the DNA intelligence center. The cytoplasm in turn is contained by a semipermeable membrane, the cell wall. It is the whole of this cell mechanism-- cell wall, cytoplasm, and nucleus--that forms the basis of human life. When a radioactive particle or ray strikes a cell, one of at least four things can happen: 1. It may pass through the cell without doing any damage; 2. It may damage the cell, but in a way that the cell can recover and repair itself before it divides; 3. It may kill the cell; 4. Or, worst of all, it may damage the cell in such a way that the damage is repeated when the cell divides. Last three of those four circumstances can have health effects. The issue of what happens to a cell once it repairs itself, for example, is the subject of scientific debate. Thousands of dead cells are eliminated from the human body every day, and thus the body has a certain tolerance for it when radiation adds to the natural toll. The prime danger from radiation striking a cell, however, comes from the potential for damage to the DNA coding and the creation of cancerous cells. If the DNA is damaged by a ray or particle, it may reproduce itself in an abnormal manner that is, in essence, the basis of radiation-induced cancer.

6 Page 6 of 8 Damage can occur to the cell wall, cytoplasm, and nucleus. There has been considerable debate among radiobiologists about how often a cell must be hit by radiation to mutate into a cancer. There is little dispute, however, over the fact that the cell is most vulnerable when it is dividing. The human fetus, infants, and young children--whose cells are multiplying most frequently--are thus the most sensitive to radiation damage; blood-forming organs such as the bone marrow are also particularly vulnerable. Radiation can also damage the body's immune system and cause a general degeneration in the health of the cell structures. Thus radiation may cause illness and premature aging without actually bringing on the more easily isolated diseases of cancer and leukemia. Is there a threshold? It has long been assumed that the most serious harm came from high-level exposures, such as those produced by the flash of the explosions at Hiroshima and Nagasaki, or those endured by scientists killed at the Los Alamos Laboratory while experimenting with primitive fission reactions. One of the most serious effects of high-level exposure to the body is the destruction of the red bone marrow. Once this occurs, a person's ability to resist infection is seriously compromised and can lead to chronic illness and early death. Other high-dose effects include skin burns, cataracts, loss of hair, loss of appetite, nausea, vomiting, sterility, and fatigue. It now appears that constant exposure to small doses of radiation may also be extremely dangerous. A 1972 study by Dr. Abram Petkau found that prolonged exposures of low-dose radiation could do more damage to cell membranes than short flashes of intense doses. This insight, along with studies of fetal irradiation over long periods of time, has lent weight to a body

7 Page 7 of 8 of evidence indicating that such doses may be causing unexpected disease among far more people than previously believed. The Petkau effect is an early counterexample to linear-effect assumptions usually made about radiation exposure. It was found by Dr. Abram Petkau at the Atomic Energy of Canada Whiteshell Nuclear Research Establishment, Manitoba and published in Health Physics March Petkau had been measuring, in the usual way, the dose that would rupture a particular cell membrane. He found that 3500 rads delivered in 2¼ hours (26 rad/min) would do it. Then, almost by chance, he tried again with much weaker radiation and found that 0.7 rads delivered in 11½ hours (1 millirad/min) would also destroy the membrane. This was counter to the prevailing assumption of a linear relationship between total dose or dose rate and the consequences. Dr. Abram Petkau discovered that at 26 rads per minute (fast dose rate) it required a total dose of 3,500 rads to destroy a cell membrane. However, at rad per minute (slow dose rate), it required only 0.7 rads to destroy the cell membrane. The mechanism at the slow dose rate is the production of free radicals of oxygen (O2 with a negative electrical charge) by the ionizing effect of the radiation. The sparsely distributed free radicals generated at the slow dose rate have a better probability of reaching and reacting with the cell wall than do the densely crowded free radicals produced by fast dose rates. The radiation was of ionising nature, and produced negative oxygen ions. Those ions were more damaging to the membrane in lower concentrations than higher (a somewhat counterintuitive result in itself) because in the latter, they more readily recombine with each

8 Page 8 of 8 other instead of interfering with the membrane. The ion concentration directly correlated with the radiation dose rate and the composition had nonmonotonic consequences. As the 1980 edition of the Encyclopaedia Britannica notes, "it can be concluded that there is no `safe' level of radiation exposure, and no dose set so low that the risk is zero." References :