The radiation produced is of three types: the alpha particle, which is a nucleus (two protons and two neutrons) of an ordinary helium atom; the beta particle, which is a high speed electron or, in some cases, a positron (the electron's antiparticle); and gamma radiation, which is a type of electromagnetic radiation with very short wavelengths. The rate of disintegration of a radioactive substance is commonly designated by its half-life, which is the time required for one half of a given quantity of the substance to decay. Radioactivity may be induced in stable elements by bombardment with particles of high energy.

Protons and neutrons are held together in the nucleus by the strong force. The energy associated with strong force is called binding energy. In some atoms, the binding energy is great enough to hold the nucleus together permanently. The nuclei of such atoms are said to be stable. In other atoms, the binding energy is not as great. The nucleus is not held together permanently. The nuclei of these atoms are said to be unstable. Atoms with unstable nuclei are radioactive. Scientists believe that unstable nuclei are caused by an imbalance in the number of protons and neutrons.

In a stable nucleus, the number of protons is equal to the number of neutrons. In an unstable nucleus, however, there are either more neutrons than protons or more protons than neutrons.

To determine which type of radiation is most harmful, we must first determine how far each one can travel. The distance that radiation travels is important for two reasons:

1.The further it travels the more easily it will get to your body. 2.The distance it travels in your body, affects how much damage it does.

There are various things to consider when determining how far radiation will travel. For a discussion on this, see How does radiation get stopped?

Gamma Rays
Alpha particles can not even penetrate a piece of ordinary paper, beta particles are stopped by a thin sheet of aluminum, while it takes an inch of lead (at least) to stop gamma rays. Thus, alpha and beta particles can not even penetrate through a person's skin, while gamma rays can get into the body and cause damage. For external radiation, GAMMA RAYS ARE THE MOST HARMFUL. Gamma Sickness

Of course, to every rule, there is at least one exception. If you eat or inhale an isotope that is an alpha emitter, it doesn't have to penetrate your skin anymore to get into the body. Some alpha emitters, when inhaled, can even pass into your blood and irradiate your whole body that way. In these situations, the alphas are most harmful because the side effect of slowing down quickly is that they deposit all their energy in a short distance, thus concentrating their effect.

It is because of this effect that smokers receive a much higher annual dose of radiation than non-smokers. Radioactive polonium (from the radon decay chain) gets deposited from the air onto tobacco leaves when they are dried. When the smoker inhales the smoke, they are also inhaling polonium and other isotopes in the decay chain, all of which are alpha and beta emitters. This radiation is harmful once it gets into the lungs.
Paul Wilson <wilson@candu.neep.wisc.edu>

Illness caused by the effects of radiation on body tissues. It may be acute, delayed, or chronic and may occur as a result of cumulative exposure to small doses of radiation (as in a plant, a laboratory, or the environment); undue exposure to solar radiation; or exposure to a nuclear explosion. Symptoms may be mild and transitory, or severe, depending on the
type of radiation, the dose, and the rate at which exposure is experienced. They include weakness, loss of appetite, vomiting, diarrhea, a tendency to bleed, increased susceptibility to infection, and--in severe cases--brain damage and death. Mild radiation sickness is a common side effect of radiation therapy for CANCER. Exposure to radiation is of concern
even in small doses because of possible long-term genetic effects and increased cancer rates in exposed people.

Effects on Human
Approximately 20 million former Soviets were exposed to radioactivity released at
Chernobyl. In 1981-1985, the number of thyroid cancer cases in Ukraine was about 5 per year. In 1986-1991, the number increased to 22 cases per year. However, during these last five years, the number has even increased to about 43 cases per year. The number of pregnancies with complications has been growing among women living in the affected area. People in those areas have become paralyzed with fear. They are afraid to marry and afraid to have families. It seems that as if the Chernobyl incident is still occurring through the a new way of attack, the psychological one, the deadly FEAR.
Herman Tjahjadi

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