In about a week ago, we were surprised by nuclear crisis on Fukushima Daiichi Complex, Japan since hit by earthquake 8.9 R.S. It is worried if the radiation of radioactive substances will blow up and contaminate many people affecting the health. But, basically we are exposed to nuclear radiation every day of our lives. Some of this radiation is from natural sources, and some results from human activity. Natural sources include cosmic radiation from space, radiation from lighter, unstable nuclei produced by the bombardment of the atmosphere by cosmic radiation, and radiation from heavy, unstable nuclei produced by the decay of a few long-lived nuclides in the earth’s crust. Artificial sources include medical procedures, commercial products that contain radioactive materials, and fallout from nuclear testing.
Nuclear radiation can cause biological damage because it is highly energetic. In passing through matter, nuclear radiation loses its energy by causing ionization in the absorbing material. For this reason, nuclear radiation is called ionizing radiation. In the ionization process, neutral atoms in the absorbing material lose electrons, forming positive ions. Frequently, the ejected electron possesses sufficient energy to cause ionizations in other atoms. The average amount of energy required to ionize an atom is about 35 electron volts (an electron volt is the amount of energy acquired by an electron accelerated in an electric field of 1 volt). The energy of a single particle from a nuclear decay can be as high as 8 million electron volts (8 MeV). This energy is dissipated by producing ions, and an 8-MeV particle can produce 2 × 10E5 ions.
The magnitude of radioactivity in a sample of radioactive material is expressed using several different units (Table 1). One type of unit focuses on the number of decaying nuclei and is called the activity. Activity is expressed in terms of disintegrations per time. The most common unit of activity is the curie (abbreviated as Ci).
It is defined as 3.7 × 10E10 disintegrations per second. This happens to be the activity of 1 gram of Ra-224, which was discovered by Marie Curie. The SI unit of activity is the becquerel, which is 1 disintegration per second. The other units of radioactivity focus on the effects of radiation on the surroundings. The exposure expresses the amount of ionization caused by radioactive material. The common unit of exposure is the roentgen, which is defined as the amount of radiation that produces, in 1 cm3 of dry air, ions having a total charge of 1 electrostatic unit. In SI units, the roentgen is equivalent to 2.58 × 10E-4 Coulomb/kg of air. The absorbed dose of radioactivity expresses the amount of energy absorbed by a substance exposed to ionizing radiation. One such absorbed dosage unit is the radiation absorbed dose, rad, which is 1 × 10E-5 Joule/g. Different kinds of radiation will cause different biological effects for the same amount of energy absorbed. For this reason, the unit called the roentgen equivalent in man, or rem, was introduced. The rem is equal to the rad multiplied by a factor, Q, which accounts for the relative biological effect of radiation on humans. For β and γ radiation (and for X-rays) Q.1, while for α particles and fast neutrons, Q.20.
The ionizing power of radiation depends on the type of radiation. An alpha particle, which is relatively massive, is quite efficient at producing ions, ionizing virtually every atom in its path. Alpha particles lose most of their energy after traveling only a few centimeters in air or less than 0.005 mm in aluminum. A beta particle, which is relatively light, ionizes only a fraction of the atoms in its path. Beta particles travel more than a meter in air or several millimeters in aluminum.
We should also point out that the location of the radiation’s origin is important, too. If the radiation is emanating from outside of your body, then you worry about how penetrating the radiation is. As we have just discussed, alpha radiation can be blocked from entering your body by a thin sheet of paper. Gamma radiation has a much greater likelihood of getting into your body and being absorbed in this case. However, if the radiation is emanating from inside your body, then alpha particles are going to have a much tougher time to not be absorbed by something in your body. In this case, gamma has a much greater likelihood of not being absorbed and getting out of your body. 
Most radiation produced by nuclear decay falls within a category called ionizing radiation. Because of its charge and/or its energy, this type of radiation has the ability to turn neutral atoms into ions. This, in and of itself, is not necessarily a bad thing. Ions are found throughout nature and are essential to many physical and biological processes. However, if this ion is created in or near certain parts of the cells in your body, it can be quite hazardous. For instance, if the ionizing radiation where to hit one of the atoms that comprise the DNA in one of your cells, it could change the bonding properties of the atom, and thereby, physically change the DNA. If this cell and DNA were to survive, it will have undergone a mutation, which could be either harmless or lethal, depending upon where in the DNA molecule it occurs.
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