2. What You Should Know About Ionizing Radiation  
Here are four key points about ionizing radiation: 
The biological effects of radiation are dose-dependent, meaning that greater amounts cause greater effects. 
The effects of a given exposure vary widely from person to person. It is impossible to predict precisely how any given individual will react to radiation -- except at lethal levels. 
Large quantities of ionizing radiation delivered over seconds, minutes or a few hours is called an acute radiation exposure. Acute exposure to the whole body is more dangerous than acute exposure to just a part of the body. 
Scientists are not sure how much harm is done by low doses of radiation over an extended time period. Some scientists believe that any amount of exposure can be harmful, that there is no minimum level of radiation that is not in some way harmful. 
Not all scientists agree about the long-term effects of low doses of radiation and that chronic doses of ionizing radiation may increase your chances of contracting cancer 

3. What is absorbed dose? 
The effects of ionizing radiation on matter are initiated by the processes in which atoms and molecules of the medium are ionized or excited. The average number of ionizations and excitations is proportional to the amount of energy given to the medium by ionizing radiation. Absorbed dose is the amount of energy given to the medium per unit mass. It is measured in gray (Gy) defined as Joule/kg. The determination of the dose due to radiation is the task of radiation dosimetry. 

4. What is radiation dosimetry? 
Dosimetry has the task of absorbed dose determination and its physical interpretation. Absorbed dose is determined with radiation measurements and calculations.Different instruments are used for absorbed dose measurements, all based on the detection of some of the physical and chemical changes caused by radiation. For example, ionization chambers, calorimeters or ferrous-sulfate dosimeters measure, respectively, the electrical charge produced by ionization of a gas inside a detector, the amount of heat produced by radiation in a piece of material, or the chemical changes in an aqueous solution of ferrous-sulfate. Most measurements therefore provide an indirect determination of the dose, and calculations are needed to convert the quantity measured into absorbed dose.Radiation dosimetry is needed in many areas, such as in cancer treatment using radiotherapy, in clinical diagnostic radiology, in environmental radiation protection, and in industrial applications such as food irradiation and sterilization of health care products. 
 
5. Can the body feel ionizing radiation? 
Ionizing radiation cannot be detected by any of our senses, even if it produces the numerous physical and chemical changes that make possible to detect radiation. Many of these effects, for example, produce heat, but not enough to be sensed by the human body. A large absorbed dose of 10 Gy, which would be lethal if delivered in a short time to a human being, produces a temperature rise of only 0.0024 deg C; the total energy imparted to a person of 70 kg would only be 700 J. Compare this to the energy of 100 g of light yogurt or in 100 ml juice, about 200,000 J. 

6. Do all kinds of ionizing radiation have the same biological effect on human beings? 
The biological effect of ionizing radiation on a human being depends on the absorbed dose, the radiation quality (i.e. gamma, beta, alpha etc. and their energy), and on the organ(s) irradiated. Besides, all human beings are not equally sensitive to radiation, and additional differences in sensitivity are attributed to age and sex. If we restrict the comparison to the various qualities of ionizing radiation and keep all other parameters constant, the biological effect from a single exposure to alpha particle radiation is about 20 times more than from gamma or beta radiation for the same dose. The biological effect from neutron radiation varies between 2-10 times that from gamma or beta radiation depending on the energy of the neutrons. If the absorbed dose to the different organs in the body and the radiation quality are known, the effective dose equivalent can be calculated. Based on the estimates of the effective dose equivalent, the risk for the late effects such as cancer, can be compared for different exposure conditions and type of radiation. 

7. When and Why Radiation Can Be Dangerous  
When you think about radiation, do you picture things like nuclear bombs or fallout? Nuclear power plants? If so, you may have the impression that all radiation is dangerous. This is not so. Some kinds of radiation are completely harmless -- such as light from an ordinary bulb. Yes, light is a form of radiation. 

The type of radiation which we do need to be concerned about is called ionizing radiation. Ionizing radiation occurs in nature and is also man-made. It is the type of radiation used in many useful machines including X-ray equipment. 

One of the problems with ionizing radiation is that we cannot see it, hear it, taste it, smell it, or feel it in any way. This type of radiation disrupts some of the atoms in its path causing them to separate into electrically charged ( + and - ) components called ions. This is why it is called ionizing radiation. 

Ionizing radiation's ability to break atoms makes it different from other types of radiation. Since our bodies are made of atoms, ionizing radiation can be harmful to living tissue. The atoms in our bodies become biologically useless if divided into ions. Trillions of atoms exist in our bodies and if a few of them are ionized, we suffer no real harm. But large doses of ionizing radiation can cause sickness, cancer and birth defects. 

Some scientists believe that long term exposure to even small amounts of ionizing radiation can cause similar problems. This is known as the Linear Non-Threshold Theory. 

8. What are the colour centers? 
Pure alkali halide crystals are transparent throughout the visible region of the spectrum. The crystals may be colored in a number of ways 
      (1) by the introduction of chemical impurities 
      (2) by introducing an excess of the metal ion 
      (3) by X-ray, g-ray, neutron and electron bombardment 
      (4) by electrolysis 
A colour centre is a lattice defect that absorbs visible light. The simplest colour center is an F-centre. The name comes from the German word for colour, Forbs. We usually produce F-centres by heating the crystal in excess alkali vapours or by irradiation. The new crystals show an absorption band in the visible or ultraviolet, whereas the original crystals are transparent in that region. This absorption band is called F-band. 
An F-centre can be regarded as a negative ion vacancy and an electron which is equally shared by the positive ions, surrounding the vacant lattice site. Conversely a hole may be trapped at a +ve ion vacancy or at a -ve ion, giving rise to V- and H-centres respectively. 

When a X-ray quantum passes through an ionic crystal, it will usually give rise to a fast photo-electron with an energy of the same order as that of the incident quantum. Such electrons, because of their small mass, do not have sufficient momentum to displace ions and therefore loose their energy in producing free electrons, holes, excitons and phonons. Evidently these while moving near the vacancies form trapped electrons as well as trapped holes.The trapped-electron or trapped-hole centres so formed can be destroyed (bleached) by illuminating the crystal with light of the appropriate wavelength or warming it. Important information about the colour centres can be obtained by thermoluminescence methods. 

9. So what is the thermoluminescence methods? 
 Please see next page