Why Measure Radiation?
Radiation including Alpha and Beta particles, and Gamma and X-rays, are useful to us in various fields like medicine, agriculture, research, power, military, and many other possible fields. But uncontrolled exposure to radiation can be harmful, even fatal. Measurement of radiation is, therefore, essential when dealing with it. Detection and measurement of any background radiation are also necessary because that too can exceed safety levels. Finally, detection and measurement are necessary to prevent illegal transfer of radioactive material across national borders.
Radiation measurement has two aspects: Measuring radiation activity (or strength) and measuring its effect on humans.
Measuring Radiation Activity– When we say Radiation Activity or Radioactivity we are referring to the amount of radiation emitted in any of these forms. Radioactivity is measured in terms of Curie or Becquerel. Both these units are named after scientists who made very prominent early contributions in this field. Curie, abbreviated as Ci, is the large older unit which is still used in the USA. Becquerel, abbreviated as Bq, is the modern SI (Standard International) unit which has become more popular the world over. One Becquerel means one particle (or photon, in case of rays) being released per second. 1 Ci= 3.7 x 1010 Bq. Radioactivity is measured using Geiger counters or scintillation sensors.
Radiation Effect-The following terms and units are used to describe the effect of radiation on tissues.
- Exposure– is the ambient level of radiation, or the amount of radiation described in terms of its ionization effect in the air at standard conditions of temperature, pressure, and humidity. Since ionization means charge separation, the SI (Standard International) unit for exposure is coulomb/kilogram (C/kg). The older unit Roentgen (R) is often used in the USA. Different types of radiation monitors are available to measure exposure. 1 R is equal to 2.58 × 10-4 Coulombs/Kg.
- Absorbed Dose– This is the energy deposited by radiation in a Kilogram of the substance (body). The SI unit is the Gray (Gy). Another unit is the RAD (Radiation Absorbed Dose). 1Gy is 100 RAD. The Gy being a rather large unit, milli-, and micro-Grays are more practical.
- Equivalent Dose– Even for the same absorbed dose, different types of radiation have the different biological effect. As an example, for the same absorbed dose, alpha particles will have twenty times the effect of gamma rays. The SI unit for equivalent dose is the Sievert (Sv). The Sv is also a large unit, and practical units are milli-Sievert and Micro-Sievert. Thus, to obtain the equivalent dose, the absorbed dose is multiplied by a radiation ‘weighting factor’ (Wr). The weighting factor for Beta particles, x-rays, and Gamma rays is one, while for Alpha particles it is 20.
- Effective Dose– For a given equivalent dose, different body tissues will get affected differently. The effective dose is, thus, the correct measure of what biological damage may occur to a given type of tissue. An experimentally evaluated ‘tissue weighting factor’ (Wt) accounts for the radiation sensitivities of different tissues. The most sensitive organs are the lungs, bone marrow, stomach, colon, and breast with a tissue weighting factor of 0.12. The least sensitive is the brain (luckily), bone surface, salivary glands, and the skin. If more than one organ is exposed then the effective dose, E, is the sum of the effective doses to all exposed organs. Dosimeters worn by vulnerable personnel are used to measure the cumulative dose. The unit of effective dose is again the Sievert (Sv).
Radiation Measuring Instruments
Because of the importance of this issue, a number of firms are producing a variety of radiation measurement devices. Ecotestgroup is a prominent producer of quality radiation measurement instruments. Their wide range includes instruments for handheld detection, personnel exposure monitors, contamination and activity measurement devices, as well as survey monitors, area monitoring devices, portal monitors, and even solutions for special purpose vehicles.