Since we all know that, we cannot see smell or taste radiation, also we depend on instrument to indicate the presence of ionization. Thus, Radiation is the form of particles or waves in bundles of energy called photons. For example, in everyday life we use microwaves used to cook food, radio waves for radio and television, light, x-rays used in medicals.
Radioactivity is a natural and spontaneous process by which unstable atoms of an element emit or radiate excess energy in the form of particles or waves. These emissions are collectively called ionizing radiations. Depending upon how the nucleus loses this energy either a lower energy atom of the same form will result, or a completely different nucleus and atom can be formed.
Ionization is a particular characteristic of the radiation produced when radioactive element decay. These radiations are of such high energy that when they interact with materials, they can remove electrons from the atom in the material. This effect is the reason why ionizing radiation is hazardous to health and provides how radiation can be detected.
A radiation detector or particles detector is a device that measures the ionization of many types of radiation like beta radiation, gamma radiation, and alpha radiation. The radiation detector is an instrument used to detect or to identify high-energy particles such as those produced by nuclear decay, cosmic radiation, or reactions in particle acceleration.
Radiation measurement
There are several common ways in which radiation levels can be measured and different units of measure apply depending on the instrument and what is being measured. A few of this common measurement for radiation includes:
- The specific energy levels of the radiation (in KV or MV)
- The count per unit time (minutes or seconds)
- The number of Roentgens in the air per unit of time (e.g. milliRoentgen per hour-nR/hr). A Roentgen is a measure of the amount of gamma or X-Rays that produce ions that carry 1 electrostatic unit of electrical charges in 1 cubic centimeter of dry air.
- The dose rate, measure in units such as the gray or rad per unit time. A gray (GY) is equal to 1 joule/kilogram or 100 rad.
- The total accumulated dose, measured in grays or rads.
- The biological risk of radiation exposure is measured in rem or Sievert (Sv).
Radiation detector-types and working principles.
There are types of detectors that are most commonly used, depending on the specific needs of the device. These are Gas-Filled Detectors, Scintillators, and Solid State detectors. Each has various strengths and weaknesses that recommend them to their specific roles.
Scintillation detectors
The basic principle behind this instrument is the use of a special material that glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produces from the scintillation process is reflected through a clear window where it interacts with a device called a photomultiplier tube. The first part of the photomultiplier tube is made of a photocathode. The photocathode produces electrons when light strikes its surface. These electrons pulled towards a series of plates called anode through the application of a positive high voltage. When an electron from the photocathode hits the first anode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is pulled toward the next anode, where more electron’s multiplication takes place. The sequence continues until the last anode is reached, where the electron pulse is now millions of times larger than it was at the beginning of the tube.
At this point, the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by the instrument.
Gas filled ionization detector
When the gas in the detector comes in contact with radiation, it reacts, with the gas becoming ionized and the resulting electronic charge is measured by a meter.
The instrument works on the principle that as the radiation passes through air or a specific gas, ionization of the molecules in the air occurs.
When a high voltage is placed between two areas of the gas-filled space, the positive ions will be attracted to the negative side of the detector (the cathode) and the free electrons will travel to the positive side (the anode). These charges are collected by the anode and cathode which then form a very small current in the wires from the cathode and anode, the small current has displayed a signal. The more radiation enters the chamber, the more current is displayed by the instrument.
The different types of gas-filled detectors are
- Ionization chambers
- Proportional counters
- Geiger-Mueller (G-M) tubes
Solid state detector
The last major detector technology used in radiation detection instruments is the solid-state detector. Generally using a semiconductor material such as silicon, they operate much like an ion chamber, simply at a much smaller scale, and at a much lower voltage. Semiconductors have electrical resistance lower than insulators and are composed of a lattice of atoms that contain “charge carriers” these being electrons available to attach to another atom or electron “holes” or atoms with an empty place where an electron would be.
Silicon solid-state detectors are composed of two layers of silicon semiconductor material, one “n-type”, which means it contains a greater number of electrons compared to holes, and one “p-type”, meaning it has a greater number of holes than electrons. An electron from the n-type migrates across the junction between the two layers to fill the holes in the p-type, creating what’s called a depletion zone. This depletion zone acts like the detection area of ion chambers.
Radiation interacting with the atoms inside the depletion zone causes them to re-ionize and create an electronic pulse that can be measured. The instruments utilizing this type of detector can have a particularly quick response time. This, when coupled with their small size, makes this type of solid-state detector very useful for electronic measurement applications. They are also able to withstand a much higher amount of radiation over their lifetime than other detectors type such as G-M Tubes, meaning that they are also useful for instruments operating in areas with a, particularly strong radiation field.