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Radiation Detector


Radiation can be categorized in many ways like ionizing and non ionizing particles and waves, hazardous and non-hazardous etc. These radiations have their own characteristics depending on the type of particles. It might be $\alpha$ ray, $\beta$ ray and $\gamma$ ray. To detect these radiations we need the detector. A radiation detector is a device that detects the high energy particles produced by nuclear decay, cosmic radiation or reactions produced in a particle accelerator.

How to Detect Radiation?

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The sensing element in nuclear measurement systems is commonly referred to as the radiation detector. The radiation detector may be in its simplest form be considered as a unit that converts radiation energy into an electronic signal. The energy of the scattered photons and so forth is lost unless these undergo further interactions in the detector. The collection and measurement of electrons are a key job in radiation detection instrumentation. In this case the type of detector used should be selected by the project health physicist to assure that the selected detector is suitable for its contaminants and is properly maintained.

In early 1980's at the national bureau of standards a fiber-optic light pipe coupled to the radiation detector was first used as a thermometer to measure the temperature of the gases.

Types of Radiation Detectors

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The radiation detectors are classified into four types based on material of detector used or its application.
  • Gas-filled detectors
  • Solid state detectors
  • Scintillation detectors
  • Passive integrating detectors

The solid state detectors are again classified into following categories:
The gaseous ionization chamber detectors are classified into the following categories- Ionization chamber, Geiger-Muller Tube, Spark chamber and proportional counter. The Geiger counter was the most popular before as it had the capacity to detect any ionizing radiation with adequate sensitivity, and even because of the pulse size being large, it did not require any high-amplifier. The counter can detect alpha, beta and gamma particles.

It consists of a Geiger Muller tube that detects the particles in the tube present at low pressure gas. Here a high voltage around 100v is applied that makes the gas discharge and hence conduct current due to the radioactive particles present in it. There will be amplification due to the further discharge of gas that amplifies the current that makes it visible or audible.

The Geiger Counters are not used much in the laboratory now for many reasons:
  • Their plateaus have a greater slope than proportional type.
  • They possess a long dead time that arises from the mechanism of discharge and cannot be reduced.
  • It cannot measure the gamma rays effectively because there are no enough particles to guarantee interaction with a neutral gamma ray.
  • Lastly they produce a pulse of constant amplitude regardless of initial ionization and hence its quite hard to distinguish between alpha and beta particles.
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A scintillation counter is a radiation counter used especially for detecting gamma radiation. It uses a transparent solid which has a higher particle density than a gas radiation present in the low-density gas interacts with the solid resulting in flashes of light which are counted by a detector. Particles in the solid absorb energy and then release some of it in flashes of light that can be counted. This light emission may continue for some time even after exposure to gamma rays is stopped. This is known as phosphorescence.

Here the light is incident on photomultiplier tube. Photoelectrons are produced at the photocathode by properly accelerating to first dynode, second dynode and so on till the secondary electrons are liberated. At the end of all stages burst of electron produces an electric pulse that gets amplified and are electronically counted.
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