The mission of the AERB is to ensure the use of ionising radiation and nuclear energy in India does not cause undue risk to the health of people and the environment.

Radiation Detection and Measurement Techniques

Radiation Detectors

Detection and measurement of nuclear radiation must be accomplished by suitable instruments, since these radiations are invisible and their presence generally cannot be sensed by human perception. All radiation monitoring devices consist of a radiosensitive detector and a means of recording the effects of radiation on the detector (i.e. the response of the detector). Detectors respond to radiation by producing various physical effects which can be measured. Ionization is one of these effects. The ion-pairs can be collected to give an electrical signal which is related to the intensity of the radiation. Some detectors will emit light pulses in response to radiation and by counting the pulses the intensity of the radiation can be detected. Others will store the effects of ionizing radiation over a long period and can then yield the information at a later time. All these devices, in one way or other, respond to the energy deposited in them by the radiation. Instruments can be designed to indicate either the rate at which the radiation is being received or the integrated amount of radiation over a certain time period. The following are the media generally used for radiation detection.

  • Gases (e.g. Ion chamber, Proportional counter, GM counter)
  • Scintillators [ Na I (Tl), Anthracene, etc.]
  • Solid state detectors [Semiconductors, Thermo-luminescent dosimeters etc.]
  • Photographic emulsions [Film]

The selection of the detector depends on a variety of factors such as type, energy, and the level of intensity of radiation to be detected in addition to other factors such as cost, size, availability, electronics needed etc. Therefore, an understanding of different types of detectors and their characteristics is an important prerequisite for their selection and optimum use in a given situation.

Principle of Gas Filled Detectors

The most common type of detector is a gas filled radiation detector. This detector works on the principle that - as the radiation pass through air or specific gas, ionization occurs in air or molecules. Fig 1 gives the features of air or gas-filled detector. The ion-pairs produced are collected and measured as current or pulses. These are generally cylindrical in shape with two electrodes, the central electrode, and the outer sheath, separated by an insulator. When a variable voltage is applied across the electrode, the positive ions will be attracted to outer electrode (the cathode) and the negative ion will travel to the positive electrode (the anode). The charges collected by anode and cathode form a very small current across the detector. By placing a sensitive current meter across the cathode and anode, the small current can be measured and displayed as a signal. More the radiation entering the chamber, more the current is displayed by the instrument. If one measures the number of ion pairs collected across the detector at different applied voltages, the following six different regions of response would be noticed (Fig.2).

Fig.1 Gas filled Detector System

Fig.2 Response of a gas filled detector at different voltages

Region of recombination
Ionizations chamber region
Region of Proportionality
Region of limited proportionality
Geiger-Muller (GM) region
Region of continuous discharge

Gas Filled Radiation Detectors

lonization chambers
Geiger-Muller (GM) counter
Proportional counters

Solid State Detectors

Scintillation Detectors
Thermo-luminescent dosimeters
Chemical Detectors [Photographic Films]

Neutron Detection

Since neutrons are uncharged particles, their detection method are different from those of charged particles and gamma radiation. Neutron detection is accomplished by methods such as a) neutron induced transformation resulting in charged particles or gamma radiation, b) neutron induced transformation resulting in radioactive product nuclei, whose subsequent decay gives the information of neutron flux, and c) elastic scattering of neutrons resulting in production of charged particles. Hence a neutron detecting system is a combination of the interacting medium in which neutron produce one of the above effects and detecting medium to detect the resulting charged particles or gamma radiation.

Thermal Neutron Detection
Fast Neutron Detection

Radiation Monitoring Instruments

Radiation hazard evaluation is necessary in order to adopt and implement suitable measures/steps to control radiation exposure. The sources used in nucleonic gauges and well logging equipment are encapsulated and sealed. Therefore, the problem of internal hazard does not arise unless the source capsule gets ruptured during use.

It is considerably easy to estimate the external hazard and there are a number of devices (of different make) suitable for this purpose, available in the market. From the application view point, these devices may be classified in to two: the area monitoring devices and personnel monitoring devices. In area monitoring application, the Geiger-Muller (GM) counter, Ionisation chamber and Proportional counter based devices are most commonly used. The devices measure either the radiation intensity or exposure (exposure rates). But in personnel monitoring, pocket dosimeters or personnel dose monitoring badge, loaded with photo-sensitive film or Thermo-Luminescent Dosimeters (TLD) are used. The personnel dosimeters recorded the cumulated radiation dose over a period of use.

Personnel Dose Monitors
Area Monitoring
Performance Tests and Maintenance of Radiation Monitors
Re-calibration

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