Explain the construction and working of scintillation counter.
Answers
A scintillation counter is an instrument which is used for detecting and measuring the ionizing radiation. It uses the excitation effect of incident radiation on a scintillator material and detects the resultant light pulses.
It consists of a scintillator and a sensitive photomultiplier tube or PMT and electronics to process the signal. The scintillator generates photons in response to incident radiation and the PMT converts the light to an electrical signal. Often a thin opaque foil like aluminized mylar is used to shield the scintillator from all ambient light.
Working
Atoms are ionized along a track when an ionizing particle passes into the scintillator material. Charged particles have the same path as the particle itself. The energy of the uncharged gamma rays is converted to an energetic electron through the photoelectric effect, pair production or Compton scattering.
A multitude of low-energy photons are produced near the blue end of the visible spectrum due to the chemistry of atomic de-excitation in the scintillator. The number of these photons is proportionate to the amount of energy deposited by the ionizing particle.
Some of these low-energy photons arrive at the photocathode of an attached photomultiplier tube. By the photoelectric effect the photocathode emits one electron for each photon that arrives. An electrical potential electrostatically accelerates and focuses the group of primary electrons so that they strike the first dynode of the tube.
A number of secondary electrons are released due to the impact of a single electron on the dynode. These are in turn accelerated to strike the second dynode. In this way there is a current amplifying effect at each dynode stage. Every stage is at a higher potential than the previous one.
The output signal at the anode is in the form of a measurable pulse. It is for each group of photons which arrived at the photocathode. It is passed on to the processing electronics.
The pulse gives the information about the energy of the original incident radiation on the scintillator. The intensity of the radiation is known by the number of such pulses per unit time.
A scintillation counter is an apparatus for identifying and regulating ionizing transmission by applying the excitation influence of disturbance distribution on a scintillating material and exposing the resultant beam pulses.
It consists of a scintillator which produces photons in acknowledgment to conflict broadcast, a delicate photodetector (normally a photomultiplier tube (PMT), a charge-coupled project which is CCD camera, or a photodiode, which transforms the light to an electrical signal and cybernetics to concoct this signal.