Scintillation Counter GR611M NaI based Scintillation detector

Principle Certain material when struck by nuclear radiation or by ionizing radiation emitted a small flash of light i.e. scintillation. These materials when coupled with an amplifying device such as photomultiplier then these radiations can be converted into electrical pulses. These output electrical pulse can then be analyses and counted electronically and gives rise to information regarding the incident radiation. The height of output pulse can be made proportional to energy dissipated by ionizing radiation. Thus this detector can be used nit only for detection but also for energy analyzing.

Construction of sodium iodide based scintillation detector

Working of sodium iodide based scintillation detector Absorption Process A charge particle falling on scintillation material loss all of its energy by elastic and inelastic collision provided that the dimension of scintillator is large as compared to range of charge particle. A γ-ray interacts with scintillator material via. photoelectric absorption, Compton scattering and pair production depending upon energy. In all cases electrons are emitted with certain kinetic energy. These electrons impart their energy for excitation and ionization of scintillation material.

Scintillation Process Due to excitation and ionization of scintillation materials absorb energy. This absorb energy appear either in form of heat or luminescence. A photon are emitted in alter case which is governed by exponential law Where Np represent the number of light photon emitted in time ‘t’ after the incidence of radiation and τ is decay time required form the emission of 63% of photon for NaI(Tl) τ=2.5 x 10-7 s. Density of material NaI= 3.67 gm/cc and λ for maximum emission is 4100 A0

Conversion of light into electrical pulse The light emitted in scintillation process is fall on photocathode coupled with scintillation and produce photoemission. Electron emitted by photocathode is incident on first dynode where it multiply and then this electron move toward second dynode. This process is repeated at every dynode. If ‘n’ be the number of electron collected at anode then

The number of primary electron is proportional to the energy of incident gamma ray. So number of photoelectron released from photocathode and charge arriving at anode and hence output voltage is proportional to energy of incoming radiation. Thus this detector is used to find the energy spectroscopy of gamma ray. This detector is not sufficient for spectroscopy of heavy charged particle like proton and alpha particle as these generally are less efficient in producing scintillation.

Block diagram of Sodium Iodide scintillation detector 1

Photomultiplier Tube

What is it? Extremely sensitive detector of light in the ultraviolet, visible and near infrared Multiplies the signal produced by incident light by as much as 108 single photons can be resolved High gain, low noise, high frequency response, and large area of collection A tiny and normally undetectable current becomes a much larger and easily measurable current

Vs. Phototube In both, the photon strikes the photocathode and emits electrons (photoelectric effect) In a phototube only these few electrons are collected It can only be used for very low signals In a photomultiplier tube these electrons are multiplied This process is called secondary emission

Components Made of a glass vacuum tube Photocathode Several dynodes One anode

How it works

How it works Electron is released by the photocathode Electron is then multiplied by the electrodes Metal channel dynodes At the end of the chain is the collection electrode Anode The current flowing from the anode to ground is directly proportional to the photoelectron flux generated by the photocathode

Dynodes Each is held at a more positive voltage than the previous one As the electron approaches the dynode, it is accelerated by the electric field On striking it, more low energy electrons are emitted and accelerated to the next dynode in the series Anode The final electrode in the chain of dynodes The electrons accumulate here This charge results in a sharp current pulse indicating the arrival of a photon

Signal Processing The output of PMT is fed to pulse shaping amplifier for shaping as well as noise reduction. The output of Amplifier fed to Single channel analyzer/Multichannel analyzer for discrimination corresponding to energy. The SCA/MCA works in three mode as per requirements viz; Integrating mode Normal mode Window mode The output of SCA/MCA the fed to scalar or computer for further analysis.

A view of NaI scintillation detector available in NIT Kurukshetra