2 constructionA GM counter consists of a fine wire mounted along the axis of a cylindrical cathode made of glass or metal with a metalized inner coating n filled with a suitable gas mixture.The gas mixture usually consists of argon (90%) at 10cm of Hg n ethyl alcohol (10%) at 1 cm of Hg.The diameter of the cylindrical cathode varies from 1cm to 5cm n its length varies from 2cm to 100cm depending on the purpose for which it is to be used. The circuit to which GM tube is connected to detect the particles is shown below.
3 workingWhen an ionizing particles passes through the gas in the GM tube ionisation takes place.The electrons produced are attracted towards the central wire n counted as a single voltage pulse.The wire (anode) is surrounded by the slow moving positive ions sheath which in turn reduces the electric field.As a result of decreased electric field the discharge stops. The discharge continues to cease till the positive ions around the wire move away from it.Since the positive ions take enough time to move away from the anode so during this period GM counter remains dead.Dead time is the time during which the counter fails to record any other ionising particle entering the counter.Positive ions move away from anode towards cathode, the value of electric field increases n another discharge takes place.It is essential that the electric discharge caused by the first ionizing particle is completely quenched before the arrival of new particle.
4 Plateau and over voltage In order to decide the operating voltage of the GM tube, a graph between anode voltage (X axis) and count rate (Y axis) is plotted.The counting rate rises n soon Geiger region is reached when pulses due to all ionizing particles become of same size n are recorded. This is called Geiger threshold.When voltage is increased beyond threshold the counting rate remains the same for large variation. This straight portion of the curve is called a plateau.This plateau continues, till the excess potential over the threshold, called the over voltage becomes so high that the counter breaks down into a continous discharge.
5 quenchingThe grid bias is adjusted in such a way that no plate current flows before the arrival of the ionizing agent. On the arrival of ionizing particle, the gas gets ionized n the current lows in the plate circuit coz the grid becomes less negative. If the change in the grid voltage is sufficient, the voltage drop in Ŕ will result in effective counter voltage dropping below threshold value. Thus the counter discharge is extinguished n the circuit recovers to its original condition.
6 Dead time and Recovery time The counter remains dead till the positive ions have moved sufficiently away from the avalanche sites to put the wire back at the Geiger threshold potential.The time for which the counter is dead is called dead time.The recovery time is the time after which the original pulse levels are restored.The usual resolving time of a GM counter is equal to the dead time plus the recovery time.
7 efficiencySuppose 2000 particles pass through and are recorded then efficiency η is given byη=1600/ *100%Therefore efficiency of counting is the ratio of the number of counts observed per unit (n°) to the number of ionizing particles (n) which pass through the counter during that timeη=n°/n
8 applicationsThe first historical uses of the Geiger principle were for the detection of alpha and beta particles, and the instrument is still used for this purpose.Geiger counters can be used to detect gamma radiation.GM counters are very useful for detecting nuclear particles.In measuring cosmic ray intensities and recording cosmic ray events.
9 DisadvantagesThe resolving power of GM counter is limited due to a large dead time and recovery time so the counter cannot rates greater particles.GM counter have a very low intrinsic efficiency for the detection of γ-radiation.GM tube have a very limited life as the quenching gases dissociate and change the pressure of the inert gases filled inside.It cannot detect uncharged particles like neutron.Because it is heavy , so it cannot be sent in balloons to sky to study cosmic rays.
10 DEFINITION: Scintillation counter A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic (usually containing anthracene) or organic liquid (see liquid scintillation counting) that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube (PMT) measures the light from the crystal, and the output signal is fed to an electronic amplifier and other electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier. Scintillation counters are widely used because they can be made inexpensively yet with good quantum efficiency.
11 PARTS :-The complete scintillation counter consist s of three basic parts:1)The scintillating material or phosphor which produces a tiny light flash when a charged particle strikes it.2)The photo-multiplier tube which detects the light flash and produce electrical pulse.3)Amplifier and electronic circuits which record and count the electrical pulses from the photo-multiplier.
12 WorkingWhen a charge particle strikes the scintillator, the phosphor's atoms are excited and emit photons, which are directed at the photomultiplier tube's photocathode which is connected to the negative of a high voltage source. Each incident photon releases an electron. A number of accelerating electrodes called dynodes are arranged in the tube at increasing positive potentials and the electron is accelerated by this electric field towards the first dynode. The incident electron causes multiple secondary electrons to be emitted, which accelerate towards and hit the second dynode. More electrons are emitted and the electron multiplication chain continues through the increasing potentials of the dynodes, with increasing numbers of electrons generated each time. By the time the electrons reach the anode, enough have been released to generate a measurable voltage pulse across external resistors. This voltage pulse is amplified and recorded by the processing electronics.
13 Schematic showing incident particles hitting a scintillating crystal, triggering the release of photons which are then converted into photoelectrons and multiplied in the photomultiplier.
14 Detection materials:- Cesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles. sodium iodide (NaI) containing a small amount of thallium is used as a scintillator for the detection of gamma waves and Zinc Sulphide is widely used as a detector of alpha particles.Detector efficiencies:-The quantum efficiency of a gamma-ray detector (per unit volume) depends upon the density of electrons in the detector, and certain scintillating materials, such as sodium iodide and bismuth germanate, achieve high electron densities as a result of the high atomic numbers of some of the elements of which they are composed. However, detectors based on semiconductors, notably hyperpure germanium, have better intrinsic energy resolution than scintillators, and are preferred where feasible for gamma-ray spectrometry. In the case of neutron detectors, high efficiency is gained through the use of scintillating materials rich in hydrogen that scatter neutrons efficiently. Liquid scintillation counters are an efficient and practical means of quantifying beta radiation.
15 Producing of a scintillation flash by the incoming ionizing particle and subsequent generation of an electrical pulse in a photomultiplier are divided into five distinct events.1)The incident radiation is first absorbed in the phosphor material and its atoms or molecules are excited.2).The excited atoms or molecules of the fluorescent material of the phosphor decay and produce a light flash of short duration.3).The emitted photons are transmitted to the photo-cathode of the photo-multiplier.4).Photo-electrons are produced due to absorption of light photons.5).Electron multiplication takes place very quickly and all these operations takes place with in about 10ˉ8 seconds.1).
16 TYPES OF SCINTILLATORS GENERALLY USED:- Sodium Iodide:-This is most commonly used scintillator in the study of gamma rays.it has one drawback, it is hygroscopic and therefore has to be sealed in an aluminium can with reflecting or diffusing walls.Zinc Sulphide:-It is extensively used for the detection of those particles which have short ranges. It can not be used in thick layers because it rapidly becomes opaque to its own radiation.CsI:-This is not hygroscopic and is therefore preferred over sodium iodide.
17 4)Anthracene and stilbene:-These are organic phosphors. For heavy particles, these have very poor efficiency.These are useful for the detection of 𝛽 particles.5)Plastic and Liquid Scintillators:-In thesescintillators the energy of excitation is transferred from thesolvent to solute. These are used in counter telescope whichare generally used in high energy physics.6)Gases:-For counting heavy charge particles in the presenceof 𝛾- radiation, Xenon is used which emits radiation in the UVregion. The most outstanding feature of scintillation counterover proportional counter is its extremely short durationpulses and higher resolution.
18 ApplicationsScintillation counters are used to measure radiation in a variety of applications.Hand held radiation survey meters•Personnel and environmental monitoringfor Radioactive contaminationMedical imagingNational and homeland securityBorder securityNuclear plant safetyRadon levels in water