nuclear radiation detectors and dielectrics, Physics

What do you mean by dead time in GM counter? Draw a neat diagram at GM counter and explain its working. Mention some of its applications. Give basic principle of Geiger-Muller counter. How quenching is achieved in Geiger-Muller counter.  What are advantages of proportional counter over GM. Counter?

Ans.:  In a gas filled detector, if voltage applied between the electrodes exceeds a limit so that the detector operates in the Geiger region. Then the charge collected is no longer proportional to the initial ionization. The gas multiplication factor M is very large in this region and a complete breakdown of the insulating properties of the gas in the chamber takes place. A single initial ion pair produced by the radiation entering the chamber can produce an electric discharge inside the chamber. Therefore, the counter becomes sensitive to even very low energy radiations. The important feature of this detector is that the size of electric pulse generated is independent of the energy of incident particles.               

Principle: It is based on the principle that a charged particle passing through a gas ionizes it. The electrons so produced during the ionization get accelerated under a high potential and produce further ionization. This ionization is called indirect ionization and ultimately a large number of electrons are produced which are collected by positive feedback. 

Construction: The GM. tube consists of a cylindrical metallic envelope which acts as cathode. A thin wire of tungsten lying on the axis of the cylinder acts as anode. A glass bead is used to cover the free and of the wire in the chamber. This bead is reseed to avoid corona discharge which may build up at a sharp point. A thin mica window is provided at one end of the chamber so that a-and b-particles may enter the chamber. In a G.M. counter, a cylindrical metal coating inside a glass envelope is also used in place of the metallic cylinder. Then the inner metal coating serves the purpose of cathode. The cathode surface is made so stainless steel of nickel of any other material having large value of work function. The G.M. tube is filled with an inert gas like Ar, Ne, He etc.   

Working:  The central wire is maintained at a high positive potential with respect to the outer cylinder. If the central wire is very thin, the electric field in its vicinity becomes very high. Suppose an incident ionizing particle produces one single ion pair in the volume enclosed by the outer cylinder. The resulting electron will be accelerated towards the positive central wire and will acquire relatively high velocity. It may itself produce a large number of additional ion pairs due to repeated collisions these now electrons are also accelerated and may produce more ion pairs. Resulting in an avalanche and a very large number of electrons reaches the anode. In this way, the initial formation of a single ion pair results in a very large pulse of current to the anode. It is found that the size of the current pulse becomes effectively independent of the initial density of ionization for quite a range of potential differences. The counter then gives a full avalanche of nearly constant size for any particle counted in this region and it is said to operate on the plateau of in the Geiger region. This is the normal mode of operation since the counter sensitivity is highest and the counting efficiency is independent of tube voltage over a reasonable range, so that the supply voltage is not critical. A counter operated in this fashion cannot give any information about the type of particle producing the count of its energy. If a resistor is connected in the Geiger counter circuit as shown in Fig. the central pulse I flowing through it produce a voltage pulse V which can be amplifier by an electronic amplifier and counted by an electronic counter.

Quenching, The positive ions produced during electron avalanche in a Geiger counter are subjected to the same electrical forces as the electrons. But because of their much larger masses, they experience smaller accelerations and hence may be considered as being virtually stationary during the electron avalanche. After this avalanche, these positive ions, which are principally formed close to the central wire, move outwards towards the cathode. When they reach there sometime later, they gave considerable energies and some of them will release electrons as they bombard the cathode surface. Starting a second avalanche, and thus a succession of discharge will follow. It should be stopped otherwise a single event will be counted twice. Hence positive ion cloud should be removed. Elimination of positive sheath around the wire is called quenching. Following quenching methods are used:                        

(1) External quenching method   (a) One method of limiting the discharge is to use a very large series resistance R. When large current pulse occurs, The momentary voltage drop across the resistance lowers the Geiger tube voltage sufficiently to extinguish the discharge. The period following one count during which the counter is insensitive is called the dead time of the counter. (b) A second and more satisfactory method for quenching the discharge is to use an electronic circuit to remove the counter voltage for a short period immediately following account. With this technique, dead times of about 40 microsecond are common. Either of this quenching process is referred to as external quenching.           (2) Self quenching Geiger Muller counter may be made in a variety of sizes but they customarily have anode wire about a tenth of a millimeter in diameter. A common filling gas is argon at a pressure of about 10 cm of mercury. For internal quenching a partial pressure of about 1 cm of ethyl alcohol is lower than Ar. So the positive ions moving towards the cathode consist of mostly of alcohol. The positive argon ions on their journey to cathode undergo many collisions with alcohol molecules and are practically neutralized by acquiring electrons from the alcohol molecules. The ions reaching the cathode ate then alcohol ions and although they are themselves completely neutralized at the cathode, the energy available is absorbed in dissociating the alcohol molecule rather than in producing further electrons from the cathode. Hence self quenching process is stopped. So a new counter is needed. Halogens are also used as self quenching agent in many of G.M. counter the life of the counter becomes longer as halogen ions gave a tendency to recombine and can neutralize themselves like ethyl alcohol but without dissociation. When the voltage is raised above this plateau of Geiger region, the quenching action is incomplete and one initial particle may give rise to more than one count. The count rate increases due to these spurious output pulses and at slightly higher voltages the counter goes into continuous discharge. This region of voltage is of no practical use and damage to the tube may be ensured.

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 to Geiger threshold potential and during this period. The counter is insensitive to further ionization pulse. The time during which the counter voltage dropped below the threshold voltage due to the presence of slowly moving sheath of positive ions towards anode is called dead of inactive time. And the time after which the original pulse level is restored is called recovery time. Now, during 1 minute N counts are observed, whole if all the particles n are detected in that time then the efficiency of the counter is from equation (b), it is clear that the efficiency of a counter depends on the counting rate N and the dead time t.

Drawbacks (1) It does not give the information about the energy of the particle counted. (2) Uncharged particle cannot be detected. (3) Permanent record of nuclear phenomenon cannot be kept. (4) The G.M. counter remains insensitive for sufficient for sufficient by long time (~500us) after each ionizing event. This limits the use of G.M. counter for lower counting rates only. (5) G.M. tube hsave a very limited life due to decomposition of organic vapors when the positive are neutralized at the cathode and break up into fragments.   

Advantages   (1) It produces a large pulse requiring no amplification. (2) It produces constant output pulse size, independent initial ionization.  (3) The G.M. counter equipped with transistorized electron is portable and is used for mineral prospecting and cosmic ray works. (4) It is very sensitive device for detecting charged particles like particles.        

Posted Date: 6/30/2012 11:47:58 PM | Location : United States

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