Gm counter& scintillation counter
- 1. PRESENTED BY : SONIYA. S I MSC BIOCHEMISTRY
- 3. GM COUNTER The most widely used apparatus for the determination of radioactive isotopes is Geiger-Muller or GM counter. What is radioactive isopotes? Its can be defined as atoms that contain an unstable nucleus(combination of neutrons and protons) and dissipate excess energy by spontaneously emitting radication in the form of alpha, beta and gamma rays . Different isopotes of the same element have the same number of protons in their atomic nuclic but different number of neutrons.
- 4. N Geiger and Muller developed a ‘Particle detector’ for measuring ‘ionizing radiation’ in 1928. They named it as ‘Geiger Muller Counter’. Ever since then it has been one of the most widely used nuclear detectors in the developmental days of Nuclear physics. The particle detector developed by Geiger and Muller is a gas filled counter. The main difference between ‘proportional counter’ and ‘Geiger-Muller Counter’ is in the formation of the avalanche. In the proportional counter, the avalanche is formed only at a point whereas in Geiger-Muller Counter it is formed in the central wire. Therefore, in GM Counter amplification is independent of initial ionization produced by the ionizing particle. The function of GM counter apparatus is based on the ability of the emitted radiation to ionize atoms. It consists of a large round tube forming cathode with a fine wire stretched in the centre as anode. The fine wire is maintained at high potential (1,000-2,500 volts) with respect to outer cathode. A GEIGER COUNTER (GEIGER-MULLER TUBE) IS A DEVICE USED FOR THE DETECTION AND MEASUREMENT OF ALL TYPES OF RADIATION: ALPHA, BETA AND GAMMA RADIATION. BASICALLY IT CONSISTS OF A PAIR OF ELECTRODES SURROUNDED BY A GAS. THE ELECTRODES HAVE A HIGH VOLTAGE ACROSS THEM. THE GAS USED IS USUALLY HELIUM OR ARGON.
- 5. CONSTRUCTION OF GEIGER- MULLUR COUNTER It consists of a hollow metal case enclosed in a thin glass tube. This hollow metal case acts as a cathode. A fine tungsten wire is stretched along the axis of the tube and is insulated by ebonite plugs. This fine tungsten wire acts as anode. The tube is evacuated and then partially filled with a mixture of 90% argon at 10 cm pressure and 10% ethyl alcohol vapours at 1cm pressure. sten The fine tungsten wire is connected to positive terminal of a high tension battery through a resistance R and the negative terminal is connected to the metal tube. The direct current voltage is kept slightly less than that which will cause a discharge between the electrodes.
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- 7. WORKING PRINCIPLE OF GM- COUNTER The basic principle of the Geiger Muller counter can be understood as follows. When an ionizing particle passes through the gas in an ionizing chamber, it produces a few ions. If the applied potential difference is strong enough, these ions will produce a secondary ion avalanche whose total effect will be proportional to the energy associated with the primary ionizing event. If the applied potential difference is very high, the secondary ionization phenomenon becomes so dominant that the primary ionizing event loses its importance. In other words, the size of the final pulse produced depends only on the triggering off of ionization by an ionizing particle but independent of the energy of this particle. A high energy particle entering through the mica window will cause one or more of the argon atoms to ionize. The electrons and ions of argon thus produced cause other argon atoms to ionize in a cascade effect. The result of this one event is sudden, massive electrical discharge that causes a current pulse. The current through R produces a voltage pulse of the order of 10μV. An electron pulse amplifier accepts the small pulse voltage and amplifies them to about 5 to 50 V. The amplified output is then applied to a counter. As each incoming particle
- 9. WORKING OF GM COUNTER . When the some external radiation source is enter into the ionizing tube such as alpha , beta, gamma. Its colloids with gas preset in tube and it ionized the gas molecules ( atom) . And it transfer energy to that molecules material medium( around voltage of +400 Volts is applied to the thin wire in the middle. When a particle arrives into the tube, it takes an electron from Argon atom) and electron of the outer shell of these molecules absorbed some energy by atom if the energy is efficient enough , the electron became free . From this ionized atom an electron is come out of atom as free electron and positive ion. Due to the high voltage in tube , it subjected to external electric field where the anode is connected to positive terminal and the cathode is connect to negative terminal of batteries ( with 1000 to3000 v of potential difference . The electron carry the negative charge move toward the anode (thin central wire) and the positive ion moves toward the( outer metallic surface ) cathode due to high potential difference. The electron is attracted to the central wire and as it rushes towards the wire, the electron will knock other electrons from Argon atoms, causing an "avalanche". Thus one single incoming particle will cause many electrons to arrive at the wire. The first electron which attracted toward anode cased further primary ionization and the electron from the primary ionization case the secondary ionization like a chain reaction
- 14. When the electron will interact with the potential field of nuclear and experience deceleration , then the electron loss energy and lead to creation of proton this kind of effect is called Bremsstrablung effect. Due to this effect, the emission of ultraviolet radiation to other cover of geiger mullar tube and create its own avalanche effect on those places . Even the presence of one electron will lead to the avalanch effect along the entire center electrode of geiger mullar counter. All of these electron reaches the center of electrode it will absorb by the anode and they complelet the circuit and potential drop at RL , This potential drop is measured by detector (creating a pulse which can be amplified and counted)and these electron complete the circuit and come back to metallic surface.
- 15. CHARACTERISTIC OF G M COUNTER THE CHARACTERISTIC SHOWS THE PLOT OF COUNT / MIN AS A FUNCTION OF VOLTAGE •FOR VOLTAGE LESS THAN 1000 VOLT THERE IS NO DISCHARGE AND HENCE NO COUNTS. •BETWEEN 1000 – 1200 VOLTS, THE NUMBER OF COUNT INCREASES LINEARLY WITH THE APPLIED VOLTAGE. THE REGION IS CALLED PROPORTIONAL REGION •ABOVE 1200 VOLT UPTO 1500 VOLT THE COUNT RATE SHOWS LEAST VARIATION, ALMOST CONSTANT THE REGION IS CALLED THE PLATEAU REGION OR GEIGER REGION OR OPERATING REGION. •IF THE VOLTAGE IS APPLIED ABOVE 1500 VOLT A CONTINUOUS DISCHARGE WILL TAKE PLACE, COUNT RATE INCREASES RAPIDLY DUE TO DISCHARGE OF ARGON GAS WHICH IS UNDESIRABLE.
- 16. Quenching • It is the process to prevent the continuous discharge. Self quenching is done by vapors of ethyl alcohol because its ionization energy is less than the ionization energy of Argon atom. Counting rate • The G M Counter can count about 5000 particles / sec. The counting rate depends upon the death and recovery time of G M Counter. Death time • In the counter, the slowly moving positive argon ion takes 200 sec to reach the cathode. If the second radiation enters the tube during this time, it will not be registered this time is called death time of the counter Recovery time • After death time the tube takes another 200 sec to regain the original working condition. This time is called recovery time of the counter. Paralysis time • The sum of death and recovery time is known as paralysis time, which is 400 sec. The tube can respond to the second radiation after 400 sec True Count Rate • If the death time of the counter is and the count which is measured by the counter is
- 17. ADVANTAGES OF GM COUNTER •It can count alpha, beta, gamma particles as well as cosmic rays. •It has high sensitivity. •Power supply need not be precisely regulated as the pulse height is constant over a large range. •Because of the fact that output pulse is very high, so the Amplification needed is also very subtle
- 18. DISADVANTAGES OF GM COUNTER •Energies cannot be measured by it as it has a lack of differentiating abilities. •It cannot detect uncharged particles like Neutrons. •It is less efficient due to the large paralysis time limits and large dead time. •Quenching agent used in this counter often decomposes, leading to less lifetime of the GM Counter. Thus, GM Counter is the one which is primarily used due to its advantages. Although it is not free from disadvantages, still its uses make it preferable over
- 19. SCINTILLATION COUNTER • The phenomenon of formation of fluorescence due to excitation by radioactivity is known as scintillation . • The light emitted in the scintillation can be detect by coupling it to the a photomultiplier which converts the photo energy into an electrical pulse. The magnitude of the generated electrical impulse proportional to the energy of the original radioactive count. • The instrument which measure scintillation are known as scintillation counter. There are two types of scintillation counter ; SOLID OR EXTERNAL SCINTILLATION COUNTER LIQUID OR INTERNAL SCINTILLATION COUNTER
- 20. HISTORY • The modern electronic scintillation counter was invented in 1944 by Sir Samuel Curran, whilst he was working on the Manhattan Project at the University of California at Berkeley. There was a requirement to measure the radiation from small quantities of uranium and his innovation was to use one of the newly- available highly sensitive photomultiplier tubes made by the Radio Corporation of America to accurately count the flashes
- 23. SOLID SCINTILLATION LIQUID SCINTILLATION SAMPLE LIGHT TIGHT COVER SAMPLE +FLUOR FLUOR PHOTOMULTIPLIER HIGH VOLTAGE SUPPLY SCALER HIGH VOLTAGE SUPPLY SCALER LEAD SHIELDING
- 24. SOLID SCINTILLATION • In solid scintillation counting, the sample is placed close to the fluor crystal ( Zn α-emitters , anthracene for β-emitters and NaI for γ-emitters ) which, in turn, is place adjacent to a photomultiplier. This photomultiplier in connected in connected to high voltage supply and a scaler. • Solid scintillation counting is useful for measurement for γ-emitting isotopes. But they are not useful for measuring weakβ-emitting radioisotopes such as 3H,14C,35S.
- 26. LIQUID SCINTILLATION COUNTER Liquid scintillation counting in the best method for counting of soft β- emitting isotopes such as 3H,14H,32P,35S. It can also used to detect α-particle positrons and weak x-rays. The sample for counting consists of three component namely the radioisotope, an organic solvent or solvent mixture and one or more organic fluors. The radiation is absorbed by the solvent ( xylene, toluene or 1,4-dioxane) which, in turn, transfers the energy to the fluor. In response to this the fluor fluoresces or scintillates. The emitted light photons are collected by a photomultiplier and converted into an amplified pulse, which is recorded as a count corresponding to the particle or radiation emitted.
- 27. β-particle primaryfluor photon amplifier sealer β-emitter solvent secondary photomultiplier pulse height fluor analyser • The above figure depicts the sequence of events occurring in a liquid scintillation counter. The β-particles from the radioactive source have a short range in the fluid and interact with solvent molecules which transfer their excitation energy to the fluor which, inturn , excited to a higher electronic energy level. • When the excited fluor returns to its ground state. It emits the energy as photons . The number of photons produced per β-particle depends on the energy of the β- particle released . The two major requirments of good fluor are, a) It should display a high efficiency of β-energy absorption. b) the absorbed energy should be re-emitted at a wavelength best suited for detection.
- 28. • But, in practice, such a fuor is not available. To achieve the above two points, a combination of two fluor is used . The primary fluor absorbs the β- particles efficiently and secondary fluor emits the transferred energy from the primary fluor to the visible region of the spectrum as photon. • In liquid scintillation counting , the excited solvent molecules emits light of a very short wavelength which fall in the range of 260nm to340nm . This range is too short to be detected . To overcome this problem , a compound known as primary fluor( eg ; PPO , Butyle-PBD) is added to the system. PPO N O
- 29. POPO N N • This compound absorbs light in the range of 260 to 340nm and emits light of a longer wavelenght (340 to 430nm). If the instrument is not sensitive to detect, a compound known as secondary fluor (eg. POPOP) is used. The secondary fluor absorbs light emitted by the primary fluor and emits light in the visible region(430 to780). The photomultiplier converts the light energy into an identical signal 9electrical) that can be easily manipulated and measured. The organic solvents usually will not accept aqueous samples. Addition of ethanol solves this problem to some extent but a loss of efficiency occurs . Emulsifier-based cocktails are frequently used for counting aqueous solution . They contain upto 50 percent water(v/v) O O
- 30. SCINTILLATION COCKTAILS The solution of primary and secondary fluor in an organic solvent is called scintillation cocktail. The efficiency of scintillation counting varies with volume of the sample taken and volume of scintillation cocktail added. As a general rule, the total volume of all samples including blank, standards and samples should be the same . Among the scintillation cocktails , toulene base cocktails are the most efficiency , but they will not accept aqueous samples. Cocktails based on 1,4-dioxane and naphthalene can accommodate samples containing water to a maximum of 20 per cent . But those cocktails are highly toxic in nature. Cocktails containing emulsifier are frequently used for counting aqueous samples. They contain an emulsifier such as triton x-100 and can accept up to 50 percent water. If the water content increases further , phase transition will occur from single phase to double phase and accurate counting cannot be done in the two phase state. A good cocktail should have the following properties, a) It is inexpensive.
- 31. A good cocktail should have the following properties, a) It is inexpensive. b) It should be clear , colorless and uniform after the addition of radioactive samples. c) It should have minimum quenching effect. d) Its components should be reasonably stable. e) It should not be toxic.
- 33. PHOTOMULTILER TUBE Photomultiplier detects the scintillations produced by the secondary fluor. A photomultiplier consists of a photocathode , containing caesium and antimony, which emits several electrons for each photon that strikes. Further, the rest of the tube is designed to multiply the emitted photoelectrons. The electrons are accelerated by a high voltage to an anode where collisions cause the emission of addition electrons ,which , in turn, are accelerated towards a second anode. This cascade process is continued upto fourteen anodes. Thus , a final pulse containing a large number of electrons produced. The number of electrons emitted at an anode for each incident electron is a function of the applied acceleration voltage. The size of the pulse can be controlled by altering the high voltage applied. The pulse generated by photomultiplier then strike an amplifier which permits additional amplification of the pulse. The pulse height analyzer comprises circuits which accept or reject pulse according to their voltage. There are two pulse height analyzers , an
- 34. The upper rejects all pulse above ancertain voltage and passes all those below it to the lower pulse height analyzer which reject all pulses below its set value and passes all those below its set value and passes all those above it to the scaler unit . Thus an energy of a set od pulses depends on the following factors. a) The energy of the β-particle b) The hight voltage setting in the photomultiplier c) The amplifier gain setting.
- 37. ADVANTAGES a) The sample preparation preparation is easy b) Any type of sample (such as liquids, solids, suspension and gels ) can be counted . c) Dual labelling experiments can be carried out . The radioactivity of two different isotopescanbe counted in the same sample. d) Higher count rate are possible in liquid scintillation counting e) counts per minte of the radioactive sample disintegrations per minute of the radioactive sample where, CE=counting efficiency f) Scintillation counters are now automated. Hundreds of samples can be counted in a short time and built in compluter facilities carry out data analysis. CE =
- 38. DISADVANTAGES The cost of counting per sample is very high. Application of high voltage leads to high back ground counts. This is referred to as photomultiplier noise. quenching is the greatest dis advantage of scintillation counting Chemiluminescence is due to chemical reactions between components of the samples to be counted and those of the scintillation cocktail. It leads to emission unrelated to excitation of the solvent and fluor system by radioactivity. Phospholuminescence affect counting efficiency the counting efficiency to prevent this effect, samples are kept in dark for sometime prior to counting and the door of The sample holder is kept closed throughout the counting process. The background count is the count recorded when the instrument is operated without any known radioactive source in it. Back ground counts is the count recorded when the instrument is operated without any radioactive source in it, Background counts will be usually in the range of 20 to 40 cpm and this value in then subtracted from the observed sample count.
- 39. The greatest disadvantage of scintillation on counting is quenching . It refers to a reduction in the efficiency of transfering energy from the β- particles to the photomultiplier. Quenching results in a decreased number of photons per particle and the production of reduced voltage . Quenching can be any of the three following kinds. a) OPTICAL QUENCHING: Its occurs if inappropriate or dirty scintillation vials are used . These vials absorb some of the light being emitted before it reaches the photomultiplier. b) COLOUR QUENCHING: Its occurs when the sample is coloured. Due to this , the emitted light is absorbed within the scintillation cocktail before it leaves the sample vial. This problem can be solved by bleaching the samples before counting c) CHEMICAL QUENCHING: It is the most difficult from of quenching to rectify. It occurs when anything in the sample interfers with the transfer of energy from the solvent to the primary fluor or from the primary fluor to the secondary fluor . This is overcome by determining the counting efficiency of each ample by converting the counts per minute to absolute counts (disintegration per second)