Geiger muller counter(GM counter) : Geiger muller counter(GM counter) G-M tube in holder scaler absorber source held in lifting tool jerin john(1025202) Radioactivity : Radioactivity is the discentigration of an unstable nuclei.
when the nuclei decays the nucleus emits alpha rays, beta rays, and gamma rays. Radioactivity jerin john(1025202) Observation of Radioactive Rays : Alpha Rays which barley penetrate a piece of paper.
Beta Rays which can penetrate 3mm of aluminum.
Gamma Rays which can penetrate several centimeters of lead. Observation of Radioactive Rays jerin john(1025202) We now know: : Alpha rays are helium nuclei
Beta rays are electrons
Gamma rays are electromagnetic radiation We now know: jerin john(1025202) Detection of Radiation : Individual particles such as electrons, protons, α particles, neutrons, and γ rays are not detected directly by our senses.
Several instruments have been developed to make up for this. Detection of Radiation jerin john(1025202) Radiation Detectors : Detectors are devices that translate radiation into recognizable signals
The signals can be electric, light or even visual
Ideal detector is:
Linear in Energy
All radiation detectors work by the principle that radiation deposits energy in matter.
Atoms are ionized and free negative charges (electrons) and positive charges (cations or holes) are created.
3 types of detectors:
Gas detectors : Geiger - Muller counter
Solid State detectors
Scintillation detectors Radiation Detectors jerin john(1025202) Geiger, Hans Wilhelm (1882 – 1945) : Geiger, Hans Wilhelm (1882 – 1945) Hans Geiger was born in Germany on Sept 30, 1882.
He studied physics and mathematics beginning in 1902 at the University of Erlangen. He graduated from the school with a PhD.
He worked with Ernest Rutherford and Ernest Marsden.
He invented the first Geiger counter.
John Nuttall, he discovered the Geiger-Nuttall law, which helped lead him to creating an atomic model with Rutherford.
Later, in 1928, he invented an improved Geiger counter, with his student, Walther Müller. This Geiger counter is called the Geiger-Müller counter.
Geiger also designed instrument capable of detecting and counting alpha particles.
Geiger died on Sept 24, 1945 in Potsdam. jerin john(1025202) Ionizing power : Ionizing power a Ionization When nuclear radiation passes through a gas, removes e from some gas molecules gas molecules are ionized - uncharged molecule +ve ion gains an e ve ion losses an e jerin john(1025202) Slide 9: a Ionization +ve & ve ions always form in pairs. Ions are free to move ionized gas can conduct electricity If intensity of radiation more ion pairs form gas is more conducting - uncharged molecule +ve ion gains an e ve ion losses an e jerin john(1025202) Geiger Counter : Geiger Counter A cylindrical metal tube filled with a certain type of gas (usually helium, neon, or argon) with a wire running down the center. The wire is kept at a very high positive voltage (slightly less than that required to ionize the gas) with respect to the cylinder.
This results in an electric field which is directed outward from the central wire to the surrounding cylinder. jerin john(1025202) Slide 11: The walls of the tube are either metal or the inside coated with metal or graphite to form the cathode while the anode is a wire passing up the centre of the tube.
Charged particles passing through the window ionize a few gas atoms.
The freed electrons accelerate toward the wire ionizing more atoms along the way. When this “avalanche” of electrons hits the wire a voltage pulse is produced which can be amplified and displayed in the form of audible clicks or by a needle meter. jerin john(1025202) Slide 12: How G-M counter works? (1) a high voltage is applied to the central wire (2) when a particle from radiation enters the tube, it pulls an e from an argon atom (3) a pulse is created due to the attraction of e to the central wire & can be counted by the G-M counter mica end-window admits nuclear radiation into the tube radiation aluminium tube () central wire (+) 400 V counter argon gas at low pressure argon atom e central wire pulse jerin john(1025202) Schematic of a Geiger counter : Schematic of a Geiger counter jerin john(1025202) Geiger-Mueller Tube : Operation
Increasing the high voltage in a proportional tube will increase the gain
The avalanches increase not only the number of electrons and ions but also the number of excited gas molecules
These (large number of) photons can initiate secondary avalanches some distance away from the initial avalanche by photoelectric absorption in the gas or cathode
Eventually these secondary avalanches envelop the entire length of the anode wire
Space charge buildup from the slow moving ions reduce the effective electric field around the anode and eventually terminate the chain reaction Geiger-Mueller Tube jerin john(1025202) Geiger-Mueller Tube : Geiger-Mueller Tube jerin john(1025202) Geiger-Mueller Tube : Gas
The main component is often argon or neon
However when the large number of these noble ions arrive at the cathode and are neutralized, the released energy can cause additional free electrons to be liberated from the cathode
This gives rise to multiple pulsing (avalanches) in the G-M tube Geiger-Mueller Tube jerin john(1025202) Geiger-Mueller Tube : Gas
Multiple pulsing can be quenched by the addition of a small amount of chlorine (Cl2) or bromine (Br2) (the quench gas)
As we mentioned earlier, collisions between ions and different species of gas molecules tend to transfer the charge to the one with the lowest ionization potential
When the halogen ions are neutralized at the cathode, disassociation can occur rather than extraction of a free electron Geiger-Mueller Tube jerin john(1025202) Geiger-Muller counters : Because of the very high voltage, a single particle can release 109 to 1010 ion pairs.
This means that a G-M counter is essentially guaranteed to detect any radiation through it.
The efficiency of all ionization chambers depends on the type of radiation.
The cathodes also influence this efficiency
High atomic number cathodes are used for higher energy radiation ( rays) and lower atomic number cathodes to lower energy radiation. Geiger-Muller counters jerin john(1025202) Higher Voltage : Higher Voltage As the voltage increases in a gas detector the ions collected increases.
The proportional region ends.
Continuous discharge Applied voltage jerin john(1025202) Continuous Discharge : Continuous Discharge Continuous discharge is due to the breakdown of gas into a plasma.
Each gas has a threshold
Example: neon lamps
Discharge is bad for detectors.
Individual signals lost
The fixed discharge threshold can be used to regulate voltage. NE-38: typical breakdown voltage 135 VDC jerin john(1025202) Multiple Avalanches : Multiple Avalanches In proportional mode a single ion pair results in an avalanche.
With higher fields electrons in the avalanche cause x-rays that start new avalanches.
The process stops when sufficient positive ions quench the avalanches.
Ions slowly drift to cathode jerin john(1025202) Geiger-Müller Region : Geiger-Müller Region In the Geiger-Müller (GM) region of operation there is a maximum amount of electrons produced in the avalanche.
Ion pair count is independent of initial ionization.
Plateau over range of voltage
The electrons are collected quickly
Less than 1 ms
Quenching gas is needed to suppress the later pulse from positive ions. jerin john(1025202) Geiger Tube : Geiger Tube Most Geiger tubes use a cylindrical geometry.
Grounded outer cathode
High voltage anode
There is usually a thin window to allow particle to enter without loss.
The output is either from case or capacitively coupled. C V + - R output jerin john(1025202) Experiment : (a) G.M. Tube Characteristics
Using handling forceps, place the radium source on the lowest shelf of the lead castle directly below the window of the G.M. tube. Switch on the counter and allow it to warm up for a couple of minutes. Increase the applied voltage from 320V in steps of 10V up to 450V. At each setting, note down the number of counts over a period of 2 minutes.
Plot a graph of count rate per minute against the applied voltage. Indicate on your graph the plateau, the Geiger threshold voltage and the operating voltage (i.e. the voltage at the middle of the plateau). Experiment jerin john(1025202) Slide 25: (b) Background Count
Remove all radioactive sources from the vicinity of the G.M. tube. Set the counter voltage at the operating voltage and take a 5-minute background count.
Note: The background count rate per minute should be subtracted from all counts in subsequent experiments in order to obtain the true count rates due to radioactive sources alone.
(c) The Resolution Time of a G.M. Counter
After a pulse is registered, a sheath of positive ions that gradually increases in radius remains about the anode wire. This effectively decreases the potential gradient near the wire and not until this space charge has drifted sufficiently far from the anode will the counter become sensitive again. The total time taken for the tube to recover to its fully sensitive state to give the next pulse, is called the resolution time. jerin john(1025202) Slide 26: (d) Verification of Inverse Square Law
Remove the G.M. tube from the lead castle and attach it horizontally to a stand provided. Using forceps, a place a radium source on another stand and align it until its active face faces the tube window and lies along the axis of the tube. Starting with a separation d between the window and the source equal to 0.7 cm and thereafter increase d successively by 0.7 cm until it reaches 12th place, note down the number of counts per second at each setting.
Correct the observed counts for background and resolution time , and hence plot the corrected count rate against to 1/d verify the inverse square law. jerin john(1025202) Slide 27: (e) Attenuation of ɣ-ray by Matter
The attenuation of a beam of ɣ-ray passing through matter depends on photoelectric absorption, Compton scattering and pail production. The relative importance of each of these processes, in any given case, is a function of the initial energy of the ɣ-photons and the atomic weight of the absorbing material. Experimentally it has been found that the attenuation follows closely the exponential law i.e. I is the initial intensity of the ɣ-ray, then after transversing a layer of matter of thickness x , its intensity I is reduced to
Where ɱ is known as the linear absorption coefficient of the matter. The value “l” of when the initial intensity is reduced to half is called the half value layer (HVL).
Note that in experiments using a G.M. counter, I is proportional to N (the counting rate corrected for background and resolution time).
Place the at a distance of about 20cm from the window of the G.M. tube. Take a one-minute count to determine the initial count rate. Without disturbing the setup, take a series of one-minute counts as a succession of aluminum sheets is placed vertically in the region between the G.M. tube and the source using the data obtained, plot a suitable graph and hence deduce the and HVL for . jerin john(1025202) Radioactive sources. : Caesium-137
It is a radioactive isotope of caesium which is formed as a fission product by nuclear fission.
It has a half-life of about 30.17 years, and decays by beta emission to a metastable nuclear isomer of barium-137.
And it is responsible for all of the emissions of gamma rays
It is responsible for the emision of ᵝ rays Radioactive sources. jerin john(1025202) Geiger-Mueller Tube : Geiger-Mueller Tube Use
Geiger tubes are often used as survey meters to detect or monitor radiation
They are rarely used as dosimeters but there are some applications
Survey meters generally have units of CPM or mR/hr but beware/check the calibration information
As radiation detector in medical application.
If calibrated, the survey meter is calibrated to some fixed gamma ray energy
For other gamma ray energies one must account for differences in efficiency jerin john(1025202) References : (1) J.B.A. England, Techniques in Nuclear Structure Physics,Part 1, Chapter 1.
(2) W.E. Burcham, Nuclear Physics An Introduction,Second Edition, Chapter 6. References jerin john(1025202) Working : Working jerin john(1025202)