Geiger- - PowerPoint Presentation

Slide1

Geiger- Muller counter

GM counterSlide2

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.Slide3

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.Slide4

Plateau and over voltageIn 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.Slide5

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.Slide6

Dead time and Recovery timeThe 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. Slide7

efficiencySuppose 2000 particles pass through and 1600 are recorded then efficiency η is given by

η=1600/2000 *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 time

η

=n°/nSlide8

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.Slide9

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.Slide10

DEFINITION: Scintillation counterA 

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

.Slide11

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.

PARTS :-Slide12

Working

When 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.Slide13

Schematic showing incident particles hitting a scintillating crystal, triggering the release of photons which are then converted into 

photoelectrons and multiplied in the photomultiplier.Slide14

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

.Slide15

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).Slide16

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. Slide17

4)Anthracene and stilbene:-These are organic phosphors. For heavy particles, these have very poor efficiency.

These are useful for the detection of particles.5)Plastic and Liquid Scintillators:-

In these

scintillators the energy of excitation is transferred from the

solvent to solute. These are used in counter telescope which

are generally used in high energy physics.

6)Gases:-

For counting heavy charge particles in the presence

of

- radiation, Xenon is used which emits radiation in the UV

region. The most outstanding feature of scintillation counter

over proportional counter is its extremely short duration

pulses and higher resolution.

 Slide18

Applications

Scintillation counters are used to measure radiation in a variety of applications.Hand held 

radiation survey meters

•Pe

rsonnel and environmental monitoring

for 

Radioactive contamination

Medical imaging

National and homeland security

Border security

Nuclear plant safety

Radon levels in water