What is accelerated Thermal Ageing Test?

 What is Thermal Ageing?

Ageing of the materials with temperature could happen and differs for various materials. Important safety systems/equipments need to be evaluated for the effect of temperature on the materials used in the system. Even storage for long years shall also be considered for some materials like polymers to estimate the ageing effect. 

Ageing effect couldn’t be estimated practically for operating conditions or storage conditions as the duration is very high (tens of years) and the qualification of product couldn’t happen. Hence, Ageing effect due to temperature could be evaluated by exposing the unit to higher temperature for accelerated time. This is called as Accelerated Thermal ageing test.

 

How to calculate time required for Thermal Ageing of an instrument at accelerated temperature?

There are different methodologies to do Thermal Ageing test. Most commonly used methodology is application of Arhenius equation. Usually, this methodology is applicable for Polymer/Organic materials for which temperature effect is more.    

Thermal Ageing is due to the chemical process induced by the temperature, which could lead to change the bulk properties of the material over a time.  

The Arhenius methodology has been developed to simulate the accelerated ageing mechanism. As per this methodology, deterioration of materials in service is due to chemical reaction. These occur internally, sometimes between components of the material, and sometimes with compounds in the environment such as oxygen or water vapor. Chemical reactions occur more rapidly at higher temperatures.

 

Arrhenius showed that temperature dependence of chemical reactions follows an exponential equation. He postulated a consistent correlation between the amount of physical change and chemical reaction so that the time to reach a selected amount of physical change will vary according to an equation. The rate of thermal aging is the slope of the graph using the Arrhenius equation.

 

Accelerated Time calculation for Thermal Ageing Test

As per Arhenius equation,

where,

‘E’ is activation energy for the material under consideration; in eV

‘K’ is Boltzman Constant i.e. 8.617 x 10^-5 eV.K^-1 

‘t1’ is ageing time required at accelerated temperature ‘T1’.

‘t2’ is service life time operating temperature ‘T2’.

‘T1’ is accelerated temperature

‘T2’ is Operating temperature

For instance, let us consider the cross linked polyolefin cables (XLPO cables) being used for an application where the normal operating temperature is 50 oC and the expected service life is 40 years. Let us calculate the accelerated time required for thermal ageing test at accelerated temperature of 135 oC.

i) Activation energy (E) for XLPO cable is considered as 0.78 eV

[Ref: An Experimental Study on Dielectric Response of the Cross linked Polyolefin (XLPO) Insulation of the Unshielded Cable Under Electric Field and Heat Flux; Publisher: IEEE]

ii) t2 = 40 years

iii) T1 = 135 degC

Time required for accelerated Thermal Ageing test for accelerated temperature of 135 oC is ~43 days.

Picture size and Resolution explained

Pixel dimensions

The number of pixels along the height and width of an image.

The display size of an image on screen is determined by the pixel dimensions of the image plus the size and setting of the monitor.

For example, a 15 inch monitor typically displays 800 pixels horizontally and 600 vertically. An image with dimensions of 800 pixels x 600 pixels would fill this small screen. On a larger monitor with an 800 x 600 setting  the same image would still fill the screen but each pixel would appear larger. Changing the setting of this larger monitor to 1024 x 768, pixels would display the image at a smaller size, occupying only a part of the screen.

Image Resolution

The number of pixels displayed per unit of printed length in an image, usually measured in pixels per inch(ppi).In photoshop, you can change the resolution of an image; in image ready, the resolution of an image is always 72ppi.

When printed, an image of high resolution contains more and therefore smaller pixels than an image with a low resolution. For instance, a 1x1 inch image with a resolution of 72ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1 x 1 inch image with a resolution of 300 ppi contains a total of 90,000 pixels.

Using too high a resolution increases the file size and slows the printing of an image.; further more, the device will be unable to reproduce extra detail provided by high resolution.

Monitor Resolution

The number of pixels or dots displayed per unit length on the monitor, usually measured in dots per inch (dpi). Monitor resolution depends on the size of monitor plus its pixel setting. 

understanding monitor resolution helps explain why display size of an image on screen often differs from its printed size. Image pixels are translated directly into monitor pixels. This means that when image resolution higher than monitor resolution the image appears larger on screen than its specified print dimensions. For example, when you display a 1 x 1 inch 144ppi image on a 72dpi monitor it appears in a 2 x 2 inch area on screen. Because the monitor can display only 72 pixels per inch. It needs 2 inches to display the 144pixels that make up one edge of the image.  

Modern Physics - Graduate level important notes for competitive exams - PART2

•  4n series is called as "Thorium" series.

• 4n+1 series is called as "Neptunium" series.

• 4n+2 series is called as "Uranium" series

• 4n+3 series is called as "Actinium" series

• X-rays & 𝛾-rays of same energies are distinguished by their frequencies

• Spin of a Deuteron  is 1.

• Spin of Muons is 1/2.

• Spin of Pions (π-mesons) is zero.

• Spin of Photon is 1.

• Spin of Graviton is 2.

• Mass of a Proton is 1.0072766 amu

• Mass of a Neutron is 1.0086654 amu

• The total quantum number 'n' specified "Energy" quantization.

• The azimuthal quantum number specified "Angular Momentum".

• The magnetic moment of proton is given by 2.8eħ/2mp

• The magnetic moment of Neutron is given by 1.9eħ/2mp.

• In Davisson-Germer experiment , the angle between incident beam and diffracted beam is called "co-lattitude".

• In heavy atoms the spin-orbit interaction is almost equal to "electrostatic interaction".

• L-S coupling is due to "electrostatic interaction".

• L-S coupling scheme breaks down in strong magnetic field and applies to light atoms.

• In light atoms, "orbit interaction" is less than "electrostatic interaction".

• According to shell model nucleons interact primarily with a general force field rather than directly with one another.

• Magic numbers: 2,8,20,28,50,82,126

• Debroglie wavelength associated with electron accelerated to a potential V is ƛ=12.26/√V A⁰

REFRIGERATION - IMPORTANT NOTES

Refrigerating machines are of two types 1) Vapour compression machine (Frigidaire)" and 2) "Vapour absorption machine". 

Refrigerant: 

The liquid which on evaporation produces cooling is called refrigerant. Some examples are Ammonia(NH₃), Sulphur dioxide, Freon.

Properties of Refrigerant:

1. It should have low boiling point & melting point
2. It should be vapor at normal temperature and pressure.
3. It should have high thermal conductivity
4. "Latent heat of vaporization" of the refrigerant must be large.  
5.  The specific volume should be small in order to reduce the size of compressors. 

The evaporation of liquid under reduced pressure is a cooling process. The reason is that when a substance changes from liquid to vapor phase, it is associated with absorption of heat. If this heat is not taken from any external source, the necessary heat for phase exchange is taken from itself which therefore cools.

Vapor Compression Machine

Production of temperature by making a liquid evaporate rapidly under reduced pressure and circulating evaporating liquid around enclosure.

Debye's theory of Atomic Heat Capacity of Solids - Important Notes

 The failure in Einsteins theory of specific heat at low temperature is due to assumption that the vibrations of all atoms are simple harmonic and have one and same frequency.

"Debye" improved the Einsteins theory by considering the atomic oscillators as a system of coupled oscillators having a continuous range of frequencies.

Essential difference between Debye & Einstein model:

Debye has considered the vibrational modes of a crystal as a whole, where as Einstein has considered the vibration of a single atom with the assumption that atomic vibrations are independent of each other.   

Debye has made following assumptions:

1. The solid is capable of vibrating elastically in many different modes.
2. The frequency of vibration is different for different modes.
3. The number of modes of vibration of waves solids are limited in number.
4. The maximum frequency is the fundamental frequency of solid. The maximum frequency is frequency of shorter waves which the solid can transmit. 

According to Debye, a solid can be treated as an elastic body in which vibrations of atoms generate "stationary waves" of both longitudinal & transverse types with velocities Vl & Vt respectively.

The velocities can be determined by elastic constants & densities of solids.

The frequencies range from zero to a definite upper limit.

The number of modes of longitudinal waves per unit volume with frequencies between '𝝂' & '𝝂+d𝝂 ' is represented by 4Π𝝂²d𝝂/(Vl)³.

The number of modes of transverse vibrations per unit volume with frequencies between '𝝂' & '𝝂+d𝝂 ' is represented by 8Π𝝂²d𝝂/(Vt)³. Here 8Π is taken in place of 4Π. Because transverse vibrations have two independent directions of vibration i.e. they are equivalent to two waves at right angles to each other.

So total number of modes of vibrations per unit volume with frequencies '𝝂' & '𝝂+d𝝂 ' is given by  

4Π[(1/Vl³)+(2/Vt³)]𝝂²d𝝂

The total number of independent modes of vibrations is given by  

4ΠV[(1/Vl³)+(2/Vt³)] ∫𝝂²d𝝂 ; V is volume of gm-mole of solid

Why U-235, as isotope of Uranium with only 0.7% abundance will undergo fission if it captures a thermal Neutron where as U-238 will not?

 From 'B/A' against A curve, we know that Binding Energy per Nucleon in Uranium region is about 7MeV, which is almost equal to height of potential barrier opposing fission. 

Now,U-238.in  when a Neutron is added to Nucleus of ₉₂U²³⁵, we get ₉₂U²³⁶ i.e. an Even-Even nuclide. The pairing term in mass formula for Uranium is about 0.6MeV. Thus, we see that Thermal Neutron capture in U-235 results in total energy release of 7.6 MeV and since to cross potential barrier, only about 7 MeV is required. So U-235 can undergo fission by Thermal Neutron capture.

On the other hand, U-238 nucleus, after Neutron capture becomes a U-239 nucleus which is an Even-Odd Nuclide. Due to absence of pairing term for U-239, the fission barrier cannot be crossed. So Thermal Neutrons will not induce fission.        

Useful Range of Materials for Vacuum Vessels and Pipes

 

Critical Energy required for fission of some target nucleus

 

Nuclear Physics - Quick Review for Graduate students

1) "Thorium" has highest half life period and "Polonium" has shortest half life period.

2) Elements lying beyond 92U238 in periodic table are called "Transuranic" elements. 

3) Effective Multiplication Factor Ke = P/(A+L); 'P' is Production of Neutrons; 'A' is Rate of absorption of Neutrons; 'L' is Rate of Leakage of neutrons.

Ke = 1   -----------------------chain reaction is "Critical"  

Ke > 1   -----------------------chain reaction is "Super-Critical"  

Ke < 1   -----------------------chain reaction is "Sub-Critical"  

Ke =NF/(A+L) as P=NF; F is rate at which Fission occur; N is average number of Neutrons emitted per Fission

Control Rods are used to keep Ke = 1

4) Slow Neutrons (Thermalized) are efficient in causing Nuclear Reaction.

5) Atoms having same 'Z' and 'A' but differ from one another in their Nuclear energy states and exhibit differences in internal structure. These Nuclei are distinguished by their different life times. Such Nuclei are called "Isomers".

6) Nuclei having same 'A' but with Proton & Neutron number interchanged  (i.e. No. of Protons in one is equal to No. of Neutrons in other) re called "Mirror nuclei". For instance, 4Be7 (Z=4 and N=3), 3Li7(Z=3 & N=4).

7) Law governing successive disintegration deals with quantity of daughter nuclei present at any instant. 

8) Formation of daughter element depends on decay constant of parent.

9) Decay of daughter element depends on decay constant of itself.

10) Condition for "Secular Equilibrium" is N₁ƛ₁ = N₂ƛ₂ ; this occurs when element-1 is long lived.

11) Condition for "Transient Equilibrium" is N₂/N₁ = ƛ₁/(ƛ₂ - ƛ₁); this occurs when element-2 has same half life as that of parent element-1 and also ƛ₁ < ƛ₂.

12) Let ‘A₀’ be the activity at time of death . Suppose this is reduced to ‘A’ after ‘t’ years. Then    

       t = 3.32 T₁/₂ log₁₀(A₀/A); T₁/₂ is 5568 yrs.

13) Age of rocks may be estimated with comparing quantities of various isotopes of Lead existing in sample.

14)  In “ GM Region",  ionization current is independent of initial ionization.

15)  35 eV is required for producing an ion pair.

14)  Proportional region is most suitable for proportional counters in which number of  pulses increases almost linearly with voltage.

15) Plateau region is most suitable for “ GM counter “ in which number of impulses remain constant. This region is function of Voltage , Resistance, nature of gas.

16) “quenching agent " should have “ Low Ionization Potential".

17)  “ Scintillation" counter depends on principle of “ fluorescence”.

18)  “ Solid state detector " depends on following principle : incoming particles into depletion region must lose all their Kinetic Energy.

19) Non integrating ionization chamber - a pulse type; Integrating Ion chamber works in  current mode.

20) "GM counter" cannot be used to detect ɑ- particles.

21)  “Cloud chamber “ cannot be used to detect ‘𝛾 ‘ rays.

22)  By counting drops in cloud track , specific ionization can be determined.

23)  Particle disintegration process ---- outgoing particle is material particle

24) Simple capture process -- outgoing particles are -γ -rays.

25) Particle disintegration process is more probable than simple capture process.

26) Nuclear Reaction Energy (Q)

Q= (M₀+M₁) - (M₂+M₃)

Q = ( E₂+E₃) - E₁

'M₀' is mass of target nucleus (A)

'M₁E₁' is Mass and Energy of projectile(B)
'M₂E₂' is Mass and Energy of product nucleus (P)
'M₃E₃' is Mass and Energy of outgoing particle(O)

CASE :

If (M₀+M₁) > (M₂+M₃), Q= +VE ;  Exothermic or Exoergic; ex:Li(P,𝛼)He⁴

If (M₀+M₁) > (M₂+M₃) , Q= -VE ;  Endothermic or Endoergic; ex: ₇N¹⁴(𝛼,P)₂He⁴

If (M₀+M₁) = (M₂+M₃), Q= 0 ;  Elastic Collision

27) Threshold energy(Eth) is minimum kinetic energy to initiate endoergic reaction.

Eth = [1+(M₁/M₀)]*Q ;  M₁ is mass of incident particle & M₀ is mass of target particle.

28)  When a parent Nucleus goes from its ground state to ground state of daughter nucleus, it emits an 𝛼-particle of maximum energy.

29) 𝛼-particle is preformed inside parent Nucleus.

Area - Conversion Factors

 

Useful Materials for Vacuum Applications

 

Throughput ranges for Vacuum Leak

 

Conversion table for Pressure Units

 

Modern Physics - Graduate level important notes for competitive exams - PART1

• Aston’s Mass Spectroscope enables identification of isotopes.

• Packing fraction (P) is mass defect for elementary particle, P = Mass defect/ Mass number.

• Negative Packing Fraction implies exceptional Nuclear Stability.

• Binding energy of Neutron is 2.23 MeV, Spin = 1.

• Binding energy of Lithium is 37.7 MeV. Binding energy per Nucleon is 37.7/7 = 5.4MeV.

• Diameter of Nucleus is of order of 10⁻¹⁴ m.

• Intensity of X- rays varies with current.

• The quality of X- rays is a function of Potential difference between Cathode and Anode.

•  Moseley’s law is based on Bohr’s theory of atom.

• Wave length of X – rays ranges from 10⁻¹⁰ to 10⁻⁸ m.

•  Speed of X- rays = 3*10⁸ m/sec.

• The short wave length of X- rays emitted from X- ray tube depends on current in tube.

• The operating Voltage in a typical X- ray tube is of order 10 KV.

• Stopping Potential is independent of intensity of incident Radiation.

• Saturation current is directly proportional to Intensity of Incident Radiation.

• Compton Effect is observed with X-Rays and Gamma Rays but not with Visible and UV Radiation.

• The constant factor of probability of transition is known as "Spontaneous Emission". It leads to broad Spectrum.

• The variable factor of probability of transition is known as "Stimulated Emission". 

• Spontaneous emission was postulated by "Neil Bohr".

• Stimulated emission was postulated by "Einstein".

• Probable rate of occurrence of absorption transition from state 1 to state 2 depends on properties of two states and is proportional to energy density.

• Probability rate of "spontaneous emission" depends on properties of states but does not depend on energy density.

• In Ruby(Al2O3) LASER, Chromium ions(Cr3+) acts as "active" material.

• Ruby(Al2O3) LASER is a “pulsed" LASER . The output beams of it have principal wave length in visible spectrum.

• Gas LASER(He-Ne) is a continuous LASER with high Monochromaticity.

• In He– Ne LASER, Helium atoms are in majority while ‘Ne’ atoms are in minority.

• In He–Ne LASER, we observe emission between 5S to 3P.

• In He- Ne LASER, Helium gas serves as a mediator in producing Population Inversion.

• Bragg’s law is a result of “ Periodicity of Lattice Points”.

• Fine structure of H⍺ line includes 7 no. of valid transitions.

• X- ray emission is referred to as inverse photo electric effect.

• Ratio of Einstien's Spontaneous & stimulated coefficients = 8Πh𝝂³/C³.

• Spectrum of non- rigid diatomic molecule is similar to that of Rigid molecule except that each line is displaced slightly to lower frequency.

• Pure "Vibrational spectra" are observed only in liquids. Because interactions between neighboring molecules prevent this Rotational motion.

The Davisson-Germer Experiment

The experiment gave the evidence of wave nature possession by materialistic particle(electron) for the first time.

The arrangement of equipment used for the experiment is as follows

                      Procedure:

An electron gun is used in order to produce electrons. The electrons so produced are accelerated by applying a high potential towards the target crystal, in this case the target crystal is nicker. The accelerated electron beam is made into fine beam by passing it through a Collimator ‘c’.

The crystal is mounted on an arrangement which could be rotated in different directions perpendicular to the plane of diagram.

The electrons are scattered in all directions by atomic planes of crystal.

The intensity of electron beam ( no. Of electrons) scattered in a particular direction is measured by electron collector which can be rotated about the same axis as target crystal.

The collector is connected to a sensitive Galvanometer whose deflection is proportional to intensity of electron beam entering collector . The electron collector is also called Faraday cylinder.

A retarding potential is applied to Faraday cylinder such that only fast electrons can reach it and secondary electrons emitted from crystals are stopped.

A graph is then plotted between galvanometer current against angle ‘θ' between incident beam and diffracted beam i.e, beam entering Faraday cylinder.



In the investigation , the electron beam accelerated by 54V and at an angle of 50 between incident and diffracted beam , a sharp maximum has occurred in electron distribution.

The incident beam and diffracted beam in this experiment make an angle of 65⁰   with Braggs plane. 

For a 54 V electron , the de-broglie wavelength associated with the electron is given by

ƛ = 12.25/√V = 12.25/√54 A⁰ = 1.66 A⁰ 

Now from Bragg’s equation for maxima in diffraction pattern for same energy electrons

2d sinθ' = nƛ; 2*0.91*10^-10*sin 65⁰ = 1*ƛ;

ƛ = 1.65 A⁰ 

Thus, both theoretical and experimental values are in excellent agreement.

Thus Davission - Germer experiment provides a direct verification of de-broglie hypothesis of wave nature of moving particles.

PIN Diode - General Description

It is a junction photo diode. 

Basic principle :- The diode is operated under reverse biased condition. Under photo excitation , photons are absorbed mainly in depletion region and also in neutral region , particularly on top where light is incident. the absorbed photons create e-h pairs

They give rise to photo current , the magnitude of which depends on quantum efficiency.

so photo excitation is therefore detected as an increase in reverse biased current of a junction photo diode.

Large depletion layer may increase no.of e-h pairs generated but it consequently increases response time of diode degrading its high speed performance.

Pin diodes have no internal gain but can have very large band widths.

Since dark current in a reverse biased junction is very small , pin diode is more sensitive device than a photo conductor.

Difference between Impedance and Reactance

For all the circuits lumped elements in which time varying voltages / current exists , we find a relation in which the voltage is proportional to Current . The proportional quantity is in general a complex number and this is called Impedance.

It is a function of frequency of ‘w’ strictly speaking impedance is sum of its real and imaginary parts.

Z= R+ IX

Impedance is equivalent to a Resistance in series with pure imaginary Impedance - called a Reactance.

The voltage drop across the resistance is in phase with the current , while the voltage drop across the purely Reactive part is out of phase with the current.

The average energy loss in an impedance Z= R + iX depends only on real part of ‘Z’ and not on complex part.

There is no energy loss in Reactive part.

GP THOMSON EFFECT - Experimental Verification of Wave nature of Matter

G P  Thomson has performed experiments with electrons accelerated from 10000 to 50000 volts.

The high energy beam of electrons is produced by a cathode ‘C’.

The experimental arrangement is as shown below:

The electron beam is excited with potential upto a maximum of 50,000 volts. A fine beam is obtained by passing it through slit or diaphragm ‘S’.

The accelerating fine beam of electrons now falls on thin gold or Aluminum film (order of 10^-6 cm thickness).

The photograph of beam from foil is recorded on photographic plate ‘P’.

After developing the plate, a symmetric pattern consisting of concentric rings about a central spot is obtained. This pattern resembles that of X- rays.

To know that this pattern is due to electrons or due to x-rays generated by electrons in their passage through foil, cathode rays in discharge tube are deflected by magnetic field.

It was observed that beam shifts correspondingly showing there by that pattern is produced by electrons and not by x-rays.  (i.e X – ray pattern is not affected by electric and magnetic fields).

As diffraction pattern can only be produced by waves and not by particles, so Thomson concluded that electrons behave like wave.

Thus, Thomson experiment clearly demonstrated the existence of matter waves.

BCS (Bardeen - Cooper - Schieffer) THEORY

The microscopic theory put forward by Bardeen, Cooper and Schreiffer (BCS), in 1957 provides the better quantum explanation of  superconductivity and explains well all the properties exhibited by superconductors.

The basis of formulation of BCS theory are two experimental conclusions namely the isotope effect and variation of specific heat of superconductors.

For isotope effect Tc M^1/2 = constant, one can infer that transition resulting in superconducting state must involve dynamics of ion motions, lattice vibrations and Phonons.

Further we note that Tc attains a value zero when 'M' approaches infinity. This all suggests that the non zero transition temperature is a consequence of finite mass of ions which can contribute Phonons by their vibrations.

Bardeen pointed out that an electron moving through a crystal lattice has a self energy accompanied by virtual Phonons. This means that an electron moving through crystal lattice distorts the lattice and lattice in turn acts on  electron by virtue of electrostatic forces between them. The oscillatory distortion of lattice is quantized in terms of Phonons and so one can interpret the interaction between lattice and electron as constant emission and re-absorption of Phonon by latter. These are called virtual Phonons.

BCS showed that basic interaction responsible for responsible for superconductivity appears to be that of a pair of electrons by means of interchange of virtual Phonons.

Suppose an electron approaches a positive ion core . It suffers attractive Coulomb interaction. Due to this attraction ion core is set in motion and consequently distorts the lattice. 

Smaller the mass of positive ion core, the greater will be the distortion. Suppose towards that site another electron comes and sees this distorted lattice. Then the interaction between the two, the electron and distorted lattice occurs which in its effect lowers the energy of second electron. 

Thus, we interpret that the two electrons interact via the lattice distortion or the Phonon field resulting in lowering of energy for electrons.

The lowering of electron energy implies that force between two electrons is attractive. This type of interaction is called electron-lattice-electron interaction. This interaction is strongest when two electrons have equal and opposite momenta and spins.

Since the oscillator distortion of Lattice is quantized in terms of Phonons, the above interaction can be interpreted as electron-electron interaction through Phonon as mediator.

Let an electron of wave vector K1 emits a virtual phonon 'q' which is absorbed by an electron with wave vector K2. K1 is thus scattered as K1-q and K2+q.

The nature of resulting electron-electron interaction depends on relative magnitudes of electronic energy and phonon energy. If the phonon energy exceeds electronic energy, the interaction is attractive.

When such interaction occurs by phonon exchange by dominating usual repulsive interaction, two such electrons from a pair called as Cooper pair.

The energy of pair of electrons in bound state is less than energy pair in free state. The difference in energy of two states is binding energy of cooper pair.

The energy difference between free state of electron and paired state appears as energy gap at Fermi surface. The normal electron states are above the energy gap and superconducting  electron states are below the energy gap at Fermi surface.   

What is Meissner Effect?

 The Meissner Effect

 

    Superconductors which are resistance less materials have an additional property of exclusion of applied magnetic field on it i.e. inside a superconducting material, we always have B=0.





 

    The property of perfect diamagnetism arises in super conductor because when a magnetic field ‘Ba’ is applied surface screening currents circulate so as to produce a flux density ‘Bi’ which every where inside the metal exactly cancels the flux density due to applied field Bi=-Ba.

For a Super Conductor μ_r=0; i.e. B=μ_rB_a=0

TThis property of exhibiting perfect diamagnetism by super conductor is known as Meissner Effect.

 

What is GM Counter and how does it operate?

 The counter is named as GM counter based on its developers ‘Geiger’ and ‘Muller’ in 1928. They are oldest type of gas filled radiation detectors.GM counters were operated in the Geiger discharge region of gas filled ion chambers.

Construction: - The counter is usually a leak tight assembly of two electrodes electrically isolated. The counter is filled to sub atmospheric pressures of few mm of mercury. A high voltage is applied to the anode electrode. 

Principle: - This counter also works on the principle of ionization caused by incoming energetic particle in the gas medium filled between anode and cathode. The electron liberated in the primary ionization event would get accelerated towards anode because of its high potential. The electron may gain sufficient energy to cause ionization of other gas molecule. This leads to a chain of ionizing events which is usually referred to as Townsend avalanche. During this process, there may be interactions in which excitation of atoms may occur due to sufficient energy of impinging electrons. Such atoms while de-exciting may emit photons which normally fall in UV or visible region. These photons which are emitted may again lead to photo electrons due to ionization of gas atoms or due to photoelectric interaction with walls of counter. Each photo electron would again cause Townsend effect. Such a series of Townsend avalanches would lead to discharge in the tube called Geiger-discharge. In such a state there is formation of dense envelope of electron-ion pairs distributed on either side of anode.

The voltage applied to anode shall be such that it is enough to trigger the avalanche mechanism and collect total charge (electrons) pertaining to single event leading to Geiger discharge.

Concept of quenching: - Practically the process would not be as simple as above. During the Geiger discharge, there is dense envelope of electrons and ions. The electrons would drift towards anode and positive ions would drift towards cathode. The positive ions which drift towards cathode having ionization potential (E) greater than the work function (W) of cathode material leads to exchange of electron from cathode and becomes neutral. The excess energy may be dissipated in two forms, one by emission of photon or an electron form cathode if excess energy is greater than the work function of the cathode material. This would again initiate another Geiger discharge. The result of this is that the tube would always be in continuous Geiger discharge and hence will not able to measure any radiation.

To overcome this problem, concept of quenching is introduced. There are two types of quenching

i)                   Organic quenching

ii)                 Halogen quenching

Organic quenching: -

This involves addition of small quantity of organic gas having complex molecule structure. This prevents the continuous Geiger discharge mechanism by charge transfer collision principle.   The positive ions on their path collide with organic molecules to get neutralized. This makes only ions of organic gas reach cathode and gets neutralized. If there is any excess energy released leads to dissociation of organic molecules. Thus multiple Geiger discharges could be avoided.

A typical filling of organic quenched GM tubes is 90% Argon and 10% of ethyl alcohol. When organic gas gets depleted to a sufficient extent there is occurrence of multiple discharges frequently and thus the plateau length gets decreased, with slope increased.

Thus the organic quenched GM tubes are characterized by short life time and thus not suitable for operation in very high fields which leads to large count rate.  To overcome this, technique of Halogen quenching is introduced.

Halogen quenching: - This involves the addition of small quantity of Halogen gas such as Chlorine or Bromine. A typical filling is about 0.1% of chlorine to Neon. The quenching action is same as that in Organic quenching process. The diatomic halogen gas molecules too gets dissociated in quenching but gets recombined to replenish the gas molecules and thus counter life gets extended.

Characteristics of GM tubes:-

The important parameters which decide the quality of functioning of Gm tubes are

i)                   Dead time

ii)                 Recovery time

iii)               Plateau length &Plateau slope

i)           Dead time: - As discussed above, the positive ions take considerable time to reach cathode tube compared to electrons. The reason is that the mobility of electrons is about 1000 times greater than that of electrons.

Due to the low drift velocity of positive ions, there is formation of cloud of positive ions which tend to electric field opposite to that of actual field. This reduces the electric field intensity due to anode potential and thus affects gas multiplication factor. This in turn affects the pulse heights.       

             In high count rates, it is more worse that there is formation of dense  positive cloud which makes the electric field intensity in the vicinity of anode wire reduce by great margin thus multiplication goes down by big margin. During this phase of detector, any new ionizing event caused by incoming particle cannot be recorded. Thus the time interval during which any event caused by newly incoming particle would not get counted and called as dead time of the country.

ii)            Recovery time: - After certain time, all the positive ions tend to reach cathode wall and thus the electric field begins to restore to actual value. When the electric field goes beyond a critical value there is again formation for pulses. But the process requires some time to give maximum pulse heights. Hence the total time required for GM tube to give maximum pulse height pulses is Recovery time.

iii)           Plateau length & Slope: 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. After applying minimum voltage to initiate Geiger discharge, the no. of pulses shall remain same in fixed radiation field exposure. But due to formation of short pulses during recovery time there is variation in count rate. Thus one of the quality parameters deciding the operation of GM tube is that plateau slope shall be less. Usually 2-3% plateau slope is a good choice.  As we go on applying voltage to the anode, the tube starts entering continuous   discharge region. Thus the slope gets worsened. The region or length of voltage region during which the plateau slope remains in desired value is called as plateau length and usually the operating voltage is chosen at the midpoint of plateau length.

You can download complete details of these counters here.  

Comparison between Frequency Modulation(FM) & Amplitude Modulation(AM)

 

Frequency Modulation	Amplitude Modulation
1) The amplitude of FM signal is constant and in depth of Modulation.	1) Amplitude of AM signal varies depending on Modulation index.
2) It requires much wider channel (7-15 times) as compared to AM	2) Band width, is very small, which is one of the biggest disadvantages.
3) Transmitters are complex and hence expensive	3) Relatively simple and cheap
4) Area of reception is small since it is limited to line of sight	4) Area of reception is large
5) Noise can be easily minimized. Amplitude variations can be eliminated by using limiter.	5) More susceptible to noise, interference & low signal to noise ratio. Difficult to eliminate effects of noise.
6) Average power in frequency modulated wave is same as contained in un-modulated wave.	6) Average power in modulated wave is greater than carrier power. This added power is provided by modulating source.
7) No restriction is placed on modulated index.	7) Maximum modulation index is 1 otherwise over modulation would result in distortion.
8) Possible to operate several independent transmitters on same frequency.	8) Not possible to operate out interference.

Advantages of Optic Fibers

1. Optical fibers have greater information carrying capacities than metallic conductors.

2. Fibers and fiber cables are very strong and flexible. So fibres are so slender that they do not break when wrapped around.

3. One of the most important advantages of fibers is that they can carry large amount of information in either digital or analog form.

4. An optical fiber is well protected from external interference and coupling with other communication channels. It is because an optic fiber made of  either glass or plastic is an insulator.

5. Electromagnetic interference caused by lightning and sparking etc doesn't effect fibers.

6. As compared to Copper, corrosion due to water or chemicals is less for glass. Glass fiber themselves can withstand high temperature before deteriorating. 

7. Fiber offers a degree of security and privacy. Because fibers do not radiate energy with in them, it is difficult for an intruder to detect signal being transmitted.

Classical Mechanics Conceptual Questions - Experts Answers

1) How does Earth's Rotation relate to time taken by aircraft to reach from say India to the United States and back?

2) Which direction would a helium filled ballon go in an enclosed car that turns right, and why?

3) If an orbiting artificial satellite were to slow down, what would happen?

4) When a car is driving up a hill, is the friction between the tires and the ground static friction or kinetic friction?

5) What is the physics involved in skydiving?

6) Do sub-atomic particles obey Newtons Laws of motion?

7) Can energy be converted back into mass?

8) The tires of airplanes (at least the big ones) are inflated by nitrogen (instead of air). Why is this done?

9) Where is the force of gravity stronger, on the top of Mt. Everest or at sea level?

How does an image change in a 3D hologram depending on angle of viewing?

 A Hologram is made by taking a single coherent beam, usually from a LASER, and splitting it into two beams.

One of the beams called a reference beam, directly hits a photographic plate where as other is reflected off an object (whose image needs to be stored in the Hologram). The interference pattern of these two beams is stored as Hologram.

When light is focussed on this Holographic plate, it reflects off the plate, but after mixing with the stored pattern.

 So if the original light beam that was used is directed at the corrected angle, it cancels out the component corresponding to reference beam and we see the object.

However, in a typical room, Light hits hologram from all angles and is also reflected back in all angles. Hence, there will be a beam that will fall on plate at same angle as that of reference and reflect the image of the object. If our eye happens to be in the path of the reflected beam then we can see the stored object.

If two interference patterns were stored simultaneously with different reference beam, we can see two different images depending on the angle of viewing. Because the light reaches each eye is not exactly the same, the 3D effect or perception of depth is produced.