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ATOMIC AND NUCLEAR PHYSICS

Atomic Physics:
      Atom is the smallest part of matter which takes part in chemical reactions. Atoms of the same element are similar in mass, size and characteristics. Atom consists of three fundamental particles electron, proton and neutron. All the protons and neutrons are present in the central core of atom called nucleus. Electrons revolve around the nucleus.
     The word ‘atom’ has been derived from the Greek word ‘a-tomio’ which means ‘uncutable’ or ‘non-divisible’. In an atom, electrons and protons are equal in number and have equal and opposite charge. Hence atom is neutral.
   The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school teacher in 1808. His theory, called Dalton’s atomic theory, regarded the atom as the ultimate particle of matter.

 

Sub atomic particles:
     Dalton’s atomic theory was able to explain the law of conservation of mass, law of constant composition and law of multiple proportion very successfully. However, it failed to explain the results of many experiments, for example, it was known that substances like glass or ebonite when rubbed with silk or fur generate electricity. Many different kinds of sub-atomic particles were discovered in the twentieth century.

 

Electron:
       In 1897, Electron was discovered by J.J.Thomson. He was discovered electron by using Cathode ray tube experiment. The result of this experiment is
(i) The cathode rays start from cathode and move towards the anode.
(ii) These rays themselves are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them. Television picture tubes are cathode ray tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials.
(iii) In the absence of electrical or magnetic field, these rays travel in straight lines.
(iv) In the presence of electrical or magnetic field, the behaviour of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons.
(v) The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube.

 

Characteristics of an Electron:
      Electron is a negatively charged particles having very small mass. Mass and charge of an electron are given below.
i. Mass of an electron (me): The mass of a electron is about 1/1840 times that of a hydrogen atom. Its absolute mass is,
       me =  9.108 × 10-31kg = 9.108 × 10-28g
     It is a very light particle, and therefore, it makes very little contribution to the total mass of the atom in which it is contained.
ii. Charge on an electron (e): An electron possess one unit negative charge, it has been found to be the smallest negative charge that any particle can carry.
     Charge on an electron, e = - 1.602× 10-19 Coulombs.

 

Wave Properties:
     In 1924, the wave-particle dualism was postulated by de Broglie (Nobel Prize 1929). All moving matter has wave properties, with the wavelength λ being related to the momentum p by
        λ= h / p = h / mv
where h : Planck constant
            m : mass
            v : velocity
    This equation is of fundamental importance for electron microscopy because this means that accelerated electrons act not only as particles but as waves too. Consequently, the wave length of moving electrons can be calculated from this equation taking their energy E into consideration. An electron accelerated in an electric field V gains an energy E = eV which further corresponds to a kinetic energy Ekin = mv2/2.
Thus: E = eV = m0v2 / 2.
From this, the velocity v of the electron can be derived:
       

where V : acceleration voltage
            e : electron charge= -1.602176487 × 10-19 C
            m0 : rest mass of the electron = 9.109 × 10-31 kg
    It follows for the momentum p of the electron:
              
Now, the wavelength λ can be calculated from the de Broglie equation according to       

     
 

Neutron:
       In 1932, James Chadwick, an English physicist who had worked with Rutherford, detected neutrons and measured their mass in an invisible game of billiards. He fired the neutrons at a block of paraffin wax, which has a high concentration of hydrogen and is therefore rich in protons. Some of the neutrons collided with protons in the wax and knocked them out. Chadwick could then detect these protons and measure their energy. Using his knowledge of energy and momentum, he was able to work out the mass of the neutrons from the range of energies of the protons that they knocked out. He found that its mass was slightly more than that of a proton.
      The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons, each with mass approximately one atomic mass unit, constitute the nucleus of an atom, and they are collectively referred to as nucleons. The nucleus consists of N neutrons, where N is the neutron number.

 

Characteristics of Neutron:
    Neutrons are electrically neutral but have a spin, or magnetic moment, so they are sensitive to magnetic sources in condensed matter and can provide images of magnetic structure. Neutrons have the ability to deeply penetrate matter, they interact with nuclei and neutrons – unlike X-rays – can distinguish light elements such as hydrogen.

Charge: It is a neutral particle because it has no charge.
Mass of neutron: Mass of neutron is 1.0086654 a.m.u. Or 1.6749 × 10-27 kg.
Comparative mass: Neutron is 1842 times heavier than an electron.
Location in the atom: Neutrons are present in the nucleus of an atom.
    Mainly Neutron has five characteristics.

 

a. Electrically neutral, they can go deep into matter:
     Neutrons are non destructive and can penetrate deep into matter. This makes them an ideal probe for biological materials and samples under extreme conditions of pressure, temperature, magnetic field or within chemical reaction vessels.

 

b. Microscopically magnetic, they can show magnetism
     Because they possess a magnetic dipole moment, neutrons are sensitive to magnetic fields generated by unpaired electrons in materials, and they can be used to analyze the magnetic properties of materials, at the atomic scale. In addition, the scattering power off an atomic nucleus depends on the orientation of the neutron and the spin of the atomic nuclei in a sample. This makes the neutron a powerful instrument of detecting the nuclear spin order.

 

c. Their Angstrom wavelengths can show structure
     Neutron wavelengths range from 0.1A to 1000A, which is comparable to the distance of neighboring atoms in solid matter. This makes them an ideal probe of atomic and molecular structure, be they single atomic species or complex, biopolymers. Like water waves at a barrier, neutrons are diffracted by the ordered atoms of a sample. The neutron diffraction angle is a sensitive measure for the distance of the atoms within the sample and therefore can give us information on the localization of atoms.

 

d. Their energies of millielectronvolts can show motion
     The energies of neutrons are of the same magnitude as the diffusive motion in solids and liquids, the coherent waves in single crystals (phonons and magnons), and the vibrational modes in molecules. It is easy to detect any exchange of energy between a sample of between 1microeV (even 1 neV with spin-echo) and 1eV and an incoming neutron.

 

e. Randomly sensitive, they can show hydrogen atoms
     With neutrons, the variation in scattering power from one nucleus to another within a sample varies in a quasi-random manner. This means that lighter atoms are visible despite the presence of heavier atoms, and neighbouring atoms may be distinguished from each other.
In addition, contrast can be varied in certain samples using isotopic substitution (for example D for H, or one nickel isotope for another); specific structural features can thus be highlighted. The neutron is particularly sensitive to hydrogen atoms; it is therefore a powerful probe of hydrogen storage materials, organic molecular materials, and biomolecular samples or polymers.

 

Properties of Neutron:
Electric charge:
    The total electric charge of the neutron is 0 e. This zero value has been tested experimentally, and the present experimental limit for the charge of the neutron is
−2(8) × 10−22 e, or −3(13) × 10−41 C. This value is consistent with zero, given the experimental uncertainties (indicated in parentheses). By comparison, the charge of the proton is, of course, +1e.

 

Electric dipole moment:
    The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanent electric dipole moment. The predicted value is, however, well below the current sensitivity of experiments. From several unsolved puzzles in particle physics, it is clear that the Standard Model is not the final and full description of all particles and their interactions. New theories going beyond the Standard Model generally lead to much larger predictions for the electric dipole moment of the neutron.

 

Magnetic moment:
    Even though the neutron is a neutral particle, the magnetic moment of a neutron is not zero. Since the neutron is a neutral particle, it is not affected by electric fields, but with its magnetic moment it is affected by magnetic fields. The magnetic moment of the neutron is an indication of its quark substructure and internal charge distribution. The value for the neutron's magnetic moment was first directly measured by Luis Alvarez and Felix Bloch at Berkeley, California in 1940, using an extension of the magnetic resonance methods developed by Rabi. Alvarez and Bloch determined the magnetic moment of the neutron to be μn = −1.93(2) μN, where μN is the nuclear magneton.

 

Mass:
    The mass of a neutron cannot be directly determined by mass spectrometry due to lack of electric charge. However, since the mass of protons and deuterons can be measured by mass spectrometry, the mass of a neutron can be deduced by subtracting proton mass from deuteron mass, with the difference being the mass of the neutron plus the binding energy of deuterium (expressed as a positive emitted energy). The latter can be directly measured by measuring the energy (B_d) of the single 0.7822 MeV gamma photon emitted when neutrons are captured by protons (this is exothermic and happens with zero-energy neutrons), plus the small recoil kinetic energy (E_{rd}) of the deuteron (about 0.06% of the total energy).
        mn = md - mp + Bd - Erd
     The energy of the gamma ray can be measured to high precision by X-ray diffraction techniques, as was first done by Bell and Elliot in 1948. The best modern (1986) values for neutron mass by this technique are provided by Greene, etc.These give a neutron mass of:
     mneutron = 1.008644904(14) u
     The value for the neutron mass in MeV is less accurately known, due to less accuracy in the known conversion of u to MeV:
     mneutron = 939.56563(28) MeV/c2.
     Another method to determine the mass of a neutron starts from the beta decay of the neutron, when the momenta of the resulting proton and electron are measured.

 

Anti-neutron
      The antineutron is the antiparticle of the neutron. It was discovered by Bruce Cork in the year 1956, a year after the antiproton was discovered. CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles, so studying antineutrons yields provide stringent tests on CPT-symmetry. The fractional difference in the masses of the neutron and antineutron is (9 ± 6) × 10−5. Since the difference is only about two standard deviations away from zero, this does not give any convincing evidence of CPT-violation.

 

Proton:
      In 1911 Ernest Rutherford who performed many experiments to explore radioactivity did an experiment in which he discovered that the atom must have a concentrated positive center charge that contains most of the atom's mass. He suggested that the nucleus contained a particle with a positive charge the proton. Atoms of different elements have different numbers of protons giving their nuclei different charges. That meant the hydrogen nucleus (it has one proton) was an elementary particle. Rutherford named it the proton, from the Greek word "protos," meaning "first."
Characteristics of proton:
Charge: Proton is a positively charged particle.
Magnitude of charge: Charge of proton is 1.6022 × 10-19 coulomb.
Mass of proton: Mass of proton is 1.0072766 a.m.u. Or 1.6726 × 10-27 kg.
Comparative mass: Proton is 1837 times heavier than an electron.
Position in atom: Protons are present in the nucleus of atom.
* Proton is a positively charged particle having an absolute charge of + 1.6 × 10-19 coulombs. Since this is the smallest positive charge carried by a particle, so this is taken as the unit of positive charge and we say that the relative charge of a proton is + 1. The mass of a proton is about 1836 times that of an electron.
* In other words, the mass of a proton is equal to the mass of 1836 electrons. The relative mass of a proton is 1 a.m.u. (1 atomic mass unit) which is the same as that of a hydrogen atom, the atom of lowest mass. The absolute mass of a proton is, however, 1.6 × 10-24 gram. Each element has a fixed number of protons in the nucleus of its atom, which is characteristic of that element. It is called atomic number of the element. No two elements can have the same number of protons in their atoms. It is due to the presence of protons that nucleus of an atom is positively charged.

 

Properties of Subatomic Particle:


 

Several Sub-atomic particles are discovered:


 

Cathode Rays:
     A cathode ray is a beam of electrons that travel from the negatively charged to positively charged end of a vacuum tube, across a voltage difference between the electrodes placed at each end. The electrode at the negative end is called a cathode; the electrode at the positive end is called an anode. Since electrons are repelled by the negative charge, the cathode is seen as the "source" of the cathode ray in the vacuum chamber.
    If the gas pressure in a discharge tube is 10-2 to 10-3 mm of Hg and a potential difference of 104 volt is applied between the electrode, then a beam of electrons emerges from the cathode which is called cathode rays. Hence cathode rays are beam of high energy electrons. Cathode is an electrode with a negative charge.

 

Properties of Cathode rays:
* Cathode rays are invisible and travel in straight line.
* These rays carry negative charge and travel from cathode to anode.
* These rays emerge perpendicular to the cathode surface and are not affected by the position of anode.
* Cathode rays travel with very high velocity.
* The velocity of cathode ray is 1/10th of velocity of light.
* These rays are deflected by electric and magnetic fields.
* These rays can ionize gases.
* These rays heat the material on which they fall.
* They can produce chemical change and thus affect a photographic plate.
* These rays can penetrate through thin metal foils.
* The source of emf used in the production of cathode rays is induction coil.
* When they strike a target of heavy metals such as tungsten, they produce x-rays.
* The nature of cathode rays is independent of nature of cathode and the gas in the discharge tube.

 

Positive or Canal rays:
    If perforated cathode is used in a discharge tube, it is observed that a new type of rays are produced from anode moving towards the cathode and passed through the holes of cathode. These rays are positively charged and are called positive rays or canal rays or anode rays. These rays were discovered by Goldstein.

 

Properties of Canal rays:
* The positive rays consist of positively charged particles.
* These rays travel in straight line.
* These rays can exert pressure and thus possess kinetic energy.
* These rays are deflected by electric and magnetic fields.
* These rays are capable of producing physical and chemical changes.
* These rays can produce ionization in gases.

 

Radioactivity:
* Radioactivity is the sending out of harmful radiation or particles, caused when atomic nuclei breakup spontaneously.
* Radioactivity was discovered by Henry Becquerel, Madame Curie and Pierre Curie for which they jointly win Noble prize.

                          
* The nucleus having protons 83 or more are unstable. They emit and γ particles and become stable. The elements of such nucleus are called radioactive elements and the phenomenon of emission of and γ particles is called radioactivity.
* γ rays are emitted after the emission of  and   rays.
* Robert Pierre and his wife Madame Curie discovered a new radioactive element radium.
* The rays emitted by radioactivity were first recognized by Rutherford.
* The end product of all natural radioactive elements after emission of radioactive rays is lead.


Difference between stable and unstable nucleus


 

Properties of and γ particles:


* With the emission -particle, atomic number is decreased by 2 and mass number is decreased by 4.
* With the emission of a  particle atomic number is increased by one and mass number does not change.
* The effect on the mass number and atomic number with the emission of  and γ rays is decided by Group-displacement law or Soddy-Fajan Law.
* Radioactivity is detected by G.M. Counter.
* The time in which half nuclei of the element is decayed is called half life of the radioactive substance.
* Could chamber: Cloud chamber is used to detect the presence and kinetic energy of radioactive particles. It was discovered by C.R.T Wilson.
* Radioactive carbon-14 is used to measure the age of fossils and plants. In this method age is decided by measuring the ratio of 6C12 and 6C14.

 

Nuclear Fission:
     Nuclear Fission is the process of causing a large nucleus to split into multiple smaller nuclei, releasing energy in the process.
                                                                     OR
    The nuclear reaction in which a heavy nucleus splits into two nuclei of nearly equal mass is nuclear fission. The energy released in the nuclear fission is called nuclear energy.

                        
* It can start when the large nuclei absorbs a neutron, causing it to become unstable to the point that it falls apart.
* This is the reaction that we use in nuclear power plants and early nuclear weapons.
* Fission is relatively easy to do, but also leaves us with lots of nuclear waste that must be stored for thousands of years before it is safe.
* Nuclear fission was first demonstrated by Strassmann and O.Hahn. They found that when U235 nucleus is excited by the capture of a neutron, it splits into two nuclei Ba142 and K92.

 

Chain Reaction:
    When uranium atom is bombarded with slow neutrons, fission takes place. With the fission of each uranium nucleus, on the average 3 neutrons and large energy is released. These neutrons cause further fission. Clearly a chain of fission of uranium nucleus starts which continues till whole of uranium is exhausted. This is called chain raction.
Chain reaction is of the following two types
    i. Uncontrolled chain reaction
    ii. Controlled chain reaction.

 

Uncontrolled Chain Reaction:
     In each fission reaction, three more neutrons are produced. These three neutrons may cause the fission of three other U235 nuclei producing 9 neutrons and so on. As a result the number of neutron goes on increasing till the whole of fissionable material is consumed. This chain reaction is called uncontrolled or explosive chain reaction. This reaction proceeds very quickly and a huge amount of energy is liberated in a short time.
                      

Atom Bomb:
     Atom bomb is based on nuclear fission. U235 and Pu239 are used as fissionable material. This bomb was first used by USA against Japan in Second World War 6th August, 1945 at Hiroshima and 9th August 1945 at Nagasaki.

 

Controlled Chain Reaction:
     A fission chain reaction which proceeds slowly without any explosion and in which the energy released can be controlled is known as controlled reaction. Actually in this situation only one of the neutrons produced in each fission is able to cause further fission. The rate of reaction remains constant.
                     

 

Nuclear Reactor or Atomic Pile:
Nuclear reactor is an arrangement which controlled nuclear fission reaction takes place. First Nuclear reactor was established I Chicago University under the supervision of Prof. Fermi. There are several components of nuclear reactor which are as follows:
Fissionable Fuel: U235 or U239 is used.
Moderator: Moderator decreases the energy of neutrons so that they can be further used for fission reaction. Heavy water and graphite are used as moderator.
Control rod: Rods of cadmium or boron are used to absorb the excess neutrons produced in fission of uranium nucleus so that the chain reaction continues to be controlled.
Coolant: A large amount of heat is produced during fission. Coolant absorbs that heat and prevents excessive rise in the temperature. The coolant may be water, heavy water, or a gas like He or CO2

 

Types of Nuclear Reactors:
i. Light water reactors:
    Of those that use water, the two most common types use a purified form of regular water (H2O), sometimes called “light water”, and are either Boiling Water Reactors (BWRs).

 

ii. Pressurized water Reactor: (PWR)
     In the more common PWR, the water that cools the nuclear fuel is at a higher pressure and does not turn into steam. However, because of the higher pressure, this primary water can reach higher temperatures and is used to convert a secondary water supply into steam and from there to the steam turbine.

 

iii. Heavy water reactor:
      Other reactors use water in which the Hydrogen contains an extra neutron and is called Deuterium (so heavy water is described as D2O). This heavy water is used in various types of Canada Deuterium-Uranium (CANDU) Reactors and allows the nuclear fuel to be in its natural form; that is, it is not enriched. Heavy water coolant passes through the reactor core and removes the heat generated by the fission chain reactions. This heated reactor coolant heats light water and converts it to steam, which drives a turbine-generator to produce electricity.

 

iv. Gas cooled Reactor HTGR:
      Reactors that use gas as a coolant (and to drive a gas turbine) are called High Temperature Gas-Cooled Reactors (HTGRs) and include the Pebble Bed Modular Reactor (PBMR). Gas such as helium or carbon dioxide is passed through the reactor rapidly to cool it.
Due to their low power density, these reactors are seen as promising for using nuclear energy outside of electricity production: in transportation, in industry, and in residential regimes.

 

v. Thorium Reactor:
    The earliest reactor used for electricity generation used Sodium to cool the nuclear fuel. Sodium, a liquid metal, allows the neutrons from the nuclear reaction to move at greater speed and so these reactors are referred to as Fast Reactors. They can also make, or breed, nuclear fuel from materials that are initially non-radioactive, and so these are also called Breeder or Fast Breeder Reactors. The UK is reported to be considering an offer from GE Hitachi to build one of these Integral Fast Reactors, called “PRISM”, to use up the approximately 100 tons of Plutonium currently being stored at many nuclear sites in that country.


Uses of nuclear reactor:
* To produce electrical energy from the energy released during fission.
* To produce different isotopes which can be used in medical, Physical and agriculture science.
Fast Breeder Reactor:
A nuclear reactor which can produce more missile fuel than it consumes is called a fast breeder reactor.
Nuclear Fusion:
    When two or more light nuclei combined together to form a heavier nucleus, tremendous energy is released. This phenomenon is called nuclear fusion.
                                                                        OR
     Nuclear Fusion is a nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy.
A typical example of nuclear fusion is
       1H2 + 1H3  2He4 + 0n1 + 11.6 Mev.
The energy released by sun and other stars is by nuclear fusion.
For the nuclear fusion, a temperature of the order of 108 K is required.

             

Hydrogen Bomb:
    Hydrogen bomb was made by American scientists in 1952. This is based on nuclear fusion. It is 1000 times more powerful than atom bomb.
Mass Energy Relation:
    In 1905 Einstein established a relation between mass and energy on the basis of special theory of relativity. According to this relation, mass can be converted into energy and vice versa, according to the relation
         E = mc2
Where c = velocity of light
             E = energy of equivalent of mass
             M = mass.
* Albert Einstein was an American scientist. He was born in Germany. He was given Nobel prize of physics in 1921.

         
* Sun is continuously emitting energy. Earth is continuously receiving 4 × 1026 joule of energy per second from sun. As a result mass of sun is decreasing at the rate of approximately 4 × 109 kg per second. But mass of sun is so large that it is estimated that the sun will continuously supply energy for next 109 years.

Posted Date : 03-02-2021

 

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