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Friday, August 31, 2012

Discovery of X-rays

Wilhelm Roentgen was professor of physics at university of Wurzburg, Germany when he discovered X-rays in 1985. The discovery was entirely serendipitous; Roentgen was merely studying a beam of electrons in a highly evacuated glass vessel. When the electrons, moving at great speed slammed into glass wall, they produced a very high penetrating radiation - a wholly unexpected occurrence. Roentgen first noticed the radiation when it caused a paper coated with Barium Platino-cyanide to glow. The chemical compound was a standard detector of UV light which causes the chemical to fluorescence i.e. to emit visible light after it has absorbed UV light. But Roentgen's evacuated vessel was tightly covered with black cardboard and so no UV light could emerge from it. The glow must be some other kind of radiation.

When he announced the discovery of the new radiation, Roentgen wrote:

"I posesss, for instance, photographs of ............the shadow of bones of hand, the shadow of a covered wire enclosed in a box.........."

Earlier in the paper, he noted that "the darker shadow of bones is seen with in the slightly dark shadow image of hand itself.

The new radiation quickly became a diagnostic tool in hospitals all over the world. Roentgen could not determine what the rays are made of and thus rays are named as X-rays.       

Thursday, August 30, 2012

How is Neutron discovered?

In 1930, Bothe & Becker bombarded Lithium, Beryllium and Boron with alpha particles from Polonium and found a very penetrating but non-ionizing radiation, they assumed that the radaition was of gamma rays type because of its high penetration.

While repeating these experiments in 1932, Dr. F. Joliot and his wife, Dr. Irene Curie Joliot found that when a sheet of Hydrogen containing material, particularly paraffin, was interposed in the path of these radiations, Protons were ejected with a considerable velocity.

From the ranges of these recoil protons, the maximum proton energy 'E' proved to be about 5.3 MeV. Assuming that the protons were produced as the result of elastic collisions with the gamma ray photons, calculations showed that each photon must have possessed an amount of energy about 52MeV. These results were entirely inconsistent with the results from experiments on absorption of these in lead(about 7 MeV).

Chadwick, in England in 1932, after performing a series of measurements of energies of recoil of Protons ejected from thin targets by the penetrating "Be-radiation", with a pulse ionizing chamber and amplifier. He said, "these results are very difficult to explain on the assumption that the radiation from Beryllium is a quantum radiation, if energy and momentum are to be conserved in the collisions. These difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0 later named as Neutrons.          

These neutrons were formed as a result of highly exoergic nuclear reactions.

Wednesday, August 29, 2012

Three stage Indian Nuclear program


The three stages of Indian Nuclear Program

STAGE-1:
Construction of pressurised heavy water reactors. These reactors use natural Uranium. Spent fuel from these reactors is reprocessed to obtain Plutonium.

STAGE-II:
Construction of fast breeder reactors fuelled by Plutonium produced in stage-I. These reactors would also breed U-233 from Thorium.

STAGE-III:
Construction of power reactors using U-233/Thorium as fuel.

[click to enlarge]

Sunday, August 19, 2012

What are Isomers?

In earlier days, if two half lives were observed in a given sample, it was assumed that two different isotopes were present, each decaying with particular half life.

One interesting example is isotope of Protactinium, Z=91, A=234 which is formed in beta decay of its parent, Thorium Z = 90, A=234[UX1]. The isotope of Protactinium was found to decay by the emission of beta particles of two distinct half lives, one of 1.18 min, the other of 6.6hr. It was assumed that these two half lives were due to two different isotopes and these were given separate names UX2 and UZ respectively.

In 1921, "Hahn" showed that these  two substances form a pair of Nuclear Isomers; i.e they are different energy states of same nucleus.

"Feather" and "Bretscher" later showed that these nuclear isomers are genetically realted; i.e. one type of nucleus is formed from other.


The nucleus called UX2 is an isomeric state of Pa-234 at an energy of 0.394MeV above ground state called UZ.

The nucleus may decay by beta particle emission directly from isomeric state of higher energy with a half life of 1.18 min or it may first emit a gamma ray photon of 0.394MeV, going to ground state of Pa, and then undergo beta decay to U-234 with a half life of 6.66 hours.

An isomeric state differs from ordinary excited state of a nucleus in that it lasts for measurable time.

Thus,

" Isomers are atoms which have same atomic number and mass number but differ from one another in their nuclear energy states and exhibit differences in their internal structure. These nuclei are distinguished by their different life times". 
 

Thursday, August 16, 2012

Determination of Avogadro's number

Quantity of any substance whose mass, in grams, is numerically equal to its molecular weight is called a mole.

The volume occupied by a mole of any gas is called gram molecular volume. At 0oC and 76 cm pressure the gram molecular volume of any gas is 22.4 liters.

On the basis of Avogadro's hypothesis, every mole of a substance contains the same number of molecules. This number is referred to as Avogadro number.

Determination of Avogadro's number:

The behavior of electrolytic cells can be summarized in terms of two laws formulated by "Faraday".

First law:

It states that the quantity of any substance liberated from the solution depends only on the total charge passing through the circuit,

M = KQ; --------------------------------(1)

where 'M' is mass of material liberated at one electrode 
         'Q' is quantity of charge transferred
         'K' is factor of proportionality called electrochemical equivalent of the substance. It is mass   liberated per unit charge transferred, usually expressed in grams per coulomb.

Second law:

For any substance, the mass liberated by the transfer of quantity of electric charge 'Q' is proportional to chemical equivalent of substance,

M = (A/V) *(1/F)* Q  ------------------------(2)

where (A/V)  is the ratio of atomic mass to the valence of element, is the chemical equivalent of the element and 'F' is a constant of proportionality known as Faraday's constant.

From equations (1) & (2) it could be noted that

F = A/KV -------------------(3)

The value of 'F' can be determined from the results of experiments on electrolysis.

For case of silver, where K=0.0011180 grams/coulomb, A = 107.88 gms/gram atomic mass and 'V' is unity; we get

F = 96,500 Coulombs.gram atomic mass.

Thus the transfer of 96,500 coulomb of charge will deposit a gram atomic mass of a monovalent element. Since the valency of silver is unity, for every atom of silver deposited on the cathode, a charge equivalent to one electron has been transferred through the solution.

If 'e' is charge of one electron, then N*e is the total charge transferred when one gram atomic mass of silver is deposited on cathode.

F = N*e = 96,500 Coulombs/gram-atomic mass

hence N = 6.022 x 10^23 gms/gram atomic mass.

The first direct determination of Avogadro number was made by "Perrin" in 1908 in an investigation of motion and distribution of very small particles suspended in a fluid.