Priyam Study Centre

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Sep 9, 2018

Radioactivity

Radioactivity:

The Beginning

The history of atomic chronicle must also induce another great discovery, namely ,the Phenomenon of Radioactivity. In 1896 the French scientist Becquerel, while investigating the nature of the mysterious X -rays discovered by Rontgen a few month earlier, found that a photographic plate wrapped in thick black paper was affected by a sample of potassium - uranyl - sulphate placed over it. In fact, any uranium compound would be effect the plate through covered by paper and kept away from light. The oblivious conclusion that some radiations emanating from the uranium compound could penetrate through the cover and attack the photographic plate. This penetrating radiation had its source in uranium itself and Becquerel christened this amazing behavior as Radioactivity. The properties of this radiations were very similar to those of X -rays.
They were highly penetrating, they effected photographic plates, they would ionize gases and would also induce the fluorescence in some substances. 
The rays are not influenced by heat, light or chemical composition.

Discovery of Radioactive Element:

Marie Curie found tat the activity of mineral pitchblende was far greater than what was expected of its uranium content. 
In 1898 Pierre and Marie Curie actually isolated two new elements Polonium and Radium which were more radioactive compared to uranium, the heaviest atom known at the time.
In 1900, Debierne and Giesel discovered actinium which was also radioactive. 
That the radioactive effects were essentially atomic was recognized early and this helps the isolation to a considerable extent.It was immaterial how uranium and radium chemically combined. The same number of radium atoms will always have the same activity independent of the physical state or the environmental conditions. The phenomenon of radioactivity is associated with atoms which are haviour than lead or bismuth.
The phenomenon of emission of radiation as a result of spontaneous disintegration in atomic nuclei was termed as radioactivity.

The Nature of Radiation :


The radiations emitted by naturally radioactive elements were shown to split by an electric or magnetic field into three distinct parts: Alpha(ɑ), Beta(β), Gamma(ɣ) Rays.

Alpha(ɑ) Rays:


These consist a stream of positively charged particle which carry +2 charge and have mass number is 4. These particles shown by Rutherford to be identical with, the nuclei of helium atom, that is, these are doubly charged helium ion Heᐩ²(atomic number 2, mass number 4).

When an Alpha particle ejected from within the nucleus the mother element loss two units of atomic number and four units of mass number.

Example:
Radioactivity Definition and Discovery  and Discovery of Alpha, Beta, Gama and Positron.
Emission of ∝ Particle

Beta Rays:

These are made up of a stream of negatively charged particles (beta particles). They have been shown to be identical with electrons from a study of their behavior in electric and magnetic fields and from the study of their e/m values (1.77 × 10⁸ coulomb/gm). The ejection of a beta particle (Charge -1, mass 0) results from the transformation of a neutron ( mass 1, charge 0) somewhere at the surface of the nucleus into a proton ( mass 1, Charge +1).
₀n¹(Neutron) ₁H¹(proton) + ₋₁e⁰(β - Particle)

When a beta particle is emitted from the nucleus, the daughter element nucleus has an atomic number one unit greater than that of the mother element nucleus.
₉₀Th²³⁴ ₉₁Pa²³⁴ + ₋₁e⁰

Although beta particles and electrons are identical in their electrical nature and charge/mass ratio, there is a fundamental difference between them.


Ejection of an electron from an atom converts a neutral atom into a positively charged ion but leaves the nucleus undisturbed.

Ejection of a beta particle changes the very composition of the nucleus and produces an atom of the next higher atomic number. 

Neutrino:
Breaking down of a neutron into a proton and a beta particle creates a problem with the principle of conservation of angular momentum. Particles like Neutron, Proton and Electron have the spin angular momentum of ±1/2 (h/2π) each. It is Thus seen that the equation :
₀n¹ ₁H¹ + ₋₁e⁰
is not balanced insofar as angular momentum is concerned. If the angular momentum of the proton and the electron are +1/2 (h/2π) they are exceed the angular momentum of the neutron. If they oppose each other then the momentum become zero in violation of that of the neutron. Pauli therefore postulated that along with the ejected beta particle another tiny neutral particle called neutrino is also ejected. This neutrino has also spin angular momentum of ±1/2 (h/2π) . The sum of angular momentum of the particles ejected {say +1/2 (h/2π) for proton, -1/2 (h/2π) for the electron and +1/2 (h/2π) for the neutrino} may now be +1/2 (h/2π) being the same as that of the neutron. the mass of the neutrino is around 0.00002 with respect to oxygen scale. Ejection of an electron from within the nucleus should be represented as:
Neutron Proton + Electron + Neutrino

Gamma Rays:
These consist of electromagnetic radiation of very short wave length (λ ∼ 0.005 - 1A). These are high energy photons. The emission of gamma rays accompanies all nuclear reactions. During all nuclear reactions there occurs a change in the energy of the nucleus due to emission of alpha or beta particles. The unstable, excited nucleus resulting from the emission of an alpha or beta particle gives off a photon and drops a lower and more stable energy state.
Gamma rays do not carry charge or mass, and hence emission of these rays cannot change the mass number or atomic of the mother nucleus.

Positrons:
Since the works of the Curies and Rutherford yet another mode of nuclear transformation has been discovered. This involves the ejection of a positron ₊₁e⁰ from within the nucleus. This ejection is made possible by the conversion of a proton into neutron.
H¹ n¹ + ₊₁e ₅₁Sb¹²⁰ ₅₀Sn¹²⁰ + ₊₁e
The ejection of positron lowers the atomic number one unit but leaves the mass number unchanged.

Characteristics of Alpha, Beta and Gamma Rays:
Alpha Beta Gamma
Charge :  Absolute 3.1 × 10⁻¹⁹ 1.6 × 10⁻¹⁹ Zero
Relative to Electron Charge +2e -1e Zero
Mass :  Absolute 6.6 × 10⁻²⁷ Kg 9.1 × 10⁻³¹ Kg Zero
Relative to Hydrogen 4 mH 1/1840 mH Zero
Velocity  :  (1.5 to 2.0) × 10⁷ m/sec 10⁸ m/sec 3 × 10⁸ m/sec
Penetrating Power  Weak Medium High
Weak: 1/10 mm Al or 3 to 8 cm air.
Medium: 5 mm Al or 1 mm Lead.
High: 25 cm Iron or 8 cm Lead
Disintegration of Radioactive Elements:

We have just seen that the radioactive elements continue to undergoes successive disintegration till the daughter elements becomes stable, non radioactive isotope of the element Lead.

Radioactivity Definition and Discovery  and Discovery of Alpha, Beta, Gama and Positron.
Disintegration Series of Radioactive Elements