Engineering Chemistry

Lesson 15. Radiation and radioactive materials

15.1 TYPE OF RADIATION:

The radioactive radiation is of three types. Alpha (α), Beta (β) and gamma (γ) differ from each other in the nature and properties. (Figure)

15.1.1 Alpha (α) Rays:

They consist of stream of α- particles. By measurement of their e/m, Rutherford showed that they have a mass of 4 amu and charge of +2. They are Helium nuclei and may be represented as  ⁴₂α or ⁴₂He. Tα  particles are ejected from radioactive nuclei with very high velocity, about 16000 km/S. because of their charge and relatively large size  α – particles have very little power of penetration through matter. They are stopped by sheet of paper, 0.01 mm thick Al foil or few centimeter of air. They cause intense ionization of a gas through which they passes. On account of their high velocity and attraction for electrons, α particle break away electron from gas molecules and convert them into positive ions.

15.1.2 Beta (β) Rays:

They are stream of β particles emitted by the nucleus from their deflection in electric and magnetic fields. Becquerel shows that β particles are identical with electrons. They have very small mass and charge of -1. A β particle is symbolized as  ⁰_-1 β or  ⁰_-1e. The velocity of β particles is around 160000 to 240000 km/S. their ionizing power is weak in comparison to α particle. β particles have 100 times more penetrating power as comparison to α - particles. This is because of their higher velocity and negligible mass. β particles can be stopped by about 1 cm thick sheet of Al or 1 meter of air.

15.1.3 Gamma (γ) Rays:

Unlike x and B rays they do not consist of particles of matters. γ rays are a form of electromagnetic radiation of shorter wavelength than X – rays. They could be thought of as high energy photon released by the nucleus during X and B emission. They have no mass or charge and may be symbolized as ⁰₀γ. γ rays travel with the velocity of light. Their ionization power is weak in comparison to α and β particles, because of their high velocity and non material nature γ Rays are most penetrating. They cannot be stopped by even by a 5 cm thick sheet of Lead.

 

15.2 DETECTION AND MEASUREMENT OF RADIOACTIVITY:

The radioactive radiation can be detected by numbers of methods. Few of them are Cloud Chamber, Bubbled chamber, Geiger –Muller Counter, Ionization chamber method and so on.

15.2.1 Cloud Chamber:

The chamber contains air saturated with water vapor. When the piston is lowered suddenly the gas expands and is super cooled. As  β particles passes through the gas, ions are created along its path these ions provides nuclei upon which droplet of water condensed. The trail of cloud thus produced, marks the track of the particles. The track can be seen through the window above an immidiatly photographed. Similarly  α or β particles form a trail of bubbles as they pass through liquid Hydrogen. The bubble chamber method gives batter photographs of the particles tracks. 

(FIGURE)

15.2.2 Geiger Muler Counte:

It consists of cylindrical metal tube (Cathode) and central wire (anode). The tube is filled with Argon gas at reduced pressure (0.1 atm). A potential difference of about 1000 volts is applied across the electrode. When an  α or β particles enters the tube through mica window, it ionizes the argon atom along its path. The Argon ion (Ar⁺) is drown to the cathode and electron to the anode. Thus for a fraction of seconds a pulse of electric current flows through the electrodes and completes the circuit around. Each electrical pulse marks the entry of one α or β particle in to the tube and is recorded in an automatic counter.  The number of such pulses registered by a radioactive material per min. gives the intensity of its radioactivity.

(FIGURE)

15.2.3 Ionization Chamber:

This is the simplest device used to measure the strength of radiation. An ionization chamber is fitted with two plates separated by air. When radiation passes through this chamber, it knocks electrons from gas molecules and positive ions are produced or formed. The freed electron migrates to the anode and positive ions to the cathodes. Thus a small current passes between the plates. This current can be measured with an ammeter and gives the strength of radiation that passes through the ionization chamber. In an ionization camber called Dosimeter, the total amount of electric charge passing between the plates in a given time is measured. This is proportional to the total amount of radiation that has gone through the chamber.

(FIGURE)

 

15.3 RADIOACTIVE DECAY:

According to the theory of Rutherford and Soddy, radioactivity is a nuclear property. The nucleus of a radioactive atom is unstable. It undergoes decay or disintrigration by spontaneous emission of an or β particle. This results in the change of proton neutron compassion of the nucleolus to form a more stable nucleolus. The original nucleolus is called parent the nucleolus and product is called daughter nucleolus.

                                          Uranium ²³⁸ ₉₂U → Thorium  ²³⁴ ₉₀ Th

There are two main type of decay: (i) α  decay   (ii)β decay

15.3.1 α Decay:

When a radioactive nucleolus decays by the emission of an  α particle (α emission) from the nucleus, the process is termed α decay. An α particle has four units of atomic mass and two units of positive charge. If Z be the atomic number and M be the atomic mass of the parent nucleus, the daughter nucleus will have

atomic mass  = M- 4

Atomic number   =  Z-2

Thus an α emission reduces the atomic mass by 4 and atomic number by 2.

For example Radium decay by  α emission to form a new element Radon

 ²²⁶₈₂Ra   → ²²²₈₆Rn

 

15.3.2 β Decay:

When a radioactive nucleus decays by β particle emission ( β emission), it is called β decay. A free β particle or electron does not exist as such in the nucleus. It is produced by the conversion of the neutron to a proton at the moment of the emission.

       =   P    +     e^-

This results in the increase of one positive charge on the nucleus. The loss of a  β particle from the nucleus dose not altered its atomic mass. For a parent nucleus with atomic mass M and atomic number Z, the daughter nucleus will have

Atomic mass = M

Atomic number = Z + 1

 Thus a β emission increases the atomic number by 1 with no change in atomic mass.

An example of B decay is the conversion of Lead (214) to Bismuth (214).

                                                 ²¹⁴₈₂Pb   -   ⁰_-1β  → ²¹⁴₈₃Bi

The Group Displacement Law:

It was stated by Fajans and Soddy in 1913, the position number of an element in a group of the periodic table corresponds to its atomic number. If the atomic number of a given element is changed, its group also changes accordingly. We know that an α particle emission decreases the atomic number of the parent element by 2 and a  β particle emission increases the atomic number by 1. Thus in an α emission, the parent element will be displaced to a group two places to the left and in a β emmision, it will be displaced to a group one place to the right.

This is called the group displacement law a also known as Fajans – Soddy Displacement Law.