Nuclear force

Substituting v = E/B

m = BB qR E

′

Ions with different masses trace semi-circular paths of different radii and produce dark lines on the plate. The distance between the opening of the chamber and the position of the dark line gives the diameter 2R from which radius R can be calculated.

Since q, B, B′, E and R are known, the mass of the positive ions and hence isotopic masses can be calculated.

8.3 Nuclear force

The nucleus of an atom consists of positively charged protons and uncharged neutrons. According to Coulomb’s law, protons must repel each other with a very large force, because they are close to each other and hence the nucleus must be broken into pieces. But this does not happen. It means that, there is some other force in the nucleus which overcomes the electrostatic repulsion between positively charged protons and binds the protons and neutrons inside the nucleus. This force is called nuclear force.

(i) Nuclear force is charge independent. It is the same for all the three types of pairs of nucleons (n−n), (p−p) and (n−p). This shows that nuclear force is not electrostatic in nature.

(ii) Nuclear force is the strongest known force in nature.

(iii) Nuclear force is not a gravitational force. Nuclear force is about 10⁴⁰ times stronger than the gravitational force.

(iv) Nuclear force is a short range force. It is very strong between two nucleons which are less than 10⁻¹⁵ m apart and is almost negligible at a distance greater than this. On the other hand electrostatic, magnetic and gravitational forces are long range forces that can be felt easily.

Yukawa suggested that the nuclear force existing between any two nucleons may be due to the continuous exchange of particles called mesons, just as photons, the exchange particle in electromagnetic interactions.

However, the present view is that the nuclear force that binds the protons and neutrons is not a fundamental force of nature but it is secondary.

8.4 Radioactivity

The phenomenon of radioactivity was discovered by Henri Becquerel in 1896. He found that a photographic plate wrapped in a black paper was affected by certain penetrating radiations emitted by uranium salt. Rutherford showed later that the radiations from the salt were capable of ionising a gas. The current produced due to the ions was taken as a measure of activity of the compound.

A few years later Madame Marie Curie and her husband Piere Curie discovered the highly radioactive elements radium and polonium.

The activity of the material has been shown to be the result of the three different kinds of radiations, α, β and γ.

The phenomenon of spontaneous emission of highly penetrating radiations such as α, β and γ rays by heavy elements having atomic number greater than 82 is called radioactivity and the substances which emit these radiations are called radioactive elements.

The radioactive phenomenon is spontaneous and is unaffected by any external agent like temperature, pressure, electric and magnetic fields etc.

8.4.1 Alpha, beta and gamma rays The existence of the three distinct types of radiations, α, β and γ−rays can be easily found by the following experiment. A small amount of radium (R) is placed at the bottom of a small hole drilled in a lead block, which is kept in an evacuated chamber (Fig. 8.3).

A photographic plate is placed at a short distance above the lead block. A strong magnetic field is applied

at right angles to the plane of the paper and acting inwards.

Three distinct traces can be seen on the photographic plate when it is developed. The trace towards left is due to positively charged particles.

They are named α-particles.The trace towards the right is due to negatively charged particles. They are named β-particles. The undeviated trace is due to neutral radiations which are called γ−rays.

If an electric field is applied, the α-rays are deflected towards the negative plate, β−rays towards the positive plate and γ−rays are not deflected.

Properties of ααααα–rays

(i) An α - particle is a helium nucleus consisting of two protons and two neutrons. It carries two units of positive charge.

(ii) They move along straight lines with high velocities.

(iii) They are deflected by electric and magnetic fields.

(iv) They produce intense ionisation in the gas through which they pass. The ionising power is 100 times greater than that of β-rays and 10,000 times greater than that of γ−rays.

(v) They affect photographic plates.

(vi) They are scattered by heavy elements like gold.

(vii) They produce fluorescence when they fall on substances like zinc sulphide or barium platinocyanide.

Fig 8.3 Radioactivity Photographic plate

Vacuum P Pump

R

LeadBlock B inwards

Properties of βββββ – rays

(i) β–particles carry one unit of negative charge and mass equal to that of electron.Therefore, they are nothing but electrons.

(ii) The β–particles emitted from a source have velocities over the range of 0.3 c to 0.99 c, where c is the velocity of light.

(iii) They are deflected by electric and magnetic fields.

(iv) The ionisation power is comparatively low (v) They affect photographic plates.

(vi) They penetrate through thin metal foils and their penetrating power is greater than that of α−rays

(vii) They produce fluorescence when they fall on substances like barium platinocyanide.

Properties of γ γ γ γ γ – rays

(i) They are electromagnetic waves of very short wavelength.

(ii) They are not deflected by electric and magnetic fields.

(iii) They travel with the velocity of light.

(iv) They produce very less ionisation.

(v) They affect photographic plates.

(vi) They have a very high penetrating power, greater than that of β-rays.

(vii) They produce fluorescence.

(viii) They are diffracted by crystals in the same way like X−rays are diffracted.