DISCOVERY OF NATURAL RADIOACTIVITY
Like many other great discoveries, the discovery of radioactivity by Henry Becquerel in 1896 was also quite accidental. At that time, Becquerel was engaged in studying the fluorescence in various compounds. It was noticed that a radiation coming from uranium salts even when coated with the thin layers of opaque materials was capable of attacking the photographic plates. Very soon it was established that the intensity of the radiation remained unchanged even in dark and was independent of the exposure to an external light. It also did not depend on the nature of uranium salts but on the presence of uranium content in the samples. Thus, the phenomenon was proved totally different from fluorescence or phosphorescence. This typical radiation emitted from uranium was named as Becquerel rays. Later, the term radioactivity was coined by a Polish student of Becquerel, Marie Sklodowska who became Madam Curie, the recipient of two Nobel prizes in Nuclear Science.
The Becquerel's discovery led the scientists to investigate for the possible radioactive elements occurring in nature. It was found that the radioactivity present in pitchblend, an ore of uranium was much greater than that shown by uranium itself. This observation led the Curie couple, Marie Curie and Pierre Curie to a search for the existence of new radioactive elements present in pitchblende. Finally the couple became successful to isolate two new radioelements, one polonium, named after Marie's native country and the other radium similar to barium. The most interesting aspect of this discovery is that the couple did the task almost without any financial assistance but with the firm belief and dedication. They had only a shed with a simple electroscope designed by Pierre himself for differentiating the active fraction from the rest. They had to take a prolonged painstaking toil to isolate the elements which were not more than a few milligrams in a tonne of the ore. By taking the help of carrier technique, polonium was separated as a sulphide along with bismuth sulphide and radium was crystallised as radium chloride along with barium chloride. Here it is worth mentioning that not the discovery of the radioelements but the way of searching stands as a milestone to all generations in the history of science.
Besides the Curies in France, many laboratories throughout the globe joined to explore the field. The works of Schmidt in Germany; Soddy, Rutherford and Ramsay in England were noteworthy. The phenomenon of spontaneous emission of powerful penetrating radiations (which are not influenced by any external factors) from some heavy elements (not always heavy elements) is called natural radioactivity, and such elements are called naturally occurring radioactive elements.
6.2 TYPES OF RADIOACTIVE EMANATIONS
From the behaviour of the radioactive emanations in a magnetic or electric field, they are found to consist of three types of radiations. (i) α-Rays, a soft radiation consisting of positively charged particles, easily absorbed by matter. (ii) β-Rays* , more penetrating radiation consisting of negatively charged particles. (iii) γ-Rays, a highly penetrating and electrically neutral radiation.
Existence of the three types of radiation was experimentally verified. A small amount of a radioactive material was placed in a cavity of a lead block and a photographic plate was placed at a short distance above the block. The whole arrangement was kept in an airtight chamber. In the absence of any external magnetic field, the radiation attacks the plate at a particular point in a line with the chamber which collimates the radiation into a narrow beam. When a magnetic field is applied in a direction perpendicular to the plane of the figure, three darkened spots instead of a single spot are obtained. Here, one point remains undeviated, while the other two points get deflected in opposite directions depending on their nature of charge. The radiation which remains undeflected is uncharged and called γ-radiation. If the magnetic field is directed from top to the bottom, the positively charged beam (i.e. α-rays) gets deflected to the left side according to Fleming's left hand rule. The radiation consisting of positively charged particles is called α-rays, while the radiation bearing the negatively charged particles is called β-rays. By using an electric field in place of a magnetic field, the α-rays are found to deflect towards the negatively charged plate and the neutral γ-radiation remains undeflected.
PROPERTIES OF ALPHA, BETA AND GAMMA RADIATIONS
6.3.1 Alpha ()-rays
(i) Nature: From the direction of deflection in an electric or a magnetic field, the particles are found to be positively charged. From the deflection, its q/M (i.e. charge to mass ratio) is found to be C/kg. From the experiments of Rutherford and H. Geiger, its charge (q) is found C which is almost double the charge carried by an electron. From these, its mass ( kg) is found to be almost four times that of a hydrogen atom. Thus it appears that an α-particle is a doubly charged helium atom.
The indistinguishability between an α-particle and a doubly charged helium atom was further confirmed experimentally. In the experiment, a small quantity of purified radon gas known to give α-radiations was placed in a thin walled tube which is surrounded by an outer tube called spectrum tube having an arrangement for electric discharge. At the beginning of the experiment, the spectrum tube was made completely evacuated, then after two days, spectroscopic examination of the gas accumulated in the spectrum tube was carried out after passing an electric discharge. It indicated the existence of helium gas accumulated in the outer tube, i.e. spectrum tube. In a separate experiment, it was shown that when helium gas was kept in the inner tube instead of the α-particle radiating material, helium gas could not penetrate the walls of the tube to appear in the spectrum tube, but the α-particles can pass the barrier. Thus the existence of helium in the spectrum tube was due to the penetration of the α-particles coming from radon. Thus the identity of the particles was experimentally confirmed.
In many ores having heavy radioelements, trapped helium gas is found. In clevite (an ore of uranium) and monazite (an ore of thorium), very often helium gas is found trapped. It is due to the fact that the α-particles radiated from the elements accept electrons from the surroundings to form helium gas.
This helium gas gets mechanically trapped in the ores. In the hot spring of Bakreswar in India, the presence of helium gas indicates the existence of α-active radioelements in its source.
(ii) Velocity and range: The initial velocity of the α-particles emitted depends on the nature of the radioelements. It is found to vary in the range . For a particular source, all the particles are of almost the same velocity. The range of the particles, i.e. the distance through which they can travel before to lose their capacity to ionise the medium, depends on the nature of their source. The range of the particles can be easily measured and the knowledge of the range provides more information regarding the nucleus.
(iii) Stopping power of a medium: Though the α-particles can travel several centimetres in air, most of the particles can be stopped by using a thin sheet of aluminium and even by a paper of ordinary thickness. Stopping power generally increases with the increase of atomic weight. During the collision between the α-particles and the atoms of the absorber, the electrons from the atoms are knocked out and thus the energy of the α-particle is lost.
(iv) Energy of the -particles: The kinetic energy of an α-particle again depends on the velocity (u) which varies from source to source. The α-particles obtained from different naturally occurring sources are found in the range of 4 to 10 MeV. However, the energy of the particles can be increased by using accelerators.
(v) Ionisation of air: An α-particle on its path collides with the molecules and atoms and knocks out the electrons to produce the ion electron pairs. In this process, the energy of the α-particle is gradually lost and it falls down to a critical value below which no further ionisation is possible. In Wilson's cloud chamber, the tracks of the α-particles are observed due to their ionising power. The α-tracks are shorter but straight.
(vi) Fluorescence: The α-particles on being incident on ZnS, produce flashes. This phenomenon is utilised in the apparatus, spinthariscope, in which the α-particles strike on a screen coated with a ZnS layer. Each flash is mechanically observed through a lens and counted.
(vii) As projectiles in nuclear reactions: The α-particles are found very much effective as projectiles in carrying out many nuclear reactions.
(viii) Scattering of -particles: The α-particles are scattered when they pass through a thin sheet of gold or silver.
(ix) Attacking the photographic plate: The particles can attack the photographic plates.
6.3.2 Beta ()-Rays
(i) Nature: The β-particles are nothing but high energy electrons. This is supported from its deflection pattern in an external magnetic or electric field. Here it is worth mentioning that the β-particles are always associated with the antineutrinos, as the β-particles and antineutrinos are simultaneously produced in the same reaction: .
(ii) Velocity: The initial velocities of the β-particles are different for different sources. Even for a particular source, all the β-particles are not monoenergic, i.e. of the same energy (difference with the α-particles). In fact, from a given source, the β-particles show a distribution of energy from zero to a certain maximum value, Emax. Actually, the energy is distributed between the antineutrino and the β-particle ejected.
The velocity of the β-particles for a particular source may vary from zero to about that of light but surely less than that of light. Thus when the velocity (u) is very high, the kinetic energy is calculated by considering the relativistic corrections:
; where
In general, the velocities of the β-particles are much higher than those of the α-particles.
(iii) Range: Though the β-particles move with a high velocity, on account of their very small mass, on collision with the molecules or atoms of the absorber, they get easily deflected. This is why, the β-particles very often travel in a zigzag (difference with the α-particles) while the α-particles travel in a more or less straight line because of their high momentum. This is why, it is very difficult to measure the exact length or range of the β-particles.
(iv) Penetrating power: The penetrating power of the β-particles is greater than that of the α-particles. The highly energetic β-particles can penetrate several mm thickness of an aluminium sheet (difference with the α-particles).
(v) Attacking the photographic plate: The β-particles affect the photographic plates more strongly than the α-particles.
(vi) Interaction with matter: (a) Absorption of β-particles by matter: The β-particles may be simply absorbed by the absorbing material. (b) Ionisation of the matter: Like the α-particles, the β-particles can also produce ion pairs on their tracks, but the ionising power is very much less compared to that of the α-particles (difference). (c) Scattering the β-particles: Because of the low momentum, the β-particles may be deflected and scattered by the absorbing material.
(vii) Fluorescence: The β-rays can produce fluorescence on barium platinocyanide, calcium tungstate, willemite, etc.
(viii) Isobaric relationship: The product and parent nuclides are isobaric. This is also true for β+ decay and electron capture process.
6.3.3 Gamma () Rays
(i) Nature: It is electrically neutral. It is an electromagnetic wave and similar to X-rays. It consists of photons and travels with the velocity of light. The wavelength lies in the range 10^−4 to 10^−1 nm. In comparison with the X-rays, it is worth mentioning that the origin of the X-rays is an extra-nuclear phenomenon while for the γ-rays it is an intra-nuclear phenomenon. The gamma-rays are emitted when the nucleus comes down from a higher energy state to some lower energy state. With the emission of α- or β-particles, the product nuclei are very often in excited states, and in returning to the ground state, the γ-rays are emitted. Besides this, in isomeric transitions (IT), the γ-rays are also given out.
(ii) Interaction with matter: This high energy electromagnetic radiation interacts with the electrons bound to the atoms of the interacting matter. They lose a large fraction or total energy in a single encounter. This is why, the term, range is not defined for the γ-rays. For the γ-rays of energy in the range 0.1 to 25 MeV, the following three interactions are important.
(a) Photoelectric effect: This process is important for the γ-photons of low energy. In this process, the total energy of the striking γ-photon is utilised in ejecting an orbital electron from an atom of the interacting matter. The kinetic energy of the ejected photoelectron is determined by the difference of the energy of the incident γ-photon and the binding energy (i.e. work function) of the electron to the atom. This phenomenon has been discussed in detail in. Thus the vacancy caused in the orbital may lead to X-ray generation or Auger effect.
(b) Compton effect: In this process, the striking γ-photon spends a fraction of its energy in a collision with an electron. The photon scatters with a higher wavelength in a new direction while the electron recoils with a higher energy. This new photon with a lower energy may initiate a further photoelectric or Compton effect. For the Compton effect, the γ-rays of medium energy in the range 0.5 to 1.0 MeV are important. This interaction has been discussed in detail in.
(c) Positron-electron pair formation: For the γ-photons of energy greater than 1.0 MeV, this process becomes important. In the region of nucleus of the interacting matter, the energy of the γ-photon gets converted to produce the particles, positron and electron, and the residual energy of the photon imparts kinetic energy to the particles of the pair. The energy is conserved as:
where, m0 stands for the rest mass of the particle, i.e. electron or positron. The threshold energy 1.02 MeV is related as : 1.02 MeV = 2mc^2. This phenomenon, pair formation has been discussed in detail in.
In contrast to the charged particles, i.e. α and β-particles, which lose their energy in the interaction with matter through several steps, the γ-rays perform the task generally in a single step.
(iii) Diffraction: Like the X-rays, the γ-rays may be diffracted by the crystals.
(iv) Attacking the photographic plate and fluorescence: The γ-rays can affect the photographic plates more intensely than the α- and β-rays. They can also produce fluorescence on suitable substances.
Comments
Post a Comment