NUCLEAR RADIATIONS

INTERACTION OF NUCLEAR RADIATIONS WITH MATTER

In the previous article we have discussed natural radioactivity in which  α and β particles and γ-rays are emitted from the disintegrating nucleus of an atom.In nuclear fission and fusion reactions neutrons and other particles together with certain radiations are emitted from the nucleus.Moreover, interaction of high energy particles (neutral or artificial) with matter produce certain nuclear or atomic reactions with emission of particles like protons,deutrons,neutrons and ionizing radiations like γ-rays from the nucleus and x-rays and ultraviolet rays from the atom.These particles and radiations have been studied carefully for their properties and effects with are as follows:

ALPHA (α)  PARTICLES

It shoots out from the nucleus with a high velocity ( 0.1 × 10^⁸ m/s ). Thus it possesses very high energy 7.7 MeV for the most energetic from R ac.Due to its large size more charge and high energy it can make very large number of collisions with the atoms and ionize them as it passes through them, before it stops. Head on collisions are rare, however,if an α-particle passes close to an atom, the strong electrostatic attraction between it and an electron tears the electron off from the atom and ionizes it.An α-particle loses about 35eV energy in each collision.Thus a 7.7 MeV α-aplha particle from RaC produces about 200000 ions before it stops.The range of α particle is small.Thus metal sheets form good shields for α-particle.The number of ions produced by an α -particle or its range in air is a measure of its energy.α particle produce fluorescence on striking certain substances each as zine sulphide and barium platinocynide.

PROTONS

A proton is also a positively charges particle with properties similar to the α particle with properties similar to the α particle.Its made is one fourth and charge is one half of that of an α particle,it is smaller in size and carries less energy at the same velocity.Obviously, it suffers fewer collisions with the atoms of the medium as compared to a greater distance (about 5 to 10 times) before stopping.Its ionizing power is also much less, about one fifth that of the α particle.The mechanism of ionization is however identical.

BETA PARTICLES

A proton also ionizes the atoms of the medium along its path but this ionization is much less than that produced by an α particle or a proton.The reason is that due to its very small size the collisions are fewer and farther apart. Even in a single collisions most of its energy is lost.Head-on collisions being rare, it can ionize an atom by strong electrostatic repulsion when it passes close to its electron.The range of  β particle in a medium is very large nearly 100 times that of an α particle of the same energy.The ionization produced by it is less than one-hundredth of that by the α particle.Alpha particles  may pass through a thick book.However, a small thickness of a heavy metal rich in electrons is enough to stop the β particle e.g.5 x 10e-3 m of Aluminium.Fluorescence is also produced when β particle strike calclumungstate and barium platinocynide.

GAMMA (γ) RAYS  

Gamma rays are very high energy electromagnetic radiations of extremely short wavelength emitted from the nuclei of radioactive atoms originating from the high energy transition of the nucleons in the nuclei.They are accompanied with the emission of α and β particles.They carry no charge and have no rest mass but possess very high energy of the order of several MeV.They penetrate for greater distance in material media as compare to α and β particles.Very energetic γ rays are capable of penetrating several centimeters of concrete.

Like ultraviolet rays and x rays,γ-rays are also capable of ionizing even for more strongly the atoms of the medium they pass through.Being a photon , γ- ray can produce ionization in three ways:

1. It may lose all its energy in a single encounter with the electron of an atom 
2. It may lose only part of its energy in an encounter 
3. Very few of very high energy γ-rays photons may impinge directly on heavy nuclei be stopped and annihilated giving rise to electron-position pairs 

Through a gas many of its photons may pass several meters without any encounter.A good many,however,do have encounters with the electrons of the atoms which are knocked off with the production of ions.This ionization is much less strong than that produced by α or β particles.Since most of the photons are absorbed by electrons and substance rich in electrons e.g. lead will stop most of the
γ-ray photons and serve as a good shield against γ-rays.

NEUTRONS 

A neutron is essentially emitted from the nucleus of an atom.It is so called because it is electrically neutral and carries no charge.Its mass is very nearly equal to that of a proton,Consequently, unlike charged particles it can neither experience or exert any electrostatic force of attraction or repulsion.Therefore, it can interact with an electron or the nucleus of an atom only by direct impact.When it hits an electron.it knocks it out from the atom (ionization) with practically no charge in its a nucleus,appreciable changes in its energy and direction of motions are very rare.Hence a neutron is highly penetrating but very slightly ionizing particle.

It is evident from the above discussion that nearly all the particles and radiations ionize the atoms in their path.This effect,therefore, is used as the basis for most of the detection devices a few of which will now be discussed here.

WILSON CLOUD CHAMBER 

Wilson cloud chamber is device for making visible the paths of ionizing particles.It helps to examine the mechanism of ionization of various ionizing radiations and the products of their interactions with the material inside the chamber.This device was devised in 1895 by the British physicist,C.T.R. Wilson.

It consists of closed cylindrical chamber with transparent glass top.T, and movable piston P at the bottom.On the sides near the top, the cylinder is provided with a glass window.L, for admitting illuminating light and an inlet.I for ionizing particles or radiations.The piston can be moved up or down by lever attached to it.

Before making the enclosed space above the piston airtight enough quantity of a low boiling point liquid such as water or alcohol is introduced in the space to produce its saturated Vapours.A small quantity of liquid stay on the piston.The vapour of a liquid usually condence at its dew point but the condensation never takes place in the absence of some particles dust particles or ions which are essential to form the nuclei (centers) of condensation.In particle free space that saturated vapour cool much below the dew point.Then they are called super saturated vapour.Under this condition if some ions are incidentally takes place a round them forming tiny droplets of of which shows itself when illuminated.This explains the underlying principle of the cloud chamber.

To investigates any ionizing particle or radiation the particle source is mounted in the chamber at the inlet.or the radiation may be admitted through the inlet window.An intense beam of light is projected into the chamber through the window, L . to illuminate the fog track and photographic camera is mounted above the glass top the chamber.

With the above previous setting of the apparatus, the piston is pulled down suddenly with the help of the lever.The saturated vapour cool down below the draw point into supersaturated vapours.If an ionizing particle or radiation passes into the chamber at the same time, the gas molecules all along its path ionize into a trail behind it.The super saturated vapours immediately condense round these ions forming tiny droplets of fog which becomes visible by the reflection of light from them.The track of the particle shows as a bright line which can be photographed at the power instant.

An α particle is highly ionizing.The ions produced are so numercus that its track is a thick and continuous line.The  β particle is much less ionizing.Its track is,therefore a thin and broken line.Gamma rays are photons emitted in a widening cone of some angle.They produce ionization by photoelectric effect distributed over a wide space.Some of the high energy photoelectrons ejected by them give tiny line tracks in random directions line β particle.The overall effect of gamma rays is that whole region exposed to radiations shows scattered dots and small lines rather like a fog and no well defined line track.

Often a magnetic field is applied vertically across the cloud chamber to cause the particles to deflect.From the detection,its direction and magnitude and the length and curvature of the paths,additional information about the charged and uncharged nature, the magnitude of the charge,the charge to the mass ration (e/m), etc of the incident particle or the particle produced by their interaction with atoms, can be obtained.By this very method a number of particles have been discovered.

GEIGER COUNTER

Geiger counter is a portable device which is widely used for the detection of ionizing particles or radiations.Figure shows that its basic construction.It consists of a hollow metal cylinder one end of which is closed by an insulating cap.At the center of the cap is fixed a stiff straight wire along the axis of the cylinder.A thin mica or glass disc closes the other end which also serves as the entrance window for the ionizing particles or radiations.The sealed tube usually contains a special mixture of (air,argon,alcohol etc) at a low pressure of 50 to 100 millimeters or mercury.A potential difference of the order of one thousand volts is applied between the meta cylinder and the axial wire through a suitable series resistor.The potential difference is only slightly less than that necessary to start a discharge between the wire and the cylinder.

When a ionizing power enter the tube through the window, it ionizes some gas molecules in it.These ions are accelerated by the strong radial electric field producing more ions by collision with the atoms and causing the ionization current to build up rapidly.So a momentary surging current flows between the wire and the cylinder and through the resistor producing a momentary potential difference across R.The ends or R are connected to a loudspeaker or an electronic counter. Thus each time a particle enters the counter an ionization current however, decays rapidly in a small fraction of a second since the circuit has a small time constant and the counter is ready to register another particle almost immediately.

In the case of ionizing radiation, the number of counts registered by the counter measures the intensity or ionizing power of incident radiation.