Physics CH # 20

Review Questions

Q.20.1 Explain how would you test whether the radiation from a radioactive source are γ, β, α Particles?
Ans. If the radiations are allowed to pass from a magnetic field then the radiations can be identified because of the deflection the α and β particles changes their path and γ rays pass went un deflected.

Q.20.2 A particle produces more ionization is less penetrating. Why?
Ans. Due to the ionization the particle losses some part of energy. so it will have small penetrating power so we can say that the grater ionization the more will be loss of energy and less will be penetrating power..

Q.20.3 It is said that α or β Particles carries an atom with out colliding with electrons. How can each do so?
Ans. As we know tat the charge on α particle is positive while β has the negative charge. Therefore the α particle ionizes the atom by attraction and  β  particle ionizes by repulsion.

Q.20.4 In how many ways γ rays produce ionization of the atoms?
Ans. (i) It may lose only a part of energy in a collision (Compton effect)
        (ii) It may lose all energy in colliding with the electron (Photoelectric Effect) 
       (iii) It may be stopped by a heavy nucleus giving rise to the pair production

Q.20.5 In what way a neutron produces ionization of an atom?
Ans. It can ionize by colliding with a substance containing a large number of hydrogen atoms and knocks out a proton.

Q.20.6 Name different electromagnetic radiations which are capable of producing ionization of atoms. By what process do they ionize?
Ans. (i) Ultraviolet rays (ii) Gamma rays and (iii) X- rays .These rays can interact with the material in elastically and remove electron from it.

Q.20.7 Why is lead better shield against γ, β, α particles than an equal thickness of water column?
Ans. As we know that the alpha and beta particles have high ionization power so they losses more energy in water than lead so water can be used for the protection against the alpha and beta particles. But gamma rays has more penetrating power so lead (which has the more electrons than water) is used.

Q.20.8 Lead is heavier than water. Yet what is more effective shield against neutrons?
Ans. If we want to stop a neutron it must collide with the nucleus of same size. When the neutron collides with the water nucleus it stops or slows down but when it collides with the nucleus of lead it bounces back with the same speed .so Water is the better shield.

Q.20.9 In an X ray photograph bones show up very clearly while the fleshy part shows very faintly. Why?
Ans. Because the X-rays can stop by the bones but can penetrate through flesh.

Q.20.10 In a cloud chamber photograph, the path of an α –particle is thick and continuous line where that of β -particle is a thin and broken line. Why?
Ans. α –particle is more ionizing than the  β –particle so it will have a clear photograph in the chamber.

Q.20.11 Why do γ-rays not give a line-track in the cloud chamber photograph?
Ans. As gamma rays don’t produces ionization directly, But it collides with the atom and eject out the electrons or beta particle from it .This beta particle also have its own track. therefore their will be many lines.

Q.20.12 A neutron can produce little ionization. is there any sure chance of getting a cloud chamber track for it or a count in the Geiger counter?
Ans. It can ionize by colliding with a substance containing a large number of hydrogen atoms and knocks out a proton. so there will be no such track on the Wilson cloud chamber and similarly it is not detected by G.M tube.

Q.20.13 A cloud chamber track of an α -particle sometimes shows an abrupt bend accompanied by small branched track. Why?
Ans. When alpha particle collides with the nucleus, the recoil nucleus will give the abrupt change in the track.

Q.20.16 Which of the rays----α, β, or γ would you advise for the treatment of: (i) Skin cancer (ii) The cancer flesh just under the skin (iii) A cancer deep inside the skin .Give reasons.
Ans. (i) For the treatment of skin cancer we use alpha particles .B/c their penetration is small
 (ii) For the treatment of cancer flesh just under the skin we use Beta particles .B/c their penetration is Medium.
(iii) For the treatment of cancer deep inside the skin we use gamma rays. B/c their penetration is very high.

Q.20.17 Two isotopes are available one of long half life and one of shorter half life, Which isotope is advisable for the treatment of patients why?
Ans. The isotope of shorter half life must be used so if some amount remains in the body must decay quickly.

Q.20.18 Why are many artificial radioisotopes are rare in nature?
Ans. The decay of these isotopes depends on their half life .All artificial isotopes has small half life.
Q.20.19 Can radiocarbon dating be used to measure the age of stone walls of ancient civilization?
Ans. No, radiocarbon dating is not used foe the measure of the age of stone walls. It is used only for the organic remains. While for stone wall uranium must be used.

What do you know about Wilson and Cloud Chamber?


Wilson Cloud Chamber is used to observe the path of ionizing particles. It helps to examine the mechanism of ionization of various ionizing radiations and the product of their interaction with material inside the chamber.

It consist of a closed cylindrical chamber with transparent glass top "I" and a movable piston on the bottom. On the sides near the top the cylindrical is provided with a glass window for light and for the ionizing particles or radiations. The piston can be moved up or down by a lever attached to it. Before making the enclosed space above the piston arright, 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 the liquid stay on the piston.

The vapours of the liquid usually condense 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 (centres) of condensation. In particle free space the saturated vapour may cool much below the dew point. Then they are called Super Saturated Vapours. Paths, additional information about the charged and uncharged nature, the magnitude of the charge, the charge to mass ration (e/m), etc of the incident particle or the particle found by their interaction with the atoms can be obtained. By this very method a number of particles have been discovered.

Explain the construction and working of Gelger Counter.
Gelger counter is a portable device which is widely used for the detaction of ionizing particles or radiations.

It consists of a hollow metal cylinder, one end of which is closed by an insulating cap. At the centre 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 all entrance window for the ionizing particles or radiations. The sealed tube usually contains a special mixture (air, argon, alcohol etc) at a low pressure of 50 to 100 millimetres of mercury. A potential difference of the order of one thousand volts is applied between the metal cylinder and difference is only slightly less than than, necessary to start a discharge between the wire and a cylinder.
When an ionizing particle enter the tube under this condition if a charged particle pass through the chamber it produces ionization along its track. The condensation of vapours takes place on ion in the form of tiny droplets of fog, which can be photographed.

1. α-Particle
An α-particle is highly ionizing the ions produced are so numerous that its trade is a thick and continuous line.

2. β-Particle
β-Particle is much less ionizing its track is therfore, a thin and broken line.

3. γ - Rays
γ - Rays are photons emitted in a widening cone of some angle. They produce ionization by photoelectric effect distributed on a wide space. Some of the photoelectrons ejected by them give tiny line tracks in directions like the β- Particles and scattered dots are produced. The γ - rays not produce well-defined line track.


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