XII BSEK ( Karachi Board)
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
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.