Question

1. (a) The mass of a nucleus is determined by using the mass spectrograph. Explain how the mass spectrograph works starting from the production of the ion (30 marks) (b) Given the following spectroscopic doublet values Find the mass of 'H, 2H and source. Draw a schematic diagram of the instrument 1C with 16O 16.000000 u 2(1 H)-2H 0.001 5483 u 3(2H)- 12C 0.042298 u C 4H)-160 0.0363390 u What happens to the values of 'H, 2H and as the standard mass? i C 12.000000 u is taken (30 marks) (c) Explain the curve of binding energy per nucleon and its stability. What is the average value for B/A? 4/8 SIF3001 (20 marks) (d) In the semiempirical mass formula, why is the symmetric term is more important in light nuclei compared to to heavy nuclei? Explain by using suitable an example of light and heavy nucle (20 marks)

Answer 1

Atoms and molecules can be deflected by magnetic fields - provided the atom or molecule is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't. The atom or molecule is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Most mass spectrometers work with positive ions. The ions are accelerated so that they all have the same kinetic energy.

The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.

The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.

c) Binding energy is the energy that holds a nucleus together and is equal to the mass defect of the nucleus. Nuclear binding energy is the energy that would be required to disassemble the nucleus of an atom into its component parts. These component parts are neutrons and protons, which are collectively called nucleons.The binding energy per nucleon and the mass number is plotted in the figure. We can see that the curve rises steeply initially and then gradually reaches maximum of 8.79 MeV at A=56. This number A=56 corresponds to Iron . The curve now drops very slowly to somewhere around 7.6 MeV at the highest known mass number. From this graph we can say that nuclei of intermediate mass are more stable since high amount of energy is required to separate these nucleons. This also means that a large amount of energy will be liberated if heavier nuclei can be divided into lighter ones in nuclear fission. so larger the binding energy the greater the stability of nucleus. Nuclei of intermediate mass are most stable as they have greatest average binding energy per nucleon of 8.5 MeV.

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