Nuclear physics
EXERCISE
MULTIPLE CHOICE QUESTIONS
Choose the best possible answer:
1.One unified mass scale (1 u) is equal to:
A. 1.660 x 1027 kg C. 931.5 MeV
B. 1.4924 × 10-10 j D. All of these
2. The number of neutrons in 'Li are
A. 7
B. 4
C. 3
D. 2
3. Sum of the masses of constituent nucleons as compared to the mass of the resultant nucleus is
A. smaller B. greater C. same
D. Infinite
4.Which one of the following nuclear radiation is similar to electrons
A. a
B. B
C. B-
D. Y
5. One curie is equal to
A. 3.70 x 10-10 Bq
B. 1 MBq
C. 1 x 1010 Bq
D. 3.70 x 10¹0 Bq
6. Half-life of the iodine-131 (1311) is 8.02 days and its weight is 20 mg. After 4 half-lives, the amount left un-decayed will be:
A. 5 mg
B. 2.5 mg
C. 1.25 mg
D. 0.625 mg
7. The unit of decay constant 'X' is
A. m B. m¹ C.s
C. M-³ D. 5-1
8. In fission of 235U, neutrons used have energies of about
A. 5 MeV B. 0.6 keV
C. 8.6 eV D. 0.04 eV
9. The nucleus that results from the reaction n+0?+ His
A. ¹⅝N
B. 17o8
C. ¹⁴8O
D. ¹⁴7N
10. The resulting nucleus in the reaction an+Ba→→?+y is
A. 138 56 Ba
B. 138 55Cs
C. 138 57 La
D. ¹⁴7N
11. The quantity of 235U in naturally occurring uranium is about
A. 0.7% D. 99.3%
B. 30% C. 70%
12. In Liquid Metal Fast Breeder Reactor (LMFBR), the type of uranium used is
A. 234U
B. 235U
C. 238U
D. 239U
13. Nuclear force exists between.
A. Proton-proton C. Neutron-Neutron
B. Proton-Neutron D. All of these
14. Which of the following is a boson
A. proton B. neutrino C. photon
D. pion
15. The hadrons formed by combination of quark and anti-quark pair are called
A. baryons B. mesons C. muons
D. neutrinos
16. All free particles have a color charge of
A. 0 B. 2/3 C. 1/3 D. 1
CONCEPTUAL QUESTIONS
1. Why do heavier nuclel have more neutrons than protons?
Ans. For the sake of stability, heavier nuclei have more neutrons than protons.
Explanation: Nucleons (protons and neutrons) are subjected to two forces in the
nucleus. 1) Coulomb's force of repulsion among protons.
2) Strong force of attraction among all the nucleons; proton and proton, proton and neutron and neutron and neutron. The strong force is a short-range force. It acts in a small range and beyond that range its effect is negligible or non-existent. On the other hand, Coulomb force is effective to a comparatively longer distance.
In the small range (very short distance) the attractive strong force is dominanover the Coulomb force of repulsion. However, in the long range, the Coulomb force becomes stronger.
Heavier nuclei have large number of nucleons. The separation among the particles also increases with the increase in volume. Consequently, the Coulomb force of repulsion is likely to override the attractive strong force. Therefore, the nucleus becomes unstable.When the number of neutrons increases, it adds to the stability of the nucleus. Because in this way the strong force increases which combines the nucleons in the nucleus. Because, with the increase of neutrons, the attractive force among all particles increases. Conclusion: With greater number of neutrons, the nucleus is more stable.
2. Radium has a half-life of about 1600 years. If the universe was formed five billion or more years ago, why is there any radium left now?
Ans. If there still exists Radium in the universe, there are two main reasons for that.
(1) The law of radioactive decay is,
N=Ne
Here No is the initial number of isotopes and N is the number of isotopes decayed in time t. However, this equation shows that for a complete decay of the No iso topes, an infinite time is required. Since the time of the formation of earth is not infinite, no matter how long it is, therefore, the Radium still exists in nature. If the earth was formed 5 billion years (5000000000) ago, there would be 3125000 half-lives of Ra and according to the above equation, there would still be some Ra on the earth. It will be as far as a single atom of Ra is there. (2) Radium is constantly being produced on the earth due to the decay of uranium and thorium and the amount of Ra is constantly added with. Therefore, there is still Ra in a sufficient amount on earth.
3. If we focus our attention on a specific nucleus in a radioactive sample, can we know exactly how long that nucleus will live before it decays?
Ans. No, it is hot possible to know exactly how long the specific nucleus will live before it decays.
Explanation: Radioactive decay is random, and measured half-lives are based on
the most probable rate. We know that a nucleus will decay at some point; we just
cannot predict when. It could be anywhere between instantaneous and the total
age of the universe. Although scientists have defined half-lives for different elements, the exact rate is completely random.
4. Why are neutrons such good projectiles for producing nuclear reactions?
Ans. Being uncharged particles, neutrons are considered to be good projectiles for producing nuclear reactions. Explanation: Since neutrons are neutral particles and carry no charge, therefore they are not affected or deflected by electric or magnetic fields. When neutron is projected at a certain nucleus, it can reach the nucleus without being deflected by the positive nucleus or negative electron cloud. Therefore, neu tron can cause nuclear transmutation easily as compared to other charged parti cles and is thus considered to be a good projectile for producing nuclear reactions.
5. Why neutron activated nuclides tend to decay by B¹ rather than ?
Ans. In neutron activated nuclides the number of neutrons is greater than number of protons and are unstable. In order to attain stability, some neutrons are con verted into proton followed by B emission.
Explanation: According to the law of conservation of charge, the total charge on both sides of a nuclear reactions should be same. Therefore, when a neutron (having no charge) is converted into proton (having positive charge), must be followed by B, in order to have net zero charge. Mathematically:
n→P+B
Since charge on LHS of the nuclear reaction is zero and the net charge on the RHS is (1-1=0) zero, and the law of conservation of charge is satisfied. Conclusion: Thus, it is concluded that a neutron activated nuclides tend to decay by B¹ rather than B. Because if neutron activated nuclides is decayed by B', then it would be the violation of law of conservation of charge.
6. Why are small and large nuclei unstable?
Ans. The binding fraction or packing fraction is the measure of stability of a nucleus. The greater the binding energy per nucleon, the more stable the nucleus is. Mathematically:
E(ZM, NM-Mic
f= A
A Small Nuclei: The decrease in Packing fraction f for small A is due to surface effect. The particles at or near the surface are less strongly bond than the nucleons at the interior. The smaller the nucleus the larger the percentage of constituents at the nuclear surface is. (e.g., He and O) and is therefore unstable. Large Nuclei: The decrease in ƒ for large nuclei is because of large number of protons. The coulomb repulsion between protons opposes the binding effect of nuclear force. Therefore, nuclei with large A are unstable. (e.g.,²²²86 Rn and ²³8 92U).
7. Why is a conventional fission nuclear reactor not able to explode as a bomb?
Ans. A conventional fission nuclear reactor does not explode as a bomb because of the controlled fission chain reaction. Explanation: The term "Nuclear Reactor" is used to refer a device in which con trolled nuclear fission chain reaction can be maintained. In such a device, neutrons are used to induce nuclear fission in controlled manner. This control can be achieved by using special type of materials like "Boron" in the core of reactor in the form of control rod. And also, multiple barrier approach scheme is adopted in reactors. This helps to prevents the radioactive material in the atmosphere in se vere conditions. And when accident happens these control rods drops automat cally in the core and stops the fission reaction. So nuclear reactors are inherently safe devices. While in nuclear bomb the fission is in uncontrolled way used for destructive pur pose. In bomb enormous power is released for a very short duration. But in reactors there is a control on power, so it can produce power for peaceful purpose.
8.Why does the fusion of light nuclei into heavier nuclei release energy?
Ans. In a fusion reaction, two light nuclel merge to form a single heavier nucleus. The process releases energy because the total mass of the resulting single nucleus is less than the sum of the masses of the two original nuclel. According to the Einstein mass energy equation, the leftover mass is released in the form of energy.
9. What factors make a fusion reaction difficult to achieve?
Ans. There are three main difficulties to achieve fusion reaction. 1) In fusion, two smaller nuclei fuse (combine) together to form a relatively large nucleus. Since both nuclei have the same nature of charge, therefore a Cou lomb repulsive force come to play. If the nuclei have to combine, they must overcome this repulsive force. A great deal of energy is required for them to attain a speed that when they collide, they blend with one another. A temper ature of approximately 108 K is required that may dispense the nuclei with such a high speed. Practically, this is not possible in our normal environment. 2) The second difficulty is confining the fusing nuclei at such a high temperature. Because at such a high temperature, the atoms are ionized and exist in plasma state. We need a container to keep the plasma in. Practically, any container will vaporize at a temperature as high as 10 K3) Fusion happens to occur in a number of processes which needs a very long time to complete.
Post a Comment