Title 27.Nuclei.pdf ✅

NEET-2022 Ultimate Crash Course PHYSICS Nuclei

POINTS TO REMEMBER 1. It is found that the size of the electron is of the order of 10-14 m (calculated on the basis of Coulomb's law of electrostatics). Obviously, not more ; a period may than two looked electrons may be accommodated in a nucleus. That too is possible i f the electrons are not moving. 2. The half-life period may be looked upon as the time over which the chance of survival of a nucleus is one half. Half-lives from 10-23s to 1020 by experiments or by indirect methods. 3. The larger the decay constant () , the more rapidly the population of a group of nuclei decrease. 4. Many homes are protected from the the hazards of fire by a small device called the smoke detector that uses the alpha decay of a man-made radioisotope americium-241 ( ) 241 95 Am . In this type of smoke detector, a minute quantity of 241 95 Am is placed between two plates connected to a battery or other source of emf. The particles emitted by the radioactive source ionise the air, allowing a measurable electric current to flow between the plates. As long as this current flows, the smoke detector remains silent. When smoke enters the detector, however, the ionised molecules tend to stick to the smoke particles and become neutralised. This reduces the current and triggers the alarm. These "ionisation" smoke detectors are more sensitive than the "photoelectric" detectors that rely on the thickness of smoke to dim a beam of light. 5. Radon-222 ( ) 222 86 Rn is an isotope produced in a radioactive decay series that includes uranium-238 and radium-226. Since uranium is naturally present M certain kinds of rocks and the soils derived from them, radon, which is a gas, can accumulate in basements and similar enclosed underground spaces that lack adequate ventilation. Radon-222 is itself radioactive, undergoing alpha decay with half-life of about 4 days, and although concentration is generally small, it may produce radiation levels great enough to be a health hazard if exposure is prolonged. Test kits to monitor the radiation levels produced by radon are available. 6. Neutron decays (n p e v) ⎯⎯→ + +− whereas proton is stable. Some recent theories, however, predict that the proton decays with a life-time of about 1051 to 1032 years. 7. The expression 'beta decay' covers all such phenomenon in which a nucleus makes an isobaric transition due to natural or induced radioactivity. The charge number (Z) changes by unity but mass number (A) remains the same. The process includes the emission of electrons or positrons from nuclei and the capture of orbital electrons. Electron and positron emission are almost similar processes and differ considerably from the third process called electron capture. 8. − decay is preceded by  or ( − particle emission. This shows that the emission of a gamma photon is associated with the daughter nucleus and not with the parent nucleus. 9. Fission has been produced in bismuth, gold, lead and rare-earths too. 10. About 99% of the neutrons ejected as a result of the fission of uranium emerge within a time interval of 10-14s and are called prompt neutrons. The remaining 1% are ejected later and are called delayed neutrons. The latter are produced when primary fission fragments further disintegrate. The fraction of neutrons which are delayed is quite small but they play a vital role in the control of nuclear reactors. 11. India exploded her first fission device on May 18, 1974 about 350 feet below the ground level at a place near Pokhran in Rajasthan. The explosive used was Pu-239 produced in the CIRUS reactor at Bhabha Atomic Research Centre and gave a yield of 12 kiloton (equivalent of 12000 tons of TNT) 12. 235U is one of the three nuclei, the other two being 235U and 239Pu which undergo fission by thermal neutrons energies — 0.025 eV). The above three and many other heavy nuclei can undergo fission by absorbing fast moving neutrons, charged particles and − rays. 13. In the process of thermal fission, in addition to two fragments and an average of 2.5 neutrons, wary − rays are emitted in each fission. Thus, fission of 235U by slow neutrons may be represented as: 235 1 236 U n U + → → 0 fission fragments + neutrons + radiation
where fission fragments are in the range A  70 to 160 and Z  30 to 65 and include isotopes of barium, lanthanum, brawn molybdenum, antimony, tellurium, caesium, iodine, krypton and xenon. 14. Most of the fission fragments are radioactive and decay by − emission and − rays. 15. Energy released ( 200 MeV) in fission of a 235U nucleus by thermal neutrons appears in the form of kinetic energy of fission fragments, neutrons, − rays, − rays and neutrinos: (i) Fission fragments 165 MeV (ii) Fission neutrons : 5 MeV (iii) − rays emitted in fission : 8 MeV (iv) − rays, − rays and neutrinos emitted in the decay of fission fragments : 22 MeV. 16. (i) If k (reproduction factor) = 1, system is critical which is the basic requirement in the operation of a nuclear power plait (ii) If k > 1, the system is supercritical which is the basic principle utilized in atomic bombs and other nuclear weapons. (iii) If k< 1, the system is subcritical. These types of systems are used in nuclear research. 17. To maintain a sustained chain reaction, there must be an adequate quantity of fissionable material. The minimum mass required t produce a sustained chain reaction is called the critical mass. When the critical mass is attained, there is enough fissionable material such that at least one neutron from each fission event, on average, goes on to fission another nucleus. 18. Fermi's original reactor, which was designed to test the basic scientific principles underlying fission reactions, produced a power of only about 0.5W when operating near its critical condition. This is the power necessary to operate a flashlight bulb. In comparison modem-day nuclear power reactors typically produce 1000 MW power, enough to power an entire city. 19. Natural uranium contains only 0.72 percent (1 part in 138) of 235U isotope. For use in nuclear reactors as well as in nuclear weapons natural uranium enriched in 235U is employed. 20. Some reactors use heavy water (D2O) in place of light water. The advantage is that deuterium (D) does not readily absorb neutrons so that the fuel can be of lower uranium enrichment. However, D2O mist first be separated horn normal water (H2O), an operation that takes energy. 21. For a nucleus, density is maximum at its centre and decreases as we move outwards from the nucleus. 22. If two small nuclei is combined to form a relatively heavy nucleus, then binding energy per nucleon increases. 23. In every nuclear reaction, following conservation laws are obeyed (i) Conservation of charge number (ii) Conservation of nucleons (iii) Conservation of energy (iv) Conservation of linear momentum 24. Nuclear force act inside the nucleus only. The nuclear force is 1038 times stronger than electrostatic forces. 25. Radioactivity is a spontaneous process. 26. Neutrons are needed for fission. But protons are needed for fusion. 27. The energy released in radioactive disintegration is smaller than energy released in fission process. 28. For stable elements, decay constant is zero.