ASRJC Nuclear Physics Notes

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 20-1 Additional Notes Name: ___________________________________ Class: 24 / ____ Topic 20: Nuclear Physics Content: A The nucleus, Isotopes B Mass defect and nuclear binding energy C Nuclear processes D Radioactive decay E Uses of radioactivity and biological effects of radiation Learning Outcomes: Candidates should be able to: (a) infer from the results of the Rutherford -particle scattering experiment the existence and small size of the atomic nucleus (b) distinguish between nucleon number (mass number) and proton number (atomic number) (c) show an understanding that an element can exist in various isotopic forms each with a different number of neutrons in the nucleus (d) use the usual notation for the representation of nuclides and represent simple nuclear reactions by nuclear equations of the form N 7 14 + He2 4 → O8 17 + H1 1 (e) state and apply to problem solving the concept that nucleon number, charge and mass- energy are all conserved in nuclear processes (f) show an understanding of the concept of mass defect (g) recall and apply the equivalence between energy and mass as represented by E = mc2 to solve problems (h) show an understanding of the concept of nuclear binding energy and its relation to mass defect (i) sketch the variation of binding energy per nucleon with nucleon number (j) explain the relevance of binding energy per nucleon to nuclear fusion and to nuclear fission (k) show an understanding of the spontaneous and random nature of nuclear decay (l) infer the random nature of radioactive decay from the fluctuations in count rate (m) show an understanding of the origin and significance of background radiation (n) show an understanding of the nature of , β and γ radiations (knowledge of positron emission is not required.) (o) show an understanding of how the conservation laws for energy and momentum in β decay were used to predic t the existence of the neutrino (knowledge of antineutrino and antiparticles is not required.) (p) define the terms activity and decay constant and recall and solve problems using A = λN (q) infer and sketch the exponential nature of radioactive decay and solve problems using the relationship x = xo exp(–λt) where x could represent activity, number of undecayed particles or received count rate (r) define and use half-life as the time taken for a quantity x to reduce to half its initial value (s) solve problems using the relation  = ln 2 t1 2 (t) discuss qualitatively the effects, both direct and indirect, of ionising radiation on living tissues and cells. Lect Concepts Qns Duration 1 A.1 – B.2 CYU 1–3, Ex 1 31 min 2 B.3 – C.4 CYU 4, Ex 2 32 min 3 C.5 – C.6 CYU 5, Ex 3–5 21 min 4 D.1 – D.4 Ex 6–7 32 min 5 D.5 – D.7 – 29 min 6 E.1 – E.2 Ex 8–10 28 min

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 20-2 Additional Notes Discovery of Protons • In 1919, using energetic -particles to bombard nitrogen atoms, Rutherford found that protons were ejected after the collision. Protons were found to be positively charged and carried a charge of +e. • It is concluded that ➢ the positive charge in the nucleus is due to the presence of protons in the nucleus. ➢ the number of protons is equal to the number of electrons in a neutral atom. ➢ the mass of a proton is approximately 1840 times the mass of an electron. Discovery of Neutrons • In 1932, James Chadwick discovered neutrons inside the nucleus. Using -particles to bombard beryllium, Chadwick found that neutrons were ejected in the collision. • It is also discovered that: ➢ neutrons carry no charge. ➢ the mass of a neutron is nearly equal to the mass of a proton. Simple Model of an Atom Structure of Nucleus An atom consists of a positively charged nucleus which makes up most of the atom’s mass. The nucleus consists of protons which are positively charged and neutrons which carry no charge. The mass of a proton is almost equal to the mass of a neutron. Strong Nuclear Force Inside the nucleus, the protons repel each other due to electrostatic repulsion of like charges. For the nucleus to be stable, there must exist other forces between the nucleons. These are called the strong nuclear forces which has a very short range. They provide a net attractive force greater than any repulsive electric forces when the nucleons are very close together. nucleus proton nucleus proton neutron electron History of the Atom

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 20-3 Additional Notes Electrons around the Nucleus Electrons are considered to be orbiting in circular motion around the nucleus at relatively large distances away. The electrostatic forces of attraction between opposite charges (protons and electrons) provides the required centripetal force for the circular motion. In the history of the atom, the concept of the atom was revisited and elaborated upon by many scientists and philosophers.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 20-4 Additional Notes A.1 Alpha-particle Scattering Experiment • In the “plum -pudding” model, first proposed by J. J. Thomson in 1904, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged “plums” surrounded by positively charged “pudding”. • In 1909, to test the “plum-pudding” model of the atom, Rutherford suggested the alpha- scattering experiment. Two of his students, Geiger & Marsden, carried out the experiment. • Experimental Setup: ➢ a narrow beam of positively charged alpha () particles (helium nuclei) were directed from a radioactive source onto a thin gold foil (~ 1 μm thick) in vacuum. ➢ a zinc sulphide screen (fluorescent material) is mounted such that it could be set at different angles to the gold foil. ➢ a scintillation is observed on the zinc sulphide screen when an -particle strikes the screen. It could be deduced that an −particle had been deflected from the gold foil to hit the zinc sulphide screen at that angle. ➢ the entire apparatus was set up in a vacuum chamber to ensure that deflection of -particles is entirely due to its interaction with gold atoms and not with the air molecules in the chamber. • Results of alpha-scattering experiment: ➢ The number of scintillations (n) observed at each scattering angle () is summarized in the following graph: A The Nucleus, Isotopes Thin metal foil Alpha particle deflections by the atoms in a gold foil gold foil α particles The ‘plum pudding’ model: A narrow beam of - particles is to ensure there is little divergence in the deflected beam to increase precision of measurement. A thin gold foil is used so that the -particles are scattered only once. It also ensures that the -particles are not absorbed by the foil. −170 170 90 −90 0 n  / o Scintillation is the process by which ionisation produced by charged particles excites a material and light is emitted by the de-excitation.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 20-5 Additional Notes Observations Interpretation (a) Majority of the scintillations were observed at a scattering angle of around 0˚ to the direct path. This indicated that a vast majority of the alpha particles were able to pass through the gold foil with little or no deflection. Much of the atom is made up of empty space, hence the nucleus has a very small volume compared to the atom. (b) A small proportion of scintillations were observed at an angle of more than about 10 ° to the direct path. The centre (or nucleus) of an atom is charged. Alpha particles, which are also