Q test remains programmed for this Friday –All problems that have been handed in are graded and are in your folder. –The Q practice test is on the front desk –The practice test for the final is on the front desk The best way to see if your response to a question on either of these practice tests is correct is to write your answer and give it to me. I will check it and get it back to you with corrections and/or comments. –We will use this Thursday (tomorrow) as a review day. We will assign some problems today that will be discussed tomorrow. –The plan is to return the Q test Monday, finish the lab sequence on Tuesday and spend all the other class time (including Thursday) reviewing for the final. –The physics party will be Friday at 7:30 at Mazzio’s– please sign the sheet indicating if you plan to come or not. If you need to arrive late, that is fine.

Chapter Q12 Introduction to Nuclear Physics and Radioactive Decay

Rutherford’s discovery of the nucleus. α (alpha) particles Thin sheet of gold foil Almost all of the particles went straight through the gold without hitting anything. A very few of the particles were strongly deflected, showing that there were very small, very massive particles present We call these small heavy particles nuclei. The nucleus has a radius about 1/100,000 that of the atom. The nucleus weighs thousands of times more than the electrons.

Nuclear notation Z = the atomic number –the number of protons in the nucleus N = the number of neutrons in the nucleus –Nuclei of the same element with different N are called isotopes. A = the mass number of the nucleus A = Z + N The standard symbol for a nucleus is Some examples are

Properties of the elementary particles The charge on the proton is positive and exactly equal in magnitude the charge on the electron which is negative. The mass of the proton and neutron are about equal –The electron’s mass is about 1/2000 as much as the proton or neutron. The most common units for measuring the nuclear mass is a unified mass unit (amu) equal to 1/12 the mass of the carbon nucleus

More nuclear data Nuclear radius r ≈ r 0 A 1/3, where r 0 = 1.2 x m = 1.2 fm The nucleus is held together by the “Strong interaction”, one of the four known fundamental forces –Binds protons to protons, neutrons to protons, neutrons to neutrons with the same force Does not effect electrons. –Has a range of about 2 fm.

Binding Energy (mass) 1 amu = MeV/c 2 Example Q12.2 calculates the binding energy of the carbon nucleus as 92 Mev. This is 10,000 times bigger than the binding energy of the electrons in this atom.

Nuclear Stability Z 100 N For Z< 15, N = Z For the nucleus to be stable at higher Z, N>Z There are no stable nuclei for Z>83

Radioactive Nuclei In 1895 Wilhelm Röntgen discovered that electrons striking a metal produced an unknown radiation, he called X-rays. –We now know these are a type of electromagnetic radiation (photons). In 1896 Henri Bacquerel discovered that uranium would fog a photographic plate in darkness. –It was soon discovered that there were three types of radiation coming from the uranium. –Alpha, beta and gamma radiation.

Types of radiation Alpha (α) – the helium nucleus, 2 protons and two neutrons –A very large heavy charged particle Beta (β) – a high speed electron, not so big and heavy as the alpha. Gamma ( γ ) – a high energy photon

Nuclear reactions Alpha decay Beta decay Gamma decay Because an electron leaves the nucleus, we are left with an additional proton. n → p + + e - The * means the nucleus is in an excited energy state. (Above ground level)

Problems to be discussed tomorrow. Q12B.1,Q12B.4, Q12B.8