PHYS 221 Recitation

1. PHYS 221 Recitation Kevin Ralphs Week 14

2. Overview • Nuclear Physics • Structure of the Nucleus • Nuclear Reactions

3. Nuclear Physics • Structure of the Nucleus • The nucleus of made of two types of particle called nucleons: protons and neutrons • A nucleus is specified with the following notation Z: Atomic Number – Number of protons A: Mass Number – Number of nucleons X: Element Symbol • Examples • Nuclei with the same number of protons, but different number of neutrons are called isotopes

4. Nuclear Physics • Structure of the Nucleus • Atomic Mass • The atomic mass of an atom is the mass of the nucleons plus the mass of the electrons • The periodic table records average values of the atomic mass weighted by the natural abundance of the isotopes • Size of the Nucleus • Scattering experiments have shown that nuclei are approximately spherical with the following approximate radius

5. Nuclear Physics • Structure of the Nucleus • Forces within the Nucleus • Electric Forces due to repulsion between protons • The strong nuclear force is an attractive force that binds nucleons together, but only acts over short distances • Stability of the Nucleus • The above forces work together to create an equilibrium within the nucleus • Although neutrons add stability by spacing the protons further apart to counterbalance their electric repulsion, adding too many create an unstable nucleus that will split

6. Nuclear Physics • Nuclear Reactions • Radioactive decay is a spontaneous process where a nuclei splits into two or more particles • The original nucleus is referred to as the parent nucleus with the final result being called decay products

7. Nuclear Physics • Nuclear Reactions • There are 3 kinds of decay that we will concern ourselves with: • Alpha Particles • An alpha particle is a cluster of 2 protons and 2 neutrons, essentially a helium nuclei • In this type of decay, the alpha particle is ejected with the appropriate numbers adjusted in the daughter nucleus • Beta Particles • Beta decay occurs when a proton turns into a neutron or the other way around • When this happens, additional particles are kicked out to conserve charge and some other properties that are beyond the scope of our class • Gamma Decay • Nucleons can become excited and emit photons, gamma rays, when they return to their ground state just like electrons

8. Nuclear Physics • Nuclear Reactions • Conservation Rules • Conservation of Mass-Energy • Conservation of Momentum • Conservation of Electric Charge • Conservation of Nucleon Number

9. Nuclear Physics • Nuclear Reactions • Binding Energy • If we were to total up the individual masses of the nucleons and compare this to the atomic mass of them together, we would notice that some of the mass is missing • We should expect this: The nucleus is stable so it should be a lower energy system than the individual nucleons • Due to relativity, we know that this lower energy has to be accounted for with lower mass • Note that when doing binding energy calculation, you have to carry many digits since the change is mass is very small

10. Nuclear Physics • Nuclear Reactions • Half-life • We cannot predict when a decay process will occur for a specific nucleus, but if there are many nuclei then we can estimate the average number of reactions that should occur in a span of time • It would make sense if the likelihood of a decay event (which is essentially the rate of decay) was proportional to the amount of nuclei that could possibly decay which suggests an exponential kind of decay • is the half-life of the element and is the period of time where we would expect that nearly half of the material had decayed • Note that N(t) is the number of nuclei left not that number that have decayed. Likewise No is the number at t=0.

11. Nuclear Physics • Nuclear Reactions • Measuring Radiation • The “strength” of a radioactive sample is called its activity and describe how many decay events occur in a span of time • Two common units of activity are the curie (Ci) and the becquerel (Bq) Ci decays/s Bq decays/s

12. Nuclear Physics • Nuclear Radiation • Measuring Radiation • Radiation can also be measured in the amount of energy absorbed weighted by the amount of material that absorbed it • Dosage can be measure in rems or rads • Rads give the raw energy absorbed • The same dosage in rads can have different effects on biological material, so the rem accounts for that Dose in rem = (dose in rads) x (RBE)