Nuclear Physics Physics 12

Protons, Neutrons and Electrons  The atom is composed of three subatomic particles: Particle Charge (in C) Symbol Mass (in kg) Electron-1.602x e-e x Proton1.602x p+p x Neutron0n0n x10 -27

Atomic Nucleus  Atom described using: X – atomic symbol A – atomic mass number (nucleon number) Z – atomic number  Number of protons and electrons = Z  Number of neutrons = A - Z

Strong Nuclear Force  The electrostatic forces inside a nucleus would rip it apart if there was not another force  By the end of the 1930’s physicists had determined that nucleons attract each other  This is the strongest force in the known universe

Stability and the Nucleus  Although the Strong Nuclear Force is strong enough to hold a small nucleus together, as the size of the nucleus becomes larger, the electrostatic forces begin to become more important  As a result, if we consider various nuclei based on their Atomic Number and Neutron Number we get the following result:

Stability and the Nucleus Each black dot represents a stable nucleus, with the number of neutrons shown on the vertical axis and the number of protons on the horizontal axis

Nuclides and Isotopes  Nuclides are different combinations of nucleons  Isotopes occur when an element (specific Atomic Number) has different numbers of neutrons (different Atomic Mass Numbers)  For example, there are three common isotopes of hydrogen:

Nuclides and Isotopes

Nuclear Binding Energy  It takes 13.6 eV to separate an electron from a hydrogen atom  However, it takes more than 20 MeV to separate a neutron from a helium- 4 atom  The energy to separate all the nucleons in a nucleus is called the binding energy

Larger nuclei are held together a little less tightly than those in the middle of the Periodic Table

Mass Defect  If you were able to apply the 20 MeV required to separate a neutron from helium-4, what would happen to it?  This is dealt with using Einstein’s Special Theory of Relativity and the fact that mass and energy are equivalent  E = mc 2  The mass of helium-4 (2p, 2n) is smaller than that of helium-3 (2p, 1n) and a neutron  The energy that was added to remove the neutron was converted into mass  The difference between the mass of a nuclide and the sum of the masses of its constituents is called mass defect

Atomic Mass Unit (u)  When dealing with nucleons, it is often more useful to deal with mass in unified atomic mass units (u) instead of kilograms Particle Mass (in kg) Mass (in u) Electron x Proton x Neutron x

Binding Energy Example  Determine the binding energy in electron volts and joules for an iron- 56 nucleus given that the nuclear mass is u

Binding Energy Example  Determine the binding energy in electron volts and joules for an iron- 56 nucleus given that the nuclear mass is u

Binding Energy Example  We would expect the binding energy per nucleon to be about 8MeV:

Radioactive Isotopes  In discussing the nucleus, we looked at a plot of stable nuclei  It is also possible to have a nucleus that is not stable (meaning that it will fall apart)  An unstable nucleus will decay following a few very specific processes  We call this decay radioactivity and classify it into one of three types

Radioactive Isotopes

Alpha Decay  An alpha particle ( α ) is a helium nucleus (two protons and two neutrons)  A nucleus that emits an alpha particle will lose the two protons and two neutrons  Large nuclei will emit alpha particles  They do not penetrate matter well and a sheet of paper or 5cm of air will stop most  They can free electrons from atoms, meaning they are a form of ionizing radiation

Alpha Decay

Beta Decay  When a nucleus emits a beta particle ( β ), it appears to lose an electron or positron from within the nucleus  There are two types of beta decay (β - and β + )  Beta particles can penetrate matter to a greater extent than alpha particles; they can penetrate about 0.1mm of lead or 10m of air  They are also a form of ionizing radiation but less damaging than alpha particles

Beta Decay (β - )  In this type of beta decay, a neutron becomes a proton and a β - particle (high energy electron) is emitted  In addition an antineutrino ( ) is emitted (antimatter) along with the beta minus particle  The nucleus’s atomic number increases by one while the atomic mass number remains the same

Beta Decay (β - )

Beta Decay (β + )  In this type of beta decay, a proton becomes a neutron and a β + particle (high energy positron or antielectron) is emitted  In addition a neutrino ( ) is emitted along with the beta plus particle  The nucleus’s atomic number decreases by one while the atomic mass number remains the same

Beta Decay (β + )

Gamma Decay ( γ )  When a nucleus goes through alpha or beta decay, the daughter nucleus is often left in an excited state  In order to reduce the energy of the nucleus, it will go through gamma decay (high energy photon) to return to the ground state  Gamma radiation can pass through 10cm of lead or 2km of air  It is the most damaging of all due to the energy of the gamma particle

Gamma Decay

Decay Series  When a large nucleus decays by alpha and beta radiation, the daughter nucleus will be more stable than the original nucleus  However, the daughter nucleus may still be unstable and will itself go through alpha or beta radiation  This leads to a decay series

Rate of Radioactive Decay  It is impossible to predict when a specific nucleus will decay  You can describe the probability of decay  The concept of half life is used with radioactive decay: the time required for half of the sample to decay  Using the half life equation, it is possible to determine how much of a sample would remain after a given period of time

Half Life  N  sample remaining  N 0  original sample  Δt  elapsed time  T  half life

Half Life