Nuclear Physics Nuclei atomic number Z = protons Neutron number N = neutrons Mass number A = nucleons ( Z+N) Isotopes same element different number of neutrons Charge of a proton=electron=1.602x 10-19C Atomic mass tied to 12C = 12U

Atomic Mass 1 U = 1.660559x10-27kg A proton = neutron = 1U an electron about 2000 times smaller. Rest energy = ER = mc2 = (1.66x10-27)(3x108)2 = 1.492x10-10J = 931.494 MeV Nuclei have same density and size about 10-15m

Nuclear Stability Nuclear force is an attraction force between all nucleons and is greater than the repulsion force between the protons. Stability of the nucleus depends on the ratio of N to Z. It is stable when N> Z. But as Z increases a greater ratio of N to Z is needed. Above a Z of 83 the repulsive force is to strong and all nuclei decay with time, (radioactive decay).

Binding Energy The total energy of the bound nucleus is less than the combined energy of the separated nucleons. The energy that must be added to separate the nucleons is called the binding energy.

Radioactivity It is the spontaneous emission of radiation resulting in the decay of the unstable nucleus (disintegration).  alpha = Helium ion = 24He  Beta = electron = e- = -10e  Gamma = high energy photon = 00 Alpha almost no penetration, beta slight penetration, gamma high penetration.

Half life The half life of a radioactive substance is the time it takes for half of the nucleus to decay. N= No(1/2)n where No = number of radioactive nuclei, n= number of half lives 1 curie (Ci) = 3.7x1010 decay/s(1g radium) 1 Bq = 1 decay/s

Alpha Decay Parent ---------> Daughter + alpha ex. Uranium to thorium 92238U -------> 90234Th + 24He An element changing to another by alpha decay is called spontaneous decay (transmutation) The mass of the parent is greater than the daughter and alpha, the difference is in energy usually kinetic carried by the alpha particle.

Beta Decay Note that although a neutron is not really composed of the addition of a proton and an electron, when it decays it produces a proton and an electron and some energy. 614C---->714C + -10e When finding the difference in mass of the parent and daughter and multiplying by MeV not all this energy is accounted for in the KE of the electron.

Neutrino (v) The difference in energy in a beta decay is attributed to the production of a neutrino. It is a mass much smaller than an electron with no charge and very weak interaction with matter.

Gamma Decay Often after radioactive decay the nucleus is left in an excited energy state. The nucleus then undergoes a second decay where protons and neutrons move from an excited energy level (high) to a lower energy level. In the process of de-excitation high energy photons are released, (gamma radiation).\ This occurs after beta decays.

Applications of Radiation 1) Carbon dating 2) Smoke detectors 3) Medical procedures

Radiation Detectors Geiger counter consists of a thin wire electrode maintained at high voltage (1000v) in a metal tube filled with low pressure gas. When radiation enters the tube some of the gas ionizes releasing electrons that are attracted to the electrode. Upon contact a sound is made which is amplified, hence the characteristic clicking. Read about semiconductor diode, scintillation counter, photomultiplier, track detectors, etc at the end of ch 29.