Nuclear and Atomic Physics Intro to Radioactivity

Isotopes Nuclear isotopes (a.k.a nuclides) have specific nuclear notation: Z = atomic number (= # protons) A = mass number ( Nucleon number) (= #protons + # neutrons) X = chemical symbol of the element

Isotopes Most elements have more than one isotope (although not always a stable one!) Isotopes are atoms of the SAME ELEMENT with DIFFERENT numbers of NEUTRONS Atomic number is ALWAYS the same for any isotope—only the mass number (nucleon number) changes Evidence for neutrons using isotopes: there is no other way to logically explain the difference in mass for various atoms of a particular element.

Isotope Practice: Isotope Mass Number Atomic Number # of protons # of neutrons # of electrons 8939Y 89 39 50 Cl-1 37 17 20 18 3516S 35 16 19 Co+2 60 27 33 25 Isotope Mass Number Atomic Number # of protons # of neutrons # of electrons 8939Y 89 39 50 Cl-1 37 17 20 18 3516S 35 16 19 60 27 25 Isotope Mass Number Atomic Number # of protons # of neutrons # of electrons 8939Y 89 39 50 Cl-1 37 16 19 60 27 25 Isotope Mass Number Atomic Number # of protons # of neutrons # of electrons 8939Y Cl-1 37 16 19 60 27 25 Isotope Mass Number Atomic Number # of protons # of neutrons # of electrons 8939Y 89 39 50 Cl-1 37 17 20 18 16 19 60 27 25

Graviton (hypothetical) Nuclear Interactions Fundamental Forces Type Relative Strength Field Particle Gravitational 1 Graviton (hypothetical) Weak nuclear 1032 W+/- and Z0 Electromagnetic 1036 Photon Strong nuclear 1038 Gluons

Fundamental Forces… Evidence: Strong nuclear force: protons do stay together in stable nuclei, even though the electromagnetic forces between them would suggest they would repel Weak nuclear force: evidence suggested during beta decay (where a neutron disintegrates into a proton and an electron…)

Radioactive Decay Discovered in 1896 by Antoine Henri Becquerel Inspired by discovery of X-rays, wanted to know connection between those and fluorescent or phosphorescent materials Experiment: Photographic paper wrapped in black paper to keep out light… Salt samples (such as Uranium) placed on the covered paper Also exposed the wrapped paper to sunlight for several hours…

Ionizing Radiation—because rays could ionize gas molecules Results: Photographic plate was NOT exposed due to the sunlight Outlines of the uranium sample clearly visible on plate: THEN manipulated: Temperature Amount of light Other physical and chemical changes NO EFFECT! Ionizing Radiation—because rays could ionize gas molecules

Radioactive Decay Marie Curie (and husband, Pierre)—followed Becquerel’s experiments to look for other substances with the same properties as Uranium… Isolated Thorium (~1898) Discovered Radium and Polonium…won Nobel Prize in Chemistry (1903) 1899—Rutherford discovered that Uranium emits 2 kinds of radiation (“alpha and beta rays”) 1900—gamma rays discovered as a 3rd type of radiation by Paul Villard

Types of Radiation Ionizing power: the ability of radiation to knock electrons out of orbit when they collide with another atom. Alpha particles (a) Helium nucleus Charge = +2e (the same as 2 protons) Mass = 4u (1u = mass of a nucleon) Type of energy: all kinetic velocity ~ 0.05c Penetration: stopped by a sheet of paper Range: a few centimeters Ionizing power—largest of the 3 types of radiation…very dangerous if ingested! On 21 August 1945, Los Alamos scientist Harry K. Daghlian, Jr. suffered fatal radiation poisoning after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.[5] On 21 May 1946, another Los Alamos scientist, Louis Slotin, accidentally irradiated himself during a similar incident (called the "Paharito accident" at the time) using the very same sphere of plutonium responsible for the Daghlian accident. Slotin surrounded the plutonium sphere with two 9-inch diameter hemispherical cups of neutron-reflecting material (beryllium); one above and one below.[6] He was using a screwdriver to keep the cups slightly apart, which kept the assembly subcritical. When the screwdriver accidentally slipped, the cups closed completely around the plutonium, sending the assembly supercritical. Immediately realizing what had happened, he quickly disassembled the device, likely saving the lives of seven fellow scientists nearby. Slotin succumbed to radiation poisoning nine days later.[7]

Very fast moving electron—emitted from the nucleus Charge = -1e Beta Particles (b) Very fast moving electron—emitted from the nucleus Charge = -1e Mass = 1/1850 u Energy = all kinetic (velocity up to 99% speed of light) Penetration: will be stopped by a few mm of aluminum Range: a few meters through air More penetrating than Alpha particles, but less ionizing.

Gamma Rays (g) High energy electromagnetic radiation Charge = neutral (very high frequency, very short wavelength) Charge = neutral Mass = 0 Energy: Photon Energy (proportional to the frequency of the ray) Velocity = speed of light (c) Penetration: can be stopped by several cm of lead or by a meter or more of concrete Range: there is no maximum range Lowest ionizing power of the 3 types of radiation

Particles can be identified based on how they interact with a magnetic field: Alpha particles will curve slightly Beta particles will be deflected significantly, and in the opposite direction from alpha Gamma rays—no charge, so no deflection at all