Nuclear Radiation

Some history Insights associated with modern physics began with qualitative study of gaseous discharge and cathode ray tubes in the 1870s and 1880s. Roentgen: x-rays in 1895 Thomson: electron ID in 1897 X-rays were discovered to ionize air Up until then, only ions observed were in solution (e.g., Na+, Cl-) Therefore, neutral atoms must possess minute electric charges as constituents Atom is a complex structure Electrical charges enter into its make-up

Anatomy of an atom Nucleus contains Protons Neutrons Mass: 1.67262 x 10-27 kg, sometimes abbreviated 1.007276 u u = ‘atomic mass unit’, defined as 1.6605 x 10-27 kg Charge: 1.60 x 10-19 C, sometimes abbreviated +1e # of protons determines location on periodic table Every atom of helium has two protons, every atom of oxygen has 8, etc. Neutrons Mass: 1.67493 x 10-27 kg, 1.008665 u Charge: 0 C # of neutrons determines isotope and atomic weight

Anatomy of an atom Nucleus is orbited (kind of) by electrons Charge: -1.67 x 10-19 C, frequently abbreviated -1e Mass: 9.11 x 10-31 kg, 0.0005549 u # of electrons typically matches number of protons Electrically neutral atoms When an atom loses an electron, it becomes a positively-charged ion When an atom gains an electron, it becomes a negatively-charged ion

Nomenclature A = Z + N Mass number, A = number of protons, Z + number of neutrons, N A = Z + N

Nomenclature There is a range of neutron numbers that form stable nucleus. Different number of neutrons  different isotopes Written as Achemical symbol 20Ne has 10 protons (all neon atoms do) and 10 neutrons 22Ne has 10 protons and 12 neutrons

Think about… Carbon is the sixth element in the periodic table. How many protons and neutrons in an isotope of 14C? Assuming it is electrically neutral, how many electrons?

Mass of nucleon The nucleus of 4He (the most common isotope of helium) is made of two neutrons and two protons. If you add the mass of two neutrons and two protons, you will find that it is slightly larger than the mass of a helium atom. The difference is the total binding energy, calculated by E=mc2 Stable nuclei cluster around line of stability No stable nuclei Z>83 (bismuth). Heavier elements, up to Z=92 (uranium) exist in nature but are radioactive Unstable nuclei are in bands on both sides of line of stability For lightest elements, stable @ N = Z @ Z = 40, N/Z ~1.2 @ Z = 80, N/Z ~1.5

Nuclear binding energy Some isotopes have more binding energy that others. More binding energy = more stable

Radioactivity Marie and Pierre Curie discover that vial of radium maintains itself permanently above room temperature and that, when placed in a container, each gram of radium gives off 80 calories per hour. Sufficient heat to melt its own weight of ice. Where does energy come from? Henri Becquerel discovers uranium can expose film and pass through objects. Rutherford discovers uranium emits two types of rays.

Types of radiation Eventually, three types of radiation were discovered Alpha,  Charge +2e Mass: 4u Beta,  Charge: -1e Mass: = electron Gamma,  Charge: 0 Mass: = photon

But… Both  and  radiation have mass Cloud chambers help illustrate How can we account for continuous emission of material particles in the apparent absence of chemical change? Cloud chambers help illustrate

Transmutation of elements Elements change from one form to another and give off energy In the form of moving mass Electromagnetic waves

Nuclear stability Stable nuclei cluster along ‘valley of stability’ No stable nuclei @ Z>83 Stable N/Z changes

Forces in the nucleus Nucleus is made of positively-charged protons in very close proximity. Why don’t it fly apart? Something must be holding it together. Strong nuclear force: Attractive force between any two nucleons Does not act on electrons Very short range, i.e., ~radius of nucleus In that range, stronger than electrostatic force Extra neutrons = extra nuclear ‘glue’

Alpha Radiation See Knight, Examples 30.2 and 30.3

Beta radiation - decay: + decay: An unstable neutron decays into a proton and spits out an electron. + decay: An unstable proton decays into a neutron and spits out a positron.

Gamma radiation Atom in an excited state makes transition to lower-energy state by emitting a powerful photon.

Radiation vs. Radioactive Energy associated with , , and  radiation is high enough to ionize nearby atoms and molecules Bad for living things: Ions drive chemical reactions Potential damage to DNA Mostly irrelevant to other objects Irradiated objects do NOT become radioactive unless their nuclei changes, which they don’t.