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Lecture

Radiation

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Department
Physics
Course
PHYS 1070
Professor
The Great Orbax
Semester
Fall

Description
November 9th, 2011 RADIATION Overview •ATOMS and the NUCLEUS •TYPES of RADIOACTIVE DECAY •Half life •Radioactivity •ABSORPTION of RADIOACTIVITY •QUIZ 4 - pretest 4, SG 5 &6, Light Absorption & Observed Absorbed - NOV 11th •QUIZ 5 - pretest 5, online quiz/lab, Electricity lab - NOV 25 •EXAM - 25 M/C questions - each question is worth 2% of final grade •Lecture 12 - REVIEW for FINAL exam The Nucleus •it was once believed that an atom was just a solid glob with a bunch of charges stuck in it (i.e. "raisin bun theory") •Now, in our enlightened age, we know it instead to be a FUZZY glob with charges SOMEWHERE inside it •but INSIDE that fuzzy glob we know there is a NUCLEUS •NEUTRONS •charge less •mass = 1.675 x 10 ^-27 kg •PROTONS •positive charge (+1 e) •mass = 1.6753 x 10 ^-27 kg •In order to balance out the positively charged nucleus we have negatively charged electrons whose location is defined by a cloud of probability •ELECTRONS •negative charges (-1 e) •mass = 9.109 x 10^-37 kg •The electron is a type of LEPTON - a very stable subatomic particle FUNDAMENTAL PARTICLES At one point we thought that particles could not be subdivided beyond the electron, the proton and the neutron It turned out we were wrong. But just this one time... All fundamental or elementary particles are either bosons or fermions, depending on their spin. Fermions are either Leptons or Quarks Fermions are very stable subatomic particles Theorized that EVERY force is an EXCHANGE of PARTICLES! Gravity - gravitons Electromagnetic - photon Weak Interaction - hadrons & leptons - Z & W particles Strong Interactions - holds together quarks - ex of GLUONS Chemicals & elements have different nuclear masses, but since the number of PROTONS is fixed, this extra mass is made up of NEUTRONS •These differently weighted versions of the same chemical are called ISOTOPES! •Luckily, we've devised notation to keep track of all this . . . A = atomic mass #, baryon #, Z + N = A Z=atomic #, # of protons in nucleus N=neutron #, # of neutrons in nucleus Ex: In the case of Carbon, this has many stable isotopes: How many electrons are there? A. 238 B. 92 C. 146 # Of protons = # of electrons in a stable atom There are: •100 elements •300 STABLE isotopes of those elements Not EVERY combination of protons and neutrons will result in a happy (or stable) nucleus But we can sure as HECK smash 'em all together! > RADIOACTIVITY In an effort to BALANCE inter (and intra) nuclear forces (ex. Coulomb repulsion and attraction) atoms will REJECT their constituents until they achieve some sort of STABILITY In the early days of radiation •It was discovered that nuclei NEVER emit singular protons and neutrons when trying to stabilize! •Instead it was noticed that three DIFFERENT types or 'particles' were emitted as the PARENT NUCLEUS radioactively decayed into a DAUGHTER NUCLEUS •ALPHA (heaviest radiation - He, no e-, just nucleus), BETA (lighter radiation, most prevalent - injection of leptons) and GAMMA (not even a particle, just ENERGY - very small electromagnetic waves - low wavelength = high energy) radiation RULES OF DECAY or BALANCING NUCLEAR EQUATIONS 1.Conserve the charge! -balance the electron number 2.Conserve the number of nucleons! -balance the number of total protons and neutrons TWO OBVIOUS RULES! ALPHA Decay •eject a tight cluster of 2 protons and 2 neutrons • •Ex. (226 = 222 + 4, 88 = 86 + 2) •alpha particle •charge of 2+ •energy ~ 5 MeV BIG mass, BIG charge interacts with matter VERY easily! > can be blocked with paper > Alpha emitters tend to be long lived BETA decay •most popular •e- or e+ •Mass defect! •Often accompanied by a charge less, 'mass less' particle which carries energy •Ex. •Ex. •MUCH lighter and LESS heavy than the alpha particles •penetration depth is LONGER and more SPREAD OUT than alpha particles GAMMA decay •Not a particle! •short wavelength E-M waves •after alpha or beta decay, it is common for the daughter nucleus to possess excess energy •EXCITED STATE! •GAMMA RAY! •allows the nucleus to readjust to the ground state •α Decay with γ decay . . . new element?? What kind of decay? BETA DECAY! Ex. RADIOACTIVE DECAY Nuclear radiation is a RANDOM event Like a hippo eating a zebra... BUT the RATE of decay for a given radioactive SPECIES is CONSTANT If we have a large number of radioactive of radioactive nuclei of a given species we can make a prediction as to the amount of radiation at any given time HALF LIFE! N = # of events at any given time, t = initial # of events = decay constant t = time EX: How long will it take Pu to decay 1% of its present value? years PROBLEM! What happens when a frog eats Pu? Through excretion, the radiation from the frog will exponentially decay at the biological decay rate But the Pu will still be decaying from inside Kermit! (The physical decay rate of the isotope). This leads to a new decay rate! An EFFECTIVE decay rate which combines BOTH types of active decay! PROBLEM! The physical ha
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