November 9th, 2011
•ATOMS and the NUCLEUS
•TYPES of RADIOACTIVE DECAY
•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
•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
•mass = 1.675 x 10 ^-27 kg
•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
•negative charges (-1 e)
•mass = 9.109 x 10^-37 kg
•The electron is a type of LEPTON - a very stable subatomic particle
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?
# Of protons = # of electrons in a stable atom
•300 STABLE isotopes of those elements
Not EVERY combination of protons and neutrons will result in a happy (or stable)
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)
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
TWO OBVIOUS RULES!
•eject a tight cluster of 2 protons and 2 neutrons
•Ex. (226 = 222 + 4, 88 = 86 + 2)
•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+
•Often accompanied by a charge less, 'mass less' particle which carries energy
•MUCH lighter and LESS heavy than the alpha particles
•penetration depth is LONGER and more SPREAD OUT than alpha particles
•Not a particle!
•short wavelength E-M waves
•after alpha or beta decay, it is common for the daughter nucleus to possess
•allows the nucleus to readjust to the ground state
•α Decay with γ decay . . . new element??
What kind of decay? BETA 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
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
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
This leads to a new decay rate! An EFFECTIVE decay rate which combines BOTH
types of active decay!
The physical ha