Transcript Nuclear Physics - Mater Academy Lakes High School

Nuclear Physics
Spring 2013
Intro:
1. What atom is this?
2. Where do you find protons?
3. Where do you find neurons?
4. Where do you find electrons?
5. How many protons does it have?
6. How many neutrons?
7. Is it a neutral atom, and how do you
know?
The Atom
In the nucleus (nucleons)
Proton- (+) charged particle
Neutron- no charge
Outside the nucleus
Electron- (-) charged particle
has almost no mass
Nucleons
• Are particles occupying the nucleus
• Consist of + charged protons and neutral
neutrons
• Have almost 2000 times the mass of
electrons
• Where can you find the number of protons?
• It’s the atomic number (found on the periodic
table)
Nuclear Notation
Atomic Number
5
B
10.811
Atomic Mass
Atomic number = no. of protons
Atomic mass = protons + neutrons
Atomic number is the same as the number of electrons
in an uncharged atom
Question 1
You may see atomic number written many ways. The smaller
number is the atomic number and the larger is the atomic
mass
1a. How many protons?
1b. How many neutrons?
1c. How many nucleons?
• has 13 protons and 14 neutrons for a
total of 27 nucleons
28
13
• has 13 protons and 15 neutrons for a
total of 28 nucleons
• The identity of an element depends
on the number of protons
Isotopes:
• Atoms of the same element with different
numbers of neutrons (different masses)
Most common
stable isotope
of carbon
Unstable
radioactive isotope
of carbon
Question 2
• List the four fundamental forces from
strongest to weakest
1.
2.
3.
4.
Review of Fundamental forces
Strongest to weakest
1.
2.
3.
4.
Strong Nuclear Force
Electromagnetic Force
Weak Nuclear Force
Gravity
Forces Acting on Nucleons:
Forces of attraction between nucleons
Strong forces
– Are independent of the charge of the nucleon
– Are short range (exist only between closest
neighbors)
Electrical force (electrostatic)
– Force of repulsions between positively
charged protons
– Are long range
When are nuclei unstable? (naturally radioactive)
a. Large nuclei (Z > 82) – electrical forces of
repulsion are greater than strong forces of
attraction
b. Wrong neutron : proton ratio
stable nucleus
no. of protons
no. of neutrons
6
6
13
14
26
30
56
81
82
125
When are nuclei unstable?
Bigger atoms require more neutrons per
proton to keep the atom stable
A radioactive isotope:
• Has an unstable nucleus
• Spontaneously emits a particle and
decays into another element (to become
more stable)
Transmutation
• Changing into another element through
radioactive decay
Who am I?
• I worked with my husband and discovered
radium, a radioactive material
Marie and Pierre Curie
• First to discover that
compounds containing
uranium emitted
penetrating rays.
• Discovered radioactive
polonium and radium
Types of Radioactive Emission
Symbol
Composition
Stopped By
Alpha
2p + 2n
(helium)
Paper
Beta
1e
(electron)
Aluminum
Energy
only
Lead
Gamma
γ
Alpha Decay
• Radiation through the loss of 2p + 2n or
(helium)
Beta Decay
• Radiation where a neutron splits, giving off
an electron and becoming a proton in the
new element
Gamma Decay
• A change energy state gives off a gamma
particle or photon
Question 3a
Balance the nuclear equation after alpha
decay
Question 3a
Balance the nuclear equation after alpha
decay
Question 3b
Balance the nuclear equation after beta decay
Remember in beta decay a neutron changes into a proton by giving off an
electron
Question 3b
Balance the nuclear equation after beta decay
Remember in beta decay a neutron changes into a proton by giving off an
electron
Extra Question
• Which radioactive isotope completes this
nuclear decay equation
6
Extra Problem
• Finish off the equation
Half Life and Half Life Calculations
• Half Life- time it takes for half
of the radioactive sample to
decay.
– Ranges from a fraction of a
second to billions of years
• Decay constant- Probability
per time that a nucleus would
decay
Section 2 Intro
1. Rewrite and balance the equation above
2. What kind of decay is shown above?
3. What is the particle given off during alpha
decay composed of?
4. What is the particle given off during beta
decay composed of?
Section 2: Nuclear Physics Math
Half Life and Half Life Calculations
y= fraction of radioactive material left
n= number of half lives
Example 1
• How much of the original radioactive
material is left after 15 half-lives?
Example 1
• How much of the original radioactive
material is left after 15 half-lives?
Half Life and Half Life Calculations
T1/2 = half life
λ = decay constant
The unit for λ and T1/2 will be in the same timeframe
Example 2
Cobalt-60, used in radiation therapy, has a halflife of 5.26 y. What is the decay constant for
cobalt-60?
Example 2
Cobalt-60, used in radiation therapy, has a halflife of 5.26 y. What is the decay constant for
cobalt-60?
Extra Examples A
• A radioactive sample has a mass of 56 mg
and a half life of 30 minutes. How much of
the sample remains after 60 minutes have
passed?
Extra Examples B
• An unknown radioactive material has a
half life of 4000 years. How much of the
sample will remain after 20,000 years?
Half Life and Half Life Calculations
N=
-λt
Noe
N = number of radioactive atoms
No = original number of radioactive atoms
t = elapsed time
λ = decay constant
e = 2.72
Example 3
Cobalt-60, used in radiation therapy, has a half-life
of 5.26 y. A sample of cobalt-60 containing 5.00 x
1012 radioactive atoms sits in a lead case in the
medical stockroom for 10.09 years. How many
cobalt-60 atoms remain after this amount of time?
(You already solved for the decay constant in example 2) λ= 0.132 y-1
Example 3
Cobalt-60, used in radiation therapy, has a half-life
of 5.26 y. A sample of cobalt-60 containing 5.00 x
1012 radioactive atoms sits in a lead case in the
medical stockroom for 10.09 years. How many
cobalt-60 atoms remain after this amount of time?
Useful applications of radioactivity
• Can be detected and therefore small
amounts can be used as tracers for
medical diagnosis
• Larger amounts can be used as
treatments for certain types of cancers
(cancer cells are killed before healthy
cells)
• Can be used to determine the age of rocks
and fossils
Show what you know
Types of Nuclear Reactions
Natural transmutation – Uranium spontaneously
decays
Artificial transmutation – bombardment of a stable
isotope to force it to decay
Question 4
• Balance the reaction after the following
artificial transmutation.
Types of Nuclear Reactions
Artificial transmutation
• First done by Earnest Rutherford
• When the bullets are positively charged,
they are repelled by the nucleus they are
bombarding. To overcome the repulsions,
they must be accelerated to very high
speeds by particle accelerators.
Nuclear Fission
• Nuclear fission - Heavy nuclei are
bombarded with neutrons and split.
plus a tremendous
amount of energy
Nuclear fission
• Mass of particles produced is slightly less
than the mass of the reactants. This mass
is converted into energy. (E=mc2)
Nuclear fission is a chain reaction. Neutrons
are needed to start and released as a
product which can start more reactions.
Critical mass: minimum mass of fissionable
material required for a chain reaction.
Problems with Fission
• Nuclear fission produces radioactive waste that has
a large half life.
U-235
Uranium 235
– Half life of U-235 is 713 million years
• We cannot get rid of this dangerous product so we
store it away from anything it can harm.
– We deeply bury
• Meltdown if cooling system fails the reactor can
overheat and melt releasing radioactive materials
• Nuclear fusion – combination of small
nuclei into larger with release of energy.
• Mass of particles produced is much less than the
mass of the reactants.
• This mass is converted into energy. (E=mc2)
• Can release up to 10 times that of fission
• Occurs naturally in our sun and other stars
• Does not give off radioactive waste
Problems with Fusion
• Fusion requires high temperatures like
those in the stars.
• We cannot sustain these temperatures
without vaporizing the container of the
fusion reaction.
• Today many are looking into ways of
making fusion work under sustainable
conditions