Download Physics 102, Class 25 The Atomic Nucleus and Radioactivity

Physics 102, Class 25
The Atomic Nucleus and Radioactivity
11/30/2005
Quantum Mechanics Review
• The PHOTOELECTRIC EFFECT can only
be understood if light has a particle nature
• At small scales, everything has both a
particle nature and a wave nature
• The particle nature is usually seen in
INTERACTIONS, the wave nature is
usually seen in PROPAGATION
Quantum Mechanics Review
• TWO BIG RULES for the Wave Nature:
– Energy is proportional to frequency, E=hf
– Momentum is inversely proportional to
wavelength, (mv)=h/wavelength
• On the exam I will give you
–
–
–
–
v=wavelength x frequency
E=hf
(mv)=h/wavelength
and the value of h, (if needed)
Quantum Mechanics Review
• Matter Waves give you PROBABILITIES, not
CERTAINTIES
• The two UNCERTAINTY PRINCIPLES comes
from thinking about standing waves in a box
– Δ(mv) x Δx is greater than or equal to hbar
– ΔE x Δt is greater than or equal to hbar
• The QUANTIZED ENERGY LEVELS seen in
atoms come from thinking about standing
waves around a nucleus. These explain the
discrete light spectra from atoms.
CPS Question
• Energy levels in atoms are discrete because
– A: electrons can orbit at any distance from the
nucleus
– B: only certain numbers of electrons are
permitted
– C: the electron’s wave function needs to be a
standing wave
CPS Question
• Energy levels in atoms are discrete because
– A: electrons can orbit at any distance from the
nucleus
– B: only certain numbers of electrons are
permitted
– C: the electron’s wave function needs to be a
standing wave
The Beginning of ““Radioactivity”:
Radioactivity”: X
-Rays
X-Rays
• Let’s apply what we learned in Quantum
Mechanics to Heavier Atoms
• We can get insight into:
– The sizes of Atoms
– The origin of X-rays
CPS Question
• X-Rays are associated with
– A: Transitions of outer electrons
– B: Transitions of inner electrons
CPS Question
• X-Rays are associated with
– A: Transitions of outer electrons
– B: Transitions of inner electrons
The Beginning of ““Radioactivity”:
Radioactivity”: Uranium
• Uranium: Who was Uranus?
– It’s the name given to the 7th planet outwards from the
Sun
learn more:
http://solarsystem.nasa.gov/planets
Wait a Minute, Look Closer at this Picture
• All the “dot” sizes are
about the same!
learn more:
http://solarsystem.nasa.gov/planets
Wait a Minute, Look Closer at this Picture
• All the “dot” sizes are
about the same
• We learned about
diffraction of light
passing through a narrow
slit
• This dot size is created
by diffraction of the light
entering the telescope
learn more:
http://solarsystem.nasa.gov/planets
Uranium
• Uranium: Who was Uranus?
– In Greek Mythology, the Titans,
who came before the Olympian
gods, were the children of Uranus
(male) and Gaea (female)
Cronus, a hungry Titan
painting by Goya
learn more at: www.timelessmyths.com/classical/titans.html
Uranium
image from: http://www.molres.org/MRI_Downloads.html
All Atoms Heavier than Lead are Radioactive
image from: http://www.molres.org/MRI_Downloads.html
Originally, There Were Three Known Types
of Radiation
• Alpha
– These were used by Rutherford’s group to find out that
atoms had nuclei. Alpha particles are helium nuclei, so they
are heavy and have a positive charge.
• Beta
– These were the “cathode rays” studied by JJ Thomson. They
are electrons, so they are light and have a negative charge.
• Gamma
– This is electromagnetic radiation, like visible light, but at
frequencies even higher than X-rays. They are emitted from
nuclear transitions. Gamma rays are massless and have no
charge.
Radioactivity Comes from the Nucleus
of the Atom
• Nucleus:
– made of protons and neutrons
– Held together by the “strong nuclear
force”
– makes up almost all the weight of the
atom
– tiny, 10-14 to 10-15 meter radius
– like a liquid drop (surface tension)
image from http://http://jazz.physik.unibas.ch/~krusche/nucleus.gif
Gamma Rays Come from Transitions
Between Nuclear Energy Levels
• quantum mechanics gives the nucleus an
internal shell structure, analogous to
electrons in atom
• when electrons in the atom change from one
quantized energy level to another, they emit
or absorb photons (X-ray or lower
frequency)
• when the nucleus changes from one
quantized energy level to another, it emits
or absorbs photons (Gamma Rays)
CPS Question
• Radioactivity originates from
– the electrons in the atom
– the nucleus of the atom
CPS Question
• Radioactivity originates from
– the electrons in the atom
– the nucleus of the atom
Alpha and Beta Particles
• Alpha particles pick up speed as they go
away from the nucleus
• Beta particles slow down as they go away
from the nucleus
Quarks
• There are something like 200 different
particles known now
• This big zoo of particles comes from
different combinations of six “Quarks”
• The six types:
– Up, Down, Strange, Charm, Top, Bottom
Go UNM!
• The name “Quark” comes
from Finnegan’s Wake
• the person who coined the
name, Murray Gell-Mann
(Nobel prize winner), founded
the Santa Fe Institute and
teaches at the UNM physics
department!
image from: http://www.bhm.ch/auxx/galerie_nachtderphysik/gross/Murray_Gell-Mann.jpg
Albuquerque Isotopes
• Isotopes have special meaning in
Albuquerque
Non
-Albuquerque Isotopes
Non-Albuquerque
• An element always has a certain number of
PROTONS
– the number of electrons equals the number of protons
– all chemistry and material properties are defined by
the number of electrons
• But an element can have different numbers of
NEUTRONS
– all have the same chemistry and material properties
• All the different versions of an element are
called “Isotopes” of the element
Example: Isotopes of Hydrogen
• 1 proton+0 neutrons
– normal hydrogen, most abundant, not radioactive
• 1 proton+1 neutron
– called “deuterium”, ingredient in “heavy water”, not
radioactive
• 1 proton+2 neutrons
– called “tritium”
– is radioactive
image from: http://images.encarta.msn.com/xrefmedia/aencmed/targets/illus/ilt/T046738A.gif
Example: Isotopes of Hydrogen
• All of these isotopes have almost exactly the
same chemical and material properties, because
those properties only depend on the electrons
image from: http://images.encarta.msn.com/xrefmedia/aencmed/targets/illus/ilt/T046738A.gif
Why Nuclei Fall Apart
• Protons REPEL each other by the electrical force
– works at all distances
• Protons and Neutrons ATTRACT other Protons
and Neutrons by the “strong nuclear force”
– only works at short distances
• If the nucleus gets big enough, then the protons
that are farther apart are REPELLED more by the
electrical force than they are ATTRACTED by the
nuclear force
– if it has the opportunity, the nucleus would like to split
into smaller pieces
Nuclei are Not Usually Round
• The nuclei have to find the best way to
balance out the electrical and nuclear forces,
so they are usually either
“Prolate” or
“Oblate”
images from http://en.wikipedia.org/wiki/Spheroid
CPS Question
• Nuclei can fall apart because
– A: The strong nuclear force can become bigger
than the electrical force for large nuclei
– B: The strong nuclear force can become smaller
than the electrical force for large nuclei
CPS Question
• Nuclei can fall apart because
– A: The strong nuclear force can become bigger
than the electrical force for large nuclei
– B: The strong nuclear force can become smaller
than the electrical force for large nuclei
What is ““Half-Life”?
Half-Life”?
• An unstable nucleus has a constant
probability per time of decaying
– probability of decay=constant x time
• The “half-life” is how much time is required
for 50% probability of decay
CPS Question
• Let’s say we start with a sample of an unstable
isotope. After 1 “half-life”, 50% of the
unstable nuclei have decayed, so 50% of the
sample remains. How much of the original
sample remains after 2 “half-lives”?
–
–
–
–
–
A: zero
B: 25%
C: 50%
D: 75%
E: Can’t say
CPS Question
• Let’s say we start with a sample of an unstable
isotope. After 1 “half-life”, 50% of the
unstable nuclei have decayed, so 50% of the
sample remains. How much of the original
sample remains after 2 “half-lives”?
–
–
–
–
–
A: zero
B: 25%
C: 50%
D: 75%
E: Can’t say
CPS Question
• If we have two radioactive samples that
both decay by emission of alpha particles,
which one will be “hotter”?
– A: the one with the longer half-life
– B: the one with the shorter half-life
CPS Question
• If we have two radioactive samples that
both decay by emission of alpha particles,
which one will be “hotter”?
– A: the one with the longer half-life
– B: the one with the shorter half-life
What
’s Left After the Nucleus Decays?
What’s
• You can figure it out by how much charge
is emitted in the decay:
– If + charges are emitted, the atomic number
goes down by the number of + charges
– If – charges are emitted, the atomic number
goes up by the number of – charges
– If neutrons are emitted, the atomic mass goes
down by the number of neutrons
– If gamma rays are emitted, atomic number and
atomic mass stay the same
• “Transmutation” is the name for changing
one element into another.
Carbon
-14 Dating
Carbon-14
• High energy radiation from space interacts with
the upper atmosphere, transmuting nitrogen to
carbon-14, which is radioactive
• Carbon is an essential part of life on Earth, and a
small but constant part of the carbon in every
living thing is this radioactive carbon-14
• After the plant or animal dies, it stops taking in the
carbon-14. We can determine the year of death
from the half-life of the carbon-14, or from the
ratio of carbon-14 to carbon-12.
Radiation and Health
• High-energy radiation can knock electrons off of
atoms and molecules in the body.
• This causes new molecules to form. Some of
them are useless and some of them are harmful.
• Because of the continuous background radiation
that we are all exposed to all the time, life has
repair mechanisms that can cope with radiation
received at constant low levels.
• Radiation at high levels will overwhelm the
body’s natural defenses.
• Natural background radiation exposure is about
equal to medical and dental X-rays, and these two
together are about 97% of all the radiation people
are usually exposed to.
Assignments
• Reading and Homework assignments are
listed on “Course Schedule” web page.