Structure of the Nucleus
•Every atom has a nucleus, a tiny but massive center.
•The nucleus is made up of particles called nucleons.
•Proton : positively charged (2 up and 1 down quark)
Neutron: no charge (1 up and 2 down quarks)
•Different atoms are have different numbers of protons
and neutrons.
•These different types of nuclei are referred to as nuclides.
Atomic Number (Z):
The number of protons in a nucleus
Atomic Mass (N):
The total number of nucleons
(protons + neutrons)
Isotopes
• are nuclei that have the same
number of protons but different
numbers of neutrons.
• Isotopes of the same atom behave
almost exactly the same.
• Different isotopes have slightly
different masses
•The total mass of a stable nucleus is always less than the
sum of the masses of its constituent protons and neutrons.
•The lost mass goes into a form of energy, such as
radiation or kinetic energy.
•The difference in mass (or energy) is called the
TOTAL BINDING ENERGY.
This is described by E = mc2
•To find the average binding energy per nucleon one must
divide the total binding energy by A (the atomic mass
number).
Radioactivity
• Radiation is electromagnetic waves or actual
particles that carry energy!!!
• Marie and Pierre Curie isolated the unknown
elements polonium and radium. These elements
were releasing energy from their nuclei. A lot of
energy. Way more than the usual energy released
by ordinary chemical processes involving
electrons.
• Rutherford classified the three types of nuclear
“rays”-alpha, beta, and gamma.
Alpha Decay
• Alpha decay occurs when the strong nuclear
force is unable to hold the nuclei together.
• When alpha decay occurs, a new element is
formed. This process called transmutation.
• The original element is called the parent
nucleus, and the resulting element is called
the daughter nucleus.
Alpha Decay cont…
• The mass of the parent nucleus is greater then the
mass of the daughter nucleus and an alpha
particle.
• This difference in mass is a result of the kinetic
energy that leaves with the alpha particles.
• The total energy released is called the
disintegration energy, or Q value.
Alpha Decay example
• This is an example of alpha decay.U is the
parent, and Th is the daughter.
• The Alpha particle is exactly the same thing
as a Helium nucleus.
• It carries away 2 neutrons + 2 protons, so it
moves the nucleus UP the periodic table.
Alpha Particles are not very dangerous
• They’re large, so it turns out they can be blocked
by even a sheet of paper.
• But you wouldn’t want to get an alpha emitter
inside you.
• And many daughter nuclei are beta emitters…
Beta Decay
Beta Decay is a result of the Weak Force.
In Beta Decay an electron and neutrino are created
from the nucleus of an unstable isotope, transmuting
it to a different element. In the nucleus, one of the
neutrons turns into a proton.
Hydrogen3 turns into Helium3, for example.
It’s Helium because it GAINS a PROTON.
A neutrino carries off energy and momentum that are
required to maintain conservation laws.
This is a Feynmann diagram.
It shows a typical Beta decay event.
The Neutron
changes to a
Proton.
A DOWN Quark
changed to an
UP Quark.
A W- Boson was released, and almost instantly
decayed to an Electron and a Neutrino.
The Neutrino was invented in 1930 to
carry away some energy.
Without it, the books don’t balance…
It would be decades before physicists
were clever enough to actually detect
neutrinos.
1st detected in 1956,
Nobel Prize awarded 40
years later !
There are three different kinds of Beta Decay:
- and + Decay & Electron Capture
In a - decay an electron and an antineutrino are emitted.
In a + decay a positron and a neutrino are emitted.
In an electron capture the nucleus absorbs its own
electron, which causes a proton to become a neutron and
a neutrino is emitted.
Electron Capture
Gamma Decay
• When a nucleus is in an excited state known
as an isomer or metastable state, it must
release energy to become stable. It does
this by emitting a high-energy gamma ray.
• Excited nucleons can also release this
energy by way of internal conversion where
the nucleus interacts with an electron and
emits an X-ray.
Conservation of Nucleon #
• In any decay reaction, all conservation laws
are observed, including conservation of the
nucleon number.
• According to this law, the total number of
nucleons (A) must remain constant in any
process, though they may change into
different types (i.e. protons to neutrons).
A-4 N+ 4 He
• Alpha decay: AZN
Z-2
2
• Nuclei do not decay all at once but rather one by one over a
period of time.
• The half-life of an isotope is the amount of time it takes for
half the original amount of the isotope to decay.
• We cannot predict exactly when one given nucleus will
decay.
• Radioactive decay law
N = N0e-lt
Where N = # of nuclei present
N0 = # of nuclei at t = 0
l = decay constant
• The half-lives of known radioactive isotopes
vary from about 10-22s to 1028s.
• The half-life bears an inverse relationship to
the decay constant.
T1/2 = 0.693 / l
• The greater l is, the more radioactive that
isotope is said to be.
Decay Series
• Usually when one radioactive
isotope decays it will decay into
another radioactive isotope,
causing a series of decays.
• These series of decay are what
produce many of the elements
found in nature that otherwise
would have decayed long ago.
• Check out the cool chart on the
right which shows the decay of
238U through many steps until it
reaches Pb, which is stable.
Radioactive Dating
•Radioactive dating is the technique used to determine the age of
ancient materials.
•The most common form, Carbon dating, is a comparison of C14
to C12 in matter. C14 is used to date once living things.
•The small amount of C14 in the atmosphere is kept more or less
constant by the Sun’s energetic particles interacting with
atmospheric carbon dioxide.
•All life forms absorb Carbon, including both C12 (common) and
C14 (relatively rare) while alive, but when they die, the C14
begins to decay, so from the ratio of C14 remaining to C12, they
can figure out the accurate age.
•Carbon dating is effective for up to 60,000 years. Beyond that
much time, there’s so little C14 left in any sample that other
isotope systems must be used.