Structure of the NucleusStructure of the Nucleus
•Every atom has a nucleus, a tiny but massive center.Every atom has a nucleus, a tiny but massive center.
•The nucleus is made up of particles called nucleons.The nucleus is made up of particles called nucleons.
•Proton : positively charged (2 up and 1 down quark)Proton : positively charged (2 up and 1 down quark)
Neutron: no charge (1 up and 2 down quarks)Neutron: no charge (1 up and 2 down quarks)
•Different atoms are have different numbers of protons Different atoms are have different numbers of protons and neutrons.and neutrons.
•These different types of nuclei are referred to as nuclides.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).
• Alpha decay: AZN A-4
Z-2N+ 42He
• 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-t
Where N = # of nuclei present
N0 = # of nuclei at t = 0 = 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 /
• The greater 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.
