Nuclear Physics - Topic 7

Nucleus

Nucleus

small, dense region consisting of protons and neutrons at the centre of the atom

Nuclide

a particle type of nucleus that is characterized by the number of protons, neutrons, and the energy state

Nucleon

a proton or neutron

Atomic Number (Z)

number of protons in nucleus

Mass Number (A)

number of protons + neutrons

Neutron Number (N)

number of neutrons in nucleus

Isotopes

nuclei with same number of protons but different numbers of neutrons

Unified atomic mass unit (u)

1/12th the mass of a carbon-12 nucleus

Atomic mass

mass number * unified atomic mass unit

Gravitational force

a long range, attractive but very weak force between masses

Colomb/Electromagnetic

long range, repulsive or attractive, magnetic or electrostatic forces with a relatively strong force

Strong nuclear force

very short range, attractive and strongest force between any two nucleons that keeps the protons together inside the nucleus

Weak nuclear force

very short range force that is involved in radioactive decay, often involves lighter particles and heavier particles

How does emission spectra work?

1. Low pressure gas is energized by applying a potential difference across it, causing it to heat up.

2)The hot gas emits light energy only at certain well-defined frequencies, as seen through a diffraction grating or prism.

How does the absorption spectra work?

1. Light is shone through a cool, low pressure gas

2. A diffraction grating or prism is used to determine the frequencies at which the gas passes through is absorbed

Spectral Lines

Lines characteristic of the particular element producing them.

What is Alpha Decay?

What is a positron?

The anti-matter version of an electron, which shares its mass but has a positive charge instead of a negative charge. It forms in beta-positive decay.

What forms during Beta-minus?

An electron

What forms during beta-positive?

Positron

Beta-minus decay

V represents an anti-neutrino particle

Beta-positive decay

e represents an anti-electron particle, and v is a neutrino

Gamma decay

y is a high energy photon

Energy spectra of radiation for each decay process

Alpha spectra - discrete

Beta spectra - continuous

Gamma spectra - discrete

Ionizing Radiation

As radiation passes through materials, it “knocks off” electrons from neutral atoms, thereby creating an ion pair: free electrons and a positive ion.

ionizing property

Allows radiation to be detected but is also dangerous since it can lead to mutations in biologically important molecules in cells, such as DNA

Summarized properties of alpha (Particle, penetration ability, material needed to absorb it, path length in air, and speed)

Particle - Helium nucleus

Penetration ability - low

Material needed to absorb it - Sheet of paper; a few cm of air

Path length in air - a few cm

Speed - about 10^7 m/s

Summarized properties of beta (Particle, penetration ability, material needed to absorb it, path length in air, and speed)

Particle - Electron or positron

Penetration ability - low

Material needed to absorb it - 1 mm of aluminium

Path length in air - less than 1 m

Speed - About 10^8 m/s, very variable

Summarized properties of gamma (Particle, penetration ability, material needed to absorb it, path length in air, and speed)

Particle - high-energy photon

Penetration ability - high

Material needed to absorb it - 10 cm of lead

Path length of air - infinite

Speed - 3 x 10^8 m/s

Geiger counter

Detects and counts the number of ionizations taking place

Half-life

The time taken for half of the nuclides in a sample to decay

Activity (A)

The number of radioactive disintegrations per unit time (decay rate)

The Radioactive Decay Law

The rate at which radioactive nuclei in a sample decay (the activity) is proportional to the number of radioactive nuclei present in the sample at any one time.

Mass defect (mass deficit)

Difference between the mass of the nucleus and the sum of the masses of its individual nucleons

Nuclear binding energy

Energy requires when nucleus is separated into its individual components

Formula for nuclear binding energy

E = mc^2

Artificial (Induces) Transmutation

A nucleus is bombarded with a nucleon, an alpha particle or another small nucleus, resulting in a nuclide with a different proton number (a different element)

Nuclear Fusion

Two light nuclei combine to form a more massive nucleus with the release of energy

Nuclear Fission

A heavy nucleus splits into two smaller nuclei of roughly equal mass with the release of energy.

Natural Radioactivity

When an unstable (radioactive) nucleus disintegrates spontaneously, the nucleus emits a particle of small mass and/or a photon.

Release of energy in nuclear reactions

Energy is usually released in the form of kinetic energy for the products.

Binding energy per nucleon

Greater for product nuclei than for original nuclei since energy is released.

What is the main source of the sun’s energy? What is the problem with using that on earth? How is is fused?

It is the fusion of hydrogen to helium. The problem with using fusion on earth is that the nuclei in fusion are positively charges and to fuse them, the need to be close enough to the point where the strong nuclear force becomes stronger than the coulomb repulsion force. This is done by colliding them at a very high speed in temperatures above 10°C.

Anti-particle

A particle with the same rest mass but opposite charge

Elementary particle

A particle with no internal structure that cannot be broken down further.

Exchange particles

A virtual particle that transfers between force between interacting particles.

Pair production

Process by which an electron and positron are produced.

Quantized energy

Energy values that are continuous

Rest mass

Mass of the object measured in the object’s rest frame

Random decay

Cannot predict when the nucleus will decay next

Quark confinement

Quarks cannot be directly observed as free particles because the energy given to nucleons creates other particles rather than freeing quarks

Standard model

The theory that describes the electromagnetic and weak interactions of quarks and leptons

Spontaneous decay

The decay cannot be modified in any way

Virtual Particle

A particle that mediated one of the fundamental forces