Ionizing radiation is made up of photons and/or moving particles that have sufficient energy to knock an electron out of an atom or molecule.

Exposure is a measure of the ionization produced in air by x-rays or gamma-rays. It is defined in terms of charge per unit mass q/m (SI is C/kg). The first radiation unit was the Roentgen (R). 1 R = 2.58 x C/kg

The absorbed dose is the energy absorbed per unit mass of the absorbing material. The absorbed dose is measured in grays (Gy). 1Gy = 1J/kg.

Another unit of absorbed dose is the rad (rd) (radiation absorbed dose). 1 rad = 0.01 gray.

Different types of radiation have different levels of biological damage. This can be measured in rems (Roentgen equivalent, man), and is the absorbed dose X relative biological effectiveness.

Radiation sickness is the general term applied to the acute effects of radiation. It can range from nausea, vomiting, fever, diarrhea, and loss of hair to death.

When a nucleus, particle, or photon makes contact with a stable nucleus and causes a change in the nucleus, it is a nuclear reaction.

An example was observed by Rutherford in 1919: 4 2 He N --> 17 8 O H This is an induced nuclear transmutation.

Ex. 2 - An alpha particle strikes an aluminum Al nucleus. As a result, a nucleus ?? and a neutron 1 0 n are produced: 4 2 He Al --> ??? n. Identify the nucleus produced.

These induced transmutations can be used to form isotopes that do not exist naturally. This is how the transuranium elements are formed.

Nuclear fission is the splitting of a more massive nucleus into two less massive fragments. 1 0 n U --> Ba Kr n

Another possible reaction in the fission of Uranium-235 is: 1 0 n U --> Xe Sr n

Some reactions produce 5 neutrons, avg is 2.5.

Roughly 200 MeV are given off per fission, primarily in the form of kinetic energy. This is approximately 10 8 times greater than the energy released in an ordinary chemical reaction.

Of the two naturally occurring types of uranium: U and U, U is readily fissionable provided the neutron is a thermal neutron.

(A thermal neutron has KE ≈ 0.04 eV or less. It is called a thermal neutron because that amount of kinetic energy is similar to that of any molecule at room temperature).

The probability of a thermal neutron causing fission is about 500 times greater than that of a neutron whose energy is high, say 1 MeV.

If the neutrons emitted by fission cause another atom to fission, and this procedure continues, it is called a chain reaction.

An uncontrolled chain reaction can reach thousands of fissions in a millionth of a second, releasing an incredible amount of energy, as in an atomic bomb.

If the number of neutrons is limited (to one per fission on average), the rate of energy production is controlled.

The neutrons released in fission of uranium-235 have more energy than 0.04 eV. Reactors need some material to slow the neutrons to thermal neutron level. This material is called a moderator. A commonly used moderator is water.

When each fission leads to one additional fission, the reactor is said to be critical. If each fission does not produce a fission, it is subcritical. If each fission produces more than one fission, the reactor is supercritical.

Control rods inserted into the reactor core absorb neutrons, thus decreasing the amount of available neutrons to cause fission.

Most nuclear power plants use steam produced by the heat released from fission to turn turbine generators.

Nuclear fusion is the combining of two very-low-mass nuclei into a single, more massive nucleus.

A fusion reaction yields more energy than a fission reaction.

The temperature needed to start a deuterium- deuterium fusion reaction is about 4 x 10 8 K. Reactions that require such temperatures are called thermonuclear reactions.

One typical thermonuclear reaction is that which occurs in stars, the proton-proton cycle.

When heated to such temperatures, the fuel ionizes and forms a plasma. The trick is to keep the plasma contained until the fusion occurs.

This has been attempted in different ways: magnetic confinement (Tokamak), inertial confinement (high- intensity lasers).

Since 1932, it has been found that several hundred elementary particles exist, and the proton and neutron are no longer believed to be elementary particles.

The particles are divided into the photon family, the lepton family, and the hadron family. The photon family: only photons.

The lepton family: the electron, the neutrinos, the positron, the muon (µ+ and µ-), and the tau.

The hadron family: the pion (π +, π -, π 0 ), the proton, and the neutron. (These lists are not complete for these families.)

It is now believed that the hadrons are made up of quarks. The six types of quarks are called: up, down, and strange and charmed, top and bottom. Each of the six has an antiquark.

The quarks have fractional charges, so combinations of quarks result in charges of +1, -1, and zero.

Molecules are composed of atoms, which are made up of a nucleus surrounded by electrons. The parts of the nucleus are protons and neutrons which are combinations of quarks.

Hubble’s Law states that the further a galaxy is from earth, the faster it is receding: v = Hd. H is the Hubble parameter: m/slight-year.

Ex. 7 - Determine an estimate of the age of the universe using Hubble’s law.

This pushes time back to the big bang. The radiation left over after the big bang has been detected.