Nuclear physics definitions Flashcards
Activity A
the number of nuclei of the isotope that disintegrate per second. The unit of activity is the Becquerel (Bq), equal to 1 disintegration per second.
Alpha decay
change in an unstable nucleus when it emits an alpha particle
Alpha radiation
particles that are each composed of two protons and two neutrons
Atomic mass unit u:
1/12 th of the mass of an atom of the carbon-12 atom, equal to 1.661 x 10-27 kg
Atomic number Z
the number of protons in the nucleus of the atom.
Background radiation
radiation due to naturally occurring radioactive substances in the environment and cosmic radiation.
Beta-minus decay
change in a nucleus when a neutron changes into a proton and an electron and an antineutrino are emitted if the nucleus is neutron-rich
Beta-plus decay
change in a nucleus when a proton changes into a neutron and a positron and a neutrino are emitted if the nucleus is neutron-rich
Binding energy of a nucleus
the work that must be done to separate a nucleus into its constituent neutrons and protons.
Binding energy per nucleon
the average work done per nucleon to separate a nucleus into its constituent neutrons and protons. Binding energy per nucleon = binding energy of nucleus/mass number A.
Chain reaction
a series of reactions in which each reaction causes a further reaction e.g. fission in a nuclear reactor. A steady chain reaction occurs when, on average, one fission neutron from each fission event produces a further fission event.
Control rods
rods made of a neutron-absorbing substance such as cadmium or boron that are moved in and out of the core of a nuclear reactor to control the rate of fission events in the reactor.
Coolant
a fluid that is used to prevent a machine or device from becoming dangerously hot. The coolant of a nuclear reactor is pumped through the core of the reactor to transfer thermal energy from the core to a heat exchanger.
Count rate
the number of count per unit time detected by a Geiger Muller tube. Count rates should always be corrected by measuring and subtracting the background count rate (the count with no radioactive source present).
Critical mass
the minimum mass of the fissile isotope in a nuclear reactor necessary to produce a chain reaction. If the mass if the fissile isotope in the reactor is less than the critical mass a chain reaction does not occur because too many fission neutrons escape from the reactor or are absorbed without fission.
Decay curve
an exponential decrease curve showing how the mass or activity of a radioactive isotope decreases with time.
Dose equivalent
a comparative measure of the effect of each type of ionising radiation, defined as the energy that would need to be absorbed per unit mass of matter from 250 k of X-radiation to have the same effect as a certain ‘dose’ of the ionising radiation. The unit of dose equivalent is the Sievert (Sv).
Fission
the splitting of a nucleus into two (approximately equal) fragments. Induced fission is fission caused by an incoming neutron colliding with a nucleus.
Fission neutrons
neutrons released when a nucleus undergoes fission and which may collide with nuclei to cause further fission.
Fusion (nuclear)
the fusing together of light nuclei to form a heavier nucleus.
Gamma radiation
electromagnetic radiation emitted by an unstable nucleus when it becomes stable.
Half-life
the time taken for the number of nuclei of a radioactive isotope to decrease to half the initial number. This is equal the to the time for its activity to halve.
Ionising radiation
radiation that produces ions in the substances that it passes through
Isotopes
atoms of the same element which have a different number of neutrons (and the same number of protons).
Inverse-square law (radiation)
the intensity of radiation from a point source that is not absorbed and spreads out equally in all directions varies with the inverse of the square of the distance from the source.
Mass defect
the difference between the mass of the separated nucleons that make up the nucleus and the nucleus itself.
Metastable state
an excited state of the nuclei of an isotope that lasts long enough after alpha or beta emission for the isotope to be separated from the parent isotope.
Moderator
substance in a thermal nuclear reactor that slows the fission neutrons down so they can go on to produce further fission.
Pair production
when a gamma photon changes into a particle and its antiparticle.
Reactor core
the fuel rods and the control rods together with the moderator substance are in a steel vessel through which the coolant is pumped.
Rest energy
energy due to the rest mass m0, equal to m0c2, where c is the speed of light in free space.
Rutherford’s alpha-particle scattering experiment
demonstrated that every atom contains a positively charged nucleus which is much smaller than the atom and where all the positive charge and most of the mass is located.
Thermal nuclear reactor
nuclear reactor which has a moderator in the core.
