Nuclear physics definitions

Question 1

Activity A

Answer 1

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.

Question 2

Alpha decay

Answer 2

change in an unstable nucleus when it emits an alpha particle

Question 3

Alpha radiation

Answer 3

particles that are each composed of two protons and two neutrons

Question 4

Atomic mass unit u:

Answer 4

1/12 th of the mass of an atom of the carbon-12 atom, equal to 1.661 x 10-27 kg

Question 5

Atomic number Z

Answer 5

the number of protons in the nucleus of the atom.

Question 6

Background radiation

Answer 6

radiation due to naturally occurring radioactive substances in the environment and cosmic radiation.

Question 7

Beta-minus decay

Answer 7

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

Question 8

Beta-plus decay

Answer 8

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

Question 9

Binding energy of a nucleus

Answer 9

the work that must be done to separate a nucleus into its constituent neutrons and protons.

Question 10

Binding energy per nucleon

Answer 10

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.

Question 11

Chain reaction

Answer 11

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.

Question 12

Control rods

Answer 12

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.

Question 13

Coolant

Answer 13

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.

Question 14

Count rate

Answer 14

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).

Question 15

Critical mass

Answer 15

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.

Question 16

Decay curve

Answer 16

an exponential decrease curve showing how the mass or activity of a radioactive isotope decreases with time.

Question 17

Dose equivalent

Answer 17

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).

Question 18

Fission

Answer 18

the splitting of a nucleus into two (approximately equal) fragments. Induced fission is fission caused by an incoming neutron colliding with a nucleus.

Question 19

Fission neutrons

Answer 19

neutrons released when a nucleus undergoes fission and which may collide with nuclei to cause further fission.

Question 20

Fusion (nuclear)

Answer 20

the fusing together of light nuclei to form a heavier nucleus.

Question 21

Gamma radiation

Answer 21

electromagnetic radiation emitted by an unstable nucleus when it becomes stable.

Question 22

Half-life

Answer 22

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.

Question 23

Ionising radiation

Answer 23

radiation that produces ions in the substances that it passes through

Question 24

Isotopes

Answer 24

atoms of the same element which have a different number of neutrons (and the same number of protons).

Question 25

Inverse-square law (radiation)

Answer 25

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.

Question 26

Mass defect

Answer 26

the difference between the mass of the separated nucleons that make up the nucleus and the nucleus itself.

Question 27

Metastable state

Answer 27

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.

Question 28

Moderator

Answer 28

substance in a thermal nuclear reactor that slows the fission neutrons down so they can go on to produce further fission.

Question 29

Pair production

Answer 29

when a gamma photon changes into a particle and its antiparticle.

Question 30

Reactor core

Answer 30

the fuel rods and the control rods together with the moderator substance are in a steel vessel through which the coolant is pumped.

Question 31

Rest energy

Answer 31

energy due to the rest mass m0, equal to m0c2, where c is the speed of light in free space.

Question 32

Rutherford’s alpha-particle scattering experiment

Answer 32

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.

Question 33

Thermal nuclear reactor

Answer 33

nuclear reactor which has a moderator in the core.