Nuclear Physics Flashcards
Alpha radiation
Made up of alpha particles (Helium nuclei). Least penetrating but most ionising form of radiation.
Beta radiation
Consists of fast moving electrons. An electron is created and emitted from a nucleus with too many neutrons as a result of a neutron suddenly changing into a proton.
Gamma radiation
Consists of electromagnetic radiation, high energy photons. Most penetrating and least ionising form of radiation.
Rutherford’s experiment
Rutherford used a narrow beam of alpha particles, all of the same kinetic energy, in an evacuated container to probe the structure of the atom. A thin metal foil was placed in the path of the beam. Alpha particles scattered by the metal foil were detected by a detector which could be moved round at constant distance from the point of impact of the beam on the metal foil. Rutherford used a microscope to observe the pinpoints of light emitted when alpha particles hit the atoms of the fluorescent screen. He measured the number of alpha particles reaching the detector per minute for different angles of deflection from zero to 180 degrees.
Results of Rutherford’s experiment
Most alpha particles passed straight through the foil with little or no deflection; about 1 in 2000 were deflected. A small percentage of alpha particles (about 1 in 10 000) were deflected through angles of more than 90 degrees.
Conclusions from Rutherford’s experiment
Most of the atom’s mass is concentrated in a small region, the nucleus, at the cetnre of the atom and the nucleus is positively charged because it repels alpha particles (which carry positive charge) that approach it too closely.
Count rate
The number of counts in a Gieger tube divided by the time taken.
Geiger tube
A sealed metal tube that contains argon gas at low pressure. The thin mica window at the end of the tube allows alpha and beta particles to enter the tube, gamma photons can enter the tube through the wall as well. A metal rod down the middle of the tube is at a positive potential, the tube wall is connected to the negative terminal of the power supply and is earthed. When a particle of ionising radiation enters the tube, the particle ionises the gas atoms along its track. The negative ions are attracted to the rod and the positive ions to the wall. The ions accelerate and collide with other gas atoms, producing more ions. These ions produce further ions in the same way so that within a very short time, many ions are created and discharged at the electrodes. A pulse of charge passes round the circuit through resistor R, causing a voltage pulse across R which is recorded as a single count by the pulse counter.
Range of alpha radiation in air
Only a few centimetres.
Range of beta radiation in air
Up to about a metre.
Range of gamma radiation in air
Unlimited.
Inverse square law for gamma radiation
I = K/x^2 where I is intensity, K is the constant nhf/4π and x is the distance.
Sources of background radiation
Air (radon gas), medical, ground and building, food and drink.
Half life of a radioactive isotope
The time taken for the mass of the isotope to decrease to half the initial mass.
Activity, A, of a radioactive isotope
The number of nuclei of the isotope that disintegrate per second. A = λN where λ is the decay constant and N is the number of atoms of a radioactive isotope.
