Radioactivity Flashcards
(37 cards)
Describe Rutherford’s experiment
He aimed a narrow beam of alpha particles, all of the same KE, at a thin gold foil and detected where they were scattered. He used a microscope to observe the pinpoints of light emitted when they hit a fluorescent screen and measured the number of alpha particles reaching the detector per minute at different angles. Only 1 in 2000 were deflected and 1 in 10,000 were deflected through angles over 90.
What conclusions were drawn from Rutherford’s experiment?
Most of the atom’s mass in concentrated in a small region, the nucleus, at the centre of the atom. The nucleus is positively charged because it repels alpha particles (positive) that approach it too closely.
What is count rate?
The number of counts per unit time detected by a Geiger Muller tube. One count is one particle of radiation that enters the tube. Value must be corrected by subtracting the count rate due to background radiation.
What completely absorbs each type of radiation?
Alpha- paper and thin metal foil
Beta- about 5mm of metal
Gamma- several centimetres of lead
How does a Geiger Muller tube work?
Contains argon at low pressure. A thin mica window allows radiation in. A metal rod down the middle of the tube is at a positive potential and tube wall is connected to the negative terminal of a power supply and is earthed. Gas atoms are ionised along the radiation’s track. The anions attracted to the rod and the cations to the wall. The ions collide with more gas atoms so more ions so, quickly many ions created and discharged at the electrodes. A pulse charge goes around the circuit to a resistor, R, causing a voltage pulse across it which is one count.
What is the intensity of radiation?
At a surface is the radiation energy per second per unit area at normal incidence to the surface
What is the intensity of gamma radiation related to distance, r, from the source?
Inverse square law. Proportional to 1/r^2
Describe electron capture
Some proton rich nuclides can capture an inner shell electron. This causes a proton in the nucleus to change into a neutron with the emission of an electron neutrino at the same time. The inner shell vacancy is filled by an outer shell electron, as a result causing an x-ray photon to be emitted by the atom.
How does ionising radiation affect living cells?
It can destroy cell membranes causing cells to die. It can damage vital molecules like DNA directly or indirectly by creating free radical ions which react with the vital molecules. Normal cell division is affected and nuclei become damaged. Damaged DNA can cause cells to divide uncontrollable causing a tumour which may be cancerous.
How does a film badge work?
It contains a strip of photographic film in a light proof wrapper. Different areas of the film are covered by absorbers of different materials and thickness. When the film is developed, the amount of exposure to each form of ionising radiation can be estimated from the blackening of the film. If the badge is overexposed, the wearer is not allowed to continue working with the equipment.
What is the dose or ionising radiation measured in terms of?
Energy absorbed per unit mass of matter from the radiation.
What causes natural background radiation?
Cosmic radiation. Radioactive materials in rocks, soil and air (eg radon gas)
What causes man made background radiation?
Medical devices. Nuclear weapons. Air travel. Nuclear power.
How should radioactive materials be stored?
In lead-lined containers because most sources produce gamma as well as alpha or beta. Lead must be thick enough to reduce the gamma radiation from the source in the container to about background radiation level. Regulations require the containers are under lock and key and a record of the sources is kept.
Regulations for using radioactive materials
Solid sources should be transferred using handling tools (eg tongs). The tools ensure the source is as far away from the user as practicable so beyond range of alpha and beta and lowest intensity gamma. Liquid, gas sources and powders should be in sealed containers so none can be breathed in, spilt or drunk. No source is allowed to touch skin and not be used longer than necessary.
Decay curve
An exponential decrease curve showing how the mass or activity of a radioactive isotope decreases with time.
Half life
The time taken for the mass of a radioactive isotope to decrease to half the initial mass.
What is the activity of a radioactive isotope?
The number of nuclei of the isotope that disintegrate per second. Unit is Bq where 1Bq is one disintegration per second. It is proportional to the mass of the isotope.
What is the power of a radioactive source?
Its activity multiplied by the energy of each particle emitted (provided all the particles have the same energy)
Decay constant
The probability of an individual nucleus decaying per second.
Which 3 variables have the same formula as the one for number of nuclei?
Mass, activity, corrected count rate
How does carbon dating work?
C-14 is formed in the atmosphere as a result of cosmic rays knocking out neutrons from nuclei which collide with nitrogen nuclei to form C-14 and a proton. CO2 taken in from photosynthesis by plants. Once dead, no more taken in so proportion of C-14 decreases as it decays. Can calculate age of sample by measuring it activity if you know the activity of the same mass of living plant.
How does argon dating work?
Argon in rocks due to electron capture in K-40. Although beta- decay to calcium is 8x more likely. For every N K-40 atoms now present, if there is 1 argon atom, there must have been N+9 K-40 atoms originally. Therefore can calculate age of rock using half life.
What characteristics should a radioactive tracer have?
Half life is stable enough for necessary measurements to be made and short enough to decays quickly after use. Emit beta or gamma radiation so it can be detected outside the flow path.
