P7 Flashcards
what is used to detect radioactivity
a geiger counter that clicks each time a particle of radiation from a radioactive substance enters the geiger tube
what is a radioactive substance
something that has an unstable nuclei that becomes stable by emitting radiation
what were the results when geiger and marsden investigated how a thin metal foil scatters a beam of alpha particles
most alpha particles passed straight through the metal foil
as angle of deflection increased, the number of alpha particles deflected per minute decreased
1 in 10,000 alpha particles were deflected by more than 90 degrees
what did rutherford deduce from the alpha particle scattering experiment
that there is a positively charged nucleus at the centre of every atom that is much smaller than the atom (because most alpha particles pass through without deflection)
most of the mass is located at the nucleus
why was rutherfords nuclear model of the atom accepted so quickly
agreed with the measurements geiger and marsden made in their expriments
explained radioactivity in terms of changes that happen to an unstable nucleus when it emits radiation
predicted the existence of the neutron (later discovered)
what was bohr’s model of the atom
electrons in an atom orbit the nucleus at specific distances and energy values
what is atomic number
number of protons in the nucleus
what is mass number
number of protons and neutrons
what are isotopes
atoms of the same element with different numbers of neutrons
what happens when an unstable nucleus emits an alpha particle
atomic number goes down by 2 and mass number goes down by 4
what is an alpha particle identical to
a helium nucleus
what happens when an unstable nucleus emits a beta particle
atomic number goes up by 1 and mass number doesn’t change
what happens when a gamma particle is emitted
no change as it is uncharged and has no mass
how do you carry out an absorption test
place the material between the geiger tube and the radioactive source then measure the count rate. add more materials until the count rate reaches 0 which is when the radiation has been stopped by the absorber material
what is ionisation
when radiation from radioactive substances knocks electrons out of atoms (atoms become charged as they lose electrons)
what is radioactive contamination
the unwanted presence of material containing atoms on other materials
what is the absorber material and range of alpha
thin sheet of paper
5 cm
what is the absorber material and range of beta
aluminium sheet (5mm thick) or lead sheet ( 2-3mm thick) 1m range in air
what is the absorber material and range of gamma
thick lead sheet ( several cm thick) or concrete ( more than 1m thick)
unlimited range in air
in what situation is alpha the most dangerous
in the body as it has the strongest ionising power
how do smoke alarms work
contain a radioactive isoptope that sends out alpha particles into a gap in a circuit in an alarm. alpha particles ionise the air in the gap so there is a current across the gap. in a fire smoke absorbs the ions so the current across the gap drops and the alarm sounds
why couldn’t beta or gamma radiation be used in a smoke alarm
they do not have strong enough ionising power to create enough ions to make the air in the gap conduct electricity
what is th activity of a radioactive source
the number of unstable atoms in the source that decay per second measured in becquerel
what is the half life of a radioactive isotope
the average time it takes for the number of nuclei of the sample to halve and for the count rate to halve its initial value
how do you calculate the count rate after x half lives
initial count rate / 2^x
what does a nucleus release when it undergoes fissions
2 or 3 neutrons at high speeds
energy in the form of gamma radiation plus the ke stored in the fission neutrons and the fragment nuclei
what is nuclear fission (induced)
nucleus of an atom of a fissionable substance is struck by a neutron causing the nucleus to split into 2 fragment nuclei and release several neutrons
what is spontaneous fission
fission that happens without a neutron being absorbed
how does a chain reaction of fission events occur
when a fission event releases several neutrons which cause other fissionable nuclei to split
what happens in a nuclear reactor
control rods absorb fission neutrons to ensure that on average only one neutron per fission goes on to produce further fission
what is nuclear fusion
2 small nuclei release energy when they are fused together to form a single larger nucleus. some of the energy is released as nuclear radiation from the larger nucleus formed
what are the difficulties of making fusion a useful source of energy
plasma of light nuclei has to be heated to very high temperatures and pressures before the nuclei will fuse. this is because 2 nuclei approaching each other will repel each other because of their positive charge but if they are moving fast enough they can overcome the force of repulsion and fuse together
what happens in a fusion reactor
plasma is heated by passing a very big electric current through it
plasma is contained by a magnetic field so it doesn’t touch the reactor walls as if it did it would go cold and fusion would stop
what are 8 sources of background radiation
cosmic rays ground and buidlings food and drink natural radioactivity in the air medical applications nuclear weapons tests air travel nuclear power
what is the main cause of background radiation in the air
radon gas that seeps from the ground from radioactive substances in rocks underground
