P7 Flashcards
(35 cards)
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
