6 Radioactivity Flashcards
alpha radiation
consists of positively charged particles.
each particle comprises 2 protons and 2 neutrons (a helium nucleus), and has charge +2e, where e is the elementary charge
beta radiation
consists of fast moving electrons (B-) or fast moving positrons (B+)
a beta-minus particle has charge -e and a beta-plus particle has charge +e
gamma radiation
(or rays)
consists of high-energy photons with wavelengths less than about 10^-13 m
they travel at the speed of light and carry no charge
all are emitted from the nuclei of atoms as a result of changes within unstable nuclei
radioactive decay
the spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter from the nucleus. It is a random process meaning that it is impossible to predict which of a number of identical nuclei will decay next. Yet each decay follows a defined pattern and given a large enough number of nuclei this yields predictable results.
effect of electric and magnetic field on radioactive decay
uniform electric field provided by two oppositely charged parallel plates can distinguish between different types of radiation
the negative B- particles (electrons) are deflected towards the positive plate, whilst he positive alpha and B+ (positron) particles are deflected towards the neg plate
alpha particles are deflected less than beta particles because of their greater mass
the paths of the B- and B+ particles are mirror images
gamma rays are not deflected, bc they are uncharged
flemings left hand rule
Hold your thumb, forefinger and second finger at right angles to each other: the forefinger is lined up with magnetic field lines pointing from north to south. the second finger is lined up with the current pointing from positive to negative.
Thumb is direction of the force
absorption of alpha radiation
the large mass of alpha particles mean they interact with surrounding particles to produce strong ionisation, and therefore they have a very short range in air
it only takes a few cm of air to absorb most alpha particles
a thin sheet of paper completely absorbs them
absorption of beta radiation
the small mass and charge of B particles make them less ionising than alpha particles
this means that they have a much longer range in air, about a metre
it takes about 1-3mm of aluminium to stop most B particles
absorption of gamma radiation
gamma rays have no charge and this makes them even less ionising than B particles
the count rate decays exponentially with the thickness of a lead absorber
you need a few cm of lead to absorb a significant proportion of gamma rays
dangers of radioactivity
all radiation causes ionisation, which means they can damage living cells
handling radioactive substances
stored in lead lined containers
use pair of tongs with long handles when handling
transmutation
the changing of one element into another by radioactive decay
parent nucleus- nucleus before decay
daughter nucleus- nucleus after decay
alpha radiation charge
+2e
beta minus radiation charge
-e
beta plus radiation charge
+e
gamma radiation charge
0
alpha decay
loss of an a particle removes 2 protons and 2 neutrons from a parent nucleus, so the nucleon number drops by 4
daughter has diff proton number so is diff element
energy is released in the decay
beta minus decay
RA nuclei that emit B- radiation are characterized as having too many neutrons for stability
the weak nuclear force is responsible for one of the neutrons decaying into a proton
an electron and electron anti neutrino are emitted
beta plus decay
RA nuclei that emit B+ radiation are characterized as having too many protons for stability
the weak nuclear force is responsible for one of the protons decaying into a neutron
a positron and electron neutrino are emitted
gamma decay
photons are emitted if a nucleus has surplus energy following an alpha or beta emission
composition of the nucleus remains the same
patterns for stability
graph of no of neutrons N against proton no Z
all stable nuclei lie on a very narrow band known as the stability band
the ration of neutrons to protons ins table nuclei gradually increases as the no of protons in the nuclei increases
only nuclei with Z less than about 20 are stable with an equal no of protons and neutrons
most nuclei have more neutrons than protons
-nuclei with more than 82 protons are likely to decay by emitting a particles
-nuclei to the right of the band have too many protons and will likely decay by B+
-nuclei to the left of the band have too many neutrons and will likely undergo B- decay
how is RA decay random?
- we cannot predict when a particular nucleus in a sample will decay or which one will decay next
- each nucleus within a sample has the same chance of decaying per unit time
how is RA decay spontaneous?
-decay of nuclei not affected by the presence of other nuclei or external factors such as pressure
half life
half life of an isotope is the average time it takes for half the number of active nuclei in the sample to decay
the number N must therefore decay exponentially with time