P7 Flashcards
Alpha radiation
Alpha particles, each composed of two protons and two neutrons, emitted by unstable nuclei
Activity
The number of unstable atoms that decay per second in a radioactive source
Beta radiation
Beta particles that are high energy electrons created in, and emitted from, unstable nuclei
Count rate
The number of counts per second detected by a Geiger counter
Ionisation
Any process in which atoms become charged
Irradiated
An object that has been exposed to ionising radiation- does not make a substance radioactive
Radioactive contamination
The unwanted presence of materials containing radioactive atoms on other materials
John Dalton model
1804- John Dalton claimed matter was made of tiny spheres “atoms” that couldn’t be broken off
JJ Thomson model
Plum pudding model - suggested atoms were spheres of positive charge with tiny negative electrons stuck in them
Who performed the first nuclear model of the atom?
Rutherford
Rutherford’s model
In 1909- scientists in Rutherford’s lab tried firing a beam of alpha particles at thin gold foil.
A few alpha particles were deflected back, with very few alpha particles deflecting through a large angle, most of the mass of the atom must be concentrated at the centre in a tiny nucleus, which must be positively charged, since it repelled the positive alpha particles.
Most alpha particles passed straight through the gold foil, so most of an atom is emoth space
Niels Bohr model
Electrons orbit the nucleus at certain distances called energy levels. Evidence from further experiments changed the model to have a nucleus made up of protons with the same positive charge that added up to the overall charge of the nucleus
Who provided evidence for the existence of the neutron?
James Chadwick in 1932
Current Atomic Model
1) The nucleus is tiny, but makes most of tne atom and has positively charged protons and neutral particles called neutrons , which gives an overall positive charge. Its radius is about 10000 times smaller than the radius of the atom.
2) The rest of the atom is mainly empty space, negative electrons whizz round the outside of the nucleus really fast.
3) Atoms have no overall charge, so number of protons = number of electrons
4) If electrons gain energy by absorbing electromagnetic radiation, they move to a higher energy level, further from the nucleus. If they release electromagnetic radiation, they move to a lower energy level closer to the nucleus.
If one or more outer electrons leaves the atom, the atom becomes a positively charged ion.
Isotope
Atoms with the same number of protons as an element, but a different number of neutrons
Radioactive decay
An unstable nucleus giving out radiation to become more stable(try to balance the number of protons and neutrons in their nucleus or get rid of excess energy). Neutrons can also be released to rebalance the number of protons and neutrons.
Are isotopes stable?
No
Types of radiation(from most penetrating to least penetrating)
1) Gamma radiation
2) Bega radiation
3) Alpha radiation
Types of radiation(from most ionising to least ionising)
1)Alpha radiation
2) Beta radiation
3) Gamma rays
Features of beta particles
A beta particle is a fast-moving electron released by the nucleus, with no mass and a charge of -1.
For every beta particle emitted, a neutron in the nucleus has turned into a proton.
Penetrate moderately far into materials before colliding
Range in air of a few metres.
Are ansorbed by a sheet of aluminium(around 5mm)- beta emitters are used to test the thickness of sheets of metal, because they’re not immediately absorbed like alpha radiation and don’t penetrate as far as gamma rays.
Can go through paper
Why are gamma rays weakly ionising?
Since they tend to pass through rather than collide with atoms
How far do alpha particles travel?
They only travel a few cm in air and are absorbed by a sheet of paper.
Gamma ray features
Gamma rays are waves of electromagnetic radiation released by the nucleus.
They travel a long distance through air
Can be absorbed by thick sheets of lead or metres of concrete
What form are nuclear equations written in?
Atom before decay: Atom after decay + radiation emitted.
The total mass and atomic numbers must be equal on both sides
What happens when an atom emits an alpha particle(in a nuclear equation)
Its atomic number reduces by 2 and its mass number reduces by 4. In nuclear equations, an alpha particle can be written as a helium nucleus: 4
He
2
The charge of the nucleus decreases, since a proton is positively charged and a neutron is neutral.
What occurs in beta decay(a nuclear equation)?
The positive charge of the nucleus increases, since the number of protons in the nucleus has increased by 1.
Since the nucleus has lost a neutron and gained a proton, the mass of the nucleus doesn’t change.
A beta particle is written as 0
e
-1
What happens to the nucleus with gamma rays?
Gamma rays are a way of getting rid of excess energy from a nucleus, so there’s no change to the atomic mass or atomic number of the atom.
Which instrument measures radiation given out by the nuclei of radioactive substances’ atoms
A Geiger-Muller tube and counter, which records the count-rate, the number of radiation counts reaching it per second.
Half-life
Time it takes for the amount of radiation emitted by a source/activith to halve. It can be used to make predictions about radioactive sources, even though their decays are random.
Activity
The rate at which a radioactive source decays.
Measured in becquerels
1 Bq=
1 decay per second
What happens to the activity when a radioactive nucleus decays to become a stable nucleus?
The activity as a whole will decrease
Model) The initial activity of a sample is 640Bq. Calculate the final activity as a percentage of the initial activity after two half-lives.
1) Find the activity after each half-life.
1-half life: 640/2=320
2 half lives: 320/2=160
2) Now divide the final activity by the initial activity, then multiply by 100 to make it a percentage.
(160/640) x 100=0.25 x 100=25%
How to find half-life from a graph
Find the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis to find a half-life.
Risks of ionising radiation
Ionising radiation can enter living cells and ionise atoms within them, which can damage the cells or kill them off completely.
How to reduce the effects of irradiation
Keeping sources in lead-lined boxes
Standing behind barriers
Remote-controlled arms to handle source in different room
Contaminated
If unwanted radioactive atoms get onto or in an object, the object is contaminated. These contaminating atoms might then decay, releasing radiation which could cause you harm.
How to reduce the risks of contamination
Gloves and tongs could be used when handling sources to avoid particles getting stuck to your skin.
Industrial workers may wear protective suits to stop them breathing in particles.
From what sources is irradiation most dangerous outside the body?
Beta and gamma sources, which penetrate the body and get to organs more than alpha sources
Which sources are most dangerous inside the body?
Alpha sources, since their damage is in a very localised area, so there’s a risk of contamination, but beta sources are less damaging as radiation is absorbed over a wider area and some passes out of the body altogether, and gamma sources are the least dangerous inside the body since they mostly pass out(lowest ionising power(,
How the alpha particle scattering experiment led to the nuclear model of the atom
1) The plum pudding model has charge scattered throughout atom and mass spread throughout atom.
2) In the alpha particle scattering experiment, alpha particles are fired at thin gold foil.
3) Most alpha particles pass through, so most of the atom is empty space.
4) Some particles changed direction more than expected, so the nucleus is positively charged and most mass is at the centre of the atom/in nucleus.
Why atoms have no overall charge
Number of protons = number of electrons
Why would beta/gamma radiation be more hazardous than alpha radiation in a smoke detector?
Is more penetrative - will irradiate people passing near the smoke detector
How does half life affect the stability of a nuclei?
A substance with a longer half life has a more stable nuclei
Uses of radiation
Checking thickness
Smoke alarms
Irradiating food
Sterilisation of equipment
Diagnosis of cancer
Radiation(to check thickness)
Use a detector to measure the rate Beta passes through paper,with thinner paper = higher betw count
Smoke alarms(radiation)
Contains a source of alpha particles
There is an electrical circuit with a gap between 2 charged plates.
Air in gap is constantly ionised therefore constant electric current
When smoke get in the alpha particles are absorbed and stops the current drops = alarm sounds
Irradiating food(uses of radiation)
Bacteria will cause food to decay or make us ill, but gamma rays kill bacteria, making food safer and longer lasting. Gamma rays don’t make food radioactive, with foods like fruit, cereals and shellfish irradiated.
Diagnosis of cancer(use of radiation)
Gamma rays used
A tracer solution is injected into the body that collects in cancers.
Gamma camera used to detect rays.
Pass through the body so easily detected
Sterilisation of equipment(use of radiation)
To kill microorganisms, surgical instrument needing to be sterilised, with some things like plastic irradiated with Gamma rays instead of using heat
