Each time an unstable nucleus decays and emits radiation, that means one more radioactive nucleus isn't there to decay later. As more unstable nuclei decay, the radioactivity of the source as a whole decrease - so the older a radioactive source is, the less radiation it emits.
HALF LIFE IS THE TIME TAKEN FOR HALF THE RADIOACTIVE NUCLEI TO DECAY.
A short half life means the activity falls quickly, because lots of the nuclei decay in a short time.
A long half life means the activity falls more slowly because most of the nuclei don't decay for a long time.
X Rays and Gamma Rays can transfer energy to the electrons. The electrons then have enough energy to escape from the atom, ionising it and leaving it positively charged.
Beta Particles can remove electrons from atoms or molecules they collide with, leaving them positively charged. A beta particle can also stick to an atom, ionising it and making it negatively charged.
Alpha Particles can remove electrons from atoms and molecules they pass by or hit, making them positive. Alpha are good ionisers because:
- Theyre relatively large - easy for them to collide with atoms or molecules
- Theyre highly charged - so they can easily remove electrons from the atoms they pass or collide with.
Lower doses of ionising radition tend to cause minor damage withouth killing the cell. This can give rise to mutant cells which divide uncontrollably - cancer. Higher doses tend to kill cells completely, which causes radiation sickness if a lot of cells all get blasted at once.
OUTSIDE the body, beta and gamma sources are the most dangerous - can still get inside to the delicate organs as they can pass through skin - alpha cannot penetrate the skin.
INSIDE the body, an alpha source is the most dangerous because they do all their damage in a very localised area.
X rays and Gamma rays are both high frequency, short wavelength electromagnetic waves. They have similar wavelengths and similar properties but are made in different ways:
- Gamma rays are released from some unstable atomic nuclei when they decay. Nuclear decay is completely random, so theres no way to control when they're released.
- X rays can be produced by firing high-speed electrons at a heavy metal like tungsten. These are much easier to control than gamma rays.
X rays pass easily through flesh but not so easily through thicker and denser materials like bones or metal.
The thicker/denser the material, the more X ray thats absorbed. So it is the varying amount of radiation that is absorbed that makes an x ray image.
Medical Uses of Radiation
Gamma rays can be used in radiotherapy- directed carefully and at just the right dosage so as to kill the cancer cells withouth damaging too many normal cells. To treat cancer:
- The gamma rays are focused on the tumour using a wide beam
- This beam is rotated round the patient with the tumour at the centre
- This minimises the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body.
Tracers - certain radioactive isotopses that emit gamma(and sometimes beta). Should have a short half life so that the radioactivity inside the person disappears quickly and can be injected, drunk, eaten or ingested. They are allowed to spread through the body and their progress can be followed using a radiation detector. e.g. Iodine-123 absorbed by the thyroid gland. All isotopes which are taken into the body must be gamma or beta as they are able to pass out of the body and be detected.
Sterilisation - Medical instruments can be sterilised by exposing them to a high dose of gamma rays which will kill all microbes. ADV - heat sensitive and plastic instruments can be totally sterilised without damaging them.
Uses of Radiation and Background Radiation
Tracers - Radioactive isotopes can be used to track the movement of waste materials, find the route of the underground pipe systems or detect leaks or blockages in pipes. To check a pipe, squirt the radioactive isotope in,go along the outside with a detector- radioactivity reduces or stops after a certain point, there must be a leak or a blockage.The isotope must be a gamma emitter - radiation can be detected through metal or earth-have a short half life
Smoke Detectors - A weak alpha radioactive source is placed in the detector, close to two electrodes.- causes ionisation of the air particles which allows a current to flow. If there is a fire, then smoke particles are hit by the alpha particles instead.This causes less ionisation of the air particles- current is reduced causing alarm to sound.
Background radiation we recieve comes from:
- (51% radon gas, 12% food, 12% Medical X-rays, 1% Nucleur Industry, 14% Rocks and building materials, 10% cosmic rays):
- radioactivity of naturally occurring unstable isotopes - a large proportion comes from these natural sources.
- Radiation from space (cosmic rays) which come from the Sun & Radiation from human activity - small proportion.
- The amount of background radiation can depend on where you are and your job.