Radioactivity

what is activity measured in

Becquerels Bq

1 Becquerel =

1 decay per second

define the term half life

The activity of a radioactive sample decreases over time.​

The half-life of a radioactive sample is the average time taken for half of the original mass of the sample to decay. ​

what is a count rate

number of radioactive particles detected per second.​

how does photographic film detect radiation

darkens on exposure to radiation and light, light cannot penetrate the badge but ionising radiation can. darkening of the film indicates that a person has been exposed to too much radiation

radioactivity can be detected using…

photographic film or a Geiger counter

how does the geiger tube detect radioactivity

radiation produces ions in a low pressure gas between a central positively charged electrode & the outer negatively charged tube. a pulse of current then flows that is registered by the counter. the thin mica window allows the least penetrating radiation (alpha) to enter the tube. Gamma radiation and most beta can enter through the sides of the metal tube

give an example of a natural substance on earth

air, food, rocks, soil

give an example of a natural background radiation from space

cosmic rays (high energy particles) mainly from the sun

give an example of radiation from living things

carbon-14

artificial background radiation due to human activity

nuclear waste, fallout from nuclear weapons testing

background radiation

background nuclear radiation low-level ionizing radiation that is produced all of the time

explain radon gas

• about 50% of natural background radiation

• isotopes: radon 222 + radon 220 are produced by the radioactive decay of uranium and thorium in the Earth’s crust

• this gas seeps into atmosphere & can build up foundations of buildings

explain cosmic rays

• produce high energy particles, bombarding the Earth

• atmosphere protects us from cosmic radiation

• natural background radiation

  • produced by nuclear reactions in stars & supernovas

explain internal radiation

• radioactive sources inside our bodies

• some are natural, others man-made

explain artificial radiation

• man-made events/procedures

• some due to leakage and accidents or due to fall-out from nuclear weapon testing

• radioactive tracers are used in industry & medicine

• normally accounts for a small % of background radiation

uses of radioactivity

smoke detectors, automatic thickness monitoring, tracing underground leaks in pipes, radiotherapy, medical tracers in diagnosis, sterilisation

radioactivity in smoke detectors

• a radioactive source inside the alarm emits alpha particles which ionise air in an air gap so that it conducts electricity

• in a fire, smoke particles block the alpha radiation, reducing ionisation

• this causes a drop in electric current which sets off the alarm

properties requires of radioactivity in smoke detectors

• a source of alpha radiation must be used

• a long half-life source must be used

radioactivity in automatic thickness monitoring

the amount of radiation received by the detector depends on the thickness of the aluminium foil

if the thickness increases then the detector reading falls and will cause the computer to bring the rollers closer together & so decrease the foil thickness

properties required of radioactivity in automatic thickness monitoring

a source of beta radiation must be used

a long half-life source must be used

radioactivity in tracing underground leaks and pipes

a radioactive tracer can be added to a fluid where a leak occurs will be shown by an increase in the count rate detected

why must you use a beta source in radioactivity in thickness monitoring

alpha wouldn’t pass through the thinnest aluminium and gamma wouldnt be affect by any thickness charge

why do you need a long half life source in radioactivity in automatic thickness monitoring

or else a false thickness increase will be detected as the activity of the source decreases

whats a suitable source with a long half life for radioactivity in automatic thickness monitoring

a suitable isotope is stronium-90, a beta emitter with a half life of 29 years

why do you need to use an alpha source in radioactivity in smoke detectors

beta or gamma wouldn’t cause sufficient ionisation & they wouldn’t come sufficient ionisation and wouldn’t be blocked by smoke

why do you need a long half life source in radioactivity in smoke detectors

or else a drop in current would set off the alarm

whats a commonly used alpha emmitter with a long half life for radioactivity in smoke detectors

the commonly used isotope Americum-241 an alpha emitter with a half life of 433 years

properties required of radioactivity in tracing underground leaks in pipes

if liquid can escape and pool in the ground, or if it is in a plastic pipe wall, what source will be used

beta can be used

if leakage is in a metal pipe underground then what source will be used

gamma

why must gamma be used if leakage is in a metal pipe underground

because gamma radiation wouldn’t give a count rate whether or not a leak was present. However, there would be an increased count rate at the site of the leak due to less absorption

why must you need a long half life for radioactivity in underground leaks and pipes

to remain reasonably active over the period of investigation but not too long so that it doesn’t remain a hazard to the environment