Ionising Radiation and Half Life

Ionising Radiation and Half Life

1. Ionising Radiation

   • Alpha Particles are helium nuclei
     • An alpha particle is two neutrons and two protons the same as a helium nucleus
       • They are relatively big, heavy and slow moving
     • They therefore dont penetrate very far into materials and are quickly stopped even when travelling through air
     • Because of there size they are strongly ionising which just means they bash into a lot of atoms and knock electrons off them before they slow down which creates lots of ions
   • Beta particles are electrons
     • Beta particles are in between alpha and gamma in terms or their properties
     • They move quite fast and they are quite small (they are electrons
     • They penetrate moderately into materials before colliding have a long range in air and are moderately ionising too
     • For every beta particle emitted a neutron turns to a proton in the nucleus
     • A beta particle is simply an electron with virtually no mass and a negative charge
   • Gamma rays are very short wavelength EM waves
     • Gamma rays are the opposite of alpha particles in a way
     • They penetrate far into materials without being stopped and pass straight through air
     • This means they are weakly ionising because they tend to pass straight through air
     • Gamma rays have no mass and no charge
   • Alpha and beta particles are deflected by electric and magnetic fields
     • Alpha particles have a positive charge, beta particales have a negative charge
     • When travelling through a magnetic or electric field both alpha and beta particles will be deflected. They are deflected in opposite directions because of their opposite charges
     • Aplha particles have a larger charge than beta particles and feel a greater force in magnetic and electric fields. But they are deflected less because they have a much greater mass
     • Gamma radiation is an electromagnetic (EM) wave and has no charge so it doesnt get deflected by electric or magnetic fields

2. Half-Life

   • The radioactivity of a sample always decreases over time
     • Each time a decay happens and an alpha, beta or gamma is given out it means one more radioactive nucleus has disappeared
       • As the unstable nuclei all steadily disappear the activity (number of nuclei that decay per second will decrease. So the older a sample becomes the less radiation it will emit
         • How quickly activity drops varies. For some it takes a few microseconds before nearly all the unstable nuclei have decayed whilst for others it can take millions of years
         • The activity never reaches 0 which is why we have to use the idea of half life to measure how quickly the activity drops of
     • A short half life means the activity falls quickly because lots of the nuclei decay quickly
     • A long half lif means the activity falls more slowly because most nuclei dont decay for a long time they just sit there basically unstable but biding their time
   • Half life is the average time it takes for the number of nuclei in a radioactive isotope sample to halve
   • The activity of a radioisotope is 640cpm (counts per minute) two hours later it has fallen to 80cpm. Find the half life
     • Initial count: 640 /2 after one half life: 320/2 after two half lives: 160/2 after three half lives: 80
       • Two hours represents three half lives so the half life is 120 mins/3 = 40 mins
   • The half-life is found from the graph by finding the time interval on the bottom axis on the vertical axis