Background radiation

Natural radioactivity was first discovered when beta particles exposed photographic plates. One of the other technologies used to investigate radioactivity was the gold leaf electroscope. Electroscopes show the presence of electric charge. A charged electroscope slowly loses charge due to the ions in the air produced by radioactive elements in the Earth’s crust. It was thought that this effect would decrease with height above the ground.

However, in 1909 it was found that the intensity of radiation was greater on top of the Eiffel Tower. Balloon flights then showed the intensity continued to increase with height, suggesting that the radiation may originate from space. So, the name ‘ cosmic rays’ was coined.

Initially cosmic rays were called ‘rays’ because they were thought to be like light. However, even though they are now known to be particles, the name has stuck. Further investigation over the following decades showed that the particles entering the Earth’s atmosphere were mainly protons. The particles seemed to come from beyond the solar system from all points of the sky. Indeed now they are thought to originate in supernovae and the centre of galaxies. They are also extremely fast and energetic. The energy of these protons is 40 million times the energy of the protons in the Large Hadron Collider used to produce the Higgs boson.

When these protons with their massive energy hit an atom in the upper atmosphere, they cause a cascade of successive collisions that produces a shower of charged particles and gamma rays at the Earth’s surface.

On average, cosmic rays contribute about 16% of your exposure to ionising radiation from natural sources. This exposure increases the more you fly in a plane and the higher you fly.

Design an experiment using the Geiger counter to measure the background radiation inside and outside of the buildings. Before performing this investigation, in which environment would you expect to detect more background radiation? Give your reasons.

How will you measure the radiation? How many tests will be enough? For how long should you do each test?