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Cosmic rays

Radiation and the atom · level 5, lesson 5 of 6

A question to start

In 1912, Victor Hess took a radiation detector up in a balloon. Everyone expected the radiation to fade as he rose away from the rocks of the ground. Instead, above about a kilometre, it grew stronger. Where was it coming from?

The idea

It was coming from space. Hess had found cosmic rays: fast particles that stream through space and hit Earth's air from every direction.

  • Most of them are protons. Some are helium nuclei, and a few are nuclei of heavier elements, up to iron and beyond.
  • Some come from the Sun. The fastest, galactic cosmic rays, come from outside the solar system, probably from exploding stars, and travel at nearly the speed of light.

Earth has two shields. Its magnetic field turns many charged particles aside. Its thick air absorbs the rest: a cosmic ray hits an atom high up and breaks into a shower of other particles, which lose energy as they come down. So the higher you go, the more cosmic radiation you get.

Shields against cosmic rays Mars Earth
Air above each square metre about 170 kg about 10,300 kg
Global magnetic field none yes
spacetop of the aira shower ofparticlesground

Worked examples

1. Height. The CNSC says a year of cosmic rays in Vancouver, at sea level, gives a dose of about 0.30 (in units met in level 6), and on top of Mount Lorne in the Yukon, at 2,000 m, about 0.84. \(0.84 \div 0.30 = 2.8\): 2 km of height nearly triples it.

2. Flying. Planes fly at about 10 km, above most of the air. Each flight adds a little cosmic radiation.

3. Mars. Mars has about 60 times less air above each square metre than Earth, and no global magnetic field. NASA says this gives minimal protection at the surface.

Try it

  1. What did Hess find as his balloon rose?
  2. What are most cosmic rays?
  3. Name Earth's two shields against cosmic rays.
  4. Why does someone on Mars get far more cosmic radiation than someone on Earth?
Answers
  1. The radiation got stronger with height, so it came from above.
  2. Protons.
  3. Its magnetic field and its thick air.
  4. Mars has very thin air and no global magnetic field.

Common mistake

"Cosmic rays are rays of light." The name is old. Cosmic rays are mostly fast particles, protons and other nuclei, not light.

Where it leads