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COSMIC RAYS - MESSENGERS FROM OUTER SPACE

Every second, billions of tiny travellers from outer space bombard the Earth's atmosphere. They trigger showers of tiny energetic particles which travel down through the atmosphere to pass through the walls and floors, and even through YOU! Can you feel them? No. We are all oblivious to this constant rain of COSMIC RAYS.

What ARE cosmic rays?

Cosmic rays are some of the smallest pieces of matter that exist. All matter here on Earth is built from building blocks called ATOMS. But the atoms themselves are built from "subatomic particles". Particles called ELECTRONS whirl around a central nucleus, which contains other particles called PROTONS and NEUTRONS.

The travellers from outer space that come into the top of the Earth's atmosphere are mainly energetic protons, although they can also be nuclei from all kinds of atom, including uranium atoms - the heaviest atoms found on Earth. These protons and nuclei are called the "primary" cosmic rays.

When a primary cosmic ray hits the nucleus of an atom in the atmosphere, the nucleus breaks up and creates a shower of new particles. The energy of the primary cosmic ray is converted into matter. The new particles do not live for long, but change into other types of particle. These are the "secondary" cosmic rays. By ground level, we are usually left with electrons, just like those in matter on Earth, together with two less familiar types of particle, called MUONS and NEUTRINOS.

Can you SEE a cosmic ray?

Cosmic rays are far too small to see directly with our eyes or even with a microscope. But we can see the "footprints" of a cosmic ray, provided it is electrically charged. So we can see where the cosmic ray has been. When a high voltage is applied in a SPARK CHAMBER, the sparks reveal the paths of cosmic rays. MUONS leave beautiful straight tracks, but neutrinos leave no tracks at all because they have no electric charge.

What ARE MUONS?

Muons are just like heavy electrons - 200 times as heavy! But they do not exist in ordinary matter like electrons do. This is because they do not live for ever, but change into electrons. We still do not understand why muons exist. This is one of the challenges of modern physics.

o Did YOU know?

The highest energy cosmic rays arriving at Earth have as much energy as a tennis ball served by a top player. But don't worry. The Earth's atmosphere protects us from their direct impact - and besides, they arrive only once every 100 years or so for every square kilometre of the Earth's surface.

COSMIC CONUNDRA

Can you STOP a cosmic ray?

The electrons in cosmic rays will stop relatively easily in solid matter as they interact with atoms. But the muons are more difficult to stop, because they are much heavier and are not slowed down so easily by matter. Some muons will stop on their way through buildings, while others can travel many metres below ground. Cosmic-ray neutrinos, by contrast, are almost impossible to stop - they usually go right through the Earth! They do not interact with atoms in the same way that electrons and muons do, because they have no electric charge.

Can cosmic rays hurt you?

On rare occasions a cosmic ray can probably cause enough damage to a cell in a human body to lead to cancer. Around 1 in every 1000 deaths from cancer in the UK may be due to cosmic rays. But the changes that cosmic rays sometimes make in a cell could also be beneficial. In this way they may have played an important role in the evolution of life on Earth.

Where do cosmic rays come from?

The simple answer to this question is that we don't really know. Most of the primary cosmic rays come from beyond the Solar System, and those with the highest energies may even come from beyond our own Galaxy. The cosmic protons and nuclei are electrically charged, and the weak magnetic fields in the Galaxy exert forces on them that bend and twist their paths. So when the cosmic rays arrive at Earth we have no way of knowing from where they came.

The Oxford Connection

Physicists from Oxford, together with colleagues from American universities, are studying cosmic rays with a 1000-tonne detector in an iron mine at Soudan in northern Minnesota. At a depth of 2500 feet, the number of cosmic ray muons has fallen to about 10 per square metre per hour, compared with 100 per second for every square metre at the Earth's surface. This low rate of muons allows the physicists at Soudan to study cosmic neutrinos - in particular the neutrinos produced in the showers in the atmosphere.

Two types of neutrino are created in these showers - MUON-NEUTRINOS, which are related to muons, and ELECTRON-NEUTRINOS, associated with electrons. On the rare occasions that they interact in the detector, the two types of neutrino lead to different patterns of tracks, so the physicists can tell them apart. So far they seem to find more electron-neutrinos than expected, but too few muon-neutrinos. One explanation is that the neutrinos change - or "oscillate" - from one type to on their way through the atmosphere. If confirmed such an effect would have an another important bearing on our understanding of the fundamental particles of matter and their interactions.

See here for more on neutrino oscillations.

For further information contact: Professor Wade Allison, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH

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Text copyright Oxford University, Department of Physics