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Watching for space invaders

9 March 2006

Every second, millions of cosmic rays come crashing through the atmosphere, showering us with subatomic particles.

Thanks to a grant from the Particle Physics and Astronomy Research Council (PPARC), a hand-held detector and a 9 volt battery, Bristol University researchers have been bringing the mysteries of cosmic rays to children at schools and science events around the country. 

Not much larger than a Coke bottle, the portable cosmic ray detector is a light-tight box containing a bundle of over 1 million scintillating glass fibres.  As ionizing radiation (such as cosmic rays) passes through the active fibre-optic material, scintillation light is produced (rather than heat).  A fraction of this light is trapped in the fibres and transmitted to the fibre ends. 

The low light level signals are amplified, while maintaining the position of the image, to levels suitable for viewing by the naked eye.  The end result is that you can view the two-dimensional path of particle as it passes through the detector as a flash of light.  Due to the size of the detector you can typically see one to two cosmic rays per minute.

Most galactic cosmic rays derive their energy from supernova explosions

Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions.  The term ‘cosmic rays’ usually refers to galactic cosmic rays, which originate in sources outside the solar system, distributed throughout our Milky Way galaxy. 

We believe most galactic cosmic rays derive their energy from supernova explosions, which occur about once every 50 years in our galaxy.  The cosmic rays are accelerated as the shock waves from these explosions travel through the surrounding interstellar gas.  Cosmic rays spend around 10 million years in the galaxy before escaping into inter-galactic space.

When high energy cosmic rays undergo collisions with atoms in the upper atmosphere, they produce a cascade of secondary particles that shower down through the atmosphere to the Earth’s surface.  The number of particles reaching the Earth’s surface depends on the energy of the cosmic ray that struck the upper atmosphere. 

Although thousands of cosmic rays pass through our bodies every minute, the resulting radiation levels are relatively low

The majority of secondary cosmic rays reaching the Earth are muons (heavier siblings of electrons), with an average intensity of about 100 per m2 per second.  Although thousands of cosmic rays pass through our bodies every minute, the resulting radiation levels are relatively low, only a few percent of the natural background radiation at sea level.

The idea of these invaders from space was first mooted in 1910 when Theodor Wulf, a Jesuit priest and German physicist measured the radiation at the bottom and the top of the Eiffel tower, using an electrometer he had designed and built himself.  The results from his experiment were the first evidence that cosmic rays might exist.  Wulf’s results were not initially accepted.

In 1912 Victor Hess took a balloon to 5000 metres and observed what he termed as “penetrating radiation” coming from space.  Hess received the 1936 Nobel Prize for his discovery of cosmic rays.  But it was not until 1929 that a cosmic ray was actually seen.  Using a newly invented cloud chamber, Dimitry Skobelzyn observed the first ghostly tracks left by cosmic rays.

By the 1930s bomber planes were being used to carry experiments to even greater heights and in 1950 the first research rocket to collect cosmic ray data was fired.  From then to today, nearly all space missions have carried a cosmic ray experiment.  Apollo 11 astronauts deployed an experiment and Apollo 17 carried a lunar surface cosmic ray experiment and we now have the Geostationary Orbiting Environmental Satellite providing cosmic ray data.

The idea of these invaders from space was first mooted in 1910

The portable cosmic ray detector has been touring schools and science events for the last two years and never fails to enthuse.  Everyone is enthralled to watch as you place your hand on the top of the detector and the rays continue to pass right through!

This simple but effective device is introducing pupils to the delights of physics and allowing us to bring a fascinating glimpse of the subject to them without the use of hot air balloons or space craft.  With the numbers of students choosing to study physics ever declining, there is a real need to spend a little time watching for space invaders.

Dr Kate Mackay/Department of Physics

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