If you have already read cosmic rays then you will know that the spark chamber detects cosmic rays. Cosmic rays, although can only be seen by detectors such as the spark chamber are all about us, indeed if it takes several minutes to read this page several hundred cosmic rays will have passed through your head and penetrated into your body. The question, therefore which must be asked is: 'Are cosmic rays harmful to humans?'
At ground level cosmic rays are generally not a problem, and certainly no direct links have been made linking cosmic rays to cancer. Yet there is a small risk of a different nature. Like with the neon-helium gas in the spark chamber (see How does a spark chamber work - Stage 2) the passage of a cosmic ray through your body can leave various fluids within your body momentarily ionised. Noble prize winner H J Muller found that mutations and changes in human genes can be caused by ionising radiation such as cosmic rays. Therefore it is thought that cosmic rays may be capable of changing your DNA make-up by hitting individual cells, and may even have human evolution implications. However it should be stressed that this is a very rare occurrence as cosmic rays of an energy high enough to cause such changes are rare at ground level.
Higher off the ground cosmic ray radiation becomes more of a problem, indeed astronauts have a problem as the cosmic rays which hit them up in space are generally of a much higher energy than the cosmic rays which reach the ground. See What does a Spark Chamber Detect?
The problem arises when high energy electrons (one form of secondary cosmic rays) are stopped suddenly inside the astronaut's space ship. This produces x-rays which can be harmful to humans if they are exposed to them for any length of time.
The astronaut has a couple of options available to him to reduce the radiation which include wearing a protective spacesuit as well as shielding the spaceship. Lead would be the ideal material for this job, however it is very heavy and would not be suitable for a spaceship. A substitute for lead therefore has to be found, often the fuel stored for use by the spaceship can also be used as a shield.
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