STAR is a Geiger counter that will measure the variation in atmospheric beta and gamma
radiation with altitude. Far above the Earth’s surface, primary cosmic rays collide with atoms and molecules in our atmosphere to produce a shower of secondary particles known as secondary cosmic rays. Secondary cosmic rays are ionizing radiation like betas (electrons) and gammas (x-rays) that can pass through our bodies.
One common human activity that exposes us to these secondary cosmic rays is simply boarding an airplane and flying somewhere. How much radiation do we receive?
Near the Earth’s surface, we are mostly subjected to terrestrial sources of radiation like radon gas. As we increase in altitude, cosmic rays slowly become a greater proportion of our total radiation exposure. This is due to the effect of the mean free path of the air in our atmosphere. The mean free path is the average distance a particle can travel before it interacts with another particle.
At the surface of the Earth, the atmosphere is dense and the mean free path is small. Very few cosmic rays, whether primary or secondary, will make it through the entire atmosphere to the surface.
At higher altitudes, the Earth’s atmosphere is thinner than at the surface, but denser than the atmosphere at very high altitudes. The mean free path is correspondingly lower than at very high altitudes, meaning fewer primary cosmic rays will penetrate to these lower altitudes, but those primary cosmic rays will have a much greater chance of striking an air molecule to produce secondary cosmic rays.
At very high altitudes, the Earth’s atmosphere is extremely thin. Therefore, the mean free path is high and a primary cosmic ray will travel a much greater distance before it strikes an air molecule and produces secondary cosmic rays. Primary cosmic rays are abundant, but few secondary cosmic rays are produced.
Therefore, what STAR expects to see as it gains altitude is that secondary cosmic rays (beta and gamma radiation) will increase to some maximum, then decrease. This maximum is known as the Regener-Pfotzer maximum and occurs around 20 km.
留言