Cosmic rays

5.14.1 Cosmic rays

Cosmic rays are energetic particles that enter the Earth’s atmosphere from space. Protons make up nearly 90% of all incoming cosmic ray particles with about 9% helium nuclei (alpha particles) and about 1% electrons along with a small compo- nent of more massive nuclei. The particles arrive individually, not in a beam, so the

term ‘ray’ is a misnomer. Some are highly energetic with energies of over 10 20 eV, considerably more than we can produce with particle accelerators here on Earth.

Introduction to Astronomy and Cosmology

2008 can produce protons with just 7 ⫻ 10 12 eV. The highest energy cosmic rays have an energy equivalent to that of a well served tennis ball!

We believe that most cosmic rays originate from galactic objects such as rotating neutron stars, supernovae, and black holes but the most energetic are thought to come from objects such as the regions surrounding a super-massive black hole at the heart of a distant galaxy. These are called active galactic nuclei and will be dis- cussed in Chapter 8. At the lower end of their energy scale are those originating from energetic processes within the Sun’s atmosphere – their numbers increasing markedly after a solar fl are.

Cosmic ray particles are divided into two groups: the primary particles , such as oxygen and carbon nuclei produced within stars and ejected in supernova explo-

sions, can interact with the interstellar medium and produce lighter secondary

particles , such as the nuclei of lithium, beryllium and boron. The rate at which cosmic rays are detected on Earth has a dependence on the interaction of the solar wind with the Earth’s magnetic fi eld. As a result, the rate depends on the solar cycle as the solar wind intensity increases at times of sunspot maximum. Some scientists believe that the extent of the Earth’s cloud cover may have a dependence on the cosmic ray fl ux and so changing solar activity may have an effect on global temperatures.

When cosmic ray particles interact with oxygen and nitrogen in the Earth’s

atmosphere they produce a cascade of lighter particles, called a cosmic ray

air shower which can contain billions of particles. In a significant reaction, neutrons can collide with a 14 N nucleus to give a proton and 14 C nucleus. Aris- ing from this process, the amount of 14 C in the Earth’s atmosphere has been kept constant for at least 100 000 years. (It increased in the latter part of the last century due to the testing of nuclear bombs.) Carbon dioxide contain-

ing a 14 C atom thus makes up a small part of the carbon dioxide that will be absorbed, for example, by a growing tree. As a result, wood initially contains

a well defined ratio of 14 C and 12 C in its makeup. However, 14 C is radioactive, having a half-life of 5730 years, with 14 C decaying to form 14 N with an elec- tron and an antineutrino. The ratio of 14 C to 12 C will thus decrease over time.

This enables the age of the wood to be found, a process known as radiocarbon

dating . In their interactions in the atmosphere, cosmic rays can produce pions and kaons which quickly decay into muons. These do not interact strongly with the atmosphere and many reach the surface as a result of the effect of time dilation as described in Section 1.6.6. At rest relative to us, muons decay into an electron and two neutrinos with a half-life of just 1.6 µs. They are produced at a height of about 10 km in the atmosphere and, travelling at ∼0.98% of the speed of light, take 34 µs to reach the ground. If time dilation did not occur, only about 1 in

3 000 000 would reach the ground. However, due to its relativistic speed, the

Observing the Universe

muon’s clock, as observed by us, is running slow by a factor of about 5, so its effective half-life is 7.8 µs. As a result, 150 000 rather than a single muon out of an initial 3 000 000 will reach the ground. In chambers 700 m below ground at the Soudan Mine in Minnesota, an experiment has detected 33 000 000 muons over a 10-year period when the Moon was in the fi eld of view. Cosmic rays are shielded by the Moon, and this led to a reduction of detections from the direction of the Moon. The ‘Moon’s shadow’ is slightly offset from its true position as the cosmic rays, being charged particles, are defl ected somewhat by the Earth’s magnetic fi eld (Figure 5.32).