Cosmic rays in the atmosphere

Radioactive or stable isotopes of noble gases are also used to determine vertical turbulent diffusion in natural water bodies. For instance, the decay of tritium (3H)— either produced by cosmic rays in the atmosphere or introduced into the hydrosphere by anthropogenic sources—causes the natural stable isotope ratio of helium, 3He/ 4He, to increase. Only if water contacts the atmosphere can the helium ratio be set back to its atmospheric equilibrium value. Thus the combined measurement of the 3H-concentration and the 3He/4He ratio yields information on the so-called water age, that is, the time since the analyzed water was last exposed to the atmosphere (Aeschbach-Hertig et al., 1996). The vertical distribution of water age in lakes and oceans allows us to quantify vertical mixing. 

Since the production rates of the cosmic ray radionuclides increase rapidly with increasing altitude in the lower atmosphere, the atmospheric concentrations and ratios of short lived cosmic ray radionuclides can be used to study rapid vertical air motions if the equilibrium concentrations of the radionuclides are known. For example, the concentrations of the short lived cosmic ray radionuclides in air which has moved upward recently from a lower altitude will be less than the equilibrium concentrations. The concentrations of the radionuclides will therefore increase with time until equilibrium is reached. However, the concentration of the shorter lived of two short lived radionuclides will increase more rapidly initially, causing the ratio of the two radionuclides of different half-lives to change with time until equilibrium is reached. Therefore, the time since the air moved from a lower altitude, the speed of the upward motion, and the altitude from which the air originated could be calculated from the concentrations and concentration ratios of cosmic ray radionuclides of different half-lives. Vertical motions of different speeds could be studied since several cosmic ray radionuclides of different half-lives are present in the atmosphere (Table I). Many other radionuclides are produced by cosmic rays in the atmosphere, but they have not yet been detected. Some of these with half-lives of a few minutes could serve as tracers of very short term processes such as post-nucleation scavenging. 

Underground detectors observing neutrinos produced by cosmic rays in the atmosphere have measured a v i/ve > 3tio much less than expected and also a deficiency of upward going compared to downward. This could be explained by oscillations leading to the disappearance of Va with Am ... 

Underground detectors observing neutrinos produced by cosmic rays in the atmosphere have measured a much less than ex-... 

Helium-3 is a decay product of radioactive tritium (3H, half-life = 12.44 years) that has been produced by nuclear bombs as well as naturally by cosmic rays in the upper atmosphere. Because virtually all 3He atoms escape from the surface ocean to the atmosphere, the 3He/tritium ratio in subsurface seawater samples indicates the time since the water s last exposure to the atmosphere. Both 3He and tritium are measured by gas mass spectrometry. Alternatively, tritium may be measured by gas counting with a detection limit of 0.05 to 0.08 tritium unit, where 1 tritium unit represents a 3H/H ratio of lxl0 18. A degassed water sample is sealed and stored for several months to allow the decay product 3He to accumulate in the container. The amount of 3He is then measured by mass spectrometry, yielding a detection limit of 0.001 to 0.003 tritium unit when 400-gram water samples are used. With this technique, the time since a water mass left the surface can be determined within a range from several months to 30 years. 

Radiocarbon Dating. This is a method of estimating Ihe age of carbon-containing materials by measuring the radioactivity of the carbon in them. The validity of this method rests upon certain observations and assumptions, of which the following statement is a brief summaiy. The cosmic rays entering the atmosphere undergo various transformations, one of which results in the formation of neutrons, which in turn, induce nuclear reactions in the nuclei of individual atoms of the adnosphere. The dominant reaction is... 

Ozone, O3, is produced when O2 molecules are subjected to electrical discharge or the action of cosmic rays in the upper atmosphere. 

Tritium is naturally produced to the extent of about 1 atom per lO hydrogen atoms as a result of nuclear reactions induced by cosmic rays in the upper atmosphere ... 

In addition to these ordinary constituents, infiltrating meteoric waters may contain more exotic solutes that are particularly useful for tracing of water flow paths and residence times. Prominent among these are radionuclides produced by the action of cosmic rays on the atmosphere. The most commonly employed are H, and C1, but many others are described in this chapter. Another category of useful tracers is that produced by human activity, whose atmospheric concentration histories are generally known. Those frequently employed for subsurface tracing include H, C, and C1 produced by atmospheric nuclear-weapons testing, Kr and released from nuclear... 

The concentration of atmospheric radionuclides has a special distribution which depends on latitudes and altitudes. Cosmogenic radionuclides have higher production rates in the stratosphere than in the troposphere, because of a higher intensity of cosmic rays in the stratosphere. Fallout nuclides have higher concentrations in mid-latitude of the Northern Hemisphere, because most atmospheric nuclear explosion experiments were made there. 

A type of radiochemical dating known as carbon dating is commonly used to measure the age of artifacts that were once part of a living organism, such as the human skeleton shown in Figure 25-14. Carbon dating, as its name implies, makes use of the radioactive decay of carbon-14. The procedure rehes on the fact that unstable carbon-14 is formed by cosmic rays in the upper atmosphere at a fairly constant rate. 

Depending on the context, the term secondary cosmic rays can refer either to particles produced by interactions of primary cosmic rays with the interstellar gas or to particles produced by interactions of cosmic rays in the Earth s atmosphere. The production mechanisms are similar, and there are some common features. 

Dorman L.I., Cosmic Rays in the Earth s Atmosphere and Underground, Kluwer Ac. Publ., 2004. 

By 1945, devices were prepared in which, when a small charge of explosive was set oflE, two pieces of uranium were driven together. Each piece by itself was below critical mass, but together diey were above it Thanks to cosmic ray bombardment, the atmosphere always contains stray neutrons, so a nuclear chain reaction starts at once in the critical mass of uranium, which explodes with fury hiflierto unimagined. 

When carbon-14 is used in radiometric dating, the technique is known as radiocarbon dating. The procedure is based on the formation of carbon-14 as neutrons created by cosmic rays in the upper atmosphere convert nitrogen-14 into carbon-14 ( FIGURE 21.7). The reacts with oxygen to form C02 in the atmosphere, and this labeled CO2 is taken up by plants and introduced into the food chain through photosynthesis. This process provides a small but reasonably constant source of carbon-14, which is radioactive and undergoes beta decay with a half-life of 5715 yr ... 

Another group of radionuclides is generated by cosmic rays in the light elements in the atmosphere. For example, nitrogen forms radioactive carbon C. The natural radioactivity in nature is mainly due to radionuclides of light elements. Approximately 90% of the radioactivity is attributable to K (it represents 0.012% of natural potassium and has a half-life of 1.3x 10 years) and almost all the rest to carbon (half life of 5730 years). Increased radioactivity in certain areas can be caused by abnormally high incidence of radioactive elements (especially uranium) in rocks. 

Cosmogenic radionuclides are formed in the upper atmosphere by the interaction of cosmic rays, primarily from the sun, with elements present in the atmosphere (e.g. 0, and " Ar). Their half-lives range from months to... 

Carbon-12 is the principal isotope of carbon, but a small proportion of carbon-14 is present in all living organisms. Its nuclei are produced when nitrogen nuclei in the atmosphere are bombarded by neutrons formed in the collisions of cosmic rays with other nuclei ... 

Most CO and CO2 in the atmosphere contain the mass 12 isotope of carbon. However, due to the reaction of cosmic ray neutrons with nitrogen in the upper atmosphere, C is produced. Nuclear bomb explosions also produce C. The C is oxidized, first to CO and then to C02 by OH- radicals. As a result, all CO2 in the atmosphere contains some 0, currently a fraction of ca. 10 of all CO2. Since C is radioactive (j -emitter, 0.156 MeV, half-life of 5770 years), all atmospheric CO2 is slightly radioactive. Again, since atmospheric CO2 is the carbon source for photos5mthesis, aU biomass contains C and its level of radioactivity can be used to date the age of the biological material. 

Suess, H. E. (1965). Secular variations of the cosmic-ray-produced carbon-14 in the atmosphere and their interpretation, /. Geophys. Res. 70,5937-5952. 

C by cosmic rays equals its rate of decay by 3 emission, so the percentage of C in the atmosphere... 

C22-0115. Carbon-14 dating gives 3250 years as the age of a charcoal sample, assuming a constant level of cosmic radiation. If the cosmic ray level in the atmosphere was 20% higher at the time the tree grew, what is the correct age of the sample ... 

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