Cosmic Ray-induced Nuclear Reactions

Tritium, which is radioactive (/T, tl/2 = 12.4 y), is made by the reaction 6Li(n,ct)3H in nuclear reactors. It is also formed in plasmas2 as 3H+ and by cosmic ray induced nuclear reactions in the upper atmosphere. The decay of 3H probably accounts for traces of 3He in the atmosphere. 

When Fritz Paneth s group in 1953 tried to determine meteorite ages by the He/U method (Paneth et al. 1953), they found much larger amounts of helium than could be accounted for by uranium decay and thus stumbled on the discovery of cosmic-ray-induced nuclear reactions in meteorites that subsequently became the subject of extensive research. Many radionuclides with half-lives ranging from days to millions of years as well as some stable spallation products have been identified in meteorites. From the amounts found, the exposure ages of meteorites in space and the average cosmic-ray flux and its time variation can be deduced (see, e.g., Schaeffer 1968). 

Be produced by cosmic-ray-induced nuclear reactions is useful for studying atmospheric transport mechanisms. Since the production of Be and other cosmogenic nuclides is directly dependent on the cosmic-ray intensity, a relationship between the production rate of these nuclides and the 11-year solar cycle has been found (Kulan et al. 2006). It is known that the galactic cosmic-ray intensity at the earth s orbit is inversely related to solar activity (Hotzl etal. 1991). 

Hydrogen occurs naturally in three isotopes. The most common ( H) accounting for more than 99.98% of hydrogen in water, consists of only a single proton in its nucleus. A second, stable isotope, deuterium (chemical symbol D or H), has an additional neutron. Deuterium oxide, D2O, is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. The third isotope, tritium, has 1 proton and 2 neutrons, and is radioactive, decaying with a half-life of 4500 days. T2O exists in nature only in minute quantities, being produced primarily via cosmic ray-induced nuclear reactions in the atmosphere. Water with one deuterium atom HDO occurs... 

Indeed, this happens every moment in the Earth s atmosphere. The upper atmosphere is bombarded with cosmic rays fast-moving subatomic particles produced by extremely energetic astrophysical processes such as nuclear fusion in the sun. When cosmic rays hit molecules in the atmosphere, they induce nuclear reactions that spit out neutrons. Some of these neutrons react with nitrogen atoms in air, converting them into a radioactive isotope of carbon carbon-14 or radiocarbon , with eight neutrons in each nucleus. This carbon reacts with oxygen to form carbon dioxide. About one in every million million carbon atoms in atmospheric carbon dioxide is C. 

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... 

The classic idea of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. The precursor rock starts out on a parent body, buried under a mantle of material many meters thick that screens out cosmic rays. At a time fj, a collision excavates a precursor rock—a meteoroid. The newly liberated meteoroid, now fully exposed to cosmic rays, orbits the Sun until a time ff, when it strikes the Earth, where the overlying blanket of air (and possibly of water or ice) again shuts out almost all cosmic rays (cf. Masarik and Reedy, 1995). The quantity ff — h is called the CRE age, f. To obtain the CRE age of a meteorite, we measure the concentrations in it of one or more cosmogenic nuclides (Table 1), which are nuclides that cosmic rays produce by inducing nuclear reactions. Many shorter-lived radionuclides excluded from Table 1 such as Na (ff/2 = 2.6 yr) and °Co ty = 5.27 yr) can also furnish valuable information, but can be measured only in meteorites that feu within the last few half-Uves of those nucUdes (see, e.g., Leya et al. (2001) and references therein). 

Tritium. In natural hydrogen it occurs in amounts of 1 in 1017-1018. It is continuously formed in nuclear reactions induced by cosmic rays, and it is radioactive. It may be made, from lithium, in nuclear reactors by the thermal neutron reaction 6Li( ,a)3H. 

There are three principal types of nuclear reactions due to the interactions of terrestrial materials with cosmic rays (i) by high-energy spallation of nucleons (E > 40MeV), principally by neutrons, (ii) by thermal neutron capture, and (iii) muon-induced nuclear disintegrations. Muon reactions become important only at depths below sea level. The estimation of the production ratio is difficult because of lack of knowledge of the probabilities of formation of nuclides in the different reactions. 

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 a strict sense, spallation is a nuclear fragmentation process in which the target nucleus loses several nucleons. As used in cosmochem-istry, however, the term is used more broadly to designate the product of any nuclear transformation induced by cosmic rays, primary or secondary, whether produced by spallation in the strict sense or by more specific reaction channels involving fewer exiting particles (e.g., (p,pn) or (n, a) reactions). 

Tritium is formed continuously in the upper atmosphere in nuclear reactions induced by cosmic rays. For example, fast neutrons arising from cosmic-ray reactions can produce tritium by the reaction 14N( , 3H)12C. Tritium is radioactive (/ ", 12.4 years) and is believed to be the main source of the minute traces of 3He found in the atmosphere. It can be made artificially in nuclear reactors, for example, by the thermal neutron reaction, 6Li(/ ,a)3H, and is available for use as a tracer in studies of reaction mechanism. 

Tritium is formed continuously in the upper atmosphere in nuclear reactions induced by cosmic rays because of its short half-life, however, only trace quantities exist naturally. The isotope can be S5mthesized in nuclear reactors by neutron bombardment of lithium-. ... 

Variations in isotopic abundances that are caused by nuclear reactions induced by cosmic rays are most commonly utilized in cosmic ray exposure dating, but this employs isotopes that are measured by either accelerator or noble gas mass spectrometry [28, 29]. In fact, there are only a very limited number of elements that are suitable for the study of cosmogenic isotopic variations, which can be readily analyzed by either TIMS or MC-ICP-MS [28]. The most important application of these techniques are studies of the secondary neutron fluxes that are generated by (primary) cosmic rays. Such measurements aim to detect anomalies in Sm, Gd, and Cd isotopic abundances that are produced by (n,y) reactions, for example " Cd(n, y) Cd. Many of these investigations were conducted by TIMS [137-139], but some cosmogenic Cd isotope variations of lunar rocks and soUs were evaluated based on MC-ICP-MS isotope ratio data that were originally acquired as part of a stable isotope study [134]. 

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