Cosmic-ray induced reaction

Examinations of the same and of other lead-bearing samples for spontaneous fission events with large proportional counters in Dubna seemed to confirm these findings, but further measurements [37] of thin samples sandwiched between two plastic fission-track detectors showed that the events were background caused by cosmic-ray induced reactions of lead. Other groups [38] found no evidence for spontaneous fission activities in lead and other samples at a lower detection limit of 10" 3 g/g achieved with the sandwich technique. Even lower limits down to 10"17 g/g can be reached by etching... 

He) were able to show that some iron meteorites and lunar rocks also have cosmogenic W isotope anomalies that are superimposed on the radiogenic isotope effects. For the lunar rocks, these anomalies were shown to be due to the Ta(n,Y) a reaction, induced by cosmic radiation, whereby the short-lived isotope a (T = 114 days) subsequently decays to [141]. For the iron meteorites, cosmic ray-induced reactions lead to both the production and consumption of various W isotopes. As a result of these reactions, many samples of both magmatic and non-magmatic irons appear to have ratios that are... 

The above examples should suffice to show how ion-molecule, dissociative recombination, and neutral-neutral reactions combine to form a variety of small species. Once neutral species are produced, they are destroyed by ion-molecule and neutral-neutral reactions. Stable species such as water and ammonia are depleted only via ion-molecule reactions. The dominant reactive ions in model calculations are the species HCO+, H3, H30+, He+, C+, and H+ many of then-reactions have been studied in the laboratory.41 Radicals such as OH can also be depleted via neutral-neutral reactions with atoms (see reactions 13, 15, 16) and, according to recent measurements, by selected reactions with stable species as well.18 Another loss mechanism in interstellar clouds is adsorption onto dust particles. Still another is photodestruction caused by ultraviolet photons produced when secondary electrons from cosmic ray-induced ionization excite H2, which subsequently fluoresces.42... 

Fig. 5. Neutron counting as detection method for spontanous fission events of superheavy nuclei. The recorded neutron rates (points) were found to follow the relative cross sections of cosmic-ray induced spallation reactions (curve) and were, thus, due to background events. The numbers are rates for natural uranium and thorium. From W. Grimm, G. Herrmann and H.-D. Schiissler [40].

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. 

Pu-Xe dating. Decay of Pu can be used as a chronometer of the first 100 Ma for some specific meteorite parent bodies. Both Pu and the LREE tend to be concentrated in refractory minerals like phosphates. Which LREE is the best proxy for Pu Various authors have suggested Nd (Lugmair and Marti 1977), Sm (Jones and Burnett 1987), or Pr or Ce (Boynton 1978). There are no neutron-induced reactions that produce a rare gas from any of the LREE, but all of these, particularly Nd, do produce the light xenon isotopes like Xe and Xe through cosmic-ray-induced spallation reactions (Wider 2002, this volume). In fact, in many cases, the LREE (and presumably Pu), are probably not fractionated much from each other. Hence, if the cosmic ray dose (i.e., the cosmic ray exposure age) is known, and the production rate of isotopes like " Xe and Xe is also known, then the abundance of the LREE can be calculated. Then the ratio of Xe244 (Pu-... 

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). 

Interaction between cosmic rays and terrestrial matter The Earth s atmosphere and, to a lesser extent, its surface are constantly bombarded with cosmic radiation which interacts with terrestrial material, resulting in isotopic variations in some elements. The best known example is the production of C from by (n,p) reaction in the atmosphere, with the neutron involved created by cosmic ray-induced spallation. a radionuclide with a half-life of 5730 years, is oxidized to CO2 and enters the food chain via photosynthesis, thus affecting the isotopic composition of C in all living organisms. 

Hasegawa n, Herbst E (1993) New gas-grain chemical models of quiescent dense interstellar clouds - the effects of H2 tunnelling reactions and cosmic ray induced desorption. Mon Not Roy Astron Soc 261 83-102... 

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... 

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. 

Neutrino detectors are placed at great depths, at the bottom of mines and tunnels, in order to reduce interference induced by cosmic rays (Fig. 5.3). Two methods of detection have been used to date. The first is radiochemical. It involves the production by transmutation of a radioactive isotope that is easily detectable even in minute quantities. More precisely, the idea is that a certain element is transformed into another by a neutrino impact, should it occur. Inside the target nucleus, the elementary reaction is... 

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. 

The Mn- Cr system can be studied by TIMS, ICPMS, and SIMS techniques. For TIMS and ICPMS work, bulk samples or mineral separates are dissolved and the solutions are passed through ion-exchange columns to produce clean solutions of manganese and chromium. For minerals with high Mn/Cr ratios SIMS can obtain isotopic data while retaining the petrographic context of the measurements. The chromium isotopic compositions may have to be corrected for small additions of chromium from spallation reactions induced by cosmic rays. This is particularly important in iron-rich meteorites. 

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... 

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. 

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