Spallations

As with synchrotron x-rays, neutron diffraction facilities are available at only a few major research institutions. There are research reactors with diffraction facilities in many countries, but the major ones are in North America, Europe and Australia. The are fewer spallation sources, but there are major ones in the United States and the United Kingdom. 

Powder diffraction studies with neutrons are perfonned both at nuclear reactors and at spallation sources. In both cases a cylindrical sample is observed by multiple detectors or, in some cases, by a curved, position-sensitive detector. In a powder diffractometer at a reactor, collimators and detectors at many different 20 angles are scaimed over small angular ranges to fill in the pattern. At a spallation source, pulses of neutrons of different wavelengdis strike the sample at different times and detectors at different angles see the entire powder pattern, also at different times. These slightly displaced patterns are then time focused , either by electronic hardware or by software in the subsequent data analysis. 

A. Balkrishnan, W. Nicolet, S. Sandhu, and J. Dodson, Galileo Probe Thermal Protection Entry Heating Environments and Spallation Experiment Design,... 

There is a very low cosmic abundance of boron, but its occurrence at all is surprising for two reasons. First, boron s isotopes are not involved in a star s normal chain of thermonuclear reactions, and second, boron should not survive a star s extreme thermal condition. The formation of boron has been proposed to arise predominantly from cosmic ray bombardment of interstellar gas in a process called spallation (1). 

Cosmogenic radionuclides are produced in the upper atmosphere by spallation reactions of cosmic rays with atmospheric elements. The most common... 

Davison, L.W. and Johnson, J.N., Elastoplastic Wave Propagation and Spallation in Beryllium, A Review, Sandia Corporation Technical Memorandum No. SC-TM-70-634, Albuquerque, NM, 44 pp., September 1970. 

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

The high depth resolution, nondestructive nature of thermal neutrons, and availability of deuterium substituted materials has brought about a proliferation in the use of neutron reflectivity in material, polymer, and biological sciences. In response to this high demand, reflectivity equipment is now available at all major neutron facilities throughout the country, be they reactor or spallation sources. 

The relative abundances of the various isotopes of the light elements Li, Be and B therefore depend to some extent on which detailed model of the big bang is adopted, and experimentally determined abundances may in time permit conclusions to be drawn as to the relative importance of these processes as compared to x-process spallation reactions. 

Be is a very interesting element, produced by spallation of galactic cosmic rays. The only two usable lines are in the extreme UV (313 nm), in a crowded spectral region, and the stellar radiation is heavily absorbed by the earth s atmosphere, so that their observations are challenging in faint stars. Only very recently (Pasquini et al. 2004) the first Be observations became available, in 2 TO stars of the nearby NGC6397. 

Two problems were identified with the GCR production, compared to me-teoritic composition the 7Li/6Li ratio ( 2 in GCR but 12 in meteorites) and the nB/10B ratio ( 2.5 in GCR but 4 in meteorites). Modern solutions to those problems involve stellar production of 70% of 7Li (in the hot envelopes of AGB stars and/or novae) and of 40% of nB (through //-induced spallation of 12C in SNII). In both cases, however, uncertainties in the yields are such that observations are used to constrain the yields of the candidate sources rather than to confirm the validity of the scenario. 

Primordial, non-standard, production during Big Bang Nucleosynthesis (BBN) the decay/annihilation of some massive particle (e.g. neutralino) releases energetic nucleons/photons which produce 3He or 3H by spallation/photodisinte-gration of 4He, while subsequent fusion reactions between 4He and 3He or 3H ere-... 

The penetration depth of cosmic radiation is of the order of 1 m and therefore isotopes are produced by spallation only in the surface layers of meteorites and the moon. After collisions of meteorites with each other or with the moon, newly formed surfaces get exposed to cosmic radiation and production of stable and radioactive isotopes starts. If P is the production rate of a... 

The discovery of cosmic ray produced 81Kr in meteorites [l]1 introduced a new method of high sensitivity measurements of 81Kr concentrations and cosmic ray exposure dating. The method consists of a direct measurement of both radioactive 81Kr atoms (Ty2 = 2.13 x 105y, [2] and of stable spallation Kr atoms by a... 

In the 81Kr-83Kr dating method, which is discussed in the next paragraph, the production ratio of radioactive and stable Kr isotopes can be evaluated directly from the Kr spallation spectrum in a meteorite according to the relation [1] ... 

As discussed earlier, the production ratio Pgi/P83 cani in general, reliably be obtained from equation (1) and, therefore, the 81Kr-Kr method avoids much of the uncertainty arising from unknown production rates or production ratios. The method derives exposure ages from Kr isotopic ratios as obtained from mass spectrometry and does not require a knowledge of the concentration of spallation Kr, which makes the method inherently more precise. 

Kr isotopic data derived from either stepwise or total sample analysis represent generally mixtures of several components, but equations (4) and (5) require the identification of the cosmic ray produced spallation component. As long as spallation Kr is the major component, the analysis of isotopic spectra is reasonably... 

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