Natural Neutron Sources

Uraninm deposits serve for weak natural neutron sources. There are a number of possible ways of the spontaneous fission of U nuclei, e.g., decay with the formation of Br and La and emission of 3 nentrons ... 

There are a number of techniques for meastuing subcritical reactivity relative to a calibrated reference control rod, in addition to soxuce multiplication. These include rod drop, rod jerk, source jerk, pulsed source and reactor power noise. Account must be taken of spatial flux transients (either by calculating them or measuring them with arrays of covmters) and of the spatial distribution of natural neutron sources due to spontaneous fission and (a,n) reactions and any fixed sources introduced to increase the subcritical flux level. The different methods have been reviewed and intercompared, for example, at the 1976 Specialists Meeting [4.87]. 

Beryllium has a high x-ray permeabiUty approximately seventeen times greater than that of aluminum. Natural beryUium contains 100% of the Be isotope. The principal isotopes and respective half-life are Be, 0.4 s Be, 53 d Be, 10 5 Be, stable Be, 2.5 x 10 yr. Beryllium can serve as a neutron source through either the (Oi,n) or (n,2n) reactions. Beryllium has alow (9 x 10 ° m°) absorption cross-section and a high (6 x 10 ° m°) scatter cross-section for thermal neutrons making it useful as a moderator and reflector in nuclear reactors (qv). Such appHcation has been limited, however, because of gas-producing reactions and the reactivity of beryUium toward high temperature water. 

Since the recognition in 1936 of the wave nature of neutrons and the subsequent demonstration of the diffraction of neutrons by a crystalline material, the development of neutron diffraction as a useful analytical tool has been inevitable. The initial growth period of this field was slow due to the unavailability of neutron sources (nuclear reactors) and the low neutron flux available at existing reactors. Within the last decade, however, increases in the number and type of neutron sources, increased flux, and improved detection schemes have placed this technique firmly in the mainstream of materials analysis. 

Radioactivity. Methods based on the measurement of radioactivity belong to the realm of radiochemistry and may involve measurement of the intensity of the radiation from a naturally radioactive material measurement of induced radioactivity arising from exposure of the sample under investigation to a neutron source (activation analysis) or the application of what is known as the isotope dilution technique. 

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

Radioisotopes that decay by spontaneous fission with the direct accompanying release of neutrons are usually associated with the natural elements of uranium and thorium and the manmade element plutonium. However, the rate of decay of these elements by fission is so slow that it is only by incorporating them into large nuclear piles or chain reactors that they can be utilized as intense neutron sources. In the US Dept of Energy National Transplutonium Program, small quantities of elements heavier than plutonium are produced for basic research studies and to discover new elements with useful properties. One of these new elements, californium-252 (2S2Cf), is unique in that it emits neutrons in copious quantities over a period of years by spontaneous fission... 

Although physics and chemistry were responsible for the conceptual framework overall, radiochemistry defined the experimental approach and provided much of the initial data. The neutron sources then in use (usually radium or radon [a source of alpha particles] mixed with powdered beryllium) were weak, with the result that the new beta activities were not much stronger than the natural radioactivity of uranium and its decay products. In 1934, the Rome group chemically separated the new activities from uranium by co-precipitating them with manganese and rhenium compounds (both transition metals), which supported the notion that these were... 

Single-crystal diffraction methods (whether based on X-ray or neutron sources) are those that carry the most complete information on the intimate nature of the crystals and therefore provide the most valuable tools for identification, characterization and comparison of polymorphs and pseudo-polymorphs. It is difficult to deny that one of the reasons for the outpouring of new results in the field of crystal engineering is the quantum leap represented by the commercial availability of single-crystal diffractometers equipped with area detectors. These devices have not only reduced the time of data collection by an order of magnitude with... 

Images obtained from electron microscopy are due to the nature/degree of electron scattering from the constiment atoms of the sample. Table 7.1 provides a comparison between electron. X-ray, and neutron sources, pertaining to their utility... 

As shown above, neutrons are in particular important for studies of hydride materials. Neutrons can be produced either in nuclear reactors or by pulsed (spallation) neutron sources. In the research nuclear reactors neutrons are produced by fission processes based on U-235 (which is 0.7% in natural uranium, but usually emiched as fuel for reactors). Since the released neutrons from these processes are very energetic, the required chain reaction for continuous production of neutrons requires moderation (to reduce the energy)... 

Nuclear energy cannot be produced by a self-sustained chain reaction in thorium alone because natural thorium contains no Bssile isotopes. Hoice the thorium-uranium cycle must be started by using enriched uranium, by irradiation of thorium in a uranium- or plutonium-fueled reactor or by using a strong external neutron source, e.g. an accelerator driven spallation source. 

Activation analysis is the other field of radiochemical analysis that has become of major importance, particularly neutron activation analysis. In this method nuclear transformations are carried out by irradiation with neutrons. The nature and the intensity of the radiation emitted by the radionuclides formed are characteristic, respectively, of the nature and concentrations of the atoms irradiated. Activation analysis is one of the most sensitive methods, an important tool for the analysis of high-purity materials, and lends itself to automation. The technique was devised by Hevesy, who with Levi in 1936 determined dysprosium in yttrium by measuring the radiation of dysprosium after irradiation with neutrons from a Po-Be neutron source. At the time the nature of the radiation was characterized by half-life, and the only available neutron sources were Po-Be and Ra-Be, which were of low efficiency. Hevesy s paper was not followed up for many years. The importance of activation analysis increased dramatically after the emergence of accelerators and reactors in which almost all elements could be activated. Hevesy received the 1943 Nobel prize in chemistry for work on the use of isotopes as tracers in the study of chemical processes . 

In a pulsed-neutron source measurement, bursts of neutrons are repetitively injected into the system for which a knowledge of the reactivity is desired. The time behavior, between bursts, of these neutrons and their progeny is accumulated by means of a series of sealers and a suitable detector properly positioned in or near the system. Such data look, ideally, like that shown in Fig. 1. If data of this nature can be obtained on a system, the degree of subcriticality can be determined without any other knowledge of the system. 

Determination of uranium in soil samples can be carried out by nondestructive analysis (NDA) methods that do not require separation of uranium (needed for alpha spectrometry or TIMS) or even digestion of the soil for analysis by ICPMS, ICPAES, or some other spectroscopic methods. These NDA methods can be divided into passive techniques that utilize the natural radioactive mission (gamma and x-ray) of the uranium and progeny radionuclides or active methods where neutrons or electromagnetic radiation are used to excite the uranium and the resultant emissions (gamma, x-rays, or neutrons) are monitored. In many cases, sample preparation is simpler for these nondestructive methods but the requiranent of a neutron source (from a nuclear reactor in many cases) or a radioactive source (x-ray or gamma) and relatively complex calibration and data interpretation procedures make the use of these techniques competitive only in some applications. In addition, the detection limits are usually inferior to the mass spectrometric techniques and the isotopic composition is not readily obtainable. 

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