Intense Neutron Generator

The current availability of small portable 14 MeV neutron generators and the future availability of high intensity 252Cf spontaneous fission neutron sources will certainly result in the wide spread use of activation techniques for non-destructive "on-stream" product analysis in industry. The cost of the required instrumentation for many types of activation analysis is not excessive, as compared to the cost of other modem analytical instrumentation. The simple off-on operation of the new sealed-tube neutron generators and minimal maintenance associated with the use of an isotopic Z5ZCf neutron source will permit operation of the analytical facility with technician-level personnel. The versatility of the activation technique justifies its inclusion among the other major analytical techniques employed in any modem analytical facility. 

In such systems (Fig. 7.5), spallation reactions induced by a high-intensity beam (10 to 250 mA) of GeV protons on a heavy target produce an intense neutron flux. These neutrons, after being more or less moderated, are used to drive a sub-critical blanket. The extra neutrons provided by the accelerator allow the maintenance of the chain reaction while burning the long-lived nuclear waste. The plant generates electricity, part of which is used to supply the accelerator. 

A nuclear fission chain reaction in a reprocessing plant is an accident that must be carefully guarded against. Although such a critical reaction is not likely to generate sufficient energy to be mechanically destructive, it emits intense neutron and gamma radiation that can kill nearby plant personnel and may release radioactive fission products outside the plant. 

Fundamentals. A collimated beam of neutrons generated in a nuclear reactor or a spallation source with wavelength X impinges onto an interface at an angle 0 (close to glancing angle). The reflected intensity R is measured as a function of momentum transfer Q... 

Activation products have also to be considered which are generated by the intense neutron flux. In contrast to atmospheric explosions, a small amount of carbon-14 is generated by activation of nitrogen-14 and a small amount of tritium. If salt water is present, the isotope sodium-24 is produced by activation of sodium-23. In the ground, silicon, aluminium and manganese are also activated, which have short half lives and rapidly decay. 

Despite this favorable record, the further development of nuclear power is greatly handicapped in many countries because of public concern over the radioactive products arising in the course of plant operation and the consequences of their possible release to the environment. Energy generation from the neutron-induced fission of heavy atoms is inevitably accompanied by the formation of radioactive nuclides. This is, first of all, the direct consequence of nuclear fission, which leads initially to fission products that are unstable due to an excess of neutrons in the newly formed nuclei. These products are transformed by a sequence of p decays (mainly with associated y emission) to stable end products. Moreover, neutron capture in the heavy atoms of the fuel results in the buildup of nuclei which are heavier than those of the starting element (uranium, plutonium) and which mostly decay — in part, with very long halflives — by a emission. Finally, from elements present in structural and cladding materials, as well as in the coolant, its additives and impurities, additional radionuclides are formed, induced by neutron capture reactions which take place in the intense neutron field inside the reactor pressure vessel. 

Second, intense neutron sources (nuclear reactors, neutron generators, isotopic sources) were generally available. Third, the high penetration depth of neutrons allows a nearly homogeneous flux density of the entire sample to be analyzed. 

With current neutron sources the times of data collection for each partial reflectivity scan will remain of the order of minutes. However, with the advent of the next generation of high intensity neutron sources being planned or built around the world significant advances can be predicted for these real time measurements. Time will tell as the saying goes. 

The correlator (6) is of the utmost importance because its generating function enters into an expression which describes the angular dependence of intensity of scattering of light or neutrons [3]. It is natural to extend expression (6) for the two-point chemical correlation function by introducing the w-point correlator ya1... (kl...,kn l) which equals the joint probability of finding in a macromolecule n monomeric units Maj.Ma> divided by (n-1) arbitrary sequences... 

Silvery white, artificial element that is also generated by intensive bombardment of plutonium with neutrons. It is a strong ("hot") neutron emitter and is used in microgram quantities in nuclear medicine. This reliable neutron source is also used in industry and science (for activation analysis). 

The most important apphcation of this metal is as control rod material for shielding in nuclear power reactors. Its thermal neutron absorption cross section is 46,000 bams. Other uses are in thermoelectric generating devices, as a thermoionic emitter, in yttrium-iron garnets in microwave filters to detect low intensity signals, as an activator in many phosphors, for deoxidation of molten titanium, and as a catalyst. Catalytic apphcations include decarboxylation of oxaloacetic acid conversion of ortho- to para-hydrogen and polymerization of ethylene. 

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