Neutron yields

This reaction offers the advantage of a superior neutron yield of in a thermal reactor system. The abiHty to breed fissile from naturally occurring Th allows the world s thorium reserves to be added to its uranium reserves as a potential source of fission power. However, the Th/ U cycle is unlikely to be developed in the 1990s owing both to the more advanced state of the / Pu cycle and to the avadabiHty of uranium. Thorium is also used in the production of the cx-emitting radiotherapeutic agent, Bi, via the production of Th and subsequent decay through Ac (20). 

One of the key advantages to time-of-flight reflectometers comes in the measurement of fluid surfaces. Simply delivering the neutrons onto the fluid surface at a fixed angle (without moving the specimen) and detecting the reflected neutrons yields the reflectivity profile. 

Neutron diffraction on Cd compounds is not feasible, as one of the Cd nuclides (113Cd) has an extremely high absorption cross-section for neutrons, yielding an average [Pg.1255]

Neutron Yields from Actinide Beryllium Alloys. Can. J. Phys. 34, 949... 

In general, scattering of thermal neutrons yields information on the sample by measurement and analysis of the double differential cross section ... 

There are many isotopes that decay by alpha emission. When these isotopes are placed in intimate contact with another material, such as berylhum, the resulting (CC,n) reaction can be used as a neutron source. Berylhum is the target material with the highest neutron yield. Other targets include Li, and Table 1... 

One of the most promising applications of polyboron hydride chemistry is boron neutron capture therapy (BNCT) for the treatment of cancers (253). Boron-10 is unique among the light elements in that it possesses an unusually high neutron capture nuclear cross section (3.8 x 10-25 m2,0.02—0.05 eV neutron). The nuclear reaction between 10B and low energy thermal neutrons yields alpha particles and recoiling lithium-7 nuclei ... 

Figure 5.2 Neutron yields vs. proton energy for the reactions shown. Reprinted from J. D. Fox, C. F. Moore, and D. Robson, Phys. Rev. Lett. 12, 198 (1964). Copyright 1964 by the American Physical Society.

The most common sources are based on the 3H(d, n) reaction. Deuterons are accelerated to 150 keV with currents 2.5 mA and strike a tritium target. They produce 2 x 1011 of 14-MeV neutrons/s under these conditions. The neutrons produced are widely used in fast neutron activation analysis for the determination of light elements. The tritium targets are typically metals such as Ti, which have been loaded with titanium tritide. The accelerators are usually small Cockcroft-Walton machines or small sealed-tube devices where the ion source and accelerator structure are combined to produce a less expensive device with neutron yields 108/s. 

The 7Li(p, n) reaction is used commonly to produce approximately monoener-getic fast neutrons. The protons are accelerated to an energy of a few MeV by a small van de Graaff accelerator and strike a cooled rotating Li target. Thick target neutron yields are > 109 n/s-pA. The energy of the neutrons can be obtained from the Q value equation (Chapter 10), which can be expressed (for 0° neutrons) as... 

A FIGURE 22.8 A representation of nuclear fission. A uranium-235 nucleus fragments when struck by a neutron, yielding two smaller nuclei and releasing a large amount of energy. 

It should be noted that commercial neutron generators are also easily adopted to the generation of 2.8 MeV neutrons produced by the 2H(2H,w)3He reaction. In most cases it is merely necessary to replace the tritium target with one containing occluded deuterium. The neutron yield from this reaction is much less than for the D—T reaction and the useful flux is often not much greater than could be obtained by use of isotopic sources. About 35 elements have been found to possess reasonably high (n,n y) or (n,y) cross sections for 2.8 MeV neutrons 41>. Since the 8 most common elements in the earth s crust are not among those readily activated, there is some potential application of 2.8 MeV neutrons in analyses for certain elements in minerals and ores, where major element interferences via 14 MeV activation may be a problem. 

Fig. 3. Variation of the relative 14 MeV neutron yield at a constant beam current as a function of deuteron energy. It should be noted that acceleration of only molecular D j ions through a potential of 200 KV results in each deuteron in the molecular ion having a energy of 200/2, or only 100 KeV. This calculated curve is based on use of a thick Ti-T target 100>...

The cross section for the 3H(maximum value at only 107 KeV incident deuteron energy. When thick ( 1 mg cm-2 thick deposit of titanium) titanium-tritium targets are used, however, the neutron yield continues to increase even above 200 KV acceleration potential. This is due to increased penetration of the deuteron beam into the tritium enriched layer. Since the penetration of molecular deuterium ions is less than that for monatomic deuterium ions for the same acceleration potential, accelerators using Penning ion sources require extremely clean vacuum systems to minimize build-up of deuteron absorbing deposits on the surface of the target. 

Inelastic scattering of neutrons yields neutron scattering spectra that measure the vibrational energy levels of the material under study. For example, the chemisorption of water on Raney nickel was shown by inelastic scattering both to produce hydroxyl groups and to chemisorb water molecules on the surface at less than monolayer coverages. 

The bombardment of natural zinc (isotopes 64. 66. 67, 68, and 70) with slow neutrons yields 3 different radioactive isotopes of zinc, which we may designate as A, B, and C. These are identified by the method of cross bombardment as follows ... 

Bombardment of natural gallium with fast neutrons yields C. Both C and B are among the products resulting from the bombardment of germanium (isotopes 70, 72, 73, 74, and 76) with fast neutrons. A is a positron emitter, whereas B and C are emitters. 

Figure 8.17. Neutron yields as a function of the mass of the primary fission fragments (according to J. Terrell, Proc. IAEA Symp. Phys. Chem. Fission, Salzburg 1965, IAEA, Vienna, Vols. 2, 3).

The deuterons are accelerated in a van de Graaff generator and hit a tritium or a beryllium target. The neutron yields of these reactions are plotted in Figs. 17.2 and 17.3. 

Figure 17.3. Neutron yield of the reaction Be(d, n) B as a function of the deuteron energy (thick Be target).

With tritium targets of 1 Ci (3.7 10 Bq) and deuteron fluxes of about 50mA, neutron yields up to about 5 10 s are obtained. The flux density depends on the distance between the tritium target and the sample. The energy of the neutrons produced by reaction (17.4) is 14 MeV and allows activation by (n,p), (n,y) or (n,2n) reactions with relatively high yields. Most cross sections of (n,2n) reactions are in the... 

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