Neutron capture reaction

The capture rate is dominated at thermal energies around 30 keV by 5-waves for which the Breit-Wigner formula gives 

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The cross sections of the thermal neutron capture reaction of 186W and 187W are... 

The amount of boron required for BNCT can be estimated using the neutron capture cross sections, which are atomic properties, and thus pertain to the number, and not the mass, of the atoms present. Conservative estimates for successful therapy result in boron concentrations of around 20 ppm in tumor tissue, to at least match the dose liberated by neutron capture reactions in the other elements of biological tissue. This would correspond to around 109 boron-10 atoms per cell, assuming that one cell corresponds to 10-9 g. 

In contrast to many chemotherapeutic agents in cancer therapy, boron compounds for BNCT do not require a tumoricidal action in their own right. For their successful application in the therapy of patients, it is important to deliver, to the tumor, a radiation dose which is higher than the radiation dose to the surrounding tissue. The demonstration that this is actually achieved lies ultimately in the treatment of the tumor in question. Because of the short range of the particles produced in the 10B(n,a)7Li reaction, it is very important where, on a cellular and subcellular dimension, the neutron capture reaction takes place. Different methods for boron detection and quantification give different resolution of the boron distribution. It is instructive to compare these methods, both for their precision and lower detection limits, as well as for their ability to yield an image of the boron distribution in tissue (Table 2.2-1). 

It should be noted that the ytterbium listed above was a mixture discovered in the mineral erbia by de Marignac in 1878 and not the neoytterbium/aldebaranium element renamed ytterbium that was foimd in the mineral ytterbia. The columbium was a mixture found in the mineral samarskite and was not the present day columbium/niobium. The ionium listed above was a mixture of terbium and gadolinium that was found in the mineral yttria and does not refer to °Th. Finally, the neptunium refers to material fovmd in niobium/tantalum minerals and does not refer to the 1940 discovery of the trans-uranium element produced via a neutron capture reaction on a uranium sample. 

Americium - the atomic number is 95 and the chemical symbol is Am. The name derives from America where it was first synthesized in a series of successive neutron capture reactions in the element plutonium, Pu, in a nuclear reactor in 1944 by American scientists under Glenn T. Seaborg at the University of California lab in Berkeley, California, using the nuclear reaction Pu ( n, y) Y) P Am. Americium is the sixth element in the Actinide... 

Am-241 may be prepared in a nuclear reactor as a result of successive neutron capture reactions by plutonium isotopes ... 

Also, Am-243 can be made from Pu-242, which can be prepared either by very intense neutron irradiation of Pu-239, or from Am-241 resulting from successive neutron-capture reactions. 

Also, higher isotopes of curium may be produced from curium-242 by neutron capture reactions. 

Heavier isotopes Es-253, Es-254 and Es-255 can be produced in a nuclear reactor by multiple neutron capture reactions that may occur when uranium, neptunium and plutonium isotopes are irradiated under intense neutron flux. These and other isotopes also are produced during thermonuclear explosions. 

Fig. 2. Schematic illustration of the ideal open nuclear fuel cycle (NRC 2003). In this case, there is no reprocessing. Interim storage may last for tens of years so that the heat and radioactivity are much less prior to handling and final disposal. The spent fuel still contains fissile nuclides, such as 235U and 239Pu (generated by neutron capture reactions with 238U).

These relations are shown in Figure 3.7. They represent the typical decay of many radionuclides prepared by neutron capture reactions, the type of reaction that commonly occurs in a nuclear reactor. 

Let us consider stripping reactions first and, in particular, the most commonly encountered stripping reaction, the (d, p) reaction. Formally, the result of a (d, p) reaction is to introduce a neutron into the target nucleus, and thus this reaction should bear some resemblance to the simple neutron capture reaction. But because of the generally higher angular momenta associated with the (d, p) reaction, there can be differences between the two reactions. Consider the A (d, p) B reaction where the recoil nucleus B is produced in an excited state B. We sketch out a simple picture of this reaction and the momentum relations in Figure 10.16. 

If the time scale of neutron capture reactions is very much less than 3 -decay lifetimes, then rapid neutron capture or the r process occurs. For r-process nucleosynthesis, one needs large neutron densities, 1028/m3, which lead to capture times of the order of fractions of a second. The astrophysical environment where such processes can occur is now thought to be in supernovas. In the r process, a large number of sequential captures will occur until the process is terminated by neutron emission or, in the case of the heavy elements, fission or (3-delayed fission. The lighter seed nuclei capture neutrons until they reach the point where (3 -decay lifetimes have... 

An alternative means of radionuclide production employs neutron capture reactions in nonfissile nuclides. Again a high-purity target is used and a mixture of the unconverted target material, the... 

Ni28+ also produces 6529Cu29+ through a neutron capture reaction followed by radioactive decay (Faure, 1998, 19 Lide, 2007) ... 

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