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Radioactivity
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
In spontaneous fission, the nucleus splits into two or more pieces with the simultaneous emission of fast neutrons. This phenomenon happens with the heaviest nuclei, and it seems to be the reason for an upper limit of the atomic numbers of approximately 110. An example of such radioactive sources used in radiation therapy is that produces α and γ particles, and neutrons with a mean energy of 2.35 MeV.
Nuclear Physics Fundamentals Milorad Mladjenovic
Published in Frank Helus, Lelio G. Colombetti, Radionuclides Production, 2019
The artificially unstable nuclides can also be divided into two groups. One consists of new elements heavier than uranium. Fifteen transuranium elements have been produced so far (up to Z = 107), and more of them might be found. As one should expect, their instability is increasing, so that those beyond Z = 102 have half-lives of a few minutes or less. The increased instability is responsible for a mode of disintegration which is characteristic for transuranium region — the spontaneous fission. There are about 70 nuclides from thorium to nobelium (Z = 102), which can undergo a spontaneous fission, with half-lives ranging from 1021 years to a few milliseconds, at the heavier end. Only some nuclides from four elements beyond plutonium (up to californium) are used in practice. The heavier ones are too expensive to produce.
Fundamental Concepts and Quantities
Published in Shaheen A. Dewji, Nolan E. Hertel, Advanced Radiation Protection Dosimetry, 2019
The emission of alpha particles, as discussed in Section 2.4.1, is energetically favorable, since the BE/A of the atom is high, leading to both a decrease in the Coulombic force of the nucleus and higher BE/A of the daughter (Eisberg and Resnick 1985). A similar effect would be seen for emission of other particles, such as or other higher Z nuclei. Emissions of larger nuclei are referred to as spontaneous fission, and become a relevant decay mode in elements with , and significant in elements with . The emission of neutrons often accompanies this decay. Typically, nuclei that undergo spontaneous fission do so for some fraction of the total decays, as alpha decay is a competing process. For example, decays via both spontaneous fission and alpha decay, with spontaneous fission occurring in about 3% of the decays (Eckerman and Endo 2007).
Neutrons are forever! Historical perspectives
Published in International Journal of Radiation Biology, 2019
Neutrons are neutral particles, composed of one up quark and two down quarks. Free neutrons have a mean lifetime of 14.7 min, whereupon they decay into a proton, an electron and an antineutrino. As a consequence, all sources of neutrons of relevance to radiobiology and human exposure arise from nuclear reactions of one type or another. Neutrons from most of the general types of source reaction summarized below were already available to radiation biology research in the 1950s, although the capabilities and sophistication of the devices have continued to develop until the present time. Two exceptions to availability were practical sources of spontaneous fission and of epithermal neutrons, which did not become available until about 1966 and the 1980s, respectively. The list below is not intended to be comprehensive, but rather to indicate the main types of neutron source that have been used for radiation biology studies and medical applications.
Design and dosimetry of a facility to study health effects following exposures to fission neutrons at low dose rates for long durations
Published in International Journal of Radiation Biology, 2021
Thomas B. Borak, Laurence H. Heilbronn, Nathan Krumland, Michael M. Weil
We selected 252Cf to provide reliable, continuous exposure conditions for periods exceeding one calendar year. This isotope is dominated by 97% alpha emission with a half-life of 2.6 years. There is a 3% branching ratio for spontaneous fission. The Savannah River Laboratory Shielding Guide, DP-1246, quotes the yield of fission neutrons as 2.4 × 1012 (n s−1g−1) with a corresponding photon yield of 1.3 × 1013 (γ s−1g−1) (Stoddard and Hootman 1971).