Multiple neutron capture

It is possible to prepare very heavy elements in thermonuclear explosions, owing to the very intense, although brief (order of a microsecond), neutron flux furnished by the explosion (3,13). Einsteinium and fermium were first produced in this way they were discovered in the fallout materials from the first thermonuclear explosion (the "Mike" shot) staged in the Pacific in November 1952. It is possible that elements having atomic numbers greater than 100 would have been found had the debris been examined very soon after the explosion. The preparative process involved is multiple neutron capture in the uranium in the device, which is followed by a sequence of beta decays. Eor example, the synthesis of EM in the Mike explosion was via the production of from followed by a long chain of short-Hved beta decays,... 

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. 

Isotopes of the elements beyond U are produced artificially, Np and Pu by neutron capture by U, Am and Cm by multiple neutron capture by Pu, and elements beyond Cm by further neutron captures or bombardment of lower atomic number actinoids with ions of He, B, C, N, or O. As the atomic number increases, the elements become more unstable and thus tend to have shorter half lives. Np-237 and Pu-239 are available in multikilogram amounts Am-241 (430 years), Am-243 (7650 years), and Cm-244 (18.1 years) in 100-g amounts Bk-249 (320 days), Cf-252 (2.6 years), and Es-253 (20 days), in milligram amounts Fm-257 in microgram quantities and Md-258 (55 days), No-259 (1.0 h), and Lr (3.0 min) in trace amounts. 

The identification of an isotope of element 95, by Seaborg, Ghiorso, James, and Leon Morgan in late 1944 and early 1945, followed the identification of this isotope of element 96 (242Cm) as a result of the bombardment of 7j Pu with neutrons in a nuclear reactor. The production reactions, involving multiple neutron capture by plutonium, are... 

Fm, which undergoes spontaneous fission (ti = 0.38 ms). This point can be passed in two ways. One is to utilize a more intense neutron flux than can be obtained in a reactor, in the form of a thermonuclear explosion, so that a product such as undergoes further neutron absorption before fission can occur. Here, in the synthesis of Fmin Tvy Mike , the world s first thermonuclear test, atEniwetok atoll on 1st November 1952, the initial product of multiple neutron capture, underwent a whole series of rapid decays, yielding Fm. 

Einsteinium (99) Es 1952 Argonne NatT Lah, Los Multiple neutron capture by fig... 

At the extremely high fluxes of a nuclear explosion, fast multiple neutron capture leads to very neutron-rich isotopes of U or Pu, respectively, changing rapidly into elements of appreciably higher atomic numbers by a quick succession of transmutations. This method of formation of heavier elements is also indicated in Fig. 14.5. The elements can be found in the debris of nuclear underground explosions. 

Derivation Multiple neutron capture in plutonium in nuclear reactors, plutonium isotopes yield241 Am and 243Am on (3 decay. The metal is obtained by reduction of the trifluoride with barium in a vacuum at 1200C. 

Pu. The isotope Pu is the longest-lived of the plutonium- isotopes, with a half-life of 8 X 10 years. It can be produced by neutron absorption in Pu, but because of the short half-life and low concentration of Pu only minute quantities of Pu, of the order of 10" percent, are present in reactor-produced plutonium [K2]. Small quantities of Pu, as well as Pu and Pu, are present in the residues from nuclear explosions, resulting from the decay of the neutron-rich uranium isotopes and formed by multiple neutron capture in the high neutron... 

Both Am and Cm are available on a lOOg scale, and multiple neutron capture followed by P -decay yields milligram amounts of Bk, Es and Es, plus... 

Fermium (100) Fm 1953 Argonne NatT Lab, Los Multiple neutron capture by flg... 

FIG. 15.2. Formation of hi er nuclides through multiple neutron capture and their associated decay chains. 

Very unexpectedly, the elements 99 einsteinium, Es) and 100 fermium, Fm) were detected in 1952 in the debris from a thermonuclear explosion. The Es and Fm isotopes found and identified by a collaborative effort of American laboratories (Ghiorso et al. 1955b) were Es (Tin = 20.5 d) and Fm (20 h). They were presumably produced by the successive capture of 15 or even more neutrons in in an enormous neutron flux on such a rapid timescale that no radioactive decay occurred between neutron captures until the P -decay half-lives of the very neutron-rich uranium isotopes became sufficiently short to compete and, thus, to feed long chains of subsequent P -decays ending in the heaviest elements. Macroscopic amounts of the elements berkelium to einsteinium were produced since the late 1960s by multiple neutron capture in curium in a high flux reactor in Oak Ridge. 

Delayed fission permits studies of fission properties of nuclei far from stability and y deexcitation of nuclear levels in the second minimum of the potential energy surface (O Fig. 18.12). There has been also considerable theoretical interest in P-delayed fission process because it may significantly affect the final abundance of heavy elements produced in the astrophysical r-process and in other multiple neutron capture process taking place in very high neutron flux, such as thermonuclear explosions (Tielemann et al. 1983 Meyer et al. 1989 Cowan et al. 1991 Staut and Klapdor-Kleingrothaus 1992). The experimental data on the delayed fission are presented in Table 18.8. 

Abstract The long quest to detect superheavy elements (SHEs) that might exist in nature and the efforts to artificially synthesize them at accelerators or in multiple-neutron capture reactions is briefly reviewed. Recent reports of the production and detection of the SHEs 113, 114, 115, 116, and 118 are summarized and discussed. Implications of these discoveries and the prospects for the existence and discovery of additional SHE species are considered. 

Such ideas spurred the quest to produce still heavier elements in subsequent thermonuclear tests. Indeed, the rather long-lived nuclide Fm (half-life Tin = 100 days) was detected in later nuclear tests, indicating capture of at least 19 neutrons in uranium. However, attempts to produce and detect still heavier elements in underground nuclear tests conducted at the Nevada Test Site all failed, thus dashing hopes that heavier long-lived elements could be produced via this multiple neutron-capture process that, as it had been postulated, might... 

Very heavy elements have been detected under circumstances where very intense neutron fluxes were produced. Such is the case for a few microseconds after a thermonuclear explosion. Isotopes of einsteinium and fermium were first discovered in the debris of the first thermonuclear explosion detonated at Eniwetok Atoll in November 1952 [2,5]. It is possible that elements of atomic number greater than 100 might have been detected had the debris been examined immediately after the explosion. The route whereby elements of high atomic number are formed in the detonation of a thermonuclear device is again multiple neutron capture in which is a component of the device. Thus, the synthesis... 

What complicates the environmental situation is that plutonium is not the only transuranium element produced in nuclear reactors. Curium and americium are also formed by multiple neutron capture (see Fig. 14.1). The amounts of long-lived actinides in spent fuel as a function of time after removal from a reactor are shown in Table 14.12. The elements americium and curium formed in the reactor undergo radioactive decay to produce radioactive daughter species [57] ... 

The main source of transuranium elements is the high-flux reactor, in which or heavier nuclei get transformed into higher-Z elements by multiple neutron capture. In the USA, there is a national program for the production of transuranium elements utilizing the high-flux reactor (HFIR) at Oak Ridge. The heaviest nuclide produced in the reactor is Fm. Neutron-deficient nuclides are synthesized in charged-particle accelerators and very neutron-rich nuclides with short half-lives are produced in reactors. 

The notations for various decay modes used in this book are a for alpha decay, for / decay, P for positron decay, EC for electron capture, IT for isomeric transition, and SF for spontaneous fission. The letter m after a mass number denotes an isomer. Isomers with a half-life of less than 1 s and fission isomers are omitted from the tables. Energies are given only for the most abundant a groups and y rays for P particles the maximum energies p , are tabulated. In the last column, only the convenient methods for the production of nuclides are given nature denotes that the nuclide occurs in nature and multiple neutron capture means that this nuclide is produced by long irradiation in a high-flux reactor. 

Pu is produced by neutron irradiation of Np ( Np (n, y/ Np,P" Pu). By multiple neutron capture Pu might be formed and present in the RTG. This would give a lower " Pu/ Pu atomic ratio than from nuclear test fallout. Figure 4 shows the activity concentrations of Pu, " Pu and the ratio Pu / Pu in a peat profile. 

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