Fermium isotopes

In der Tat zeigen die bislier bekannten Fermium-Isotope den er-warteten Effekt. Die spontanen Kemspaltungshalbwertszeiten nehmen in der Reihe ... 

All the radioisotopes of fermium are dangerous radiation hazards. There is htde chance of coming in contact with one of fermiums isotopes given that they all have very short half-lives and do not exist for long periods. In addition, very small amounts are produced and mainly available for research purposes. 

The ion Es1 is stable. The isotopes of mass numbers 245. 252. 253 and 254 decay by alpha-particle emission that of mass number 250 by electron capture, those of mass numbers 24ft. 248. 249. and 251 by both of these processes, while those of mass numbers 255 and 256 emit electrons to form the corresponding fermium isotopes. 

There is, however, an interesting exception in the figure, namely Md, which shows a neutron yield of about 2 rather than an extrapolated value of 4. The nucleus of Md is the only nucleus among those shown in Fig. 4.20 with a symmetric yield distribution as seen in O Fig. 4.16. (The fermium isotopes with A = 254, 256, and 257 (N = 154, 156, and 157) shown also in Fig. 4.20 have an asymmetric mass distribution (see Fig. 4.16)). Unfortunately, no values of prompt neutron emission are known for the other isotopes with Z > 100 that show a symmetric mass distribution ( Fig. 4.16). 

Einsteinium-99 was found in the debris from the first thermonudear explosion on November 1,1952, at Enewetok Atoll. The einsteinium isotope obtained has a half-life of 20.5 days Fermium-100 was found in the debris from the first thermonuclear explosion at Enewetok Atoll on November 1,1952. The fermium isotope obtained has a half-life of 20.1 hours Mendelevium-101 is obtained by bombardment of einsteinium with helium ions. The mendelevium isotope obtained has a half-life of 1.3 hours... 

Nurmia, M., Sikkeland, T., Silva, R., Ghiorso, A. Spontaneous fission of light fermium isotopes new nuclides Fm and m. Phys. Lett. B26, 78-80 (1967)... 

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

Sixteen isotopes of fermium are known to exist. 257Fm, with a half-life of about 100.5 days, is the longest lived. 250Fm, with a half-life of 30 minutes, has been shown to be a decay product of element 254-102. Chemical identification of 250Fm confirmed the production of element 102 (nobelium). 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Isotopes sufficiently long-Hved for work in weighable amounts are obtainable, at least in principle, for all of the actinide elements through fermium (100) these isotopes with their half-Hves are Hsted in Table 2 (4). Not all of these are available as individual isotopes. It appears that it will always be necessary to study the elements above fermium by means of the tracer technique (except for some very special experiments) because only isotopes with short half-Hves are known. 

The effects of a rather distinct deformed shell at = 152 were clearly seen as early as 1954 in the alpha-decay energies of isotopes of californium, einsteinium, and fermium. In fact, a number of authors have suggested that the entire transuranium region is stabilized by shell effects with an influence that increases markedly with atomic number. Thus the effects of shell substmcture lead to an increase in spontaneous fission half-Hves of up to about 15 orders of magnitude for the heavy transuranium elements, the heaviest of which would otherwise have half-Hves of the order of those for a compound nucleus (lO " s or less) and not of milliseconds or longer, as found experimentally. This gives hope for the synthesis and identification of several elements beyond the present heaviest (element 109) and suggest that the peninsula of nuclei with measurable half-Hves may extend up to the island of stabiHty at Z = 114 andA = 184. 

Half-lives can be interpreted in terms of the level of radiation of the corresponding isotopes. Uranium has a very long half-life (4.5 X 109 yr), so it gives off radiation very slowly. At the opposite extreme is fermium-258, which decays with a half-life of 3.8 X 10-4 s. You would expect the rate of decay to be quite high. Within a second virtually all the radiation from fermium-258 is gone. Species such as this produce very high radiation during their brief existence. 

For example, the most stable isotope of element 100, fermium, has a half-life of only 4.5 days ... 

Self-Test 17.5A The decay constant for fermium-254 is 210 s. What mass of the isotope will be present if a sample of mass 1.00 xg is kept for 10. ms ... 

H. G. Jackson Isotopes of Einsteinium and Fermium Produced by Neutron Irradiation of Plutonium. Report AECL-287. Phys. Rev. 102, 203 (1956). 

Abb. 14. oc-Zerfallsenergien der Fermium-Isotopc in Abh ngigkeit der Massenzahlen. x Geradzahlige Isotope o ungeradzahlige Isotope. 

Einsteinium s most stable isotope, einsteinium-252, with a half-life of472 days, decays into berkelium-248 through alpha decay, and then into californium-252 through beta capture. It can also change into fermium-252 through beta decay. 

ISOTOPES There are a total of 21 isotopes of fermium. Their half-lives range from fer-mium-258 s 370 microseconds to fermium-257 s 100.5 days, which is the longest of all its isotopes. None of fermium s isotopes exist in nature. All are artificially produced and are radioactive. 

Fermium does not exist in nature. All of it is artificially produced in cyclotrons, isotope particle accelerators, or nuclear reactors by a very complicated decay process involving six steps of nuclear bombardment followed by the decay of beta particles, as follows ... 

Because such small amounts of fermium are produced and because the half-hves of its isotopes are so short, there are no commercial uses for it except for basic scientific research. 

Nohehum may he synthesized by several methods involving irradiation of isotopes of curium, plutonium, and fermium in the form of thin targets with heavy ions of boron, carbon, and oxygen using double-recod technique. The nuclear reaction in the synthesis of No-254 carried out by Ghiorso and his group is as foUows ... 

EINSTENIUM. CAS 7429-92-71. Chemical element symbol Es, at. no. 99. at. wt. 254 (mass number of the most stable isotope), radioactive metal of the Actinide series, also one of the Transuranium elements. Both einsteinium and fermium were formed tit a thermonuclear explosion that occurred in the South Pacific in 1952. The elements were identified by scientists from the University of California s Radiation Laboratory- the Argonnc National Laboratory, and the I. os Alamos Scientific Laboratory. It was observed that very heavy uranium isotopes which resulted from the action of the instantaneous neutron dux on uranium (contained in the explosive device) decayed to form Es and Fm. The probable electronic configuration of Es is... 

The discovery of fermium (also einsteinium) was not the result of very carefully planned experiments, as in the cases of the other trans uranium elements, bill fermiuni and einsteinium were found in Ihe debris of an atomic weapon lest in the Pacific in November 1952. Researchers, using the Oak Ridge High Flux Isotope Reactor (HFIR) which produced 3.2-hour " Fm. determined ihe magnetic moment of the atomic ground state of the neutral fermium atom with a modified atomic beam magnetic resonance... 

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