stylistic:style_transfer, information:extraction
Isotopes

33 isotopes of actinium and eight excited isomeric states of some of its nuclides are known, ranging in mass number from 204 to 236. Three isotopes, Ac, Ac and Ac, were found in nature and the others were produced in the laboratory; only the three natural isotopes are used in applications. Actinium-225 is a member of the radioactive neptunium series; it was first discovered in 1947 as a decay product of uranium-233 and it is an α-emitter with a half-life of 10 days. Actinium-225 is less available than actinium-228, but is more promising in radiotracer applications. Actinium-227 (half-life 21.77 years) occurs in all uranium ores, but in small quantities. One gram of uranium (in radioactive equilibrium) contains only 2 × 10 gram of Ac. Actinium-228 is a member of the radioactive thorium series formed by the decay of Ra; it is a β emitter with a half-life of 6.15 hours. In one tonne of thorium there is 5 × 10 gram of Ac. It was discovered by Otto Hahn in 1906.

There are 32 known isotopes of thorium ranging in mass number from 207 to 238. Of these, the longest-lived is Th, whose half-life of 1.4 × 10 years means that it still exists in nature as a primordial nuclide. The next longest-lived is Th, an intermediate decay product of U with a half-life of 75,400 years. Several other thorium isotopes have half-lives over a day; all of these are also transient in the decay chains of Th, U, and U.

28 isotopes of protactinium are known with mass numbers 212–239 as well as three excited isomeric states. Only Pa and Pa have been found in nature. All the isotopes have short lifetimes, except for protactinium-231 (half-life 32,760 years). The most important isotopes are Pa and Pa, which is an intermediate product in obtaining uranium-233 and is the most affordable among artificial isotopes of protactinium. Pa has convenient half-life and energy of γ-radiation, and thus was used in most studies of protactinium chemistry. Protactinium-233 is a β-emitter with a half-life of 26.97 days.

There are 27 known isotopes of uranium, having mass numbers 215–242 (except 220). Three of them, U, U and U, are present in appreciable quantities in nature. Among others, the most important is U, which is a final product of transformation of Th irradiated by slow neutrons. U has a much higher fission efficiency by low-energy (thermal) neutrons, compared e.g. with U. Most uranium chemistry studies were carried out on uranium-238 owing to its long half-life of 4.4 × 10 years.

There are 25 isotopes of neptunium with mass numbers 219–244 (except 221); they are all highly radioactive. The most popular among scientists are long-lived Np (t = 2.20 × 10 years) and short-lived Np, Np (t ~ 2 days).

There are 20 known isotopes of plutonium, having mass numbers 228–247. The most stable isotope of plutonium is Pu with half-life of 8.13 × 10 years.

Eighteen isotopes of americium are known with mass numbers from 229 to 247 (with the exception of 231). The most important are Am and Am, which are alpha-emitters and also emit soft, but intense γ-rays; both of them can be obtained in an isotopically pure form. Chemical properties of americium were first studied with Am, but later shifted to Am, which is almost 20 times less radioactive. The disadvantage of Am is production of the short-lived daughter isotope Np, which has to be considered in the data analysis.

Among 19 isotopes of curium, ranging in mass number from 233 to 251, the most accessible are Cm and Cm; they are α-emitters, but with much shorter lifetime than the americium isotopes. These isotopes emit almost no γ-radiation, but undergo spontaneous fission with the associated emission of neutrons. More long-lived isotopes of curium (Cm, all α-emitters) are formed as a mixture during neutron irradiation of plutonium or americium. Upon short irradiation, this mixture is dominated by Cm, and then Cm begins to accumulate. Both of these isotopes, especially Cm, have a longer half-life (3.48 × 10 years) and are much more convenient for carrying out chemical research than Cm and Cm, but they also have a rather high rate of spontaneous fission. Cm has the longest lifetime among isotopes of curium (1.56 × 10 years), but is not formed in large quantities because of the strong fission induced by thermal neutrons.

Seventeen isotopes of berkelium have been identified with mass numbers 233, 234, 236, 238, and 240–252. Only Bk is available in large quantities; it has a relatively short half-life of 330 days and emits mostly soft β-particles, which are inconvenient for detection. Its alpha radiation is rather weak (1.45 × 10 % with respect to β-radiation), but is sometimes used to detect this isotope. Bk is an alpha-emitter with a long half-life of 1,380 years, but it is hard to obtain in appreciable quantities; it is not formed upon neutron irradiation of plutonium because β-decay of curium isotopes with mass number below 248 is not known. (Cm would actually release energy by β-decaying to Bk, but this has never been seen.)

The 20 isotopes of californium with mass numbers 237–256 are formed in nuclear reactors; californium-253 is a β-emitter and the rest are α-emitters. The isotopes with even mass numbers (Cf, Cf and Cf) have a high rate of spontaneous fission, especially Cf of which 99.7% decays by spontaneous fission. Californium-249 has a relatively long half-life (352 years), weak spontaneous fission and strong γ-emission that facilitates its identification. Cf is not formed in large quantities in a nuclear reactor because of the slow β-decay of the parent isotope Bk and a large cross section of interaction with neutrons, but it can be accumulated in the isotopically pure form as the β-decay product of (pre-selected) Bk. Californium produced by reactor-irradiation of plutonium mostly consists of Cf and Cf, the latter being predominant for large neutron fluences, and its study is hindered by the strong neutron radiation.

Among the 18 known isotopes of einsteinium with mass numbers from 240 to 257, the most affordable is Es. It is an α-emitter with a half-life of 20.47 days, a relatively weak γ-emission and small spontaneous fission rate as compared with the isotopes of californium. Prolonged neutron irradiation also produces a long-lived isotope Es (t = 275.5 days).

Twenty isotopes of fermium are known with mass numbers of 241–260. Fm, Fm and Fm are α-emitters with a short half-life (hours), which can be isolated in significant amounts. Fm (t = 100 days) can accumulate upon prolonged and strong irradiation. All these isotopes are characterized by high rates of spontaneous fission.

Among the 17 known isotopes of mendelevium (mass numbers from 244 to 260), the most studied is Md, which mainly decays through electron capture (α-radiation is ≈10%) with a half-life of 77 minutes. Another alpha emitter, Md, has a half-life of 53 days. Both these isotopes are produced from rare einsteinium (Es and Es respectively), that therefore limits their availability.

Long-lived isotopes of nobelium and isotopes of lawrencium (and of heavier elements) have relatively short half-lives. For nobelium, 13 isotopes are known, with mass numbers 249–260 and 262. The chemical properties of nobelium and lawrencium were studied with No (t = 3 min) and Lr (t = 35 s). The longest-lived nobelium isotope, No, has a half-life of approximately 1 hour. Lawrencium has 14 known isotopes with mass numbers 251–262, 264, and 266. The most stable of them is Lr with a half life of 11 hours.

Among all of these, the only isotopes that occur in sufficient quantities in nature to be detected in anything more than traces and have a measurable contribution to the atomic weights of the actinides are the primordial Th, U, and U, and three long-lived decay products of natural uranium, Th, Pa, and U. Natural thorium consists of 0.02(2)% Th and 99.98(2)% Th; natural protactinium consists of 100% Pa; and natural uranium consists of 0.0054(5)% U, 0.7204(6)% U, and 99.2742(10)% U.