From Current Literature

Elements with Atomic Numbers 97 (Berkelium, Bk) and 98 (Californium, Cf)

Some time ago, two new transuranium elements with atomic numbers 97 and 98 were obtained. The first was given the name “berkelium” (Bk), the second “californium” (Cf). Their principal properties, as well as the methods of their preparation, are described below.

The study of the chemical and physical properties of americium and curium, obtained in 1944[^1], confirmed the supposition that, similarly to the lanthanides, the radioactive elements beginning with actinium should be combined into a single transition group, since their \(5f\) shell is being filled. On this basis it became possible to predict the chemical properties of transuranium elements not yet in existence[^2]. Knowledge of these properties is necessary for separating new elements, during their synthesis, from target elements, since chemically they are almost indistinguishable and so far can be obtained only in ultramicroscopic quantities. (The number of atoms of the californium isotope obtained in several irradiations was less than 10 thousand!)

On the path to obtaining new transuranium elements[^3], very great technical difficulties were expected, connected with their short lifetime and with the laborious procedures for separating them from the lanthanides and from the other elements of the actinide series; these, moreover, had to be carried out very rapidly, which required not only the design and development of new techniques, but also the necessity of an enormous separation factor from the target atoms (\(10^6\) for berkelium) at a high percentage yield.

Three possible routes for obtaining transuranium elements were available: 1) irradiation of the first transuranium elements, present in large quantities, with a beam of ions having a large nuclear charge. Although this method is the most promising, it was rejected, since the authors of the papers reviewed did not have intense beams of high energies. 2) Irradiation of \(\mathrm{Am}_{95}^{241}\) with helium ions or of \(\mathrm{Cm}_{96}^{242}\) with deuterons to obtain element 97, and irradiation of \(\mathrm{Cm}_{96}^{242}\) with helium ions to obtain an isotope of element 98. This route was used to obtain both elements. 3) Irradiation of \(\mathrm{Cm}_{96}^{242}\) with neutrons to obtain intermediate nuclei unstable with respect to \(\beta\)-decay and transforming into elements 97 and 98. This method was used only in some experiments to obtain element 97, since it was impossible to predict even the order of magnitude of the reaction cross section for the formation of element 98—there was an uncertainty in the irradiation time of several years.

The starting material for obtaining element 97 was \(\mathrm{Am}_{95}^{241}\), obtained from \(\mathrm{Pu}_{94}^{239}\) by irradiation with fast neutrons according to the reaction:

\[ \mathrm{Pu}_{94}^{239}\ (n,\gamma)\ \mathrm{Pu}_{94}^{240}\ (n,\gamma)\ \mathrm{Pu}_{94}^{241} \]

\[ \mathrm{Pu}_{94}^{241}\xrightarrow[\tau=10\ \text{yr}]{\beta}\mathrm{Am}_{95}^{241} \]

The degree of purity of the \(\mathrm{Am}_{95}^{241}\) thus obtained for the target (in an amount of several milligrams) was about 97% (the principal contaminants were Na—1.5%; Zn—1.0%; Ti—0.2%; Ca—0.2%; Al—0.5%). Irradiation of the target was carried out in a special apparatus serving to cool the target with water and to prevent scattering into the surrounding medium. Irradiation with 30- and 35-MeV helium ions was usually carried out for about 6 hours. The intensity of the beam, measured by the charge accumulated on the insulated target and foils serving as windows of the vacuum chamber, was \(2\ \mu\text{A}\) per \(\text{cm}^2\) of target. Chemical separation procedures\(^{6,7}\), carried out immediately after irradiation of \(\mathrm{Am}_{95}^{241}\), performed in accordance with the expected chemical properties of element 97, separated the activity in the eka-terbium position, i.e., corresponding to the position of element 97. Ion exchange was carried out at high temperature with Dowex-50 resin\(^{3}\). As expected, the behavior of this activity with various carriers and oxidizing agents showed that it had III and IV oxidation states; the ion-exchange separation process showed that in the III oxidation state the separated activity had a great similarity to curium, analogous to the similarity of terbium to gadolinium (see Fig. 4), and the course of the ionic radius of the actinides was disrupted at the isolated element just as the course of the ionic radius of the lanthanide ions was disrupted at terbium. The precipitate of a radiochemically pure fraction isolated in this way, attributed to element 97, was investigated for \(\alpha\)-, \(\beta\)-, and \(\gamma\)-activities on a differential pulse analyzer\(^{10}\), at whose output there stood 48 mechanical registers with a large number of counts. All \(\alpha\)-particle pulses produced by the medium under study in the ionization chamber were differentiated by energy. Three groups of \(\alpha\)-particles were found with energies \(6.72\ \text{MeV}\)—30%; \(6.55\ \text{MeV}\)—53%; and \(6.20\ \text{MeV}\)—17% (Fig. 1). The energy of these groups of \(\alpha\)-particles was measured by the comparison method

Fig. 1.

with $\alpha$-particles of known energies. The high energy of the $\alpha$-particles is a sign indicating that the atomic number of this element must undoubtedly be greater than 94. This is also confirmed by measurements of the energy of the x-ray $L$-radiation. The decay rate for each group of $\alpha$-particles, measured from the area of each peak at definite instants of time, remained unchanged for all three groups and was equal to $4.6 \pm 0.2$ hours. Thus all three groups of $\alpha$-particles had to be assigned to one isotope. This was also confirmed by the fact that changing the energy of the helium ions did not affect the ratio of the intensities of these three groups of $\alpha$-particles. This result was somewhat unexpected: at first it had been assumed (in view of the presence of three groups of $\alpha$-particles) that several isotopes had been obtained. The presence of three groups of $\alpha$-particles with comparable intensities indicates that the transition to the ground state is strongly forbidden (by a factor of $10^3$–$10^5$), for which reason transitions to excited states are possible with comparable probabilities.

Fig. 2.

Measurements of $\gamma$- and $\beta$-radiation were made with Geiger counters with a mica window of about $3 \, \mathrm{mg}/\mathrm{cm}^2$, filled, to increase the counting efficiency for x-ray radiation, with amyl acetate up to $0.8 \, \mathrm{cm}$ and with xenon up to $11 \, \mathrm{cm}$ Hg. To distinguish pulses produced by x-rays and by conversion electrons, various filters were placed between the activity and the counter. The investigated $\gamma$-radiation, typical for $\beta$-capture, had energies from $2$ to $10 \, \mathrm{keV}$ (the cerium $L$ series) and $70 \, \mathrm{keV}$ (the $K$ series). The energy of the conversion electrons (5% of all $\beta$-capture) was $\sim 0.5 \, \mathrm{MeV}$. No search was made for the existence of harder $\gamma$-rays. Measurement of the $\beta$-capture half-period was carried out from the x-ray $L$-radiation and gave the value $4.6 \pm 0.2$ hours. According to the results obtained, the branching of $\alpha$-decay amounts to $0.1\%$ of $\beta$-decay, whence the half-period of the $\alpha$-decay is about $1$ year. The cross section for formation of the berkelium isotope by $35 \, \mathrm{MeV}$ helium ions is equal to $10^{-26} \, \mathrm{cm}^2$; for $30 \, \mathrm{MeV}$ ions it is approximately two times smaller. Investigation of the daughter products of the berkelium isotope was carried out after its complete decay, i.e., after 27 hours (the berkelium residue was $\sim 2\%$); two groups of $\alpha$-particles were found with energies $5.89 \, \mathrm{MeV}$—15% and $5.79 \, \mathrm{MeV}$—85%. (In connection with incomplete initial separation, a small amount of the curium isotope $\mathrm{Cm}^{242}_{96}$ was present in the berkelium fraction—see Figs. 1 and 2.) Chemical separation of the decay products of berkelium showed the presence of Am (the $\alpha$-decay product of berkelium and of the isotope Cm (the $\alpha$-emitter)—the $\beta$-capture product of berkelium. The results of the investigation of the daughter-

of the products not only confirmed the fact of obtaining element 97, but also made it possible to determine its mass number. The small amounts of Am separated from the berkelium fraction make it difficult to determine precisely the half-life of its decay; therefore the value obtained, 15 hours, should be regarded as agreeing well with the half-life of 12 hours for the known isotope \(\mathrm{Am}^{239}_{95}\). On this basis, mass number 243 should be assigned to the isotope of berkelium. The data on the analysis of the Cm isotope separated from the same berkelium fraction (see Fig. 2) are not in contradiction with this. For the half-life of the daughter Cm, a value of 100 years was obtained (energies of the \(\alpha\)-particles 5.89 and 5.79 MeV). Therefore it cannot be identified with the known isotope \(\mathrm{Cm}^{242}_{96}\)*) because of the disagreement of all the decay parameters. Nor can a mass number smaller than 242 be assigned to the isotope of berkelium,

Fig. 3.

since this is inconsistent with the reaction of its formation from americium: \(\mathrm{Am}^{241}_{95}(\alpha,4n)\mathrm{Bk}\) at a helium-ion energy of \(30\) MeV. For this reaction the threshold is above \(30\) MeV. Comparison with the empirical regularities in the systematics of \(\alpha\)-activity \(^{4,5}\) gives the best agreement if mass 243 is assigned to the daughter isotope of curium. No other isotopes of berkelium present in significant quantities were found.

After completing the work with berkelium, Seaborg had to overcome a whole series of difficulties. The accumulation of empirical data in the field of heavy nuclei made it possible to predict more accurately the decay parameters of elements not yet obtained. Methods of chemical separation were also improved \(^{6,7}\). As mentioned, synthesis of the element with atomic number 98 could be carried out only by irradiating \(\mathrm{Cm}^{242}_{96}\) with helium ions. In this case it was possible to predict the number of atoms of element 98 obtained, which was necessary in order to know the optimal irradiation time of the target. In the irradiation process, the following isotopes could arise: \(98^{243}\), \(98^{244}\), \(98^{245}\). Of these, only for \(98^{244}\) was a decay period longer than 30 minutes expected (from 30 to 60 min.). The expected energy for the \(\alpha\)-particles was 7.0–7.3 MeV. The presence of \(\alpha\)-particles of such high energy with a very small number of atoms of element 98 obtained was very convenient; their detection could serve as sufficient proof of the obtaining of element 98, since the known emitters of \(\alpha\)-particles of such energy—\(C'\)-products and Th—could not be produced when \(\mathrm{Cm}^{242}_{96}\) was irradiated with helium ions of energy less than 40 MeV, because of the excessively

*) An \(\alpha\)-emitter with \(\alpha\)-particle energy 6.08 MeV and a period of 10 years.

...of small mass \(^{12}\). For a further increase in the sensitivity of the measurements it was necessary to limit the total \(\alpha\)-activity of the investigated sample to a value of less than \(10^4\) pulses per minute, which imposed still more stringent requirements (than for berkelium) on the separation factor for element 98 (up to \(10^7\)). The target of \(\mathrm{Cm}^{242}_{96}\) was irradiated with 33 MeV helium ions for 2 to 3 hours. The intensity of the ion beam was \(3\ \mu\text{A}\) per \(1\ \text{cm}^2\) of target (the target area was \(\sim 0.5\ \text{cm}^2\)). To facilitate the separation of very small amounts of the isotope californium and to make it possible to compare its position with respect to other actinides during ion exchange, \(\mathrm{Am}^{241}_{95}\), which on irradiation gives \(\mathrm{Bk}^{243}_{97}\), was added to the target in some experiments; for the same purpose a small amount of deuterons was added to the ion beam.

Fig. 4.

By chemical separation methods based on predictions of the expected properties, activity was isolated in the eka-dysprosium position (see Fig. 4), i.e. corresponding to element 98. Its position in the fractions isolated by ion exchange with respect to the actinides was in the same agreement as the position of dysprosium with respect to the lanthanides. Measurement of the energy of the \(\alpha\)-radiation of this fraction gave a value of 7.1 MeV (Fig. 3). The half-life proved to be 45 min. The existence of this activity was observed in four experiments. Investigation of daughter products in its fractions was not carried out because of the small amounts of the element obtained. The approximate value of the cross section for the production of californium by 33 MeV helium ions, based on measurements of only the \(\alpha\)-particle emission, is \(3 \cdot 10^{-27}\ \text{cm}^2\). Since this value of the reaction cross section for 30–40 MeV helium ions is typical also for other transuranium elements, it follows that the \(\alpha\)-radiation must be the dominant type of decay. The question of the mass number from these experiments cannot be finally resolved. Tentatively, from the considerations indicated, it is assigned the value 244.

B. Shulyak

CITED LITERATURE

1. G. T. Seaborg, The Transuranium Elements: Research Papers (1949).
2. G. T. Seaborg, Nucleoniks 5, 16 (1949).
3. S. G. Thompson, Phys. Rev. 80, 781, 790 (1950).
4. Perlman, Phys. Rev. 77, 26 (1950).

1. S. G. Thompson, Phys. Rev. 76, 319 (1949).
2. Asprey, J. Am. Chem. Soc. 72, 1425 (1950).
3. K. Street, J. Am. Chem. Soc. 72, 2790 (1950).
4. S. G. Thompson, Phys. Rev. 77, 838 (1950).
5. Kirkwood, Phys. Rev. 74, 497 (1948).
6. Ghiorso, National Nuclear Energy Series, see reference 1, Paper No. 16, 7.
7. S. G. Thompson, Phys. Rev. 78, 298 (1950).
8. G. T. Seaborg, Rev. Mod. Phys. 20, 585 (1948).