Reports of the Academy of Sciences of the USSR

1. Volume 112, No. 3

PHYSICS

G. M. GORODINSKII, A. N. MURIN, V. N. POKROVSKII,
B. K. PREOBRAZHENSKII, and N. E. TITOV

RADIOACTIVE ISOTOPES OF THE RARE EARTHS FORMED IN A DEEP-SPALLATION REACTION

(Presented by Academician L. A. Artsimovich, 27 X 1956)

Radioactive isotopes of the rare earths obtained by irradiating tantalum with protons of energy 340 MeV were studied by Nervik and Seaborg (^1). In this work isotopes of erbium, thulium, ytterbium, and lutetium were isolated; the lighter fractions, with the exception of cerium, were not investigated.

We isolated and chromatographically separated (^2) long-lived radioisotopes of the rare earths obtained by irradiating tantalum with protons of energy 680 MeV in the synchrocyclotron of the Joint Institute for Nuclear Research.

Determination of half-lives, type and energy of radiation, as well as repeated recording of γ-spectra on a scintillation spectrometer (NaJ(Tl) and CsJ(Tl) crystals), made it possible to identify a number of known radioisotopes, to clarify certain genetic relationships, to discover new gadolinium isotopes, and to indicate a number of new lines in the γ-spectra of the nuclides studied by us. A brief account of the results obtained for the separate fractions is given below.

1. Cerium. The isotopes Ce^134 and Ce^139 were found; their radioactive characteristics completely coincide with those reported in the literature (^3).

2. Neodymium. In this fraction the only radioisotope found was Nd^140 ($T = 3.3$ days), whose γ-spectrum had not previously been studied. According to our data, in addition to an intense annihilation gamma line with energy 0.51 MeV

[
\left(
\mathrm{Nd}^{140}
\ \xrightarrow[3.3\ \text{days}]{k}\
\mathrm{Pr}^{140}
\ \xrightarrow[3.5\ \text{min.}]{k,\ \beta^{+}}\
\mathrm{Ce}^{140};
\ \text{the corresponding positrons of energy } \sim 2.3\ \text{MeV were observed with a magnetic } \beta\text{-spectrometer}
\right)
]

there is a close γ-line of energy 0.5 MeV, detected from the shape of the annihilation line. In the energy region 0.26–0.32 MeV there is a group of weak lines; there is a 0.19 MeV line (apparently double), and, finally, a weak 0.11 MeV line.

1. Europium. The isotopes Eu^145 ($T = 5$ days) and Eu^147 ($T = 24$ days) were found; their γ-spectra are not described in the literature. The γ-spectrum of Eu^145 consists of four weak lines: 0.630, 0.660, 0.730, and 0.890 MeV. Study of the γ-spectrum in the region 0.080–0.300 MeV is hindered by the presence of the intense γ-spectrum of Eu^147; in the region 0.30–0.45 MeV there are a number of very weak lines, poorly resolved by the spectrometer. The spectrum of Eu^147 consists of a weak, apparently strongly converted, line at 0.080 MeV and two intense lines at 0.124 and 0.200 MeV.

2. Gadolinium. Activities with half-lives of 1.5 days, 9 days (Gd^149), and $70 \pm 5$ days were found. In the separated daughter substances accumulating in the gadolinium fraction, Eu^147 was found with its characteristic γ-spectrum. Repeated separation of Eu suggests that the parent substance of Eu^147 is a Gd isotope with a half-life of 1.5 days, to which, consequently, mass number 147 should be assigned. The mass number of the other radioisotope ($T = 70$ days) has not been definitively established, but it has been proposed to be 145 on the basis of the observed

in the daughter fraction from the traces of Eu({}^{145}). In the spectrum of Gd({}^{147}) three (\gamma)-lines were found, 0.232, 0.373, and 0.385 MeV, with successively decreasing intensity; in the previously unknown spectrum of Gd({}^{149}), four lines (given in order of decreasing intensity): 0.154, 0.292, 0.350, and 0.505 MeV. The spectrum of Gd({}^{145(?) }) consists of three lines: 0.115 (weak), 0.638, and 0.750 MeV.

1. Terbium. Activities were observed with half-lives of 20 h (Tb({}^{151}) or Tb({}^{154})), 4.5 days (Tb({}^{153}) or Tb({}^{156}); Tb({}^{157}), according to work ({}^{(4)}), has a half-life of less than 10 min or greater than 25 years). The (\gamma)-spectrum of the Tb fraction is very complex and is poorly resolved. Lines with energies of 0.345 and 0.465 MeV belong to the Tb isotope with half-life (\simeq 20) h. In the daughter fraction, Gd with a half-life (\simeq 70) days and with (\gamma)-line energies of 0.100 and 0.150 MeV was found. Further study of the Tb fraction is quite necessary.

2. Dysprosium. In the Dy fraction (separated from Y), activity with a half-life (\simeq 8) h, which we attribute to Dy({}^{157}), was found. In the (\gamma)-spectrum of this isotope, (\gamma)-lines of 0.233 and 0.325 MeV were observed, of which the latter agrees with the literature data ({}^{(5)}).

3. Fractions of holmium, erbium, and thulium. The principal data agree satisfactorily with the results of Nervik and Seaborg and other authors ({}^{(1,3,6,7)}).

The radioisotope Ho({}^{160}) ((T = 5) h), in addition to the (\gamma)-lines indicated in ({}^{(1)}), has two weak lines with energies of 0.29 and 0.51 MeV. The structure of the spectrum of Tu({}^{166}) is indicated below.

1. Ytterbium. Activity was found with half-lives of 60 h (Yb({}^{166})) and 31 days (Yb({}^{169})). In ({}^{(1)}) the (\gamma)-spectrum of the equilibrium mixture was given

[
\mathrm{Yb}^{166}
\ \xrightarrow[\text{60 h}]{k}\
\mathrm{Tu}^{166}
\ \xrightarrow[\text{7.7 h}]{k,+\beta}\
\mathrm{Eu}^{166}.
]

Separation of the daughter fraction (Tu({}^{166})) made it possible to separate the (\gamma)-spectra of Yb({}^{166}) and Tu({}^{166}). The spectrum of Tu({}^{166}) consists of lines 0.080, 0.180, 0.690, and 0.780 MeV. Lines with energies of 0.112 and 0.140 MeV should be attributed to the isotope Yb({}^{166}). The presence is confirmed of a (\gamma)-line with energy 0.260 MeV in the spectrum of Yb({}^{169}), indicated in ({}^{(8)}).

1. Lutetium. Activities were found with half-lives (\simeq 2) days, 7–8 days, about 35 days, and with a longer period. The supposition of Nervik and Seaborg that the Lu radioisotope with a period of 1.7 days (2 days according to our measurements) is the parent substance of Yb({}^{169}) with a half-life of 32 days is confirmed, since the spectrum of Yb separated from the Lu fraction fully coincides with the well-studied spectrum of Yb({}^{169}). The activity observed in the Lu fraction with a period of 32 days belongs precisely to this daughter isotope. Thus, the period of 1.7 days corresponds to Lu({}^{169}), which does not exclude the possibility of formation of Lu({}^{170}) with a half-life close to the indicated one ((\simeq 2) days). The spectrum of long-lived Lu (apparently Lu({}^{174})) consists of a (\gamma)-line with energy 0.265 MeV. Let us note that we observed electrons with an energy of 0.6 MeV, appearing in the transformation Lu({}^{174} \xrightarrow{\beta^-}) Hf({}^{174}).

A detailed description of the method and discussion of the results will be published.

The authors express their deep gratitude to the operating staff of the synchrocyclotron and especially to V. P. Dzhelepov for assistance in the work.

V. G. Khlopin Radium Institute
Academy of Sciences of the USSR

Received
17 X 1956

CITED LITERATURE

1. W. E. Nervik, G. T. Seaborg, Phys. Rev., 97, 1092 (1955).
2. B. K. Preobrazhenskii, O. Lilova et al., Zhurn. neorg. khim., No. 9 (1956).
3. I. M. Hollander, I. Perlman, G. T. Seaborg, Rev. Mod. Phys., 25, 469 (1953).
4. T. H. Handley, W. E. Lyon, Phys. Rev., 99, 1415 (1955).
5. T. H. Handley, E. L. Olson, Phys. Rev., 90, 500 (1953).
6. T. H. Handley, E. L. Olson, Phys. Rev., 92, 1260 (1953).
7. T. H. Handley, Phys. Rev., 94, 945 (1954).
8. S. A. E. Johansson, Phys. Rev., 100, 835 (1955).