O.B. Nielsen

Levels in Gd156 excited in the decay of 5.6 d Tb156

Abstract The decay of 5.6 day Tb 156 has been studied by means of a six gap β-ray spectrometer, scintillation spectrometers, and coincidence techniques. Conversion electron and γ-ray coincidence measurements have been used extensively for obtaining quantitative estimates of γ-ray intensities. A description of the electronic equipment is given, including a scaler and gate unit, which improves the performance of a relatively slow ( τ = 700 μ sec) multichannel analyzer. 24 transitions are assigned to Tb 156 . Multipole orders are given in most cases. The following levels are proposed and their quantum numbers ( K , I , π ) given in parenthesis: 89 keV (0, 2 + ), 199 keV (0, 4 + ), 584 keV (0, 6 + ) 1126 keV (tentative), 1152 keV (2, 2 + ), 1246 keV (2, 3 + ), 1507 keV (4, 4 + ), 1620 keV (4, 5 + ), 1931 keV (3 − ), and 2042 keV (4 − ). Violation of K -selection rules and significant deviations from the expected branching ratios for transitions to a rotational band are observed. These features are discussed in terms of a probable mixing of the rotational bands.

Strength-function phenomena in electron-capture beta decay

Abstract Approximate beta strength functions have been derived from measurements of the electroncapture beta feed to the high-lying excited states in about 40 neutron-deficient nuclei. These nuclei, available at the ISOLDE on-line facility at CERN, ranged from isotopes of iridium ( Z = 77) to those of radon ( Z = 86). The beta feed was determined from the gamma spectra by a total absorption spectrometry technique involving two 15 cm diam. × 10 cm height NaI (T1) crystals. Two sum spectra, one with and one without a coincidence demand, were recorded simultaneously for each isotope. An average multiplicity of three to four high-energy γ-rays per cascade was deduced from the ratio of these two spectra. In general, the strength functions vary smoothly with energy as well as with the neutron number N and the proton number Z . Strong peaks appear in the energy region 1.5–3.5 MeV. Relatively little strength is found below an excitation energy in the daughter nucleus corresponding approximately to the energy required for forming a (pn) pair. This overall behaviour can be understood by assuming that the main part of the strength is associated with transitions that excite protons that are initially in paired orbits. An average value S β of the beta strength function for a given isotope may be calculated if the Q -value and the gross electron-capture half-life are known. It was found that S β varies strongly but systematically with N and Z ; from the light Pt isotopes with N ≈ 100 to isotopes just below the 126-neutron closed shell, it decreases by a factor of about 300. Fine structure in this dependence may be evidence for Subshell effects in deformed nuclei. The results are discussed in terms of the shell model.

Some neutron-deficient isotopes of thulium and erbium: A survey of half-lives and γ-ray spectra with the use of mass-separated samples

Abstract Neutron-deficient isotopes of Tm and Er were produced in Er(p, xn)-reactions. Chemical separation, as well as mass-separation of the products, was performed, and the half-lives and γ-ray spectra of the individual samples were measured. Definite mass assignments of the activities were made, and previously reported results essentially confirmed.

The decay of the 1.14 min isomer of Pa234 (UX2)

Abstract Sources of 24 d Th234(UX1) in equilibrium with its daughters, 1.14 min Pa234(UX2) and 6.75 h Pa234(UZ) were studied with a six-gap β-spectrometer, scintillation spectrometers and with e-γ and β-γ coincidence techniques. Quantitative chemical separation of the three nucleides was performed and the individual γ-spectra and disintegration rates were measured. The short lived Pa234 isomer was found to populate three intrinsically excited states in U234, a state with (K, I, π) = (0, 1−) at (790±5) keV, a (0, 0+) state at 811 keV and another (0, 0+) state at 1045 keV. Detailed results on β-, γ- and electric monopole transitions are presented and discussed. By applying three independent methods the relative disintegration rates of UX2 and UZ were found to be 100 to 0.13±0.03. A 70–73 keV isomeric transition of corresponding intensity was found in the conversion line spectrum. The possible assignments of the Pa234 levels are discussed.

The light terbium isotopes a survey of half-lives and γ-ray spectra with the use of mass-separated samples

Abstract Gadolinium oxide was bombarded with 65 MeV protons to produce terbium nuclides. Following a chemical separation the terbium fraction was mass separated. The different mass samples were studied for their half-lives. γ-Ray spectra were taken in most cases. New information was found concerning γ-rays of the extremely light isotopes, i.e., 149Tb through 152Tb. 150Tb was discovered for the first time in this research. The results obtained on the remaining isotopes are essentially in agreement with the data reported earlier and summarized in the most recent Table of Isotopes (April 1958).

The decay scheme of Pb211

Abstract The disintegration scheme of the 36.1 min β− emitter Pb211 has been studied by means of a six-gap β-ray spectrometer, scintillation spectrometers, coincidence techniques, directional correlation measurements and lifetim m measurements. The β− spectrum of Pb211 has four components with endpoint energies: 1355 keV (92.4%), 951 keV (1.4%), 525 keV (5.5%) and 251 keV (0.7%). The following transitions in Bi211 were observed: 404 keV (55% M1 and 45% E2), 426 keV (M1), 700 keV (M1), 830 keV (M1) and 1104 keV. A decay scheme is proposed with levels in Bi211 at 0 keV ( 9 2 − ), 404 keV ( 7 2 − ), 830 keV ( 9 2 − ) and 1104 keV. The transition energy between Pb211 and Bi211 is 1355 keV. The half life of the 404 keV level is (3.15 ± 0.20) × 10−10 sec. The E2 lifetime of the 404 keV level is compared with theoretical estimates. The half life of the 830 keV level is less than 0.9 × 10−10 sec. In addition it was possible to set an upper limit of 0.6 × 10−10 sec on the half life of the 351 keV level in Tl207. The levels of Bi211 are discussed in terms of single-proton states or states involving the single proton and two neutrons outside the doubly closed shell.

0+ states of Sm152 and Gd152

Abstract The decay of Eu152m (9.2 h) is studied by means of internal and external conversion and e−-γ coincidence measurements with particular interest focused on the possible low-lying 0+ levels in the transition region between spherical and non-spherical nuclei. Such levels are now found at 615.3 keV in the near-spherical nucleus Gd152 and at 685.0 keV in the strongly deformed nucleus Sm152. The experimental K-conversion coefficients for the 0+ → 0+ transitions are ≧ 0.770 and ≧ 0.250 , respectively. The e K − γ branching ratios μ from the 0+ levels to the ground states and to the first excited 2+ states are compared with the theoretical values: GD 152 : μ exp = 0.10±0.03, μ theory = 0.085; SM 152 : μ exp = 0.13±0.001, μ theory = 0.13.

Levels in Ho162 and Dy162

Abstract An activity with half-life 11.8 min, representing the ground state of Ho 162 , has been isolated by a recoil method from the 68 min Ho 162 activity which belongs to an isomeric level about 90 keV above the ground state. A decay scheme for the Ho 162 levels is proposed, indicating the following levels in Ho 162 : ground state, ( K, I π ) = (1, 1 + ); 38.5 keV (1, 2 + ); 86.5 keV (1.3 + ); ≈ 90 keV (6, 6 − ). In Dy 162 the following levels are proposed: 81 keV (0, 2 + ); 266 keV (0, 4 + ); 549 keV (0, 6 + ); 1485 keV (5, 5 − ). Single-particle assignments in the Nilsson-Mottelson scheme can be proposed for the unpaired particles of the ground state and the 90 keV states of Ho 162 as well as of the 1485 keV level of Dy 162 , based on the abovespins and the fact that the K-capture transitions from Ho 162 are allowed unhindered.An activity with a half life of 11.8 min, representing the ground state of Ho/sup 1//sup 6//sup 2/, is isolated by a recoil method from the 68 min Ho/ sup 1//sup 6//sup 2/ activity, which belongs to an isomeric level about 90 kev above the ground state. A decay scheme for the Ho/sup 1//sup 6//sup 2/ levels is proposed, indicating the following levels in Ho/sup 1//sup 6//sup 2/: ground state, (K, I pi ) = (1, 1+); 38.5 kev (1, 2/sup +/); 86.5 kev (1.3/sup +/); approx equal 90 kev (6, 6/sup -/). In Dy/sup 1//sup 6//sup 2/ t he following levels are proposed: 81 kev (0, 2/sup +/); 266 kev (0, 4/sup +/); 549 kev (0, 6/ sup +/); 1485 kev (5, 5/sup -/). Single-particle assignments in the Nilsson- Mottelson scheme can be proposed for the unpaired particles of the ground state and the 90 kev states of Ho/sup 1//sup 6//sup 2/, and for the 1485 kev level of Dy/sup 1//sup 6//sup 2/, ba sed on the above spins and the fact that the K-capture transitions from Ho/sup 1//sup 6//sup 2/ are allowed unhindered. (auth)

The decay of 37.5 min 164mHo

Abstract It is shown that a 37.5−0.5+1.5 min activity of 164Ho belongs to a 139 keV state, whereas the ground state has a half-life of 29±2 min. The 139 keV level has probably spin 6− and decays through the 3+, 2+ and 1+ members of the K = 1+ ground state rotational band by transitions of 45.9 keV (E3), 56.1 keV (M1) and 37.0 keV (M1). This decay scheme is analogous to that of the 68 min 162Ho state at 106 keV. The log ft values for the β−-branches from the 164Ho ground state to the 0+ and 2+ states of the 164Er ground state rotational band are 5.5±0.2 and 5.8±0.2, whereas log ft ≤ 4.8 for the transition to the 164Dy ground state. Based on these allowed unhindered β− and electron capture transitions single-particle assignments can be made for the isomeric state as well as for the ground state of 164Ho. The difference in ft values for the transitions to the two even daughter nuclei is interpreted as an indication of individual behaviour of the single particles in the pair scattering process.

Measurements on isotope separated β-ray sources of Re186 and Re188

Abstract Isotopically pure β-ray sources of Re186 and Re188 were prepared from pile irradiated natural Re in an isotope separator. The spectrum of the β-rays from Re188 to the ground state of Os188 was found not to have the allowed shape. The deviations are of the type earlier observed in the decay Re186 → Os186. Both transitions probably proceed from a 1− to a 0+ state. The K-conversion lines for the transitions from the 633 keV level in Os188 to the ground state and to the 155 keV 2+ level were observed. The conversion coefficients indicate E2 nature for these transitions in agreement with an angular correlation experiment. The level schemes of Os186 and Os188 indicate that these nuclei lie in the transitional region between the strongly deformed nuclei and those of spherical equilibrium shape.