ALBERT

Notes: Metal colloids in CaF2 created by low energy electron irradiation are identified by their optical absorption bands. A mean colloid radius of 18 nm is deduced from a narrow colloid band found in crystals irradiated at room temperature. Absorption data reveal a strong influence of crystal temperature during irradiation on metallization efficiency, mean colloid size, and colloid size distribution. A reversible absorption band shift is observed when turning the electron beam on and off, which may indicate a decrease of colloid size during electron irradiation. © 1995 American Institute of Physics.

Notes: Abstract Multiple Coulomb excitation measurements on198, 200Hg have been performed with 5MeV/amu208Pb projectiles andB(E2)-values are determined for transitions between states up to spin 8+. In198Hg a reduction of theB(E2)-value for the yrast transition 8+→6+ by a factor of 3 as compared to the rigid rotor prediction is observed, which supports the earlier proposed idea that the ground state band is crossed between the 6+ and 8+ state by a weakly interactingvi 13 2/2 rotation aligned band. In each of the two nuclei,198Hg and200Hg, a state with a possibleI π=8+ assignment is observed, which is tentatively interpreted as the 8+ member of the ground state band.

Notes: Abstract The nuclearg-factor of the 2·083 MeV-state of Ce140 has been determined as:g=1·11±0·04 This value suggests the interpretation of the level as the (g7/2,d5/2)4 two proton shell model configuration. The investigation implies an integral measurement as well as a time dependent differential measurement (spin rotation method) of the rotation of the 329 keV-487 keVγγ-angular correlation in strong external magnetic fields. Liquid sources of lanthanum nitrate solved in 3n HNO3 were used in these experiments. Some cerium (IV)nitrate was added to make sure that the 4f;-electronic shell remained empty after theβ −-decay. This prevented paramagnetic effects. Additional runs were performed without adding cerium (IV)nitrate and finally by using liquid sources of lanthanum chloride solved in 2n HCl with some iron (II)-chloride added. It was expected that the Fe++ ions would reduce the primarily formed Ce++++ ions into the Ce+++ state. The Ce+++ shell is paramagnetic and enlarges the effective magnetic field at the position of the nucleus by about 36%. As no definitely faster rotation could be observed, one has to conclude that the ion reaction Ce+++++Fe++⇒Ce++++Fe+++ takes more than about 10−8s. The half life of the 2·083 MeV state has been redeterminded asT 1/2=(3·41±0·04)·10−9s. Differential measurements of the angular correlation of the 329 keV-487 keV cascade as a function of the time delay showed that perturbations by internal fields were very small. The coefficientsA 2 andA 4 of the angular correlation and the attenuation parameterλ 2 were determined from measurements with sources of the first type as:A 2=−0·105±0·003A 4=−0·001±0·002 andλ 2=(0·0065±0·0040)·109S−1=(0·032±0·020)·λ A comparison with the theoretical coefficients of a (3-4-2)-cascade shows that the multipolarity of the 329 keV radiation is a mixture of 99·83%M1 and (0·17±0·03) %E2.

Notes: Abstract The angular correlation of the 133 keV-482 keV-yy-cascade in the decay of Hf181 is strongly attenuated if solid sources of hafniumammoniumhexafluoride are used. The unperturbed correlation was observed however when a single crystal of hafniumammoniumhexafluoride was used whose main axis pointed into the direction of one of the two detectors. This proves that the perturbation is static and that the maximum component of the electric field gradient at the position of the hafnium nucleus coincides with the direction of the main axis of the crystal. The anisotropy of the angular correlation was measured as a function of the direction of the crystal axis. The results agree with the theoretical predicted functions for a strong electric quadrupole interaction. Then we combined the intrinsic electric field with an external magnetic field. The magnetic field direction was chosen parallel to thez-axis of the electric field gradient and perpendicular to the plane of the detectors. The theory for axially symmetric field gradients predicts a maximum of the anisotropy of the angular correlation for a magnetic field strength at which resonance exists between electric and magnetic precession. For a strong electric interaction the maxium anisotropy has half the value of the unperturbed correlation. In our case the electric quadrupole interaction was so strong that we could not reach the resonance even when we applied external magnetic fields up to 48000 gauss. The observed anisotropies were too large however to be fitted by theoretical curves which were calculated under the assumption that the field gradient has axial symmetry. Therefore we developed the theory for non-axially symmetric electric field gradients. Now a fit was possible and gave unique solutions for the strength of the electric hyperfine interaction as well as for the asymmetry coefficient of the electric field gradient tensor. The accuray of these results was not very high but the strength of the electric hyperfine interaction was found to be small enough to make a direct observation of the electric spin rotation by the differential angular correlation method possible. The observed pattern confirmed the non-axially symmetry of the electric field gradient and we derived the following parameters: $$\omega _{E_0 } = \left( {570 \pm 30} \right)MHz\left( {\omega _{E_0 } = electric interaction frequency = \frac{{6eQ \cdot \left| {V_{zz} } \right|}}{{4I \cdot \left( {2I - 1} \right) \cdot \rlap{--} h}}} \right)$$ $$\eta = 0,43 \pm 0,04\left( {\eta = asymmetry parameter = \frac{{V_{xx} - V_{yy} }}{{V_{zz} }}} \right)$$ . As the hafnium nucleus is strongly deformed, a value for the electric quadrupolemoment of the 482 keV state can be derived from coulomb excitation work. UsingQ(432 keV)=(2·45±0·1)·10−24 cm2 one gets the following absolute values for the electric field gradient tensor components: $$\begin{gathered} V_{xx} = \mp \left( {1 \cdot 0 \pm 0 \cdot 1} \right) \cdot 10^{17} V/cm^2 \hfill \\ V_{yy} = \mp \left( {2 \cdot 6 \pm 0 \cdot 2} \right) \cdot 10^{17} V/cm^2 \hfill \\ V_{zz} = \pm \left( {3 \cdot 6 \pm 0 \cdot 2} \right) \cdot 10^{17} V/cm^2 \hfill \\ \end{gathered} $$ .

Notes: Abstract The following angular correlations in the decay of Hf180m have been measured: 215 keV-93 keVγγ-cascade: $$W(\Theta ) = 1 + (0 \cdot 093 \pm 0 \cdot 007) P_2 + (0 \cdot 014 \pm 0 \cdot 009) P_4 $$ 332 keV-215 keVγγ-cascade: $$W(\Theta ) = 1 + (0 \cdot 090 \pm 0 \cdot 010) P_2 + (0 \cdot 006 \pm 0 \cdot 010) P_4 $$ 501 keV-332 keVγγ-cascade: $$W(\Theta ) = 1 + (0 \cdot 004 \pm 0 \cdot 007) P_2 - (0 \cdot 002 \pm 0 \cdot 012) P_4 .$$ Liquid sources of HfF4 solved in concentric HF were used for these experiments. The angular correlation of the 215 keV-93 keV-cascade was found to be slightly attenuated by internal fields. A measurement of the differential angular correlation for different delays gave: $$\lambda _2 = (0 \cdot 073 \pm 0 \cdot 036) \cdot 10^9 \sec ^{ - 1} assuming A_2 (t) = A_2 (0) \cdot e^{ - \lambda _2 t} .$$ The above correlation functions agree with the interpretation of the low lying states of Hf180 as a (0+, 2+, 4+, 6+)-rotational band. The 501 keV-332 keV angular correlation shows that the multipolarity of the 501 keV-γ-transition is a mixture of (3.5±0.5)%M2 and (96.5±0.5)%E3. The rotation of angular correlations in a strong external magnetic field has been observed and the gyromagnetic ratios of the 2+ and the 4+ rotational states were determined as: $$g_R (2^ + level) = + 0 \cdot 371 \pm 0 \cdot 032$$ and $$g_R (4^ + level) = + 0 \cdot 5 \pm 0 \cdot 1.$$

Notes: Abstract A differential measurement of the spin rotation of Dy160 in the 2+ rotational state was performed by using liquid sources of TbCl3 solved in 3M HCl and applying an external magnetic field of 33 500 Gauss. No change of the Larmor precession frequency could be detected within the first 10·10−9 s. It is concluded that the ground state of the electronic shell of Dy+++ is reached in 6·10−10 s after theβ-decay of Tb160. The valueg R=+0.364±0.011 was derived using 〈r−3〉eff=8.92 a. u. for the 4f-shell of Dy+++. A comparison with the result ofCohen who studied the Mössbauer-effect in Fe2Dy shows that the value of 〈r−3〉eff must be 10% larger in this compound. A measurement of the effective magnetic field at the position of the nucleus in a source of terbium metal was performed for different temperatures. It revealed a temperature dependence which is very similar to the paramagnetic susceptibility χ(T). We observed a strong attenuation ofγ γ-angular correlations in the 2+ rotational state. For liquid sources of TbCl3 solved in 3M HCl the following attenuation parameters were measured: $$\begin{gathered} \lambda _2 = (0.122 \pm 0.013) \cdot 10^9 {\text{s}}^{ - {\text{1}}} , \hfill \\ \lambda _4 = (0.235 \pm 0.024) \cdot 10^9 {\text{s}}^{ - {\text{1}}} . \hfill \\ \end{gathered}$$ The ratioλ 4/λ 2 deviates from the value which is expected for the attenuation by internal fields of the 4f-electronic shell. Possible reasons are discussed. We derived the following multipole mixtures ofγ-transitions in the decay of Tb160: $$\begin{gathered} \delta (879 {\text{keV) = + 17}}{\text{.7}}_{ - {\text{2}}{\text{.8}}}^{{\text{ + 5}}{\text{.2}}} (M1 + E2) \hfill \\ \delta (299 {\text{keV) = }} - 0.029 \pm 0.005 (E1 + M2) \hfill \\ \delta (329 {\text{keV) = }} - 0.023 \pm 0.007 (E1 + M2). \hfill \\ \end{gathered}$$ The half life of the 2+ rotational state was redetermined as $$T_{\tfrac{1}{2}} = (1.99 \pm 0.04) \cdot 10^{ - 9} {\text{s}}{\text{.}}$$

Notes: Abstract The rotation of the angular correlation of the 631 keV-137keVγ-γ-cascade in the decay of Re186 in an external magnetic field of 53 500 gauß was determined to:ωτ=0·098 ± 0·008. The half life of the 137 keV level was measured as: T1/2=(0·84 ± 0·03) · 10−9 sec. The coincidences between the 137 keVγ-radiation and theβ-group of 927 keV maximum energy were used in connexion with a time to pulse height converter circuit. The coefficients of the angular correlation of the 631 keV-137 keV γ-γ-cascade were found as: A2=− 0·073 ± 0·010; A4=+ 0·310 ± 0·014. These values imply corrections for 3·9% admixture of internal bremsstrahlung and a 1 % contribution by K-X-radiation. The solid angle-corrections were done according to the formula given by E.Rose. The comparison with the theoretical coefficients for a (2+ 2+ 0+)-cascade shows that the multipolarity of the 631 keV radiation is pure E2, the M1 admixture being less than 0· 1%. This result is in agreement with the K-selection rule. There is no appreciable attenuation by internal fields. The comparison with the theoretical angular correlation gives for the integral attenuation factor: G4=0·95±0·04; Assuming only attenuation by electric quadrupole interaction one gets for: G2=0·92±0·07; The nuclear g-factor of the 137 keV rotational state was derived from these results without any further correction as: gR= + 0·316±0·028.

Notes: Abstract. Excited bands of 228Th were studied in the 226Ra(α,2n) reaction by gamma-ray and conversion-electron spectroscopy. The first-excited K π=0+ and 2+ bands were identified up to I π= 12+ and 10+, respectively. The spin dependence of the moments of inertia is discussed. The experimental data indicate a structural change of these bands from 228Th to 230Th and 232Th.

Notes: Abstract: The electron-capture decay of 228Pa to levels in 228Th was studied using mass-separated sources and a γ-ray detection system consisting of five Compton-suppressed Ge detectors. A total of 87 levels were observed up to an excitation energy of 2 MeV which are connected by approximately 500 γ-ray transitions. The complete octupole quadruplet, three excited K π=0+ bands and two K π=2+ bands were identified below 1.4 MeV. The observed level structure is compared to calculations within the quasiparticle-phonon nuclear model. The surprisingly good agreement indicates that 228Th has less transitional character than hitherto assumed.