The four quasi-particle 178Hf isomeric state,its excitation energy and multipolarities of deexciting transitions

The decay properties of the 31 y ¹⁷⁸Hf isomeric state have been investigated by means of a Ge(Li)-NaI(Tl) Compton suppression device and a Si(Li) detector, as well as with Ge(Li) and Ge(Li), intrinsic Ge and Si(Li) coincidence arrangements. From the E3 character of the unobserved isomeric transition (11.7 ≦ E_IT ≦ 16.7 keV), deduced from L-subshell and M/L ratios, I^π, K of the isomeric level was determined as 16⁺, 16. The {p[404] ↓ + p1514] ↑ + n[514] ↓ + n[624]↑}_16⁺ + four quasi-particle configuration was assigned to this level at 2447.5 ± 2.5 keV. As proposed earlier the isomeric E3 transition decays to the 13^? level of the band built upon the I^π = 8^? isomer. The |g_K?g_R| values derived from $\text{E}\text{2}\text{M}\text{1}$ ratios of interband transitions show that the 8^? isomer contains about 36 % of the p[404] ↓ + p[514] ↑ configuration and about 64 % of the n[624] ↑ + n[514] ↓ configuration.     

On the level scheme of the159Tm nucleus

Level in¹⁵⁹Tm have been excited in the¹⁵⁰Sm(¹⁴N,5n reaction. The yrast band 7/2^?[523] observed up to spin 45/2^? is built on a isomeric state at 166 keV (T _1/2=37±5ns). Connections between the 7/2^?[523] and 7/2⁺ [404] bands fix the band-head energies.     

Properties of the 13/2+ isomeric decay in201Hg

The isomeric decay of theT _1/2=94 us isomeric state in²⁰¹Hg was studied in beam via the reaction¹⁹⁸Pt(α, n). Gamma-ray and conversion-electron spectra were recorded. From the conversion coefficients, the multipolarities of the two depopulating transitions were determined to beM 2 + 10%E 3 andE 2, respectively. These assignments give the depopulating transitions to be 13/2⁺ → 9/2^? and 9/2^? → 5/2^?, and these thus substantiate the previous suggestion that the isomeric state hasJ ^π=13/2⁺. For the 13/2⁺ → 9/2^? M 2 transition a detailed study ofE 3 andM 4 admixed amplitudes was done. The microscopic properties of the 13/2⁺ and 9/2^? states are discussed, and a comparison to the corresponding ones in^199,203,205Hg is made.     

The odd-parity level at 687.59 keV in236U

The spin-parity of the 687.59 keV level in²³⁶U populated in the decay of²³⁶Np is determined to be 2^? in disagreement with recent 1^? assignments from reaction data, but in accordance with previous radioactive²³⁶Np decay studies. The 687 keV ground state transition is shown to have a pureM2 character. The 642 keV 2^?→2⁺ transition hasE1:M2:E3 mixing ratios of 64∶30∶6. The half-life of the 2^? state is 3.78 ± 0.09 ns. The hindrance factor of the E1-part fits an E1 transition withΔK=2, theM2-part has the same transition rate as the M2 transitions in the neighbouring²³⁷Np nucleus and the E3-part is enhanced by two orders of magnitude relative to the Weisskopf estimate. The data are compared to analogous transitions in the rare earth region.     

Half-life measurements of intrinsic states in deformed odd-mass nuclei

The half-lives of the following intrinsic states in deformed odd-mass nuclei has been measured by delayed coincidences with a time-to-amplitude converter:

1. 5/2 5/2⁺[642] at 86.5 keV in¹⁵⁵Gd:T _1/2=6.7±0.3 ns, which results in the determination of theE1,ΔK=1 transition probability to the ground state 3/2 3/2^?[521] and first rotational state 5/2 3/2^?[521], yielding hindrance factors ofF _N ≈5.5 and ≈1.8 (F _W =3.1×10⁴ and 2.3×10⁴) respectively.
2. (3) 5/2 5/2^?[512] at 191.4 keV in¹⁶⁹Yb:T _1/2=3.35±0.15 ns and at 122.39 keV in¹⁷¹Yb:T _1/2=265±20 ns which results in the determination of the transition probabilities of theE1,ΔK=1 transitions to the ground states 7/2 7/2⁺[633], of theK-forbiddenM1 transitions to the 5/2 and 3/2 1/2^?[521] and of theE2 transitions to the 5/2, 3/2 and 1/2 1/2^?[521] states in both nuclei.

TheE1 transition probabilities are compared to the transitions between the same Nilsson states in¹⁷³Yb and¹⁷⁵Hf discussing the influence of the position of the Fermi surface — obtained from recent stripping and pick-up reactions — on these transition probabilities. Additional information on the decay scheme of¹⁷¹Lu→¹⁷¹Yb is obtained by delayed coincidence measurements. For testing the used time-to-amplitude converter the well known half-lives of the 482 keV level in¹⁸¹Ta (T _1/2=10.4±0.3 ns) and of the 279 keV level in²⁰³Tl (T _1/2=0.285 ±0.015 ns) were measured, in good agreement with other measurements.     

Isomeric states in180Os

Two new isomers have been observed in¹⁸⁰Os. A high-K isomer withI, K ≧20 and a half-life ofT _1/2=12+4 ns have been established. It deexcites via two transitions into the 18⁺ level of the yrare band indicating an unusually smallK-hindrance factor. Evidence for an isomer withI, K>16 and a half-life ofT _1/2=41±10 ns was found. A half-life of 17±3 ns was measured for the previously known 7^? state at 1862 keV. The decay scheme of the previously known 7^? isomer at 1928 keV has been extended and a revised version is presented.     

The 61/2+ yrast isomer in149Tb

An isomeric state (T _1/2=4.0±0.2ns, E ^*=9.915 MeV,I _π=61/2⁺)has been found in ¹⁴⁹ Tb by measuring the half-life of the K internal conversion line of the 184 keV E2 deexciting transition. The breaking of the neutron core is theoretically expected at that spin resulting in sizeable increase of the oblate deformation.     

Gammaübergänge und Energieniveaus in Dy 165

220 neutron capture Gamma rays of Dy 165 were measured with the Risø curved crystal spectrometer. The intense transitions permitted the construction of a level scheme for this oddN nucleus. The rotational bandsK=7/2⁺[633],K=1/2^?[521] andK=5/2^?[512] were precisely localized, including the spin 11/2 states. Gammavibrational bands built on these Nilsson orbits were observed with their band heads at 538.6, 573.5 and 570.2 keV. TheI=5/2 term at 533 keV can be considered as the 5/2^?[523] Nilsson state. The branching ratios allowed the determination of (g _K?g^R)²-factors for the [633], [521] and [512] bands and yieldedE 1-hindrance factors.     

Die Konversion des86mRb

^86mRb was produced by irradiation of natural Rb and Rb enriched in⁸⁷Rb (99.2%) with fast neutrons. The half-life ofT _1/2=(61.2±1.0) sec and the energy ofE _ρ=(556.03±0.25) keV were remeasured. The experimental conversion coefficients α_tot=0.0184±0.0015 andα _K =0.0158±0.0015 yield the multipolarity of E4 for the isomeric transition. The spin of 6^? was assigned to the isomeric state. It is proposed that the spin 6^? is formed by coupling ap _3/2-proton and ag _9/2-neutron according to Nordheim's “weak” rule.     

Excited states in91Nb populated in the89Y(α, 2nγ) reaction

The reaction⁸⁹Y(α, 2n γ) was used to populate excited states in⁹¹Nb. The de-excitation of these states was studied by in-beam gamma-ray spectroscopy, involving the measurement of excitation functions, angular distributions and life-times. A 17/2^? isomeric state at 2034 keV and withT _1/2=3.8±0.2 μs was observed; it is interpreted as the highest-spin state of the {(g _9/2)² (p _1/2)} proton configuration. The experimental results are compared with other published data and with shell-model calculations.     

Observation of the weakly deformedvi 13 2/2 band in188Hg

High-Spin states in¹⁸⁸Hg have been investigated using the¹⁶⁸Er(²⁴Mg,4n) reaction. The rotation aligned vi ₁₃ ^2/2 band, which intersects the weakly oblate ground state band atI≈10, is observed up toI ^π =20⁺ with an isomeric 12⁺ level with T_1/2=134(15)ns. The coexisting strongly deformed band, built on the 825 keV 0⁺ level, is observed up to the 14⁺ state. The systematics of the vi ₁₃ ^2/2 rotation aligned bands is discussed.     

Investigation of the ground state rotational bands in233U and239Pu in the (α, 3n) reactions

The ground state rotational bands in²³³U and²³⁹Pu were investigated in (α, 3n) reactions. Conversion electrons were measured with an iron free orange spectrometer in order to suppress the background from fission. Levels up toI ^π=33/2⁺ of theK=5/2 band in²³³U andI ^π=31/2⁺ of theK=1/2 band in²³⁹Pu were identified ine ^?γ coincidence measurements. The level energies of both rotational bands can be well described up to the highest observed spins by a two-parameter angular velocity expansion. The electromagnetic properties of theK=1/2 band in²³⁹Pu are discussed.     

Nuclearg-factors of theJ π=21/2+isomer in112In and theJ π=6+ 112Sn

Theg-factor of theJ ^π=21/2⁺ isomeric state in¹¹¹In (T _1/2=13.3 ns) and of theJ ^π=6⁺ isomeric state in¹¹²Sn (T _1/2=13.7 ns) were measured using the spin rotation method. The result obtained for theJ ^π=21/2⁺ level in¹¹¹In,g=+0.47 (2), indicates that this state has an almost pure ((πg _9/2)^?1 νg _7/2 νd _5/2) shell model configuration. The experimental valueg=+0.04 (3) for theJ ^π=6⁺ isomer in¹¹²Sn agrees with the theoretical value calculated within the frame of the BCS model.     

Niederenergetische Gammalinien vom Neutroneneinfang in Lu 175 und Lu 176

The low energy gamma-rays from neutron-capture in Lu 175 and Lu 176 have been investigated by means of the bent crystal-spectrometer at the DR-3-reactor at Risø. From the transitions in Lu 177 3 rotational bands have been determined. The levels of the (404)K=7/2⁺ groundstate rotational band are: 121,62 keV (I=9/2), 268,79keV (I=11/2), 440,66 keV (I=13/2), 636,22 keV (I=15/2), 854,34 keV (I=17/2). The level-sequence of the (514)K=9/2^?-band is: 150,39 keV (I=9/2), 288,99 keV (I=11/2), 451,49 keV (I=13/2), 637,05 keV (I=15/2) and 844,88 keV (I=17/2). At 457,92 keV is the basis for the (402)K=5/2⁺-band the higher levels of which are 552,05 keV (I=7/2), 671,89 keV (I=9/2), 816,63 keV (I=11/2), 985,23 keV (I=13/2), 1176,73keV (I=15/2) and probably 1389,5 keV (I=17/2). The energies of the levels apart from the 1389 keV-level have an accuracy of 7×10^?5. The energy differences between the 3 bands agree very well with the values expected from the Bohr-Mottelson-formulaE=A·I(I+1)+B·I ²(I+1)². The calculated branching-ratios within the 3 bands are in fairly good agreement with the experimental values. Theg _K -factors have been determined for 2 bands: It was found that for the (514)-bandg _K =1,16±0,04 and for the (402)-bandg _K =1,33±0,07.     

Half-life,g-factor and relaxation of the nuclear orientation of the isomeric 11/2− state in139Pr

By time-differential observation of the perturbed angular distribution in an external magnetic field, the g-factor of the 11/2^? isomeric state in¹³⁹Pr has been measured, g = +1.20 ± 0.09. The angular distribution also shows an attenuation with relaxation time, τ _R = 19 ± 6 ns. This relaxation is due to paramagnetic interaction of the nucleus with the 4f-electrons. Our determination of the isomeric half-life,T _1/2= 45.5 ±0.9 ns does not agree with previous measurements.     

New results in the decay of150Ho and152Tm

A new isotope¹⁵²Tm (T _1/2=5.2±0.6 s) and a new isomeric state in¹⁵⁰Ho (T _1/2=88±15 s) were observed with the on-line isotope separator ISOCELE II at Orsay. The decay of¹⁵²Tm is similar to the one of the high spin state (9⁺) in¹⁵⁰Ho. The new¹⁵⁰Ho isomer, probably with a low spin (2 or 3), decays mainly to the 2⁺ level in¹⁵⁰Dy.     

E1-M2-E3 mixing of the 1241.7 keV transition in147Yb

The half-life of theI,K ^π=2, 2^? state at 1318 keV in¹⁷⁴Yb has been measured in the¹⁷³Yb(n, γ) reaction to be 0.486±0.015 ns. This half-life determines the absolute transition probabilities of the gamma-ray transitions to the ground state rotational band yielding the following hindrance factors relative to the Weisskopf estimate:F _W(M 2; 2, 2^?→0, 0⁺)=350;F _W(M2; 2, 2^?→2, 0⁺)≧77F _W(M 2;2, 2^?→4, 0⁺)>740;F _W(E 1; 2, 2^?→2, 0⁺)=4.5×10⁶;F _W(E 3; 2, 2^?→2, 0⁺)≈0.1;F _W(E 3; 2, 2^?→4, 0⁺)≈0.4. These data are compared to analogous transitions in neighboring nuclei.     

Coulomb excitation of the Kπ=16+ rotational band in178Hf

A heavy-ion multiple Coulomb excitation experiment on a very exotic target containing microweight quantities of¹⁷⁸Hf in the K^π = 16⁺ isomeric state has been performed at 4.77 MeV/u²⁰⁸Pb beam energy. The first excited I^π = 17⁺ state has been observed at an excitation energy of 357.4±0.3 keV with respect to the isomeric state. The intrinsic electric quadrupole moment ofQ ₀=8.2±1.1 b has been derived from the experimental data within the rigid rotor model.     

Neutroneneinfangs-Gammaspektrum und Niveauschema von Erbium 167

The (n, γ)-spectrum of Er 167 was investigated by means of the Risø bent crystal spectrometer. The energies and intensities of 47 transitions were measured. 22 of them were fitted into a level scheme. The rotational bands of theK=7/2⁺[633],K=1/2^?[521], andK=5/2^?[512] states were found up to the spin valuesI=11/2⁺, 9/2^?, and 7/2^?. Gammavibrational states of these Nilsson orbitals were determined or proposed. The energy of theK=5/2^?[523] state was suggested. Branching ratios of gamma transitions were compared with theoretical predictions.     

High-spin states in206Bi populated in the205Tl(α, 3n) reaction

Using alpha-particles in the energy range 35–51MeV and in-beam gamma ray and conversion electron spectroscopy techniques the reaction²⁰⁵Tl(α, 3n)²⁰⁶Bi was studied. A 15±1ns isomeric 15⁺ state was found at an excitation energy of 3147keV in²⁰⁶Bi. The main configuration of the isomeric state is suggested to beπh _9/2 vp ₁ ^2/?1 i ₁₃ ^2/?2 . The isomeric state decays mainly through a stretched cascade of five gamma rays to the previously known 0.88ms 10^? state of theπh _9/2 vi ₁₃ ^2/?1 (j ^?2)₀₊ configuration at an excitation energy of 1045 keV. A shell model calculation of the yrast states has been performed and it is found that the calculation agrees fairly well with the experiments. The average deviation between experimental and calculated energies for the yrast states with angular momenta in the region 6–18 is +4keV and the root mean square deviation is 22 keV.