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[en] The ¹⁵⁴Tb nucleus lies in the threshold of the rotational mass region. The shift in structure dynamics and deformations make its level scheme both complicated and interesting. There are no recent experimental studies on this nucleus. In an attempt to unambiguously establish its level scheme, we carried out experimental studies using the INGA facility at VECC. An (α, n) reaction was performed on nat. Eu target in powdered oxide form at beam-energy 39 MeV. On-line coincidence spectroscopy was performed for ~24 h followed by off-line decay runs. Our analysis has led to a lot of interesting observations which will significantly add to the existing experimental data on ¹⁵⁴Tb structure

[en] Complete text of publication follows. The precise knowledge of the half-life of the reaction product is of crucial importance for a nuclear reaction cross section measurement carried out with the activation technique. The cross section of the ¹⁵¹Eu(α,n)¹⁵⁴Tb reaction has been measured recently using the activation method, however, the half-life of the m1 isomer in ¹⁵⁴Tb has a relatively high uncertainty (T_1/2^m1 9.4±0.4 h) and ambiguous values can be found in the literature. This uncertainty introduces an error in the measured cross section and therefore the precise determination of the half-life is highly needed. For the half-life determination, several ¹⁵⁴Tb sources have been prepared by the ¹⁵¹Eu(α,n)¹⁵⁴Tb reaction. Highly enriched ¹⁵¹Eu targets have been irradiated with the alpha-beam of the cyclotron of ATOMKI. The targets have been prepared by vacuum evaporation. By choosing different bombarding energies and irradiation times, source activities in the range of 1-100 kBq have been obtained. The γ-radiation following the β-decay of ¹⁵⁴Tb has been measured with a shielded 40% relative efficiency HPGe detector. The length of the countings varied between 20 and 60 hours (more than 6 half-lives) and the spectra were stored in every hour. For the analysis the 540 keV line, the strongest γ-radiation which comes exclusively for the m1 isomer of ¹⁵⁴Tb has been selected. In order to obtain a high precision half life value, all possible systematic uncertainties have been carefully investigated. The five measured sources gave the same half-life value within their statistical uncertainty. This shows the robustness of the counting system. However, to increase the reliability of the measurements, the half-life of a reference source is also determined. During α-bombardment the ⁶³Cu(α,n) reaction takes place with high cross section in the target backing producing ⁶⁶Ga. The half-life of this isotope is similar to that of ¹⁵⁴Tb^m1 (9.49±0.07 h) and it is precisely known. The obtained half-life results for this reference isotope are in all cases in good agreement with the literature value, which shows the reliability of the measurement. Further systematic uncertainties may arise from the complicated decay scheme of ¹⁵⁴Tb. The m2 isomer (T_1/2^m2 =22.7±0.5 h) is known to have a weak decay branch to the m1 state. This contribution has been taken into account by measuring the production ratio of the two isomers. Although the 540 keV radiation has been reported to come exclusively from the m1 isomer, a hypothetical contribution of the ¹⁵⁴Tb ground state decay to this γ-line has been considered. Using a χ² analysis, an upper limit could be assigned to this contribution. A final result of T_1/2 =9.994±0.039 h has been obtained for the half-life of the m1 isomer in ¹⁵⁴Tb. Compared to the literature value, the half-life is increased significantly and its uncertainty is reduced by one order of magnitude. With this precise measurement one source of uncertainty of the ¹⁵¹Eu(α,n)¹⁵⁴Tb cross section measurement is avoided. Further details can be found in Ref. 1.

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[en] The study of level schemes of odd-odd deformed nuclei has always been challenging due to multiple band interactions and the existence of low-lying isomeric states. The light rare-earth region is where the rotational deformations begin. One of the features of this region is the existence of isomer triplets in lighter rare-earths of which many have not been fully characterized. One such nucleus, ¹⁵⁴Tb (Z=65), is a case where two low-lying isomeric states, other than the gs, have been found experimentally but lack definite assignments. The band structure and spin-parity assignment of the levels in the existing level scheme is based on comparison with the neighbouring isotopes and isotones. Except the three known low-lying isomeric states, the spin parity values of the rotational bands are all tentative. The parity of the I=0 gs state has also not been experimentally established. Earlier studies on the level structure of ¹⁵⁴Tb have mostly focused on expanding the level scheme to higher spin states without much experimental observations on the lower members of the band structures. With an aim to resolve the ambiguity and characterize the low-lying level structure of ¹⁵⁴Tb, in-beam gamma spectroscopic studies were carried out. We report here the preliminary results of this experimental study

[en] The experimentally observed four bands of ¹⁵⁴Tb are analyzed in detail using the particle number conserving method for treating cranked shell model with monopole and quadrupole pairing interactions. Besides analyzing their behaviors at higher frequencies beyond the observed frequency range, we further predict the sixteen bands based on the low excited states in ¹⁵⁴Tb. Our calculated results show that below the rotational frequency -bar ω∼0.45 MeV, the νi_13/2 and their interferences are responsible for all band crossings and above the frequency -bar ω∼0.45 MeV, besides the νi_13/2 orbitals, the π[660]1/2 and ν[505]11/2 orbitals begin to influence rotational alignments

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[en] The measurement of decay half-life provides valuable information for the measurement of nuclear reaction cross-section as well as validation of different statistical model calculations and choosing appropriate input parameters. The half-life values are also required for modelling the nucleosynthesis of heavy elements. Again an inaccurate nuclear input parameters used in the reaction rate calculations may lead to the fact that the nucleosynthesis models are not able to reproduce well the heavy element abundances observed in nature. In the present work, determination of half-life of ¹⁵⁵Tb and different isomers of ¹⁵⁴Tb has been attempted. Tb isotopes have been produced both from alpha and proton induced reactions on natural Eu and enriched Gd targets respectively. Enriched Gd-target contains 67% ¹⁵⁴Gd and 21% ¹⁵⁵Gd isotopes

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