Conversion Electron Measurements Research Articles

Decay Studies on Neutron-Rich Isotope 151Ce

The β− decay of 151Ce produced by the thermal neutron-induced fission of 235U has been studied using an isotope separator on-line. From γ-ray singles, γ-γ coincidence and internal conversion electron measurements, the decay scheme of 151Ce with a half-life of 1.72(5) s was proposed for the first time.

Half-life and internal conversion electron measurements in low-lying levels of ^{125,127}Ba

Decay spectroscopy using on-line mass separators has the advantage of allowing the study of level properties in the low energy region, including band head information, since g transitions between low-spin states can be measured more intensively under lower background conditions than can those with in-beam spectroscopy measurements. Neutron-deficient nuclides of 125,127 Ba have been studied by means of in-beam spectroscopy, and the level structures for high-spin states were successfully interpreted within the framework of the IBFM model @1‐5#. Decay studies of these nuclides have rarely been reported. Therefore, the last evaluations in Refs. @6, 7# were mainly based on in-beam studies. In 1991, the possibility of static octupole deformation, similar to that of the A5145 and 225 regions proposed, was proposed for the A5130 region by Cottle @8#. The presence of static octupole deformation is represented by parity doublets, which have equal spins and opposite parity levels of rotational bands with alternating bands. The E1 transitions between parity doublets are characterized by a two to four orders of magnitude enhancement compared to those of more normal cases. The 127‐130 Ba isotopes were successively studied by in-beam conversion electron measurements to investigate for parity doublets, and no evidence of them was observed @9,10#. These studies, however, focused only on the conversion electron measurements; the transition probabilities were not measured. It is necessary to measure not only the conversion electrons, but also the half-lives of the excited states in order to explain this feature. The aim of this investigation was to study the level properties of 125,127 Ba in the low energy region, focusing on a search for E1 transitions, and to study their properties by measuring the half-lives of low-lying excited states. The half-lives and conversion electrons were measured for the first time through the decays of 125,127 La, by means of a delayed coincidence technique and a cooled Si~Li! detector, respectively. The expected enhanced E1 transitions were not observed. Level properties based on the Nilsson model are proposed.

Recent Results from Heavy Element Research at JYFL

The gas-filled recoil separator RITU in conjunction with germanium detector arrays and the SACRED conversion electron array has been used for studies of nuclear structure in the region around 254No. Rotational spectra have been extracted using in-beam measurements with recoil gating or recoil decay tagging. Results from completed in-beam γ-ray studies of 252, 254No as well as preliminary data from in-beam conversion electron measurements of 253, 254No and γ-ray studies of 250Fm are presented. These measurements give strong support to the prediction that nuclei in this region of the nuclear chart are quadrupole-deformed in their ground state with β2∼0.27. There are indications of an upbend occurring in the moment of inertia of 252No and 250 Fm at a frequency around 180 keV. Future prospects for these studies as well as for focal plane decay spectroscopy in the transfermium region using RITU are discussed.

Decay scheme of [formula omitted]La isomers

The β +/EC decay of 126 La produced by the 94 Mo( 36 Ar, 3pn) reaction has been studied using the on-line isotope separator connected to an AVF cyclotron. Gamma-ray singles, γ– γ angular correlation and internal conversion electron measurements were carried out. From analysis of these data, 33 new levels in 126 Ba were found and the decay scheme containing 137 γ-rays and 50 excited states was constructed. The probable spin and parity of the two β-decaying states in 126 La were found to be 4 ±, 5 ± for the high-spin isomer with a half-life of 54(2) s and 0 −, 1 ±, 2 − for the low-spin isomer with a half-life shorter than 50 s.

The r-process and neutrinos

The structure of 229Ra has been studied in the β− decay of 229Fr. Spins and parities have been determined from the conversion electron measurements, while half-lives for the 137.5, 142.7, 168.8, 213.0 and 479.0 keV levels have been measured in the ps and ns ranges via the fast timing βγγ(t) method. Our data confirm the previous results and enrich the knowledge of the structure of this nucleus by 25 new states and several strong transitions. Structural similarities are established between 229Ra and its N = 141 isotone 231Th. The experimental results are compared with the theoretical predictions of the quasiparticle-phonon model with inclusion of Coriolis coupling. The reduced transition probabilities, and especially the moderately fast B(E1) rate for the 137.5 keV transition connecting the band-heads of the Kπ = 52± parity doublet bands, reveal the presence of significant octupole correlations in 229Ra.

Persistence of octupole correlations in [formula omitted]Ra

The structure of 231 Ra, the heaviest odd Ra nucleus currently accessible for detailed spectroscopic investigation, has been studied in the β − decay of 231 Fr. The main purpose has been to verify whether fast B(E1) rates and significant octupole correlations recently established in 229 Ra persist in 231 Ra. The excited states in 231 Ra have been determined for the first time. Spins and parities have been deduced from conversion electron measurements, while level half-lives in the ps and ns ranges have been measured with the Advanced Time Delayed βγγ(t) method. The moderately fast B(E1) rates found for transitions connecting the lowest-lying K π=5/2 ± and K π=1/2 ± bands, reveal the persistence of octupole collective effects in 231 Ra, although the B(E1) rates are lower than in 229 Ra. These findings reinforce the differences in the B(E1) systematics between isotonic Ra and Th nuclei.

Low-lying levels in119Xe

The decays of ${}^{119m}\mathrm{Cs}$ and ${}^{119g}\mathrm{Cs}$ to ${}^{119}\mathrm{Xe}$ have been studied on mass-separated samples, using $\ensuremath{\gamma}$-ray and internal conversion electron measurements. Several new low-lying levels have been established in the ${}^{119}\mathrm{Xe}$ level scheme. Half-life evaluations for ${}^{119m}\mathrm{Cs}$ and ${}^{119g}\mathrm{Cs}$ have been revisited. The results are compared with other experimental data known in light odd-mass xenon isotopes and with calculations performed in the frame of the multishell interacting boson-fermion model.

Detailed Spectroscopy in the Superdeformed Second Minimum of 240 Pu

Spectroscopic studies in the superdeformed shape isomer of 240Pu using γ-spectroscopy, conversion electron spectroscopy and transmission resonance spectroscopy have been performed. In a high-resolution and high-efficiency γ-spectroscopy experiment the out-of-band decays of several excited superdeformed rotational sequences with K=2− and 1− could be identified together with evidence for a weakly populated 0− octupole band. Surprisingly, no low-lying collective quadrupole excitations could be observed. Complementary information could be obtained in conversion electron measurements in coincidence with isomeric fission, resulting in the first identification of the lowest s-vibrational K=0+ band. For all rotational bands the variation of the moment of inertia with spin could be studied. A predominant population of negative parity states in the second well could be observed, which can be explained by the selective population and depopulation of the second minimum. Complementary transmission resonance measurements have been performed, yielding new information on the fine structure of high-lying (s-)vibrational multi-phonon states. A new method could be established to determine the excitation energy of the fission isomer ground state from measured level densities.

On-line conversion electron measurements with a magnetic guide in heavy-ion-induced reactions

A practical description of a solenoidal electron guide and its adaptation on-line with heavy-ion accelerators is given. This includes details concerning the use of silicon detectors and a new preamplifier circuit. Examples of low-energy spectra obtained in γ-electron coincidence measurements carried out at the SARA and VIVITRON facilities, respectively, are shown and commented. Some conclusions are drawn from the experiments.

High-resolution measurements of low-energy conversion electrons

High-resolution measurements of low-energy conversion electrons have been performed in odd and odd–odd nuclei with masses around A=182. The experimental setup, a magnetic spectrograph coupled to a specific tape transport system, is described. Three experiments have been performed and the main results are presented.

Efficiency calibration of low-energy electron detectors by means of β-ray emitters

The relative efficiency of a channel electron multiplier for 2–9 keV electrons was determined using the β-spectra of vacuum evaporated 241 Pu and 63 Ni sources. The results agreed within ±0.8% (1 σ). Conversion electron measurements of 57 Co and 169 Yb supported the result. The electron scattering and energy losses within the sources were taken into account by means of Monte Carlo simulations of individual elastic and inelastic collisions. The detector was calibrated in the same geometry as it works inside the cylindrical mirror analyzer.

Octupole correlations in 229Ra

The structure of 229Ra has been studied in the β − decay of 229Fr. Spins and parities have been determined from the conversion electron measurements, while half-lives for the 137.5, 142.7, 168.8, 213.0 and 479.0 keV levels have been measured in the ps and ns ranges via the fast timing βγγ( t) method. Our data confirm the previous results and enrich the knowledge of the structure of this nucleus by 25 new states and several strong transitions. Structural similarities are established between 229Ra and its N = 141 isotone 231Th. The experimental results are compared with the theoretical predictions of the quasiparticle-phonon model with inclusion of Coriolis coupling. The reduced transition probabilities, and especially the moderately fast B(E1) rate for the 137.5 keV transition connecting the band-heads of the K π = 5 2 ± parity doublet bands, reveal the presence of significant octupole correlations in 229Ra.

Beta decay of 125Sb and level structures in 125Te

The decay of 2.76y 125Sb to levels of 125Te has been studied using an HPGe detector for gamma-ray and a mini orange electron spectrometer for conversion electron measurements. We identify 38 transitions in this decay, including 13 gamma rays and 4 conversion electron lines being reported for the first time. New results also include E1 multipolarity assignments to 3 newly observed transitions and M-shell conversion coefficient for the 109 keV M4 transition. A revised 125Te level scheme is constructed using Ritz combination principle. While confirming the existence of 10 well established levels below 700 keV excitation, we introduce 3 other levels at 402.0, 538.6 and 652.9 keV. Interpretation of the observed levels in terms of various theoretical approaches is briefly discussed. The newly introduced 538.6 keV (1/2+) and 652.9 keV (3/2+) levels are seen as the two missing members of the (s 1/2 ⊗ 2+) and (d 3/2 ⊗ 2+) sextuplet in the quasiparticle-phonon coupling scheme.

A new isomer in 125La

Levels in 125La have been studied via β+/EC decay of on-line mass-separated 125Ce using the HIGISOL technique. A new (390 ± 40) ms isomer is definitely attributed to 125La by conversion electron measurements of the 107 keV E3 isomeric transition.

Isomeric M3 transitions in 183Pt and 181Os

The M3 transitions linking the ν 7 2 [514] and ν 1 2 [521] states in 78 183Pt 105 and 76 181Os 105 have been searched by high-resolution conversion-electron measurements. The reduced M3 transition probabilities have been estimated and compared to the values obtained in neighbouring isotones ( 74 179 W 105 and 79 184Au 105). All the known M3 transitions are presented and their properties discussed, especially as a function of deformation. The present work supports the π h 9 2 ⊗ν 1 2 [521] and π h 9 2 ⊗ν 7 2 [514] particle configuration assignments proposed for the 184Au isomeric and ground states, respectively.

Conversion electron measurements of 152Nd

Multipolarities of γ-rays in 152Nd have been studied by conversion electron measurements in the β− decay of 152Pr. The praseodymium activities were obtained by the thermal neutron fission of 235U and followed with the on-line isotope separator KUR–ISOL. Conversion electrons and γ-rays were measured with a Si(Li) detector and a HPGe detector, respectively. The two γ-rays, 226 keV and 285 keV, were decided to have the multipolarities of the E1 transition. The spins and parities of the 1600 keV and 1542 keV levels were determined to be 3+ and 2+, respectively.

Quadrupole deformed and octupole collective bands in 228Ra

Spins and parities for collective states in 228Ra have been determined from conversion electron measurements with a mini-orange β spectrometer. The fast-timing βγγ( t) method has been used to measure lifetimes of T 1 2 = 550(20) ps and 181 (3) ps for the 2 1 + and 4 1 + aembers of the K = 0 + band, and T 1 2 ⩽ 7 ps and ⩽6 ps for the 1 1 − and 3 1 − members of the K = 0 − band, respectively The quadrupole moments, Q 0 deduced from the B (E2; 2 1 + → 0 1 +) and B (E2; 4 1 + → 2 1 +) rates are in good agreement with the previously measured value and the systematics of the region. However, the B(E1) rates of ⩾4 × 10 −4 e 2 fm 2, which represent the first B(E1) measurements for this nucleus, are at least 25 times larger than the value previously suggested for 228Ra. The new results are consistent with the B(E1) rates recently measured for the neighbouring 227Ra and reveal octupole correlations in 228Ra.

Gamma-decay of the 27/2− 0. 6s E3-isomer in 15168Er83

The nucleus 151Er was investigated through γ-ray and conversion electron measurements from its 27/2− isomer decay produced in a HI reaction at the GSI on-line mass separator. Inaddition tothe known intense yrast cascade,a complex 8% decay branch to above-yrast states was identified. Among these, the 11/2+ member of the νf7/2 × 3− septet was unambiguously characterised. Its energy implies that the quadrupole moment of the 3− core has changed sign with respect to the lighter even-Z N=83 isotones.

Yrast states in the π- particle ν- hole nucleus 14665Tb81

The nucleus 146Tb was studied from in beam γγ-and conversion electron measurements. The level scheme was established up to ∼5MeV above the (πh11/2vd3/2−1)5−β-isomer. In addition to the known (πh11/2vh11/2−1)10+ E3-isomer, the 8+ and 11+ members of this configuration were located. The levels at the yrast line are dominated by the couplings of the πh11/2vh11/2−1 valence nucleons to the collective 3− octupole state and to the πh11/2d5/2−1 and πh11/2g7/2−1 particle-hole excitations of 146Gd.

Beta Decay of153Ndand Reflection Asymmetric Deformation in153Pm

The level structure of 153 Pm has been studied from the decay of 153 Nd. The radioactivities of 153 Nd were separated from the fission products of 235 U using the upgraded KUR-ISOL. A precise decay scheme of 153 Nd has been constructed up to 2 MeV and 115 γ-rays and 36 levels were involved in the decay scheme, in which 107 γ-rays and 28 levels were newly observed. The multipolarities and E2/M1 mixing ratios of 11 γ-transitions were deduced from the measurement of internal conversion electrons. From the results of the angular correlation measurements, the spin values of 450.5, 105.4 and 32.3 keV levels were deduced to be 3/2 + , 7/2 + and 5/2 + , respectively and the previous assignments were confirmed. The half-lives of 32.3 and 105.4 keV levels were determined to be 1.2(1) ns and 0.44(2) ns, respectively and the enhanced E1 transitions with B ( E1)=10 -3 ∼10 -4 W.u. were observed. The 153 Pm level structure was compared with the result of a calculation with a rotation-vibration coupling model (RVCM) and ...