In-beam measurements of magnetic moments of excited nuclear states

In-beam measurements of magnetic moments of very short-lived nuclear states employing the transient field technique are presented. Particular features of this experimental method relevant for its successful application are discussed. Instead of surveying a large body of data significant details of the capacity of this technique are demonstrated by giving preference to a few selected examples. Particular emphasis is put on high-spin states of deformed nuclei whose structure is dominated by rotationally-aligned single particle configurations.     

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.     

Half-life measurements of excited levels in 122I

The half-lives of three low-lying levels in ¹²²I have been measured using delayed coincidence techniques. The following results were obtained: $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(61.8}\text{keV}\text{) = 7.4 ± 0.5}\text{ns}\text{, T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(90.7}\text{keV}\text{) = 1.9 ± 0.3}\text{ns and}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(148.8}\text{keV}\text{) ≦ 80}\text{ps}$. All transitions depopulating these levels are mainly of M1 multipolarity; they are hindered with factors up to 295 as compared to the single particle estimates.     

Lifetime measurements of some excited DyI states

The method of single-photon-counting, using the multichannel analyser and the time-amplitude-converter, was applied to lifetime measurements of some atomic levels of neutral Dysprosium. The different energy levels were populated by nitrogen-lasers pumped dye-lasers employing excitation by either one or two photons. Because of the lack of experimental data concerning lifetime measurements, the present results were compared with theA-values theoretically calculated by Cowan, particularly for the levels arising from the excited state configuration 4f ¹⁰6s 6p. The spectroscopic properties of this configuration allow such a comparison of data to be made.     

Lifetime measurements on excited states in100Rh

Lifetime measurements of three excited states in¹⁰⁰Rh populated by the decay of¹⁰⁰Pd have been carried out. A two parameter delayedγ-γ-coincidence experiment using an intrinsic Ge and a plastic scintillation detector resulted in the following values:T _1/2(32.72 keV,¹⁰⁰Rh)=(27.6 ±0.6) nsT _1/2(74.8keV,¹⁰⁰Rh)=(213.6±2) nsT _1/2(158.8 keV,¹⁰⁰Rh)<0.5 ns. For the levels at 32.72 keV and 158.8 keV no former values are known.     

Fine-structure measurements for highly excited F states of cesium

The fine-structure splittings of highly excited F-states in Cs were measured using high-resolution laser spectroscopy. The 12 and 13²F states were found to be inverted with splittings ΔE (12²F)=?522(3) MHz and ΔE (13²F)=?416(3) MHz.     

Lifetime measurements of ultrashort-lived excited states in Be-like ions

We propose to measure the lifetime of short-lived excited states in highly charged ions by pump-probe experiments. Utilizing two synchronized and delayed Femtosecond pulses allows accessing these lifetimes with Femtosecond precision. Such measurements could provide sensitive tests of state-of-the art atomic structure calculations beyond the capabilities of established methods.     

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 excitation scheme of 139Cs

¹³²Te was produced via the ²³²Th (α, fission) reaction at the Variable Energy Cyclotron , Calcutta. ¹³²Te was separated in carrier-free form from the fission products by radiochemical separation and the half-lives for the 50 keV and 162 keV states of ¹³²I were determined to be 2.94±0.11 ns and 0.55±0.04 ns respectively. The present results differ significantly from the previously reported half-lives of 7.14 ns and 3.57 ns for the 50 keV and 162 keV states respectively. Received: 28 July 1998 / Revised version: 23 September 1998     

Study of magnetic moments of nuclear excited states at Tristan

Facilities at the TRISTAN mass separator for measurement of magnetic moments of nuclear excited states are described and recent results for the N=82 isotones are presented.     

Isospin effects upon the decay of highly excited nuclear states

It is proposed that T_< states which strongly mix with analog states to give states of mixed isospin results in the enhancement of the probability of proton decay from highly excited nuclear states. Such considerations are shown to explain the anomalies observed in the ($\text{p, n}\text{p}$) reactions.     

Recoil distance half-life measurements of the excited states in 145Sm

A recoil distance method was used to measure half-lives of the excited states of ₁₄₅Sm. The reaction used was ₁₃₉La(₁₀B, 4n)₁₄₅Sm. A plunger system was used. Half-lives were determined for two excited states for the first time. The yrast 27/2₊ state was found to have a half-life of 1.1 ± 0.2 ns corresponding to the retardation of 3.1 × 10₋₄ comparing with the single particle estimate of M1. The excitation energy of this state was well reproduced by the shell model calculation having a mixed configuration of [π{h^11/2(g^7/2)₋₂ (d^5/2)₋₁}¹⁰⁻, νf^7/2] + [π{h^11/2(g^7/2)₋₁}⁹⁻,νh^9/2]. Another retarded E1 transition was also found in a decay of a 21/2₊ state. Its retardation was 1.6 × 10₋₄ comparing with the single particle value. Received: 9 September 1997 / Revised version: 12 June 1998     

Role of excited nuclear states in photoabsorption shadowing at medium energies

The effect of intranuclear motion of nucleons on the photoabsorption shadowing due to the conversion of a γ-ray photon into a vector meson in nuclear matter is analyzed. It is shown within the method of time-correlation functions that, in comparison with the results of the standard approach, the correction to the total photoabsorption cross section due to the nuclear shadowing is determined by two additional factors: the Fermi motion of nucleons in a nucleus and nucleon recoil.     

Lifetimes of nuclear excited states with neutron gated recoil distance method

A motor driven plunger has been constructed for measuring lifetimes of nuclear excited states in pico second region. An array consisting of six neutron detectors was used to clean up γ-spectra obtained with CS-HPGe detectors. Lifetimes of low excited states in neutron deficient nuclei with low production cross-section e.g.⁸¹Y,⁷⁷Kr and⁷⁸Rb are reported.     

Isotope shift in the resonance lines of the arc spectrum of133Cs,135Cs,and137Cs

Reactor Cs and mixtures of electromagnetically enriched Cs were used to investigate the isotopic shift of the two resonance lines. By a digital evaluation method anomalies in the isotope shift were found.     

Microstructure of excited nuclear states in the vicinity of an yrast line

The microscopic model describing the wobbling motion in fast rotating nuclei is presented and the connection of the parameters describing it with the Coriolis effects at low spins is discussed.     

Long-lived excited states at surfaces: Cs/Cu(111) and Cs/Cu(100) systems

One-electron and multielectron contributions to the decay of transient states in the Cs/Cu(111) and (100) systems are studied by a joined wave-packet propagation and many-body metal response approach. The long lifetime of these states is due to the Cu L and X band gaps which reduce the electron tunneling between Cs and Cu. In the (111) case, the decay is mainly by inelastic e-e interaction, whereas in the (100) case, electron tunneling is dominating. This accounts very well for the experimental findings [Bauer et al., Phys. Rev. B 55, 10 040 (1997) and Ogawa et al., Phys. Rev. Lett. 82, 1931 (1999)].     

NewQ β -values of139–146Cs

In continuation to ourβ-endpoint measurements of shortlived Rb-isotopes at ISOLDE we have determined preciseβ-endpoint energies of^{139,140,141,142,143,144,145,146}Cs using our achromatic magnetic electron separator that eliminates low energy electrons. Here only electrons in the neighbourhood of theβ-endpoint are recorded in a low background environment.     

Preparation of 137Cs Pollucite Source Core

A method for the preparation of ¹³⁷Cs ceramic pollucite source core is presented. The amount of cesium introduced into formed zeolite pellets is 30%wt. The transformation of zeolite loaded with cesium into the pollucite was investigated thermoanalytically. The structure of the obtained pollucite was determined by the X-ray diffraction method. The leachability and vaporization of cesium from the obtained ceramics was investigated.

Es wird eine Methode zur Herstellung einer keramischen ¹³⁷Cs Polluzil-Strahlenquelle beschrieben. Die Menge an eingetragenem Caesium in die gebildeten Zeolithpellets betragt 30 Gew.?% und die Umwandlung des Cs-beladenen Zeotiths in Polluzit wurde thermoanalytisch verfolgt. Die Struktur des erhaltenen Polluzits wurde mit Hilfe der Röntgenbeugungs-methode bestimmt. Die Auslaugbarkeit und die Verdampfung des Caesiums aus den erhaltenen Keramiken wurde ebenfalls untersucht.     

Half-life measurements in59Co,141Pr and227Ac

The half-lives of the 1291.6 keV level in⁵⁹Co, 145.43 keV level in¹⁴¹Pr and 27.35 keV level in²²⁷Ac have been measured using leading edge and zero-crossover timing techniques. The decay curves analysed by moments, Laplace transform and slope methods gave the following half-life values: $$\begin{gathered} T_{\tfrac{1}{2}} (1291.6 keV level in {}^{59}Co) = (0.538 \pm 0.004) ns \hfill \\ T_{\tfrac{1}{2}} (145.43 keV level in {}^{141}\Pr ) = (1.82 \pm 0.04) ns \hfill \\ T_{\tfrac{1}{2}} (27.35 keV level in {}^{227}Ac) = (41.0 \pm 1.1) ns. \hfill \\ \end{gathered} $$ From the measured half-lives, the reduced transition probabilitiesB(M1)↓,B(E2)↓ for gamma transitions de-exciting the above mentioned levels in⁵⁹Co and¹⁴¹Pr are determined and compared with single particle, intermediate coupling and Sorensen estimates. In²²⁷Ac, absolute transition probability for the 27.35 keV transition is determined and compared with single particle and Nilsson estimates.