Lifetime measurements of the low-lying excited states of ²⁰⁸Po

In this study we present the preliminary results about the lifetimes of the 2₂⁺, 4₁⁺ states of ²⁰⁸Po and the upper limit of the lifetime of the 2₁⁺ state. For measuring the lifetimes of the 2₁⁺ and 4₁⁺ states the Recoil Distance Doppler Shift (RDDS) method and for the lifetime of the 2₂⁺ state the Doppler Shift Attenuation method (DSAM) were used. The resulting absolute transition strength B(M1 ; 2₂⁺ → 2₁⁺) ≥ 0.122(20) μN² reveals the predominant isovector nature of the 2₂⁺ state of ²⁰⁸Po

Lifetime measurements of the low-lying excited states of ²⁰⁸Po

In this study we present the preliminary results about the lifetimes of the 2₂⁺, 4₁⁺ states of ²⁰⁸Po and the upper limit of the lifetime of the 2₁⁺ state. For measuring the lifetimes of the 2₁⁺ and 4₁⁺ states the Recoil Distance Doppler Shift (RDDS) method and for the lifetime of the 2₂⁺ state the Doppler Shift Attenuation method (DSAM) were used. The resulting absolute transition strength B(M1 ; 2₂⁺ → 2₁⁺) ≥ 0.122(20) μN² reveals the predominant isovector nature of the 2₂⁺ state of ²⁰⁸Po

Revisiting the measurement of absolute foil-separation for RDDS measurements and introduction of an optical measurement method

Uncertainties in absolute plunger distances are an often overlooked component in measurements with the Recoil Distance Doppler-shift (RDDS) method when the Differential Decay Curve (DDC) method cannot be used for the analysis. It is shown how these uncertainties arise when the capacitance method is used to determine absolute distances and how they influence the obtained lifetime values of a plunger experiment. Furthermore, a new approach to obtain absolute foil separations for plunger measurements using a high precision optical distance probe is introduced, which can reduce these uncertainties by a large margin. The performance of this method is demonstrated by comparisons to the capacitance method as well as to a precise measurement of absolute distances using known lifetimes in 181Ta

Lifetime measurement of excited states in Ce-144: Enhanced E1 strengths in a candidate for octupole deformation

A lifetime measurement of excited states in Ce-144 using the (142)ce(O-18, O-16) reaction with a beam energy of 67 MeV and the recoil distance Doppler-shift method was performed at the Cologne FN Tandem accelerator. Lifetimes of the three lowest yrast states in Ce-144 have been measured as well as for the 3(1)(-) state and an effective lifetime of the 4(2)(+) state. Reduced E2 transition strengths determined using these results have been compared to predictions from recent shell-model calculations. From the interband transitions reduced E1 strengths could be determined, which are strongly enhanced

Evolution of collectivity is Xe-118

A recoil-distance Doppler shift experiment has been performed using the Pd-102(F-19, p2n) reaction at a beam energy of 73 MeV to measure the lifetime of excited states in Xe-118. The differential decay-curve method using gamma gamma coincidences and a gating procedure that allows to extract the lifetime without feeding assumptions has been employed. The lifetimes obtained for the yrast states up to spin-parity 8(+) are compared with interacting boson model calculations and Xe-118 can be classified as a transitional nucleus between the spherical and a deformed shape. Systematics of the B(E2) values for the 2(+) -> 0(+) and 4(+) -> 2(+) transitions in the isotopic chains of tin, tellurium and xenon are presented. It is proposed that a critical point exists at which the B(E2; 4(+) -> 2(+))/B(E2; 2(+) -> 0(+)) ratio drops to unity for lower neutron numbers within the isotopic chain. The position of the critical point varies with proton number, i.e., it is presumed to be located at the same mass number A = 114 in the Sn, Te, and Xe isotopes

Lifetimes and structures of low-lying negative-parity states of Po-209

The 5/2(1)(-), 9/2(1)(-), and 11 /2(1)(-) states in Po-209 were populated in the beta decay of At-209 and their lifetimes measured using the electronic gamma-gamma fast timing technique. The lifetime of the 9/2(1)(-) state is measured for first time. The lifetime of the 5/2(1)(-) is measured to be shorter than the value adopted in the literature while the lifetime of the 11/2(1)(-) state agrees well with the previous measurement. In order to get deeper insight into the structure of the states, a shell-model calculation was carried out adopting a microscopic effective interaction derived from the realistic CD-Bonn potential. The comparison between theoretical and experimental data for the low-lying negative-parity states of Po-209 supports the reliability of the predicted wave functions, which are found to be dominated by the coupling of a neutron hole to the yrast states of Po-210. However, it also points to the important role played by minor wave-function components in describing the reduced electromagnetic strengths, suggesting the need of additional configuration mixing for achieving a better quantitative agreement

Lifetimes in At-211 and their implications for the nuclear structure above Pb-208

Lifetimes of excited states in At-211 were measured using the electronic gamma-gamma fast timing technique. The nucleus of interest was populated in a Pb-208(Li-6, 3n)(211) At fusion-evaporation reaction at the FN Tandem accelerator of the Institute for Nuclear Physics, University of Cologne. The lifetimes of the 17/2(1)(-) and 23/2(1)(-) states were determined, together with an upper limit for the 13/2(1)(-) state. The experimental results are compared to two shell-model calculations, one using a semiempirical interaction for three particles in a single j = 9/2 shell and the other using the modified Kuo-Herling interaction in a multi- j model space

X-ray–<math><mi>γ</mi></math> fast-timing lifetime measurement of the <math><msubsup><mn>6</mn><mn>1</mn><mo>+</mo></msubsup></math> state in <math><mmultiscripts><mi>Po</mi><mprescripts/><none/><mn>206</mn></mmultiscripts></math>

International audienceLow-lying states in Po206 were investigated using the fast-timing technique with LaBr3(Ce) and high-purity germanium detectors. The excited states in this nucleus were populated via two consecutive electron capture decays from Rn206. The parent isotope was produced in the Pt194(O16,4n)Rn206 fusion-evaporation reaction at the FN-Tandem facility at the Institute for Nuclear Physics, University of Cologne. The previously known value for the lifetime of the 41+ state in Po206 was confirmed using the generalized centroid difference (GCD) method. The lifetime of the 61+ state was determined from β decay in the first application of the GCD method for x-ray–γ coincidences. A Monte Carlo based approach was applied to address the indirect population of the 61+ state by the decay of At206. The experimental results were examined in the context of the transition of single-particle excitations to collective behavior in the neutron-deficient Po isotopes. The obtained B(E2;61+→41+) value suggests that for the 61+ states in even-even Po isotopes the transition from a noncollective regime at N=126 to a collective regime occurs at N≤120

Lifetimes of the 4(1)(+) states of Po-206 and Po-204: A study of the transition from noncollective seniority-like mode to collectivity

Low-lying yrast states of Po-204 and Po-206 were investigated by the gamma-gamma fast timing technique with LaBr3(Ce) detectors. Excited states of these nuclei were populated in the Au-197(B-11, 4n)Po-204 and the Pt-198(C-12, 4n )Po-206 fusion-evaporation reactions, respectively. The beams were delivered by the FN-Tandem accelerator at the University of Cologne. The lifetimes of the 4(1)(+) states of both nuclei were measured, along with an upper lifetime limit for the 2(1)(+) state of Po-204. A comparison between the derived B(E2; 4(1)(+) -> 2(1)(+)) values and results from simplified empirical two-state mixing calculations suggests that for the 4(1)(+) states of even-even polonium isotopes the transition from single-particle mode at N = 126 to collective mode, when reducing the number of neutrons, occurs above N = 122