[en] The absolute alpha-decay widths in Po isotopes are calculated using a shell-model description of the alpha-particle formation. It is found that high-lying shell-model configurations greatly enchance both the alpha-clustering features and the calculated absolute alpha-decay widths. (author)

[en] The energy systematics of low-lying polonium states show sudden changes near N = 114. The observed drops in the low-lying levels of ^196,198Po relative to the heavier isotopes indicate significant changes in the underlying structure of these nuclei. It is thought that this change is due to the onset of vibrational collectivity brought about by the quadrupole interaction between neutron and proton-pairs. In order to extend the Po systematics even further, we measured, for the first time, states in ^194,195,197Po using the ²⁸Si + ¹⁷⁰Yb reaction at a beam energy of 142 MeV. The beam was supplied by ATLAS, and the data were taken with 10 Compton-suppressed Ge detectors placed at the target position of the Fragment Mass Analyzer. Preliminary level schemes were constructed for ^194,195,197Po based on γ-γ and γ-FMA coincidences. The results for ¹⁹⁴Po show that the 2⁺ - 0⁺ transition energy decreased in energy by 140 keV relative to ¹⁹⁶Po suggesting that this nucleus moved beyond the vibrational limit to more collective motion. An extrapolation of the systematics predicts that the 2⁺ energy could drop another 140 keV between ¹⁹⁴Po and ¹⁹²Po which would indicate the onset of rotational motion. Currently, we have an approved experiment to investigate the decay of yrast isomers in ¹⁹⁴Po which will allow us to (1) confirm our earlier level scheme of ¹⁹⁴Po, and (2) assess the experimental conditions needed for a future study of ¹⁹²Po

[en] Alpha- and beta-activities per unit volume of air due to radon (²²²Rn), thoron (²²⁰Rn) and their progenies were measured in the air of natural caves and ancient mines as well as inside different reference atmospheres by using CR-39 and LR-115 type II solid state nuclear track detectors (SSNTDs). In addition, the radon concentration was continuously measured inside one of the studied caves by using the SSNTDs' method and AlphaGuard counter. Equilibrium factors between radon and its daughters and between thoron and its progeny were evaluated in the studied atmospheres. Alpha-activities due to ²¹⁸Po and ²¹⁴Po short-lived radon decay products were determined in different compartments of the respiratory tract of members of the public. The committed equivalent doses due to the ²¹⁸Po and ²¹⁴Po radon short-lived progeny were evaluated in different tissues of the respiratory tract of the visitors of the considered caves and ancient mines. Annual effective doses due to radon progeny from the inhalation of air by the visitors of the studied caves and ancient mines were evaluated. (author)

[en] The response of some SSNTDs was calculated theoretically taking into consideration the major parameters that affect them. These parameters are related to the behavior of alpha emitters, the detectors types and the used diffusion chambers. The obtained results showed that the response of the filtered CR-39 detector is about 5 times higher than that of the LR-115 detector and more than 6 times higher in case of bare track detectors. This calculation was used to determine the optimum diffusion chamber dimensions with experimental verification for different diffusion chambers. The cup of 3 cm radius and 5 cm height gives a good agreement with the theoretical value

[en] Polonium nuclei, with two valence protons, provide fertile ground for the study of the onset of collectivity: from the textbook two-particle levels of ²¹⁰Po to the phonon-like multiplets of ¹⁹⁶Po. In their earlier work the authors described the low-lying structure of even-A Po nuclei down to ¹⁹⁶Po as two protons interacting via a surface-δ interaction and adiabatically coupled to a vibrating core. However, this particle-core model (PCM) fails to adequately describe the recently measured level scheme of ¹⁹⁴Po. Also because it assumes a pre-existing vibrational core, this model does not help to understand the single particle contributions to the collective motion. Therefore, the authors have also studied the Po systematics using a more microscopic model: the quasi-particle random phase approximation (QRPA). They will present a phenomenological analysis of the rapidity of the onset of collectivity for the Po isotopes as well as results of both PCM and QRPA calculations. The microscopic nature of the collectivity for the even-A Po isotopes will be discussed

[en] Continuos measurements of the natural radioactive gas sup(222)Rn are made at the Brazilian Antarctic station Comandante Ferraz (62 sup(0)S, 58 sup(0)W) to study the continental influence of air masses over the Antarctic peninsula. The technique employed in the measurements is base on the process of electrostatic collection of the ionized Polonium isotopes (the Radon decay products). The efficiency of collection is of the order of 50% to 57% for an applied electric potential of 17 kV. The collection chamber has a hemispheric shape in order to optimize the collection yield. The observed radioactivity at the Ferraz station for the period March to November 1986 and 1987 exhibited different behaviors. The year 1986 showed an average radioactivity of (2.6 ± 1.8) x 10 sup(-2) Bq.m sup(-3), while in 1987 a 50% decrease in the concentration was observed with a value of (1.44 ± 0.8) x 10 sup(-2) Bq.m sup(-3). Associated variations of wind velocity and continental aerosol loads were also observed. Periodicities in the radon time series displayed more intense peaks at 25 to 30 day periods. No seasonal trend was observed. Transient increases of radon correlated with short deicing periods. The average flux of Rn were calculated from a sampling network consisting of 20 points spread over the exposed regoliths and outcrops of the King George island, with values of 250 atoms m sup(-2) s sup(-1) for the sup(220)Rn and 7500 atoms m sup(-2) s sup(-1) for the sup(222)Rn isotope. (author)

[en] The evolution of the potential energy landscape is analysed in detail for ten even–even polonium isotopes in the mass range $188<<!--\; <\; -->A<<!--\; <\; -->220$ as obtained within the macroscopic–microscopic approach, relying on the Lublin–Strasbourg drop model and the Yukawa-folded single-particle energies for calculating the microscopic shell and pairing corrections. A variant of the modified Funny–Hills nuclear shape parametrization is used to efficiently map possible fission paths. The approach explains the main features of the fragment partition as measured in low-energy fission along the polonium chain. The latter lies in a transitional region of the nuclear chart, and will be essential to consistently understand the evolution of fission properties from neutron-deficient mercury to heavy actinides. The ability of our method to predict fission observables over such an extended region looks promising. (paper)

[en] Fine-structure α-decays of the odd mass ^191-197Po identifying proton based intruder states in the daughter lead nuclei have been observed, leading to a systematics of intruder states in odd mass lead isotopes from ¹⁹⁷Pb down to ¹⁸⁷Pb. The interpretation of these states involves the coupling of the i_13/2 or p_3/2 odd neutron to the oblate deformed even lead core

[en] The γ-ray spectroscopy of even- and odd-mass isotopes of polonium have been studied using arrays of Ge detectors coupled to recoil-mass analyzers, including recoil-decay tagging techniques. The level energies and B(E2) branching ratios can be reproduced by theoretical frameworks which do not explicitly include proton particle-hole excitations across the Z = 82 shell, conclusions in contrast to those deduced from alpha-decay measurements

[en] The ground states of the Z = 82 (Pb) isotopes are known to be spherical also far from the doubly magic ²⁰⁸Pb. Near midshell with N = 104 where the number of neutron valence particles is at the maximum, np-nh excitations across the closed shell are energetically favoured. As a result, there are low-lying excited 0+ states in Pb isotopes with N ∼ 104. Calculations using the deformed mean- field approach lead us to connect the lowest excited 2p-2h and 4p-4h 0⁺ states with ablate and prolate deformation, respectively. Recently, fine structure in the alpha decay of ¹⁹²Po to low-lying 0⁺ states in ^188Pb was studied at JYFL using the reaction ³⁶Ar + ¹⁶⁰Dy at the recoil separator RITU. Evidence for the ablate and the prolate bandheads was found by using the method of alphaconversion electron correlations. These studies have now been extended to the fine structure alpha decay of ^190Po using the SHIP velocity filter at GSI and the reaction ⁵²Cr + ¹⁴²Nd. It was found that in ¹⁸⁶Pb₁₀₄ the three lowest states all have 0⁺ spin and parity. The mid-shell Pb nuclei seem to be unique in the sense that such a shape triplet can only exist here, and the energetically most favourable method of excitation, instead of pair breaking or nuclear vibrations or rotations, is a macroscopic shape change. Further studies such as the Recoil-Decay-Tagging measurements using RITU and JUROSPHERE at JYFL used to identify the prolate rotational band in ¹⁸²Pb, will give further information on the nature of the low-lying 0⁺ states (author)