A long-standing prediction of nuclear models is the emergence of a region of long-lived, or even stable, superheavy elements beyond the actinides. These nuclei owe their enhanced stability to closed shells in the structure of both protons and neutrons. However, theoretical approaches to date do not yield consistent predictions of the precise limits of the 'island of stability'; experimental studies are therefore crucial. The bulk of experimental effort so far has been focused on the direct creation of superheavy elements in heavy ion fusion reactions, leading to the production of elements up to proton number Z = 118 (refs 4, 5). Recently, it has become possible to make detailed spectroscopic studies of nuclei beyond fermium (Z = 100), with the aim of understanding the underlying single-particle structure of superheavy elements. Here we report such a study of the nobelium isotope 254No, with 102 protons and 152 neutrons--the heaviest nucleus studied in this manner to date. We find three excited structures, two of which are isomeric (metastable). One of these structures is firmly assigned to a two-proton excitation. These states are highly significant as their location is sensitive to single-particle levels above the gap in shell energies predicted at Z = 114, and thus provide a microscopic benchmark for nuclear models of the superheavy elements.
An atmospheric-pressure, uniform, continuous, glow plasma was produced in ambient air assisted by argon feeding gas, using a 13.56 MHz rf source. Based on the measured current–voltage curve and optical emission spectrum intensity, the plasma showed typical glow discharge characteristics, free from streamers and arc. The measured rotational and vibrational temperatures were in the range of 490 to 630 K and 2000 to 3300 K, respectively, within the operation range of argon flow rate and rf power. From the spatial measurement of total optical emission intensity, and rotational and vibrational temperatures, the plasma shows very high uniformity (over 93%) in the lengthwise direction. The plasma size for this study was 200 mm×50 mm×5 mm, although a plasma was produced in the scaled-up version of 600 mm in length, aiming for large-area plasma applications.
Discharge modes, α and γ, of a radio-frequency helium capacitively coupled discharge at atmospheric pressure were investigated with the discharge gap distance between electrodes varied from 1 to 5mm. As similarly observed in other experiments, the α and γ mode and the α–γ mode transition were observed with large drops in the voltage (310–179V) and the phase angle between the voltage and current (54°–18°), and a contraction of the plasma volume (8.5–0.17cm3, at 3mm gap distance). The discharge voltage where the α–γ mode transition occurred versus the gap distance showed a similar behavior with the Paschen curve for a gas breakdown. Depending on the gap distance, normal and abnormal glow regimes were observed in the αmode. At 1 and 2mm, the α mode remained in the abnormal glow discharge until the α–γ mode transition occurred as the discharge current increases. At 3mm, however, the α mode was excited as a normal glow discharge with a constant current density (17mA∕cm2) but it became an abnormal glow discharge as the current increased. At 4mm, the α mode was sustained as a normal glow discharge, then the transition to the γ mode occurred. Using a simple resistor-capacitor circuit model and a α sheath breakdown model, the discharge modes and the mode transition properties were studied.
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