Improvements in the characterization of actinide targets by low solid-angle counting

In a paper presented in the Proceedings of the 19th World Conference of the INTDS, 1998, the results of a blind comparison between groups at NIST and the IRMM showed excellent agreement in characterization of actinide targets by alpha-particle counting with low solid-angle geometry. At that time the most accurate NIST results estimated an uncertainty of 0.03 % for the solid-angle determination. The NIST Neutron Physics Group now has improved the accuracy of its solid-angle determination by about a factor of 4, based on an improved aperture design, improved metrology, and comparisons among independent metrology techniques at NIST. Applications of this improved technology are expected to include improved NIST measurements of the free neutron lifetime and a recalibration of the NIST standard for neutron emission rate, NBS-I.     

Measurement of the neutron lifetime using a proton trap

We report a new measurement of the neutron decay lifetime by the absolute counting of in-beam neutrons and their decay protons. Protons were confined in a quasi-Penning trap and counted with a silicon detector. The neutron beam fluence was measured by capture in a thin 6LiF foil detector with known absolute efficiency. The combination of these simultaneous measurements gives the neutron lifetime: tau(n)=(886.8+/-1.2[stat]+/-3.2[syst]) s. The systematic uncertainty is dominated by uncertainties in the mass of the 6LiF deposit and the 6Li(n,t) cross section. This is the most precise measurement of the neutron lifetime to date using an in-beam method.     

Paramagnetic resonance of V in tetragonal GeO2

Anisotropic g and A tensors are found for V⁴⁺ in single crystal tetragonal GeO₂. The orbital mixing parameter λ, normalized radial average 〈1/r³〉/N and core polarization x/N are calculated, and λ is not anomalously low as reported from polycrystalline measurements.     

Set the controls for the heart of the Sun

In this paper we describe experiments conducted with high-power lasers that are attempting to replicate, for a very short time and in miniature, conditions found in the Sun. Experiments to date have reached conditions in the outer part of the Sun. To reach the Sun's centre requires compression of material to very much greater than solid density and heating to over ten million degrees. To achieve this, a new class of experiments and a new generation of high-power lasers are required.     

Raman spectroscopy of arsenolite: crystalline cubic As4O6

The complete Raman spectrum of arsenolite, cubic crystalline As₄O₆, is reported for the first time. The previously unseen E_g mode has been found at 443 cm⁻¹. Further, there is additional support for the assignment of the 415 cm⁻¹ mode as T_2g.