Novel search for heavy nu mixing from the beta+ decay of 38mK confined in an atom trap

A new technique, full neutrino momentum reconstruction, is used to set limits on the admixture of heavy neutrinos into the electron neutrino. We measure coincidences between nuclear recoils and positrons from the beta decay of trapped radioactive atoms and deduce the neutrino momentum. A search for peaks in the reconstructed recoil time-of-flight spectrum as a function of positron energy is performed. The admixture upper limits range from 4 x 10(-3) to 2 x 10(-2) and are the best direct limits for neutrinos (as opposed to antineutrinos) for the mass region of 0.7 to 3.5 MeV.     

Chemical oxidation of endohedral metallofullerenes: identification and separation of distinct classes

Chemical oxidation is useful for solubilizing and separating endohedral metallofullerenes, as demonstrated here by a separation of three categories of Gd@C2n species and by the solubilization of Tm@C60+ and Tm@C70+ for the first time.     

Gapless color superconductivity

We present the dispersion relations for quasiparticle excitations about the color-flavor locked ground state of QCD at high baryon density. In the presence of condensates which pair light and strange quarks there need not be an energy gap in the quasiparticle spectrum. This raises the possibility of gapless color superconductivity, with a Meissner effect but no minimum excitation energy. Analysis within a toy model suggests that gapless color superconductivity may occur only as a metastable phase.     

The effects of selective consonant amplification on sentence recognition in noise by hearing-impaired listeners

Weak consonants (e.g., stops) are more susceptible to noise than vowels, owing partially to their lower intensity. This raises the question whether hearing-impaired (HI) listeners are able to perceive (and utilize effectively) the high-frequency cues present in consonants. To answer this question, HI listeners were presented with clean (noise absent) weak consonants in otherwise noise-corrupted sentences. Results indicated that HI listeners received significant benefit in intelligibility (4 dB decrease in speech reception threshold) when they had access to clean consonant information. At extremely low signal-to-noise ratio (SNR) levels, however, HI listeners received only 64% of the benefit obtained by normal-hearing listeners. This lack of equitable benefit was investigated in Experiment 2 by testing the hypothesis that the high-frequency cues present in consonants were not audible to HI listeners. This was tested by selectively amplifying the noisy consonants while leaving the noisy sonorant sounds (e.g., vowels) unaltered. Listening tests indicated small (～10%), but statistically significant, improvements in intelligibility at low SNR conditions when the consonants were amplified in the high-frequency region. Selective consonant amplification provided reliable low-frequency acoustic landmarks that in turn facilitated a better lexical segmentation of the speech stream and contributed to the small improvement in intelligibility.     

Beta–neutrino correlation experiments on laser trapped 38mK, 37K

Laser trapping and cooling techniques are now being applied to the study of nuclear β-decay at several labs. A magneto-optical trap (MOT) provides a localized source of atoms suspended in space, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with the β. This allows reconstruction of the neutrino momentum, and the deduction of the β-v correlation in a more direct fashion than previously possible. In addition, the nuclei can be polarized by atomic techniques, opening a new class of spin correlation measurements to test the degree to which parity is maximally violated in the weak interaction. Our present experiment has detected several hundred thousand recoil-β⁺ coincidences from the 0⁺ → 0⁺ pure Fermi decay of ^38mK, produced at the on-line isotope separators TISOL and ISAC at TRIUMF. Our goal is to set constraints on non-Standard Model scalar bosons competitive with high-energy colliders and more conventional β-v correlation experiments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ensembles of engineered cardiac tissues for physiological and pharmacological study: heart on a chip

Traditionally, muscle physiology experiments require multiple tissue samples to obtain morphometric, electrophysiological, and contractility data. Furthermore, these experiments are commonly completed one at a time on cover slips of single cells, isotropic monolayers, or in isolated muscle strips. In all of these cases, variability of the samples hinders quantitative comparisons among experimental groups. Here, we report the design of a "heart on a chip" that exploits muscular thin film technology--biohybrid constructs of an engineered, anisotropic ventricular myocardium on an elastomeric thin film--to measure contractility, combined with a quantification of action potential propagation, and cytoskeletal architecture in multiple tissues in the same experiment. We report techniques for real-time data collection and analysis during pharmacological intervention. The chip is an efficient means of measuring structure-function relationships in constructs that replicate the hierarchical tissue architectures of laminar cardiac muscle.     

Rapid thermal nitridation of thin chromium films

Thin Cr films were nitrided during rapid thermal annealing for 3 min in NH₃ between 500 and 850°C. Nitridation begins at 500°C and occurs in a layer-by-layer fashion, first forming Cr₂N then CrN. At 550°C, Cr₂N begins to nitride to CrN despite the existence of unreacted Cr, such that a CrN/Cr₂N/Cr structure exists. At 650°C, no unreacted Cr remains and the layer consists of CrN on Cr₂N; this configuration remains at 700°C although CrN grew at the expense of Cr₂N. At 750°C, the reaction is complete, forming CrN. Oxygen accumulates in front of the growing nitrides until the temperature is high enough to allow redistribution. This occurs at lower temperatures in Cr₂N than in CrN, which may be correlated to the enhanced oxidation resistance of CrN. Phase identification was performed by X-ray diffraction, composition determination by Rutherford backscattering spectrometry, and elemental profiling by secondary ion mass spectrometry. Ion beam resonances of the forms ¹⁶O(,)¹⁶O and ¹⁴N(,)¹⁴N provided additional elemental profiling. Vickers microhardness was measured and was observed to increase sharply once a surface nitride had formed, then remained nearly constant upon further annealing.