Quantum reflection of atoms from a solid surface at normal incidence

We observed quantum reflection of ultracold atoms from the attractive potential of a solid surface. Extremely dilute Bose-Einstein condensates of ^{23}Na, with peak density 10^{11}-10^{12}atoms/cm^3, confined in a weak gravito-magnetic trap were normally incident on a silicon surface. Reflection probabilities of up to 20 % were observed for incident velocities of 1-8 mm/s. The velocity dependence agrees qualitatively with the prediction for quantum reflection from the attractive Casimir-Polder potential. Atoms confined in a harmonic trap divided in half by a solid surface exhibited extended lifetime due to quantum reflection from the surface, implying a reflection probability above 50 %.Comment: To appear in Phys. Rev. Lett. (December 2004)5 pages, 4 figure

Dynamical Instability of a Doubly Quantized Vortex in a Bose-Einstein condensate

Doubly quantized vortices were topologically imprinted in $|F=1>$ $^{23}$Na condensates, and their time evolution was observed using a tomographic imaging technique. The decay into two singly quantized vortices was characterized and attributed to dynamical instability. The time scale of the splitting process was found to be longer at higher atom density.Comment: 5 pages, 4 figure

Psychological Safety and Norm Clarity in Software Engineering Teams

In the software engineering industry today, companies primarily conduct their work in teams. To increase organizational productivity, it is thus crucial to know the factors that affect team effectiveness. Two team-related concepts that have gained prominence lately are psychological safety and team norms. Still, few studies exist that explore these in a software engineering context. Therefore, with the aim of extending the knowledge of these concepts, we examined if psychological safety and team norm clarity associate positively with software developers' self-assessed team performance and job satisfaction, two important elements of effectiveness. We collected industry survey data from practitioners (N = 217) in 38 development teams working for five different organizations. The result of multiple linear regression analyses indicates that both psychological safety and team norm clarity predict team members' self-assessed performance and job satisfaction. The findings also suggest that clarity of norms is a stronger (30\% and 71\% stronger, respectively) predictor than psychological safety. This research highlights the need to examine, in more detail, the relationship between social norms and software development. The findings of this study could serve as an empirical baseline for such, future work.Comment: Submitted to CHASE'201

The Observations Of The X-Ray Source Hz Herculis-Hercules X-1

NASAESASRCAstronom

Fifty Years of IMF Variation: The Intermediate-Mass Stars

I track the history of star count estimates of the Milky Way field star and open cluster IMFs, concentrating on the neglected mass range from 1 to 15 M${_\odot}$. The prevalent belief in a universal IMF appears to be without basis for this mass range. Two recent estimates of the field star IMF using different methods and samples give values of the average logarithmic slope $\Gamma$ between -1.7 and -2.1 in the mass range 1.1 to 4 M${_\odot}$. Two older estimates between 2 and 15 M${_\odot}$ disagree severely; the field IMF in this range is essentially unknown from star counts. Variations in $\Gamma$ among open cluster IMFs in this mass range have not decreased despite numerous detailed studies, even for studies using homogeneous data and reduction procedures and including only clusters with a significant mass range. These cluster variations \textit{might} be due to the combined effects of sampling, systematic errors, stellar evolution uncertainties, dynamical evolution, and unresolved binaries. If so, then the cluster data are consistent with a universal IMF, but are also consistent with sizeable variations. The cluster data do not allow an estimate of an average IMF or $\Gamma$ because the average depends on the choice of weighting procedure and other effects. If the spread in cluster IMFs is in excess of the effects listed above, real IMF variations must occur that do not depend much on physical conditions explored so far. The complexity of the star formation process seen in observations and simulations suggests that large realization-to-realization differences might be expected, in which case an individual cluster IMF would be in part the product of evolutionary contingency in star formation, and the function of interest is the probability distribution of IMF parameters.Comment: 18 pages, including 4 figures: invited talk presented at the conference on "IMF@50: The Stellar Initial Mass Function Fifty Years Later" held at Abbazia di Spineto, Siena, Italy, May 2004; to be published by Kluwer Academic Publishers, edited by E. Corbelli, F. Palla, and H. Zinnecke

High LET, passive space radiation dosimetry and spectrometry

The development of high linear energy transfer (LET), passive radiation dosimetry and spectrometry is needed for the purpose of accurate determination of equivalent doses and assessment of health risks to astronauts on long duration missions. Progress in the following research areas is summerized: intercomparisons of cosmic ray equivalent dose and LET spectra measurements between STS missions and between astronauts; increases LET spectra measurement accuracy with ATAS; space radiation measurements for intercomparisons of passive (PNTD, TLD, TRND, Emulsion) and active (TEPC, RME-111) dosimeters; interaction of cosmic ray particles with nuclei in matter; radiation measurements after long duration space exposures; ground based dosimeter calibrations; neutron detector calibrations; radiation measurements on Soviet/Russian spacecraft; space radiation measurements under thin shielding; and space radiation

Improvement of neurovascular imaging using ultra-high-resolution computed tomography angiography

Objective To evaluate diagnostic image quality of ultra-high-resolution computed tomography angiography (UHR-CTA) in neurovascular imaging as compared to normal resolution CT-angiography (NR-CTA). Material and Methods In this retrospective single-center study brain and neck CT-angiography was performed using an ultra-high-resolution computed tomography scanner (n = 82) or a normal resolution CT scanner (NR-CTA; n = 73). Ultra-high-resolution images were reconstructed with a 1024 × 1024 matrix and a slice thickness of 0.25 mm, whereas NR-CT images were reconstructed with a 512 × 512 matrix and a slice thickness of 0.5 mm. Three blinded neuroradiologists assessed overall image quality, artifacts, image noise, overall contrast and diagnostic confidence using a 4-point Likert scale. Furthermore, the visualization and delineation of supra-aortic arteries with an emphasis on the visualization of small intracerebral vessels was assessed using a cerebral vascular score, also utilizing a 4-point Likert scale. Quantitative analyses included signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), noise and the steepness of gray value transition. Radiation exposure was determined by comparison of computed tomography dose index (CTDIvol), dose length product (DLP) and mean effective dose. Interrater agreement was evaluated via determining Fleiss-Kappa. Results Ultra-high-resolution CT-angiography (UHR-CTA) yielded excellent image quality with superior quantitative (SNR: p < 0.001, CNR: p < 0.001, steepness of gray value transition: p < 0.001) and qualitative results (overall image quality: 4 (Inter quartile range (IQR) = 4–4); p < 0.001, diagnostic confidence: 4 (IQR = 4–4); p < 0.001) compared to NR-CT (overall image quality: 3 (IQR = 3–3), diagnostic confidence: 3 (IQR = 3–4)). Furthermore, UHR-CT enabled significantly superior delineation and visualization of all vascular segments, from proximal extracranial vessels to the smallest peripheral cerebral branches (e.g., UHR-CTA PICA: 4 (3–4) vs. NR-CTA PICA: 3 (2–3); UHR-CTA P4: 4 (IQR = 3–4) vs. NR-CTA P4: 2 (IQR = 2–3); UHR-CTA M4: 4 (IQR = 4–4) vs. NR-CTA M4: 3 (IQR = 2–3); UHR-CTA A4: 4 (IQR = 3–4) vs. NR-CTA A4: 2 (IQR = 2–3); all p < 0.001). Noteworthy, a reduced mean effective dose was observed when applying UHR-CT (NR-CTA: 1.8 ± 0.3 mSv; UHR-CTA: 1.5 ± 0.5 mSv; p < 0.001). Conclusion Ultra-high-resolution CT-angiography improves image quality in neurovascular imaging allowing the depiction and evaluation of small peripheral cerebral arteries. It may thus improve the detection of pathologies in small cerebrovascular lesions and the resulting diagnosis

Towards quantum magnetism with ultracold atoms

22nd International Conference on Atomic PhysicsAt ICAP we presented the efforts and progress at MIT towards using ultracold atoms for the realization of various forms of quantum magnetism. These efforts include a study of fermions with strong repulsive interactions in which we obtained evidence for a phase transition to itinerant ferromagnetism, the characterization of cold atom systems by noise measurements, and a new adiabatic gradient demagnetization cooling scheme which has enabled us to realize temperatures of less than 350 picokelvin and spin temperatures of less than 50 picokelvin in optical lattices. These are the lowest temperatures ever measured in any physical system

Velocity Dispersion of Dissolving OB Associations Affected by External Pressure of Formation Environment

This paper presents a possible way to understand dissolution of OB associations (or groups). Assuming rapid escape of parental cloud gas from associations, we show that the shadow of the formation environment for associations can be partially imprinted on the velocity dispersion at their dissolution. This conclusion is not surprising as long as associations are formed in a multiphase interstellar medium, because the external pressure should suppress expansion caused by the internal motion of the parental clouds. Our model predicts a few km s$^{-1}$ as the internal velocity dispersion. Observationally, the internal velocity dispersion is $\sim 1$ km s$^{-1}$ which is smaller than our prediction. This suggests that the dissipation of internal energy happens before the formation of OB associations.Comment: 6 pages. AJ accepte