We have produced a sample of free sodium atoms at rest in the laboratory by decelerating atoms in an atomic beam using momentum transfer from a counterpropagating, resonant laser beam. These atoms have a density of about 10 5 cm -3 and a velocity spread of about 15 m/s full width at half maximum corresponding to a kinetic temperature less than 100 mK.PACS numbers: 32.80.PjIn this Letter we report the production of a stationary "gas" of atoms whose effective temperature is 100 mK. The measured velocity distribution of the atoms has a full width at half maximum (FWHM) as low as 15 m/s and can be centered at v = 0. The interest in such an atomic sample lies in its possible application to such diverse problems as elimination of motional effects in high-resolution spectroscopy or precision measurement, ulta-low-energy collision, and neutral-atom trapping. This latter possibility is especially interesting: Although neutral-atom trapping has historically attracted much attention, 1 " 8 and recent specific proposals for stable optical or laser traps look quite promising, 7,8 trapping has not been realized largely because suitably slow atoms were not available. 9 While previous laser-cooling efforts in our laboratory at the National Bureau of Standards 10 " 12 and in Moscow 13,14 produced very slow atoms, the shallow laser and magnetic traps being proposed require a still slower sample. The present work provides just such a sample.Our previous technique for laser deceleration and velocity compression (cooling) of an atomic beam has been described in detail elsewhere. 1,2,15,16 Briefly, atoms in a thermal atomic beam with a mean velocity of about 1000 m/s are cooled by directing a nearresonant laser beam opposite to their motion. The atoms absorb and then fluoresce the light, changing their velocity by hv/Mc for each absorption. As the atoms in the beam scatter light and slow down, their changing Doppler shift would take them out of resonance with the laser so that they would eventually cease to celerate. However, we compensate for the changing Doppler shift by Zeeman tuning the atomic levels along the beam path using a spatially varying magnetic field produced by a nonuniform solenoid.With this compensation all atoms initially slower than some selectable maximum are decelerated to a final narrow velocity group. This process is appropriately called laser cooling. Both the selectable maximum and the final velocity are determined by the laser frequency and intensity and by the magnetic field strength and its gradient. A continuous atomic beam with final velocity as low as 200 m/s can be achieved, or the atomic beam can actually be stopped in the solenoid.We measure the velocity distribution by detecting the fluorescence from atoms excited by a second, very weak laser propagating nearly parallel to the atomic beam. This signal is proportional to atomic density. Because of the Doppler shift, the intensity of this fluorescence depends resonantly on the atomic velocity, and a slow scan of this laser's frequency results in a fluo...

BackgroundBalance assessment and training is utilized by clinicians and their patients to measure and improve balance. There is, however, little consistency in terms of how clinicians, researchers, and patients measure standing balance. Utilizing the inherent sensors in every smartphone, a mobile application was developed to provide a method of objectively measuring standing balance.ObjectiveWe aimed to determine if a mobile phone application, which utilizes the phone’s accelerometer, can quantify standing balance.MethodsThree smartphones were positioned simultaneously above the participants’ malleolus and patella and at the level of the umbilicus. Once secured, the myAnkle application was initiated to measure acceleration. Forty-eight participants completed 8 different balance exercises separately for the right and left legs. Accelerometer readings were obtained from each mobile phone and mean acceleration was calculated for each exercise at each ankle and knee and the torso.ResultsMean acceleration vector magnitude was reciprocally transformed to address skewness in the data distribution. Repeated measures ANOVAs were completed using the transformed data. A significant 2-way interaction was revealed between exercise condition and the body position of the phone (P<.001). Post-hoc tests indicated higher acceleration vector magnitude for exercises of greater difficulty. ANOVAs at each body position were conducted to examine the effect of exercise. The results revealed the knee as the location most sensitive for the detection of differences in acceleration between exercises. The accelerometer ranking of exercise difficulty showed high agreement with expert clinical rater rankings (kappa statistic>0.9).ConclusionsThe myAnkle application revealed significantly greater acceleration magnitude for exercises of greater difficulty. Positioning of the mobile phone at the knee proved to be the most sensitive to changes in accelerometer values due to exercise difficulty. Application validity was shown through comparison with clinical raters. As such, the myAnkle app has utility as a measurement tool for standing balance.

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