579 research outputs found
Opportunities for Electron Microscopy in Space Radiation Biology
Densely ionizing, particulate radiations in outer space are likely to cause to mammalian tissues biological damage that is particularly amenable to examination by the techniques of electron microscopy. This situation arises primarily from the fact that once the density of ionization along the particle track exceeds a certain value, small discrete lesions involving many adjacent cells may be caused in organized tissues. Tissue damage produced by ionization densities below the critical value also afford opportunities for electron microscopic evaluation, as is shown by the damage produced in optic and proximate tissues of the New Zealand white rabbit in terrestrial experiments. Late radiation sequelae in nondividing, or terminally differentiating, tissues, and in stem cell populations, are of special importance in these regards.
It is probable that evaluations of the hazards posed to astronauts by galactic particulate radiations during prolonged missions in outer space will not be complete without adequate electron microscopic evaluation of the damage those radiations cause to organized tissues
Coulomb crystallization in expanding laser-cooled neutral plasmas
We present long-time simulations of expanding ultracold neutral plasmas,
including a full treatment of the strongly coupled ion dynamics. Thereby, the
relaxation dynamics of the expanding laser-cooled plasma is studied, taking
into account elastic as well as inelastic collisions. It is demonstrated that,
depending on the initial conditions, the ionic component of the plasma may
exhibit short-range order or even a superimposed long-range order resulting in
concentric ion shells. In contrast to ionic plasmas confined in traps, the
shell structures are built up from the center of the plasma cloud rather than
from the periphery
Instability Heating of Sympathetically-Cooled Ions in a Linear Paul Trap
Sympathetic laser cooling of ions stored within a linear-geometry, radio
frequency, electric-quadrupole trap has been investigated using computational
and theoretical techniques. The simulation, which allows 5 sample ions to
interact with 35 laser-cooled atomic ions, revealed an instability heating
mechanism, which can prevent ions below a certain critical mass from being
sympathetically cooled. This critical mass can however be varied by changing
the trapping field parameters thus allowing ions with a very large range of
masses to be sympathetically cooled using a single ion species. A theoretical
explanation of this instability heating mechanism is presented which predicts
that the cooling-heating boundary in trapping parameter space is a line of
constant (ion trap stability coefficient), a result supported by the
computational results. The threshold value of depends on the masses of
the interacting ions. A functional form of this dependence is given
Collisions of cold magnesium atoms in a weak laser field
We use quantum scattering methods to calculate the light-induced collisional
loss of laser-cooled and trapped magnesium atoms for detunings up to 30 atomic
linewidths to the red of the 1S_0-1P_1 cooling transition. Magnesium has no
hyperfine structure to complicate the theoretical studies. We evaluate both the
radiative and nonradiative mechanisms of trap loss. The radiative escape
mechanism via allowed 1Sigma_u excitation is dominant for more than about one
atomic linewidth detuning. Molecular vibrational structure due to
photoassociative transitions to bound states begins to appear beyond about ten
linewidths detuning.Comment: 4 pages with 3 embedded figure
Two-Photon Doppler cooling of alkaline-earth-metal and ytterbium atoms
A new possibility of laser cooling of alkaline-earth-metal and Ytterbium
atoms using a two-photon transition is analyzed. We consider a -
transition, with excitation in near resonance with the
level. This greatly increases the two-photon transition rate, allowing an
effective transfer of momentum. The experimental implementation of this
technique is discussed and we show that for Calcium, for example, two-photon
cooling can be used to achieve a Doppler limit of 123 microKelvin. The
efficiency of this cooling scheme and the main loss mechanisms are analyzed.Comment: 7 pages, 5 figure
Doppler cooling and trapping on forbidden transitions
Ultracold atoms at temperatures close to the recoil limit have been achieved
by extending Doppler cooling to forbidden transitions. A cloud of ^40Ca atoms
has been cooled and trapped to a temperature as low as 6 \mu K by operating a
magneto-optical trap on the spin-forbidden intercombination transition.
Quenching the long-lived excited state with an additional laser enhanced the
scattering rate by a factor of 15, while a high selectivity in velocity was
preserved. With this method more than 10% of pre-cooled atoms from a standard
magneto-optical trap have been transferred to the ultracold trap. Monte-Carlo
simulations of the cooling process are in good agreement with the experiments
All Optical Formation of an Atomic Bose-Einstein Condensate
We have created a Bose-Einstein condensate of 87Rb atoms directly in an
optical trap. We employ a quasi-electrostatic dipole force trap formed by two
crossed CO_2 laser beams. Loading directly from a sub-doppler laser-cooled
cloud of atoms results in initial phase space densities of ~1/200.
Evaporatively cooling through the BEC transition is achieved by lowering the
power in the trapping beams over ~ 2 s. The resulting condensates are F=1
spinors with 3.5 x 10^4 atoms distributed between the m_F = (-1,0,1) states.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Effects of urban living environments on mental health in adults
Urban-living individuals are exposed to many environmental factors that may combine and interact to influence mental health. While individual factors of an urban environment have been investigated in isolation, no attempt has been made to model how complex, real-life exposure to living in the city relates to brain and mental health, and how this is moderated by genetic factors. Using the data of 156,075 participants from the UK Biobank, we carried out sparse canonical correlation analyses to investigate the relationships between urban environments and psychiatric symptoms. We found an environmental profile of social deprivation, air pollution, street network and urban land-use density that was positively correlated with an affective symptom group (r = 0.22, P perm < 0.001), mediated by brain volume differences consistent with reward processing, and moderated by genes enriched for stress response, including CRHR1, explaining 2.01% of the variance in brain volume differences. Protective factors such as greenness and generous destination accessibility were negatively correlated with an anxiety symptom group (r = 0.10, P perm < 0.001), mediated by brain regions necessary for emotion regulation and moderated by EXD3, explaining 1.65% of the variance. The third urban environmental profile was correlated with an emotional instability symptom group (r = 0.03, P perm < 0.001). Our findings suggest that different environmental profiles of urban living may influence specific psychiatric symptom groups through distinct neurobiological pathways
Pion and Sigma Polarizabilities and Radiative Transitions
Fermilab E781 plans measurements of gamma-Sigma and -pion
interactions using a 600 GeV beam of Sigmas and pions, and a virtual photon
target. Pion polarizabilities and radiative transitions will be measured in
this experiment. The former can test a precise prediction of chiral symmetry;
the latter for a_1(1260) ----> pi + gamma is important for understanding the
polarizability. The experiment also measures polarizabilities and radiative
transitions for Sigma hyperons. The polarizabilities can test predictions of
baryon chiral perturbation theory. The radiative transitions to the
Sigma*(1385) provide a measure of the magnetic moment of the s-quark. Previous
experimental and theoretical results for gamma-pi and gamma-Sigma interactions
are given. The E781 experiment is described.Comment: 13 pages text (tex), Tel Aviv U. Preprint TAUP 2204-94, uses
Springer-Verlag TEX macro package lecproc.cmm (appended at end of tex file,
following \byebye), which requires extracting lecproc.cmm and putting this
file in your directory in addition to the tex file (mmcd.tex) before tex
processing. lecproc.cmm should be used following instructions and guidelines
available from Springer-Verlag. Submitted to the Proceedings of Workshop on
Chiral Dynamics, Massachusetts Institute of Technology, July 1994, Eds. A.
Bernstein, B. Holstein. Replaced Oct. 4 to add TAUP preprint number. Replaced
Oct. 12 to correct Pb target thickness from 1.3% interaction to 0.3
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