119404050
Constraining the cosmic abundance of stellar remnants with multi - TeV gamma-rays
If stellar remnants are cosmologically significant, the infrared flux from the remnant progenitors would contribute to the opacity of multi-TeV γ-rays. The multi-TeV γ-ray horizon is established to be at a redshift z > 0.034 by the observation of the blazar Mrk 501. By requiring that the optical depth due to γγ → e+e- be less than 1 for a source at z = 0.034, we limit the cosmological density of stellar remnants, Ωrm ≤ (2-4) × 10-3 h (h70 is the Hubble constant in units of 70 km s-1 Mpc-1), and thus strongly constrain stellar remnants as a cosmologically significant source of dark matter.
119396716
Big Bang nucleosynthesis constraints on brane cosmologies
Abstract We examine constraints from Big Bang nucleosynthesis on type II Randall–Sundrum brane cosmologies with both a dark radiation component and a quadratic term that depends on the 5-dimensional Planck mass, M 5 . Using limits on the abundances of deuterium and helium-4, we calculate the allowed region in the M 5 –dark radiation plane and derive the precise BBN bound on M 5 alone with no dark radiation: M 5 >13 TeV.
14347496
The BBN manifesto
Abstract In this manifesto I review the current status of standard big bang nucleosynthesis in light of recent observational data and discuss the importance of near-future observations as direct tests of the standard BBN.
123382656
PRIMORDIAL NUCLEOSYNTHESIS AS A PROBE OF PARTICLE PHYSICS AND COSMOLOGY
115618593
The nineteen neutrinos of SN 1987A: mass or no mass?
18626728
Testing Two Nuclear Physics Approximations Used in the Standard Leaky-Box Model for the Spallogenic Production of LiBeB
The spallative production rates of lithium, beryllium, and boron (LiBeB) are a necessary component in any calculation of the evolution of these nuclei in the Galaxy. Previous calculations of these rates relied on two assumptions relating to the nuclear physics aspects: the straight-ahead approximation that describes the distribution of fragment energies and the assumption that the major contributor to the production rate arises from single-step reactions between primary cosmic-ray projectiles and interstellar medium targets. We examine both assumptions by using a semiempirical description for the spall's energy distribution and by including the reactions that proceed via intermediary fragments. After relaxing the straight-ahead approximation, we find that the changes in the production rates and emerging fluxes are small and do not warrant rejection of this approximation. In contrast, we discover that two-step reactions can alter the production rate considerably, leading to noticeable increases in the efficiency of producing the LiBeB nuclei. Motivated by this result, we introduce a cascade technique to compute the production rates exactly and find that the results differ only slightly from those of our two-step calculations. We thus conclude that terminating the reaction network at the two-step order is sufficiently accurate for current studies of spallation.
350710
Magnetocardiography with a modular spin-exchange relaxation-free atomic magnetometer array.
We present a portable four-channel atomic magnetometer array operating in the spin-exchange relaxation-free regime. The magnetometer array has several design features intended to maximize its suitability for biomagnetic measurement, specifically foetal magnetocardiography, such as a compact modular design and fibre-coupled lasers. The modular design allows the independent positioning and orientation of each magnetometer. Using this array in a magnetically shielded room, we acquire adult magnetocadiograms. These measurements were taken with a 6-11 fT Hz(-1/2) single-channel baseline sensitivity that is consistent with the independently measured noise level of the magnetically shielded room.
55911739
Spin exchange collisions and their consequences for spin polarized gas targets of hydrogen and deuterium
Abstract The effects of spin-exchange collisions on the polarization of dense spin-polarized samples of hydrogen and deuterium are analyzed. It is shown that even in large magnetic fields spin-exchange collisions transfer angular momentum between the electrons and the nuclei. This effect has important implications for the operation of densed spin-polarized targets and sources of hydrogen and deuterium. For tensor polarized targets care will be required to obtain a high tensor polarization, especially at low fields. For the specific case of sources that are spin-polarized by spin-exchange collisions with optically pumped alkali atoms, spin-exchange not only polarizes the hydrogen and deuterium electron spins, but polarizes the nuclear spins as well. For high-density vector-polarized targets this may eliminate the need for rf transitions to polarize the nuclei.
8930049
Spin-exchange optical pumping of noble-gas nuclei
Spin-exchange optical pumping of mixtures of alkali-metal vapors and noble gases can be used to efficiently polarize the nuclei of the noble-gas atoms. Liters of noble gases at standard temperature and pressure and with nuclear spin polarizations of several tens of percent are now used in many applications. The authors describe the basic phenomena that govern the spin-exchange process and review the physics of angular momentum transfer and loss in optical pumping and spin-exchange collisions. {copyright} {ital 1997} {ital The American Physical Society}
32501332
Hybrid spin-exchange optical pumping of 3He.
We demonstrate spin-exchange optical pumping of 3He using a "hybrid" K-Rb vapor mixture. The Rb atoms absorb light from a standard laser at 795 nm, then collisionally polarize the potassium atoms. Spin-exchange collisions of K and 3He atoms then transfer the angular momentum to the 3He with much greater efficiency than Rb-3He. For a K-rich vapor, the efficiency of the hybrid spin-exchange collisions approaches 1/4, an order of magnitude greater than achieved by pure Rb pumping. We present the first measurements of actual photon efficiencies (polarized nuclei produced per absorbed photon), and show that a new parasitic absorption process limits the total efficiencies for both hybrid and pure Rb pumping.