arXiv subjects

M. H. Lee

Publications and source records attributed to M. H. Lee.

99 records · Page 6Linked to original sources

arXiv2008

Measurement of cosmic-ray low-energy antiproton spectrum with the first BESS-Polar Antarctic flight

The BESS-Polar spectrometer had its first successful balloon flight over Antarctica in December 2004. During the 8.5-day long-duration flight, almost 0.9 billion events were recorded and 1,520 antiprotons were detected in the energy range 0.1-4.2 GeV. In this paper, we report the antiproton spectrum obtained, discuss the origin of cosmic-ray antiprotons, and use antiprotons to probe the effect of charge sign dependent drift in the solar modulation.

astro-ph↗

arXiv2008

Measurements of cosmic-ray secondary nuclei at high energies with the first flight of the CREAM balloon-borne experiment

We present new measurements of heavy cosmic-ray nuclei at high energies per- formed during the first flight of the balloon-borne cosmic-ray experiment CREAM (Cosmic-Ray Energetics And Mass). This instrument uses multiple charge detectors and a transition radiation detector to provide the first high accuracy measurements of the relative abundances of elements from boron to oxygen up to energies around 1 TeV/n. The data agree with previous measurements at lower energies and show a relatively steep decline (~E$^-0.6$ to E$^-0.5$) at high energies. They further show the source abundance of nitrogen relative to oxygen is ~10% in the TeV/n region.

astro-ph↗

arXiv2007

Anisotropy and Penetration Depth of MgB$_{2}$ from ${}^{11}$B NMR

The ${}^{11}$B NMR spectra in polycrystalline MgB$_2$ were measured for several magnetic fields (1.97 T and 3.15 T) as a function of temperature from 5 K to 40 K. The composite spectra in the superconducting state can be understood in terms of anisotropy of the upper critical field, $γ_{H}$, which is determined to be 5.4 at low temperature. Using Brandt's algorithm\cite{Brandt} the full spectrum, including satellites, was simulated for the temperature 8 K and a magnetic field of 1.97 T. The penetration depth $λ$ was determined to be $1,152\pm50$ Å, and the anisotropy of the penetration depth, $γ_λ$, was estimated to be close to one at low temperature. Therefore, our findings establish that there are two different anisotropies for upper critical field and penetration depth at low temperatures.

cond-mat.supr-con↗

arXiv2005

Beam test calibration of the balloon-borne imaging calorimeter for the CREAM experiment

CREAM (Cosmic Ray Energetics And Mass) is a multi-flight balloon mission designed to collect direct data on the elemental composition and individual energy spectra of cosmic rays. Two instrument suites have been built to be flown alternately on a yearly base. The tungsten/Sci-Fi imaging calorimeter for the second flight, scheduled for December 2005, was calibrated with electron and proton beams at CERN. A calibration procedure based on the study of the longitudinal shower profile is described and preliminary results of the beam test are presented.

physics.ins-det↗

arXiv2002

Neutron beam test of CsI crystal for dark matter search

We have studied the response of Tl-doped and Na-doped CsI crystals to nuclear recoils and $γ$'s below 10 keV. The response of CsI crystals to nuclear recoil was studied with mono-energetic neutrons produced by the $^3$H(p,n)$^3$He reaction. This was compared to the response to Compton electrons scattered by 662 keV $γ$-ray. Pulse shape discrimination between the response to these $γ$'s and nuclear recoils was studied, and quality factors were estimated. The quenching factors for nuclear recoils were derived for both CsI(Na) and CsI(Tl) crystals.

nucl-ex↗

arXiv1999

Test of CsI(Tl) crystals for the Dark Matter Search

Searches for weakly interacting massive particles(WIMP) can be based on the dete ction of nuclear recoil energy in CsI(Tl) crystals. We demonstrate that low energy gamma rays down to few keV is detected with CsI(Tl) crystal detector. A clear peak at 6 keV is observed using X-ray source. Good energy resolution and linearity have been achieved down to X-ray region. In addition, we also show that alpha particles and gamma rays can be clearly separated using the different time characteristics of the crystal.

physics.ins-det↗

arXiv1997

Compensation of Magnetic Impurity Effect in Superconductors by Radiation Damage

Compensation of the reduction of $T_{c}$ caused by magnetic impurities has been observed as a consequence of radiation damage. Using the recent theory by Kim and Overhauser (KO) we consider the effect of radiation damage on the $T_{c}$ of superconductors having magnetic impurities. We find a good fitting to the experimental data. It is also pointed out that Gor'kov's formalism with the pairing constraint derived from the Anomalous Green's function leads to KO theory.

cond-mat.supr-con↗

arXiv1997

Impurity scattering in a d-wave superconductor

The influence of (non-magnetic and magnetic) impurities on the transition temperature of a d-wave superconductor is studied anew within the framework of BCS theory. Pairing interaction decreases linearly with the impurity concentration. Accordingly $T_{c}$ suppression is proportional to the (potential or exchange) scattering rate, $1/τ$, due to impurities. The initial slope versus $1/τ$ is found to depend on the superconductor contrary to Abrikosov-Gor'kov type theory. Near the critical impurity concentration $T_{c}$ drops abruptly to zero. Because the potential scattering rate is generally much larger than the exchange scattering rate, magnetic impurities will also act as non-magnetic impurities as far as the $T_{c}$ decrease is concerned. The implication for the impurity doping effect in high $T_{c}$ superconductors is also discussed.

cond-mat.supr-con↗

arXiv1995

Water Chemisorption and Reconstruction of the MgO Surface

The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with Delta E = -90.2kJ/mol. We conclude that protonation stabilizes the otherwise unstable (111) surface and that this, not the bare (001), is the most stable surface of MgO under ambient conditions.

mtrl-th↗