| The Hard X-ray Modulation Telescope(HXMT)operates in near-earth orbit with an altitude of 550 km,where exist high flux charged particles that may induce a massive X-ray background in the main detectors.Anticoincidence detectors are designed to actively shield against the incident charged particles that arrive from the front 2π solid angle,and then improve the sensitivity of the main detectors.The anticoincidence detectors of HXMT consist of 6 top and 12 side detectors.Each detector should have detection efficiency over 95% and dead time in main detectors less than 0.1%.Each detector assembly is composed of a large thin flat plastic scintillator(0.225 m for top board and 0.1m for side board,both 6.4mm thickness)coupled to a photomultiplier tubearound the center of the scintillator.The large thin configuration results in too low light transmission efficiency to reach the design specifications.In this work we use optical simulations to study the light transfer behavior in scintillators and propose optimal reflector configurations to improve the performances of the anticoincidence detectors.The cosmological model,which is based on the cosmological principle and the Einstein field equation,is consistent with most current observations.In this model,however,there are several theoretical and observational questions left unsolved and being argued.The geometric degeneracy between the space curvature and the dark energy arise difficulties in finding constrains on both parameters simultaneously.Several works tries to derive the curvature via pure geometric manner using current observations of distance and Hubble expansion rate,but the result is weak as compared to one constrained by cosmic microwave background.In this work we present a model-independent determination of the curvature parameter using the distance and Hubble parameter from recent baryon acoustic oscillation measurements.The method is without any assumption of the parameterization of dark energy,thus break the degeneracy,and is expected to make better constrains on the curvature parameter. |
