C. S. J. Pun

Thin accretion disks in f(R) modified gravity models

We consider the basic physical properties of matter forming a thin accretion disc in the static and spherically symmetric space-time metric of the vacuum f(R) modified gravity models. The Lagrangian of the generalized gravity theory is also obtained in a parametric form, and the conditions of the viability of the model are also discussed. The exact Schwarzschild-type solution of the gravitational field equations in the f(R) gravity contains a linearly increasing term, as well as a logarithmic correction, as compared to the standard Schwarzschild solution of general relativity, and it depends on four arbitrary integration constants. The energy flux and the emission spectrum from the accretion disk around the f(R) gravity black holes are obtained, and they are compared to the general relativistic case. Particular signatures can appear in the electromagnetic spectrum, thus leading to the possibility of directly testing modified gravity models by using astrophysical observations of the emission spectra from accretion disks.

Thin accretion disks onto brane world black holes

The brane-world description of our universe entails a large extra dimension and a fundamental scale of gravity that might be lower by several orders of magnitude as compared to the Planck scale. An interesting consequence of the brane-world scenario is in the nature of the vacuum solutions of the brane gravitational field equations, with properties quite distinct as compared to the standard black-hole solutions of general relativity. One possibility of observationally discriminating between different types of black holes is the study of the emission properties of the accretion disks. In the present paper we obtain the energy flux, the emission spectrum and accretion efficiency from the accretion disks around several classes of static and rotating brane-world black holes, and we compare them to the general relativistic case. Particular signatures can appear in the electromagnetic spectrum, thus leading to the possibility of directly testing extra-dimensional physical models by using astrophysical observations of the emission spectra from accretion disks.

Development of a Bonner Sphere neutron spectrometer from a commercial neutron dosimeter

Bonner Spheres have been used widely for the measurement of neutron spectra with neutron energies ranged from thermal up to at least 20 MeV . A Bonner Sphere neutron spectrometer (BSS) was developed by extending a Berthold LB 6411 neutron-dose-rate meter. The BSS consists of a 3He thermal-neutron detector with integrated electronics, a set of eight polyethylene spherical shells and two optional lead shells of various sizes. The response matrix of the BSS was calculated with GEANT4 Monte Carlo simulation. The BSS had a calibration uncertainty of ± 8.6% and a detector background rate of (1.57 ± 0.04) × 10−3 s−1. A spectral unfolding code NSUGA was developed. The NSUGA code utilizes genetic algorithms and has been shown to perform well in the absence of a priori information.

Measurement of cosmic-ray muons and muon-induced neutrons in the Aberdeen Tunnel Underground Laboratory

In this study, we have measured the muon flux and production rate of muon-induced neutrons at a depth of 611 m water equivalent. Our apparatus comprises three layers of crossed plastic scintillator hodoscopes for tracking the incident cosmic-ray muons and 760 L of a gadolinium-doped liquid scintillator for producing and detecting neutrons. The vertical muon intensity was measured to be Iμ = (5.7±0.6)×10–6 cm–2 s–1 sr–1. The yield of muon-induced neutrons in the liquid scintillator was determined to be Yn = (1.19 ± 0.08(stat) ± 0.21(syst)) × 10–4 neutrons/(μ•g•cm–2). A fit to the recently measured neutron yields at different depths gave a mean muon energy dependence of 0.76±0.03 for liquid-scintillator targets.