  We performed a detailed modelling of the background counts observed in a
phoswich scintillator X-ray detector at balloon altitude, used for astronomical
observations, on board small scientific balloon. We used Monte Carlo simulation
technique in Geant4 simulation environment, to estimate the detector background
from various plausible sources. High energy particles and radiation generated
from the interaction of Galactic Cosmic Rays with the atmospheric nuclei is a
major source of background counts (under normal solar condition) for such
detectors. However, cosmogenic or induced radioactivity in the detector
materials due to the interaction of high energy particles and natural
radioactive contamination present in the detector can also contribute
substantially to the detector background. We considered detailed 3D modelling
of the earth's atmosphere and magnetosphere to calculate the radiation
environment at the balloon altitude and deployed a proper mass model of the
detector to calculate the background counts in it. The calculation
satisfactorily explains the observed background in the detector at 30 km
altitude (atmospheric depth: 11.5 $g/cm^{2}$) during the balloon flight
experiment from a location near 14.5$^{\circ}$N geomagnetic latitude.
