The Initial Research Of Monte Carlo Calibration Method On Airborne HPGe Detector

There is important practical valuation to employ airborne HPGe detector in the early stage monitoring of nuclear accidents or anti-terror radiation monitoring. The calibration method using Monte Carlo calculation theory is an interesting and significative research topic. In this paper, we do some work on the un-ignored problems lies in the HPGe detector response function calculation and radiation field simulation of aerial monitoring survey.In the part of detector response function simulation, the dead layer thickness is adjusted from 0.7mm supplied by manufactory to 2.2mm based on comparison of efficiency calculation result with varied of thickness of "dead layer" to the experimental data. Then between 0.2MeV~1.5MeV the deviation of efficiency calculation results to experimental ones is no more than 4%. To get a better simulation spectrum, we adjust the model of detector physics construction in geometry description of MCNP input file. An electron collection efficiency function in transition zone, T(x), is introduced into the simulation procedure to improve the detector response to low energy scatter photon. As the result, we get a simulated spectrum which is well according to the experimental spectra and the deviation of sum of counts is 12.6% between 0~500keV.The main problem consists in the simulation procedure of airborne gamma radiation field is the deep penetration and huge calculation space, which results in very low calculation efficiency. As countermeasures two reduction variance techniques are used in the radiation field simulation procedure in our testing calculation, which are the next estimator tally and the geometry splitting with Russian roulette. With the appropriate technique the flux spectrum at 60m far from a 16.5mci ~137Cs point source is obtained in a 1km radius air semi-sphere space.The experimental spectrum is obtained by a portable HPGe detector and the energy spectrum is translated to incidence photon flux spectrum at this point through a data transform program which comprise of three parts. First, the background spectrum is subtracted from experimental one; Second, by the spectrum stripping, the counts came from partial energy deposition of incidence photon in HPGe crystal, such as Compton scatting or pair production; the next, transform the experimental spectrum to the incidence photon flux spectrum.