Monte-Carlo Study of Energy Losses in Hot Stage of Electronic Excitation Relaxation in Scintillators

  • PDF / 330,099 Bytes
  • 6 Pages / 414.72 x 648 pts Page_size
  • 29 Downloads / 150 Views

DOWNLOAD

REPORT


Moscow

State

University,

ABSTRACT The results of computer simulation of the fast stages of energy relaxation in insulators after the VUV or XUV photon absorption are presented. The simulation involves two stages: the inelastic scattering of excitations with production of secondary excitations and thermalization through phonon emission. The main attention is focused on the spatial distribution functions of excitations, i.e. one-particle and two-particle distribution functions. The latter Function determines the energy transfer at final stages of energy relaxation and is important for different quenching processes and the acceleration of the luminescence decay. The Monte-Carlo simulation was carried out for BaF2 crystal for photon energies from 20 eV to 100 eV. The simulation shows that the two-particle distribution functions and thus the kinetics depend on the energy of excitation.

INTRODUCTION The efficiency and decay characteristics of fast luminescence in wide-bandgap insulators mainly depends on two factors, each of them characterizing different stages of the relaxation of

electronic excitations: o The number N(h v) of secondary electronic excitations (EEs) created by inelastic scatterings of the primary EE produced by a photon absorption. This number can be estimated for high photon energies hvas N(hv) = hv/Eo, where F,, is the mean energy required for the creation of an electron-hole pair. (/'o (1.5 to 3)Eg, lE' is the forbidden gap.). The number of such secondary excitations can be calculated either with the help of Monte-Carlo methods (e.g. [1,2]) or by solving of the system of kinetic equations for the hot stage of relaxation (e.g. [3-5]). e The spatial distribution of secondary EEs formed both at hot and thermalized stages of their relaxation controls the yield of luminescence and the decay law [6]. For instance, for a crystal with two types of electronic excitations n and c, both of which are created by the same photon (n is the concentration of the emission-active excitations, and c is the concentration of the quenching excitations) one can obtain the decay law vs time:

I/() = /o0exp

- t -_c(O) d-r

Sf

K~ ) K(r)+

G(r,O) 1-exp r,

T___

387 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society

,

(1)_

where r, and r, are the radiative life-time for n-type excitations and the life-time for c-type excitations, respectively, K(r) is the probability of the multipolar transfer from n to c (K(r) =(l•/r)" for dipole-dipole transfer), and G(r,0) is the two-particle n-c distribution function. The equation (I) describes a nonexponential function (e.g. (7,81)so 1(t) is determined by the spatial distribution of excitations. The aim of the present investigation is the Monte-Carlo simulation of these two stages of relaxation of excitations resulting in evaluation of N(hv) and G(r,0) as a function of the excitation photon energy h v. The simulation program was tested for BaF-, fast luminescence originating from the radiative transition of the valence electron to the highest lying core hole.