We have measured the temperature dependence of the radiative lifetime
between 4 and 300 K. This dependence is interpreted as due to the exch
ange splitting E(ex) = 10 meV of an electron-hole pair confined in a s
ilicon crystaIlite for a luminescence wavelength equal to 0.7 mu m. Th
en we present a variational calculation taking into account the correl
ation of the electron-hole pair due to the Coulomb interaction. This l
eads us to conclude that the measured exchange splitting corresponds t
o a (spherical)crystallite of diameter 32 Angstrom.