  The accretion of hot, dense matter which consists of heavy nuclei, free
nucleons, degenerated electrons, photons and neutrinos is studied. The
composition of free nucleons and their chemical potentials are provided through
the equation of state for hot, dense matter proposed by Lattimer and
Swesty(1991). When the mass of a central black hole and the accretion rate are
selected as $M_{BH} \approx 3{\rm M}_\odot$ and $\dot{M} \approx 0.1{\rm
M}_\odot$sec$^{-1}$, which provide the typical luminosity of gamma ray
bursts(GRBs), the fraction of free protons in the accreting matter becomes very
small, $Y_p \approx 10^{-4}$, while that of free neutrons is closer to unity,
$Y_n \approx 0.7$. Then the antielectron neutrinos $\bar{\nu}_e$ can freely
escape through the disk but the electron neutrinos $\nu_e$ are almost absorbed
in the disk. Thus the frequent collisions of $\bar{\nu}_e$ with $\nu_e$ over
the disk couldn't be occurred. The accretion disk is cooled mainly by
$\bar{\nu}_e$, which suppresses the increase of temperature and increases the
density in the accreting matter such as $T \approx 3\times 10^{10}$K and $\rho
\approx 3\times 10^{13}$g/cm$^3$ at the inner side of the disk. The scattering
optical depths of $\bar{\nu}_e$ and $\nu_e$ then reach to be very large,
$\tau_s(\bar{\nu}_e) \approx \tau_s({\nu}_e) \approx 10^2$. Thus the accretion
disk could be thermally unstable within the duration of diffusion time of
neutrinos, $t_{diff} \approx 10$(ms).

Astrophysics