\magnification=\magstep2


$$
{\rm neutrino\ scattering\ cross\ section\ \ } \sigma \quad \sim \quad 10^{-47} {\rm m}^2
$$

$$
{\rm volume\ swept\ out\ \ } V_s \quad = \quad L \sigma 
$$

$$
{\rm mass\ of\ cube\ \ } M \quad = \quad \rho V \quad = \quad \rho \thinspace (1 {\rm m}^3)
$$

$$
{\rm number\ of\ nucleons\ per\ cubic\ meter\ \ } N \quad = \quad {{ M } \over {m_p}} \quad 
                 = \quad {{\rho} \over {m_p}}
$$

$$
{\rm volume\ per\ nucleon\ \ } V_N \quad = \quad {{ m_p } \over {\rho}} 
$$


$$
\eqalign{
{\rm volume\ swept\ out\ \ } V_s \quad &= \quad {\rm volume\ per\ nucleon\ \ } V_N  \cr
         \phantom{=} \cr
                  L \sigma \quad &= \quad {{ m_p } \over {\rho}}  \cr
         \phantom{=} \cr
                  L \quad &= \quad  {{ m_p } \over {\rho \sigma}}  \cr
}
$$

\bye
