Neutron Scattering

d) f, = v a, , q - ti = P a, - 0 .37 Q / (47c Ls 3 /3) 'T, (1 .1)
where the slowing down density q is the number of neutrons slowed down to thermal energies
(i .e . to about 25 meV) per unit volume and per second. For a point source of strength Q in the
center of a spherical moderator volume of radius r = L s we obtain what we have inserted for q
in (2 .1) . The life time (also called relaxation time) is given by [2] ti = (Y-abs'vth + D'BZ)-1
where Y- .,b, and D are the coefficients of absorption and diffusion of neutrons, respectively,
and B2 = (Tc/L s )2 a geometrical factor, the "buckling", which is a measure for the spatial flux
distribution . Inserting numerical values, Ls = 29 cm, Eabs = 3x 10 -5 cm- ' and D = 2x10 5 cm2/s
for heavy water (Jülich's research reactor FRJ-2 is heavy water moderated), we obtain with
the source strength Q - 3x10 16 n/(s MW) a thermal neutron flux fia, = 1 .1x 10 13 n/(cm2s MW) .
Extrapolating this to 23 MW, the power of the FRJ-2, we obtain a)a, = 2.5x1014 n/(cm2 s) . This
is only 25% too big, a suiprisingly good result taking into accourt the non realistic assumption
Fie. 1 . 1 Horizontal cut through the reactor block of the Jülich research reactor FR7-2.
(The numbers below the acronyms are the beam charnel heights above thefloor of the experimental
hall.)