Neutron Scattering

For these reasons a pulsed spallation source will have small (V - 1 .5 liter) H20-moderators
for thermal neutrons . The corresponding storage time of such H20-moderators bas been
measured and is i = 150 ps [3], which is in good agreement with the estimate according to
(1 .4) . Small size and absorption diminish in any case the tune average neutron yield . In order
to improve this without deteriorating the peak fluxes, two tricks are used.
Firstly, a moderator
is enclosed by a so-called reflector (see Fig .
1 .5), a strongly scattering ("reflecting") but non
moderating material, i .e . a heavy element with a large scattering cross section like lead . Secondly,
the leakage probability from the moderator interior, i .e .
a region of higher flux due to
geometrical buckling (Chapter 1 .3), is enhanced by holes or grooves pointing toward the neutron
beam holes. Both measures give gain factors of 2 each, whereby the reflector gain is so to
speak "for free", because the anyway necessary lead or iron shielding bas the saine effect.
A
reflector can be imagined to effect such that it
scatters fast neutrons back, which penetrated
the moderator without being or insufficiently slowed down .
Similar considerations hold as
well for cold moderators employed with spallation sources (Fig . 1 .5) .
As a final remark let us point out that the overall appearance of a target station can hardly
be told from a reactor hall with the respective experimental equipment in place .
In both cases
neutrons are extracted from the moderators by beam channels or guide tubes and transported
to the various scattering instruments .
In the following Table 1 .2 the expected and experimentally supported flux data of ESS are
shown and compared te , those of existing sources .
High flux reactor
(HFR)
Grenoble R
Pulsed reactor
HiR-H
Dubna (R
Spallation source
ISIS
Chilton
ESS
Hg-Target
H2O Moderator
4 [cm -' s-' ] 10 15 2 - 1016 4 .5- 10 15 1 .4- 10 17
(U[cm-2S-' 1 10 15 2 . 1013 7 - 1012 0.6 .
()15
Pulse repetition rate v [s-1 1 - 5 50 50
Pulse duration [10-6 s] - 250 30 165
dl0"cm -2 S -1 ] l* 1 2 .2 70
* with neutron chopper 100 s 1
Tab .
1 .2 Comparison ofthe performance of varions modern neutron sources
1-12