Neutron Scattering

If we set aside the costs and ask about safety, we can assert that accelerator driven sources
(e .g . spallation sources) are inherently safc, because no critical configuration is needed for the
neutron production . A pulsed reactor, on the other hand, bas to run periodically through a
prompt super critical configuration . Therefore the external control mechanisms (absorbers) of
the continuously operating reactor will not work. The power excursion must be limited by inherent
mechanisms, e .g . by the temperature rise of the fuel . Although it may be unlikely in reality,
malfunctions of the necessarily mechanical insertion of excess reactivity (rotating parts
of fuel or reflector) may lead to substantial damage of the reactor core . No problems exist in
that respect with a spallation neutron source. Furthermore, the proton beam can be shut down
within a few milliseconds . Neutron generation by protons enables the shaping of pulse structures
(pulse duration below 1 microsecond, arbitrary pulse repetition rates) basically unfeasible
with mechanical devices .
1 .5 The Spallation Neutron Source
1 .5 .1 The spallation reaction
For kinetic energies above about 120 MeV, protons (or neutrons) cause a reaction in atomic
nuclei, which leads to a release of a large number of neutrons, protons, mesons (if the proton
energy is above 400 MeV), nuclear fragments and y-radiation . This kind of nuclear disintegration
has been named spallation, because it resembles spalling ofa stone with a hammer .
The spallation reaction is a two stage process, which can be distinguished by the spatial
and spectral distribution of the emitted neutrons . This is depicted schematically in Figure 1 .2 .
l1Et1a tiu I?~i t Yittcl 1lFt-l,i1
f -t
f',',l' c', t
P'ntsPtfy ~ ;Si1
Fig. 1.2 The spal1crlion process