Neutron Scattering

In the 60's the ferst high flux reactor specially designed for beam hole experiments became
critical in Brookhaven (USA) . It provided a flux of 10 15 n/cmZs1. For research reactors this
level of flux was not significantly surpassed since then. Finally, 1972 the high flux reactor at
the Institute Laue Langevin in Grenoble (France) went into operation . This reactor since then
constitutes the most powerful neutron source worldwide .
In parallel using proton accelerators already beginning in the 60's, another path for neutron
production was developed . Pioneering work was performed at the Argonne National
Laboratory (USA) . At present the most powerful Neutron Spallation Source is situated at the
Rutherford Laboratoiy in Great Britain which bases on a proton beam of about 200KW beam
power. The future of neutron scattering will most probably go along the lines of spallation
sources . At present in the United States the construction of a 2.5MW spallation source has
commenced wich the aim to get operational in 2005 . European plans to build a Megawatt
Spallation Source are still under development and hopefully a European decision for the
European Spallation Source (ESS) will be reached in the year 2003 .
After the war, Germany was late in the development of neutron tools for research . Only in
1955 international agreements allowed a peaceful use of nuclear research. In the saine year
the frst German Research Reactor became critical in Garching . In the early 60's powerful
research reactors were build like for example the FRJ-2 reactor in Jülich which provides a
flux of 2. 10 14 n/cxri Zs ' . Instrumental developments became a domain of German neutron
research. A number of important German contributions in this field are the backscattering
spectrometer, the neutron small angle scattering, the instruments for diffuse neutron scattering
and high resolution time of flight machines .
2 .2 Properties of the neutron
The neutron is a radioactive particle with a mass of m = 1.675 . 10 -27kg . It decays alter a
mean lifetime of z= 889 .1 ± 1 .8s into a proton, an electron and an antineutrino (/3 decay) .
n -> p+ + e + v (+ 0 .77 MeV) (2 .1)
2-3