Neutron Scattering

Period Example Flux (D [1013
1950-60 FRM-1 München - 1
1960-70 FRJ-2 Jülich -10
1970-80 HFR Grenoble -100
CM-2
S-1 ]
1980-90 ? -1000 ???
Table 1 .1
Development ofthermalfuxes ofresearch reactors
A flux increase by a factor of about 6 over that ofthe Grenoble reactor had been envisaged
for a new research reactor in Oak Ridge, USA . This enhancement would have been only possible
by a power increase to 350 MW with a simultaneous increase of the average power density
by a factor of 4 compared to Grenoble . After ten years of planning, the US Department of
Energy decided not to build this so called ANS (Advanced Neutron Source).
At this point we have earnestly to ask, whether the decision was adequate to build ever
more powerful but continuously operating reactors . From a technical point of view is was
perhaps the easiest path, from the point of view of neutron scattering, on the other hand, it
was by no means necessary or economic . In order to accept this we only have to realize that
the two standard methods of neutron scattering, i .e. crystal and time of flight techniques, in
any case only use a minute fraction (10-2 . . . 10-4 ) of the source flux. Monochromatization
and/or chopping the primary beam as well as collimation and source to detector distance
(shielding!) may even reduce the source flux by factors of 10-8 to 10-11 , depending on resolution
requirements .
Time of flight spectroscopy inefficiently utilizes the continuous reactor flux for two reasons,
because it requires both a monochromatic and a pulsed beam . Crystal spectrometers and
diffractometers use an extremely narrow energy band, too . The rest of the spectrum is literally
wasted as heat . Obviously, time of flight techniques wich pulsed operation at the same average
source power yield gain factors equal to the ratio of peak to average flux . With crystal techniques
higher order Bragg reflections can be utilized, because they become distinguishable by
their time of flight. In other words, the peak flux will be usable between pulses as well .
So, without increasing the average power density, pulsed sources can deliver much higher
peak fluxes, e.g. 50 times the HFR flux . Now, which type of pulsed source is to be preferred :
a pulsed reactor or an accelerator driven source? This question is not easy to answer. Possibly
it depends on the weights one is willing to assign to the particular arguments . Important arguments
are cost, safety, pulse structure or the potential for other uses than neutron scattering .