Experimental Study of the Phenomenology of
Spallation Neutrons in a Large Lead Block
The purpose of PS211 is to determine how neutrons, produced by spallation
inside a large Lead volume are slowed down by undergoing a very large number
of scatterings, losing each time a small fraction ( 1%) of their kinetic
energy. The focus is in determining the probability for a spallation neutron
produced at an energy of several MeV or more, to survive capture on Lead
resonances and to reach resonance energies of materials to be transmuted,
such as 5.6 eV for
99Tc. This process, of Adiabatic Resonance
Crossing, involves a subtle interplay between the capture resonances of the
Lead medium and of selected impurities. This phenomenology of spallation
neutrons in a large Lead volume, is the physics foundation of the Fast Energy
Amplifier proposed by C. Rubbia, and could open up new possibilities in the
incineration of long-lived nuclear waste such as Actinides or Fission Fragments
(e.g.
99Tc,
129I, etc.).
334 tons of high purity Lead, installed in t7, are exposed to a primary proton
beam of 3.5 GeV/c or 2.5 GeV/c (107 to 1010 protons per PS shot) in the fast
extraction mode, and 2.5 to 0.6 GeV/c in the slow extraction mode (103 protons
per PS shot). Neutron energy spectra are measured with various techniques
over an energy dynamic range of 8 orders of magnitudes. Both prompt and delayed
measurements are performed to study the capture rates in various elements
of interest. The correlation between neutron capture time and energy is determined.
A pneumatic system ("rabbit") allows to perform delayed measurements
of short-lived daughters activity, during the 14.4 seconds available between
two consecutive PS shots. In addition, various types of standard activation
measurements are performed.
Data taking started in April 1996 and was completed by July 1997. The Collaboration
is now mainly engaged in a large analysis effort.
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