26.08.2015   ACCELERATOR DRIVEN SUBCRITICAL REACTORS

In-medium measurements

An alternative way to measure neutron multiplicities has been used by the TARC collaboration [57]. The goal of the TARC experiment was to evaluate the possibility to transmute long-lived fission products like 99Tc by adiabatic resonance crossing, as discussed in section 3.2.5. To this aim a large 330 ton

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p — energy (GeV)

Figure 6.10. Mean neutron multiplicity per incident proton on uranium, as a function of the proton energy. Solid circles: data of Hilscher et al. [110]. Open circles: moderator measurement of Fraser et al. [118].

block of pure lead was used as a target for the CERN PS protons at various energies. A number of holes were drilled through the block, allowing the placement of different types of neutron detector inside it. In particular, 3He gas proportional counters and silicon detectors viewing 6Li or 233U targets allowed the measurement of the neutron fluxes. In the range between 1 eV and 1 keV, neutron energies were determined by their slowing down time as discussed in section 3.2.5. A detailed mapping of the neutron flux could, therefore, be obtained. This mapping was reproduced by a Monte Carlo simulation, as shown in figure 6.11.

The figure shows excellent agreement between the different types of measurement and the calculated values. The simulation found a multiplicity of 31/p/GeV for neutrons falling below the threshold of 20MeV, in agree­ment with the HERMES calculation of Hilscher et al. [110].