High-energy in-beam neutron measurements of metal-based shielding for accelerator-driven spallation neutron sources

Metal-based shielding plays an important role in the attenuation of harmful and unwanted radiation at an accelerator-driven spallation neutron source. At the European Spallation Source, currently under construction in Lund, Sweden, metal-based materials are planned to be used extensively as neutron guide substrates in addition to other shielding structures around neutron guides. The usage of metal-based materials in the vicinity of neutron guides however requires careful consideration in order to minimize potential background effects in a neutron instrument at the facility. Therefore, we have carried out a combined study involving high-energy neutron measurements and Monte Carlo simulations of metal-based shielding, both to validate the simulation methodology and also to investigate the benefits and drawbacks of different metal-based solutions. The measurements were carried out at The Svedberg Laboratory in Uppsala, Sweden, using a 174.1 MeV neutron beam and various thicknesses of aluminum-, iron-, and copper-based shielding blocks. The results were compared to geant4 simulations and revealed excellent agreement. Our combined study highlights the particular situations where one type of metal-based solution may be preferred over another.

Figure 1

The incident neutron spectrum used in the TSL measurements, calculated using the algorithm in [26], and the ${}^{238}\mathrm{U}(n,f)$ cross section [27], which highlights the sensitive range of the neutron fluence monitor used in the measurements.

Figure 2

Comparison of the geant4 simulations and TSL measurements for varying thicknesses of shielding materials. The points represent the measured data and the error bars are smaller than the markers. The simulations were carried out for the same thicknesses and the lines are drawn to guide the eye. The statistical error for the simulated data is less than 0.2%. The quantity $R/R_0$ represents the counts in the fluence monitor as measured and simulated for the indicated thickness of a shielding block divided by the counts without any shielding block in the beam.

Figure 3

Comparison of the geant4 simulated neutron energy spectra at the monitor position for Cu-HCP (10 cm), steel CK45 (10 cm) and AL6082-T6 (30 cm). The simulations used the QGSP_INCLXX_HP physics list.

Figure 4

Comparison of the geant4 simulated Fe-Cu and Cu-Fe energy spectra as used in the TSL measurements. Fe refers to steel CK45 and Cu to Cu-HCP. The simulations used the QGSP_INCLXX_HP physics list.