FLUKA benchmark of high-energy neutron spectra outside shielding of a hadron accelerator

1. FLUKA benchmark of high-energy neutron spectra outside shielding of a hadron accelerator Stefan Roesler SC-RP/CERNon behalf of the CERN-SLAC RP Collaboration

2. Motivation (1) • The radiation field around loss points at a high-energy hadron accelerator (e.g., SPS, LHC) is characterized by • wide range of secondary particles (p, n, p, g,..) • wide range of energies (thermals up to TeV) • Stray radiation field and dose outside shielding of a high-energy hadron accelerator (e.g., SPS, LHC) is dominated by • neutrons (thermals up to GeV) and photons • about 50% of the dose equiv. is caused by high-energy neutrons (E>20MeV)

3. Motivation (2) • Modern Monte Carlo transport codes allow detailed calculations of the radiation field. • How accurate are these predictions? • How much differ predictions obtained with different codes from each other? • The answers can only be given by accurate experimental benchmark data, however • available (good) data still scarce • difficult to measure neutron energy spectra above 20MeV with low uncertainty

4. Benchmark Experiment - The CERF Facility 120 GeV/c hadron beam facility Neutron Calibration field outside the shield (concrete or iron)  Calibration for various kinds of dosimeter, counter Calibrated Dose rates are given at marked measuring positions

5. I3 I2 I1 Target-A I3 I2 I1 Beam I2’ B5 B4 Beam B3 B2 B1 Target-B Beam A3 A2 A1 Beam Benchmark Experiment – Measurement Locations Top view Side view Side Concrete Iron roof 80-cm thick 160-cm thick 40-cm thick Location Angle A3 A2 A1 40 90 133 A3 A2 A1 40 90 133 A Location Angle • B5 B4 B3 B2 B1 • 26 50 90 110 • B5 B4 B3 B2 B1 • 26 50 90 110 I3 I2 I2’ I1 35 90 90 130 i3 i2 i2’ i1 35 90 90 130 B

6. Two Veto counters to reject charged particles (NE102A plastic scintillator 5-mm thick) Benchmark Experiment – Instruments NE213 organic liquid scintillator (f 5’’ x 5’’ thick)

7. Simulations – General Benchmark of three different Monte Carlo codes: FLUKA(Version 2005) MARS(Version 15, update Feb. 2006) PHITS(Version 1.97) Emphasis on identical input parameters: - Geometry - Material definitions (composition, densities) - Beam parameter (2/3 pions, 1/3 proton, 120GeV/c, Gaussian) - Scored quantities (tracklength of neutrons)

8. Simulations – Code Specific FLUKA (Version 2005) • transport of all hadrons until absorbed or stopped • no electromagnetic cascade • region-importance biasing in the shielding • average over a large number of beam particles (56 Mio.) MARS (Version 15, update Feb. 2006) • transport of neutrons, protons, pions and muons down to 1 MeV • MCNP-option for transport of neutrons below 14.5 MeV • no variance reduction techniques • detector volumes artificially increased to reduce uncertainties PHITS (Version 1.97) • transport of neutrons, protons, pions, kaons and muons down to 1 MeV • LA150 cross sections for neutrons below 150 MeV • JAM model for high energy interactions (>3.5 GeV for nucleons, >2.5 GeV • for mesons), Bertini model at lower energies • evaporation using GEM model • cell-importance biasing in the shielding

9. Concrete, 80cm

10. Concrete, 80cm

11. Concrete, 160cm

12. Iron, 40cm

13. Code Results – Ratios of Integrated Fluences

14. Code Results – Discussion andUncertainties • backward direction and at 90 degrees:good agreement between spectra of all codes • forward direction: FLUKA and PHITS similar fluence, MARS tends to be lower than FLUKA • and PHITS • good description of exp. data within their uncertainties below ~100 MeV • tendency of overestimation of experimental data above ~100 MeV, especially FLUKA and • PHITS • Does it indicate a lack in the models ? • Could it be caused by difficulties in reduction and analysis of exp. data ? • (e.g., uncertainties in response of detector for non-vertical incidence or false signals in Veto counter) • measurements behind iron difficult due to large background (muons, neutrons) • Study of observed features and open question with simplified, cylindrical geometry

15. Simplified Geometry – 120 GeV protons 120 GeV proton

16. Simplified Geometry – 120 GeV protons 120 GeV proton

17. Simplified Geometry – 120 GeV protons 120 GeV proton

18. Simplified Geometry – 120 GeV protons 120 GeV proton

19. Simplified Geometry – 120 GeV protons 120 GeV proton

20. Simplified Geometry - Ratios of Integrated Fluences FLUKA / MARS • ratios increasing in forward • direction • results behind shield reflect • differences in source • generally good agreement in • backward direction and at 90 • degrees

21. Summary and Conclusions • The measurements for the concrete shield confirm the calculated spectra within • the uncertainties below 100 MeV and tend to be lower, especially at 90 degrees • and backward angles at higher energy.. • Result obtained with the different codes in the energy range of the experimental • data (32 MeV - 380 MeV) show agreement within about 20% for backward • and 90 degree angles. • Furthermore, predictions of MARS and FLUKA for high-energy neutron spectra • were studied in more detail with a simplified, cylindrical geometry. The simulations • revealed differences by up to a factor of two between the neutron fluences emitted • from the target. • This study clearly shows the need for experimental verification of the particle • spectra around the loss point and a more detailed simulation of the setup of the • present experiment.

22. References N.Nakao et al., “Measurement of Neutron Energy Spectra behind Shielding at 120 GeV/c hadron Beam Facility” N.Nakao et al., “Calculation of high-energy neutron spectra with different Monte Carlo transport codes and comparison to experimental data obtained at the CERF facility” SATIF-8, Pohang Accelerator Laboratory, Korea, 22-24 May 2006