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

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Presentation on theme: "FLUKA benchmark of high-energy neutron spectra outside shielding of a hadron accelerator Stefan Roesler SC-RP/CERN on behalf of the CERN-SLAC RP Collaboration."— Presentation transcript:

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

2 Stefan Roesler FLUKA meeting 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, , ,..)  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 Stefan Roesler FLUKA meeting 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 Stefan Roesler FLUKA meeting 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 Benchmark Experiment - The CERF Facility

5 Stefan Roesler FLUKA meeting B3 B2 B1 B5 B4 I3 I2 I1 Beam Target-A A3 A2 A1 Beam Target-B Top view Side view A3 A2 A B5 B4 B3 B2 B Side Concrete Iron roof 80-cm thick 160-cm thick 40-cm thick A B Location Angle Location Angle i3 i2 i2’ i A3 A2 A B5 B4 B3 B2 B I3 I2 I2’ I I3 I2 I1 I2’ Benchmark Experiment – Measurement Locations

6 Stefan Roesler FLUKA meeting Two Veto counters to reject charged particles (NE102A plastic scintillator 5-mm thick) NE213 organic liquid scintillator (  5’’ x 5’’ thick) Benchmark Experiment – Instruments

7 Stefan Roesler FLUKA meeting Simulations – General FLUKA (Version 2005) MARS (Version 15, update Feb. 2006) PHITS (Version 1.97) Benchmark of three different Monte Carlo codes: 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 Stefan Roesler FLUKA meeting 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 Stefan Roesler FLUKA meeting Concrete, 80cm

10 Stefan Roesler FLUKA meeting Concrete, 80cm

11 Stefan Roesler FLUKA meeting Concrete, 160cm

12 Stefan Roesler FLUKA meeting Iron, 40cm

13 Stefan Roesler FLUKA meeting Code Results – Ratios of Integrated Fluences

14 Stefan Roesler FLUKA meeting Code Results – Discussion and Uncertainties - 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 Stefan Roesler FLUKA meeting 120 GeV proton Simplified Geometry – 120 GeV protons

16 Stefan Roesler FLUKA meeting 120 GeV proton Simplified Geometry – 120 GeV protons

17 Stefan Roesler FLUKA meeting 120 GeV proton Simplified Geometry – 120 GeV protons

18 Stefan Roesler FLUKA meeting 120 GeV proton Simplified Geometry – 120 GeV protons

19 Stefan Roesler FLUKA meeting 120 GeV proton Simplified Geometry – 120 GeV protons

20 Stefan Roesler FLUKA meeting 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 Stefan Roesler FLUKA meeting 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 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 Stefan Roesler FLUKA meeting 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, May 2006


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