Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator
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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. Motivation (1). The radiation field around loss points at a high-energy hadron

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

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Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

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


Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

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)


  • Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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


    Two veto counters to reject charged particles ne102a plastic scintillator 5 mm thick

    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)


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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)


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Concrete, 80cm


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Concrete, 80cm


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Concrete, 160cm


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Iron, 40cm


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Code Results – Ratios of Integrated Fluences


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Simplified Geometry – 120 GeV protons

    120 GeV proton


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Simplified Geometry – 120 GeV protons

    120 GeV proton


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Simplified Geometry – 120 GeV protons

    120 GeV proton


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Simplified Geometry – 120 GeV protons

    120 GeV proton


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    Simplified Geometry – 120 GeV protons

    120 GeV proton


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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.


    Fluka benchmark of high energy neutron spectra outside shielding of a hadron accelerator

    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


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