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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

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

slide2

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)

slide3

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

slide4

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

slide5

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)

slide7

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)

slide8

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
slide14

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
slide20

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
slide21

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.
slide22

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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