1. Stefan Roesler SC-RP/CERN on behalf of the CERN-SLAC RP Collaboration
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. 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)
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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
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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
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5. Benchmark Experiment – Measurement Locations
Beam
Target-A
A A2 A1
Beam
Target-B
I I2 I1
I2’
I I2 I1
B B2 B1
B B4
Top view
Side view
Side Concrete Iron roof
80-cm thick cm thick cm thick
Location
Angle
A A A1
A A A1 A
Location
Angle
B B4 B B B1
B B4 B B B1
I I2 I2’ I1
i i i2’ i1 B
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6. NE213 organic liquid scintillator
Benchmark Experiment – Instruments
Two Veto counters to reject charged particles (NE102A plastic scintillator 5-mm thick)
NE213 organic liquid scintillator
(f 5’’ x 5’’ thick)
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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)
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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
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9. Concrete, 80cm
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10. Concrete, 80cm
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11. Concrete, 160cm
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12. Iron, 40cm
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13. Code Results – Ratios of Integrated Fluences
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14. 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
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15. Simplified Geometry – 120 GeV protons
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16. Simplified Geometry – 120 GeV protons
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17. Simplified Geometry – 120 GeV protons
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18. Simplified Geometry – 120 GeV protons
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19. Simplified Geometry – 120 GeV protons
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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
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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 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.
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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, May 2006
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