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/CERN

on behalf of the CERN-SLAC RP Collaboration

FLUKA meeting Stefan Roesler

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)


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


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


211.5 181

250 100

115

133

957812

6

104.5

59.5

164

160

240 24016

0

80

480

80

160

100

580 (between target A to B)

333

333

Q

P

250 115

235

127

80 20

Cu target7 diam. x 50 long

20

7.57.57.5

QP

B3 B2 B1B5 B4

I3 I2 I1Beam

Beam

Target-A

A3 A2 A1

Beam

Beam

Target-B

Top view Side view

A3 A2 A140 90 133

B5 B4 B3 B2 B113 26 50 90 110

Side Concrete Iron roof80-cm thick 160-cm thick 40-cm thick

A

B LocationAngle

LocationAngle

i3 i2 i2’ i135 90 90 130

A3 A2 A140 90 133

B5 B4 B3 B2 B113 26 50 90 110

I3 I2 I2’ I135 90 90 130

I3 I2 I1

I2’

Benchmark Experiment – Measurement Locations


Two Veto counters to reject charged particles (NE102A plastic scintillator 5-mm thick) np

NE213

Large VETO

Small VETOShielding

NE213

S-Veto

L-Veto

Iron roof shield

NE213 organic liquid scintillator ( 5’’ x 5’’ thick)

Benchmark Experiment – Instruments


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)


Simulations – Code SpecificFLUKA (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


Concrete, 80cm


Concrete, 160cm


Iron, 40cm


Code Results – Ratios of Integrated Fluences


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


120 GeV proton

Simplified Geometry – 120 GeV protons


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


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.


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

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

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