Transcript Measurements and Simulation of Induced Activity at the

CERF Benchmark Study of
Radionuclide Production
with FLUKA
M. Brugger on behalf of the CERN-SLAC RP Collaboration
Motivation For Experiments
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Motivation
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Beamline components at high-energy hadron colliders can become
highly activated due to various beam loss mechanisms.
The activation of components and equipment is an important
radiation safety concern
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Rather accurate calculations of the radionuclide inventory are
required in order to avoid
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during the operation of facility: dose to personnel during maintenance
interventions
decommissioning of facility and disposal of activated materials
unjustified doses to personnel and environment during operation due to
underestimates in the design phase
excessive costs (e.g., for waste disposal) caused by overly conservative
estimates
Modern Monte Carlo codes (FLUKA) allow a detailed assessment
of isotope production by high-energy particle beams.
However, processes and methods involved are very complex and
benchmark studies are essential.
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CERN – Accelerators
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Injectors
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SPS
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Extraction
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North Experimental Hall
H6 Beamline
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CERF
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CERN-EU High-Energy Reference
Field (CERF) facility
Location of
Samples:
Behind a 50 cm
long, 7 cm
diameter copper
target,
centered with the
beam axis
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Target Setup
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Samples were placed
downstream and lateral to the
CERF copper target
(50cm long, 7cm diameter)
A custom built holder ensured a
quick exchange of the samples
and a correct alignment with
the target
The induced radioactivity in the
target itself was also
determined using a mobile
Gamma Spectroscopy
instrument and a proper
analysis software, so to
account for its dimensions as
well as the angle and distance
to the detector
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Beam Conditions
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120 GeV secondary SPS mixed
hadron beam
(p 34.8%,  60.7% and  4.5%)
16.8s spill cycle, 4s burst
~ 5x1010 (short) - 1x1012 (long)
particles hit the target during
irradiation
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Particles hitting the target were
measured using a Precision
Ionization Chamber
~ 1x108 particles/spill
~ 6x106 particles/second
Beam Profile (approx.
Gaussian): measured with
multi-wire prop. Chamber,
 ~ 10 mm
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Details of Samples
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Large variety of
samples
(typical for the LHC)
Different irradiation
times
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Different locations
during the irradiation
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Short (~ 8 hours)
Long (~ 1 week)
downstream
lateral
Various measurements
at different cooling
times
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Chemical Composition
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Gamma Spectrometry
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Low background coaxial High Precision Germanium detector
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Genie-2000 (Ver. 2.0/2.1) spectroscopy software by Canberra and
the PROcount-2000 counting procedure software
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Canberra: two different detectors 90 cm3 sensitive volume, 60% and 40% relative
efficiency at 1.33 MeV
include a set of advanced spectrum analysis algorithms:
e.g. nuclide identification, interference correction, weighted mean activity,
background subtraction and efficiency correction.
comprise well-developed methods for peak identification using standard or usergenerated nuclide libraries
High accuracy for the measurements is achieved via regular quality
assurance
Use of user-generated nuclide libraries, based on nuclides expected
from the simulation and material composition
Manual revision of results in case ambiguities (overlapping peaks,
contributions from different isotopes, etc.)
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Dose Rate Measurements
Portable spectrometer Microspec
(Bubble Technologies Ind.)
 NaI detector, cylindrical shape, 5 x 5 cm
 sensitivity between 60 keV and 3 MeV
 dose rates (H*(10)) up to 100 mSv/h
 folds spectrum with detector response
(“calibrated” with 22Na source)
 physical centre of detector determined
with additional measurements with known
sources (60Co, 137Cs, 22Na) to be 2.4 cm
Thermo-Eberline dose-meter FHZ 672
 organic Scintillator and NaI detector,
cylindrical shape, 9 x 9 cm
 H*(10) covering a range from
48 keV to 6 MeV
 dose rates up to 100 mSv/h
 assumes average detector response
 physical centre of detector determined as above to be 7.3 cm
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FLUKA Simulations
First Step:
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simulation of isotope production by high-energy processes and low-energy neutron
interactions
calculation of build-up and decay of radioactive isotopes for arbitrary irradiation pattern
and cooling times including radioactive daughter products
storage of information on produced radionuclides in an external file (mass, charge,
position of creation, activity, weight)
Second Step:
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sampling of photons, electrons, and positrons from radioactive decay assuming
isotropic emission and taking into account correct branching ratios, intensities and
energy spectra (positrons and electrons, obtained from program NUCDECAY)
simulation of the electromagnetic cascade induced by these particles, e.g., in the
beamline and shielding components or in air
calculation of dose equivalent rate by folding fluence with energy-dependent dose
equivalent conversion factors, at any points of interest
The calculation of residual nuclei and dose rates is also available as one step
method implemented in FLUKA, however for the benchmark it is necessary to
change the geometric configuration between the calculation of the radioisotopes
and the residual dose rates which is only possible in the two step approach.
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Particle Spectra at Sample Positions
Copper Sample
downstream of target
forward direction,
thus dominant +, 31st August 2006
Aluminium Sample
laterally to target
lateral spectra,
but close to the target,
thus dominant 1MeV n
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Aluminium Sample
Cooling Times:
(4) 25m and (5) 1h 09m, (1) 1d 16h 55m, (2) 16d 8h 56m, (3) 51d 9h 47m
FLUKA Old
0.36
very short
half-life,
thus unc.
in EOI
high
Exp. Error!
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FLUKA Old
Copper
Sample
0.05
0.66
fragmentation
0.27
A>24
very short
half-life,
thus unc.
in EOI
Cooling Times:
(1) 34m
(2) 1h 7m
(3) 2d 5h 28m
(4) 48d 3h 21m
problem in GS
(48Sc, 48V)
50/50%
assumption
GS (65Ni, 65Zn)
FLUKA
xSection
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Example: CERF Isotope Production
Ratio FLUKA/Exp
0.8 < R < 1.2
0.8 < R ± Error < 1.2
Exp/MDA < 1
R + Error < 0.8 or
R – Error > 1.2
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Iron – Dose Rates
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A very good agreement
can be observend for the
Microspec Instrument
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For the Eberline
instrument only few data
points are available
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As for the observed
systematic discrepancy
for the Eberline
instruments, this can most
probably explained by:
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its varying energy response with respect to the calibration using Cs137
its calibration in homogenous fields at large distances
(under investigation)
measured data below 10 nSv/h were excluded from graphs (except for Al)
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Aluminium – Dose Rates
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Good agreement for the
Microspec Instrument
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Positrons annihilate in the
sample and contribute to dose
rate via the two 511 keV
photons (only at short cooling
times tc < 1h)
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Errors of Measurements include
the following:
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±2 mm of the effective centre of the
detector as well as the positioning of the samples
Eberline: a statistical error obtained from repetitive measurements
Microspec: 5% general uncertainty as specified in the manual
a systematic instrument uncertainty of 1 nSv/h
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Concrete – Dose Rates
contributing
Isotope?
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Again good agreement for the Microspec instrument
the Eberline instrument shows systematically higher values, however the effect is
bigger after 2 hours of cooling, where different isotopes dominate
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Concrete Contribution
Gamma vs. Beta+ Emitter
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Beta+ emitters are
dominant up to 2 hours
of cooling
gamma Emitter!
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Concrete - Contributors
Contribution of gamma and positron emitting isotopes to total dose rate at 12.4 cm
Assumption: sample is point-source of photons (2 x 511keV in case of positrons)
24Na:
Ratio ~ 0.7
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dD
10-8 A(tc) ∑ Ig Eg
=
x
dt
7
r2
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Parent Reactions
partl. coming
directly
from Parents
produced
Al
Cu
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Production
sign.significant
also
lowEn.
Neutron
p/n
production
Production
mainly
-production
Al
Cu
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7Be
from Aluminium
Ratio calculated / measured activity: default evaporation 0.36
new evaporation 0.71
Response
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  (E) 
d ( E )
d ln E
(arbitrary units)
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54Mn
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from Copper
FLUKA/Exp: default evaporation 1.18
new evaporation 1.19
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Test Case: Be7 on Cu
Downstream:
- Dominated by high energy hadrons
with energies above ~5 GeV
- Sensitive to Cross Section above
Threshold (~16mb)
- Cross Section Overestimated (?)
by FLUKA
Laterally:
- Mainly Produced by Hadrons with
Energies of a few GeV
- Very Sensitive to the Cross Section at
the Production Threshold
- Well Reproduced by FLUKA
“Response”
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  (E) 
CERF Benchmark Study of Radionuclide Production with FLUKA
d ( E )
d ln E
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Test Case: Be7 on Cu
Confirmed by Cross Section Data !
Slightly
Underestimated
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Overestimated
(?)
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The Four Laws of MC Simulations
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Par. 1)
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Par. 2)
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In case of measurements deviating from the
simulation results refer to Par. 1)
Par. 3)
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The simulation is always right
In the immediate urge of publications scale the
measured values accordingly
Par. 4)
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!!! DO NOT TELL !!!
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Conclusions
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The FLUKA code allows one to calculate in detail
induced radioactivity and residual dose rates for
arbitrary materials and configurations.
The predictions given by FLUKA were benchmarked
with experimental data. Agreement was found to be
within 20% in most cases.
A detailed comparison with xSection data and reaction
channels is mandatory in order to understand the origin
of uncertainties or disagreements between
measurements and simulations.
As a result radionuclide inventories and residual dose
rates can be predicted in detail.
Currently a new approach of modelling the high-energy
hadron-nucleus part in FLUKA is in its testing phase,
extending PEANUT to energies > 5 GeV
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