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SINBAD ABSTRACT NEA-1552/26
CERF Residual Dose Rates (2003)
1. Name of Experiment:
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BENCHMARK STUDY OF RESIDUAL DOSE RATES WITH FLUKA
2. Purpose and Phenomena Tested:
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Samples of materials, typical for accelerator machines as well
as for shielding and construction components, were irradiated in
the stray radiation field of the CERN-EU high-energy Reference
Field facility (CERF). The samples included pure materials such
as aluminium, copper, iron and titanium as well as composites
like concrete. Emphasis was put on an accurate recording of
the irradiation conditions, such as irradiation profile and
intensity, and on a detailed determination of the elemental
composition of the samples. After the irradiation the residual
dose rate was measured at different cooling times ranging from
about twenty minutes to one month. Furthermore, the irradiation
experiment was simulated with the FLUKA [1, 2] Monte Carlo
code and residual dose rates were calculated using a new
method simulating the production of the various isotopes and
the electromagnetic cascade induced by the radioactive decay
at a certain cooling time. In general good agreement was
found giving confidence in the predictive power of FLUKA and
tools for the calculation of residual rates important to
estimate in a detailed way individual and collective doses to
personnel during interventions at accelerators.
The presented data is published in Ref. [3] and references listed
therein.
3. Description of Source and Experimental Configuration:
----------------------------------------------------
All samples were irradiated at the CERN-EU high-energy Reference
Field (CERF) facility [4]. At this facility a pulsed, 120 GeV/c
mixed hadron beam (1/3 protons, 2/3 positively charged pions) from
the Super Proton Synchrotron (SPS) accelerator is aimed at a 50 cm
long copper target creating a stray radiation field around the
target that is similar to beam loss regions at high-energy
accelerators (collimators, dumps, etc.). The samples were either
laterally attached to the target or placed on a sample holder,
located immediately downstream of the target and centred with its
axis. A list of the samples is given in Table 1.
The actual alignment of the target with respect to the beam
axis was measured at the up- and downstream faces of the
target using Polaroid films and was then also taken into
account for the simulations. The samples were irradiated
with exposure times ranging from a few hours to several days
and a total number of accumulated beam particles ranging from
3.9 x 10^11 to 1.6 x 10^12. The lateral beam profile as well
as the number of particles in each beam spill (cycle length
of 16.8 s) were recorded for later use in the simulation as well
as during the post-processing of the FLUKA results.
The geometry of the target is detailed in file geometryDescription.htm,
and shown on Fig. 1 and Fig. 2.
4. Measurement System and Uncertainties:
------------------------------------
Following the irradiation of each sample, residual dose rates
were measured with a Microspec portable spectrometer by Bubble
Technology Industries (BTI) at various cooling times and
distances to the surface of the samples.
The instrument is based on a NaI crystal of cylindrical shape
with a diameter and height of about 5 cm. The scintillation
light is detected by a photomultiplier tube, which converts
the scintillation light into an electronic signal and amplifies
the signal. Dose rates can be measured up to 100 µSv/h in a range
from 60 keV to 3 MeV. Before each use the spectrometer was
calibrated with a 22Na source according to the
manufacturer's recommendation. To determine dose rates the device
measures energy spectra which are then internally folded with the
detector response as calculated by the manufacturer.
Since an absolute comparison of measured and calculated dose
rates (especially on contact) requires knowledge of the effective
centre of the detector it was determined in the CERN
calibration laboratory. The dose rates from three different
calibration sources (60Co, 137Cs and 22Na) were measured at
distances R between the source and the surface of the detector
varying between contact (R=0) and 30 cm and the results for
each source were fitted, resulting in an average value of
2.4 cm for the centre of the detector.
For the dose rate measurements the irradiated samples were
placed on a holder to allow for distances of 12.4 cm, 22.4 cm and
32.4 cm, between the surface of the sample (the surface which was
facing the CERF target during the irradiation) and the centre of
the detector. In addition, the samples were directly placed in
contact with the detectors. All measurements were carried out
in a laboratory with a low background radiation dose rate of 55
nSv/h.
All measured data points carry errors which include the
following uncertainties: a 2 mm uncertainty for the
determination of the effective centre of the detector, a 2
mm uncertainty for the positioning of the sample with the
holder (i.e., distance to the detector), and a systematic
instrument uncertainty of 1 nSv/h corresponding to the last
significant figure on the display of the respective devices.
Except for the aluminium sample measured data below 10 nSv/h
were systematically excluded from the comparison due to their
proximity to the background value and the lower measurement
threshold as indicated in the user -manual of the instrument.
In case of aluminium they were kept in order to indicate the
behaviour of the dose rate at large cooling times.
Uncertainties which could arise from deviations of the actual beam
-shape, i.e., spatial distribution of beam particles, from a
Gaussian distribution (which is assumed in the simulations) would
affect most the results for the samples irradiated downstream of
the CERF target. Thus, an additional simulation was performed with
a pencil beam source instead of a Gaussian distribution among the
beam particles in lateral directions. Results from this additional
simulation for the residual dose rate from an iron sample are
shown in Figure 3 together with the results of the default
(Gaussian) beam definition. The difference is considerable with
the dose being higher by about 70 % for a pencil beam. However, it
should be noted that it represents the maximum possible effect,
while the actual uncertainties of the (measured) Gaussian
distribution are assumed to be much smaller.
Similarly, uncertainties arising from deviations of the actual
beam orientation from the assumed one (see direction cosines in
x- and y-direction at the beam spot) could lead to uncertainties
in the results for the samples attached laterally to the CERF
target. Again, the effect has been estimated with an additional
simulation, i.e., by aligning the source particles with the
z-axis (zero direction cosines). The result can be seen in
Figure 4. Here, the effect is much smaller and only about 10% at
maximum.
5. Description of Results and Analysis:
-----------------------------------
Both the specific activities of different radionuclides in the
samples and the residual dose rates at various distances
were calculated with FLUKA. For the isotope production the
simulations were based on a detailed description of the
experimental setup containing the copper target, the holder with
the samples, as well as the concrete enclosure of the beam-line
shielding. According to the beam profile measurements the
beam was assumed to be rectangular with a Gaussian profile
of 2.1 cm and 2.6 cm full widths at half maximum (FWHM), in
the lateral directions. In addition, the small offset of the
beam axis with respect to the axis of the copper target was
included into the simulation. Furthermore, the elemental
compositions of the samples were considered as given in Table 2.
Residual dose rates were calculated following a two-step
approach based on specifically developed user routines. For the
first step (i.e., the calculation of isotopes), the FLUKA
implementation of the geometry of the CERF experimental area
includes all details as described above. In order to increase the
statistical significance of the results for the relatively small
samples, particle transport into the sample regions was biased
using region importance factors. Isotope information was written
into files for a total of 12 cooling times ranging from 6
minutes to 1000 hours (~42 days) and for the exact profile of
the respective irradiation considering each beam pulse and the
actual number of particles. Table 3 lists FLUKA input-files and
the files containing the irradiation profiles.
In the second step of the simulation (i.e., the calculation
of residual dose rates) the FLUKA geometry consisted only of
the respective sample surrounded by air which roughly
represents the situation during the dose rate measurements in
the laboratory. Backscattering of photons from the walls of
the laboratory (concrete) was found by MC simulation to
have only a minor influence on the dose rate results. Therefore,
the laboratory walls were neglected in the simulations as was
also the sample holder which provided only a small volume for
scattering. A dedicated simulation of the electromagnetic
cascade caused by gamma and positron emitter was performed for
each cooling time and the dose equivalent was calculated by
folding the particle fluence with appropriate fluence-to-dose
equivalent conversion factors [5]. The emission of electrons was
neglected as it was found to give only a negligible contribution
to the total dose rate.
Results of these calculations were compared to the
experimental values. As mentioned above, values are compared for
four distances between the sample surface and the centre of the
detector: contact, 12.4 cm, 22.4 cm, 32.4 cm, respectively.
Experimental and simulation results are given in the files listed
in Table 4.
6. Special Features:
----------------
None
7. Author/Organizer:
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Experiment:
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M. Brugger, S. Mayer, S. Roesler, L. Ulrici
CERN SC-RP
CH-1211 Geneva 23
Switzerland
H. Khater, A. Prinz, H. Vincke
SLAC, M.S.48
2575 Sand Hill Road
Menlo Park, CA 94025
U.S.A.
Compilation of data for SINBAD:
-------------------------------
M. Brugger, S. Roesler
CERN SC-RP
CH-1211 Geneva 23
Switzerland
Reviewer of compiled data:
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I. Kodeli
OECD/NEA, 12 bd des Iles, 92130 Issy les Moulineaux, France
8. Availability:
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Unrestricted
9. References:
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[1] A. Fasso`, A. Ferrari, J. Ranft and P.R. Sala," FLUKA: a
multi-particle transport code", CERN-2005-10 (2005),
INFN/TC_05/11, SLAC-R-773
[2] A. Fasso`, A. Ferrari, S. Roesler, P.R. Sala, G. Battistoni,
F. Cerutti, E. Gadioli, M.V. Garzelli, F. Ballarini, A.
Ottolenghi, A. Empl and J. Ranft, "The physics models of
FLUKA: status and recent developments", Computing in High
Energy and Nuclear Physics 2003 Conference (CHEP2003), La
Jolla, CA, USA, March 24-28, 2003, (paper MOMT005), eConf
C0303241 (2003), arXiv:hep-ph/0306267
[3] M. Brugger, H. Khater, S. Mayer, A. Prinz, S. Roesler, L.
Ulrici and H. Vincke, "Benchmark studies of induced radioactivity
produced in LHC materials, Part II: Remanent dose rates",
Radiation Protection Dosimetry 116 (2005) 12-15
[4] Mitaroff, A. and Silari, M. The CERN-EU high-energy
reference field (CERF) facility for dosimetry at commercial flight
altitudes and in space. Radiat. Prot. Dosim. 102, 7-22 (2002).
[5] M. Pelliccioni, "Overview of fluence-to-effective dose and
fluence-to-ambient dose equivalent conversion coefficients for
high energy radiation calculated using the FLUKA code",
Radiation Protection Dosimetry 88 (2000) 279-297
10. Data and Format:
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DETAILED FILE DESCRIPTIONS
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Filename Content
------------------------ -------------
1 cerf_dr3-a.htm This information file
2 cerf_dr3-e.htm Tables with numerical data
3 NewFullCERFGeometry.pdf Fig. 1: Experimental Geometry
4 aug03c3.pdf Fig. 2: Irradiation configuration
5 geometryDescription.htm Geometry description
6 DR-SSZ_04ICRSpencil.pdf Fig. 3: Residual dose rate from an iron sample
(pencil beam source)
7 DR-Fe_03ICRSnooff.pdf Fig. 4: Residual dose rate from an iron sample
(z-axis alligned source)
8 aug03c1offp-neweva.inp FLUKA input data for Monte Carlo Simulation
9-53 Experimental results, FLUKA input and output files listed in cerf_dr3-e.htm
54 slac-pub-11812.pdf Ref. [3]
File cerf_dr3-e.htm contains the following table:
Table 1: list of the samples.
Table 2: Material composition.
Table 3: List of FLUKA input-files and files containing the irradiation profiles.
Table 4: Experimental and calculated results.
Figures are included in the PDF formats.
SINBAD Benchmark Generation Date: 11/2008
SINBAD Benchmark Last Update: 11/2008
