CERF RADIONUCLIDE PRODUCTION (~2003)

SINBAD ABSTRACT NEA-1552/27

CERF Radionuclide Production (~2003)

1. Name of Experiment:
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BENCHMARK STUDY OF RADIONUCLIDE PRODUCTION WITH FLUKA

2. Purpose and Phenomena Tested:
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The activation of beamline components at high-energy accelerators, such as the Large Hadron Collider (LHC),
is of concern with regard to radiation safety, not only during the operation of the machine and possible
maintenance but also later for decommissioning and final disposal of the activated materials.
In all cases accurate calculations of the radionuclide inventory are required which are typically performed
with modern particle interaction and transport codes, in case of the LHC mainly with the Monte Carlo code FLUKA [1, 2].

In contrast to integral quantities, such as the dose equivalent to personnel by prompt radiation or the
dose to components, predictions for the production of individual isotopes can be much less reliable as they depend
strongly on details of the models implemented in the Monte Carlo (MC) code. Thus, benchmark experiments are of
utmost importance in order to verify the accuracy and limits of applicability of these models.
Unfortunately, activation experiments are not trivial and are prone to uncertainties of various natures.

Samples of different materials to be used for machine and shielding components of the Large Hadron Collider
were irradiated in a high-energy stray radiation field. The materials included steel, copper, titanium,
concrete as well as light materials such as carbon composites and boron nitride. The induced specific activities
of radionuclides, as measured with gamma spectrometry at different cooling times (from about twenty minutes to two months),
then served as benchmark of the FLUKA Monte Carlo code.

The study aimed at high accuracy in all aspects of measurements and simulations, including elemental analyses of
the irradiated materials with different methods, detailed revision of the spectrometry results as well as low
statistical uncertainties in the predictions of FLUKA. Furthermore, the experiment allowed for an accurate recording
of the irradiation conditions, such as irradiation profile and intensity.

The presented data is published in Ref. [3] and references listed therein.

3. Description of the 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.

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 6.7 x 10^10 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
------------------------------------
The specific activities of the irradiated samples were measured at different cooling times ranging from about 20 minutes
to two months. The gamma spectrometry measurements were performed with a high-sensitivity, low-background High-Purity Germanium (HPGe)
detector by Canberra (245 cm3 sensitive volume, 60% efficiency at 1.33 MeV). The data acquisition and analysis was carried
out using the software GENIE 2000 (version 2.1) by Canberra and the PROCOUNT-2000 counting procedure software. This system
was complemented with LABSOCS (Laboratory SOurceless Calibration Software, version 4.1.1), a mathematical efficiency calibration
software by Canberra taking into account geometrical effects and also correcting for self-absorption in the samples.

The samples were positioned on a custom-made sample holder at different reproducible distances from the detector. The distance
for each sample was chosen on the basis of its remanent dose rate and on the respective dead time of the measuring system.
For each distance the efficiency of the detector was calculated using LABSOCS.

The analysis performed by the GENIE 2000 software includes advanced analysis algorithms for nuclide identification,
interference correction (resolution of overlapping peaks into individual components), calculation of specific activities,
background subtraction and efficiency correction. The nuclide identification within GENIE 2000 is based on standard or
user -specific libraries. The latter were created for each sample material taking into account the chemical composition,
the possible activation reaction channels as well as the cooling time. All results of the analyses were revised manually since,
for some radionuclides (e.g., in case of interference in the gamma energies of different nuclides), the semi-manual calculation
of the specific activity turned out to be the most accurate method.

Stated uncertainties are calculated by the analysis software based on the shape of the peak in the spectra, the respective
signal to background ratio and furthermore includes parameters based on the efficiency curve of the detector, the correction
factor coming from LABSOCS as well as possible uncertainties coming from stated half-lives and branching ratios of mother-daughter chains.

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
-----------------------------------
A total of 9 different materials used in the construction of the LHC were selected: aluminum, steel, iron, copper, titanium,
concrete, carbon composite, boron nitride and resin. Of these materials, 19 samples (see Table 1)
were machined to a size of about 2x2x2 cm3, except for the concrete sample, which was powdered and filled into boxes of
about 4 cm length and diameter. The elemental compositions of the materials were analysed by a number of different outside
companies and institutes using a variety of techniques. The materials discussed in this paper, their elemental composition
as well as their densities are summarized inTable 2.

Specific activities of different radionuclides in the samples 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 detailed elemental compositions of the samples were considered as given in Table 2.
The full hadronic cascade was simulated in the target, in the samples and in the beamline enclosure. Neutrons were
transported down to thermal energies; all other hadrons were transported until stopped. The electromagnetic cascade was not
simulated as activation by photonuclear interactions can be neglected in hadron-induced cascades. FLUKA input file names
are listed in Table 3 for the different configurations. Separate simulations
were performed for proton and pion beam particles and their results were combined in a post-processing step according
to the actual beam composition. The total yield of all produced radionuclides was scored separately for all samples
and the results were written into output files. These data files were then post-processed, and specific activities were
calculated taking into account the decay chains and build-up of isotopes, as well as the correct intensity profile of the
respective irradiation experiment (see Table 3 for the file names of the irradiation profiles).

In Tables 4-12the calculated and measured specific activities
are compared for each material. All cooling times (see Table 1) refer to the beginning of the respective gamma
spectrometry measurement. The first gamma spectrometry measurement was typically performed after a cooling
time of between 20 minutes and one hour. This allowed for the identification of isotopes with half-lives of less than one hour.
Tables 4-12 show the experimentally measured specific activities in Becquerel per gram, together with their corresponding
errors in percent, as well as the ratio between the simulated and experimental results. In addition to the ratios of calculated
to measured specific activities, the ratios of measured specific activities to the so -called "Minimal Detectable Activity" (MDA)
of the gamma spectrometry measurement are listed. Only those isotopes are given which were identified by both the experiment
and the simulation, as well as where the respective measured activity was above the given MDA value.

For the benchmark experiment, many isotopes were detected at different cooling times and their specific activities were determined.
However, for the final comparison with FLUKA predictions, only one measurement result was selected for each nuclide based
on the following criteria: smallest experimental uncertainty, largest ratio between the measured specific activity and
the respective MDA, as well as the appropriate cooling time as compared to the half-life of the respective isotope.
The experimental errors contain both statistical and systematic uncertainties of the spectrometry analysis. For the calculated
values the errors represent statistical uncertainties only. Isotopes predicted by FLUKA with an uncertainty larger than 20 % are
not listed. Uncertainties in the half-lives used to follow the radioactive decay chains were found to have a negligible
influence on the results. The errors of the ratios of calculated to measured specific activities were obtained by summing up
the relative errors of the latter.

6. Special Features:
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None

7. Author/Organizer:
----------------

Experiment:
-----------
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:
--------------------------
I. Kodeli
OECD/NEA, 12 bd des Iles, 92130 Issy les Moulineaux, France
e-mail: ivo.kodeli@oecd.org

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, A. Ferrari, S. Roesler and L. Ulrici, "Validation of the FLUKA Monte Carlo
code for predicting induced radioactivity at high-energy accelerators", Proceedings of the 7th International
Conference on Accelerator Applications, Venice, Italy (2005). Nuclear Instruments and Methods A 562, 814-818 (2006).

[4] A. Mitaroff and M. Silari, "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. Brugger, H. Khater, S. Mayer, A. Prinz, S. Roesler, L. Ulrici and H. Vincke,
"Benchmark studies of induced radioactivity produced in LHC materials, Part I: Specific Activities",
Radiation Protection Dosimetry 116 (2005)

10. Data and Format:
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DETAILED FILE DESCRIPTIONS
--------------------------
Filename Content
------------------------ -------------
1 cerf_ac5-a.htm This information file
2 cerf_ac5-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
9-29 FLUKA input files listed in cerf_ac5-e.htm
30 RefAC.pdf Ref. [3]
31 slac-pub-11811.pdf Ref. [5]

File cerf_ac5-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 (Copper).
Table 5: Experimental and calculated results (Iron).
Table 6: Experimental and calculated results (Titanium).
Table 7: Experimental and calculated results (Stainless Steel).
Table 8: Experimental and calculated results (Aluminum).
Table 9: Experimental and calculated results (Concrete).
Table 10: Experimental and calculated results (Carbon Composite).
Table 11: Experimental and calculated results (Boron-Nitride).
Table 12: Experimental and calculated results (Resin).

SINBAD Benchmark Generation Date: 11/2008
SINBAD Benchmark Last Update: 11/2008