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SINBAD ABSTRACT NEA-1552/26

CERF Residual Dose Rates (2003)



 1. Name of Experiment:
    ------------------
    BENCHMARK STUDY OF RESIDUAL DOSE RATES WITH FLUKA

 2. Purpose and Phenomena Tested:
    ----------------------------
    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 ÁSv/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:
    ----------------

    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

 8. Availability:
    ------------
    Unrestricted


 9. References:
    ----------

    [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:
    ---------------

    DETAILED FILE DESCRIPTIONS
    --------------------------
     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