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IPPE neutron transmission through thorium shell

1. Name of Experiment:
   IPPE neutron transmission benchmark experiment with 14 MeV and Cf-252 
   fission neutrons through thorium shell.

2. Purpose and Phenomena Tested
   Neutron leakage spectra between 0.2 MeV and 15 MeV from thorium shell
   were measured by the time-of-flight technique using a 14 MeV neutron
   generator and Cf-252 fission ionization chamber. 
   The sphere had outer radius 13.0 cm, inner radius 3.0 cm, wall
   thicknesses 10.0 cm.
   The experiments were performed in the period from 1987 to 1992.

3. Description of Experimental Set-up and Measuring Procedure
3.1. Th shell specification
   Th spherical shell of pure thorium had outer radius 13.0 cm, inner
   radius 3.0 cm, and cylinder hole with radius 2.5 cm to accomadate either
   14 MeV or Cf-252 neutron sources. The weight of the sphere was measured
   and atom density was assessed as 0.0293E+24 atoms/cc. The material
   composition was 100% Th.

3.2. Experiment with 14 MeV neutron source
   A Cockroft-Walton type accelerator, the KG-0.3 pulse neutron generator
   in Obninsk, was used to accelerate deuterons to a kinetic energy of
   280 keV. The layout of the experiment is shown in Figure 1.

   The deuterons were led through a conical aluminium tube of only 0.5 mm
   wall thickness and collimated by a diaphragm with an 8 mm hole to a solid
   Titanium-Tritium target backed by a copper radiator 0.8 mm thick and with
   diameter 11 mm. Beam spot diameter was 5 mm.

   The center of the target is located in the geometrical center of the thorium
   shell. For monitoring the neutron source strength, the alfa particles
   generated in the deuterium-tritium reaction were detected at 175 deg. through
   a 1 mm diameter collimator by a silicon surface barrier (SSB) detector.

   The ion pulse width is 2.5 ns. The repetition period of the pulse can be set
   arbitrarily to multiples of 200 ns. The mean beam current for 800 ns period
   was 1 microampere.

   The mean energy and yield of the '14 MeV' neutron source peak are slightly
   angle dependent, as shown in Figure 2.

   The detector used was a fast scintillator detector located at an angle of 8
   deg. relative to beam trajectory extension and at a flight path of 682.5 cm.
   The detector was installed in a lead house behind a concrete wall. A conical
   hole drilled through the wall acted as a collimator (see Figure 1).

   The detector itself consisted of a cylindrical paraterphenyl cristal of 5 cm
   diameter and 5 cm height. It was coupled to a FEU-143 photomultiplier.

   The time-of-flight measurement is made in the usual inverse method, i.e.
   using the detector signal as a start signal and the delayed deuteron pick-up
   pulse as a stop signal. In this way only the useful neutron bursts, i.e.
   those producing a signal in the detector are used, so avoiding dead time

   The experimental spectra were corrected for the background effects. To 
   measure the background neutron spectra, a 1 m long by 18 - 26 cm diameter
   iron shadow bar and a 30 cm long borated polyethylene cylinder were placed
   between the detector and the sphere (Figure 1).

3.3. Experiment with Cf-252 neutron source

   The neutron leakage with a Cf-252 neutron source has been measured by 
   time-of-flight method using a fast ionization chamber (Figure 3). The latter
   had 34 mm diameter and 120 mm length and was filled with Ar-CO2 gas. The one
   disc electrode had Cf-252 layer with intensity of 4E+5 n/s. 
   The output of the chamber, with discrimination against pulses from alpha-
   particles, supplied the stop pulses for the TOF measurement as well as the
   total number of disintegrations during the experiment. 

   The scintillator detector (dia.6.3 cm by 5.0 cm height stilben crystal +
   photomultiplier FEU-30) was located at 385.5 cm flight path from the sphere
   center (Figure 3). The angle between the axis of the hole in the sphere and
   the direction to the detector was 135 deg. to reduce the influence of the
   streaming of source neutrons through the hole. 
   For background measurements an iron shadow bar was installed between
   detector and sphere. The efficiency of the detector was measured
   employing the same Cf-252 neutron source by removing the thorium sphere.
   This results in elimination of part of the experimental uncertainties. 

3.4 Uncertainties
   The estimated uncertainties of the experimental data and their main
   components are listed in Table 1. During the experiment the main
   spectrometer parameters (detector efficiency, absolute normalization
   factor, etc.) were measured several times, hence the stability of the
   spectrometer could be estimated by calculating the mean square deviation
   of individual runs.

   Two radioactive reference sources were used, Cf-252 for neutron detector
   calibration and Pu238 for alfa detector calibration, with their specific
   uncertainties. The uncertainties of corrections for Cf-chamber scattering
   and time-of-flight conversion to energy, calculated with MCNP, were
   estimated at about 1-2%.

   In Table 1 the quadratic sum of components 2-5, considered as systematic,
   is calculated and its quadratic summation with the statistical uncertainty
   gives the total uncertainty of the experimental data.

4. Description of Results and Analysis:
   The measured TOF spectra were corrected for the background effects and
   converted to the energy spectrum. The leakage spectrum, L(E), representing
   the differential fluence of leakage neutrons, integrated over the full
   sphere (4 pi sr) and normalised to 1 source neutron, was then calculated
   from the following expression:


      N(E)  = neutron energy spectrum, converted from measured TOF spectra,
     eps(E) = neutron detection efficiency,
     dOmega = detector solid angle (=(pi*r*r)/(L*L), where r is the detector
              radius and L the distance from the sphere to the detector),
       Nn   = number of source neutrons. 
   The experimental results are presented in:
   Table 2 for thorium shell with 14 Mev neutron source, 
   Table 3 for thorium shell with Cf-252 neutron source.

   as leakage spectrum in terms of neutrons per MeV and per source neutron. 

   MCNP-4C input data for Th shell are given in files mcnp_th_14.inp (14 MeV
   source) and mcnp_th_Cf.inp (Cf-252 source). In the models the spheres
   and the neutron source are described precisely, including anisotropic
   energy and yield distributions of the T(d,n) source and energy
   distribution of Cf-252 source.

   For an adequate comparison of measurements and analytical calculation,
   the convolution with the spectrometer response function, describing the
   energy resolution of the spectrometer, is necessary. It is presented in
   Table 4 and in DetResponse.jpg.
   It corresponds to the neutron spectra measured without the Th splere.

   Further details on experiment and data analysis could found in Refs. [1],
   [2] and [3]. Ref. [4] discusses in details the corrections for time-
   of-flight measurements with bulk spherical samples and non-spherical
   effects, which should be taken into account in case of 1-dimensional
   (spherical) calculations using codes like ANISN, ONEDANT, ANTRA-1

5. Special Features:
6. Author/Organizer:
   Experiment and Analysis:
   S.P. Simakov, B.V. Devkin, M.G. Kobozev, V.A. Talalaev (Inst. of Physics
   and Power Engineering, Obninsk)

   Reviewer of Compiled Data
   I. Kodeli
   OECD/NEA, 12 bd. des Iles,
   92130 Issy les Moulineaux, France

7. Availability:

8. References:
[1]  S.P. Simakov, A.A. Androsenko, P.A. Androsenko, S.I. Dubrovina,
     B.V. Devkin, M.G. Kobozev, A.A. Lychagin, V.A. Talalaev, D.Yu. Chuvilin,
     V.A. Zagraydsij, “Neutron leakage spectra from Be, Al, Fe, Ni, Pb, LiPb,
     Bi, U and Th spheres with T(d,n) and 252Cf neutron sources”,
    (SOFT-17, Rome, Sept. 1992), Fusion Technology, Elsevier, 1993, v. 2,
     p. 1489
[2]  S.P. Simakov, A.A. Androsenko, P.A. Androsenko, B.V. Devkin, B.V. Zhuravlev,
     M.G. Kobozev, V.A. Zagraydsij, D.V. Markovskij, D.Yu. Chuvilin,
     "Neutron leakage spectra from U and Th spheres with Cf neutron source",
     Report FZK-646, Dresden, 1988, p. 111. 
[3]  S.P. Simakov, B.V. Devkin, M.G. Kobozev, A.A. Lychagin, V.A Talalaev,
     A.A. Androsenko, “14 MeV Facility and Research in IPPE”,
     Report INDC(CCP)-351, IAEA, Vienna, 1993;
     Voprocy Atomnoy Nauki i Tehniki, Seriya Yadernye Konstanty, Obninsk,
     1997, no. 3-4, p. 93.
[4]  B.V. Devkin, M. G. Kobozev, S.P. Simakov, U. Fischer, F. Kappler
     U. von Möllendorff, “Evaluation of Corrections for Spherical-Shell
     Neutron Transmission Experiments by the Monte-Carlo Technique”,
     Report FZKA 5862, Karlsruhe, 1996;
     Voprocy Atomnoy Nauki i Tehniki, Seriya Yadernye Konstanty, Obninsk,
     1997, no. 1-2, p. 38.

9. Data and Format:
    No. Filename       Size(KB) Content
    --  --------       -------- -------
    1  ippe_th-a.htm      12    This information file
    2  ippe_th-e.htm      11    Experiment Description
    3  mcnp_th_14.inp      7    MCNP-4C input for Th sphere with 14 MeV neutron source
    4  mcnp_th_Cf.inp     13    MCNP-4C input for Th sphere with Cf-252 neutron source
    5  ippe_th_fig1.jpg   35    Fig. 1: Experimental setup  with 14 MeV source
    6  ippe_th_fig2.pdf    9    Fig. 2: Angular/energy distribution of '14 MeV' source peak
    7  ippe_th_fig3.jpg   32    Fig. 3: Experimental setup  with Cf-252 source
    8  DetResponse.jpg    20    Fig. 4: Detector Experimental setup  with Cf-252 source
    9  soft-17.pdf       659    SOFT-17 Paper (Ref. 1)

SINBAD Benchmark Generation Date: 12/2005
SINBAD Benchmark Last Update: 3/2006