SINBAD ABSTRACT NEA-1517/88
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
losses.
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:
L(E)=4*pi*N(E)/(eps(E)*dOmega*Nn)
where:
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
etc.
5. Special Features:
-----------------
None
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:
-------------
Unrestricted
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
