Measurement of the 232Th capture cross section in the energy region 5 keV – 150 keV
G. Lobo, P. Schillebeeckx, A. Brusegan, A. Borella, F. Corvi
EC-JRC-IRMM, 2440 Geel, Belgium
N. Janeva and K. Volev
INRNE, 1784 Sofia, Bulgaria
The 232Th(n,) neutron capture cross-section is of great importance for accelerator driven reactor
(ADS) systems based on the Thorium-Uranium fuel cycle. An analysis of the required nuclear
data1, reveals that the status of the 232Th capture data is far from the requested 2 % uncertainty
level. Recently 232Th average capture measurements, between 5-200 keV neutron energy, were
performed at the FzK Karlsruhe (G) 2. A comparison of the measured averaged capture cross
section with the evaluated data files shows a reasonable agreement in the neutron energy range
above 15 keV. However, discrepancies of up to 40 % at lower neutron energies are observed.
The same order of discrepancies is observed when comparing their results with the results
obtained by Macklin et al.3,4 at ORELA. To clarify these discrepancies we measured at IRMM
the average capture cross-section at the GEel LINear Accelerator (GELINA).
The measurements were performed at a 14.37 m flight-path using the Time-Of-Flight (TOF)
method. The gamma rays, originating from the 232Th(n,) reaction, were detected by a pair of
C6D6-based liquid scintillators applying a pulse-height weighting method. The neutron flux was
measured with an ionisation chamber placed at 80 cm before the Thorium sample. This chamber
has a cathode loaded with two back-to-back layers of about 40 g/cm2 10B. The sample consisted
of a metallic natural thorium disc of 8 cm diameter and 0.5 mm thick, corresponding to a
thickness of 1.588 10-3 at/b.
The background for the capture measurements consists of a time independent and time
dependent component. The former, mainly produced by the radioactive decay of the sample, was
deduced from measurements with a closed beam. The latter was measured by replacing the
thorium sample with a 0.5 mm thick 208Pb sample of the same size. Such a Pb sample has
practically the same scattering probability as the thorium sample and has a negligible capture
yield. Therefore, the 208Pb run provides a good estimate of both the so-called “open beam”
background and of the contribution due to scattered neutrons.
The normalisation constant was determined from a resonance shape analysis of the well-isolated
and nearly saturated resonances at 21.8 eV and 23.5 eV, with a peak transmission of respectively
4.7% and 0.9%. To estimate the systematic uncertainty related to the normalisation procedure,
the experimental data were fitted in different energy regions, using resonance parameters from
several evaluation data file. The final normalisation and energy calibration will be obtained with
resonance parameters resulting from recent transmission measurements. As discussed in Ref.5
we used the SESH code6 to correct for self-shielding and multiple scattering effects was .
The prelimanary capture cross-section values are plotted in fig.1, together with the ENDF-B VI
values and the experimental data obtained by Wisshak et al.2, Macklin et al.3,4 and Karamanis et
al.7. Our data in the 5-100 keV region, agree within a 7 % systematic uncertainty with the data
obtained by Macklin et al.3,4. We do not confirm the large discrepancies at lower neutron
energies reported by Wisshak et al.2. Our data between 5 –80 keV are systematically 10% higher
compared to the evaluated data. In the 80-100 keV region the differences are much smaller. To
1
confirm the present data an additional measurement campaign, including the measurement of the
Au(n,) cross-section, was performed. The analysis of this data is in progress.
232
Th(n,)
1500
Present work
2
Wisshak
3,4
Macklin
7
1000 Karamanis
ENDF/B-VI
(n,) / mb
500
0
4 5
10 10
Energy / eV
Fig.1 A comparison of the average cross-section obtained at the IRMM with the ENDF/B-VI
evaluation, the data from Wishak et al.2, Macklin et al.3,4, and Karamanis et al.7.
References
1 B.D. Kuzminov, V.N. Manokhin, “Analysis of nuclear data for the thorium fuel cycle”, Proc.
Int. Conf. Nuclear Data for Science and Technology, Trieste, Italy, 19-24 May 1997, Part. II,
p.1167, (1997).
2 K. Wisshak, F. Voss, and F. Käppeler, “Neutron capture cross section of 232Th”, Nucl. Sci.
Eng., 137, 183 (2001)
3 R.L. Macklin and J. Halperin, “232Th (n,) cross sections from 2.6 to 800 keV”, Nucl. Sci.
Eng., 64, 849 (1977).
4 R.L. Macklin and R.R. Winters, “Stable isotope capture cross sections from the Oak Ridge
Electron Linear Accelerator”, Nucl. Sci. Eng., 78, 110 (1981).
5 F.H. Froehner, Report GA-8380, Gulf General Atomic (1968)
6 A. Lukyanov, N. Koyumdjieva, N. Janeva, K. Volev, A. Brusegan, P. Schillebeeckx, G. Lobo
and F. Corvi, “Neutron Capture of 232Th in The Unresolved Resonance Region – Data
Corrections and Analysis “ Nuclear Mathematical and Computational Sciences: A Century in
Review, Anew, Gatlinburg, Tennesse, April 6-11,2003
7 D. Karamanis et al. , Nucl. Sci. & Eng, 139, 282 (2001)
2