Analysis of cross sections on iron and oxygen using Cf-252 neutron source

The leakage neutron spectra measurements have been done on benchmark spherical assemblies with Cf-252 source in center of 1) heavy water sphere with diameter of 30 cm (with Cd cover) and of 2) iron spheres with diameter of 100 cm and 50 cm. It has been stated for years that transport calculations by iron overestimate measured spectra in energy region around 300 keV by about 20-40 % (calculation to measurement ratio C/E = 1.2-1.4). The influence of an artificial changes in cross-section XS-Fe-56 (n,elastic)designed by IAEA, Nuclear Data Section, has been studied on the iron spheres. Influence of those XS-corrections to calculated neutron spectrum is presented.


Introduction
Validation of 56 Fe and 16 O cross section has been performed in Research Centre Rez (RC) in accordance with CIELO [1] project requirements. It has been stated for years that calculations for iron overestimate measured spectra in energy region around 300 keV by about 20-40 %, i.e., determination of calculation to experiment ratio C/E = 1.2- 1.4 and also around 600 keV by about 12-15 %. The C/E around 300 keV grows with iron thickness, B. Jansky [2], [3]. R [4]) to test influence of such change.

Experimental setup
Determination of C/E ratio of leakage neutron spectra from iron and heavy water spheres with Cf-252 neutron source in center was done for 4 libraries IND-R22, IND-R34, CIELO and JEFF 3.2T2 for 56 Fe and CIELO for 16 O. The following assemblies were used for measurement and calculation: 1) heavy water sphere with diameter of 30 cm (with Cd cover) and 2) iron spheres with diameter of 100 cm and 50 cm. Each assembly had certain acronym, e.g., FE100R150 is Fe assembly with the diameter of 100 cm and the distance of 150 cm between sphere and detector (center to center). At first, the leakage neutron (and gamma) spectrum from spherical assemblies with Cf-252 in center was measured together with background. Then, the background itself was measured with shielding cone placed between the sphere and detector. To get "pure" leakage spectrum, the measured background was subtracted from the first measurement. * e-mail: bohumil.jansky@cvrez.cz

Calculation
The calculations were performed using Monte-Carlo code MCNP with nuclear data library CIELO (i.e., ENDF/B-VIII.0) and JEFF-3.2T2. As for calculation geometry description, a simplified model was used, which substitutes assembly elements with concentric spherical shells around the source. Also, the MCNP model of detector is represented by a 1 cm thick spherical shell with radius equal to the real distance R of detector and source. For each calculation 10 8 -10 9 particle histories were computed.

Spectrometry
Two hydrogen proportional spherical detectors (HPD) K4 and K8 with diameter of 40 mm were used in neutron spectrometer [5]. The detector K4 with pressure 400 kPa was used for measurement in the energy range E n = 0.01-0.7 MeV, the detector K8 with pressure 1000 kPa was used for measurement in the energy range E n = 0.2 -1.3 MeV, see Figure 1. Stilbene spectrometer was used for measurement in the range E n = 1-10 MeV. The HPD measured and calculated spectra were evaluated in two group structures: 40 gpd, it corresponds to the lethargy step about 6 % and in structure 200 gpd, i.e., with lethargy step 1 %. The common energy structure for stilbene had step of 100 keV.

Uncertainties
Uncertainty of single measurement is composed of uncertainty of the "A-type" that includes statistical uncertainty in measurement (in channel) and consequent calculation of each energy group and uncertainty of "B-type" that includes influence of apparatus instability, of benchmark geometry and detector position, neutron source position, detector discharges, energy calibration, during time remote    4

Influence of spectrometer resolution on spectra assessment
Measurements and calculations use 200 gpd energy structure, i.e., lethargy step is about 1 %. Spectrometer with HPD has relatively good resolution, because in the energy interval 200-400 keV 4-6 peaks are visible: 218, 242, 272, 309, 352 and 375 keV. See Figure 5. Other spectrometers that use stilbene or TOF method resolve usually only one "thick" peak at 300 keV in the region 200-400 keV. Results are in Figure 4 and Figure 5. Correction of 56 Fe(n,tot) cross section is in the Figure 8, where CIELO is compared with IND-R34 data version.