Use of nickel sphere and copper cube with 252 Cf neutron source in the centre for test of nuclear data libraries ENDF/B-VII.1, ENDF/B-VIII.0, JEFF-3.3, BROND-3.1

. The leakage neutron spectrum measurements have been done on benchmark spherical assembly-nickel sphere with a diameter of 50 cm and a copper cube (block) with dimensions of 49.5 x 49.5 x 48 cm 3 in Research Centre Rez (RC Rez). The 252 Cf neutron source was placed into the centre of nickel sphere and copper cube. The proton recoil method was used for the neutron spectrum measurement using spherical hydrogen proportional detectors (HPD) with pressure of 400 and 1000 kPa (diameter of detectors is 4 cm) and scintillation stilbene (ST) detector (diam. of 1 x 1 cm). The neutron energy range of spectrometer is from 0.04 MeV to 1.3 MeV for HPD and from 1 MeV to 12 MeV for ST. The adequate MCNP neutron spectrum calculations based on data libraries ENDF/B-VII.1, ENDF/B-VIII.0, JEFF-3.3, BROND-3.1 were done and compared with the experiment, i.e., calculation to experiment ratio C/E was determined.

Secondarily, there is also an interest in the following elements (isotopes): D, Li, Be, B, C, Na, Cr, Ni, Mo, 240,241 Pu and 241 Am.
Neutron nuclear data libraries for transport were tested in RC Rez for these elements: Fe, O, H, D, Ni and Na as well as benchmarks. Sodium fast reactor mock-up, i.e., full scale, was studied in cooperation with FEI Obninsk (Russia) [2][3][4]. Materials that are of current interest are: Cu, Ni and stainless steel (SS).

Experimental setup
The following two assemblies were used for measurement and calculation: 1) Copper cube with dimensions of 49.5 x 49.5 x 48 cm 3 and 2) Nickel sphere with diameter of 50 cm, see Fig.1 and Fig.2. The 252 Cf neutron source was placed into the centre of copper block and nickel sphere, see Fig.3 and Fig.4. The proton recoil method was used for neutron leakage spectrum measurement using spherical hydrogen proportional detectors (HPD) with pressure of 400 (K4) and 1000 kPa (K8) with diameter of 4 cm [5] and scintillation stilbene (ST) detector with dimensions of 1x1 cm [4], see The neutron energy range of spectrometer is from 0.04 MeV to 1.3 MeV for HPD and from 1 MeV to 11 MeV for stilbene. At first, the leakage neutron spectrum was measured together with background. Then, the background itself was measured with a shielding cone placed between the sphere and detector, see the Fig.1 and Fig.2. To get "pure" leakage spectrum, the measured background was subtracted from the first measurement.

Spectrometry
The detector K4 with pressure 400 kPa was used for measurement in the energy range En = 0.01-0.7 MeV, the detector K8 with pressure 1000 kPa was used for measurement in the energy range En = 0.2-1.3 MeV. A stilbene spectrometer was used for measurement in the range En = 1-10 MeV. The detector photos are in Figure 5. The spectra measured and calculated with HPD were evaluated in two group structures: 40 gpd (groups per decade), 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 "Btype" that includes influence of apparatus instability, of the benchmark geometry and the detector position, the neutron source position, the detector discharges, the energy calibration, during repeated measurements distant in time.
The "A-type" uncertainties of the integral values presented in tables 1-3 are in the interval from 0.3 % to 6 %. The "B-type" uncertainty is estimated from 2 % to 4 %. Statistical uncertainties ("A-type") of MCNP calculations are better than 1 % in energy interval from 0.1 MeV to 1.3 MeV and better than 5 % in energy interval from 1 to 5 MeV.

Spectrum processing
The results of spectrum calculation and measurement of the neutron flux density φn(E), i.e., the "Flux" in graphs, are normalised in the following way: where R [cm] is the distance between detector and neutron source (centre to centre) and Q [1/s] is the neutron source emission. The integral values presented in tables 1 and 2 are also with the dimension of 1.
To obtain the calculated spectrum in the form "how a spectrometer (HPD) can see it", the calculated spectra were smoothed by Gaussian with the constant percentage resolution Δ (FWHM):

Results
The experimental (E) and calculated (C) neutron spectra were compared, and the C/E ratio determined. The results are: 1) For Cu -in Figures 6 and 7

Conclusions
The conclusions are listed in several points below: 1) Tab.1 (Cu) and Tab.2 (Ni): The colour classification (namely red and dark blue cells in tables) of the results says at first glance that the overall agreement between the measurement and the calculation is better for nickel than for copper This process was used in laboratory also for evaluation of bare 252 Cf and Fe sphere with a diameter of 50 cm and other assemblies.
Then, the I(1E-6 MeV,10 MeV), calculated integral over total energy range for mentioned experimental assemblies is following: The total integral values for Cu and Ni are more or less close to 1.
Therefore, the data in tables 1 and 2 more or less indicate the proportion of the energy group in the entire spectrum.
This process cannot be applied for materials with higher neutron absorption, e.g., water, because total integral is much less than 1.