Results of the 244 Cm, 246 Cm and 248 Cm neutron-induced capture cross sections measurements at EAR1 and EAR2 of the n_TOF facility

. Accurate neutron capture cross section data for minor actinides (MAs) are required to estimate the production and transmutation rates of MAs in light water reactors, critical fast reactors like Gen-IV systems, and other innovative reactor systems such as accelerator driven systems (ADS). In particular, 244 Cm, 246 Cm and 248 Cm play a role in the transport, storage and transmutation of the nuclear waste of the current nuclear reactors, due to the contribution of these isotopes to the radiotoxicity, neutron emission, and decay heat in the spent nuclear fuel. Also, capture reactions in these Cm isotopes open the path for the formation of heavier elements. In this work, the results of the capture cross section measurement on 244 Cm, 246 Cm and 248 Cm performed at the CERN n_TOF facility are presented. It is important to notice that the Cm samples used in the experiment at n_TOF have been used previously in an experiment at J-PARC, this experiment and the previous one done in the 70s with a nuclear explosion were the only previous capture experiments for these isotopes. At n_TOF, the capture cross section measurements of 244 Cm, 246 Cm and 248 Cm were performed at the 20 m vertical ﬂight path (EAR2) with three C 6 D 6 total energy detectors. In addition, the cross section of 244 Cm was measured at the 185 m ﬂight path (EAR1) with a Total Absorption Calorimeter (TAC). The combination of measurements in EAR1 and EAR2 has contributed to controlling and reducing the systematic uncertainties in the results. The compatibility of the di ﬀ erent measurements performed and the techniques to obtain the results are presented in this paper as well as the procedure to obtain the resonance parameters.


Introduction
The safe and efficient management of the high-level waste produced in the operation of nuclear reactors requires more accurate nuclear data. In particular, inventory calculations of the spent nuclear fuel (SNF) and the derived magnitudes such as the decay heat, radiotoxicity or neutron and gamma dose, among others, rely on the accuracy of neutron induced reaction cross sections ruling the burnup in the reactor. The Cm isotopes require special attention due to their various implications along the fuel cycle. For instance, 244 Cm is responsible for ∼10% of the radiotoxicity and the decay heat in the spent nuclear fuel in a Light Water Reactor (LWR) during the first fifteen years after unloading the SNF from the reactor. Furthermore, the neutron emission in the spent fuel is dominated by the 244 Cm and 246 Cm spontaneous fission during the first 10 4 years of disposal, see Figure 1. For LWR sensitivity analyses performed in [1] indicate that uncertainties in the 244 Cm capture cross section need to be reduced to 4.1% between 4 and 22.6 eV and 14.4% between 22.6 and 454 eV. Last, but not least, more accurate knowledge of the capture cross sections of 244 Cm, 246 Cm and 248 Cm (hereafter 244,246,248 Cm) is required for improving the calculations on the formation of heavier isotopes such as Bk, Cf and other Cm isotopes.
There are only three previous capture measurements of these isotopes, the one performed in 1969 using the neutrons of an underground nuclear explosion [3], and the most recent ones performed at J-PARC with the same set * e-mail: victor.alcayne@ciemat.es

The experiment
The 244,246,248 Cm cross section has been measured at the n_TOF spallation neutron-time-of-flight facility at CERN. In order to reduce the systematic uncertainties and to cross-check the results the measurements were performed in two experimental areas, with two different detectors and using two different samples. The experiment was performed in Experimental Area 1 (EAR1) located horizontally at 185 m from the target [6] and in Experimental Area 2 (EAR2) located at 21 m vertically [7].
The samples used in the experiment were provided by the Japan Atomic Energy Agency (JAEA), the same batch material has been used in the previous experiments at J-PARC [4,5]. Sample 1 contains approximately ∼1.3 mg of actinides and is prepared to measure the cross section of 244 Cm whereas sample 2 is prepared to measure the cross section of 246,248 Cm containing approximately ∼1.8 mg of actinides. The total masses of the samples are not known precisely, whereas the relative abundances are known with small uncertainties, as presented in Table 1. For this reason, the normalization has been performed to the first resonance of 240 Pu.

The measurement at EAR1
At EAR1 sample 1 was measured to obtain the resonance parameters of 244 Cm up to 100 eV [8]. The TAC detector used in the experiment consists of 40 BaF 2 crystals covering ∼95% of the solid angle [9]. In order to extract the neutron yield the following equation is used: where C is the total counting rate, B is the background counting rate, ε is the capture detection efficiency and φ n is the number of neutrons impinging in the sample per unit of time.
In order to reduce the background, the analyses were performed using coincidences between the crystals defining events. The cuts applied in the events are deposited energy sum 2.5 < E sum (MeV) < 6 and the number of crystals recording signals in the event higher than 2. The efficiency to detect the cascades with these analyses cuts are 0.589 ± 0.07 for 240 Pu and 0.588 ± 0.07 for 244 Cm, these values were determined with the cascades obtained with NuDEX and Geant4 simulations [10][11][12]. The uncertainty obtained in the normalization to the first resonance of 240 Pu considering also the uncertainties in the abundances is 3.3%. Also, the uncertainties in the neutron fluence and the background subtraction were considered in the yield determination.

The measurements at EAR2
At EAR2 samples 1 and 2 were measured to obtain the resonance parameters from 244,246,248 Cm. Three BICRON C 6 D 6 detectors [13] were placed at 5 cm from the sample, see Figure 3. The yields were obtained with the To-  [14]. A weighting function is used to weight each detected count with an energy (pulse height) factor to fulfil the necessary conditions of the TED technique. The equation to obtain the yields with these techniques is: where C w is the total weighted counting rate, B w is the background weighted counting rate, S n,i is the neutron separation energy of the isotope i, F PHWT,i is the factor for correcting the deviations from the ideal situation with the PHWT, and φ n is the number of neutrons impinging in the sample per unit time. In order to reduce the uncertainties, the unweighted yield would be used normalized to the weighted one using the technique described in reference [15]. The F PHWT,i factors are determined with Monte Carlo simulations using the γ-ray cascades fitted with NuDEX. The factors consider the counts lost below the detection threshold of 0.12 MeV, the effect of detecting various γrays of the cascade or the same γ-ray in various detectors and the γ-ray summing effect. In Table 2 the correction factors for each isotope are presented. The uncertainties in these factors, the normalization, the neutron fluence and the background subtraction were considered in the determination of the yields.

Resonance analysis of the capture yields
The three capture yields obtained in EAR1 and EAR2 with the two samples have been analysed to obtain the Resonance Parameters (RP) of the 244,246,248 Cm and 240 Pu isotopes in the Resolved Resonance Region (RRR). The resonances have been fitted with SAMMY [16], a code widely used in the nuclear data community, applying the R-matrix formalism and the Reich-Moore approximation [17]. The different experimental effects consider in the SAMMY analysis are: • The Doppler broadening is caused by the thermal motion of the target nuclei. In this experiment, the free gas model has been used.
• The multiple scattering effects, which take into account that the neutrons can be captured after one or more elastic scatterings. SAMMY calculates this effect by performing dedicated Monte Carlo simulations.
• The resolution broadening is caused by the different TOF of the neutrons of the same energy arriving at the sample. The time-energy distribution of the neutrons is given by the resolution function (RF). In EAR1 the standard RF was taken [6], whereas for EAR2 it was necessary to calculate a particular RF for the experiment fitting the resonances of 197 Au [18].
The E n and Γ n parameters of 244,246,248 Cm and 240 Pu has been obtained for different energy regions. The rest of the RP (Γ f , Γ γ and spin) are taken from the JENDL-4 library [19]. As an example in Figures 4 and 5 the fits performed for the first resonances of each isotope are presented. The energy ranges analysed for each isotope are presented in Table 3. For the first time at n_TOF a capture measurement has been performed at EAR1 and EAR2 with different detection setups, as presented in Figure 6. The results

Summary and conclusions
This work presents a series of measurements of the capture cross section of 244,246,246 Cm from 1 to 400 eV using two complementary experimental areas of the n_TOF facility and different samples to improve the present evaluations. The experiment was the first capture measurement at n_TOF performed in the two experimental areas, also for the experiments, two different setups were used (TAC and C 6 D 6 detectors). The efficiencies and the different corrections needed were obtained considering the cascades ob- tained with NuDEX, these accurate work leads to small uncertainties in the absolute yields. The results obtained in the two areas with different detectors are compatible. The work that is already ongoing is to combine the information of the various measurements to obtain the final RP with their corresponding uncertainties, these results will be published in dedicated papers and sent to the EXFOR database.