Absolute cross section measurements of 238U(n,f) and 237Np(n,f) in the neutron energy range 1-2.4 MeV

Abstract. New standard (n,f) cross sections other than 235U are important to study the relevant cross sections for Generation-IV power plants. A specific need for such standards is for performing new experiments with quasimonoenergetic neutron beams, such as those produced by Van de Graaf accelerators. Neutrons down-scattered to low energies in the experimental environment, so called room-return, become relevant for this type of measurements. Hence, a standard (n,f) cross section with a fission threshold is of great interest, in order to suppress the contribution from room-return background. For this reason we have performed two experiments at the VDG of the National Physical Laboratory to measure absolutely the (n,f) cross sections of 235U, 238U and 237Np in the fast neutron energy region. Our preliminary results are in agreement with the most up-to-date evaluations.


Introduction
Modelling of Generation-IV nuclear power plants requires highly accurate values of cross sections in the fast energy region. Cross section measurements are usually performed relative to the primary standard 235 U(n,f), but in environments where the thermal and epi-thermal neutron background is non-negligible, such as the target hall of a Van de Graaff accelerator, other isotopes with a fission threshold should be preferred. Up to now, none of the isotopes with a fission threshold has been considered a primary standard. Two isotopes, for which the experimental database is sufficient, could potentially fill this role: 238 U(n,f) and 237 Np(n,f). 238 U(n,f) has a fission threshold at around E n = 1.6 MeV and is a secondary standard from E n = 2 MeV. Although the JEFF 3.2 evaluation showed discrepancies up to 7% in the range 1.5 MeV < E n < 5 MeV with respect to ENDF/B-VIII.0, the new JEFF 3.3 evaluation is in agreement with ENDF/B-VIII.0. 237 Np(n,f) would be more suitable as a standard because of its lower fission threshold (E n = 0.5 MeV) and its higher cross section above E n = 1.0 MeV, but some discrepancies between measurements have been found in the last years [1].
To address these issues, two experimental campaigns have been performed at the Van de Graaff accelerator of the National Physical Laboratory (NPL,UK) under the European CHANDA project. The Nuclear Metrology group at NPL is known worldwide for its capability to provide very accurate and precise neutron flux measurements by using a well characterized long counter. Within their facilities there is also a large low-scatter target hall ideal for * e-mail: paula.salvador.castineira@npl.co.uk cross section measurements. A twin Frisch-grid ionization chamber was used as fission fragment detector. Measurements were done absolutely, by using the long counter, and relatively, by placing two samples in a back-to-back configuration in the fission chamber. The first campaign was performed in January 2016 and the isotopes measured were 235 U(n,f), 238 U(n,f) and 237 Np(n,f). For the second campaign, performed in January 2017, measurements were performed for 235 U(n,f), 237 Np(n,f) and 242 Pu(n,f). Results from both campaigns are presented, although results on the 242 Pu(n,f) cross section will be reported in a future publication as they are outside the scope of the present work. This paper presents a follow-up on the experiment reported on [2].

Experimental setup
The experimental setup consisted of a long counter (LC) and a twin Frisch-grid ionization chamber (TFGIC). The measurements were done in the following chronological order: first, a LC measurement was performed with the desired neutron energy to determine the neutron fluence per monitor count. The monitor is measuring the current on the neutron producing target can produced by the incident ion beam. Then, a shadow cone was used to subtract the contribution from scattered neutrons to the neutron fluence measured by the LC. Finally, the TFGIC was placed in front of the neutron producing target.
Two proton-induced reactions were used during the experiment: 7 Li(p,n) 7 Be for producing E n = 0.565 MeV and 3 H(p,n) 3 He for producing E n = 0.9-2.4 MeV.

Long counter
The LC consists of a BF 3 tube surrounded by a moderating layer and an outer shield. It has a high sensitivity, a negligible gamma response and a nearly flat fluence response from 1 keV up to 7 MeV [3]. Figure 1 (left) shows the LC placed in front of the beam line and with the shadow cone in place.

Fission Fragment detector
Two different TFGICs were employed as fission fragment detector with very similar dimensions, one in 2016 and another one in 2017. For the measurements reported here the configuration was the same as in [2] (see Fig. 1 (right)).

Fissile samples
During both campaigns five different samples were used. Their properties are listed in table 1.

Measurements
Two measurement campaigns have been performed, one in 2016 and one in 2017. In 2016 the focus was on the ability to reproduce the 235 U(n,f) cross section, thus proving our experimental setup and our data analysis. In addition, the 238 U(n,f) and the 237 Np(n,f) cross section were measured. As it will be shown in Sect. 5, the results of the 2016 measurement on the 235 U(n,f) cross section were systematically higher than predicted by the evaluations. For this reason, the same cross section was measured in 2017 using two different samples, together with the 237 Np(n,f) and 242 Pu(n,f).

Neutron fluence at the sample positions
The neutron fluence determined by the LC needs to be corrected by several effects that are related with the fact that we could not perform a shadow cone measurement with the TFGIC. In addition, the code used to obtain the neutron fluence at the sample positions from the LC measurement treats the system as a point-to-point problem, it considers the neutron producing target as a point in the space and the fission samples as a point. However, this was no longer applicable when we measured with the TFGIC because the edge of the fission chamber was only around 3 cm away from the neutron producing target, therefore the total distance between the neutron producing target and the fission samples was only around 9 cm. For this reason MCNP6 simulations [4] were performed to allow for the neutron fluence emitted by the disk-like neutron producing target and transmitted through the disk-like samples. This correction is around 2-4% depending on the incident neutron energy.

Target can scattering
The target can scattering was taken into account as this is subtracted in the shadow cone measurement performed with the long counter, but could not be taken into account in the TFGIC measurement due to the close distance between the TFGIC and the neutron producing target. The calculation was made using MCNP6 and the correction is of 2-3% depending on the incident neutron energy.

Neutron attenuation on the TFGIC
Finally, the attenuation of neutrons not only on the front face of the TFGIC, but on the first anode and grid, needs to be considered as these layers will be contributing to the neutron slowing down and, therefore, change the spectrum of the neutrons impinging on the samples in the cathode. The calculation was made using MCNP6 and the correction is of 1.5-2% depending on the incident neutron energy.

Fission fragment determination
The fission fragments were detected using the TFGIC. The signal used in our experiments corresponded to the anodes and grid only. A 2D grid vs anode pulse height was used to clean the anode pulse height (PH) distribution from any α-particle background. In addition, the anode projected PH was extrapolated to lower PH values to account for FFs of lower energy than the level of the electronic threshold, this was done using a straight line. This correction is about 2-5%.

Reaction rate due to lower energy neutrons
An MCNP6 simulation was needed to simulate the target hall to account for all scattered neutrons that will be impinging on the samples with lower energy than expected. These neutrons will be inducing fissions in the samples as a function of their corresponding (n,f) cross section. Therefore, this correction will have more impact on the 235 U samples as its cross section increases as the neutron energy decreases. The correction is of 4-10% depending on the sample and the incident neutron energy.

Cross section determination
The cross section has been determined as: where C corr are the corrected counts for the electronic threshold, accounts for the FF loss within the sample, A is the atomic number of the sample, m its mass, N A is Avogadro's number and Φ n (E n ) is the absolute fluence. The correction factor k FF,low accounts for the fission fragments produced by neutrons of lower energy than the main peak, k PP-DD corrects for the point-to-point to disk-to-disk difference when extracting the absolute fluence, k TS corrects for the neutrons scattered in the target can and k AttFC corrects for the attenuation of neutrons in the front face of the TFGIC as well as the anode and grid facing the neutron beam.

Preliminary results and discussion
The preliminary results are presented in Fig. 2. All results presented include both statistical and systematic uncertainties. In all case, the most significant ones are the counting statistics and the uncertainty related to the neutron fluence determination at the position of the samples.

235 U(n,f)
The 235 U(n,f) cross section was measured in both campaigns, 2016 and 2017. In 2016, only one sample was employed, namely 235 U 2016 , whilst in 2017 we measured with 235 U 2016 and with a new sample, namely 235 U 2017 . The main reason to measure with different samples was the increased cross section values found in 2016. In order to confirm that the issue was due to the sample, in 2017 we repeated the same measurement with the same sample and measured as well with another sample. The results we have obtained for the 235 U(n,f) cross section are in very good agreement with the present evaluations when using the 235 U 2017 sample (red squares). However, there was clearly an issue with the 235 U 2016 sample (green and blue squares). This unidentified issue increased the measured cross section by around 5% in the investigated neutron energy range. Although additional experiments are planned to further study this effect, the most probable hypothesis is that the high-Z backing of the 235 U 2016 sample could have an impact on the amount of backscattered FFs detected.

238 U(n,f)
The 238 U(n,f) absolute cross section measured in this experiment agrees within uncertainties with the present evaluations.

237 Np(n,f)
In the case of 237 Np(n,f) measurements were performed in 2016 and 2017 with some neutron energies repeated, mainly due because in 2017 the 242 Pu(n,f) cross section was measured at the same time. The 237 Np(n,f) absolute cross section measured in this experiment agrees within uncertainties with the present evaluations and the newest data from Diakaki [5]. Therefore, does not confirm the 5% increase seen by Paradela [1]. (The ratio σ ENDF /σ meas is not shown as its value is out of scale.)

Conclusions
The neutron-induced fission cross section has been measured absolutely for 235 U, 238 U and 237 Np. The results show a very good agreement with the present evaluations for 238 U(n,f) and 237 Np(n,f). In the case of 235 U(n,f) our data agree with evaluation when using one of the samples ( 235 U 2017 ), but we obtain a higher cross section when using the 235 U 2016 sample. However, we suspect that further investigations will be able to explain this mismatch. Our uncertainties are dominated by the statistical uncertainty due to the beam current limitation of the facility as well as the fluence uncertainty. A follow up experiment is planned to resolve the discrepancy obtained with the 235 U 2016 sample. Our preliminary results agree with the latest evaluations in the neutron energy ranges studied in this work and we believe that they could be used, when finalized, along with other high quality data in a possible evaluation of 238 U(n,f) and 237 Np(n,f) as standards in these energy ranges.
This work was partly funded by the CHANDA project (EC-FP7-Fission-2013).