First results of the 230Th(n,f) cross section measurements at the CERN n_TOF facility

The study of neutron-induced reactions on actinides is of considerable importance for the design of advanced nuclear systems and alternative fuel cycles. Specifically, 230Th is produced from the α-decay of 234U as a byproduct of the 232Th/233U fuel cycle, thus the accurate knowledge of its fission cross section is strongly required. However, few experimental datasets exist in literature with large deviations among them, covering the energy range between 0.2 to 25 MeV. In addition, the study of the 230Th(n,f) cross-section is of great interest in the research on the fission process related to the structure of the fission barriers. Previous measurements have revealed a large resonance at En=715 keV and additional fine structures, but with high discrepancies among the cross-section values of these measurements. This contribution presents preliminary results of the 230Th(n,f) cross-section measurements at the CERN n_TOF facility. The high purity targets of the natural, but very rare isotope 230Th, were produced at JRC-Geel in Belgium. The measurements were performed at both experimental areas (EAR-1 and EAR-2) of the n_TOF facility, covering a very broad energy range from thermal up to at least 100 MeV. The experimental setup was based on Micromegas detectors with the 235U(n,f) and 238U(n,f) reaction cross-sections used as reference.


Introduction
The Th/U fuel cycle has several advantages concerning radioactive waste management and non-proliferation, compared with the conventional U-Pu fuel cycle [1]. In order to achieve improved design calculations for thorium based reactors, the determination of neutron induced reaction cross-sections on isotopes of thorium is required.
Specifically, the 230 Th isotope is produced from the αdecay of 234 U as a byproduct of the 232 Th/ 233 U fuel cycle. Regarding 230 Th(n,f) cross-section, the existing experimental datasets cover the energy range between 0.2 to 25 MeV with large deviations among them. Furthermore, interesting structures appear in the cross section of this isotope at the fission barrier, giving insight to the fission potential wells. In this context, the 230 Th(n,f) cross-section was measured at the CERN n_TOF facility, with the aim of covering with high accuracy data over a broad energy range from thermal up to at least 100 MeV.
The measurement was performed with the same experimental setup at both experimental areas EAR-1 [2] and EAR-2 [3] of the CERN n_TOF facility, exploiting their different neutron beam characteristics, in order to cover the largest possible energy range and minimize the systematic uncertainties of the final cross-section results. In the present contribution, the experimental setup and the analysis, along with first results above ∼500keV, will be presented and discussed. * e-mail: veatriki.michalopoulou@cern.ch 2 Experimental setup

The n_TOF facility
The measurement was performed at the experimental areas EAR-1 and EAR-2 of the CERN n_TOF facility. Neutrons are produced by proton-induced spallation on a lead target with energies ranging from thermal up to the GeV region. In order to reach the experimental areas, the neutrons travel flight paths of 185 m horizontally and 19.5 m vertically to EAR-1 and EAR-2, respectively. This difference in the flight paths results in different neutron beam characteristics in the two experimental areas.
Specifically, experimental area EAR-1 offers better neutron energy resolution combined with the capability to reach higher neutron energies, while experimental area EAR-2 offers a higher neutron flux [4]. The experimental setups at experimental areas EAR-1 and EAR-2 are shown in Figure 1 and Figure 2 respectively.

Samples
Seven high purity thorium samples, prepared at JRC-Geel, were used for the measurements, with a total 230 Th mass of ∼32 mg , while ∼3.6 mg 235 U and ∼14.4 mg 238 U samples were used as references. The material was deposited on a 0.025 mm thick aluminum backing in the form of 8 cm diameter disks.
In the thorium samples, Pu contaminants which have a significant contribution in the yield below the fission threshold were present and need to be considered in the analysis. A detailed characterization of the thorium and  uranium samples is ongoing, thus, the mass and contaminants of the samples are currently known with a high uncertainty.

Detectors
The measurements were carried out using a set-up based on Micro-Bulk Micromegas (Micro-Mesh Gaseous Structure) detectors [5]. The detector volume is divided into two parts by a thin electrode with holes, called micromesh: the drift region with an electric field of ∼1kV/cm and the amplification region with an electric field of ∼50 kV/cm. The samples and detectors were placed in pairs, as shown in Figure 3, in an aluminum chamber filled with a mixture of Ar:CF 4 :isoC 4 H 10 (88:10:2) at atmospheric pressure and room temperature.
The actinide samples act as the drift electrode of each detector. When a fission event occurs the heavy and the light fission fragments move in opposite directions due to the kinematics of the reaction, one of the two entering the drift region of the detector, while the other is not detected. In the drift region the fission fragment creates primary electrons, which are guided towards the mesh electrode from the electric field applied. Upon entering the mesh region the electrons are multiplied. The positive ions created in the amplification region are collected from the mesh and result in the formation of the signals. A schematic representation of the operating principle of the Micromegas detector is shown in Figure 4.

Data analysis
The detector signals were digitized and then analyzed by means of a pulse shape recognition routine [6], in order to determine the amplitude of each pulse and its arrival time, which is correlated to the energy of the neutron that caused the fission event. Data without the neutron beam were regularly recorded in order to a) check the stability of the detectors and b) determine the amplitude distributions which originate from the alpha activity of the targets. Depending on the amplitude, each pulse is either attributed to the alpha activity of the samples or to a fission event. Fission signals with an amplitude lower than the applied pulse-height threshold are rejected, in order to completely reject the alpha counts from the analysis. The fraction of these rejected fission events is estimated by FLUKA [7,8] simulations of the detector, using the GEF [9] code as a fission event generator. This work is in progress, while a typical experimental pulse height spectrum is shown in Figure 5.
The arrival time of each signal is determined relative to the "γ-flash" signal. In addition to neutrons, the interaction of the protons with the lead target results in the production of γ-rays and other relativistic particles which reach the experimental areas at (almost) the speed of light. This "γ-flash" causes a high amplitude signal, lasting a few hundred ns. In order to mitigate this effect, an average "γ-flash" shape is calculated for all detectors separately, as shown Figure 6. The average "γ-flash" shape is subtracted from each event and normalized to the amplitude of the pulse. The energy of the neutron that caused the fission event is calculated from the time of flight difference between the "γ-flash" and the fission pulse.
The 230 Th(n,f) cross-section was calculated using the 235 U(n,f) and 238 U(n,f) reactions as references, assuming all targets receive the same flux. Since the final mass of the targets and the contaminants characterization is in progress, and the correction for the fission fragments which are recorded below the amplitude threshold is not yet finalized, the cross-section results presented are preliminary.

Results
The preliminary 230 Th(n,f) cross-section results, with the 235 U(n,f) cross-section used as reference, deduced as the mean value from the seven thorium targets up to 100 MeV are presented in Figure 7, along with the previous datasets. As seen in Figure 7 the preliminary cross-section results are in agreement within errors with the data from Muir and Vesser [17] at the region of the resonance (from 650 to 840 keV). For energies higher than 800 keV the crosssection results are in agreement within errors with the data from Meadows [14], as well as with the data from Muir and Vesser, with exception of the energy region between 840 keV to 1 MeV. Regarding the data from the surrogate method from Goldblum et al. [10] and Petit et al. [11], the cross-section results are in good agreement within errors for energies higher than 1 MeV, while for energies higher than 7 MeV the data from Petit et al. are systematically lower and for energies higher than 22 MeV the data from Goldblum et al. [10] are systematically higher than the present data. Large deviations are observed between the present data and the data of Boldeman and Walsh [13], Blons et al. [15], James et al. [16] and Kazarinova et al. [18] at the resonance region until 4 MeV. At the energy of ∼14 MeV the present data are in agreement within errors with the available data points of Meadows [12], Kazarinova et al. and Goldblum et al.. The analysis of the high energy region from the experimental area EAR-1 measurement is in progress in order to extend the results to higher energies and to estimate the necessary correction factors such as amplitude cut correction, dead time, contribution from contaminants etc. Regarding the EAR-2 measurement, the analysis is ongoing, in order to correct for the plutonium contaminants, present in the 230 Th samples, which dominate the yield below the fission threshold.

Conclusions
The 230 Th(n,f) measurement was performed at the experimental areas EAR-1 and EAR-2 at CERN's n_TOF facility and preliminary cross-section results in the energy range between 600 keV to 100 MeV are presented. The analysis is ongoing but the measurement at both experimental areas is expected to cover a very wide energy range, taking advantage of the substantially better energy resolution of EAR-1 to measure the high energy region, and the higher neutron flux of EAR-2 to measure the lower energy region, where the cross-section is expected to be small. The data from the present work will provide for the first time results at energies higher than 25 MeV and useful information for the improvement of the evaluations.