Neutron Capture and Transmission Measurements of 54 Fe at the RPI LINAC

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Introduction
Fe is an important part of various nuclear systems and has a variety of applications in fuel storage systems, reactors, and radiation shielding. As such, it is important to have highly accurate nuclear data that will reduce uncertainties. Natural Fe and 56 Fe cross sections have been studied extensively [1], unlike the less abundant 54 Fe. Prior to the measurements discussed in this work, there were no extensive radiative capture datasets available on the Experiment Nuclear Reaction Data (EXFOR) database [2], providing some motivation to perform new radiative capture cross sections for 54 Fe. Additionally, some discrepancies were observed between the capture cross sections reported in different evaluation nuclear data libraries, in particular ENDF/B-VIII [3] and JEFF3.3 [4]. As a result, neutron TOF measurements were conducted at the RPI LINAC to study both the radiative capture cross section and the total neutron cross section of 54 Fe. In the future, these data will be used in tandem with other relevant data from EXFOR to perform resonance analysis on 54 Fe.

Previous Measurements
Several previous measurements exist in EXFOR that can be compared to the RPI data and used for future resonance analysis. An overview of some of these measurements is shown in table 1. Following the start of the RPI capture measurements, a high-resolution radiative capture * e-mail: singhs7@rpi.edu dataset from the neutron time-of-flight facility (nTOF) at the European Council for Nuclear Research (CERN) was uploaded to EXFOR. [5] These data are reported and useful up to 1 MeV. Additionally, several transmission measurements were found, each with varying resolutions and energy regions. All experiments were conducted enriched 54 Fe samples. The relative lack of radiative capture data provided motivation to perform a capture measurement, and discrepancies observed between some transmission datasets at low keV neutron energies also provided motivation for a transmission measurement.

Radiative Capture Measurement
Differential neutron capture cross section measurements were conducted at the RPI LINAC using the time-of-flight technique. For the capture measurements discusses in this work, an array of seven deuterated benzene (C 6 D 6 ) liquid scintillators were used to measure the capture yield from a 54 Fe sample. These detectors are designed to have a low sensitivity to scattered neutrons, which is especially important for samples with large Γ n /Γ γ ratios. In the case of the RPI capture array, the system is mounted and supported by materials with low neutron capture cross sections, such as Al-6061. Work is currently underway to experimentally quantify the system's neutron sensitivity and validate previous MCNP results that show negligible neutron sensitivity. The detectors are placed at back angle of 125 degrees relative to the beam to remove any sensitivity to anisotropic capture cascades and reduce the in-beam time dependent gamma background which mostly scatters in a forward direction from the sample. Data acquisition is handled by a SIS-3305 10-bit digitizer that allows for all detector pulses to be saved for later robust analysis.
The system is reliant on the total-energy detection (TED) principle [10], which requires a low overall photon detection efficiency. The details of this method, pulse height weighting technique, and the weighting methods used to reduce the raw capture data are further discussed in Refs. [11]. The capture yield is corrected for an inbeam gamma background shape, open beam background, as well as any beam intensity fluctuations via flux monitor normalizations. Additionally, due to the upgrade to seven total C 6 D 6 modules, a coincidence algorithm was implemented to ensure only one photon capture event was being detected per cascade, which would otherwise invalidate the TED principle. An overview of the experiment is shown in Table 2. The raw count rate obtained from the 54 Fe sample along with the open sample and room background count rates are shown in Figure 3. Discrepancies are shown in Figure 4 between the RPI  data and other evaluations, with the nTOF resonance parameters providing the best agreement to the preliminary RPI capture yield. To help resolve some of these differences and constrain the neutron width in resonance analysis, a transmission experiment was conducted with the same RPI Fe sample.

Transmission Measurement
A transmission experiment was conducted at the RPI LINAC using the 35m Li-6 glass DIABLO detector system [12]. An overview of the experiment is shown in Table  3. The transmission data was taken using an analog electronics setup and reduced using the methods described in   [13,14]. In addition to the 54 Fe sample and open samples, a 2.0 cm thick natural Fe sample and a 625 mil depleted Uranium sample were also measured. To characterize the background shapes for each transmission sample, several fixed notch materials (Al + Co) were placed in-beam for the duration of the experiment. The presence of these materials create fixed depressions in the sample count rates corresponding to black resonances in the notch material's total neutron cross section, as shown in Figure 5. These fixed notch resonances, along with the presence of a flat constant background below an energy of 10 eV allow for characterization and subtraction of the neutron and gamma backgrounds for each sample. Transmission values can then be easily obtained for each sample after correcting for effects such as dead-time loss and beam intensity fluctuations followed by the background subtraction for each sample. Comparison of the RPI data to different eval- Al + Co uations, as shown in Figure 6 show that there is indeed room for improvement for some resonance parameters of existing evaluations, and highlights the importance of per- forming resonance evaluation while considering multiple datasets in both transmission and capture.

Conclusions
Preliminary neutron capture and transmission measurements of 54 Fe were conducted at the RPI LINAC. The current results show discrepancies between the RPI experiment and existing evaluations for prominent capture resonances in the low-keV neutron energy region. More work is currently underway to quantify the neutron sensitivity of the capture detector array and obtain final results and uncertainties for both sets of measurements. Moving forward, these new data will allow for more robust resonance analysis of 54 Fe.

Acknowledgements
Part of this work was funded by the Nuclear Criticality Safety Program, funded and managed by the National Nuclear Security Administration for the Department of Energy. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessary reflect the views of the DOE. The 54 Fe used in this research was supplied by the U.S. Department of Energy Isotope Program, managed by the Office of Isotope R&D and Production.