The nuclear data group at the RPI Gaerttner LINAC Laboratory uses a 60 MeV pulsed electron LINAC to produce short pulses of neutrons and perform cross section and other nuclear data measurements in a wide energy range from below 1 meV to about 20 MeV. This paper will cover several recent activities that are of interest to nuclear applications. Interest in thermal neutron scattering evaluations prompted the need for accurate thermal total cross section measurements for validation. To improve the neutron flux in the sub-thermal region (below 0.01 eV) a cold moderator was designed and installed. A polyethylene moderator operating at about 26 K resulted in a factor of 8 increase in neutron flux below 0.01 eV. Using this new capability, several transmission measurements were performed with samples of polyethylene, polystyrene, Plexiglas, and yttrium hydride. Neutron capture and transmission measurements in the keV energy range were made for 54Fe, which will be used in an evaluation effort that is underway. Capture measurements were collected on an array of C6D6 detectors that was expanded from 4 to 7 detectors, and a complementary transmission measurement was also performed. Finally, research aimed at experimental validation of neutron capture gamma production is in progress. Energy dependent capture gamma cascades are measured with the RPI 16-segment gamma multiplicity detector. Measurements are then compared to capture gamma cascades generated from nuclear structure evaluations processed with DICEBOX and transported with a modified version of MCNP. This system provides important information on the completeness of primary gamma-ray databases.

Yttrium hydride serves as a neutron moderator material that enables compact, high temperature nuclear reactors. However, in order to accurately design and simulate a nuclear system relying upon yttrium hydride, the fundamental nuclear data of yttrium hydride must be well understood. Thermal neutron scattering law (TSL) evaluations represent an important aspect of nuclear data as thermal scattering can drastically alter the neutron multiplication factor of a system. Therefore, to support evaluation and validation of thermal neutron scattering for yttrium hydride, researchers at Rensselaer Polytechnic Institute (RPI) performed total thermal neutron cross section measurements for YH 1.68 and YH 1.85 over the energy range of 0.0005 - 3 eV. These measurements represent the first total cross section measurements for yttrium hydride that encompass the entire thermal region. Comparisons were made against the ENDF-B/VIII.0, Zerkle & Holmes and Oak Ridge National Laboratory TSL evaluations, where generally good agreement was found.

Hydrogen dense polymers, specifically polyethylene, polystyrene, and Plexiglas, have served as neutron moderator and reflector materials in hundreds of separate critical benchmark experiments because of their low cost and abundance of hydrogen. In order to accurately model and simulate these critical benchmarks, the thermal scattering law (TSL) evaluation that governs how neutrons will thermalize must be well understood and rigorously validated. To support this validation, researchers at Rensselaer Polytechnic Institute performed total neutron cross section measurements for high-density polyethylene & polystyrene over the energy range 0.0005–20 eV and for Plexiglas G & Plexiglas G-UVT over the energy range 0.0005–3 eV. Comparisons were made between the measured cross section and that predicted by the ENDF/B-VIII.0 and Oak Ridge National Laboratory/European Spallation Source/Rensselaer Polytechnic Institute TSL evaluations for polyethylene, Plexiglas, and polystyrene, as available. These experiments represent the first total neutron cross section measurements for polystyrene.

This presentation discusses the experimental, simulation, and nuclear data methods that were validated for the RPI γ-Multiplicity Detector. When the neutron capture γ-cascade data is well-known, the γ-emission spectra can be accurately calculated using the modified simulation tools. The RPI γ-Multiplicity Detector system is now ready for analysis and recommendations for isotopes with deficiencies in γ-ray data. The presentation also discusses future work which includes developing a method for analyzing and adjusting nuclear data for 59Co, 55Mn and other measured isotopes including 181Ta. Additionally, future work includes comparing experimental γ-emission spectra with MCNP-6.2/DICEBOX simulations for 238U and 235U. In summation, new capture and transmission measurements for 54Fe will help improve resonance parameter evaluation. Neutron capture gamma cascade spectra and yields were measured in the resolved resonance region and compared to evaluations. In addition, the pulsed neutron die-away method was developed as a tool to provide data for validation of TSLs.

A new sub-thermal neutron transmission capability at RPI LINAC has been used to validate Be (metal). NNL transmission measurements validated NCSU Be and Be+Sd TSLs for ENDF/B-VIII.1 and showed improvements WRT ENDF/B-VII.1 and ENDF/B-VIII.0. Amplitude differences highlighted the importance of sample characterization. Additional measurement is in progress for CY2022. PNDA is being developed as an alternative to critical experiment validation.

The neutron microscopic capture cross section for U-235 is a critical parameter for the design and operation of nuclear reactors. The evaluated nuclear data libraries of ENDF/B-VII.1 and JENDL-4.0 have nearly identical values for the neutron capture cross section for neutron energies below 0.5 keV. In the most recent release of the JENDL library the onset of the unresolved resonance region was changed from 2.25 keV to 0.5 keV. In the energy region from 1.5 keV to 2.25 keV the average neutron capture cross section from ENDF/B-VII.1 is about 10% higher than that from JENDL-4.0. In an attempt to address the discrepancies between the libraries, a measurement of the neutron capture cross section of U-235 was conducted at the Gaerttner LINAC Center located at Rensselaer Polytechnic Institute. This measurement used a 16-segment gamma-multiplicity NaI(Tl) detector to detect the prompt gammas emitted from neutron interactions with a highly enriched U-235 sample. Using the time-of-flight method, detected events were recorded and grouped based on the total gamma energy per interaction and observed multiplicity. A method was developed to separate fission from capture based on total energy deposition and gamma multiplicity. Application of this method in the thermal and resonance region below 0.5 keV for both the fission and capture produced cross sections that are in good agreement with both ENDF/B-VII.1 and JENDL-4.0 evaluations. The measurements support a lower U-235 neutron capture cross section in the energy range 0.5 to 2.25 keV, which is closer to JENDL 4.0.

The Gaerttner LINAC Center at RPI uses a 60 MeV electron linear accelerator to produce short pulses of neutrons with duration of 5-5000 ns. The main research thrust at the Center is nuclear data for nuclear reactors and criticality safety applications. The Center includes several setups for time-of-flight measurements including neutron transmission, capture and scattering detectors, and a lead slowing-down spectrometer. Experiments were designed to produce neutron interaction cross sections that cover the energy range of 0.01 eV to 20 MeV. Recently added experiments include: setups for keV and fast neutron transmission, a C6D6 detector array for keV neutron capture measurements, and a fast neutron scattering system. Results discussed here include fast neutron scattering and angular distributions for natFe, iron capture measurements for incident neutrons from 1 keV to 2 MeV, fast neutron transmission through W and H2O samples, and keV transmission through Mo isotopes.

Accurate isotopic molybdenum nuclear data are important because molybdenum can exist in nuclear reactor components including fuel, cladding, or as a high yield fission product. High-resolution time-of-flight neutron transmission measurements on highly enriched isotopic metallic samples of Mo-95, Mo-96, Mo-98, and Mo-100 were performed in the resonance energy range from 1 to 620 keV. The measurements were taken with the newly developed modular Li-6-glass transmission detector positioned at the 100-m experimental flight station. In the unresolved energy region (URR), new comprehensive methods of analysis were developed and validated in order to obtain accurate neutron total cross-section data from the measurement by correcting for background and transmission enhancement effects. Average parameters and fits to the total cross section for 95Mo were obtained using the Hauser-Feshbach statistical model code FITACS, which is currently incorporated into the SAMMY code. The fits to the experimental data deviate from the current evaluated nuclear data file/B-VII.1 isotopic Mo evaluations by several percent in the URR.

A series of new total cross section measurements for the stable molybdenum isotopes of 92,94,95,96,98,100Mo covering the energy range between 1 keV and 620 keV was performed at the Gaerttner LINAC Center at Rensselaer Polytechnic Institute. New high-accuracy resonance parameters were extracted from an analysis of the data using the multilevel R-matrix Bayesian code SAMMY. In the unresolved resonance region, average resonance parameters and fits to the total cross sections were obtained using the Bayesian Hauser-Feshbach statistical model code FITACS.

A new method for the simultaneous measurement of the neutron capture and fission cross sections of U has been developed at the Gaerttner LINAC Laboratory located at Rensselaer Polytechnic Institute. This method uses a 16 segment γ-multiplicity NaI(Tl) detector. The prompt γ rays from a neutron interaction with the highly-enriched U sample are detected and categorized based on the total energy deposited and the multiplicity (number of γ rays detected). Spectra of the events are recorded using the time-offlight method and grouped based on γ energy and observed multiplicity. Above the resolved resonance region for U (2.25 keV), the cross sections are presented as averaged values. The results of this research can be used to improve the accuracy of the capture-to-fission ratio for U in the resolved resonance region and to provide additional capture cross section data to the limited amount that currently exists for U in the unresolved resonance region.

The development of a new energy dependent double differential resonance scattering kernel by Rothenstein & Dagan, Annals of Nuclear Energy (1998) was shown to have a significant impact on core calculations as far as their criticality, Doppler Effect and the nuclide inventory is concerned. Thereafter, it was of great interest to experimentally validate this scattering kernel in addition to analytically proving its consistency with the integral Doppler broadened cross section, which was achieved by integrating the new kernel over all angles and all scattered energies. This study deals with the unique experiment suggested by Y. Danon at the Gaerttner Linear Accelerator Laboratory at Rensselaer Polytechnic Institute (RPI). The main advantage of this facility is the ability to move the neutron production source off axis relative to the detector beam line. It was, therefore, possible to position the sample, from which the neutron were scattered, on the same axis as the detector. In this way it was possible to directly measure the angular distribution of scattered neutron from heavy nuclides with pronounced resonances. In this study the previous results obtained for (238)U were extended to (232)Th. Improvements were made to the new resonance scattering kernel by development of a stochastic formalism known as DBRC (Doppler Broadened Rejection Correction) which was implemented by Becker et al. in several Monte Carlo codes. Based on the good agreement between this DBRC model and the measurements presented in this paper, it was shown that the standard asymptotic back angle scattering used previously in Monte Carlo codes differs by almost 80% for highly scattering resonances. Moreover, the scattering angle measurements and the ability to simulate it accurately by means of stochastic methods emphasized the deficiencies of the current methods which use only transmission and capture measurements.

The nuclear data program at the Rensselaer Polytechnic Institute (RPI) is centered around a 60 MeV pulsed electron Linear Accelerator (LINAC) configured to deliver pulsed neutron beams. The LINAC electron pulse width can vary between 5 ns and 5 us, a repetition rate of 1–500 pulses per second and neutron yield of up to 1013 n/s. Over the years several experimental setups and techniques were developed to enable a variety of measurement capabilities. The experiments cover the energy range from 0.001 eV to 20 MeV and included neutron transmission, capture, scattering and fission measurements. The facility is also equipped with a Lead Slowing-Down Spectrometer (LSDS) producing a high neutron flux that is used for simultaneous measurements of the fission cross section and fission fragment mass and energy distributions. Detectors for (n, alpha) and (n, p) cross section measurements using the LSDS were also developed and measurements of (n, alpha) cross section were completed. The high neutron flux inside the LSDS enables measurements on small samples (sub micrograms) or samples with small cross section (sub millibarns). In order to fully utilize the capabilities of the facility, several measurement techniques were developed; high accuracy (<1%) total cross sections were measured using iron and uranium filtered neutron beams. A system for fast neutron scattering measurements using an array of liquid scintillators and a digital data acquisition system was recently developed and used for several measurements and is now being adapted for fission neutron studies. Methods for simultaneous measurements of fission and capture cross sections using the RPI multiplicity detector are under development and new modular detectors for both fast and resonance region neutron detection were developed and deployed. These developments result in numerous measurements on different materials and provide input to improved evaluated nuclear data libraries and thus help improve the accuracy of calculations of nuclear reactors and other nuclear systems.

New high energy (0.5-20 MeV) beryllium total cross sections have been measured at the Gaerttner Linac Laboratory located at Rensselaer Polytechnic Institute. The transmission measurement combines fast detector response and electronics, a narrow neutron pulse width and 100 meter flight path. The detector system was validated using the well known cross sections of carbon. An innovative background determination technique was developed and applied. Cross sections derived from both the carbon and beryllium data are compared to ENDF/B-VII.0. This effort validates the newly developed 100 meter time of flight system and data analysis methods and provides useful information to improve the current data for beryllium.

Recently a method for high accuracy total cross section measurement in the energy range of 24 keV to 940 keV using an iron filtered beam was developed at RPI. Measurements the total cross section of carbon and beryllium are discussed. A new neutron detection system was developed at RPI and the first measurement with this system is reported here.

A brief overview of the history of the Gaerttner LINAC Laboratory since its beginning of operation in 1961 is given. This includes a short description of some of the upgrades necessary for continuous operation of more than 40 years. An overview of the current and future plans of the research activity in the laboratory is also provided. ——— * Corresponding author. e-mail: danony@rpi.edu.