Allan D. Carlson

The neutron cross section standards, evaluations and applications

The measurement of a neutron cross section can be simplified if it is measured relative to a neutron cross section standard. Then it is not necessary to measure the neutron fluence. The standards in effect provide determinations of the neutron fluence. These standards are not defined standards but instead working standards. As such it is important to make measurements to improve the quality of the standards. Also better techniques for evaluating the standards must be promoted. The history of the evaluation process for the standards is discussed from the first rather primitive one to the latest very sophisticated evaluation that was an international cooperative effort. An overview of the main detectors used for applications of the standards is given.

An International Evaluation of the Neutron Cross Section Standards

Work is reported here on the process and present results of an international evaluation of the neutron cross section standards. The evaluations include the H(n,n), 3He(n,p), 6Li(n,t), 10B(n,α), 10B(n,α1γ), 197Au(n,γ), 235U(n,f), and 238U(n,f) standard reactions as well as the 238U(n,γ) and 239Pu(n,f) reactions. This evaluation was performed to include new experiments on the standards that have been made since the ENDF/B-VI evaluation was completed and to improve the evaluation process. Evaluations have been completed for the 6Li(n,t), 197Au(n,γ) and 238U(n,γ) cross sections. Also below 20 MeV the H(n,n), 235U(n,f), 238U(n,f) and 239Pu(n,f) cross sections are completed. Many of the cross sections being evaluated are used in neutron dosimetry for fluence determination. The general trend observed for the evaluations is an increase in the cross sections for most of the reactions from fractions of a percent to several percent compared with the ENDF/B-VI results.

Measurement of the 235U Neutron-Induced Fission Cross Section at 14.1 MeV

The /sup 235/U neutron-induced fission cross section was measured at a neutron energy of 14.1 MeV using the time-correlated associated-particle technique with the /sup 3/H(d,..cap alpha..)n reaction at the National Bureau of Standards 3-MV Van de Graaff Laboratory. The areal density and total mass of the /sup 235/U deposits were measured relative to the standard /sup 235/U reference deposit (Los Alamos National Laboratory Spare Number 1) using thermal-neutron-induced fission counting. The total mass was also determined from the alpha-particle decay rate. The measured /sup 235/U cross section at 14.1 + or - 0.1 MeV is 2.080 + or - 0.030 b where the uncertainty i one standard deviation. This value agrees within 1% with other recent measurements using this technique and with the ENDF/B-V evaluation.

Measurements of the {sup 235}U(n,f) cross section in the 3 to 30 MeV neutron energy region

To improve the accuracy of the {sup 235}U(n,f) cross section, measurements have been made of this standard cross section at the target 4 facility at Los Alamos National Laboratory (LANL). The data were obtained at the 20-meter flight path of that facility. The fission reaction rate was determined with a fast parallel plate ionization chamber and the neutron fluence was measured with an annular proton recoil telescope. The measurements provide the shape of the {sup 235}U(n,f) cross section relative to the hydrogen scattering cross section for neutron energies from about 3 to 30 MeV neutron energy. The data have been normalized to the very accurately known value near 14 MeV. The results are in good agreement with the ENDF/B-VI evaluation up to about 15 MeV neutron energy. Above this energy differences as large as 5% are observed.

A New Methodology for Adjustment of Iron Scattering Cross Sections Using Time-of-Flight Spectroscopy

Monte Carlo analysis of a time-of-flight experiment was performed to investigate the iron scattering cross section. Experimental data were utilized to build a Monte Carlo source for simulation of a time-of-flight (ToF) experiment with a spherical iron shell, different source spectra, and different angular beam source alignments. For this study, two pointwise cross section libraries, ENDF/B-VI and JEFF 3.0 are examined. Comparison of calculation and experimental results indicates differences that are larger than experimental and calculation errors. To reduce the observed discrepancy, pointwise data in the ENDF/B-VI iron scattering cross section file were adjusted at a few energy intervals using a utility code developed for this purpose. Adjusted cross sections resulted in closer agreement of calculated results with the experimental Time-of-Flight data. Multigroup cross sections were generated with the adjusted cross sections and results indicate pressure vessel fluence may be underestimated.

Status of the Neutron Cross‐Section Standards Database

A new evaluation of the neutron cross‐section standards is now underway. This evaluation has been supported by the Working Party on International Evaluation Cooperation (WPEC), the Cross Section Evaluation Working Group (CSEWG), and an International Atomic Energy Agency Coordinated Research Program (CRP). The CRP has had the dominant role in producing these evaluations. An important goal is to produce the standards needed for the upcoming new ENDF/B‐VII library. Since most neutron cross‐section measurements are made relative to neutron cross‐section standards, the standards evaluation is of crucial importance. The standard reactions to be evaluated are: H(n,n), 3He(n,p), 6Li(n,t), 10B(n,α), 10B(n,α1γ), C(n,n), Au(n,γ), 235U(n,f), and 238U(n,f). These standards should receive international acceptance to ensure that all evaluation projects use the same set of standards. The last complete evaluation of the standards dates back almost 20 years. In the meantime quite a number of new and improved measurements h...

Fission Cross-Section Measurements of the Odd-Odd Isotopes 232Pa, 238Np, and 236Np

Transmutation of actinide waste into fission products could be enhanced by using resonance fission of odd-odd target materials; those of interest are 232Pa, 238Np, and 242Am. Fission cross-section measurements of two of these short-lived materials were performed at Los Alamos National Laboratory. Samples were produced by the (d,2n) reaction in the Los Alamos Ion Beam Facility followed by fast radiochemistry to separate the odd-odd target of interest. The fission cross section of the nanogram samples was measured in a high intensity pulsed neutron beam produced by 800-MeV proton spallation. Using this procedure, the fission cross sections of the 1.3-day 232Pa and 2.1-day 238Np were successfully measured in the energy range from 0.01 eV to 50 keV. The fission cross section of the relatively long-life isotope 236Np was also measured in the same system while the short half-life isotopes were being prepared. The results and resonance analysis are presented.

Novel Investigation of Iron Cross Sections via Spherical Shell Transmission Measurements and Particle Transport Calculations for Material Embrittlement Studies

Abstract We have carried out a multifaceted research project to improve our knowledge of the iron nonelastic scattering cross sections. Spherical shell transmission measurements were made using time-of-flight techniques with neutrons from the 15N(p,n)15O and D(d,n)3He source reactions. For the 15N(p,n)15O work, measurements were made with a proton energy of 5.1 MeV. Measurements were made from 3 to 7-MeV deuteron energy for the D(d,n)3He work. For both source reactions, the angular range was as large as 15 to 135 deg. Two shell thicknesses were used. Comparisons are given between Monte Carlo predictions and experimental data. Utilizing a new tallying option, the estimated total iron cross sections at energies corresponding to the peak of the spectra for the 0-deg experiments were calculated to within 1% of the data in the ENDF/B-VII library. A processing code was developed to adjust ENDF format files to obtain closer agreement between measurements and calculations. Sensitivity analyses were performed at energies corresponding to the 0-deg beam angle neutrons. Using cross sections where the nonelastic and elastic cross sections were adjusted while constraining the total cross section to be constant, differences between experiment and calculation were reduced by ~;40% for a pressure vessel calculation. Such fluence calculations with adjusted cross sections indicate possible underestimation of neutron fluence, and therefore more material damage.

Status of the International Neutron Cross-Section Standards File

A report is given of the progress achieved in an IAEA Co‐ordinated Research Project (CRP) to improve the cross‐section standards. The objectives of the CRP, started in 2002, were initially the understanding of the origin of the strong uncertainty reduction in R‐matrix model fits and the improvement of the evaluation methodology. These aims were extended in 2003 to the preparation of new evaluations for the standard 6Li(n,t), 10B(n,α), 10B(n,α1), 197Au(n,γ), 235U(n,f), and 238U(n,f) reactions. The methodology, codes, and experimental database developed by Poenitz and Hale for the ENDF/B‐VI standards evaluation were taken as the basis for the new evaluation. The major results achieved by the CRP participants include the testing and intercomparison of a number of codes that can be used in the standards evaluation, updating the database of experimental results, analysis of the reasons leading to the strong uncertainty reduction in model fits, and a study of the bias in evaluated data caused by the Peelles’s P...

Calibration of the NBS black neutron detector at 2.3 MeV using the time-correlated associated-particle method

Abstract A time-correlated associated-particle measurement capability using the D(d, n)3He source reaction has been developed at the National Bureau of Standards 3-MV positive-ion Van de Graaff Accelerator Laboratory. The facility has been used to measure the efficiency of the NBS Black Neutron Detector at a neutron energy of 2.3 MeV. The associated 3He particles are detected at an angle of 45° with respect to the beam axis which is a more forward angle than conventionally employed. The kinematically more energetic 3He particles detected at the forward angle are readily separated from scattered deuterons at an incident beam energy of 250 keV. The time-correlated coincidence requirement on events detected in the Black Neutron and associated-particle detectors virtually eliminates the need for background corrections to the Black Neutron Detector rate. A result for the efficiency of the Black Neutron Detector at 2.3 MeV has been obtained with an accuracy of about ± 1% and agrees well with a Monte Carlo calculated value. The measurement extends the usefulness of the Black Neutron Detector as an absolute neutron flux monitor to the higher energy region.