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Uncertainties associated with short lived (half-lives less than 1 day) fission product yields listed in databases such as the National Nuclear Data Center’s ENDF/B-VII are large enough for certain isotopes to provide an opportunity for new precision measurements to offer significant uncertainty reductions. A series of experiments has begun where small samples of 235U are irradiated with a pulsed, fission neutron spectrum at the Nevada National Security Site and placed between two broad-energy germanium detectors. The amount of various isotopes present immediately following the irradiation can be determined given the total counts and the calibrated properties of the detector system. The uncertainty on the fission yields for multiple isotopes has been reduced by nearly an order of magnitude.

A new metal matrix composite material has been developed to serve as a thermal neutron absorber for testing fast reactor fuels and materials in an existing pressurized water reactor. The performance of this material was evaluated by placing neutron fluence monitors within shrouded and unshrouded holders and irradiating for up to four cycles. The monitor wires were analyzed by gamma and X-ray spectrometry to determine the activities of the activation products. Adjusted neutron fluences were calculated and grouped into three bins—thermal, epithermal, and fast—to evaluate the spectral shift created by the new material. A comparison of shrouded and unshrouded fluence monitors shows a thermal fluence decrease of ~11 % for the shielded monitors. Radioisotope activity and mass for each of the major activation products is given to provide insight into the evolution of thermal absorption cross-section during irradiation. The thermal neutron absorption capability of the composite material appears to diminish at total neutron fluence levels of ~8 × 1025 n/m2. Calculated values for dpa in excess of 2.0 were obtained for two common structural materials (iron and nickel) of interest for future fast flux experiments.

A dosimetry experiment used to measure the neutron flux and spectrum of a D-T neutron generator is presented. The D-T generator at Pacific Northwest National Laboratory is installed in the middle of a large room to minimize scatter of neutrons back to the sample. The efficacy of maintaining a pure fast neutron field for the sample is investigated. Twenty-one positions within 13 cm of the neutron source contained foils or wires of Fe, Ni, and Al with additional Au, and in monitors at some locations. Spectral adjustment of the neutron flux at each position based on the measured reaction rates and theoretical Monte Carlo calculations show that at least 99.1% of the spectrum lies above 110 keV for all measured positions, and neutrons above 14 MeV can account for as much as 91% at locations along the axis of the generator and close to the source. The 14 MeV component drops to 77% in radial positions far from the source. The largest total flux observed was 8.29E+08 n/cm 2 s (±1.4%) in the center of the cooling cap, although additional experiments have shown this value could be as high as 1.20E+09 n/cm 2 s.

The 27Al(n,2n)26Al reaction is of considerable interest to the fusion reactor program. Aluminum is an attractive material for many structural applications and the (n,2n) reaction is the major source of long-lived activity (26Al, g.s. T12 = 7.34 × 105 y). The threshold for this reaction falls within the spread of neutron energies generated by a D-T plasma. Its cross section is therefore a steeply rising function of energy for the primary fusion neutrons. This special feature makes it possible to use this reaction to measure plasma ion temperatures as well as neutron yields and neutron spectral shapes. The above applications require accurate measurements of the 27Al(n,2n)26Al cross section near threshold, but no data was available in the 14–15 MeV energy range of interest so a series of experiments1,2 were performed using aluminum samples irradiated at the neutron sources at LLL (RTNS-II), 3,4 and at PPL (electrostatic generator).2 In some experiments the new technique of accelerator mass spectrometry1,5 was used to measure the production rate of 26A1. The new measurements1 suggest that most of the (n,2n) yield near threshold is associated with the direct production of the 3+ state of 416.9 keV rather than the 5+ ground state. This raises the Q-value and neutron threshold energy for this reaction by a similar amount and greatly reduces the effective cross section of the (n,2n) reaction for D-T fusion neutrons. The effective cross section for a 9 keV D-T plasma is reduced by a factor of 5 by this shift in Q-value. A similar reduction occurs for the long-lived radioactivity.