B.A. Pohl

The effects of truncation in nuclear Hartree-Fock calculations

Abstract Hartree-Fock calculations were carried out for the doubly even nuclei with A = 4 to A = 80 in a space consisting of the 1 s 1 2 to the 1 i 1 2 harmonic oscillator states inclusive. The results are compared to results of a previous HF calculation in which the space was truncated at the top of the 2p-1f shell. The effects of the truncation are found to be more pronounced for deformed nuclei than for the spherical nuclei, where convergence has been previously studied. As expected, the quadrupole moment is found to be quite sensitive to this extension of the space, while the binding energy, r.m.s. radius and the kinetic energy per particle are not. Center-of-mass effects, compressibility and deformability are also studied, and a few nuclei with 80 ≦ A ≦ 208 are considered.

Measurements and calculations of the leakage multiplication from hollow beryllium spheres

Using the Pulsed-Sphere Method, the leakage spectra from hollow Be spheres of 4.5, 13.8 and 19.9 cm thickness have been measured. The predicted copious production of epithermal and thermal neutrons from the 13.8 and 19.9 cm spherical shells has been verified. A quantitative comparison of measured and calculated time-of-arrival count spectra over the energy range from thermal to ..integral.. 15 MeV indicates that the ENDL-84 library overestimates the leakage spectra between 2 and 10 MeV and in the epithermal and thermal energy regions. In the remaining regions, the leakage spectra are underestimated. Because of the above compensation the inferred leakage multiplication for the 19.9 cm Be is ..integral.. 3% higher than calculations. In the case of the 13.8 cm Be, the compensation is less exact and the inferred leakage multiplication is ..integral.. 9% higher than calculations.

The (α, n) cross sections on 17O and 18O between 5 and 12.5 MeV

Abstract The excitation functions for the (α, n) reaction on 17 O and 18 O have been measured from 5 to 12 MeV using long counters. Also, the neutron differential cross sections were measured at 9.8, 11.6 and 12.2 MeV, using the Livermore neutron time-of-flight facility. Neutron spectra were obtained every 15° between 3° and 135° with gas targets of enriched 17 O and 18 O. The angular distributions are roughly symmetric about 90°. Small forward peaking is observed at the higher bombarding energies for some of the states, mainly for the ground and first excited states.

Neutron and Gamma-Ray Spectra from 232Th, 235U, 238U, and 239Pu After Bombardment with 14-MeV Neutrons

AbstractThe neutron and gamma-ray leakage spectra from pulsed spheres having a radius equal to 1.0 mean-free-path (mfp) for 14-MeV neutrons in 232Th, 235U (with radii of 0.7 and 1.5 mfp), 228U (with radii of 0.8 and 2.8 mfp), and 239Pu (with radii of 0.7 and 1.25 mfp) were measured by using time-of-flight techniques. The neutron spectra were measured between 0.9 and 15 MeV using stilbene and NE-213 scintillators. For the gamma rays, the electron recoil spectra were measured between 0.35 and 8 MeV with the NE-213 detector only. Pulse-shape discrimination and flight paths of ∼10 m were used in these measurements. The measured spectra are compared with calculations carried out with TARTNP, a coupled neutron-photon Monte Carlo transport code. The Lawrence Livermore Laboratory ENDL (neutron and photon cross sections) and ENDF/B-IV libraries were used in these calculations.In the region between 10 and 15 MeV, the calculated neutron spectra are in good agreement with the measurements (±5%). The maximum discrepan...

Neutron and photon transport in seagoing cargo containers

Factors affecting sensing of small quantities of fissionable material in large sea-going cargo containers by neutron interrogation and detection of {beta}-delayed photons are explored. The propagation of variable-energy neutrons in cargos, subsequent fission of hidden nuclear material and production of the {beta}-delayed photons, and the propagation of these photons to an external detector are considered explicitly. Detailed results of Monte Carlo simulations of these stages in representative cargos are presented. Analytical models are developed both as a basis for a quantitative understanding of the interrogation process and as a tool to allow ready extrapolation of the results to cases not specifically considered here.

ELASTIC SCATTERING OF 7- TO 14-MeV NEUTRONS FROM NITROGEN.

Abstract The differential cross sections for the scattering of neutrons with incident energies of 6.78, 7.41, 7.93, 8.35, 8.57, 9.38, 10.10, 10.93, 11.55, 12.25, 13.50 and 13.96 MeV from nitrogen have been measured for lab angles between 15° and 135° in 15° steps. The measurements were made with a liquid target in cylindrical geometry with a thickness of approximately one half of the total mean free path. The neutrons were produced by the D(d, n) 3 He reaction in the Livermore variable-energy cyclotron; the average neutron energy spread was ±200 keV. A multiple detector time-of-flight system was used for simultaneous recording of the data. The experimental differential cross sections are analysed in terms of the optical model, with only the real and imaginary potentials varied.

Analysis of decay dose rates and dose management in the National Ignition Facility.

AbstractA detailed model of the Target Bay (TB) at the National Ignition Facility (NIF) has been developed to estimate the post-shot radiation environment inside the facility. The model includes the large number of structures and diagnostic instruments present inside the TB. These structures and instruments are activated by neutrons generated during a shot, and the resultant gamma dose rates are estimated at various decay times following the shot. A set of computational tools was developed to help in estimating potential radiation exposure to TB workers. The results presented in this paper describe the expected radiation environment inside the TB following a low-yield DT shot of 1016 neutrons. General environment dose rates drop below 30 &mgr;Sv h−1 within 3 h following a shot, with higher dose rates observed in the vicinity (∼30 cm) of few components. The dose rates drop by more than a factor of two at 1 d following the shot. Dose rate maps of the different TB levels were generated to aid in estimating worker stay-out times following a shot before entry is permitted into the TB. Primary components, including the Target Chamber and diagnostic and beam line components, are constructed of aluminum. Near-term TB accessibility is driven by the decay of the aluminum activation product, 24Na. Worker dose is managed using electronic dosimeters (EDs) self-issued at kiosks using commercial dose management software. The software programs the ED dose and dose rate alarms based on the Radiological Work Permit (RWP) and tracks dose by individual, task, and work group.

PLANNING TOOLS FOR ESTIMATING RADIATION EXPOSURE AT THE NATIONAL IGNITION FACILITY

Abstract A set of computational tools was developed to help estimate and minimize potential radiation exposure to workers from material activation in the National Ignition Facility (NIF). AAMI (Automated ALARA-MCNP Interface) provides an efficient, automated mechanism to perform the series of calculations required to create dose rate maps for the entire facility with minimal manual user input. NEET (NIF Exposure Estimation Tool) is a web application that combines the information computed by AAMI with a given shot schedule to compute and display the dose rate maps as a function of time. AAMI and NEET are currently used as work planning tools to determine stay-out times for workers following a given shot or set of shots, and to help in estimating integrated doses associated with performing various maintenance activities inside the target bay. Dose rate maps of the target bay were generated following a low-yield 1016 D-Tshot and will be presented in this paper.

Post-Shot Radiation Environment Following Low-Yield Shots Inside the National Ignition Facility

Abstract A detailed model of the Target Bay (TB) at the National Ignition Facility (NIF) has been developed to estimate the post-shot radiation environment inside the facility. The model includes large number of structures and diagnostic instruments present inside the TB. These structures and instruments are activated by the few nanosecond pulse of neutrons generated during a shot and the resultant gamma dose rates are estimated at various decay times following the shot. The results presented in this paper are based on a low-yield D-T shot of 1016 neutrons. General environment dose rates drop to below 3 mrem/h within three hours following a shot with higher dose rates observed at contact with some of the components. Dose rate maps of the different TB levels were generated to aid in estimating worker stay-out times following a shot before entry is permitted into the TB.

Double differential cross sections for (p, xn) reactions of64Zn,65Cu and89Y with 26MeV protons

Double differential cross sections for the inclusive production of neutrons from64Zn,65Cu and89Y bombarded with 26.0 MeV protons have been measured with time-of-flight techniques. The data reduction is discussed with respect to the influence of background corrections in the continuous part of the spectra, and a short comparison with preequilibrium model calculations is presented, showing good agreement with predictions of both, the hybrid and the geometry-dependent hybrid model including multiple particle emission. The question of how to treat pairing remains open.