Preequilibrium Asymmetries in the

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of ^{239}Pu is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

Bright laser-driven neutron source based on the relativistic transparency of solids.

Neutrons are unique particles to probe samples in many fields of research ranging from biology to material sciences to engineering and security applications. Access to bright, pulsed sources is currently limited to large accelerator facilities and there has been a growing need for compact sources over the recent years. Short pulse laser driven neutron sources could be a compact and relatively cheap way to produce neutrons with energies in excess of 10 MeV. For more than a decade experiments have tried to obtain neutron numbers sufficient for applications. Our recent experiments demonstrated an ion acceleration mechanism based on the concept of relativistic transparency. Using this new mechanism, we produced an intense beam of high energy (up to 170 MeV) deuterons directed into a Be converter to produce a forward peaked neutron flux with a record yield, on the order of 10(10) n/sr. We present results comparing the two acceleration mechanisms and the first short pulse laser generated neutron radiograph.

New precision measurements of the 235U(n,γ) cross section.

The neutron capture cross section of (235)U was measured for the neutron incident energy region between 4 eV and 1 MeV at the DANCE facility at the Los Alamos Neutron Science Center with an unprecedented accuracy of 2-3% at 1 keV. The new methodology combined three independent measurements. In the main experiment, a thick actinide sample was used to determine neutron capture and neutron-induced fission rates simultaneously. In the second measurement, a fission tagging detector was used with a thin actinide sample and detailed characteristics of the prompt-fission gamma rays were obtained. In the third measurement, the neutron scattering background was characterized using a sample of (208)Pb. The relative capture cross section was obtained from the experiment with the thick (235)U sample using a ratio method after the subtraction of the fission and neutron scattering backgrounds. Our result indicates errors that are as large as 30% in the 0.5-2.5 keV region, in the current knowledge of neutron capture as embodied in major nuclear data evaluations. Future modifications of these databases using the improved precision data given herein will have significant impacts in neutronics calculations for a variety of nuclear technologies.

Kerma factor of carbon for 14.1-MeV neutrons.

Using microdosimetric techniques, a direct measurement was made of the kerma factor of carbon for 14.1-MeV neutrons. Kerma was inferred from charged particle energy depositions measured with a small graphite-walled proportional counter. Measurements with an ionization chamber and a proportional counter, both constructed with A150 plastic walls, as well as induced 24Na activity from the 27Al(n, alpha) reaction, determined the neutron fluence. The resulting carbon kerma factor was 0.178 +/- 0.11 X 10(-8) cGy X cm2 which is lower than published tabulations but in agreement with recent microscopic cross-section measurements.

Measurement of dose distributions of linear energy transfer in matter irradiated by fast neutrons.

A detector has been developed and used to measure dose distributions versus linear energy transfer to thin gas targets in spherical geometry from fast neutron irradiation of tissue-equivalent plastic and carbon. The detector is a hemispherical proportional counter with a Cs(T1) scintillator at the center of the hemisphere. The coincidence of the proportional counter signals constrain the measurements to charged particles traversing the radius of the hemisphere. The charged particle energy deposition distributions are directly measured for a known pathlength. The A-150 kerma factor was measured at a neutron energy of 14.8 MeV and is in agreement with tabulated values. The carbon kerma factor measurements are less than the tabulated value at 14.8 MeV. The alpha-particle production in carbon was measured for neutron energies from 14.1 to 14.8 MeV and is compared with existing data.

Morbidity in the first year of a family practice and its comparison to the Virginia study.

In the first year of practice two family physicians recorded 8,795 new problems. The problems were ranked in order of decreasing frequency and compared to the results of the Virginia study. The results support the concept that a curriculum for family practice residency training based on results of the Virginia study is a valuable concept. The study also sheds light on the patterns of disease in a new practive as well as suggesting applications for the use of the disease index in practice management.

Primary hyperparathyroidism in the primary care setting.

A series of 36 patients with surgically proved primary hyperparathyroidism is reported. From this group a clinical profile consisting of obesity, anxiety, and/or depression in a mildly hypertensive, middleaged female was derived. Women constituted 92 percent of the patients. The serum calcium was confirmed again as the single most valuable test in the evaluation of this disorder.

Low back pain in the primary care setting.

A study of the natural presentation, course, and treatment of low back pain in the primary care setting was undertaken. One hundred and forty-four charts listing low back pain as a problem were reviewed at a family practice center for a period of one year. A profile of the patient evaluated by the primary care physician emerged, revealing a high incidence of associated weight problems (70 percent), psychologic problems (33 percent), and hypertension (19 percent). The rate of actual or suspected disc disease (1.4 percent) was much lower than that reported in series from referral centers. This study of low back pain in the primary care setting illustrates the usefulness of outpatient study in defining a problem category, recognizing disease as a symptom complex, suggesting modalities of treatment, and designing a curriculum for the primary care physician.

Sideline evaluation of neck pain: when is it time for transport?

Both ambulatory and nonambulatory athletes with neck pain require careful evaluation for possible cervical spine injury. The fact that an athlete is ambulatory should not lull the physician into a false sense of security (we offer a case study of a high school football player that makes this point dramatically) and the decision to return a player to the game should not be taken lightly. Symptoms such as neck pain, numbness, loss of range of motion of the neck, paralysis, or loss of consciousness require immediate action to prevent further, and possibly permanent, cervical spinal cord damage.

A study of accuracy of pharmacy technicians working in a unit dose system.

The incidence of undetected errors in one aspect of unit dose dispensing found in two methods of quality control is compared. In one method, a technician fills the unit dose box and a pharmacist checks each box. In the second method, a technician fills the box; a second technician checks the box. Pharmacist control and responsibility are maintained by having a pharmacist check a random sample of filled boxes. It was determined in this study that the incidence of undetected errors did not differ significantly between these two control methods.

Plastic fiber scintillator response to fast neutrons.

The Neutron Imaging System at NIF uses an array of plastic scintillator fibers in conjunction with a time-gated imaging system to form an image of the neutron emission from the imploded capsule. By gating on neutrons that have scattered from the 14.1 MeV DT energy to lower energy ranges, an image of the dense, cold fuel around the hotspot is also obtained. An unmoderated spallation neutron beamline at the Weapons Neutron Research facility at Los Alamos was used in conjunction with a time-gated imaging system to measure the yield of a scintillating fiber array over several energy bands ranging from 1 to 15 MeV. The results and comparison to simulation are presented.

Neutron cross-sections for next generation reactors: new data from n_TOF.

In 2002, an innovative neutron time-of-flight facility started operation at CERN: n_TOF. The main characteristics that make the new facility unique are the high instantaneous neutron flux, high resolution and wide energy range. Combined with state-of-the-art detectors and data acquisition system, these features have allowed to collect high accuracy neutron cross-section data on a variety of isotopes, many of which radioactive, of interest for Nuclear Astrophysics and for applications to advanced reactor technologies. A review of the most important results on capture and fission reactions obtained so far at n_TOF is presented, together with plans for new measurements related to nuclear industry.