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1.
Phys Rev Lett ; 122(7): 072503, 2019 Feb 22.
Article in English | MEDLINE | ID: mdl-30848631

ABSTRACT

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.

2.
Rev Sci Instrum ; 85(11): 11E607, 2014 Nov.
Article in English | MEDLINE | ID: mdl-25430353

ABSTRACT

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.

3.
Phys Rev Lett ; 110(4): 044802, 2013 Jan 25.
Article in English | MEDLINE | ID: mdl-25166169

ABSTRACT

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.

4.
Phys Rev Lett ; 109(20): 202506, 2012 Nov 16.
Article in English | MEDLINE | ID: mdl-23215479

ABSTRACT

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.

5.
Appl Radiat Isot ; 68(4-5): 643-6, 2010.
Article in English | MEDLINE | ID: mdl-20096595

ABSTRACT

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.


Subject(s)
Neutron Capture Therapy/instrumentation , Neutron Capture Therapy/methods , Nuclear Reactors , Equipment Design , Equipment Failure Analysis , Neutrons , Reproducibility of Results , Sensitivity and Specificity
6.
Phys Rev Lett ; 94(5): 052701, 2005 Feb 11.
Article in English | MEDLINE | ID: mdl-15783632

ABSTRACT

Prompt-fission-neutron multiplicities were measured for 238U(n,f) and 235U(n,f) from 0.4 to 200 MeV. The data are of great importance in connection with accelerator-coupled nuclear reactor systems incinerating actinides. We report that fission induced by 200 MeV neutrons produces approximately 10 more prompt neutrons than fission induced by reactor neutrons. Most neutrons are evaporated from the fission fragments and the prefission compound nucleus, as the preequilibrium emission of energetic neutrons accounts for a maximum of 15% of the prompt neutrons at 200 MeV.

7.
Phys Rev Lett ; 93(16): 161103, 2004 Oct 15.
Article in English | MEDLINE | ID: mdl-15524972

ABSTRACT

The151Sm(n,gamma)152Sm cross section has been measured at the spallation neutron facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The new facility combines excellent resolution in neutron time-of-flight, low repetition rates, and an unsurpassed instantaneous luminosity, resulting in rather favorable signal/background ratios. The 151Sm cross section is of importance for characterizing neutron capture nucleosynthesis in asymptotic giant branch stars. At a thermal energy of kT=30 keV the Maxwellian averaged cross section of this unstable isotope (t(1/2)=93 yr) was determined to be 3100+/-160 mb, significantly larger than theoretical predictions.

8.
Phys Sportsmed ; 29(3): 45-62, 2001 Mar.
Article in English | MEDLINE | ID: mdl-20086566

ABSTRACT

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.

9.
Phys Rev B Condens Matter ; 53(23): 15738-15742, 1996 Jun 15.
Article in English | MEDLINE | ID: mdl-9983409
10.
Phys Rev Lett ; 76(11): 1912-1915, 1996 Mar 11.
Article in English | MEDLINE | ID: mdl-10060552
11.
Appl Opt ; 35(33): 6445-8, 1996 Nov 20.
Article in English | MEDLINE | ID: mdl-21127669

ABSTRACT

I describe the generation of broadly tunable harmonic photons with energies to 80 eV from a laser system that produces 200-fs pulses of 0.5-mJ 610-nm light at a repetition rate of 540 Hz. The harmonics are produced by focusing of the 610-nm light into high densities of rare gas atoms produced at the output of a pulsed valve. Harmonic photons are wavelength selected by a grazing incidence toroidal grating and refocused onto samples that reside in a vacuum analysis chamber. Valence and atomic core-level photoemission spectra generated with this light are described.

12.
Phys Rev B Condens Matter ; 49(19): 13859-13862, 1994 May 15.
Article in English | MEDLINE | ID: mdl-10010333
13.
Phys Rev Lett ; 70(25): 3979-3982, 1993 Jun 21.
Article in English | MEDLINE | ID: mdl-10054014
15.
Phys Rev Lett ; 67(9): 1153-1156, 1991 Aug 26.
Article in English | MEDLINE | ID: mdl-10045089
16.
Med Phys ; 17(1): 1-9, 1990.
Article in English | MEDLINE | ID: mdl-2308539

ABSTRACT

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.


Subject(s)
Fast Neutrons , Neutrons , Radiometry/instrumentation , Energy Transfer , Equipment Design , Graphite , Plastics , Radiation Dosage
18.
Phys Rev Lett ; 56(26): 2846-2849, 1986 Jun 30.
Article in English | MEDLINE | ID: mdl-10033110
19.
20.
Phys Rev Lett ; 54(12): 1302-1305, 1985 Mar 25.
Article in English | MEDLINE | ID: mdl-10030990
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