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Scaling laws, transient times and shell effects in helium induced nuclear fission

Description: Fission excitation functions are analyzed and discussed according to a method which allows one to check the validity of the transition state rate predictions over a large range of excitation energies and a regime of compound nuclei masses characterized by strong shell effects. Once these shell effects are accounted for, no deviation from transition state rates can be observed. Furthermore, shell effects can be determined directly from the experiment by using the above described procedure. In contrast to the standard method, there is no need to include liquid drop model calculations. Finally, plotting the quantity R{sub f} allows one to search for evidence of transition times (discussed in a series of papers): our results set an upper limit of 10{sup {minus}20} seconds.
Date: February 1, 1996
Creator: Rubehn, T.; Jing, Kexing; Moretto, L.G.; Phair, L.; Tso, Kin & Wozniak, G.J.
Partner: UNT Libraries Government Documents Department

Alpha-physics and measurement requirements for ITER

Description: This paper reviews alpha particle physics issues in ITER and their implications for alpha particle measurements. A comparison is made between alpha heating in ITER and NBI and ICRH heating systems in present tokamaks, and alpha particle issues in ITER are discussed in three physics areas: `single particle` alpha effects, `collective` alpha effects, and RF interactions with alpha particles. 29 refs., 4 figs., 4 tabs.
Date: December 31, 1995
Creator: Zweben, S.J.; Young, K.M.; Putvinski, S.; Petrov, M.P.; Sadler, G. & Tobita, K.
Partner: UNT Libraries Government Documents Department

{sup 3}He- and {sup 4}He-induced nuclear fission -- A test of the transition state method

Description: Fission in {sup 3}H and {sup 4}He induced reactions at excitation energies between the fission barrier and 140 MeV has been investigated. Twenty-three fission excitation functions of various compound nuclei in different mass regions are shown to scale exactly according to the transition state prediction once the shell effects are accounted for. New precise measurements of excitation functions in a mass region where shell effects are very strong, allow one to test the predictions with an even higher accuracy. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign limits for the fission transient time. The precise measurement of fission excitation functions of neighboring isotopes enables one to experimentally estimate the first chance fission probability. Even if only first chance fission is investigated, no evidence for fission transient times larger than 30 zs can be found.
Date: January 1, 1997
Creator: Rubehn, T.; Jing, K.X.; Moretto, L.G.; Phair, L.; Tso, K. & Wozniak, G.J.
Partner: UNT Libraries Government Documents Department

On the unexpected oscillation of the total cross section for excitation in He{sup 2+} + H collisions

Description: Recent calculations and measurements have revealed unexpected oscillations of the total cross section for excitation in low- to intermediate-energy He{sup 2+} + H collisions. A physical explanation of this behavior is given here stemming from analysis of classical trajectory Monte Carlo simulations, molecular orbital close coupling calculations, and solution of the time-dependent Schroedinger equation on a numerical lattice. These results indicate that the observed behavior should be characteristic of a wide range of reactions in ion-atom collisions.
Date: October 1, 1996
Creator: Schultz, D.R.; Reinhold, C.O. & Krstic, P.S.
Partner: UNT Libraries Government Documents Department

The {alpha}-induced thick-target {gamma}-ray yield from light elements

Description: The {alpha}-induced thick-target {gamma}-ray yield from light elements has been measured in the energy range 5.6 MeV {le} E{sub {alpha}} {le} 10 MeV. The {gamma}-ray yield for > 2.1 MeV from thick targets of beryllium, boron nitride, sodium fluoride, magnesium, aluminum and silicon were measured using the {alpha}-particle beam from the Lawrence Berkeley Laboratories 88 in. cyclotron. The elemental yields from this experiment were used to construct the {alpha}-induced direct production {gamma}-ray spectrum from materials in the SNO detector, a large volume ultra-low background neutrino detector located in the Creighton mine near Sudbury, Canada. This background source was an order of magnitude lower than predicted by previous calculations. These measurements are in good agreement with theoretical calculations of this spectrum based on a statistical nuclear model of the reaction, with the gross high energy spectrum structure being reproduced to within a factor of two. Detailed comparison of experimental and theoretical excitation population distribution of several residual nuclei indicate the same level of agreement within experimental uncertainties.
Date: October 1994
Creator: Heaton, R. K.
Partner: UNT Libraries Government Documents Department

Mass measurements of highly neutron-excess nuclei in the light elements

Description: The $sup 26$Mg($sup 7$Li,$sup 8$B)$sup 25$Ne reaction at 78.9 MeV was used to measure the mass-excess of $sup 25$Ne (-2.18 +- 0.10 MeV) and several low- lying excited states. Mass-excesses for $sup 43$Ar (-31.98 +- 0.07 MeV), $sup 45$Ar (-29.727 +- 0.06 MeV), and $sup 46$Ar (-29.732 +- 0.07 MeV) were determined from the $sup 48$Ca($alpha$,$sup 9$,$sup 7$Be)$sup 43$,$sup 45$Ar reactions at 77.7 MeV and the $sup 48$Ca($sup 6$Li,$sup 8$B)$sup 46$Ar reaction at 80.1 MeV. Several excited states of $sup 43$Ar and $sup 45$Ar were observed. These and the excited states of $sup 25$Ne are compared with theoretical predictions. Ground state differential cross sections are given for all reactions, ranging from 0.1 to 1 $mu$b/sr at forward angles. Upper limits of about 1 to 25 nb/sr were determined for the ground state cross sections of the reactions $sup 48$Ca($alpha$,$sup 8$,$sup 10$,$sup 11$B)$sup 44$,$sup 42$,$sup 41$Cl and $sup 48$Ca($alpha$,$sup 10$-$sup 13$C)$sup 42$-$sup 39$S at 110 MeV. Recoil coincidence techniques were used to measure the mass-excess of the particle- unbound nucleus $sup 10$Li(33.83 +- 0.25 MeV) by the $sup 9$Be($sup 9$Be,$sup 8$B)$sup 10$Li reaction at 121 MeV; the ground state cross section was 30 nb/sr. The $sup 14$C($sup 9$Be,$sup 8$B)$sup 15$B reaction at 120 MeV was also investigated using coincidence techniques. Contamination problems only permitted determining a ground state cross section upper limit of 50 nb/sr. A simple mass relation is derived, similar in approach to the Garvey-Kelson method but taking more explicit account of shell effects. Comparison is made with the Garvey- Kelson relation, and predictions of masses and of the stability of neutron-excess light nuclei are given for both methods. The modified mass relation is shown to offer better account for highly neutron-excess nuclei, including the values reported here for $sup 25$Ne and $sup 43$,$sup 45$,$sup 46$Ar. (26 figures, 2 ...
Date: May 1, 1975
Creator: Wilcox, K.H.
Partner: UNT Libraries Government Documents Department

Benchmarking the External Surrogate Ratio Method using the (alpha,alpha' f) reaction at STARS

Description: We measured the ratio of the fission probabilities of {sup 234}U* relative to {sup 236}U* formed via an ({alpha},{alpha}{prime}) direct reactions using the STARS array at the 88-inch cyclotron at the Lawrence Berkeley National Laboratory. This ratio has a shape similar to the ratio of neutron capture probabilities from {sup 233}U(n; f) and {sup 235}U(n; f), indicating the alpha reactions likely formed a compound nucleus. This result indicates that the ratios of fission exit channel probabilities for two actinide nuclei populated via ({alpha}, {alpha}{prime}) can be used to determine an unknown fission cross section relative to a known one. The validity of the External Surrogate Ratio Method (ESRM) is tested and the results support the conclusions of Burke et al. [1].
Date: January 9, 2008
Creator: Lesher, S R; Bernstein, L A; Ai, H; Beausang, C W; Bleuel, D; Burke, J T et al.
Partner: UNT Libraries Government Documents Department

Emission of complex fragments for compound nuclei formed far from the stability line for A{approx}120

Description: Experiments and calculations are presented for the emission of complex fragments (Z>2) from compound nuclei around mass 120 and for bombarding energies below 15 MeV/nucleon. It is shown that a strong enhancement in the emission of complex fragments (especially for Z=6) is expected for compound nuclei formed close or at the N=Z line. Several predictions are given that could be tested with the radioactive beams soon to be available. Also results from a recent experiment that utilizes the emission of complex fragments for {gamma}-ray nuclear spectroscopy studies are presented.
Date: October 1, 1996
Creator: Gomez Del Campo, J.
Partner: UNT Libraries Government Documents Department

A photon shield capsule design for an {sup 241}Am/Be ({alpha},n) source using high density tungsten alloy

Description: A photon shield capsule made of high density tungsten alloy was designed for a 400 GBq {sup 241}Am/Be ({alpha},n) NIST-traceable source using Monte Carlo calculations. The {sup 241}Am/Be ({alpha},n) source replaces a {sup 239}Pu/Be ({alpha},n) source used in the Los Alamos Neutron Well for dose rate calibrations of portable and fixed neutron rem meters. Potential operator exposure due to {sup 241}Am photon emission (E{sub {gamma}} = 59.5 keV, Y{sub {gamma}} = 0.357 {gamma} d{sup -1}) is a major practical concern in using this type of source. This has been recognized by the International Organization for Standardization (ISO 8529:1989), which recommends wrapping the source in a 1 mm thick lead shield. However, the optimum photon shield capsule design depends on source construction and other considerations. These considerations include minimizing source spectrum degradation and inelastic gamma production from shielding, structural integrity, toxicity, and cost effectiveness of available materials and construction. Investigations of several materials and combinations using stainless steel, high density tungsten alloy (composed of 90%W, 6% Ni and 4% Cu) and lead with various capsule thicknesses were simulated using the Los Alamos Monte Carlo N-Particle Transport Code. The final design was based on a 2 mm thick capsule using the high density tungsten alloy. This material resulted in a small change in the neutron spectrum accompanied with only a slight increase in inelastic gamma production, and unobservable 59.5 keV photon emissions compared to the bare {sup 241}Am/Be ({alpha},n) source.
Date: January 1, 1997
Creator: Clement, R.S.; Hsu, H.H.; Olsher, R.S. & Aikin, D.J.
Partner: UNT Libraries Government Documents Department

Helium-induced reactions in astrophysics

Description: Helium-induced reactions play a crucial role in stellar nucleosynthesis. Carbon and oxygen are produced mainly during the helium-burning phase by the chain of reactions {sup 8}Be({alpha}, {gamma} + e{sup +}e{sup {minus}}){sup 12}C({alpha}, {gamma}){sup 16}O. The first step, often called triple-{alpha} capture, was proposed by Hoyle to bypass the mass stability gap at {sup 8}Be. The second step gives rise to the largest uncertainty in most of the calculated stellar abundances. Later {alpha}-captures on {sup 13}C are believed to be a major source of s-process neutrons. The status of each of these important reactions is reviewed here.
Date: November 1, 1997
Creator: Hale, G.M.
Partner: UNT Libraries Government Documents Department

Dependence of the yield of the ({alpha},n) reaction on alpha particle energy

Description: Neutron yields for alpha particle reactions with beryllium, boron, carbon, magnesium, and oxygen are calculated. The dependence on energy of the incoming alpha particles is analyzed for yield. Calculations are varied for different sets of cross section data.
Date: March 27, 1967
Creator: Tsenter, E. M. & Silin, A. B.
Partner: UNT Libraries Government Documents Department

Low-energy {sup 4}He{sup +} scattering from deuterium adsorbed on stepped Pd(331)

Description: We have taken angle-resolved data for the scattering of low-energy (< I keV) {sup 4}He{sup +} from deuterium adsorbed on a stepped Pd(331) surface. The impact geometry was up the staircase, that is, the {sup 4}He{sup +} beam was perpendicular to and directly incident onto the unshadowed <011>Pd ledge atoms. A strong quasi-elastic scattering signal of {sup 4}He{sup +} from D ({sup 4}He{sup +}/D) was observed at a forward scattering angle of {theta} = 25{degrees} and an incidence angle of {alpha} = 76{degrees} from the (331) normal. The results agree with shadow cone calculations of scattering first from Pd ledge atoms followed by a second event, {sup 4}He{sup +}/D. The resultant adsorption geometry shows D to reside in the quasi- threefold ledge site on the surface directly above the bulk fcc octahedral void. These results are consistent with the previous{sup 4}He{sup +} scattering study of the geometrically related Pd(110)- D(ads) system.
Date: July 1, 1996
Creator: Ellis, W.P. & Bastasz, R.
Partner: UNT Libraries Government Documents Department

The astrophysical reaction rate for the {sup 18}F(p,{alpha}){sup 15}O reaction

Description: Proton and alpha widths for a 3/2{sup +} ({ell}{sub p} = 0) state in {sup 19}Ne at E{sub x} = 7.1 MeV have been extracted using the results of recent measurements of the {sup 18}F(p,{alpha}){sup 15}O reaction. This {ell}{sub p} = 0 resonance dominates the astrophysical reaction rates at temperatures T{sub 9} > 0.5.
Date: March 1, 1996
Creator: Rehm, K.E.; Paul, M. & Roberts, A.D.
Partner: UNT Libraries Government Documents Department

Dielectron production in nucleus {plus} nucleus collisions at 1.05 GeV/nucleon

Description: Measurements of dielectron production in heavy-ion collisions are valuable probes into the dynamics of the collision process. While the hadronic participants of the collision are subject to strong final state interactions, the coupling of the electron-positron pair to the collision medium is electromagnetic. Dielectrons, therefore suffer little rescattering leaving the interaction and can retain information about their production origins, probing even the early stages in the evolution of the collision. The DiLepton Spectrometer (DLS) collaboration`s original measurements of dielectron production established the existence of the signal at Bevalac energies. The 1992- 93 DLS measurements in nucleus+Nucleus collisions at a kinetic beam energy of 1.05 {ital GeV/nucleon} are the subject of this presentation.
Date: June 28, 1996
Creator: Porter, R.J.; Beedoe, S.; Bougteb, M.; Hallman, T. & Wang, Z.F.
Partner: UNT Libraries Government Documents Department

Addendum to: Applications of evaluated nuclear data in the LAHET code

Description: This document is intended to provide additional information supplementing a prior publication. The discussion describes the details of the implementation of the proton nonelastic cross section parameterization for LAHET usage. It also documents extensions of the method to stable nuclei with 2 {le} Z {le} 5.
Date: May 6, 1997
Creator: Prael, R.E. & Chadwick, M.B.
Partner: UNT Libraries Government Documents Department

A six-body calculation of the alpha-deuteron radiative capture cross section

Description: The authors have computed the cross section for the process d + alpha -&gt; {sup 6}Li + gamma at the low energies relevant for primordial nucleosynthesis and comparison with laboratory data. The final state is a six-body wave function generated by the variational Monte Carlo method from the Argonne v-18 and Urbana IX potentials, including improved treatment of large-particle-separation behavior. The initial state is built up from the alpha-particle and deuteron ground-state solutions for these potentials, with phenomenological descriptions of scattering and cluster distortions. The dominant E2 cross section is in reasonable agreement with the laboratory data. Including center-of-energy and other small corrections, the authors obtain an E1 contribution which is larger than the measured contribution at 2 MeV by a factor of three. They calculate explicitly the impulse-approximation M1 contribution, which is expected to be very small, and obtain a result consistent with zero. They find little reason to suspect that the cross section is large enough to produce significant {sup 6}Li in the big bang.
Date: September 1, 2000
Creator: Nollett, K. M.; Wiringa, R. B. & Schiavilla, R.
Partner: UNT Libraries Government Documents Department

SOURCES 4A: A Code for Calculating (alpha,n), Spontaneous Fission, and Delayed Neutron Sources and Spectra

Description: SOURCES 4A is a computer code that determines neutron production rates and spectra from ({alpha},n) reactions, spontaneous fission, and delayed neutron emission due to the decay of radionuclides. The code is capable of calculating ({alpha},n) source rates and spectra in four types of problems: homogeneous media (i.e., a mixture of {alpha}-emitting source material and low-Z target material), two-region interface problems (i.e., a slab of {alpha}-emitting source material in contact with a slab of low-Z target material), three-region interface problems (i.e., a thin slab of low-Z target material sandwiched between {alpha}-emitting source material and low-Z target material), and ({alpha},n) reactions induced by a monoenergetic beam of {alpha}-particles incident on a slab of target material. Spontaneous fission spectra are calculated with evaluated half-life, spontaneous fission branching, and Watt spectrum parameters for 43 actinides. The ({alpha},n) spectra are calculated using an assumed isotropic angular distribution in the center-of-mass system with a library of 89 nuclide decay {alpha}-particle spectra, 24 sets of measured and/or evaluated ({alpha},n) cross sections and product nuclide level branching fractions, and functional {alpha}-particle stopping cross sections for Z &lt; 106. The delayed neutron spectra are taken from an evaluated library of 105 precursors. The code outputs the magnitude and spectra of the resultant neutron source. It also provides an analysis of the contributions to that source by each nuclide in the problem.
Date: September 1, 1999
Creator: Madland, D.G.; Arthur, E.D.; Estes, G.P.; Stewart, J.E.; Bozoian, M.; Perry, R.T. et al.
Partner: UNT Libraries Government Documents Department

LRAD-based airflow monitors

Description: Conventional alpha detectors depend upon detecting the alpha particle directly. This is often difficult because typical alpha particles generated by uranium or plutonium travel less than 3 cm in air. In contrast, the long-range alpha detector (LRAD) technology relies on detecting the ions generated by alpha particles as they lose their energy in air, rather than detecting the alpha particles themselves. We have designed, built, and used LRAD-based airflow monitors to detect alpha contamination. The airflow monitor is composed of a sample chamber with air and ion filters at one end and an ion detector at the opposite end. A set of small fans is located just outside the ion detector to provide an airflow that transports the ions into the ion detector. Ambient air enters through the particulate and ion filters, passes over and through an object located in the sample chamber, through the ion detector, then leaves the system through the fans. The number of ions present is proportional to the amount of contamination on the object`s surface. This work describes the design, construction, and testing of the LRAD-based airflow monitors.
Date: March 1, 1994
Creator: Caress, R. W.; MacArthur, D. W.; Allander, K. S.; Bounds, J. A.; Catlett, M. M.; Garner, S. E. et al.
Partner: UNT Libraries Government Documents Department

Application of the Long-Range Alpha Detector (LRAD) to the detection of Natural-Occurring Radioactive Materials (NORM)

Description: Standard fieldable alpha detectors are severely limited when monitoring alpha contamination on equipment, large surfaces, or the inside surfaces of pipes. New regulations are driving a need for new technologies to address these problem areas. The Long-Range Alpha Detector (LRAD) technology addresses these problems by detecting ion pairs created by an alpha particle in ambient air, rather than the alpha particle directly. These ion pairs, whose lifetime is several seconds, are transported to a collection grid by using either an airflow or an electric field. When collected, these ion pairs create a small electric current (typically 10{sup {minus}13} to 10{sup {minus}14} A) that is read by an electrometer and displayed on a data acquisition system. This method of detection is also being used to create monitoring systems for health physics, environmental site characterization decontamination characterization, and it is adaptable to cover the new needs of the power industry.
Date: April 1, 1994
Creator: Allander, K. S.; Bounds, J. A. & MacArthur, D. W.
Partner: UNT Libraries Government Documents Department

Empirical observations on the unpredictable behavior of nuclear matter

Description: While many aspects of matter are unpredictable from basic principles, there are some that are susceptible to empirical descriptions which can be quite accurate and beautiful. One such example from the field of ``Nuclear Matter Under Extreme Conditions`` is the distribution of the number of particles produced, or alternatively, of the energy carried by these particles, in energetic collisions of atomic nuclei. The present work consists of a series of published scientific papers on measurements of the distribution of particles produced, or the energy carried by these particles, in collisions of various nuclei, spanning more than a decade of research. Due to the unpredictability of the theory, the work includes empirical studies of the regularity of the measured distributions from which significant knowledge is gained. The aesthetics of this subject derives from the physical beauty of the measured curves, the characteristic changes of shape with different species of nuclei, and the deep understanding obtained by the use of a simple and elegant mathematical function to describe the data.
Date: January 19, 1994
Creator: Tannenbaum, M. J.
Partner: UNT Libraries Government Documents Department

Potential problems in U-233 production

Description: Interaction of {alpha} particles with elements of low atomic number leads to the emission of neutrons by the ({alpha}, n) reaction. The light element content may become one of the limiting factors in the production of U-233 acceptable for projected uses. Neutron production by the ({alpha}, n) reaction was calculated as functions of U-232 and light element contents.
Date: November 19, 1963
Creator: Kofoed, R. J.
Partner: UNT Libraries Government Documents Department

Reaction rate sensitivity of 44Ti production in massive stars and implications of a thick target yield measurement of 40Ca(alpha,gamma)44Ti

Description: We evaluate two dominant nuclear reaction rates and their uncertainties that affect {sup 44}Ti production in explosive nucleosynthesis. Experimentally we develop thick-target yields for the {sup 40}Ca({alpha},{gamma}){sup 44}Ti reaction at E{sub {alpha}} = 4.13, 4.54, and 5.36 MeV using {gamma}-ray spectroscopy. At the highest beam energy, we also performed an activation measurement which agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower {sup 44}Ti production in scenarios for the {alpha}-rich freeze out. Special attention has been paid to assessing realistic uncertainties of stellar reaction rates produced from a combination of experimental and theoretical cross sections. With such methods, we also develop a re-evaluation of the {sup 44}Ti({alpha},p){sup 47}V reaction rate. Using these two rates we carry out a sensitivity survey of {sup 44}Ti synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in {sup 44}Ti synthesis as that produced by different treatments of stellar physics.
Date: February 16, 2010
Creator: Hoffman, R D; Sheets, S A; Burke, J T; Scielzo, N D; Rauscher, T; Norman, E B et al.
Partner: UNT Libraries Government Documents Department