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Extended TDHF approximation illuminates the approach to thermal equilibrium

Description: The region of validity of the TDHF approximation is extended by including the collisions between particles. The resultant equations of motion consist of a modified TDHF equation and a master equation for the occupation numbers. Concepts such as entropy, temperature, and thermal equilibrium can be quantitatively introduced. The well-known H-theorem that entropy never decreases is proved for nuclear systems. New types of level crossing formulas are obtained to shed light on how a nonequilibrium finite nuclear system approaches thermal equilibrium. 2 figures.
Date: January 1, 1979
Creator: Wong, C.Y. & Tang, H.H.K.
Partner: UNT Libraries Government Documents Department

Polarized electronic sources for future e{sup +}/e{sup {minus}} linear colliders

Description: Polarized electron beams will play a crucial role in maximizing the physics potential for future e{sup +}/e{sup {minus}} linear colliders. We will review the SLC polarized electron source (PES), present a design for a conventional PES for the Next Linear Collider (NLC), and discuss the physics issues of a polarized RF gun.
Date: May 1, 1997
Creator: Tang, H.; Alley, R.K. & Clendenin, J.E.
Partner: UNT Libraries Government Documents Department

Preparation of GaAs photocathodes at low temperature

Description: The preparation of an atomically clean surface is a necessary step in the formation of negative electron affinity (NEA) GaAs. Traditional methods to this end include cleaving, heat cleaning and epitaxial growth. Cleaving has the advantage of yielding a fresh surface after each cleave, but is limited to small areas and is not suitable for specialized structures. Heat cleaning is both simple and highly successful, so it is used as a preparation method in virtually all laboratories employing a NEA source on a regular basis. Due to its high cost and complexity, epitaxial growth of GaAs with subsequent in vacuo transfer is not a practical solution for most end users of GaAs as a NEA electron source. While simple, the heating cleaning process has a number of disadvantages. Here, a variety of cleaning techniques related to preparation of an atomically clean GaAs surface without heating to 600 C are discussed and evaluated.
Date: October 1, 1996
Creator: Mulhollan, G.; Clendenin, J. & Tang, H.
Partner: UNT Libraries Government Documents Department

The SLAC polarized electron source

Description: The SLAC polarized electron source employs a photocathode DC high voltage gun with a loadlock and a YAG pumped Ti:sapphire laser system for colliding beam experiments or a flash lamp pumped Ti:sapphire laser for fixed target experiments. It uses a thin, strained GaAs(100) photocathode, and is capable of producing a pulsed beam with a polarization of {ge}80% and a peak current exceeding 10 A. Its operating efficiency has reached 99%. The physics and technology of producing high polarization electron beams from a GaAs photocathode will be reviewed. The prospects of realizing a polarized electron source for future linear colliders will also be discussed.
Date: June 1, 1995
Creator: Tang, H.; Alley, R. & Frisch, J.
Partner: UNT Libraries Government Documents Department

Study of non-linear photoemission effects in III-V semiconductors

Description: Our experience at SLAC with photoemission-based polarized electron sources has shown that charge limit is an important phenomenon that may significantly limit the performance of a photocathode for applications requiring high intensity electron beams. In the process of developing high performance photocathodes for the ongoing and future SLC high energy physics programs, we have studied the various aspects of the charge limit phenomenon. We find that the charge limit effect arises as a result of non-linear response of a photocathode to high intensity light illumination. The size of the charge limit not only depends on the quantum efficiency of the cathode but also depends critically on the extraction electric field. In addition, we report the observation of charge oversaturation when the intensity of the incident light becomes too large.
Date: October 1, 1993
Creator: Tang, H.; Alley, R. K. & Aoyagi, H.
Partner: UNT Libraries Government Documents Department

A polarization study of strained GaAs photocathode structures

Description: The polarized electron source at SLAC has performed extremely well during recent years supplying electrons having a spin polarization of 78% (85%) for high (low) current operation with beam current limited primarily by experimental requirements. However, there is room for improvement in the electron polarization. The less-than-ideal polarizations are a result of both imperfections and depolarizing mechanisms within the photocathode. The structure of the photocathode used at SLAC in the polarized electron source is a single-strained emitting layer structure grown atop a GaAs substrate. Here, the properties of several types of strained GaAs and GaAsP photocathodes have been studied using x-ray diffraction and photoemission.
Date: October 1, 1996
Creator: Mulhollan, G.; Garwin, E.L.; Maruyama, T.; Tang, H.; Mair, R. & Prepost, R.
Partner: UNT Libraries Government Documents Department

High level vibration test of nuclear power piping: Overall plan, input motion development and analysis

Description: As part of cooperative agreements between the United States and Japan, tests have been performed on the seismic vibration table at the Tadotsu Engineering Laboratory of Nuclear Power Engineering Test Center (NUPEC) in Japan. The tests involved increasing the excitation up to the limits of the vibration table in order to induce inelastic response in a reactor coolant system piping model. The model was subjected to a maximum acceleration well beyond what nuclear power plants are designed to withstand. The High Level Vibration Test (HLVT) model was constructed by modifying the 1/2.5 scale model of one loop of a PWR primary coolant system which was previously tested by NUPEC as part of their seismic proving test program. The upper and middle steam generator shell supports of the model, which simulated the actual plant condition, were removed and the steam generator shell was truncated. Furthermore, the four lower support columns for the steam generator were replaced by a pin-type support. A modified earthquake excitation was used to drive the structure to a condition of substantial strain. Since the piping was pressurized, and the high level input motion was repeated several times, it was possible to investigate the effects of ratchetting and fatigue as well. An isometric view of the test model and support frame is shown in Figure 1. Further details of the hot leg pipe are shown in Figure 2. The piping in the model is stainless steel, 14-15 inches in diameter and 1 to 1 1/4 inches thick. This paper describes the overall plan, the input motion development and pre-test analysis results. 4 refs., 8 figs., 1 tab.
Date: January 1, 1989
Creator: Hofmayer, C.H.; Curreri, J.R.; Park, Y.J.; Kato, W.Y.; Costello, J.F.; Tang, H.T. et al.
Partner: UNT Libraries Government Documents Department

Experimental studies of the charge limit phenomenon in GaAs photocathodes

Description: GaAs photocathodes have been in use for generating high intensity polarized electron beams (up to a peak current of 6 A in 2 ns pulses) for the SLC high energy physics program. If the quantum efficiency (measured at low light intensities) of a GaAs photocathode is below a certain level, the maximum photoemitted charge is found to be limited by the intrinsic properties of the cathode instead of by the space charge limit. We have studied this charge limit phenomenon in a variety of GaAs photocathodes. The effects of the quantum efficiency, excitation laser wavelength, and extraction electric field on the charge limit have been examined. The temporal behavior of the charge limit as manifested in both intrapulse and interpulse effects has also been studied. These results will be discussed in light of possible mechanisms.
Date: October 1, 1993
Creator: Tang, H.; Alley, R.; Aoyagi, H.; Clendenin, J.; Saez, P.; Schultz, D. et al.
Partner: UNT Libraries Government Documents Department

Spontaneous Generation of Voltage in Single-Crystal Gd5Si2Ge2 During Magnetostructural Phase Transformations

Description: The spontaneous generation of voltage (SGV) in single-crystal and polycrystalline Gd{sub 5}Si{sub 2}Ge{sub 2} during the coupled magnetostructural transformation has been examined. Our experiments show reversible, measurable, and repeatable SGV responses of the materials to the temperature and magnetic field. The parameters of the response and the magnitude of the signal are anisotropic and rate dependent. The magnitude of the SGV signal and the critical temperatures and critical magnetic fields at which the SGV occurs vary with the rate of temperature and magnetic-field changes.
Date: April 19, 2006
Creator: Zou, M.; Tang, H.; Schlagel, D.L.; Lograsso, T.A.; K.A. Gschneidner,jr. & Pecharsky, V.K.
Partner: UNT Libraries Government Documents Department

Design of the NLC positron source

Description: The design of the positron source for the Next Linear Collider (NLC) is presented. The key features of this design include accelerating positrons at an L-band frequency (1428 MHz) and using a rotating positron target with multi-stage differential pumping. Positron yield simulations show that the L-band design yields at the source 2.5 times the beam intensity required at the interaction point and is easily upgrade to higher intensities required for the 1 TeV NLC upgrade. Multi-bunch beam loading compensation schemes in the positron capture and booster accelerators and the optics design of the positron booster accelerator are described. For improved source efficiency, the design boasts two parallel positron vaults adequately shielded from each other such that one serves as an on-line spare.
Date: August 1, 1996
Creator: Tang, H.; Emma, P.; Gross, G.; Kulikov, A.; Li, Z.; Miller, R. et al.
Partner: UNT Libraries Government Documents Department

The NLC positron source

Description: A baseline design for the NLC positron source based on the existing SLC positron system is described. The proposed NLC source consists of a dedicated S-band electron accelerator, a conventional positron production and capture system utilizing a high Z target and an adiabatic matching device, and an L-band positron linac. The invariant transverse acceptance of the capture system is 0.06 m{center_dot}rad, ensuring an adequate positron beam intensity for the NLC.
Date: May 1, 1995
Creator: Tang, H.; Kulikov, A.V.; Clendenin, J.E.; Ecklund, S.D. & Miller, R.A.
Partner: UNT Libraries Government Documents Department

Magnetic Field Induced Phase Transitions in Gd5(Si1.95Ge2.05)Single Crystal and the Anisotropic Magnetocaloric Effect

Description: The magnetization measurements using a Gd{sub 5}(Si{sub 1.95}Ge{sub 2.05}) single crystal with the magnetic field applied along three crystallographic directions, [001], [010] and [100], were carried out as function of applied field (0-56 kOe) at various temperatures ({approx}5-320 K). The magnetic-field induced phase transformations at temperature above the zero-field critical temperature, i.e. the paramagnetic (PM) {leftrightarrow} ferromagnetic (FM) transitions with application or removal of magnetic field, are found to be temperature dependent and hysteretic. The corresponding critical fields increase with increasing temperature. The magnetic field (H)-temperature (T) phase diagrams have been constructed for the Gd{sub 5}(Si{sub 1.95}Ge{sub 2.05}) single crystal with field along the three directions. A small anisotropy has been observed. The magnetocaloric effect (MCE) has been calculated from the isothermal magnetization data, and the observed anisotropy correlates with H-T phase diagrams. The results are discussed in connection with the magnetic-field induced martensitic-like structural transition observed in the Gd{sub 5}(Si{sub 2}Ge{sub 2})-type compounds.
Date: September 30, 2004
Creator: Tang, H.; Pecharsky, V.K.; Pecharsky, A.O.; Schlagel, D.L.; Lograsso, T.A. & K.A. Gschneidner,jr.
Partner: UNT Libraries Government Documents Department

Valence band hybridization in N-rich GaN1-xAsx alloys

Description: We have used photo-modulated transmission and optical absorption spectroscopies to measure the composition dependence of interband optical transitions in N-rich GaN{sub 1-x}As{sub x} alloys with x up to 0.06. The direct bandgap gradually decreases as x increases. In the dilute x limit, the observed band gap approaches 2.8 eV; this limiting value is attributed to a transition between the As localized level, which has been previously observed in As-doped GaN at 0.6 eV above the valence band maximum in As-doped GaN, and the conduction band minimum. The structure of the valence band of GaN{sub 1-x}As{sub x} is explained by the hybridization of the localized As states with the extended valence band states of GaN matrix. The hybridization is directly confirmed by soft x-ray emission experiments. To describe the electronic structure of the GaN{sub 1-x}As{sub x} alloys in the entire composition range a linear interpolation is used to combine the effects of valence band hybridization in N-rich alloys with conduction band anticrossing in As-rich alloys.
Date: May 4, 2004
Creator: Wu, J.; Walukiewicz, W.; Yu, K.M.; Denlinger, J.D.; Shan, W.; Ager III, J.W. et al.
Partner: UNT Libraries Government Documents Department

Polarized source performance in 1992 for SLC--SLD

Description: In its initial operation, the SLC Polarized Electron Source successfully met the SLC goals for 1992 for intensity and efficiency. However, the stability of the beam at the source was marginal, and the polarization was only [approximately]28%. The SLC goal to provide > 10,000 Z events for the SLD from polarized electrons was met.
Date: February 1, 1993
Creator: Schultz, D.; Alley, R.; Clendenin, J.; Frisch, J.; Garden, C.; Hoyt, E. et al.
Partner: UNT Libraries Government Documents Department

Canted magnetic moments at the Gd(0001) surface

Description: With spin polarized electron spectroscopies, the authors have investigated ordered Gd(0001) films deposited on W(110). The photoemission features of the gadolinium 5d surface state, the 4f levels, and the background exhibit considerable spin polarization along the same direction in the plane of the film, indicative of ferromagnetic coupling between the surface and the bulk. The 4f spin polarized photoemission data provides strong evidence that the surface 4f polarization differs from the bulk 4f polarization for Gd(0001). The temperature dependent measurements with spin polarized secondary electron spectroscopy conclusively establishes that the surface of clean Gd(0001) possesses a perpendicular polarization component which persists to an enhanced surface Curie temperature. Small amounts of contamination at the surface result in the disappearance of the perpendicular component and, therefore, a more perfect ferromagnetic coupling between the surface and the bulk.
Date: January 1, 1993
Creator: Li, Dongqi; Zhang, Jiandi; Dowben, P.A. (Syracuse Univ., NY (United States)); Garrison, K.; Johnson, P.D. (Brookhaven National Lab., NY (United States)); Tang, H. et al.
Partner: UNT Libraries Government Documents Department