摘要:

The Advanced Neutron Source (ANS) is a new user experimental facility planned to be operational at Oak Ridge in the late 1990’s. The centerpiece of the ANS will be a steady‐state research reactor of unprecedented thermal neutron flux (&fgr;th≊8×1019m−2.s−1) accompanied by extensive and comprehensive equipment and facilities for neutron‐based research.

ISSN:0094-243X    

DOI:10.1063/1.41180

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The availability of cold neutron beams can improve the quality of results obtainable from a capture gamma‐ray measurement. Although capture gamma‐ray instruments that use cold neutrons are less susceptible to problems that limit the capabilities of thermal instruments, new design parameters become important to consider. We discuss some of the questions that relate to neutron beam handling and present techniques for improving the quality of the capture gamma‐ray instrument.

ISSN:0094-243X    

DOI:10.1063/1.41297

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Arguments are presented that the addition of cold neutron beam facilities at intermediate flux reactors is an effective and feasible way to upgrade their value as research facilities by expanding the range of topics in which they can carry on competitive and useful research. These include prompt gamma‐ray analysis, basic level scheme studies with capture gamma‐ray and conversion electron spectroscopy in which very pure or very low background beams are required, development and testing of new methods and instruments for research with cold neutrons, and pedagogical experiments that demonstrate quantum mechanical interference and related phenomena at the macroscopic level. Reasons are suggested that may explain why the only US reactors with cold sources are at Brookhaven National Laboratory (BNL) and the National Institute of Standards and Technology (NIST). It is also noted that only the latter has a system of cold neutron beams comparable to those at European laboratories. Design considerations for installing cold neutron beams at intermediate flux reactors, including safety and operational simplicity, are discussed in general terms. Some examples of specific design compromises are drawn from the Cornell project that has been underway for several years.

ISSN:0094-243X    

DOI:10.1063/1.41181

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Intense positron sources from pair production of high energy prompt &ggr;‐rays from neutron capture reactions are described. A titanium‐platinum target placed in a thermal neutron flux of 3.3×1014n/cm2s was used in conjunction with the BILL beta spectrometer as a high intensity, monochromatic and tunable positron source. The system yielded a 2.5×106e+/s in 10cm of the focal plane with a momentum dispersion of &Dgr;p/p=1.5×10−3per cm. It was applied for the search for neutral resonances superimposed on the excitation function of elastice+e−(Bhabha) scattering. These resonances had been proposed as an explanation of the GSI positron lines from heavy ion reactions. No such resonances were found and very stringent limits were set for their intrinsic lifetimes. In addition, the possibility of constructing a very intense source of moderated positrons based on the same principle is discussed.

ISSN:0094-243X    

DOI:10.1063/1.41182

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

In November 1989, we carried out a measurement of the fission cross section of247Cm,250Cf, and254Es on the Rensselaer Intense Neutron Source (RINS) at Rensselaer Polytechnic Institute (RPI). In July 1990, we carried out a second measurement, using the same fission chamber and electronics, in beam geometry at the Los Alamos Neutron Scattering Center (LANSCE) facility. Using the relative count rates observed in the two experiments, and the flux‐enhancement factors determined by the RPI group for a lead slowing‐down spectrometer compared to beam geometry, we can assess the performance of a spectrometer similar to RINS, driven by the Proton Storage Ring (PSR) at the Los Alamos National Laboratory. With such a spectrometer, we find that it is feasible to make measurements with samples of 1 ng for fission 1 &mgr;g for capture, and of isotopes with half‐lives of tens of minutes. It is important to note that, while a significant amount of information can be obtained from the low resolution RINS measurement, a definitive determination of average properties, including the level density, requires that the resonance structure be resolved.

ISSN:0094-243X    

DOI:10.1063/1.41183

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The use ofn‐capture gamma rays for studies of some topics in condensed matter physics is reviewed. These include the study of Debye temperatures of some metallic elements and molecular gases. Properties of 2‐dimensional systems such as adsorbed gases on graphite and also of graphite intercalated compounds are discussed in detail. In particular, the zero‐point energies of the vibrational and librational potentials of the molecules with respect to the adsorbing surfaces was determined.

ISSN:0094-243X    

DOI:10.1063/1.41184

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The internal oxidation of dilute solid solutions of Sb and In in single crystals of silver was studied with nuclear reaction analysis, depth profiling and channeling using the18O(p,&agr;)15Nreaction. Combining the results obtained for theOconcentration depth profiles and theOlattices sites with results of hyperfine interaction measurements, a detailed picture of the internal oxidation process was obtained. It was shown that isolated molecules of SbO2and InO2embedded in the Ag matrix are created, which diffuse as an entity at elevated temperatures to form an oxide layer at the surface or to form precipitates in the Ag matrix. At still higher temperatures these oxides decompose and the Sb or In redissolves in the silver. The information obtained for this model system may be important, among other things, to understand the formation of buried nitride layers in metals or silicide layers in silicon by high‐energy implantation.

ISSN:0094-243X    

DOI:10.1063/1.41185

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Resonant capture reactions are a versatile tool to measure concentration profiles of light elements in near‐surface layers. They allow us to monitor, with nanometer depth resolution, atomic transport processes caused by thermal diffusion, ion irradiation, nucleation and chemical forces. We have investigated properties of ion‐implanted super saturated Na solutions in metals by using the 309 keV23Na(p,&ggr;) resonance. Results on Na‐nucleation, outdiffusion and hydrogen decoration are presented; theH‐profiles were measured with the resonant1H(15N,a&ggr;) reaction. The second example refers to 30–300 nm magnetron sputtered TiN and Cr2N hard coatings irradiated with 100‐900 keV Ar, Kr and Xe ions. The resonant15N(p,a&ggr;) reaction served to monitor the composition of the coatings and ion‐induced surface sputtering and interface mixing effects.

ISSN:0094-243X    

DOI:10.1063/1.41186

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The gamma rays escaping from a planet can be used to map the concentrations of various elements in its surface. In a planet, the high‐energy particles in the galactic cosmic rays induce a cascade of particles that includes many neutrons. The &ggr; rays are made by nuclear excitations induced by these cosmic‐ray particles and their secondaries (especially neutron capture or inelastic‐scattering reactions) and by the decay of the naturally‐occurring radioelements. After a short history of planetary &ggr;‐ray spectroscopy and its applications, the &ggr;‐ray spectrometer planned for the Mars Observer mission is presented. Laboratory experiments that simulate the cosmic‐ray bombardments of planetary surfaces or measure cross sections for the production of &ggr; rays are reviewed. Theoretical calculations for the processes that make and transport neutrons and &ggr; rays are discussed. The emphasis here is on studies of Mars and on new ideas, concepts, and problems that have arisen over the last decade, such as Doppler broadening and peaks from neutron scattering with germanium nuclei in a high‐resolution &ggr;‐ray spectrometer.

ISSN:0094-243X    

DOI:10.1063/1.41188

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The neutron current density can be increased by using a converging guide; this can be particularly useful as the last section of a long straight guide. We give the expected flux gains for such a system. This increased density is obtained at the expense of increased divergence of the beam, which is unimportant in neutron absorption techniques, such as prompt gamma activation analysis and neutron depth profiling. These experiments can benefit from improved detection limits available when the neutron beam is focused by a neutron optical element such as a converging guide.

ISSN:0094-243X    

DOI:10.1063/1.41153

出版商:AIP    

年代:1991

数据来源: AIP