Pulsed-Neutron Techniques

The data displayed in Fig 1 can be readily undostood in terms of the local configurations of water molecules since the peak position ate simply related to the distances of a hydrogen-bonded pair as shown in the insets. Most mtdecular dynamics studies using a range of interaction potentials ate able to reproduce these featmes quite well on a qualitative basis. More recently, Soper and Finney (3) have used pulsed neutron techniques and obtained similar data for the gOD(r) arid gDD(r) frinctions but now have a gooCf) curve with a smaller peak height that is in better agreement with the results obtained from X-ray diffraction studies. 

Pulsed neutron techniques were used to measure neutron lifetimes on most of the cores. 

The final grouping of experiments, still in part under development, includes a presentation of some of the methods of basic research in physical science. Neutron diffraction is demonstrated and the distribution of thermal neutrons measured to familiarize the student with this useful technique. Neutron time-of-flight techniques are used to measure energy-dependent cross sections and reactor neutron spectra, and a recently developed experiment permits the student to measure prompt-neutron lifetime and shortterm kinetic reactor response directly by pulsed neutron techniques. 

CONTROL ROD WORTHS BY PULSED NEUTRON TECHNIQUES, AGN-201 REACTOR... 

This experiment is designed to determine the reactivity worth of the control rods by a pulsed neutron technique. A burst of neutrons is injected into the reactor, and the decay rate of the resultant neutron flux is measured. The decay rate measured is that of the prompt fission neutrons and is proportional to the prompt critical reactivity of the reactor. Measurements will be made with the reactor in subcritical conditions and at delayed critical. The decay rate at delayed critical yields the constant of proportionality between the decay rate of the neutron flux and the reactivity in dollars. This constant is equal to the ratio of the effective delayed-neutron fraction to the prompt-neutron lifetime. 

The three methods for measuring reactivity in a nuclear reactor are the inverse multiplication method, the positive-period and rod-drop methods based on reactor kinetics equations, and the pulsed neutron technique. This experiment will acquaint the participant with the latter technique. The pulsed neutron technique has two advantages over the others. The first is that large negative reactivities can be measured with good accuracy. Secondly, the method obtains its own calibration at delayed critical, relating the prompt-neutron decay constant to the reactivity of the system. 

TIME AND SPACE DEPENDENCE OF NEUTRON FLUX USING PULSED NEUTRON TECHNIQUES... 

High-intensity pulsed-neutron sources provide a powerful tool in nuclear physics generally, and in reactor physics in particular. Basically, the pulse technique consists of observing the transient behavior of neutron intensity following a burst of neutrons injected into the medium under study. Using pulsed-neutron techniques, one can determine neutron spectra, flux densities, capture and scattering cross sections, diffusion constants, neutron lifetime, neutron age, and reactivity. 

Introduction to pulsed-neutron techniques, pulsed-neutron generators, and measurements will be the subject of this experiment. 

In Fig. 17.5 are presented the data measured with pulsed-neutron techniques. The pulsed length was 200 jusec the measurements were started 75 /usee later and taken at intervals of 250 /jsec. In Fig. 17.5, the dotted line represents the fundamental mode, and it can be easily seen that after 1275 /usee the fundamental mode alone is present. 

G, R. Keepin, Pulsed-Neutron Techniques, LAMS-2215 (April 1958). 

Fischer, P, Zolliker, P, Meier, B. H., Ernst, R. R., Hewat, A. W., Jorgensen, J. D. and Rotella, F. J. (1986). Structure and dynamics of terephthalic acid from 2 to 300K. I. High resolution neutron diffraction evidence for a temperature dependent order-disorder transition—a comparison of reactor and pulsed neutron source powder techniques. J. Solid State Chem., 61, 109-25. [115t]... 

SANS has historically been the domain of cold neutrons on reactor based sources, and impressive instrumentation, such as notably D22 at the Institute Laue Langevin, is currently available. The more recent emergence of neutron reflectivity as a probe of surface structure has been very much linked to the development of pulsed neutron sources and much of the initial impact of this technique has arisen from pulsed source instrumentation, such as the CRISP and SURF reflectometers at ISIS. - ... 

A laboratory technique that is commonly used to calibrate particle velocity is the tracer technique. The tracers can be optically illuminating particles or short-lived radioactive tracers. We will briefly discuss two techniques that can be used to produce radioactive tracers on-line pulsed neutron activation (PNA) and off-line irradiation. 

Structures of powdered P-rhombohedral boron and amorphous boron were investigated with pulsed neutron diffraction techniques (Delaplane et al. 1988). To avoid intensive neutron absorption by °B nuclei, samples were "B isotopically enriched up to 97.1% and 99.1%, respectively. Earlier neutron diffraction studies based on nuclear reactor data did not permit the derivation of a meaningful radial distribution of atoms in amorphous material due to limited range of the neutron wave vector (<10.8 A" ). The obtained static structural factor and derived radial distribution function supported a structural model of amorphous boron based on building blocks of B,2 icosahedra resembling those found in p-rhombohedral boron, but with disorder occupying in the linking between ico-sahedral subunits. The intensity data indicated that amorphous samples contained 5% of a mixture of crystalline a- and p-rhombohedral boron. 

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