Pulsed neutron beam experiment

This case obviously corresponds to the critical system, since the time-dependent function in the expression (9.70) now vanishes. Thus a second important fact to be obtained from the pulsed neutron-beam experiment is the critical dimensions of a given geometry. Another fact to be learned from the experiment is the diffusion coefficient of the mixture and the absorption cross section of the fuel, since for small specimens... 

It is perhaps evident from these observations that the relatively modest and inexpensive pulsed neutron-beam experiment can yield a great deal of information about a particular reactor material. The outstanding... 

The first experiments with the TOF method were accomplished on stationary reactors, obtaining a pulsed neutron beam by mechanical choppers. The real potential of the method was realized after the building of pulsed neutron sources, such as pulsed reactors, electron linear accelerators and proton synchrotrons - spallation sources. In electron linear accelerators (LINAC) and proton synchrotrons, targets from 239 j heavy atoms are used. Slowing-... 

It is clear that although the exponential pile and criticality experiments would be required ultimately in any large-scale reactor development program, it would be desirable to obtain some preliminary experimental verification of reactor calculations by means of other more modest tests. One experiment which appears to be eminently suited to this purpose is based on the pulsed neutron-beam technique. This technique has been applied by several investigators to the determination of the thermal-diffusion coefficient and macroscopic absorption cross sections of reactor materials.More recently, it has been used by E. C. Campbell and P. H. Stelson in the study of short-lived isomers and for the measurement of reactor parameters of multiplying media. The experiment con-... 

The other source is the continuous wavelength spectrum of neutrons produced by stopping an accelerated beam of electrons, i.e., the spallation source . Since the electron beam is pulsed, so is the neutron beam [230]. The diffraction experiment uses the Laue method and the wavelengths are measured by their time of flight (TOF). In place of Bragg s law, dhk) = X/2 sin 0hk), the TOF relationship is... 

But arguably, the most exciting future development is the SNS, which is currently being built in Oak Ridge, Tennessee. When fully completed (scheduled to take place around 2007-2008), this facility is estimated to provide neutron beams between 50 and 100 times the intensity of current pulsed neutron sources. As far as single-crystal neutron diffraction experiments are concerned, the instrument currently being built for medium-sized unit cells is TOPAZ (Figure 3), which is scheduled to be available to users some time in 2009 and is planned to handle crystals... 

Knowledge of the spectrum of higher energy neutrons (/- 1.4X 10 Mev) from proton synchrotrons is still rather preliminary, and neutrons associated with meson production probably introduce an appreciable low energy tail. Experiments with neutrons at these energies resemble those with cosmic rays in that the detection technique determines the effective energy spread as in the experiments of CooR et al.. Their neutron beam was the order of 10 neutrons per cm per pulse of 10 protons. 

Financial support (W.H.D. and H.F.v.G.) was given by the Dutch Department of Economic Affairs, as part of the "lOP-Katalyse" programme. Beam time both at Daresbury Laboratory and Rutherford Appleton laboratory were provided by the SERC/NWO agreement on use of synchrotron radiation and pulsed neutron source respectively. We thank Drs. Wim Bras (NWO/SERC) for his assistance at SAXS and dr. Richard Heenan (SERC) for his assistance at SANS. Minispec experiments were performed at Bruker Spectrospin N.V., Wormer, The Netherlands and we appreciate the assistance of Piet Ruigrok. 

To perform the experiment [41] an intense beam of high-energy K was constructed at Fermilab. After the 400 GeV/c proton beam struck a beryllium target, a series of collimators and magnets defined the beam and swept charged particles from the flux of secondaries emerging in the forward direction (Fig. 5). The average K momentum was about 75 GeV/c, and typical intensities were about 107 K s and 109 neutrons per accelerator pulse. The setup detected the pions and muons from the decay... 

On the contrary, the results of experiments presented here have been produced with continuous neutron sources (controlled chain reactions). The beams thus produced, are more suitable to study long polymers than the pulsed beams in fact, such studies require neutrons with a large wavelength, i.e. slow neutrons. 

The more typical NPL reported has been pumped by a pulsed or steady-state nuclear reactor. Reactor pumped lashhers (RPLs) are in a sense laboratory laser devices, even though in practice there are only a few laboratories with the required reactor facilities. The relatively few workers in the RPL field is probably related to the scarcity of these facilities. In these RPL experiments an intense pulse of neutrons triggers a nuclear reaction, which emits heavy charged particles into a gaseous laser medium. This results in excitation processes qualitatively similar to those produced by an intense electron beam. 

The TOF method is applied to elastic and inelastic scattering instruments. In both cases, neutron choppers are used to produce pulsed beams for time of flight experiments. TOF instruments use time of flight to give a continuous wavelength scan at one or many fixed angles. 

The diffusion coefficient and the diffusion length are fundamental macroscopic properties of a material which are useful in the one-velocity formulation. Both quantities can be measured directly in the laboratory by suitable experiments. The direct measurement of the diffusion coefficient, however, entails the use of a pulsed beam of neutrons. Inasmuch as this experiment involves a time-dependent phenomenon, a discussion of the experiment will be deferred until after a suitable model has been developed for the analysis of nonstationary problems in neutron diffusion. An experiment for the direct determination of the diffusion length, however, is based on a steady-state phenomenon, and the important features of this experiment can be displayed by means of the models and concepts already developed. Because of the close relationship between the parameters L and D, it would be desirable to examine the techniques for their measurement simultaneously. This is not possible because of the complications mentioned above thus for the present we confine our attention to the study of the diffusion length and an experiment for its measurement. 

---