Sources, neutron

Since Chadwick first produced free neutrons in 1932, neutrons have been used in investigations in physics, chemistry, material science, and biology. 

A typical neutron scattering beam hall layout. 

Differences between a pulsed source and a reactor from the view point of beam port 

The energy of the neutrons from a spallation source is selected using time of flight techniques, while a chopper or a crystal monochromator is used to select the neutron energy for neutrons from a reactor. For a reactor, the double chopper inelastic spectrometer is used while for the pulsed spallation source, one chopper is used for matching the neutron pulse duration to that from the moderator. 

The production and the control of a neutron beam requires a lot of equipment of monumental size. Accordingly, the number of installations providing proper 

The heat produced during the fission process is removed by circulation of a cooler. Even if the heat outflow (which equals 6 x 107 at the ILL at Grenoble) is maximized, the neutron flux must be maintained below 1016 cm 2 s in order to ensure that the temperature of the reactor elements remains under a critical technological threshold. The flux is low as compared to the photon flux obtained with a light source (I019 cm 2s-1 for a laser with a cross-section of 1(T2 cm 2 and a power of 10-i watt).+ However, neutrons can be used to perform experiments that could not be carried out with photons (lack of contrast, for instance). 

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. 

Some materials have a spontaneous decay process that emits neutrons. Some shortlived fission products are in this class and are responsible for the delayed neutron emission from fission events. Another material in this class is Cf that has a spontaneous fission decay mode. Cf is probably the most useful material to use as a source of neutrons with a broad energy spectrum. 

There are many isotopes that decay by alpha emission. When these isotopes are placed in intimate contact with another material, such as berylhum, the resulting (CC,n) reaction can be used as a neutron source. Berylhum is the target material with the highest neutron yield. Other targets include Li, and Table 1 

Neutron detectors are often separated into two types low-energy neutron and fast neutron detectors. Low-energy neutrons are typically detected through the use of a 

Self-powered neutron detectors (SPNDs) use a material such as cadmium that has a high cross section for low-energy neutrons and produces copious gammas or 

Fast neutron detection sometimes uses a hydrogenous moderator to slow down the neutrons and then employs a low-energy neutron detector as described above. One common fast neutron detector is a Bonner sphere. In this detector, a scintillator is placed in the center of a polyethylene sphere. Radiation transport calculations are used to produce efficiency curves that depend on the energy of the incident neutron. Another common fast neutron detector is a long counter. This detector uses a slow neutron detector (originally a BF3 chamber) at the center of a cylindrical moderator designed so that the detector is only sensitive to neutrons incident from one side. 

A nuclear reactor is, of course, itself a copious source of neutrons, and many experiments have been carried out using intense beams of neutrons extracted from an operating reactor. For many applications, however, it is desirable to have a cheap and compact source of neutrons with a defined energy distribution. 

One of the most convenient mechanisms for producing neutrons is the bombardment of beryllium with a particles from a radioactive source. The reaction involved is 

This process is exothermic, the sum of the masses of the beryllium nucleus and the a particle exceeding the sum of the product masses by an amount equivalent to an energy of 5.7 MeV. 

A common form of (a, n) source is a finely divided mixture of a radium salt with powdered beryllium. Because of the long half-life of Ra  

Where it is desirable to have a monoenergetic source, i.e., one where all the neutrons are emitted with the same energy, use can be made of the (y, n) reaction, in which the target nucleus is excited by the capture of a y ray, and subsequently decays by the emission of a neutron photoneutron source). The only practical targets are beryllium or deuterium (in the form of heavy water), in both of which the neutron binding energy has a particularly low value. The reactions are 

At room temperature, thermal neutrons have energies near 0.03 eV, velocities of 2 x 10 cm/sec, and wavelengths of 2 A. All known sources are characterized by being continuous spectrum sources with a spectrum resembling a Maxwellian distribution. Most diffraction techniques utilize a monochromatic beam, and for the neutron case this means a selection of a monochromatic band of radiation by diffraction from a crystal, or by some alternative mechanical system. This has two implications (a) a large fraction of the neutron intensity 

Because of the slow rate of neutron data collection, automatic operation of spectrometers is a necessity and it is fair to say that the neutron technology is as advanced here as in X-ray methods. Very elaborate, computer-controlled neutron spectrometers have been designed and are in operation at many reactor installations. 

The following sources and instruments dominate studies in the area of liquids and amorphous materials. Although there are a number of sources available, each is optimized for a particular class of experiment. The sources can be split into two types pulsed neutron sources and reactor sources 

The basic essentials required to carry out an analysis of samples by NAA are a source of neutrons, instrumentation suitable for detecting y-rays, and a detailed knowledge of the reactions that occur when neutrons interact with target nuclei. Brief description of various neutron sources and y-ray detection systems are given in the following subsections  

There are several types of neutron sources viz. radioisotopic neutron emitters like Pu-Be, particle-accelerators (to produce high flux of neutrons through (p,n), (d,n), (a,n) or other reactions) and nuclear reactors (which are sources of neutron due to fission reaction). Nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions (thermal, epithermal, and fast) and fluxes due to the materials used to moderate the 

A comparison between Cf and Pu-Be for partial-body in vivo NAA (Morgan et al. 1981) shows that although the depth distributions of thermal neutron fluences in a water phantom are very similar for the two sources yet with respect to the fiuence-to-dose ratio, the Cf neutrons have an advantage 

Neutron Beam Using Radioisotopes Through (a, n) and (y, n) Reactions 

Neutron beam can be produced from ( , n) reaction using cx-radioactive sources and Be-target or using photon-sources interacting on Be/D2 0 targets are presented in Tables 6.1 and 6.2. 

In this chapter we will describe how these are accomplished and how the sample may be introduced so as to optimise the measurement. Some safety aspects of neutron experimentation will also be presented ( 3.6). More detailed discussion of neutron sources and instrumentation can be found in [1-3]. 

The experiment was successful because the nucleons (protons and neutrons) are arranged in shells in the core, analogous to electron shells, and in Be the highest energy neutron is the only nucleon in the outermost shell. Thus exactly as sodium has a low first ionisation energy because the 3 s electron is outside a filled shell, the highest energy neutron in Be is comparatively weakly bound and so is expelled when 

A number of methods have been used to generate sufficiently strong neutron sources, but all current facilities are either fission reactors or spallation sources, so we will consider only these two methods. 

We should not leave our discussion of nuclear reactors without mentioning the Oklo phenomenon. In 1972, French scientists analyzing uranium ore from the Oklo uranium mine in Gabon found ore that was depleted in 235U. Further investigation showed the presence of high abundances of certain Nd isotopes, which are formed as fission products. The relative isotopic abundances of these isotopes were very different from natural abundance patterns. The conclusion was that a natural uranium chain reaction had occurred 1.8 billion years ago. 

252Cf acts as the basis of a radionuclide neutron source because 3.2% of its decays are by spontaneous bssion, yielding 3.76 neutrons per bssion. The neutron emission rate/Ci of material is quite high and these sources have found widespread use. 

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The application of IP can facilitate or even make feasible some experimental techniques in NR, where the neutron source intensity poses a problem for imaging with radiographic films. 

Neutron source Neutron sources Neutrons, thermal Neutropenia... 

Experiments were conducted during the Metallurgical Project, centered at the University of Chicago, and led by Enrico Fermi. Subcritical assembhes of uranium and graphite were built to learn about neutron multiphcation. In these exponential piles the neutron number density decreased exponentially from a neutron source along the length of a column of materials. There was excellent agreement between theory and experiment. 

Among other isotopes produced at SRP were uranium-233 for breeder research, cobalt-60 [10198-40-0] for irradiators, plutonium-238 for spacecraft such as V ojager 2in.d lunar research power suppHes, and califomium-252 as a fast neutron source. The accomplishments of Du Pont at SRP are well chronicled (53). 

Uses of Plutonium. The fissile isotope Pu had its first use in fission weapons, beginning with the Trinity test at Alamogordo, New Mexico, on July 16, 1945, followed soon thereafter by the "Litde Boy" bomb dropped on Nagasaki on August 9, 1945. Its weapons use was extended as triggers for thermonuclear weapons. This isotope is produced in and consumed as fuel in breeder reactors. The short-Hved isotope Tu has been used in radioisotope electrical generators in unmanned space sateUites, lunar and interplanetary spaceships, heart pacemakers, and (as Tu—Be alloy) neutron sources (23). 

Beryllium has a high x-ray permeabiUty approximately seventeen times greater than that of aluminum. Natural beryUium contains 100% of the Be isotope. The principal isotopes and respective half-life are Be, 0.4 s Be, 53 d Be, 10 5 Be, stable Be, 2.5 x 10 yr. Beryllium can serve as a neutron source through either the (Oi,n) or (n,2n) reactions. Beryllium has alow (9 x 10 ° m°) absorption cross-section and a high (6 x 10 ° m°) scatter cross-section for thermal neutrons making it useful as a moderator and reflector in nuclear reactors (qv). Such appHcation has been limited, however, because of gas-producing reactions and the reactivity of beryUium toward high temperature water. 

It is supposed to apply this neutron source to nondestmctive evaluation of products using neutron radiography and elemental analysis of materials by detection of capture gamma rays. 

NAA involves the bombardment of the sample with neutrons, which interact with the sample to form different isotopes of the elements in the sample (14). Many of these isotopes are radioactive and may be identified by comparing their radioactivity with standards. This technique is not quite as versatile as XRF and requires a neutron source. 

Alternatives to XRD include transmission electron microscopy (TEM) and diffraction, Low-Energy and Reflection High-Energy Electron Diffraction (LEED and RHEED), extended X-ray Absorption Fine Structure (EXAFS), and neutron diffraction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivity in neutron diffraction is quite different from XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron diffraction is a good alternative for low-Z materials and for materials where the magnetic structure is of interest. 

Though a powerfiil technique, Neutron Reflectivity has a number of drawbacks. Two are experimental the necessity to go to a neutron source and, because of the extreme grazing angles, a requirement that the sample be optically flat over at least a 5-cm diameter. Two drawbacks are concerned with data interpretation the reflec-tivity-versus-angle data does not directly give a a depth profile this must be obtained by calculation for an assumed model where layer thickness and interface width are parameters (cf., XRF and VASE determination of film thicknesses. Chapters 6 and 7). The second problem is that roughness at an interface produces the same effect on specular reflection as true interdiffiision. 

Since the recognition in 1936 of the wave nature of neutrons and the subsequent demonstration of the diffraction of neutrons by a crystalline material, the development of neutron diffraction as a useful analytical tool has been inevitable. The initial growth period of this field was slow due to the unavailability of neutron sources (nuclear reactors) and the low neutron flux available at existing reactors. Within the last decade, however, increases in the number and type of neutron sources, increased flux, and improved detection schemes have placed this technique firmly in the mainstream of materials analysis. 

Two types of sources are used. Originally developed in the 1940s, nuclear reactors provided the first neutrons for research. While reactors provide a continuous source of neutrons, recent developments in accelerator technology have made possible the construction of pulsed neutron sources, providing steady, intermittent neutron beams. 

For further discussion of neutron sources, see R. B. Von Dreete. Reviews in Mineralogy. Volume 20 Modem Powder Diffraction. 333,20, 1990. 

For a detailed discussion of pulsed neutron sources, see J. D. Jorgensen andj. Faber. ICANS-II, Proceeding of the Sixth International Collaboration... 

At the end of the irradiation, the samples are withdrawn from the reactor and y-ray spectroscopy is carried out. Most often the laboratory performing the y-ray spectroscopy is located in a different city, in which case the samples are shipped and the reactor serves as a neutron source only. Many reactors also have y-ray spectroscopy capability so that measurements can be made at the reactor site as well. 

One example of a pulsed neutron source is to be found at ISIS, at the Rutherford Appleton Laboratory, UK. This source has the highest flux of any pulsed source in the world at present, and is therefore one of the most suitable for isotopic substitution work, as this class of experiment tends to be flux-limited. At ISIS, two stations are particularly well set up for the examination of liquids. 

ISIS is only one pulsed source available for the study of liquids. Both the USA and Japan have facilities similar to SANDALS and GEM for studying liquids, but with slightly lower neutron intensity in the forms of the IPNS (Intense Pulsed Neutron Source) at the Argonne National Eab. on the instrument GEAD, and the KEK Neutron Scattering Eacility (KENS) on the instrument ELit II, respectively. 

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