Slow neutron detector

Two different types of information can be obtained by bombarding soil with neutrons. Fast neutrons are slowed when they interact with water and thus can be used to measure the amount of water present. This type of analysis is most often conducted in the field rather than in the laboratory. Figure 8.2 illustrates the use of a fast neutron source and a slow neutron detector to measure the moisture content of soil. This method depends on the interaction of neutrons with hydrogen and so it is not as useful in soils with significant or highly variable organic matter contents. 

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. 

Thresholds for slow neutron production in dn) reactions have been observed by Bonner and Cook the ratio of the counting rate in a slow neutron detector to that in a fast neutron counter increases sharply when the incident deuteron energy becomes sufficient to excite a new level of the residual nucleus [see for example Be [dn) Sect. 50]. 

The activation [672] of Lil with Eu2+ and the use of an activated Lil phosphor as a scintillation detector for slow neutron detection [673] has been investigated. Blue, fluorescent Lil (0.03 mole % Eu) phosphor was found to be the most useful [673] phosphor because of its ease to growth, relatively high light output, chemical stability and good match with spectral characteristics of the 6260 type photomultiplier. Lil (Eu), however, does have an interfering y radiation sensitivity. Fast neutron scintillation spectra of Li6(w, a)H3 in Eu doped Lil crystals has also been investigated [674]. 

The measurement of neutron fluxes by foil activation is more complicated because the neutrons are not monoenergetic and the monitor cross sections are energy dependent. The simplest case is monitoring slow neutron fluxes. Radiative capture (ivy) reactions have their largest cross sections at thermal energies and are thus used in slow neutron monitors. Typical slow neutron activation detectors are Mn, Co, Cu, Ag, In, Dy, and Au. Each of these elements has one or more odd A isotopes with a large thermal (n,y) cross section, 1-2000 barns. The (n,y)... 

Neutrons are slowed down most effectively by light elements ( 12.6). As a consequence, neutron scattering can be used for the analyses of light elements, particularly hydrogen. In one type of instrument the radiation source consists of Cf which produces fast neutrons (from spontaneous fission), while the detector is sensitive only to slow neutrons. This system is used for studies of ground water and analysis of bore holes in wells (Fig. 6.29, G). These analyses are usually combined with density determinations using a y-source, thereby making it possible to identify strata of water, oil, coal, etc. 

In scintillation detectors, incoming radiation strikes a thin layer of crystals or a solution of organic materials that produce fight. Light output is proportional to the radiation absorbed. These devices measure alpha, beta, gamma, and slow neutron sources. 

Besides gamma peaks, the p spectra from the decays of Ge with the maximum energy of 1.2 MeV and with a half-life of 83 min, and of Ge (frpmax = 2.2 MeV, Tv2 = 11.3 h) can be seen. The p particles are in coincidence with the detected decay gammas from the daughter isotopes, partly shifting the p spectrum. Under steady-state operational conditions one count in the 139.7 keV peak from Ge indicates about two counts of P decay from the 83 min half-life isotope, and simUarly one count in the 159.7 keV peak from Ge accompanies about two counts from the 11.3 h half-life isotope. The conclusion is that one has to wait several days to make low-background measurements with detectors exposed previously to slow neutrons. 

I said No, no. I ll have a go, 1 just haven t thought about that before . Pause. I don t think you would detect any significant difference in reactor coolant outlet temperature. The instrument response time will be too slow . Pause. However, 1 know the sodium coolant absorbs some neutrons, because that is how radioactive sodium-24 forms in the primary coolant. So, if bubbles of argon were passing up through the reactor core, the absence of sodium would mean that the neutron flux would increase a bit and we would see some noise on the signals from the neutron detectors . 

There is, however, one drawback in this method of neutron detection. °Bs has a large absorption cross section for low energy neutrons, but a very low absorption cross section for high energy neutrons. Therefore, since the neutron must be captured by a boron atom for detection, fast neutrons are not detected. To detect fast neutrons, a moderating material is placed around the outside of the detector. This moderator material slows the neutrons enough for the B to absorb it. It is very important for one to know whether the neutron detector he is using has... 

Li, is a common reaction employed in neutron detectors, and these detectors are especially sensitive to slow neutrons because of large thermal neutron cross section of B -°. Another neutron detecting scheme using secondary charged particles to ionize the gas is the fission counter. Here, fission fragments do the ionizing and this detector type is also primarily sensitive to slow neutrons. Most detectors used in reactors and health physics instruments detect slow neutrons by one of the above (or similar) reactions. 

Why is the proton recoil neutron detector used for fast but not slow neutron detection ... 

Scintillators which have hydrogen as a constituent, such as organic liquids for example, may be used for fast neutron detection, since the protons produced by fast neutron collisions create the ionization required to operate the detector. In order to adapt a sodium iodide scintillator for the detection of slow neutrons, a small concentration of boron may be distributed in the crystal, giving a particles on neutron capture as discussed above. Alternatively, it is possible to add a neutron absorber which emits 7 rays following the (n, y) capture reaction. Another possibility is the use of lithium iodide (Lil) which, in addition to its own suitability as a scintillator, interacts with neutrons through the reaction... 

A small and simple instrument for measuring slow neutron fluxes in operating reactors is the self-powered, or Hilborn, detector. This consists of a length of rhodium wire (the emitter) separated by a layer of insulating material from a surrounding cylindrical metal tube (the collector). Slow neutron capture in Rh ° leads to the production of the short-lived )8 emitter which decays with a half-life of 43 s to Most of the energetic jS... 

For this experiment, four neutron detectors will be used with two being cadmium covered to decrease their sensitivity. During the course of the experiment, the new rod position will be decided upon on the basis of data from the four detectors. The emphasis is on a slow, conservative approach to critical, just as it would be for an actual approach. 

For the neutron porosity measurement, fast neutrons are emitted from a 7.5-curie (Ci) americium-beryllium (Am-Be) source. The quantities of hydrogen in the formation, in the form of water or oil-filled porosity as well as crystallization water in the rock if any, primarily control the rate at which the neutrons slow down to epithermal and thermal energies. Neutrons are detected in near- and far-spacing detectors, located laterally above the source. Ratio processing is used for borehole compensation. 

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