Measurement of neutrons

Straume, T., T.C. Kwan, L.S. Goldstein, and R.L. Dobson. 1991. Measurement of neutron-induced genetic damage in mouse immature oocytes. Mutat. Res. 248 123-133. 

Estimates of primordial helium and deuterium with improved measurements of neutron half-life restrict number of light neutrino families to about 3. Similar, firmer result from LEP measurements at CERN of width of Z° resonance. 

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)... 

The expected advantages of the proposed experiment are of three kinds. First, this experiment will operate on isolated atoms, thus avoiding all possible parasitic effects related to macroscopic samples. Second, its sensitivity is expected to be at least comparable and hopefully larger than the one achieved by the best previous experiments reported below. Third, one can perform the experiment with the two natural isotopes of lithium, 6Li and 7Li and comparing these two results will lead to an independent measurement of neutron charge. 

On a personal note, the mathematical description of this series of neutron captures called the s process was solved by the writer in his Ph.D. thesis, and was published in 1961. This provided me the opportunity to assist in small ways with the development of programs of the measurement of neutron-capture cross sections for the s process, first at Oak Ridge and later at Karlsruhe. These were great privileges in a fortunate life. 

Use Measurement of neutron flux, reactor fuels, fluorescence activator in phosphors. 

Nuclear and pion related 7-rays provide important information about the spectra of protons and ions accelerated in solar flares [e.g. Hua and Lingen-felter, 1987 Murphy et al., 1987 Lockwood et al., 1997 Hua et al., 2002], However, nuclear 7-ray lines probe the proton spectrum only up to 40 MeV, while 7-rays from pion decays are only observed in the most intense flares. In addition, any spectral break in the proton spectrum is likely to he below the pion production threshold. Neutrons produced at the solar surface over a wide range of energies may provide important information from the 50-300 MeV regime, complementing 7-ray observations. Due to the long neutron thermal-ization time ( 100 s) the 2.223 MeV neutron capture line is only a limited measure of neutron production. The spectrum of accelerated and interacting protons can be deduced more reliably from direct neutron measurements. 

More recently, Rawiso, Duplessix, and Picot17 made very precise absolute measurements of neutron scattering by long polystyrene chains in CS2. The chains consisted of monomers labelled in the following manner... 

Sands DG, De Laeter JR, Rosman KJR (2001) Measurements of neutron capture effects on Cd, Sm and Gd in lunar samples with implications for the neutron energy spectrum. Earth Planet Sci Lett 186 335-346 Schaeffer OA (1975) Constancy of galactic cosmic rays in time and space. 14th Inti Cosmic Ray Conf 3508-3520... 

The time-of-flight (TOF) method, which is also used for the measurement of neutron energy (see Sec. 14.8), has been applied successfully for the determination of the mass of fission fragments and other heavy ions. 

Fission counters are used extensively for both out-of-core and in-core measurements of neutron flux in nuclear flux in nuclear reactors. In out-of-core situations, they monitor the neutron population during the early stages of power ascension when the neutron flux level is very low. For in-core measurements, fission counters are used for flux mapping (and consequently, determination of the core power distribution). They are manufactured as long thin q lindrical probes that can be driven in and out of the core with the reactor in power. Typical commercial fission counters for in-core use have diameters of about 1.5 mm (0.06 in), use uranium enriched to at least 90 percent in as the sensitive material, and can be used to measure neutron fluxes up to 10 neutrons/(m s) [10 neutrons/(cm s)]. 

The measurement of neutron energy with a crystal spectrometer is based on the same principle of Bragg diffraction as is the measurement of X-ray energy (see Sec. 12.10). 

The electric conductivity of various samples of germanium and silicon doped with °B during the neutron irradiation was measured by Kervalishvili et al. (1993). In particular, measurements of neutron fields of various nuclear plants with sensors made of these materials have been carried out. The selection of the base material and the concentration of °B impurity allowed the efficiency of neutron detection and flux measurement to be varied. Such neutron dosimeters possess memory and can be used as witnesses of unusual situations in nuclear plants. 

E. Greenspan, Theory and Measurement of Neutron Importance in Nuclear Reactors, CURL-15 Cornell Univ. Reactor Lab. (1966) see also E. Greenspan and K. B. Cady, Trans. Amer. Nucl. Soc. 9, 243 (1966). 

Boron Thermopiles. The boron thermopiles are used for an additional check on neutron flux level. They are located, in the graphite. (VG-27, 28, 42, 44, 56 see Fig.- 3.F) all at the same horizontal level. These instruments consist of a large number of thermocouple junctions in series, with alternate junctions coated with boron. The coated junctions become warm owing to absorption of neutrons, thus yielding a net voltage which is a measure of neutron flux. The instrument case is about H in. in diameter by 6 in. long. The approximate sensitivity is 6 mv in a flux of 10. Internal resistance is of the order of 3 to 4 ohms. By proper mounting and thermal insulation of the thermocouples the instruments may be made quite insensitive to ambient temperature (say, 40 /xv/°C). For transient conditions the time constant is about 1 sec. 

Measurements of neutron and gamma fluxes inside and above the fuel, in the graphite reflector outside the tank, and in the thermal shield. 

APPARATUS FOR THE MEASUREMENT OF NEUTRON ABSORPTION Filed March 4, 1947... 

This invention relates to an improved apparatus for the measurement of neutron absorption characteristics of materials. More specifically the invention relates to an improved apparatus for measuring the effect of the presence of the sample under measurement on the neutron reproduction factor, and thus the power output, of a neu-tronic reactor capable of sustaining a nuclear fission chain reaction. 

It is, therefore, the principal object of this invention to provide an improved apparatus for the measurement of neutron absorption characteristics of samples of materials. 

As far as provisions affecting both the design and the operation are concerned, the most relevant one is the experimental programme for the measurement of neutron embrittlement. This programme should offer a good indication of the state of the material in the areas of interest (base metal, welds, heat-affected zones) well ahead of time the specimens, that is, must be located where the neutron... 

A. E, Profto et al., "Time-of-Flight Measurements of Neutron Spectra in U-235 and Tungsten, GA-7427, General Atomic (1966). 

W. U. McELROY, "Measurement of Neutron Flux Spectra by a Multiple Fmi Activation Rerative Method and Comparison with Reactor Physics Calculations and Spectrometer Measurements," Ihicl. Sei. Eng.. 36, 109 (1969). 

T. W. BONNER, "Measurements of Neutron Spectra from fission," Mtcl. Phys., 23, 116 (1961). 

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