Neutron foil activation

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)... 

Neutron detection by foil activation is based on the creation of a radioisotope by neutron capture, and subsequent counting of the radiation emitted by that radioisotope. Foil activation is important not only for neutron flux measurements but also for neutron activation analysis, which is the subject of Chap. 15. This section presents the basic equations involved. 

Determination of the Neutron Flux by Counting the Foil Activity... 

Foil activation may be used for detection of the number of either fast or thermal neutrons. The use of foils for fast-neutron energy measurements is discussed in Sec. 14.6. Foil activation is not used generally for measurement of the energy of thermal neutrons. 

The LSL-M2 program package determines the neutron energy spectrum based on information obtained from a combination of neutron flux calculations and threshold foil activation measurements. The results of LSL-M2 are used primarily for the determination of radiation damage to reactor components and... 

Neutron dosimetry by foil activation is not used so much to record doses received by personnel as it is to record doses to materials, instruments, or other components that may suffer radiation damage as a result of neutron bombardment. The principle of this method was presented in Secs. 14.4 and 14.6. A target, in the form of a thin small foil, is exposed to the neutron field and becomes radioactive. The relationship between activity and neutron flux is... 

Like Fermi s and Anderson s previous experiment, the new project involved measuring neutron production in a tank of liquid. For a more accurate reading the experimenters needed a longer exposure time than their customary rhodium foils activated to 44-second half-life would allow. They planned instead simply to fill the tank with a 10 percent solution of manganese, an ironlike metal that gives amethyst its purple color and that activates upon neutron bombardment to an isotope with a nearly 3-hour... 

The neutron flux calculation error rate was evaluated to be less than 5% in the fuel region according to the comparison between MAGI and reactor dosimetry test results (see Table 5). Figure 10 shows an example of adjusted neutron spectrum based on the foil activation method at the core center position of the MK-II. 

Detailed parameter measurements made in the SE included determinations of the thermal-neutron distributions in terms of subcadmium foil activation thermal-neutron temperatures in terms of Lu/l/v activity ratios epithermal-neutron fractions in terms of Th capture and u fission cadmium ratios U production in terms of neutron captures in Th and lattice fissions in terms of the Th/ U fission ratios. The HAMMER computations generally agreed with the intracell activation profiles. The spectral index measurements agreed poorly because the computations overestimated the spectral index in all coolants. 

W. N. McELROY, S. BERG, T. B. CROCKETT, and R. J. TUTTLE, Measurement of Neutron Flux Spectra by a Multiple Foil Activation Iterative Method and Comparison with Reactor Physics Calculations and Spectrometer Measurements, IVucl. Sci. Eng., 36, 15 ( 969). 

Foil activation Reactivity-compensating changes In fuel density, neutron spectrum, and power profile Needed to Supplement reactivity measurement. Limited sample size because of counting requirements and short half-lives. 

A measurement of the quantity A soo is obtained from a knowledge of the actual flux distribution in the test specimen. The usual experimental procedure is to obtain a measure of the relative flux at various stations along the axis of the parallelepiped by means of metallic foils. The activation of these foils due to neutron captures is directly proportional to the flux level at the foil. Thus a plot of the foil activities as a function of position along the axis will yield a curve having the spatial form of the axial thermal flux. Moreover, if this curve is drawn on semilog paper, it will display a linear behavior in the intermediate range AS, as shown in Fig. 5.266. The slope of this curve yields the quantity fcaoo. But, from (5.243) we have that... 

Knowing the mass, density and reaction cross section of the foil material, and the efficiency of the counting system, the neutron flux, nv, can be derived. Corrections, of course, have to be made for decay of the foil activity during any time lapse between removal from the reactor and start of the counting sequence. 

The ratios of the various capture and fission rates, relative to the Pu fission rate, are obtained by the standard methods, such as foil activation (see Section 1.8). Measurements of this kind are among those which have ted to the abandonment of the concept of the steam-cooled fast reactor, which was at one time considered as a competitor for the liquid-metal- and gas-cooled systems. Measurements of (Xg showed that the degradation of the neutron spectrum had increased the capture-to-fission ratio to such an extent that the breeding ratio was no longer economic. 

In order to determine the indium-foil activation due to the fast neutrons present, it is suggested that we cover the foils with cadmium. Discuss the merits of covering the entire box with cadmium as compared to covering each individual foil with cadmium. 

Below the pile is a 2-ft-high graphite pedestal which contains the antimony-beryllium neutron source. Neutron-flux measurements are made in the horizontal and vertical directions, either with a neutron detector or by foil-activation techniques. The neutron distribution is then fitted to the theoretical distribution determined for a pile of this geometry. The fitting of the theoretical distribution with that obtained experimentally yields the parameters necessary for the determination of the thermal-neutron diffusion length. 

Measurements of the neutron-flux distribution can be made by foil-activation techniques. Indium has been chosen as the activation detector because of its high activation cross section for thermal neutrons and its convenient half-life of 54 min. The circular foils are 1 i. in. in diame-... 

A j = equivalent thermal-neutron-induced activity of the foil ... 

R = fraction of total foil activity due to thermal neutrons ... 

The equivalent saturated thermal-neutron-induced activity of the aluminum-covered foil irradiated in the reactor is computed, and the absolute thermal-neutron-induced disintegration rate of the saturated foil is determined. The activation cross section of indium is then computed. 

P. M. Uthe, Jr., Attainment of Neutron Flux Spectra from Foil Activations, USAFIT-Technical Report, 57-3. 

Whenever it is necessary to measure reactor neutron flux profiles, a section of wire or foil is inserted directly into the reactor core. The wire or foil remains in the core for the length of time required for activation to the desired level. The cross-section of the flux wire or foil must be known to obtain an accurate flux profile. After activation, the flux wire or foil is rapidly removed from the reactor core and the activity counted. 

Activated foils can also discriminate energy levels by placing a cover over the foil to filter out (absorb) certain energy level neutrons. Cadmium covers are typically used for this purpose. The cadmium cover effectively filters out all of the thermal neutrons. 

The neutron activation method for the determination of arsenic and antimony in seawater has been described by Ryabin et al. [66]. After coprecipitation of arsenic acid and antimony in a 100 ml sample of water by adding a solution of ferric iron (10 mg iron per litre) followed by aqueous ammonia to give a pH of 8.4, the precipitate is filtered off and, together with the filter paper, is wrapped in a polyethylene and aluminium foil. It is then irradiated in a silica ampoule in a neutron flux of 1.8 x 1013 neutrons cm-2 s 1 for 1 - 2 h. Two days after irradiation, the y-ray activity at 0.56 MeV is measured with use of a Nal (Tl) spectrometer coupled with a multichannel pulse-height analyser, and compared with that of standards. 

Two thin 1-mg samples of dysprosium are irradiated and counted in a similar manner, except for the use of a Cd cover foil on one sample. A Cd ratio of 7 is measured, with the bare foil saturation activity of 1 x 104 dpm. Calculate the thermal neutron flux at the irradiation position in the reactor. 

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