Neutron fluence rate

The water thickness measurement uncertainty due to neutron counting statistics can be calculated from Poisson counting statistics. For a random process, the standard deviation, Ah in the observed counts I is A/ = y7. The number of neutrons in the incident, or open beam, I0, is the product of the neutron fluence rate (cur2 s 1), integration time T (s), integration area A (cm2), and neutron detection efficiency, ip... 

E10.05 E0481-97 Standard Test Method for Measuring Neutron Fluence Rate by Radioactivation of Cobalt and Silver... 

When a reactor is not available, or a portable neutron source is required, isotopic sources (Shani 1990), small mobile reactors (Chung 1990), or neutron generators (Csikai 1987) are possible alternatives. The attainable neutron fluence rates of the various neutron sources are compared in Table 30.1. 

HQ. 3. Inteiference-hee limits of detection bv NAA in nanograms for the elements based on irradiation for 5 hr at a neutron fluence rate of lo cm sec and typical counting conditions [6]. 

Limitations in radiation dose, neutron fluence rate, cross-sections, half-life, and so forth may be partially offset by longer irradiation, larger detectors, larger samples, pulsed operation, etc. 

Typical factors that may increase the overall variance d are temporal or spatial variations in neutron fluence rate during activation that affect replicate samples in a different way. Particularly in the case of in vivo analysis, unavoidable small differences in counting geometry may give rise to additional variability among replicate determinations. 

A filtered, collimated neutron beam with the characteristics discussed above is required to perform BNCT. There are several ways to obtain such a beam. A popular method moderates the fast neutrons leaking from the core down to the appropriate energy. This works well in facilities that have a large thermal column that can be unstacked and replaced with appropriate moderators and filters. The addition of a fission converter consisting of a row of fuel elements can significantly increase the epithermal neutron fluence rate at a distance from the core. If the facility does not incorporate a thermal cavity or other large void space near the core, shield removal is an alternative to provide direct access to the core or for the installation of a converter. 

The protection system monitors relevant plant variables. These may be process variables such as neutron fluence rates (fluxes) or coolant temperatures and pressures, or they may be variables specific to anticipated operational occurrences or... 

The Fluence Rate and Approximate LID for the Five Apertures Available at the NIST Neutron Imaging Facility. The Slits in Positions 1 and 2 Reduce the Geometric Blur Along One Direction by About a Factor of 5 over the Previous Configuration for Use in High-Resolution Imaging of the Through-Plane Water Content. 

Figures 3(a) and (b) show fast neutron fluence dependence of conductivities for the specimens (a) with and (b) without H at ionizing dose rates of 0.5 and 1.8 Gy/s, respectively. The irradiation temperatures for the specimens with and without H were 384-401 and 473 K at the first cycle and 484-519 and 673 K at the second cycle. It was found that the RIC increased with raising the irradiation temperature. Particularly, the RIC with H greatly corresponds to the change of the irradiation temperature. On the other hand, the RIC without H quickly decreased at the initial fluence and hereafter gradually as the fast neutron fluence increased, even if the irradiation temperature was constant.

The most stable graphites are the near-isotropic materials ". The effects of irradiation on the properties of graphite are governed by neutron energy, neutron fluence, temperature, temperature gradients, and structure and type of graphite used. The effects of irradiation are dimensional changes, reduction in thermal conductivity, and enhancement of creep rates. 

The models and material property data for predicting fission metal release from fuel particles and fuel elements are described in Ref. 4. The transport of fission metals through the kernel, coatings, fuel rod matrix, and fuel element graphite is modeled as a transient diffusion process in the TRAFIC code (Section 4.2.5,2.2.1.2). The sorption isotherms which are used in the calculation of the rate of evaporation of volatile metals from graphite surfaces account for an increase in graphite sorptivity with increasing neutron fluence. 

Regarding the neutron fluence value at the time of annealing, no substantial effects were found. Residual embrittlement, as well as the rate of further re-embrittlement, is practically independent of this neutron fluence. The lateral (horizontal) shift approach was also confirmed in the IAEA Round Robin Exercise on radiation embrittlement of WWER-440 weld metal, as shown in Rg. 5.16. The microstructure investigation results demonstrate that copper clusters, which are formed in material under primary irradiation, are not recovered during annealing to a structure of solid solution such as that in the unirradiated state. 

ASTM (1998) Standard practice for determining neutron fluence, fluence rate, and spectra by radioactivation techniques (E261), Report E 261-98. ASTM International, West Conshohocken... 

PGNAA can also be carried out with a nuclear reactor as a neutron source, provided a beam of neutrons can be extracted from the reactor to obtain a reasonably low background during irradiation. Superior results are obtained by the use of cold neutrons, which may be guided to positions where the background is negligible. Although die fluence rate is low, the low velocity of the neutrons results in increased capture cross-sections for many elements [9]. 

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