Cadmium ratio

The detector elements of JP-A-61268075 are formed in mesas where the mercury to cadmium ratio is increased towards the surface of the mesas. The design provides an imager with a low level of surface leakage-currents. 

A silicon dioxide layer 3 is formed on an insulating CdTe substrate 1. A photo-resist coating 5 is formed over the silicon dioxide layer. The photo-resist layer is patterned and the silicon layer is partly etched away. The photo-resist layer is removed and a film of HgCdTe 9 of a first mercury to cadmium ratio is deposited by liquid phase epitaxial deposition over the entire surface of the substrate. The HgCdTe film is only formed at regions where the CdTe substrate is exposed and does not adhere to the silicon dioxide. Next, the silicon dioxide layer is removed. In order to increase the window of frequency response of the detectors, the process is repeated using a second mercury to cadmium ratio different from the first ratio. 

No clear-cut method for estimating the neutron current leaving the thermal shield was established analytically. Therefore, the final analysis of the concrete with respect to neutron effects was held in.abeyance until certain experimental data were available from the Mock-Up. These data are principally measurenents of therma1-neutron flux and of the cadmium ratios at various points in the reactor. 

The cadmium ratios on the outer surface of the graphite are very insensitive to changes in distance from the core. This is interpreted to indicate that the spectrum at all positions on the outside of the graphite is nearly at equilibrium. 

With these data it sedms reasonable to postulate that (a).the observed thermal flux at the surface of the graphite is proportional to the total flux there, and (b) the cadmium ratios at this location are proportional to the fraction of fast neutrons (of the iron-penetrating variety) in the spectrum. 

The calculation of the neutron current depends on the choice of the cadmium ratio as discussed earlier in this section. Although examination of the Mock-Up experimental results indicates values ranging from 1200 to 2000 at this distance from the core, a value of 1000 is used for design in an effort again to be conservative. From Eq. (7),... 

Figures A4-R and S show the data for the mid-plane and top plane of the active lattice. Typical curves showing vertical distribution of neutron fluxes in several holes in the graphite reflector are presented in Figs. A4.T. U, and V. Table A4.G lists the therma1-neutron flux values at various mid-plane positions in the graphite reflector with the cadmium ratio at the points where it was measured.

Table A4.H lists the flux at mid-plane in most of the holes in the northwest quadrant of the graphite reflector and the cadmium ratio in the positions where it was measured. Table A4.I shows the measurements taken on the top and bottom of the graphite reflector. Traverses on the west face of the graphite behind hole 17 with and without the partial steel thermal shield section in place are presented in Table A4.J.

The problem of measuring an unknown cross section by the activation method involves the determination of < o and 4>ep by choosing appropriate monitors and the determination of R. Usually, the amount of product formed is determined by its radioactivity or by its mass using a mass-spectrometer, making a correction for any product destroyed by other reactions (spontaneous decay or neutron transmutation). A usual procedure is to measure both 4>o and cadmium metal covers or irradiations can be carried out simultaneously at two or more different positions in a reactor if the relations between the fluxes are known. The cadmium ratio (CR), that is, the rate of product formed without cadmium to that with a cadmium cover is given by the equation... 

It is important to use different monitors for the resonance and thermal fluxes, and to use positions of different < o/< ep ratios because the error in the measured < o/< ep ratio is proportional to CR — 1) (see Eq. (38.28)) and this error will be greater under conditions where CR is near unity (in which case CR — 1 approaches 0). In order to minimize the errors in the experimental value of o/ep> it is necessary to use a resonance flux monitor with a large Jo/o o ratio to minimize thermal absorption and the significance of uncertainties in the cutoff. On the other hand, a nearly 1/v thermal flux monitor should be used to minimize epithermal absorption. This source of error may also be reduced by irradiating in a position with a higher thermal to resonance flux ratio. For an unknown sample of low cadmium ratio, only this latter approach will minimize the uncertainty. It has been customary to report values of (To and Iq rather than Cth and Iq since experimental data. These distinctions have not usually been made clear, probably because tbe difference between (To and (7th is small for a 1/v absorber. 

The thermal utilization of the lattice was determined using the known volume fractions of the constituents of a unit cell and the corresponding macroscopic cross sections along with the thermal-neutron distrlbutitms in these constituents. The distributions were obtained from activations of sets of bare and cadmium-covered dysprosium foils placed in the various parts of the central cell at a distance of 18 in. above the bottom of the assembly where the cadmium ratio no longer varied with height. The value of f obtained was 0.9445 0.0009. 

The actual spectrum near the reflector-core interface Is obviously a - rapidly varying function of the space coordinates. As shown by C. Kelber and P. iQer and P. Michael, the Influence of the soft neutrons entering the core from the reflector should be limited to a regioi of the order of the relaxation length of such neutrons in the core which in C-1 was of the order of 3.2 cm. Thus, it can be expected that the calculations will not give good results for the cadmium ratios of various absorbers within 3 cm of the Interfaces. 

Since the cadmium cut-off was assumed to be 0.53 ev, the estimated cadmium ratio, CR, is given by... 

The cadmium ratio of U-238 was measured in two cores with the highest D O concentrations to separate thermal and epithermal absorptions. Two different tech-... 

Core No. DtO Boron Cone Cone (Mole%) (g B/1) No. of Fuel Rods Critical Radius (cm) Critical Height (cm) Critical Buckling (xl0 cm" ) Thermal DisadvanU e Factor Cadmium Ratio ... 

Since the desired experimental parameter is pu, which is defined as the reciprocal of (Cn-1), and since Cu is close to unity, relatively large errors in pu could not be avoided. Therefore, direct measurements of pn were attempted using a third technique by which epicadmium absorptions in U-238 were compared with thermal absorptions in another material possessing a high cadmium ratio in these lattices. By this technique Cu-1 was measured to be 0.059 e 0.002 in Gore III and 0.090 w 0.004 in Core IV. These results suggest that the technique is much more sensitive than the standard cadmium ratio measurements, but the possibility of systematic errors has not been eliminated completely. 

Using the methods described, the effective multipUca4 tlon factor (kg), U-235 fission cadmiu ratios (Cis)r U-238 cadmium ratios (Cnl, and thermal disadvantage factors, the dm/ F Assemblies I tiumugh VI were calculated and compared with experiment. Table H presents this comparison. 

WESTCOTT, C. H., et al, Effective Cross Sections and Cadmium Ratios for the Neutron Spectra of Thermal Reactors, AECL-612 also 1958 Geneva Ctmfer-ence, Vol. 16, p. 70 (paper P/202). 

The observed cadmium ratios were constant out to about 75% of geometrical radius for each core and increased rapidly for larger radii. 

FASTRUP, B., On Cadmium Ratio Measurements and Their Interpretation in Relation to Reactor Spectra, RISO-11 (1959). 

The disadvantage factor was obtained by the so-called integral technique. Foils of a uranium-aluminum alloy of 17.5 wt% uranium that was enriched to 92.75 wt% U-235 were shaped to measure the U-235 fission rate in representative portions of the fuel and moderator volumes of a unit cell. Cadmium ratios in both fuel and moderator sections were determined with 0.051-cm cadmium covers. Various thicknesses (0.0051 to 0.066 cm) of foils were used. The foil activities were corrected for thermal self-shielding. Ih the cadmium-covered irradiations, various masses of cadmium were used and the cadmium ratios were corrected to zero cadmium mass. 

Pitch (cm) Water-to-Oxide Volume Ratio. Fuel-to- Cadmium Ratio Moderator to-Cadmium Ratio Bare-Moderator -to-Bare-Fuel Ratios t Thermos Number Density Thermos U-235 Fission B692/RP Flux... 

The modified conversion ratio, U-238 to U-235 fission ratio (5 ), U-238 capture cadmium ratio, and thermal disadvantage factor have been measured in the lull range of lattices, while critical bucklings and reflector savings liave been measured in the looser assemblies. The measurements of lattice and ceU parameters were carried out as previously described. ... 

The standard cadmium ratio technique wais used for the measurement of p , the ratio of epi- to subcadmium captures in Th. The 27.4-day half-life Pa produced in ThO, wafers was y counted. The differential technique was used to determine lny> dysprosium thermal disadvantage factor. Dysprosium aluminum foils 1.19-mm diam x 0.127-mm thick were distributed throughout a unit cell. 

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