1 - IAEA Nuclear Data Services

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homogeneity of the layers. Satisfactory results
(probably accurate within 0.3%) are found for the
comparison of direct weighing in vacuum and relative
neutron measurements for layer thicknesses between
2
about 20 and 80 v-g/cm . These layers will be used
as standards for fission cross-section measurements.
Intercomparison of Fissile Layers
The same equipment as used for the boron comparisons
is used for the intercomparison of fissile layers.
In this case the fission- and a-particle pulse-height
spectra are recorded with beam-on and beam-off. The
fission- and a-counting rates are deduced respectively
per monitor count and per unit-time. The ratios of aand
fission counting-rates can be compared. With nonhomogeneous
sources they may differ.
Measurements were performed on electro-sprayed uranylacetate
sources and evaporated UF¿,-layers made from
NBS-930 ^U-material and agreement was found within
statistical errors proving their good homogeneity.
Another method to compare the quality of the foils
from different preparation methods is to compare the
fission spectrum characteristics of foils with the
same thickness in the same geometry. A particularly
sensitive parameter, e.g., is the ratio Nj- / N-y of
the light fragment peak counting-rate to the valley
counting-rate. From these spectra also the low energy
tailing can be evaluated and possible multiplication
effects (at the high energy-side of the spectrum) of
the surface-barrier detector used can be detected.
They also enabled us to determine the limit thickness
of applicability of the preparation methods to be more
2
than 500 ug/cm for electrosprayed and probably more
than 1 mg/cm
2
for evaporated sources.
2200 m/s Fission Cross-Sections
The boron standard foils and the evaporated ^UF.-