Fluence measurements made with accelerator-based neutron fields in standards laboratories have been compared at neutron energies of 0,144,0,565,2,5, $0 and 14,8 MeV. The method used was to compare the neutron sensitivities of a pair of fission chambers, one containing 235U and the other containing 238U fissile deposits. The uncertainty contribution from the transfer method is generally smaller than the uncertainty in the fluence determination, and the results of the intercomparison are in satisfactory agreement. The weighted least squares estimates of the measured chamber neutron sensitivities at each energy confirm the ENDF/B-VI evaluations of the fission crosssections.

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Specifications for isotopic targets of both stable and actinide nuclides for nuclear measurements are very varied and individually very stringent. The majority of the targets are required to be of specified isotopic and chemical purity, well characterised and uniformly deposited over a precisely defined area. Mass assay of actinide targets may be requested to an accuracy of + 0.1%, resolutions of <10 KeV FWHM may be requested for infinitely thin alpha sources and the Schmidt and Pleasonton criteria must be met for 252Cf fission fragment energy calibration sources. In order to cater for this wide range of requirements it is necessary to have available many preparation and calibration techniques. Actinide sources with activities in the range 0.01 pCi to several hundred mCi and targets up to a few mg/cm2 are prepared by one of the following techniques, vacuum evaporation, painting, electrodeposition or the use of TEG as a spreading agent. These targets may be assayed quantitatively by absolute alpha counting and weighing and qualitatively by alpha spectrometry. Stable isotope targets with thickness varying from a few pg/cm2 to several hundred mg/cm2 are prepared primarily by vacuum evaporation and rolling. Mass assay is to + 10% or better, the achievable accuracy being a function of substrate and target thickness. Activities of particular interest include the preparation of fission foils to be used in a series of flux inter-comparison measurements being organised by the Bureau International des Poids et Mesures, the preparation of stable isotope alloy foils for Mossbauer studies and the preparation of actinide targets on thin substrates.

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