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The solution of scientific and technical problems in nuclear physics, biology, solid-state physics, and nuclear technology depends on the accuracy of measuring neutron fluxes and the energy spectrum of radiation.A description is now given of a metrological scheme for the reproducibility of units and it is shown that it is preferable to use master (reference) neutron sources based on nuclear reactors and neutron generators when measuring flux densities of fast neutrons in the range 1 The primary State standard with its checking scheme is mainly used for the purposes of dosimetry and radiation safety. The inventory of service measuring instruments includes a wide spectrum of radionuclide sources and radiometers with different sensitivities, in a broad interval of neutron energies. The main load falls on the master measuring instruments, and this makes it necessary when performing successive transfer to maintain high-quality metrological characteristics of the primary and working standards. A high accuracy of the primary standard makes it possible to certify working measuring instruments with an accuracy of 12-30%.The special State standard is mainly directed at metrological support of neutron measurements in steady-state, quasisteady-state, and pulsed radiation fields of nuclear reactors, accelerators, and other nuclear physics devices which are sources of neutrons. The radiation fields of nuclear physics devices are characterized by a neutron flux density in the range from 105 to 2 • 1019 s -1 "cm -2, a spectral variety in the range from thermal energies to 20 MeV, and the presence of background radiation.Working measurements made on nuclear physics devices include determining the flux (fluence), flux density, and spectral characteristics of the neutron radiation field at the investigated points. A feature of the measurements is that the solution of many scientific and technical problems requires an accuracy close to the maximum attainable with the present-day state of development of science in measurement technique. There is almost no accuracy margin between the standard and working measuring instruments. This approach makes it impossible to implement metrological support based on the principle of the successive transfer of unit sizes. Metrological support of neutron measurements in the radiation fields of nuclear physics devices is therefore based on the principle of directly creating working standards and master

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