Standard neutron source based on the F-1 reactor and standard calibration facility – effective tool for determining the sensitivity of gas-filled ionization chambers

Abstract

The F-1 reactor is a controlled uranium-graphite assembly equipped with horizontal and vertical channels, a thermal column, and a "cold assembly." The diameter of the open vertical channel is 89 mm. The channel passes along the central axis of the graphite insert in a well with dimensions 600 × 600 mm, set into the core at a depth of 1 m. The flux density of the thermal neutrons in the reactor channels lies in the interval 1·10 9 -6·10 13 sec -1 ·m -2 .It is shown for the first time on the basis of a standard source that the dose rate of the accompanying photon radiation in the reactor channel is described by a linear function of the reactor power or the neutron flux density.Standard Source of Thermal Neutrons. A state primary standard of thermal-neutron flux was created in the 1980s at the Mendeleev All-Union Research Institute for Metrology [1]. However, it is impossible to determine the neutron sensitivity of the ionization chambers in the primary standard. Together with the primary standard, in accordance with the state calibration scheme [2] a secondary standard of the unit of measurement of thermal-neutron flux density was developed on the basis of the F-1 reactor [3]. The thermal-neutron flux density Φ th with energy below the cadmium limit and temperature T n at different locations of the reactor are presented in Table 1. The error in determining the neutron flux density does not exceed 2% with confidence probability 0.99. The spectral density of the intermediate neutrons at the center of the core of the F-1 reactor is described by the function E -(0.965±0.01) , where E is the energy of the neutrons. The fraction of the intermediate neutrons in the full flux is 0.066. In the vertical channel, the spectral density of the intermediate neutrons varies as E -(0.995±0.01) , the fraction of the intermediate neutrons is 0.036. The error in determining the fraction is 0.003 arb. units.The spatial distribution of the flux density of the thermal neutrons over the height of the irradiation cavity of the vertical channel, shown in Fig. 1, is normalized to the neutron flux at a point 20 cm from the bottom of the channel. The yearly certification over a period of 25 yr attests to the high stability of the neutron source based on the F-1 reactor.Standard Calibration Facility. Together with the standard source of neutrons in the F-1 reactor, a radiometric facility, containing a set of certified gas-filled KNT and KNK ionization chambers, a power supply, and an electric current meter, was developed. The chambers were calibrated in the air cavity of the vertical channel at fixed reactor power. The error in determining the sensitivity to thermal neutrons K n is 3.5% with confidence probability 0.95 (Table 2). The unit of neutron flux is transferred by the substitution method. The flux of thermal neutrons in the channel is proportional to the reactor power and the indications of the fission chamber.