Here we propose a method of determining the sensitivity of a thermal-neutron detector in a form independent of the thermal-neutron temperature, which is designed mainly for neutron detectors in monitoring systems for power and research thermal reactors. However, the area of application is unrestricted, and the method can readily he extended to neutron monitoring and measurement detectors no matter what the spectral characteristics of the neutron field at the point of application.The sensitivity r/ of such a neutron detector is defined in the following form [1] no matter what the mode of response (the current for an average-current detector, the number of counts for a pulse detector, and the square of the alternating component of the current or voltage for the fluctuation mode of operation and so on):in which R is the response and ,I, the thermal-neutron flux density at the point of calibration. As regards the purpose of the detectors, the choice of the normalizing thermal-neutron flux density is not a matter of indifference, and as regards uniqueness in the metrological support and the actual use, it is of considerable interest. Thermal neutrons are by definition [2, 3] ones whose differential flux density (spectrum) in an unbounded nonabsorbing medium can be described by a maxwellian distribution:in which % = f ~(L3dE , k is Boltzmann's constant, T the temperature of the neutron gas in thermodynamic equilibrium o with the medium, and E neutron energy. However, when there is even slight absorption, (2) becomes in which where ~a and E s are the macroscopic neutron absorption and scattering cross sections respectively, ~ the mean logarithmic energy loss, and A(E/kTef) the function for the transitional region to epithermal neutrons. It follows from (3) that absorption in the moderator requires one to introduce the effective neutron temperature Tel, which is higher than the equilibrium temperature T O to an extent that increases with the absorption. The moderators in reactors are as a rule absorbers, so the actual thermal-neutron patterns are characterized by a considerable variety in Tef.
~(~3 = e,h{ [E/(~Tef ) ~1 cxp(-E/kTef ) + 2 [A(E/kTefThe detector's response in a thermal-neutron field is governed in the main by the neutron interaction cross section for the radiator material, which is related to the neutron temperature by 0(7") = g(T) (q-~'/2) ~ ~(Vo),in which T O is a temperature taken as 293.6K (20.4~ a0'0) is the cross section for a neutron velocity of 2200 m/sec, and g(T) a parameter that corrects for deviation of the cross section from a 1/p law.
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