The measured parameters of the electron beams generated by small-sized nanosecond direct-action êÄÑÄç-220 and êÄÑÄç-ùäëèÖêí accelerators are presented. The measuring techniques and the designs of electron detectors developed for this purpose are described. The fluence of electrons and the energy density, uniformity, and energy spectrum of the electron beam have been measured. Fig. 1. External appearance of the êÄÑÄç-ùäëèÖêí accelerator.

A new approach is proposed and investigated for reconstructing the neutron spectra of reactors from activation measurements: the parameterization of the neutron spectrum is changed and the spectrum is represented in the form of a B spline. A Monte Carlo method is proposed for taking account of the distortions of the neutron spectrum by the measuring design of the detectors. The results of the activation measurements show that the approach developed can be used to reconstruct the neutron spectra of BARS-5, IGRIK, and YAGUAR reactors.The determination of neutron spectra ϕ(E) from activation measurements reduces to solving a system of integral equations of the form [1,2] (1)where Q i is the number of reactions on one nucleus of the main isotope of the ith indicator, measured with absolute rms error ∆Q i (1σ); E is the energy of the neutron; i is the number of the indicator; σ i (E) is the activation cross section of the ith indicator; N is the number of indicators used to measure the neutron field. Relation (1) is approximate, since it lacks the distortion which the measurement design of the activation detectors introduces into the neutron field. A more accurate relation is (2) where ϕ i (E) is the spectrum in the ith detector. The spectra ϕ i (E) are unknown and depend on the activation measurement scheme used.For further analysis, it is convenient to write system of equations (2) in the form (3) where S i (E) = ϕ i (E)/ϕ(E) are correction functions which take account of the distorting effect of the measurement construction. Calculating the spectra ϕ i M (E), ϕ M (E) by the Monte Carlo method, taking account, to the maximum extent possible, of the design of the reactor setup and the measurement scheme, and assuming the average-group values of the functions S i (E) to be Q ES E E d E i N i i i = = ∞ ∫ σ ϕ ( ) ( ) ( ) , ,..., , 0 1 Q E E d E i N i i i = = ∞ ∫ σ ϕ ( ) ( ) , ,..., , 0 1 Q E E d E i N i i = = ∞ ∫ σ ϕ ( ) ( ) , ,..., , 0 1

3HeDf'LiD thermal-to-fusion (E=14 MeV) neutron converters have been developed at the IWW-2M reactor to simulate the irradiation of structural materials in a fusion reactor. These converters increase the highest energy part of the neutron spectrum (E > 14 MeV) by =10lo n/ (cm2*s) in the reactor's test channel (=6-cm dim.), corresponding to a 14-MeV neutron fluence of =2*1016 dcm' per 500-hour reactor cycle. However, the high energy release (up to 1-2 kW/gr) in the converters from the 6Li(n,T)a and 3He(n,T)p exothermal reactions created there can be a critical problem in conducting experiments of this kind.

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A new approach to reconstructing the neutron spectra of reactor facilities on the basis of activation measurements is proposed and investigated. A new parameterization of a neutron spectrum is introduced; this representation is in the form of a special "neutron" spline. A generalized algorithm for minimizing the directed deviation is used for the reconstruction. To analyze the possibilities of the approach developed, the spectra of the BARS-5, IGRIK, and YaGUAR reactor facilities are reconstructed on the basis of activation measurements. The reconstructed spectra agree well with previous results.The determination of neutron spectra ϕ(E) on the basis of activation measurements ordinarily reduces to solving a system of integral equations of the form [1,2] (1)where Q i is the number of reactions on a nucleus of the main isotope of the ith indicator, measured with absolute standard error ∆Q i (1σ); E is the neutron energy; σ i (E) is the activation cross section of the ith indicator; i is the number of the indicator; and N is the number of indicators used in the measurements. The existence of a solution of the (1) problem is guaranteed by its physical content, but it is well known that it is an improperly posed problem [1-3]. The many methods existing for finding a solution differ by the form and method of introduction of a priori information. Thus, together with the fact that the solution sought must be positive, the requirements that ϕ(E) be smooth and close to a certain trial function ϕ 0 (E), and so forth are often imposed [1][2][3][4].We note that a great deal of significance has always been attached at the All-Russia Research Institute of Theoretical Physics to the choice of the initial approximation for the neutron spectrum ϕ 0 (E). For this, it was calculated by the Monte Carlo method for each specific case of measurement of a neutron spectrum. A suitable choice of ϕ 0 (E) is especially important in the energy range 10 keV ≤ E ≤ 0.6 MeV; this is due mainly to the inadequacy of the information which sets of activation indicators provide in this region of the spectrum.A group representation of the spectrum ϕ(E) is often used to solve the (1) problem, and the problem reduces to solving a system of linear algebraic equations. Since the cross-sections of many activation reactions are of a resonance character, the number of groups must be large so that the quadrature error due to the transition to a system of equations would be small compared with the measurement errors ∆Q. As a result, the uncertainty (or degeneracy) of the system is large. It has been proposed on the basis of a priori considerations concerning the typical reactor spectrum that a prescribed set of model functions or a piecewise representation of the neutron spectrum be used to decrease the number of parameters sought [4,5]. For Q E Ed E i N i i = = ∞ ∫ σ ϕ ( ) ( ) , ,..., , 0 1

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