Experimental procedure. The samples under study were irradiated by 14 MeV neutrons produced by neutron generator NG-300/15. The reaction T(d,n)4He was used. Ions D2+ of the beam was accelerated to energy 220 keV. The diameter of deuteron beam on the T-Ti target was 10 mm. The average neutron flux density at the irradiation points was determined by using monitor-samples; to do this, the samples were covered by two foils of Nb, Zr or Al. The reference reactions for the flux density measurements were 27Al(n,p)27Mg, 27Al(n,a)24Na, 90Zr(n,2n)89(g+0.9377m)Zr and 93Nb(n,2n)92mNb. The energy dependence of the cross sections for reactions 93Nb(n,2n)92mNb and 90Zr(n,2n)89(g+0.9377m)Zr were evaluated. These excitation functions were used for determination of average neutron energy at the irradiation points by Zr/Nb method. The samples of Y and Ta were covered by Cd foils to reduce the influence of the thermal neutrons. The samples of natural Y, La, Ta, Pb and Bi were examined on purity by preliminary X-ray fluorescent analysis and by neutron activation method. As a first stage, the samples were irradiated in "close" geometry (distance to T-Ti target - 4 mm). Diameter of samples were 12-15 mm with thickness from 10 mm to 2 mm. The average neutron flux density at this geometry was ~1.5*10^9 [cm^(-2)*s^(-1)]; corresponding average neutron energy (14.50±0.04) MeV. The energy resolution in this case was 0.18 MeV=(FWHM)/2. The value of full width at half maximum (FWHM) was derived from the calculated neutron energy profile. The conditions of experiment for measurements of excitation functions of (n,x) reactions on Ta and Pb were optimized using results obtained for 14.5 MeV neutron average energy ("close" geometry of irradiation). As a result of this optimization the following conditions have been chosen: distance to T-Ti target - 75 mm; diameter of samples - 30 mm; thickness of samples - 100-800 mm. The angles of the irradiation position to the D2+ beam were 0°, 30°, 60°, 90°, 120° and 150°. The average neutron flux density at this geometry was ~1.3*10^7 [cm^(-2)*s^(-1)]. The spectra of activation products were measured both by in-line gamma-spectrometer with Ge(Li)- and HPGe- detectors and by four-crystals Compton suppression spectrometer. Some of the gamma-spectra were measured using W or Pb filters in order to reduce the influence of intensive low energy gamma-radiation. Most of the spectra were measured in close geometry. The corrections on true coincidence summing were applied. The method was controlled experimentally by measurements of some spectra in "close" and "far" geometries. The corrections on self-absorption of gamma-rays in the sample material were made by Monte-Carlo method. These corrections also were experimentally checked by comparing the corrected results with that ones obtained with thin samples when self-absorption was negligible. All data on the decay of radioactive nuclei were taken from systematics of Firestone (1996 and 1999). Information on the Q-values of the nuclear reactions also was taken into account. Main experimental results. New experimental results were obtained for 138La(n,a)135mCs, 139La(n,n'a)135mCs and for 181Ta(n,a)178gLu nuclear reactions. Determined results for 181Ta(n,d)180mHf and 208Pb(n,p)208Tl significantly improve existing experimental information. Experimental results for 181Ta(n,a)178mLu and 208Pb(n,a)205(m+g)Hg reactions remove the ambiguity concerning the values of the cross sections of these nuclear reactions. Measured values of nuclear reactions cross sections could significantly affect evaluated data for subsequent nuclear reactions: 181Ta(n,a)m+g, 206Pb(n,a), 208Pb(n,p), 208Pb(n,a)m+g. Theoretical calculations. The code STAPRE-H95 was used for theoretical calculations of the nuclear reactions cross sections. The nuclear reactions cross sections were calculated within the framework of the multistep statistical model. The preequilibrium processes were taken into account by means of exciton model. The unified set of theoretical parameters for this region of neutron energies wereused. The theoretical parameters were taken from systematics. A comparison of the theoretical calculations with experimental data shows the great contribution of the preequilibrium processes to the nuclear reaction cross sections, in particular it reaches almost 99 % in (n,p) reactions.
