Some Peculiarities in the Interaction of 6 He with 197 Au and 206 Pb 41Fig. 2. Proˇle of the 6 He beam extracted from the U400 cyclotron (a) and its energy resolution (b)Our aim was to study the interaction of 6 He with different target nuclei. In the presentpaper, results of measurements of the excitation functions for fusion and transfer reactionsinvolving the accelerated 6 He ions are presented. The measurement of the yields of theproducts of the fusion reaction after the evaporation from the compound nucleus of x neutronsand of the transfer reactions was performed by the activation method. Two stacks of foilswere placed in the reaction chamber of the magnetic spectrometer MSP-144 on the way ofthe 6 He beam: ˇrst Å a stack of 13Ä50 μm thick gold foils, and further downstream asecond stack of 206 Pb targets, 600Ä700 μg/cm 2 each (Fig. 3). In order to tune the 6 He beamand to measure its intensity and spatial distribution, the multiwire proportional chamber forbeam diagnostics was placed in front of the stacks. After passing through the two stacks thebeam entered the magnetic spectrometer MSP-144, which gave a precise measurement of theresidual energy of the beam. The 6 He energy and the energy loss in each layer of the stackswas calculated with the LISE code  and the calculated residual energy was compared tothe value measured by the magnetic spectrometer. In this way, in spite of the rather largeenergy dispersion of the beam at the end of the stacks (±2 MeV), the absolute value of theenergy was determined with good accuracy (not worse than 1 MeV).Fig. 3. Schematic layout of the activation experiments using the 6 He beam: 1 Å miltiwire proportionalchamber, specially designed for beam diagnostics; 2 Å stack of gold foils; 3 Å stack of thin 206 Pbtargets. The incident and ˇnal energies of the beam are also shownAfter the irradiation the γ activity induced in the gold foils was measured off-line usingHPGe detectors with high efˇciency (about 10% for E γ = 662 keV) and high energy resolution(1.5 keV for the γ transition at 1800 keV). Peaks in the γ spectra could be identiˇedas belonging to the Tl isotopes, which are the decay products of the compound nucleus 203 Tl
42 Penionzhkevich Yu. E. et al.after the evaporation of 2Ä7 neutrons. The table contains the energies, half-lives and relativeyields of the most intensive γ transitions in the corresponding fusion reaction decay products,which have been used for their identiˇcation.Characteristics of the decay products of the compound nucleus 203 Tlxn Decay product Half-life T 1/2 ,h Eγ, keV(I %)2n3n4n5n6n7n201 Tl 72.91 167.4 (10%)200 Tl 26.1 367.9 (87%)199 Tl 7.42 247.26 (9.3%)198 Tl 5.3 675.88 (11%), 587.2 (52%)197 Tl 2.84 152.2 (7.3%)196 Tl 1.84 344.9 (2%)In addition to the Tl isotopes, γ transitions of the isotopes 196 Au, 198 Au and 199 Au couldbe identiˇed in the spectra measured for the gold foils. The isotope 196 Au could be formedas a result of the stripping of one neutron, 198 Au and 199 Au Å after the pickup of one andtwo neutrons, respectively, in the interaction of the 6 He beam with the 197 Au target nuclei.The 206 Pb stack was measured using an α spectrometer and the excitation function forthe formation of the compound nucleus 212 Po and its decay by emission of two neutrons( 206 Pb( 6 He, 2n) 210 Po) was obtained in the beam-energy range 13Ä24 MeV (the Coulombbarrier for the given reaction is 20 MeV). The 210 Po isotope was identiˇed by the α-particleenergy (E α =5.3 MeV) and its half-life (T 1/2 = 138 d). The energy resolution of theα spectrometer amounted to about 50 keV, and the total efˇciency of registration of the αparticles was about 50%.2. RESULTS AND ANALYSISOn the basis of the measured yields of the isotopes, formed after the evaporation from thecompound nucleus 203 Tl of 2 to 7 neutrons, taking into account the 6 He beam intensity andthe target thicknesses, we could determine the cross sections for the formation of the differentisotopes and their dependence on the bombarding energy (the excitation functions). The sameprocedure was applied for 210 Po, which was formed in the 206 Pb( 6 He, 2n) 210 Po reaction.The excitation functions measured for the reaction channels 6 He + 197 Au → 203−x Tl areshown in Fig. 4. The analysis of the obtained data was performed using the ALICE-MPcode [15, 16]. The values of the parameters used were taken from analyses of experimentaldata on the cross sections of evaporation reaction channels induced by heavy ions in therange of medium and heavy nuclei. The solid curves in Fig. 4 represent the results of thecalculations. It can be seen that the experimental reaction cross sections are in agreementwhat concerns the maxima of the xn-channel distributions. The channel with the emissionof two neutrons, in which the 201 Tl nucleus is produced, is not well described by the givenmodel. As can be seen from the ˇgure, the isotope 201 Tl is formed with a cross section largerthan expected in the model. This may be connected with the fact that the reaction with totalabsorption of 6 He by the 197 Au target nucleus has a Q value equal to +12.2 MeV, which
Some Peculiarities in the Interaction of 6 He with 197 Au and 206 Pb 43Fig. 4. Experimental excitation functions for the197 Au + 6 He → 203−xn Tl reaction, where x =2−7.The symbols denote: ◦ Å2n, ▽ Å3n, □ Å4n,△ Å5n, ♦ Å6n, Å7n evaporation channels;the curves are the calculations with the ALICE-MPcode [15, 16] using the following parameters for theinteraction potential: r 0 =1.29 fm, V = −67 MeVand d =0.4 fm. B c is the Coulomb barrier for the6 He + 197 Au reactionmakes the reaction with the evaporation of two neutrons deeply subbarrier. The calculations,where fusion is described as the penetration of 6 He through the barrier, should result indecreased values of the cross sections. A similar situation arises for the 206 Pb( 6 He, 2n) 210 Poreaction (Fig. 5). However, in this case the reaction Q value is equal to +4.2 MeV, whichmust lead to somewhat larger cross-section values.Fig. 5. Excitation function measured for the206 Pb( 6 He, 2n) 210 Po reaction. Å experimentalvalues of cross sections for the formation of 210 Po;dashed line Å calculations within the framework ofthe statistical model; solid line Å calculations usingthe two-step fusion model, taking into account theprocess of consecutive transfer of neutrons . B cis the Coulomb barrierThis difference is particularly well seen in Fig. 5, where the excitation function for the206 Pb( 6 He, 2n) 210 Po reaction is shown. The cross section for this reaction at the maximum,according to the statistical model calculations (the dashed line), should be small, because thereaction takes place at energies below the Coulomb barrier and is strongly suppressed. But, ascan be seen from the presented data, even at energies 7 MeV below the Coulomb barrier forthe 206 Pb + 6 He reaction, the cross section for formation of 210 Po, i.e. for the evaporationfrom the compound nucleus of two neutrons, is rather large and amounts to ∼10 mb. Thus,due to the observation of the reaction with the evaporation of two neutrons we could draw
46 Penionzhkevich Yu. E. et al.11. Kuznetsov V. D. et al. FLNR Scientiˇc Report 2001Ä2002 / Ed. A. G. Popeko. Dubna, 2003.P. 223; 224.12. Astabatyan R. A. et al. FLNR Scientiˇc Report 2001Ä2002 / Ed. A. G. Popeko. Dubna, 2003. P. 212;FLNR Scientiˇc Report 2003Ä2004 / Ed. A. G. Popeko. Dubna, 2005.13. Skobelev N. K. et al. // Nucl. Instr. Meth. B. 2005. V. 227. P. 471.14. http://dnr080.jinr.ru/lise/15. Muzychka Yu. A., Pustylnik B. I. // Proc. of the Intern. School-Seminar on Heavy-Ion Physics,Alushta, 1983. Dubna, 1983. P. 420.16. Penionzhkevich Yu. E. et al. // Phys. At. Nucl. 2002. V. 65, No. 9. P. 1563.17. Zagrebaev V. I. // Phys. Rev. C. 2003. V. 67. P. 061601(R); Prog. Theor. Phys. Suppl. 2004. V. 154.P. 122.Received on August 10, 2005.