Investigations with two-neutron transfer reactions on the even isotopes of samarium

The (p, t) reaction on the even isotopes of samarium has been investigated at a proton energy of 25.5 MeV. Angular distributions were obtained in the range 18° ≦ θ ≦ 148° with angular steps between 2° and 5°. The experimental energy resolution varied between 35 keV and 50 keV FWHM. Spin and parity assignments are performed by comparing the measured angular distributions to zero-range DWBA calculations. Some difficulties of DWBA calculations for (p, t) reactions are pointed out. The relative cross sections for transitions to different levels of the final nuclei are compared with other (p, t) and (t, p) measurements in the same region of the rare earth isotopes. The dependence of the (p, t) cross sections for different transitions on the neutron number of the final nuclei is discussed. Some 2⁺ states observed in (p, t) and (t, p) reactions are described in the quadrupole pairing vibrational picture.     

Finite-range calculation of two-neutron transfer reactions in deformed nuclei

Finite-range DWBA calculations have been done for the reaction ¹⁵⁴Sm(p, t)¹⁵²Sm. The results have been compared with those of the zero-range DWBA calculations. It is found that the finite-range effects are considerably large to the shapes of the angular distributions, but are small to the relative magnitude of the cross section for the 2⁺ and 4⁺ ground-band members to the ground-state transition.     

Extended basis shell-model study of two-neutron transfer reactions

A method to calculate two-nucleon overlaps is presented. The approach is based on shell-model techniques and the effect of one and two nucleons in the continuum are included implicitly. The reactions ¹⁸O(p, t)¹⁶O and ¹⁶O(t, p)¹⁸O (ground and excited 0⁺ states) are analysed within the zero-range and finite-range DWBA framework. The differential cross sections calculated using the overlaps obtained by the present method are compared with the ones obtained by the standard well depth (WD) and Jaffe-Gerace (JG) techniques. It is shown that the WD approach yields cross sections smaller than the experimental values by an order of magnitude while the JG method predicts undue energy dependence in the cross section. Our approach removes these difficulties and is shown to be consistent with the one-nucleon transfer calculation.     

Exact finite range calculations of light-ion induced two-neutron transfer reactions

Exact finite-range (EFR) distorted-wave Born approximation calculations were performed for light-ion induced two-neutron transfer reactions, by using a technique to calculate the form factors rather fast. The use of this method made it possible to carry out calculations even when realistic light-ion wave functions and multi-configurational two-neutron wave functions were used and large transferred angular momenta were considered. It was found that, at lower bombarding energies, the predictions of the EFR and zero-range calculations agree very closely both in angular distributions and relative magnitudes of the cross sections, though they differ significantly in absolute magnitude. As the bombarding energy increases, the discrepancy between the predicted absolute magnitude becomes still larger, and noticeable differences are seen even in relative cross sections. For all the energies considered, the EFR calculations predicted the absolute magnitudes of the experimental cross sections to within a factor of several units.     

Low-lying 0+-states in a three-alpha model of12C

The nucleus¹²C is treated as a system composed of three alpha particles. Separable potentials are used for thes-wave andd-wave interactions between two alpha particles. Applying Faddeev's theory two 0⁺-states are found in the neighbourhood of the three-particle threshold.     

Analyzing power in two-neutron stripping reactions

Analyzing powers have been measured for six states from the ${}^{208}\text{Pb(}\text{t}\text{, p)}{}^{210}\text{Pb}$ reaction and seven states from the ${}^{90}\text{Zr(}\text{t}\text{, p)}{}^{92}\text{Zr}$ reaction. These studies were performed with a beam from a new polarized triton source, and the reaction protons were detected in a Q3D spectrometer. The analyzing powers and differential cross sections are adequately described by DWBA calculations for both the medium and heavy targets up through L-transfers of 4. Above this value, some disagreement is seen.     

Excitation of3 G 0-states of titanium, zirconium, and hafnium atoms by electron impact

Detailed information about excitation cross-selections of spectral lines of titanium, zirconium, and hafnium atoms obtained using extended crossed beams are used for calculation of the excitation cross-sections of the³G⁰-levels of these atoms. Results are compared with cross-sections calculated in Born’s approximation. For titanium: it is found that theoretical predictions agree well with experimental data. Differences found for zirconium are explained by the single-configuration approximation used in the calculation. Moscow Power Institute, Russia, 14 Krasnokazarmennaya St., 111250, Moscow E-250, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 707–710, November–December, 1997.     

Breakup reactions of two-neutron halo nuclei

Investigation of reactions initiated in collisions of beams of unstable nuclei with nuclear targets has resulted in the discovery of a new type of nuclear structure, the halo, in some light nuclei at the boundary of nucleon stability. A survey of different reactions with two-neutron halo nuclei and their use for gaining information on the structure of exotic nuclei is presented. Kinematically complete breakup reactions, which provide the possibility of obtaining quite reliable information on the structure of the ground state and the continuum of nuclei, are considered in detail. The microscopic four-body model of breakup reactions for two-neutron halo nuclei with account of characteristic specific features of their structure is formulated. The model is based on the distorted wave approximation and is applicable for analysis of low excitations of the continuum near the breakup threshold, the region most sensitive to manifestation of specific features of the halo structure. The described approach enables the calculation of all observables of kinematically complete experiments in nucleus-nucleus collisions at intermediate energies for which one-step processes dominate, and creates the basis for the spectroscopy of continua via successive analysis of various correlation cross sections available in kinematically complete experiments.     

Strong enhancement of two-neutron transfer in the system 206Pb+118Sn

One and two neutron transfer has been measured in the heaviest asymmetric nuclear system with semi magic nuclei showing superfluid properties, in ²⁰⁶Pb+¹¹⁸Sn collisions at an energy well below the Coulomb barrier with scattering orbits covering the largest angles. Particle-γ coincidence techniques using 5 Euroball-Cluster detectors (EB) combined in a set-up with the Heidelberg-Darmstadt NaI-Crystal Ball (CB) have been used. Transfer channels are identified with EB via their known γ-decays of the lowest excited states. Using the unique feature of the set-up with the CB, transfer to well defined final states with known quantum numbers (without feeding) are selected using the high efficiency multiplicity filter of the CB (no second γ-ray). The data are analysed using the semiclassical approach and transfer probabilities are obtained. The enhancement for the two-neutron transfer populating the low lying superfluid 2⁺ state in ¹²⁰Sn (and ¹¹⁶Sn), while the Pb-branch is in the groundstate is deduced by comparison with the strongest single neutron transfer transition. Large enhancements (EF ≃ 10³) are observed. This is the first direct measurement of enhancement for a heavy nuclear binary system with experimentally separated levels suggesting a strong contribution from superfluid pair transfer. Received: 18 December 1998 / Revised version: 23 February 1999     

Excitation of 56Co by two-particle transfer reactions

The reactions ⁵⁴Fe(³He, p)⁵⁶Co and ⁵⁸Ni(d, α)⁵⁶Co have been investigated at energies of 12.0 and 7.0 MeV, respectively. Ground-state Q-values of 7.408±0.015 MeV for the (³He, p) and 6.506±0.010 MeV for the (d, α) reactions were measured. Angular distributions for eight of the (³He, p) transitions were measured and analysed using the DWBA in order to extract L-values. The level scheme of ⁵⁶Co is discussed in terms of these results. The analogue of the ⁵⁶Fe ground state was identified, and a reduced Coulomb energy ΔE_C = 8.949±0.025 MeV was extracted.     

One- and two-neutron transfer reactions on 12C with the weakly bound 6He projectile

Using the framework of the coupled reaction channels (CRC) the one- and two-neutron transfer process initiated by the weakly bound nucleus ⁶He on ¹²C at an energy of E _L = 5.9 MeV is studied. The absolute cross-sections for a few states in ¹⁴C are well reproduced within a factor 2 in second order, using microscopic wave functions of ⁶He and ¹²C. Only a small dependence of the cross-section on details of the ⁶He wave function is observed. Good fits to the data are obtained in a calculation with full coupling (25 iterations) with renormalised optical potential parameters and spectroscopic amplitudes of ⁶He. Received: 13 June 2000 / Accepted: 24 July 2000     

The two-neutron transfer reactions (O, O) with O, C and Mg targets

Excitation functions, angular distributions and differential ranges were measured for the ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg reaction at ¹⁸O beam energies of 20–45 MeV. Excitation functions only were measured for the reactions ¹⁴C(¹⁸O, ¹⁹O)¹³C, ¹⁴C(¹⁸O, ¹⁶O)¹⁶C, ¹⁴C(¹⁸O, ²⁰O)¹²C, ¹⁴C(¹⁸O, ¹⁵N)¹⁷N and ¹⁸O(¹⁸O, ¹⁹O)¹⁷O, ¹⁸O(¹⁸O, ¹⁶O)²⁰O, ¹⁸O(¹⁸O, ¹⁵N)²¹F at ¹⁸O beam energies of 13–41 MeV. We have identified these as direct reactions in which a single neutron, a two-neutron cluster, a deuteron and a triton are transferred between projectile and target.

The cross sections for two-neutron transfer reactions were found to be relatively high and those for the ¹⁸O+¹⁸O and the ¹⁴C+¹⁸O reactions were higher than the ones of single-neutron transfers over most of the energy range.

Attempts were made to apply the theory of Buttle and Goldfarb for single-neutron transfer to the case of two-neutron transfer in the ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg reaction below the Coulomb barrier. It is shown that for those reactions for which the assumptions, implicit in the model, are valid, good agreement is obtained with experiment. We also tried to apply the diffraction model of Dar and Kozlovsky to the calculation of the angular distribution of these reactions. A good fit to the experimental results could be obtained if quite different sets of parameters were used in the calculations for the two bombarding energies.     

Two-nucleon T = 0 transfer reactions in the 0p shell

The ¹⁶O(d, α)¹⁴N, ¹⁴N(d, α)¹²C and ¹²C(d, α)¹⁰B reactions at E_d = 40MeV and the ¹²C(α d)1¹⁴N at E_α = 55 MeV were investigated. A total of seventeen transitions are analysed in terms of one-step, zero-range DWBA calculations, using the two-particle coefficients of fractional parentage obtained from the Cohen-Kurath Op shell wave functions. For most transitions, fair agreement is obtained between experiment and calculation, possible exceptions being the transition to the E_x = 4.43 MeV, J^π = 2⁺ state in ¹²C and to the E_x = 2.15 MeV, J^π = 1⁺ state in ¹⁰B, for which the calculations predict too much L = 0 strength. Where possible, a comparison with previous (p, ³He) results is made. In ¹⁴N a state at E_x = 11.04 MeV was observed for which the values (J_π; T) = (3⁺; 0) are suggested. In ¹²C we found, in addition to the well known T = 0 states, two relatively sharp T = 0 states at E_x = 19.50 ± 0.10 and 20.55 ± 0.10 MeV. The shape and strength of the angular distribution for the transitions to these states can be approximately accounted for by the calculations, although no one-to-one correspondence between observed and predicted levels could be established.     

Excitation energy transfer between Er3+ and Tm3+ in LiNbO3

Conclusions Nonradiative energy transfer between similar and different rare-earth ions in LiNbO₃ has been evidenced for the first time. The contribution of nonradiative energy transfer to the decay of the¹G₄ and³H₄ levels of thulium and the⁴S_3/2 and⁴I_13/2 levels of erbium is significant for activator concentrations of the order of several thousand ppm in LiNbO₃. The excitation of higher-lying levels of both activators is transferred faster in codoped crystal to the lowest luminescent levels, from which the luminescence in the 1–2-μm spectral region occurs. This accelerated relaxation is advantageous for broadband optical pumping since the contribution of radiative transitions competing with the³F₄−³H₆ laser transition of thulium is reduced. On the other hand, it appears that erbium ions will influence adversely the efficiency of a laser-diode-pumped LiNbO₃∶Er, Tm laser operating near 1.8 μm since the part of the³H₄ excitation transferred to erbium ions is emitted from the long-lived⁴I_11/2 level of Er³⁺. Furthermore, the⁴I_13/2 excitation is not transferred fully to thulium ions and is lost in⁴I_13/2−⁴I_15/2 emission. Lithium niobate singly doped with thulium is more promising since the cross-relaxation process that transfers the excitation from the³H₄ level to the³F₄ level appears to be efficient in this host. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 5, pp. 125–133, September–October, 1995.     

Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1₃ A′ (π → π*) and 1₃ A″ (n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1³ A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2₁ A′(π → π*) state is excited with almost identical efficiency at different residual energies.     

Strong sequential transfer processes in 0+ → 0+ (p,t) reactions on Pb isotopes

A marked difference of experimental analyzing powers $\text{A (θ)}\text{for}{}^{208}\text{Pb}\text{(}\text{p}\text{,}\text{t}\text{)}{}^{206}\text{Pb}\text{(0}{}_{\mathrm{<mtext>g</mtext>}}{}^{\mathrm{+}}\text{and}\text{0}{}_2{}^{\mathrm{+}}\text{)}$ reaction is explained by considering sequential transfer as wesses as well as the one-step process. The calculated A(0θ) for the 0₂⁺ state is very sensitive to the wave functions employed. An enhancement of the cross sections for the ground-state transitions of three Pb isotopes is found to be due to sequential transfer processes.