Neutron Pickup Research Articles

Dynamics of low-energy heavy-ion fusion

Preliminary data on the fusion of 36S+96Zr are reported; the excitation function near the barrier is intermediate between those of 40Ca+90,96Zr. The peculiar role of the strong 3− octupole vibration of 96Zr is pointed out, in addition to the couplings to neutron transfer channels with positive Q-values. Recent data on 40Ca+124Sn are also shown; for that system the fusion barrier distribution is wide without separated peaks, similar to the case of 40Ca+96Zr. Simplified coupled-channel calculations have been performed, including surface vibrations and sequential neutron pick-up channels, with form factors that fit the single- and multi-nucleon transfer data for 40Ca+124Sn. A good agreement with the data is found.

Measurement of the ground state 2n pickup probability for 28Si+68Zn and its role in sub-barrier fusion enhancement

The ground state and excited state transfer yields for the 2-neutron pickup channel in the 28Si+68Zn system have been measured explicitly. The recoil mass separator at the nuclear Science Centre, New Delhi was used for the measurement. A NaI(T1) detector was used for detecting the deexcitation γ’s from the transfer products. The kinematic coincidence technique was employed for the transfer measurement. Simplified coupled channels calculations show that out of all transfer channels the major contribution to the sub-barrier enhancement comes from the ground state 2 neutron pickup channel with a ground state Q-value of+1.83 MeV.

Fusion of40Ca+ 124Sn at subbarrier energies

Fusion-evaporation cross-sections of40Ca +124Sn have been measured, following a recent experiment on multinucleon transfer processes in the same system. The40Ca XTU Tandem beam was used (~5 pnA), and the evaporation residues were detected at 0° and at small angles by an Energy-TOF telescope following an electrostatic beam deflector. High statistical accuracy was achieved (~1% near the barrier). A few energy points were measured for40Ca +116Sn, too. The two fusion excitation functions are similar to each other and are greatly enhanced at low energies over the simple one-dimensional barrier penetration estimate. The fusion barrier distribution of40Ca +124Sn was extracted from the data as the second derivative ofE × σ with respect to the energy. Its shape is wide without separated peaks, similar to the recent result for40Ca +96Zr, where it was attributed to a dominant effect of couplings to neutron transfer channels with positiveQ-values. Such channels are available in40Ca +124Sn as well and they are known to be very strong from the multinucleon transfer experiment. Simplified coupled-channel calculations including surface vibrations and sequential neutron pick-up channels are unable to account for the data. The peculiar role of the strong 3 octupole vibration of96Zr is pointed out.

Single particle excitations in 129Ba from a ( [formula omitted], t) reaction study

The 130Ba( d, t) 129Ba reaction was studied with 25 MeV deuterons and excellent energy resolution. About 50 excited states have been observed up to E x = 2.0 MeV and for 42 of them transferred ℓ values and/or J π values were assigned on the basis of the measured angular distributions and asymmetries. The level scheme of 129Ba, the neutron pickup strength distribution and the known electromagnetic decay scheme are compared with results of calculations with the interacting boson-fermion model. These simple multi-shell IBFM-1 calculations describe well the main features of the experimental data.

Multinucleon transfer reactions of

The experimental investigation of multi-nucleon transfer channels of has been triggered by the very different fusion excitation functions recently observed in these two systems. The cross sections of the transfer reactions in the two systems have been measured at two energies close to the Coulomb barrier. The time-of-flight spectrometer PISOLO allowed us to identify a multitude of transfer channels, even very weak ones, with high resolution and efficiency. Especially for the neutron pick-up channels, the cross sections are much larger for the target than for with evidence, in the Q-value spectra, of large energy losses (20 - 30 MeV) involved in the transfer mechanism at forward angles.

Evidence of complex degrees of freedom in multinucleon transfer reactions of48Ca+124Sn

Multinucleon transfer reactions in ${}^{48}$Ca+${}^{124}$Sn have been studied at 174 MeV with a time-of-flight magnetic spectrometer. The pickup and stripping of several protons and neutrons have been observed. The experimental isotope yields for the $\ensuremath{-}2p$ and $+2p$ channels cannot be reproduced with calculations that incorporate only independent single-nucleon transfer modes. The discrepancies can be accounted for by explicitly including nucleon pair and $\ensuremath{\alpha}$-cluster degrees of freedom.

A mechanism for excitation of metastable levels by(γ,γ') reactions

The analysis of the cross-section of 77Se and 87Sr isomeric states excitation in the (γ, γ′) reaction, was carried out using neutron pick-up and stripping reaction results. It allows the shell configurations of the transitions to the activation states to be determined. To check thus obtained conclusions the energies of Jπ=1- excited states for 90Zr and 138Ba were calculated, which are in good agreement with the resonance structure at the γ-quanta elastic scattering in the 5 to 10 MeV energy range.

Depopulation of 180Tam by Coulomb excitation and possible astrophysical consequences.

The depopulation of [sup 180]Ta[sup [ital m]] by Coulomb excitation was investigated in a thick-target experiment using beams of [sup 32]S and [sup 36]S with energies of 70--130 MeV on disks of natural tantalum. The resulting [sup 180]Ta ground-state population was measured by detecting its eight-hour decay. At the higher incident energies the observed [sup 180]Ta yield is dominated by sub-Coulomb neutron pickup transfer reactions on the 10[sup 4] times more abundant [sup 181]Ta. At the lower incident energies in [sup 180]Ta ground-state yield is found to be compatible with Coulomb excitation of the isomer to an intermediate level with energies [ital E][sub [ital x]][lt]1 MeV and a reduced transition probability [ital B]([ital E]2)[congruent]0.02--0.25 Weisskopf units. Consequences for the astrophysical production mechanism of [sup 180]Ta are discussed.

One neutron pickup reactions in the 32S+144Sm and 32S+166Er systems at energies close to the Coulomb barrier.

One neutron transfer cross sections in the systems $^{32}\mathrm{S}$${+}^{144}$Sm and $^{32}\mathrm{S}$+ $^{166}\mathrm{Er}$ have been measured at energies close to the Coulomb barrier. Mass and charge identification was achieved using characteristic x rays. Transfer probabilities were analyzed by considering barrier penetration mechanisms. Possible enhancement effects due to permanent quadrupole deformations are investigated and compared with the experimental results.

Escape and spreading widths of the isobaric analog states of Sn and Te isotopes

The escape widths for direct proton decay from isobaric analog states in Sb and I nuclei have been obtained by combining calculated single-particle widths with experimental spectroscopic information for neutron pickup reactions on the respective Sn and Te target nuclei. They display a pronounced odd- A/even- A staggering effect with very small widths for the even- A isotopes due to the lower proton decay energies. Only data for odd- A nuclei are available, and there is good agreement. Combining the calculated escape widths with the known experimental total widths yields values for the spreading widths. As expected, and in agreement with theoretical predictions, the spreading widths are essentially constant and dependent only weakly on neutron excess. Interestingly, the spreading widths are independent of whether statistical isospin-violating neutron decay is energetically possible or not, as is the case for the isobaric analog states of the light Sn isotopes with A < 116. The branching ratios for the isospin-forbidden neutron decays from the isobaric analog states average about 95% for the heavier even- A Sb isotopes, and about 60% for the odd- A isotopes. Gamma-ray deexcitation and direct and statistical proton decay compete in the decays of the IAS of the light Sn isotopes, and a branching ratio of about 90% for gamma-ray deexcitation is expected for the IAS in 112Sb.

3He and 4He production by 800 MeV protons from 12C, Ti, and Pb at forward angles.

The doubly differential cross section for the production of $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ by 800 MeV protons from $^{12}\mathrm{C}$, Ti, and Pb has been measured at laboratory angles of 6\ifmmode^\circ\else\textdegree\fi{} and 15\ifmmode^\circ\else\textdegree\fi{}. The momentum of the detected helium nuclei varied from 1 to 2 GeV/c, the maximum being well above the incident proton momentum of 1.46 GeV/c. The cross sections were found to increase with increasing target mass and decrease with increasing momentum and scattering angle. In our momentum region, the $^{3}\mathrm{He}$ production cross section is 1.5--10 times larger than $^{4}\mathrm{He}$ depending on the target and the momentum. The data are consistent with the hypothesis that the dominant reaction mechanism is a direct process where the initial nucleon-nucleon scattering is followed by a sequential pickup of neutrons.

Proton-induced break-up of 3He at 35MeV

The reactions 3He + p → d + p + p, and 3He + p → p + p + p + n have been studied at 35MeV. These reactions are dominated by quasi-two-body processes: pp and pd quasi-free scattering, pp final-state interactions, and neutron pick-up. We compare our data with phenomenological calculations which describe these processes and establish significant discrepancies. Our data are also compared with four-body calculations based on exact integral equations, which give a much better description and we show that if one would use potentials that yield the correct 3He binding energy and if one would take even approximately into account the Coulomb interaction a much better agreement would be achieved with all the data.

One-nucleon-transfer reactions induced by 20Ne at 500 and 600 MeV.

The ${(}^{20}$Ne${,}^{19}$Ne), ${(}^{20}$Ne${,}^{21}$Ne), and ${(}^{20}$Ne${,}^{21}$Na) reactions on $^{90}\mathrm{Zr}$ and $^{208}\mathrm{Pb}$ have been investigated at 500 and 600 MeV incident energies. Experimental spectra have been compared with predictions of distorted-wave Born approximation calculations. The different shapes observed for proton and neutron pickup spectra are explained quite well by the calculations. In particular, the broad peak observed at about 6.5 MeV in the ${(}^{20}$Ne${,}^{21}$Ne) spectra is shown to originate from ejectile excitation in the 1${\mathit{d}}_{3/2}$, 1${\mathit{f}}_{7/2}$, and 2${\mathit{p}}_{3/2}$ orbitals. An important component of the ${(}^{20}$Ne${,}^{19}$Ne) cross section is due to three-body processes, such as the elastic breakup of $^{20}\mathrm{Ne}$ into $^{19}\mathrm{Ne}$ and one neutron. In addition, bumps are observed at about 1.5 and 10 MeV excitation energy in $^{209}\mathrm{Pb}$ and 2.5 and 14 MeV in $^{91}\mathrm{Zr}$. The low energy peaks are shown to originate mainly from neutron transfer to high spin orbitals, i.e., 1${\mathit{i}}_{11/2}$ and 1${\mathit{j}}_{15/2}$ in $^{209}\mathrm{Pb}$ and 1${\mathit{g}}_{7/2}$ and 1${\mathit{h}}_{11/2}$ in $^{91}\mathrm{Zr}$. Because of the selectivity of the ${(}^{20}$Ne, $^{19}\mathrm{Ne}$) reaction for large angular momentum transfer, it is proposed that the structures observed at 14 MeV in $^{91}\mathrm{Zr}$ and 10 MeV $^{209}\mathrm{Pb}$ are due to a neutron transfer to high spin orbitals, such as 1${\mathit{i}}_{13/2}$ and 1${\mathit{k}}_{17/2}$ in $^{91}\mathrm{Zr}$ and $^{209}\mathrm{Pb}$, respectively.

Study of 116sn via neutron pickup and proton stripping reactions

The nucleus 116Sn has been studied via the neutron pickup reactions 117Sn(d,t) 116Sn and 117Sn( 3He, α) 116Sn, and via the proton stripping reactions 115In( 3He, d) 116 and 115In(α,t) 116Sn. Spectroscopic factors have been deduced from the 117Sn(d,t) and the 115( 3He, d) reactions for final states in 116Sn below E x = 3.8 MeV. By combining the ( l-transfers, observed in these reactions, spin assignments for various states can be given or limits on them can be set. Notably an unambiguous assignment of I π = 3 + can be made for the states at 2.997, 3.180, 3.416 and 3.709 MeV. Spectroscopic factors and their fragmentation are compared with those derived from the broken pair model and from a large-basis shell-model calculation involving both neutron quasi-particle excitations and proton 1p1h excitations.

Structure of 14c studied by the 14c(p,d) 13c reaction

Cross sections for the neutron pickup reaction 14C(p, d) 13C were measured at E p = 35.0 MeV for the states below E = 10 MeV, and at E p = 40.1 MeV for the states between E x = 11 and 18 MeV. Obtained angular distributions were compared with exact finite-range DWBA calculations to extract spectroscopic factors. Second-order DWBA calculations were also made in some cases. Almost full 0p 1 2 strength was found, while the observed 0p 3 2 strength was 80% of the simple sum-rule limit. The main part of the remaining strength was found in the 0d 5 2 orbit, and, in a lesser degree, in the 1s 1 2 and 0d 3 2 orbits. Small fractions of the 0f 5 2 and Og 9 2 strengths were also necessary to explain the angular-distribution shapes and the cross-section magnitudes of the predominantly multistep transitions to the 5− 2 and 5+ 2 states.

Neutron pickup strength in 87Sr from the (p

Differential cross section and analyzing power angular distributions have been obtained for the $^{87}(\mathit{p}\ensuremath{\rightarrow}$,d${)}^{86}$Sr reaction in two measurements using different magnetic spectrometers at incident proton energies of 94.2 and 91.8 MeV. Typical energy resolutions of 70 and 40 keV, respectively, were achieved. Optical-model parameters were obtained by fitting the elastic scattering data measured to laboratory angles of 90\ifmmode^\circ\else\textdegree\fi{} for protons at 94.2 MeV and to 120\ifmmode^\circ\else\textdegree\fi{} for deuterons at 88.0 MeV. Standard exact finite range distorted-wave Born approximation calculations using these optical-model parameters failed to describe the experimental data, while the adiabatic approximation improved the description to some extent. Orbital and total angular momenta, l and j, of the picked-up neutron were extracted from the angular distributions of the differential cross section and analyzing power for all the residual states observed. The rms radius of the 1${g}_{9/2}$ neutron orbital, obtained from magnetic electron scattering, was used in the extraction of strength for this orbital, while rms radii from Hartree-Fock calculations were used for the 2${p}_{1/2}$, 2${p}_{3/2}$, 1${f}_{5/2}$, and 1${f}_{7/2}$ orbitals. The observed 1${g}_{9/2}$ strength, spread over 20 states up to 5.4-MeV excitation, indicates a depletion of about 43% compared to simple shell-model expectations. Similarly, the observed 1${f}_{5/2}$ strength, spread over 17 states, indicates a depletion of about 32%. The weighted summed spectroscopic strength for the 1${g}_{9/2}$ orbital, deduced from the present study, is lower than that obtained from magnetic electron scattering.

Neutron pickup and four-body processes in reactions of 16O+ 197Au at 26.5 and 32.5 MeV/nucleon

Projectile breakup in reactions of 424 MeV and 520 MeV 16O+ 197Au was studied via coincidence measurements of projectile-like fragments and light particles. The quasi-elastic breakup of the projectile was found to proceed via sequential decay. In addition, the 12C- α channel showed important contributions from neutron pickup followed by successive neutron and alpha-particle emission, a mechanism generating four-body final states.

Tensor polarisation of [formula omitted] in the [formula omitted] reaction at Ep = 41.3 MeV

Angular correlation measurements have been made between the deuterons and the 12C g.s. product nuclei in the reaction 13 C( p,d) 12 C ∗(15.11 MeV, 1 +)(→ 120 C g.s. + γ) at deuteron scattering angles of 40°, 50°, 60°, and 75°, and a proton bombarding energy of 41.3 MeV. These measurements yield information concerning the k = 2 polarisation tensors ( t 2q) of the excited 12C nucleus produced in the direct (p, d) reaction. Cross-section angular distributions to the strongly excited g.s., 4.43, 12.71, 15.11 and 16.11 MeV states are also reported. Monte Carlo simulations of the laboratory distribution functions associated with the individual t kq were performed, incorporating the effects of finite geometry (beam spot and detector sizes), beam divergence, energy loss and multiple scattering of the 12C nuclei in the target. Finite range DWBA calculations were performed describing the polarisation tensors and cross sections, assuming a 1p neutron pickup. DWBA calculations for both conventional (elastic) and adiabatic d + 12C potentials were performed. Also, the effects of spin-orbit and the T r and T ℓ tensor potential were studied. The results confirmed that the “standard” DWBA formalism using an elastic d + 12C potential is inadequate for describing the recoil nucleus polarization. The use of adiabatic potentials, which arise from three-body effects, produced an improvement in the description of the extracted data. The DWBA calculations showed only a modest sensitivity to inclusion of spin-orbit and the T r or T ℓ tensor terms in the deuteron optical potential.

Study of the165Ho(?,n)164Ho reaction

The spectrum of neutrons following photo-excitation of165Ho with 8,999 keVγ-rays has been observed. The data indicate a neutron separation energy of 7,987 (2) keV. Eleven excited states are identified, with excitation energies generally in good agreement with those observed in the neutron pickup reaction. Significant discrepancies between observed and theoretical branching are noted.

Charge exchange, scattering, and transfer reactions for 9Be+6Li and analysis of 14C(6Li,6He).

Angular distributions for the /sup 9/Be(/sup 6/Li,/sup 6/He) charge exchange reaction, /sup 9/Be(/sup 6/Li,/sup 6/Li) elastic and inelastic scattering, and the /sup 9/Be(/sup 6/Li,/sup 7/Li) transfer reaction have been measured at E = 32 MeV. Limited measurements at 36 MeV showed no change in the cross sections at this energy compared with those at 32 MeV. Coupled channels calculations are necessary to describe the inelastic scattering data to the /sup 9/Be, 2.43 MeV, (5/2)/sup -/ state. The deformation length obtained is in good agreement with the electron scattering value. A microscopic analysis of the charge exchange reaction shows that the M3Y interaction gives a good description of the /sup 9/Be ground state to /sup 9/B ground state transition, but underpredicts the cross section to the /sup 9/B, 2.36 MeV, (5/2)/sup -/ state by a factor of 3. The same calculations carried out for previously reported /sup 14/C(/sup 6/Li,/sup 6/He) data reproduce the magnitude of the data provided that Visscher-Ferrell densities are used for the /sup 14/C to /sup 14/N transition. The results of finite range distorted-wave Born approximation calculations for /sup 9/Be(/sup 6/Li,/sup 7/Li/sub 0,1/) /sup 8/Be/sub 0/ show the neutron pickup spectroscopic factors to be in good agreement withmore » the Cohen-Kurath calculations.« less