MeV Excitation Energy Research Articles

Experimental study of the 18O(d, p)19O reaction and the ANC Method

The neutron capture rate 18O(n, γ)19O is important for analysis of nucleosynthesis in inhomogeneous Big Bang models and also for models of helium burning in massive red giant stars and AGB stars. Angular distributions of the 18O(d, p)19O reaction were measured at a deuteron energy of 16.3 MeV in NPI in Rez, Czech Republic, with the aim to determine Asymptotic Normalization Coefficients which can then be used for indirect determination of the direct contribution to the 18O(n, γ)19O process. In the experiment, the gas target with 18O isotope of high purity 99.9% was used thus eliminating any contaminating reactions. Reaction products were measured by the set of 8 ΔE-E telescopes consisting of thin and thick silicon surface-barrier detectors. Angular distributions of proton transfers corresponding to 6 levels of 19O up to the 4.1093 MeV excitation energy were determined. The analysis of angular distributions in the angular range from 6 to 64 degree including also the angular distribution of elastically scattered deuterons was carried out by means of ECIS and DWUCK codes. From the determined ANCs the direct contribution to the radiative capture 18O(n, γ)19O was deduced.

Investigation of0+states in198Hg after two-neutron pickup

We studied ${0}^{+}$ states in ${}^{198}$Hg after the ${}^{200}$Hg${(p,t)}^{198}$Hg transfer reaction up to 3 MeV excitation energy. The experiment was performed using the high-resolution Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory Tandem accelerator in Munich. In total, only four ${0}^{+}$ states were observed in ${}^{198}$Hg, significantly fewer than in other experiments of the $(p,t)$ transfer campaign. We discuss the low-energy ${0}^{+}$ state density as a function of the valence nucleon number ${N}_{\mathrm{val}}$ and test if the ${0}^{+}$ density can be used as a signature for the prolate-oblate shape-phase transition in the Hf-Hg region.

Absolute decay width measurements in 16O

The reaction 126C(63Li, d)168O* at a 6Li bombarding energy of 42 MeV has been used to populate excited states in 16O. The deuteron ejectiles were measured using the high-resolution Munich Q3D spectrograph. A large-acceptance silicon-strip detector array was used to register the recoil and break-up products. This complete kinematic set-up has enabled absolute α-decay widths to be measured with high-resolution in the 13.9 to 15.9 MeV excitation energy regime in 16O; many for the first time. This energy region spans the 14.4 MeV four-α breakup threshold. Monte-Carlo simulations of the detector geometry and break-up processes yield detection efficiencies for the two dominant decay modes of 40% and 37% for the α+12C(g.s.) and a+12C(2+1) break-up channels respectively.

States of 14C and 15C via the (18O,16O) two-neutron transfer reaction at 84 MeV

A study of the 14C and 15C states was pursued at the Catania INFN-LNS laboratory by the 12C(18O,16O)14C and 13C(18O,16O)15C reactions at 84 MeV incident energy. The 16O ejectiles were detected at forward angles by the MAGNEX magnetic spectrometer. Exploiting the large momentum acceptance (20%) and solid angle (50 msr) of the spectrometer, energy spectra were obtained with a relevant yield up to 20 MeV excitation energy. The application of the powerful trajectory reconstruction technique did allow to get energy spectra and angular distributions with resolution of about 160 keV and 0.3°. In the energy spectra several known low lying states of 14C and 15C have been observed and some unknown resonant structures at about 10.5 and 13.6 MeV in 15C and 16 MeV in 14C appear.

Experimental investigation of shell-model excitations of89Zr up to high spin

Near yrast states in ${}^{89}$Zr were investigated up to high spin using the fusion evaporation reaction ${}^{80}$Se(${}^{13}$C, 4$n$) at an incident beam energy of 50 MeV. Excited levels of ${}^{89}$Zr have been observed up to $\ensuremath{\sim}$10 MeV excitation energy and spin $\ensuremath{\sim}$37/2$\ensuremath{\hbar}$ using the prompt gamma spectroscopy technique with the Indian National Gamma Array (INGA). The angular distribution, directional correlation, and polarization measurements were carried out to assign the spin and parity of the newly reported states. The structures of both the positive and negative parity states up to highest spin observed in the present experiment have been compared with shell-model calculations using two recently developed residual interactions, JUN45 and jj44b. The role of proton excitations from ${p}_{3/2}$ and ${f}_{5/2}$ orbitals to the ${g}_{9/2}$ orbital for the higher spin states has been discussed.

Nuclear structure of 216 Ra at high spin

High-spin states of 216Ra (\(\emph{Z}=88\), \(\emph{N}=128\)) have been investigated through 209Bi(10B, 3n) reaction at an incident beam energy of 55 MeV and 209Bi(11B, 4n) reaction at incident beam energies ranging from 65 to 78 MeV. Based on γγ coincidence data, the level scheme for 216Ra has been considerably extended up to ~ 33\(\hbar\) spin and 7.2 MeV excitation energy in the present experiment with placement of 28 new γ-transitions over what has been reported earlier. Tentative spin-parity assignments are done for the newly proposed levels on the basis of the DCO ratios corresponding to strong gates. Empirical shell model calculations were carried out to provide an understanding of the underlying nuclear structure.

High spin band structures in doubly odd194Tl

The high-spin states in odd-odd $^{194}$Tl nucleus have been studied by populating them using the $^{185,187}$Re($^{13}$C, xn) reactions at 75 MeV of beam energy. $\gamma-\gamma$ coincidence measurement has been performed using the INGA array with a digital data acquisition system to record the time stamped data. Definite spin-parity assignment of the levels was made from the DCO ratio and the IPDCO ratio measurements. The level scheme of $^{194}$Tl has been extended up to 4.1 MeV in excitation energy including 19 new gamma ray transitions. The $\pi h_{9/2} \otimes \nu i_{13/2}$ band, in the neighboring odd-odd Tl isotopes show very similar properties in both experimental observables and calculated shapes. Two new band structures, with 6-quasiparticle configuration, have been observed for the first time in $^{194}$Tl. One of these bands has the characteristics of a magnetic rotational band. The cranked shell model calculations, using a deformed Woods-Saxon potential, have been performed to obtain the total Routhian surfaces in order to study the shapes of the bands and the band crossing in $^{194}$Tl. The semiclassical formalism has been used to describe the magnetic rotational band.

Energy separation of the 1+/1− parity doublet in 20Ne

The parity doublet of 1+/1− states of Ne-20 at 11.26 MeV excitation energy is one of the best known test cases to study the weak part of the nuclear Hamiltonian. The feasibility of parity violation experiments depend on the effective nuclear enhancement factor (RN/|E(1+) − E(l−)|) which amplifies the impact of the matrix element of the weak interaction on observables indicating parity mixing. An extreme large value of Rn/|E(1+) − E(l−)| = (670 ± 7000) MeV−1 was reported for the doublet in 20Ne. The large uncertainty depends amongst others on the large uncertainty of |E(1+) − E(l−)| = 7.7±5.5 keV of the parity doublet. Nuclear resonance fluorescence (NRF) experiments with linearly and circularly polarized photon beams were performed at the High Intensity Gamma-Ray Source at Duke University, Durham, NC, USA, to determine the energy difference of the parity doublet with higher precision. The different angular distributions for 0+ → 1− → 0+ and 0+ → 1+ → 0+ NRF cascades in polarized γ-ray beams were used to determine the energy difference of the parity doublet to 2.9(13) keV.

High-spin states with seniorityv=4, 5, and 6 in119–126Sn

The ${}^{119--126}$Sn nuclei have been produced as fission fragments in two reactions induced by heavy ions: ${}^{12}\text{C}+{}^{238}\text{U}$ at a bombarding energy of 90 MeV and ${}^{18}\text{O}+{}^{208}\text{Pb}$ at 85 MeV. Their level schemes have been built from $\ensuremath{\gamma}$ rays detected using the Euroball array. High-spin states located above the long-lived isomeric states of the even- and odd-$A$ ${}^{120--126}$Sn nuclei have been identified. Moreover, isomeric states lying around 4.5 MeV have been established in ${}^{120,122,124,126}$Sn from the delayed coincidences between the fission fragment detector SAPhIR and the Euroball array. The states located above 3 MeV excitation energy are ascribed to several broken pairs of neutrons occupying the $\ensuremath{\nu}{h}_{11/2}$ orbit. The maximum value of angular momentum available in such a high-$j$ shell, i.e., for midoccupation and the breaking of the three neutron pairs, has been identified. This process is observed for the first time in spherical nuclei.

Role of momentum transfer in the quenching of Gamow-Teller strength

Background: Differential cross sections for the $(p,n)$ and $(n,p)$ reactions on ${}^{90}$Zr over the interval of $0--50$-MeV excitation energy were used to determine the corresponding Gamow-Teller (GT) strengths, and the resulting quenching factor is $\ensuremath{\approx}$0.9 with respect to the Ikeda sum rule. In this procedure the contribution of the isovector spin monopole (IVSM) strength was subtracted from the total strength without taking into account the interference between the GT and the IVSM modes.Purpose: To determine the quantitative effect of the IVSM excitation mode on the $L=0$ strength in charge-exchange reactions on several closed-shell nuclei and the Sn isotopic chain.Method: The fully consistent relativistic Hartree-Bogoliubov model plus proton-neutron relativistic quasiparticle random-phase approximation ($pn$-RQRPA) are employed in the calculation of transition strength in the ${\ensuremath{\beta}}^{\ensuremath{-}}$ and ${\ensuremath{\beta}}^{+}$ channels.Results: The inclusion of the higher-order terms, that include the effect of finite momentum transfer, in the transition operator shifts a portion of the strength to the high-energy region above the GT resonance. The total strength is slightly enhanced in nuclei with small neutron-to-proton ratios but remains unchanged with increasing neutron excess.Conclusions: Terms that include momentum transfer in the transition operator act mostly to shift the strength to high excitation energies but hardly affect the total strength. Based on the strength obtained using the full $L=0$ transition operator in the $pn$-RQRPA calculation, we have estimated the impact of the IVSM on the strength measured in the charge-exchange reactions on ${}^{90}$Zr and found that the data are consistent with the Ikeda sum rule.

Small quadrupole deformation for the dipole bands in112In

High spin states in $^{112}$In were investigated using $^{100}$Mo($^{16}$O, p3n) reaction at 80 MeV. The excited level have been observed up to 5.6 MeV excitation energy and spin $\sim$ 20$\hbar$ with the level scheme showing three dipole bands. The polarization and lifetime measurements were carried out for the dipole bands. Tilted axis cranking model calculations were performed for different quasi-particle configurations of this doubly odd nucleus. Comparison of the calculations of the model with the B(M1) transition strengths of the positive and negative parity bands firmly established their configurations.

First observation of high spin states and isomeric decay in210Fr

The first observation of the prompt and the delayed $\ensuremath{\gamma}$ transitions involving the high spin states in ${}^{210}$Fr is reported. The decay of the high spin states and the isomeric levels of ${}^{210}$Fr, identified for the first time from the known sequence of low-lying transitions found earlier in the $\ensuremath{\alpha}$ decay of ${}^{214}$Ac, were studied. High spin states of the doubly-odd ${}^{210}$Fr, which were produced by the fusion evaporation reaction ${}^{197}$Au (${}^{16}$O, $xn$) ${}^{213\ensuremath{-}x}$Fr, were populated and the subsequent emitted $\ensuremath{\gamma}$ rays were detected through the high-sensitivity germanium clover detector array INGA. The level scheme up to yrast levels of 5.3 MeV excitation energy and $\ensuremath{\sim}20\ensuremath{\hbar}$ angular momentum could be established for the first time through $\ensuremath{\gamma}\ensuremath{\gamma}$, $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\Delta}T$ coincidence, and DCO ratio measurements. A new low-lying isomeric transition at ${E}_{\ensuremath{\gamma}}=$ 203(2) keV was observed. The half-life was measured to be ${T}_{1/2}=$ 41(2) ns. The measured half-life was compared with the corresponding single-particle estimate, based on the level scheme obtained from the experiment.

Medium-spin states in neutron-rich83As and81As

The ${}^{83,81}\phantom{\rule{-0.16em}{0ex}}$As nuclei have been produced as fission fragments in the fusion reaction ${}^{18}$O + ${}^{208}$Pb at 85 MeV bombarding energy and studied with the Euroball array. Medium-spin states of ${}^{83,81}\phantom{\rule{-0.16em}{0ex}}$As have been established up to $\ensuremath{\sim}3.5$ MeV excitation energy. From angular correlation analysis, spin values have been assigned to most of the ${}^{81}\phantom{\rule{-0.16em}{0ex}}$As excited states. The behaviors of the yrast structures identified in this work are discussed in comparison with the general features known in the mass region. Then they are compared to the results of two theoretical approaches: the ``rotor + quasiparticle'' for ${}^{81}\phantom{\rule{-0.16em}{0ex}}$As and the shell model using the effective interactions JUN45 for ${}^{83,81}\phantom{\rule{-0.16em}{0ex}}\phantom{\rule{-0.16em}{0ex}}$As.

A novel manifestation of α clustering in 212Po: Pure α-208Pb states revealed by their enhanced E1 decays

Excited states in 212Po have been populated by α transfer using the 208Pb(18O, 14C) reaction and studied with the Euroball γ multidetector array. The level scheme of 212Po has been extended up to ∼ 3.2 MeV excitation energy. Spin and parity values have been assigned thanks to the measurements of γ angular distributions and γ-γ angular correlations. Several γ lines have been found to be shifted by the Doppler effect, allowing for the measurements of the associated lifetimes by the DSAM method. The values, found in the [0.1-1.4] ps range, lead to very enhanced E1 transitions. The emitting states, which have non-natural parity values, are the fingerprints of the "α-208Pb" structure present in 212Po. They are in the same excitation-energy range as the states issued from shell-model configurations.

Two-Neutron Excitations in light nuclei via the (18O,16O) reaction at 84 MeV

A study of light neutron rich nuclei was pursued at the Catania INFN-LNS laboratory by the (18O,16O) reaction at 84 MeV incident energy on 9Be, 11B and 13C targets. The 16O ejectiles were detected at forward angles by the MAGNEX magnetic spectrometer. The energy spectra of 15C show several known low lying states up to about 7 MeV excitation energy as well unknown resonant structures at higher excitation energy. The strong excitation of these latter together with the measured width of about 2 MeV FWHM could indicate the presence of collective modes of excitation connected to the transfer of a correlated neutron pair. Similar features characterize the energy spectra of the 11Be and 13B nuclei.

High-precision28Si(p,t)26Si reaction to determine22Mg(α,p)25Al reaction rates

The rise time of stellar x-ray bursts is a signature of thermonuclear runaway processes in the atmosphere of neutron stars and is highly sensitive to a series of ($\ensuremath{\alpha}$,$p$) reactions via high-lying resonances in $sd$-shell nuclei. Lacking data for the relevant resonance levels, the stellar reaction rates have been calculated using statistical, Hauser-Feshbach models, assuming a high-level density. This assumption may not be correct in view of the selectivity of the ($\ensuremath{\alpha}$,$p$) reaction to natural parity states. We measured the ${}^{28}$Si($p$,$t$)${}^{26}$Si reaction with a high-resolution spectrometer to identify resonance levels in ${}^{26}$Si above the $\ensuremath{\alpha}$-emission threshold at 9.164 MeV excitation energy. These resonance levels are used to calculate the stellar reaction rate of the ${}^{22}$Mg($\ensuremath{\alpha}$,$p$)${}^{25}$Al reaction and to test the validity of the statistical assumption.

Status of vibrational structure inNi62

Measurements consisting of $\ensuremath{\gamma}$-ray excitation functions and angular distributions were performed using the ($n,{n}^{\ensuremath{'}}\ensuremath{\gamma}$) reaction on $^{62}\mathrm{Ni}$. The excitation function data allowed us to check the consistency of the placement of transitions in the level scheme. From $\ensuremath{\gamma}$-ray angular distributions, the lifetimes of levels up to $~3.8$ MeV in excitation energy were extracted with the Doppler-shift attenuation method. The experimentally deduced values of reduced transition probabilities were compared with the predictions of the quadrupole vibrator model and with large-scale shell model calculations in the $\mathit{fp}$ shell configuration space. Two-phonon states were found to exist with some notable deviation from the predictions of the quadrupole vibrator model, but no evidence for the existence of three-phonon states could be established. $Z=28$ proton core excitations played a major role in understanding the observed structure.

Investigation of low-lying electric dipole strength in the semimagic nucleusCa44

The dipole-strength distribution in the semimagic nucleus $^{44}\mathrm{Ca}$ has been measured up to 10 MeV excitation energy in photon-scattering experiments using bremsstrahlung and monoenergetic 100% linearly polarized photon beams. The combination of both measurements allows a clear determination of spin and parity quantum numbers of the excited states as well as absolute cross sections and transition probabilities. The results show that the majority of the dipole strength in $^{44}\mathrm{Ca}$ below 10 MeV is due to $E1$ transitions while $M1$ strength plays only a minor role. The experimental results are compared to the strength in the neighboring doubly magic nuclei $^{40,48}\mathrm{Ca}$ and to microscopic calculations within the extended theory of finite Fermi systems in order to investigate the evolution of the low-lying $E1$ strength in this isotopic chain. Both, experiment and calculations, show a nontrivial dependence of the total $E1$ strength as a function of the neutron number.

High-spin states in the five-valence-particle nucleusPo213

Excited states in {sup 213}Po have been populated using the {sup 18}O+ {sup 208}Pb reaction at 85 MeV beam energy and studied with the Euroball IV {gamma} multidetector array. The level scheme has been built up to {approx}2.0 MeV excitation energy and spin I{approx}25/2({h_bar}/2{pi}) from the triple {gamma} coincidence data. Spin and parity values of several yrast states have been assigned from the {gamma} angular properties. The configurations of the yrast states are discussed using results of empirical shell-model calculations and by analogy with the neighboring nuclei. The spin and parity values of several low-spin states of {sup 213}Po previously identified from the {beta} decay of {sup 213}Bi are revised.

Structures in 20O from the 14C(7Li, p) reaction at 44 MeV

We have studied the multi-nucleon transfer reaction 14 C( 7 Li ,p )a tELab( 7 Li) = 44 MeV popu- lating states of the neutron-rich oxygen isotope 20 O. The experiments have been performed at the Munich Tandem accelerator using the high-resolution Q3D magnetic spectrometer, with an overall energy reso- lution of 45 keV. States were populated up to 20 MeV excitation energy —65 states have been identified in the analysis, among which 42 are new. Rotational bands are proposed in terms of underlying intrinsic reflection-asymmetric cluster and prolate molecular structures (namely 14 C ⊗ 2n ⊗ α) as parity doublet bands. A rectangular oblate structure is suggested for some very narrow states at high excitation energies.