Excitation Energy Region Research Articles

Measurement of inelastic cross sections for low-energy electron scattering from DNA bases

Purpose: To determine experimentally the absolute cross sections (CS) to deposit various amount of energies into DNA bases by low-energy electron (LEE) impact.Materials and methods: Electron energy loss (EEL) spectra of DNA bases were recorded for different LEE impact energies on the molecules deposited at very low coverage on an inert argon (Ar) substrate. Following their normalisation to the effective incident electron current and molecular surface number density, the EEL spectra were then fitted with multiple Gaussian functions in order to delimit the various excitation energy regions. The CS to excite a molecule into its various excitation modes were finally obtained from computing the area under the corresponding Gaussians.Results: The EEL spectra and absolute CS for the electronic excitations of pyrimidine and the DNA bases thymine, adenine, and cytosine by electron impacts below 18 eV were reported for the molecules deposited at about monolayer coverage on a solid Ar substrate.Conclusions: The CS for electronic excitations of DNA bases by LEE impact were found to lie within the 10216 to 10218 cm2 range. The large value of the total ionisation CS indicated that ionisation of DNA bases by LEE is an important dissipative process via which ionising radiation degrades and is absorbed in DNA.

EXPERIMENTAL INVESTIGATION OF 6Be CLUSTER DECAY

A complete kinematics measurement was performed using the 6 Li (3 He ,t) charge exchange reaction to populate 6 Be at Elab = 50 MeV . Decay mode of the ground and first excited states were studied through the analysis of the energy and angular distributions of the break-up particles in the 6 Be center of mass frame. Two-body decay is observed above the first excited state but not firmly assigned to a 6 Be resonant state. Experimental challenges regarding the identification of true 6 Be events in the excitation energy region 5 < Ex < 20 MeV are discussed.

Probing the nature of particle–core couplings in 49Ca with γ spectroscopy and heavy-ion transfer reactions

Neutron rich nuclei around 48Ca have been measured with the CLARA–PRISMA setup, making use of 48Ca on 64Ni binary reactions, at 5.9 MeV/A. Angular distributions of γ rays give evidence, in several transfer channels, for a large spin alignment (≈70%) perpendicular to the reaction plane, making it possible to firmly establish spin and parities of the excited states. In the case of 49Ca, states arising from different types of particle–core couplings are, for the first time, unambiguously identified on basis of angular distribution, polarization and lifetime measurements. Shell model and particle–vibration coupling calculations are used to pin down the nature of the states. Evidence is found for the presence, in the same excitation energy region, of two types of coupled states, i.e. single particle coupled to either 48Ca or 50Ca simple configurations, and particle–vibration coupled states based on the 3− phonon of 48Ca.

Conformationally resolved structures of jet-cooled acetaminophen by UV–UV hole-burning spectroscopy

The conformational structures of jet-cooled acetaminophen were investigated in the gas phase by resonant 2-photon ionization and UV-UV hole-burning spectroscopy. In contrast to the results from a previous study, two nearly isoenergetic conformers were distinctly found in a supersonic molecular beam expansion and positively identified as the cis and trans isomers of acetaminophen by UV-UV hole-burning spectroscopy. The 0-0 bands of the cis and trans isomers were found at 33518.7 and 33485.6 cm(-1), respectively. The vibronic bands of the two isomers are close-lying and/or partially overlapping due to the small energy difference (33 cm(-1)) between the two 0-0 bands. As a consequence, the recorded resonant 2-photon ionization spectrum is highly congested in the low excitation energy region, which develops continuously into a featureless, broadened spectrum in the high energy region.

Absolute cross sections for electronic excitations of cytosine by low energy electron impact.

The absolute cross sections (CSs) for electronic excitations of cytosine by electron impact between 5 and 18 eV were measured by electron-energy-loss (EEL) spectroscopy of the molecule deposited at low coverage on an inert Ar substrate. The lowest EEL features found at 3.55 and 4.02 eV are ascribed to transitions from the ground state to the two lowest triplet 1 (3)A(')(π→π(∗)) and 2 (3)A(')(π→π(∗)) valence states of the molecule. Their energy dependent CSs exhibit essentially a common maximum at about 6 eV with a value of 1.84×10(-17) cm(2) for the former and 4.94×10(-17) cm(2) for the latter. In contrast, the CS for the next EEL feature at 4.65 eV, which is ascribed to the optically allowed transition to the 2 (1)A(')(π→π(∗)) valence state, shows only a steep rise to about 1.04×10(-16) cm(2) followed by a monotonous decrease with the incident electron energy. The higher EEL features at 5.39, 6.18, 6.83, and 7.55 eV are assigned to the excitations of the 3 (3,1)A(')(π→π(∗)), 4 (1)A(')(π→π(∗)), 5 (1)A(')(π→π(∗)), and 6 (1)A(')(π→π(∗)) valence states, respectively. The CSs for the 3 (3,1)A(') and 4 (1)A(') states exhibit a common enhancement at about 10 eV superimposed on a more or less a steep rise, reaching, respectively, a maximum of 1.27 and 1.79×10(-16) cm(2), followed by a monotonous decrease. This latter enhancement and the maximum seen at about 6 eV in the lowest triplet states correspond to the core-excited electron resonances that have been found by dissociative electron attachment experiments with cytosine in the gas phase. The weak EEL feature found at 5.01 eV with a maximum CS of 3.8×10(-18) cm(2) near its excitation threshold is attributed to transitions from the ground state to the 1 (3,1)A(")(n→π(∗)) states. The monotonous rise of the EEL signal above 8 eV is attributed to the ionization of the molecule. It is partitioned into four excitation energy regions at about 8.55, 9.21, 9.83, and 11.53 eV, which correspond closely to the ionization energies of the four highest occupied molecular orbitals of cytosine. The sum of the ionization CS for these four excitation regions reaches a maximum of 8.1×10(-16) cm(2) at the incident energy of 13 eV.

New Scalings in Nuclear Fragmentation

Fragment partitions of fragmenting hot nuclei produced in central and semiperipheral collisions have been compared in the excitation energy region 4-10MeV per nucleon where radial collective expansion takes place. It is shown that, for a given total excitation energy per nucleon, the amount of radial collective energy fixes the mean fragment multiplicity. It is also shown that, at a given total excitation energy per nucleon, the different properties of fragment partitions are completely determined by the reduced fragment multiplicity (i.e., normalized to the source size). Freeze-out volumes seem to play a role in the scalings observed.

Tensor Effects on Spin-Dependent Charge-Exchange transitions

The tensor terms of the Skyrme effective interaction are included in the self-consistent Hartree-Fock plus Random Phase Approximation (HF+RPA) model. The Gamow-Teller (GT) and Spin-Dipole (SD) strength functions of 208Pb are calculated with and without the tensor terms. The main GT peak is moved downwards by about 2 MeV and about 10 % of the total strength is shifted to the excitation energy region above 30 MeV by the RPA tensor correlation. The SD 0 − peaks are moved upward while the 1 − peaks are moved downward dramatically, by the RPA tensor correlation. An qualitative understanding of these effects are given by using a separable tensor force.

Carrier relaxation dynamics in SnxNy nanowires grown by chemical vapor deposition

Carrier relaxation dynamics in tin nitride (SnxNy) nanowires have been investigated using femtosecond transient absorption spectroscopy. The nanowires were grown directly on quartz using chemical vapor deposition and had diameters ≤200 nm and lengths up to 2 μm. Steady state optical transmission measurements suggest that the band gap is ∼2.9 eV while time resolved measurements reveal that free carrier absorption dominates the carrier dynamics and overcomes state filling within 0.5 ps of the incoming excitation pulse even when probing above the band edge. This is a unique and markedly different behavior compared to what we have observed in other semiconductor nanowires and it is attributed to fast scattering of the photogenerated carriers out of the excitation energy region and possible rise in the lattice temperature due to energy relaxation. Carrier relaxation occurs through two channels with a fast time constants of ≈200 ps and a slow time constant ranging between 5 and 8 ns while intensity measurements reveal negligible contribution from nonlinear effects such as Auger recombination.

Weak-interaction strength from charge-exchange reactions versusβdecay in theA=40isoquintet

We report a measurement of the Gamow-Teller (GT) strength distribution for $^{40}\mathrm{Ar}\ensuremath{\rightarrow}^{40}\mathrm{K}$ using the ${0}^{\ifmmode^\circ\else\textdegree\fi{}}(p,n)$ reaction. The measurement extends observed GT strength distribution in the $A=40$ system up to an excitation energy of $~8$ MeV. In comparing our results with those from the $\ensuremath{\beta}$ decay of the isospin mirror nucleus $^{40}\mathrm{Ti}$, we find that, within the excitation energy region probed by the $\ensuremath{\beta}$-decay experiment, we observe a total GT strength that is in fair agreement with the $\ensuremath{\beta}$-decay measurement. However, we find that the relative strength of the two strongest transitions differs by a factor of $~1.8$ in comparing our results from ($p,n$) reactions with the $\ensuremath{\beta}$ decay of $^{40}\mathrm{Ti}$. Using our results we present the neutrino-capture cross section for $^{40}\mathrm{Ar}$.

Binary reaction decays fromMg24+C12

Charged-particle and $\ensuremath{\gamma}$ decays in $^{24}\mathrm{Mg}{}^{*}$ are investigated for excitation energies where quasimolecular resonances appear in $^{12}\mathrm{C}+^{12}\mathrm{C}$ collisions. Various theoretical predictions for the occurrence of superdeformed and hyperdeformed bands associated with resonance structures with low spin are discussed within the measured $^{24}\mathrm{Mg}{}^{*}$ excitation energy region. The inverse kinematics reaction $^{24}\mathrm{Mg}+^{12}\mathrm{C}$ is studied at ${E}_{\mathrm{lab}}(^{24}\mathrm{Mg})=130$ MeV, an energy that enables the population of $^{24}\mathrm{Mg}$ states decaying into $^{12}\mathrm{C}+^{12}\mathrm{C}$ resonant breakup states. Exclusive data were collected with the Binary Reaction Spectrometer in coincidence with Euroball IV installed at the Vivitron tandem facility at Strasbourg. Specific structures with large deformation were selectively populated in binary reactions, and their associated $\ensuremath{\gamma}$ decays studied. Coincident events associated with inelastic and $\ensuremath{\alpha}$-transfer channels have been selected by choosing the excitation energy or the entry point via the two-body $Q$ values. The analysis of the binary reaction channels is presented with a particular emphasis on $^{24}\mathrm{Mg}\text{\ensuremath{-}}\ensuremath{\gamma}$, $^{20}\mathrm{Ne}\text{\ensuremath{-}}\ensuremath{\gamma}$, and $^{16}\mathrm{O}$-$\ensuremath{\gamma}$ coincidences. New information (spin and branching ratios) is deduced on high-energy states in $^{24}\mathrm{Mg}$ and $^{16}\mathrm{O}$, respectively.

Hyperdeformed sub-barrier fission resonances observed inU232

The fission probability of $^{232}\mathrm{U}$ has been measured using the $^{231}\mathrm{Pa}$($^{3}\mathrm{He}$,df) reaction with an energy resolution of 11 keV in the excitation energy region of ${E}^{*}=4.0\text{\ensuremath{-}}6.4$ MeV. A number of sub-barrier fission resonances have been observed for the first time in the excitation energy range below ${E}^{*}=4.8$ MeV and interpreted as rotational bands with a rotational parameter characteristic to a hyperdeformed nuclear shape ($\ensuremath{\hbar}{}^{2}/2\ensuremath{\Theta}=1.96\ifmmode\pm\else\textpm\fi{}0.11$ keV). The angular distribution of the associated fission fragments was measured to deduce the $K$ value of the rotational bands. The fission barrier parameters of $^{232}\mathrm{U}$ have been determined by analyzing the overall features of the fission probability. A deep third minimum with an excitation energy of ${E}_{\mathrm{III}}=3.2(2)$ MeV and rather low inner barrier height of ${E}_{A}=4.0(3)$ MeV could be established.

Gamow-Teller transitions studied in the high-resolutionNi64(He3,t)Cu64reaction

To study the Gamow-Teller (GT) transitions to the pf-shell nucleus Cu-64, the Ni-64(He-3,t)Cu-64 experiment was performed at the Research Center for Nuclear Physics (RCNP) Ring Cyclotron, Osaka, using a He-3 beam of 140 MeV/nucleon. The outgoing tritons were momentum analyzed by the Grand Raiden spectrometer at 0(degrees). A high energy resolution of 32 keV (full width at half-maximum) allowed the separation of individual levels in the excitation-energy region from 0 to 3.5 MeV. In addition to the ground state (gs), known to be a J(pi)=1(+) GT state, many low-lying states showed L=0 nature, suggesting that they are candidates for GT states. Because the GT strength B(GT) for the gs transition is known from the beta-decay measurement, the strengths for the excited states could be determined using the proportionality between the B(GT) and the reaction cross section extrapolated to q=0 momentum transfer. At higher excitation energies, the level density becomes high and the so-called GT giant resonance dominates the spectrum. The lower and the upper limits of the strength contained in this energy region were estimated. Our results show that less than 55% of the strength predicted by the Ikeda sum rule is located in the excitation-energy region from 0 to 17 MeV..

Identification of theg9/2-proton bands in the neutron-richGa71,73,75,77nuclei

Excited states in the odd-A {sup 71,73,75,77}Ga nuclei have been populated in deep-inelastic reactions of a {sup 76}Ge beam at 530 MeV with a thick {sup 238}U target. High-spin sequences built upon the 9/2{sup +}, 5/2{sup -}, and 3/2{sup -} states were identified in all four isotopes. A comparison of the observed structures with the yrast positive-parity states in the neighboring even-even Zn cores indicates that the newly identified levels may be regarded as arising from the relatively weak coupling of the odd proton to the core states. However, significant contributions from broken pairs are expected to be present in this region of excitation energy. The present data set also provides clarification of previously reported decay paths of the low-energy levels in {sup 71,73,75,77}Ga.

Statistical multifragmentation model with Skyrme effective interactions

The statistical multifragmentation model is modified to incorporate the Helmholtz free energies calculated in the finite temperature Thomas-Fermi approximation using Skyrme effective interactions. In this formulation, the density of the fragments at the freeze-out configuration corresponds to the equilibrium value obtained in the Thomas-Fermi approximation at the given temperature. The behavior of the nuclear caloric curve at constant volume is investigated in the micro-canonical ensemble, and a plateau is observed for excitation energies between 8 and 10 MeV per nucleon. A kink in the caloric curve is found at the onset of this gas transition, indicating the existence of a small excitation energy region with negative heat capacity. In contrast to previous statistical calculations, this phase transition takes place even when the system is constrained to fixed volume. The observed phase transition takes place at approximately constant entropy. The charge distribution and other observables also turn out to be sensitive to the treatment employed in the calculation of the free energies and the fragment volumes at finite temperature, specially at high excitation energies. The isotopic distribution is also affected by this treatment, which suggests that this prescription may help obtain information on the nuclear equation of state.

Effect of tensor correlations on Gamow–Teller states in 90Zr and 208Pb

The tensor terms of the Skyrme effective interaction are included in the self-consistent Hartree–Fock plus Random Phase Approximation (HF + RPA) model. The Gamow–Teller (GT) strength function of 90Zr and 208Pb are calculated with and without the tensor terms. The main peaks are moved downwards by about 2 MeV when including the tensor contribution. About 10% of the non-energy weighted sum rule is shifted to the excitation energy region above 30 MeV by the RPA tensor correlations. The contribution of the tensor terms to the energy weighted sum rule is given analytically, and compared to the outcome of RPA.

Multiphoton ionization/dissociation dynamics of formyl fluoride by velocity mapping ion imaging

The dissociation dynamics of HFCO(+) ion has been studied using the velocity map ion imaging technique. The HFCO(+) ion is prepared by one-photon resonant three-photon ionization in the region of 43100-43860 cm(-1) excitation energy. The HFCO(+) ions, produced by multiphoton ionization, have sufficient internal energy to dissociate into the F and HCO(+) fragments without further absorption of another photon. Images of HCO(+) have been recorded at various excitation energies. It is noticed that the angular distributions of HCO(+) change dramatically from parallel distribution to perpendicular distribution and then back to parallel distribution in a very narrow excitation energy region of 43 473-43 500 cm(-1). Analysis of anisotropy parameters of beta(n) (n = 2, 4 and 6) reveals that the electronic states in the three-photon excitation of HFCO are mainly: HFCO(X(1)A') --> HFCO(A(1)A'') --> HFCO(A') --> HFCO(+)(A(2)A'';B(2)A'). The purely perpendicular resonant transitions are likely responsible for the perpendicular angular distribution of the HCO(+) ion fragment.

Excited states of 8Be populated via the p(7Li, 4He)4He and p(7Li, 4He*)4He resonant reactions

A measurement of the p(7Li, 4He)4He resonant reaction on a thick mylar target was used to probe resonances in 8Be in the excitation energy range 20–25 MeV. The 7Li beams, of energies 25.8 and 58 MeV, were stopped in the target and the resulting α-particles were detected at zero degrees. Evidence was found for resonances at ∼20 and ∼25 MeV with Jπ = 0+ and 2+ character, respectively. A simultaneous measurement of the p(7Li, 4He* → p + t)4He reaction was performed, in which one of the two final-state α-particles was produced in the 20.2 MeV excited state. These events were observed to arise from an excitation energy region 23–25 MeV and have been associated with the decay of the 25.2 MeV 2+ 8Be resonance. The 25.2 MeV resonance is found to have a dominant 4He + 4He* reduced width.

Multinucleon transfer reactions in closed-shell nuclei

Multinucleon transfer reactions in Ca40+Zr96 and Zr90+Pb208 have been measured at energies close to the Coulomb barrier in a high-resolution γ-particle coincidence experiment. The large-solid-angle magnetic spectrometer PRISMA coupled to the CLARA γ array has been employed. Trajectory reconstruction has been applied for the complete identification of transfer products. Mass and charge yields, total kinetic energy losses, γ transitions of the binary reaction partners, and comparison of data with semiclassical calculations are reported. Specific transitions in Zr95 populated in one-particle transfer channels are discussed in terms of particle-phonon couplings. The γ decays from states in Ca42 in the excitation energy region expected from pairing vibrations are also observed.3 MoreReceived 26 April 2007DOI:https://doi.org/10.1103/PhysRevC.76.024604©2007 American Physical Society

Excitation-energy dependence of the resonant Auger transitions to the4p4(D1)np(n=5,6)states across the3d3∕2−15pand3d5∕2−16presonances in Kr

The energy dependencies of the intensities and angular distribution parameters $\ensuremath{\beta}$ of the resonant Auger final states $4{p}^{4}(^{1}D)np$ $(n=5,6)$ of Kr were determined experimentally in the excitation-energy region of the overlapping $3{d}_{3∕2}^{\ensuremath{-}1}5p$ and $3{d}_{5∕2}^{\ensuremath{-}1}6p$ resonances. The experimental results were compared with the outcome of multiconfiguration Dirac-Fock calculations. Combining experimental and calculated results allowed us to study interference effects between the direct and several resonant channels that populate the $4{p}^{4}(^{1}D)np$ states. The inclusion of the direct channel was crucial in order to reproduce the observed energy behavior of the angular distribution parameters. It was also important to take into account experimentally observed shake transitions.

Proton scattering at intermediate energies onNi58: How well is it understood?

The excitation-energy region up to 23 MeV in $^{58}\mathrm{Ni}$ has been investigated with the ($\stackrel{\ensuremath{\rightarrow}}{p},{\stackrel{\ensuremath{\rightarrow}}{p}}^{'})$ reaction at an incident energy of 172 MeV and forward angles favoring the excitation of the spin-dipole mode in the continuum. The measured cross sections, spin-flip cross sections, and spin-flip probabilities are compared to microscopic reaction-model calculations using various effective projectile-target interactions. The nuclear structure input, which takes into account one-particle one-hole (1p1h) as well as two-particle-two hole (2p2h) excitations, allows for a reasonable description of the continuum region well above the particle-decay thresholds. However, considerable problems remain in the description of spin-flip resonances at lower excitation energies, which cannot be explained by a quenching of the spin-isospin interaction.