Excitation Energy Region Research Articles

A high spin γ-decaying state at 16.9 MeV in 24Mg

States with excitation energy > 15 MeV populated in the reaction 12C( 16O, αγ) 24Mg have been studied using a total energy γ-ray spectrometer for E lab( 16 O) between 40 and 67 MeV. A previously unknown state has been identified at 16.904 ± 0.003 MeV with Γ γ / Γ=0.13±0.02, mean lifetime <10 fs, a predominant γ-decay branch to the yrast (8 +) state at 11.86 MeV and possible un-natural parity. These measurements also show that all other states produced in this excitation energy region have Γ γ/Γ less than 0.02. Possible points of correspondence between the observed properties of the 16.9 MeV level and the predictions of shell model calculations are noted.

Fission decay of the isoscalar giant quadrupole resonance in 24Mg

The 24 Mg(α, α′) 12 C+ 12 C reaction was studied by measuring 12C fragments in coincidence with inelastically scattered α-particles at E α = 120 MeV. Both 12C fragments were identified using the ΔE − E technique. The measured angular correlations indicate that the 12C gs+ 12C gs decay channel is dominated by decay of L = 2 strength, which yields an integrated fraction of 0.14% of the E2 EWSR strength in comparison with 22.2% of the E2 EWSR strength observed in singles inelastic α-scattering experiments in the same excitation energy region.

Excitation of isovector giant resonances in 27Al by heavy ion charge exchange reaction.

Excitation of isovector giant resonances in $^{27}\mathrm{Al}$ was studied by using the $^{27}\mathrm{Al}$${(}^{7}$Li, $^{7}\mathrm{Be}$${)}^{27}$Mg reaction at incident energies of 100, 150, and 180 MeV. We observed the analog of the giant dipole resonance at ${E}_{x}$=14.3 MeV in $^{27}\mathrm{Mg}$. From the observed excitation function, we suggest that a peak at ${E}_{x}$=9.5 MeV is a resonance with a higher multipolarity than one. In the excitation energy region higher than the giant dipole resonance, no resonance was observed.

Isoscalar monopole and dipole strength between 10 and 20 MeV in 24Mg from inelastic alpha scattering at and around 0 degrees.

Differential cross sections for inelastic \ensuremath{\alpha} scattering from $^{24}\mathrm{Mg}$ in the region of 10--20 MeV excitation have been measured at ${E}_{\ensuremath{\alpha}}$=120 MeV in the angular range of 0--8\ifmmode^\circ\else\textdegree\fi{}. The strength distribution for isoscalar monopole and dipole strength in $^{24}\mathrm{Mg}$ between 11--20 MeV was determined. 90\ifmmode\pm\else\textpm\fi{}20% of the E0 energy-weighted sum rule and 56\ifmmode\pm\else\textpm\fi{}12% of the E1, \ensuremath{\Delta}T=0 energy-weighted sum rule are found to be exhausted in this excitation energy region.

Multiple photoionization from 3p excitation of Kr and 4p excitation of Xe

The photoionization cross sections for multiply charged ions produced by 3p excitation of Kr and 4p excitation of Xe have been obtained by means of a time-of-flight mass spectrometer and synchrotron radiation. It is found that the main formation of doubly to quadruply charged ions in both Kr and Xe is caused from the each initialp-hole state through a Coster-Kronig transition followed by Auger or double Auger processes. The formation of singly charged ions in these excitation energy regions is caused by direct photoionization from outermost shell electrons in both Kr and Xe. Triply charged ions are prominently produced among the multiply charged ions. The quadruple photoionization cross sections show clearly the structures due to the Rydberg series, 3p−1nl of Kr and 4p−1nl of Xe. Their main structures were assigned to the 3p−1nd series in Kr and the 4p−1nd series in Xe.

Ground state photoneutron reactions in 17O.

Photoneutron angular distributions and differential cross sections have been measured for the reaction $^{17}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{16}$O over the excitation energy region from 10 to 24 MeV. The angle-integrated ground state cross section was obtained and compared to the total photoneutron cross section. The comparison indicates that the ground state channel dominates the pygmy resonance region below ${E}_{\ensuremath{\gamma}}$=16 MeV but contributes very little (&lt;10%) to the giant resonance region (near 22 MeV). Legendre coefficients extracted from the angular distribution data suggest that electric dipole transitions make up nearly all of the absorption strength in the region studied except in narrow regions near 10.8, 15.1, 17.3, and 22.3 MeV, where small but significantly nonzero values of the ${a}_{1}$ coefficient are observed. Values of the ${a}_{2}$ coefficient, interpreted in an E1 approximation, suggest a relatively pure single-particle composition of many of the states observed in the pygmy resonance region from 10 to 16 MeV.

Determination of the spreading width for the collective transition strength at finite temperature.

The experimentally observed structure of the $E2$ transitions in the quasicontinuum for $^{168}\mathrm{Yb}$ is analyzed. The valley ridge structure along the ${E}_{\ensuremath{\gamma}1}={E}_{\ensuremath{\gamma}2}$ diagonal in $\ensuremath{\gamma}$-energy correlation spectra has been reproduced by a model simulation in which a spreading of the rotational decay strength ${\ensuremath{\Gamma}}_{\mathrm{rot}}$ is introduced in regions of nuclear excitation energy $Ug{U}_{0}$ where the density of bands with slightly differing moments of inertia and alignment is high. The values of ${\ensuremath{\Gamma}}_{\mathrm{rot}}$ and ${U}_{0}$ vary from 75 to 110 keV and from 1.25 to 1.0 MeV, respectively, in the energy range $780l{E}_{\ensuremath{\gamma}}l1160$ keV.

Proton threshold states in 26Al

The energy levels of 26Al between E x = 6.3 and 6.5 MeV, corresponding to proton threshold energies in the 25Mg + p reaction from E p = 0 to 200 keV, have been investigated using the reactions 27Al(su3 He, α) 26Al and 24Mg( 3He, p) 26Al. Despite early work reporting a doublet at E x = 6346 keV and E x = 6362 keV, most subsequent work reported a single state with conflicting spin and parity assignments. Our work clearly establishes the presence of the doublet and resolves the conflicts. We find that there are six states in this excitation energy region, including a new state at E x = 6410 ± 5 keV. The l-values leading to possible spin and parity assignments for all these states have been made using DWBA. We conclude however that only three of these six states may contribute to the production of 26Al by the 25Mg(p, γ) reaction in the MgAl cycle and the new structure of 26Al reported here can substantially increase the production rate of 26Al in stellar reactions.

Ground-state photoneutron reactions in 14C.

Photoneutron angular distributions were measured by time-of-flight techniques for the reaction /sup 15/N(..gamma..,n/sub 0/) /sup 14/N over the region of excitation energy from 15 to 25 MeV. Ground state cross sections were obtained by stepping the bremsstrahlung end point over the energy region of interest in 2 MeV intervals. By fitting the spectral data to a series of Legendre polynomials, angular distribution coefficients were extracted and interpreted on the basis of a simple single particle model. It appears that a large fraction of the photoabsorption strength leading to decays via the ground state channel is due to the formation of J/sup ..pi../ = (3/2)/sup +/, T = (1/2) states in /sup 15/N which decay by d-wave neutron emission. The data support an approximation of purely electric dipole absorption in the region measured. Some small amount of s-wave neutron emission interfering with the dominant p/sub 1/2/..-->..d/sub 3/2/ transition is consistent with an observed value for the a/sub 2//a/sub 0/ coefficient of -0.7 +- 0.2. The (..gamma..,n/sub 0/) cross section integrated between threshold and 30 MeV is estimated to represent about one-third of the total strength in the neutron channel. A state identified at 17.3 MeV is consistent in energy and compositionmore » with a theoretical prediction based on a shell model calculation using a residual interaction with a Soper mixture of exchange forces.« less

15N( [formula omitted], po) 15N and the levels of 16O for Ex = 14.8–18.6 MeV

Angular distributions of cross section and analyzing power for elastic scattering of protons from 15N have been measured for E p = 2.7–7.0 MeV. A phase-shift analysis of the data yields spin-parity assignments and level parameters for seventeen states of 16O in the excitation energy region 14.8–18.6MeV. Three of the resonances have not previously been identified, among them being a broad J π = 2 − level at E p = 6.1 MeV which is almost certainly the analog of the 2 − 1p1h state with configuration ( d 3 2 , p 1 2 −1 ) at E x ∽ 5.0 MeV in 16N. The broad level previously reported near E p = 5.0 MeV has been observed and its parameters determined. A resonance analysis of the phase shifts yielded values of E r , Γ and Γ p for all of the levels. The J π assignments are in agreement with previously reported values. For resonances having J = l, the data can usually be fit with a resonant phase shift corresponding to either J = l + 1 or J = l − 1, in addition to the phase shift for J = l. Which of the two spurious- J solutions occurs seems to depend on whether the partial wave through which the resonant state is formed is J = l + 1 2 or J = l − 1 2 .

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X1Σ+ → (1π−14σ)A1Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π−1 npπ)-valence (3σ−14σ) mixing in X1Σ+ → 1Σ+ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X1Σ+ → 1Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X1Σ+ → 1Σ+ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X1Σ → (1π−14σ)A1Π, → (1π−13pπ)B1Σ+, → (1π−13sσ)C1Π, and → (3σ−14σ)D1Σ+ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π−1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F2, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.

Microscopic background calculations for the 90Zr(p, n)-reaction at E = 200 MeV

Using a microscopic one-particle—one hole doorway model for nuclear excited states we calculate the continuous forward angle energy spectra of the 90Zr(p, n)-reaction at 200 MeV incident energy. We find that a large fraction of the “missing” Gamow-Teller strength is located in the high excitation energy region 18 ≤ E x ≤ 50 MeV. Then approximately 80% of the total GT strength is seen in the (p, n) experiments. Similarly the strength of the ΔL = 1, J π = 0 −, 1 −, 2 − resonance is spread out over a large energy range. There are indications that the transition strength of the spin-flip giant dipole ( ΔL = 1, ΔS = 1, ΔT = 1) resonance is quenched by roughly 50%

Fine Structure and Spin Excitations in the Giant Resonance Region ofZr90

Cross sections, analyzing powers, and spin-flip probabilities have been measured at small angles in the reaction $^{90}\mathrm{Zr}(p,{p}^{\ensuremath{'}})^{90}\mathrm{Zr}^{*}$ at 319 MeV. A rich fine structure is observed in the previously proposed $M1$ giant resonance region; angular distributions of most of these structures are consistent with $M1$ excitation. The spin-flip measurements reveal a large cross section for spin excitations distributed roughly uniformly over the excitation energy region from about 8 to 25 MeV.

High-resolution (e, e') study of isovector M1 and M2 transitions in the oxygen isotopes: (III). 18O

The excitation energy region in 18O from about E x = 11–27 MeV has been studied with low-momentum transfer, but high-resolution inelastic electron scattering. Two sharp lines are prominent in the spectra, corresponding to the excitation of T = 2 levels at 16.399 ± 0.005 MeV and 18.871 ± 0.005 MeV of J π = 2 − and 1 +, respectively. In contradiction to theoretical predictions no more strong M2 transitions could be found. Broad peaks were observed at 18.5, 19.7, 20.2, 22.5 and 23.8 MeV, the latter two are due to the giant dipole resonance as known from photonuclear reactions. The spectra show in addition considerable fine structure and the application of a cross correlation function technique for its analysis resulted in the location of twelve more low multipolarity weak transitions in the excitation energy range between 16 and 19 MeV. Tentative J π assignments are given for these levels. The spectra of isospin T = 2 states of A = 18 nuclei are discussed in view of the existing experimental and theoretical work. Finally, the pattern of the isovector M1 and M2 strength distributions of all the three oxygen isotopes 16,17,18O is discussed.

High-resolution (e, e') study of isovector M1 and M2 transitions in the oxygen isotopes: (II). 17O

In a high-resolution electron scattering experiment, six isospin T = 3 2 levels in the excitation energy region of 11–15.3 MeV were populated in 17O. Five transitions at E x = 11.08, 12.47, 12.99, 14.23 and 14.75 MeV are predominantly of M2 nature with transition strengths B(M2, k)↑ = 6.1 ± 1.9, 6 ± 3, 6 ± 3, 46 ± 7 and 27 ± 9 μ N 2 · fm 2, respectively. The transition to the 15.10 MeV level is mainly M1 with B(M1, k)↑ = 0.14 ± 0.04 μ N 2. For this level, a tentative spin-parity assignment of J π = 3 2 + is possible. Also, a spin assignment of J π = 9 2 − is preferred for the 14.75 MeV level. The comparison of the transition strength to the level at 11.08 MeV with the prediction from the analogous unique first-forbidden β − decay of the 17N ground state indicates that it may be important to include p 3 2 -hole configurations in the ground-state wave function of 17O. A comparison made with the corresponding E1 transitions in 17F, by estimating the possible longitudinal component for the 14.23 MeV transition and assuming that the 12.99 MeV transition is pure E1, shows that mirror asymmetries in the mass 17 pair may be of non-zero magnitudes.

( 6Li, d) reactions on 24Mg and 26Mg at 73 MeV

The 24, 26Mg( 6Li, d) 28, 30Si reactions have been studied at 73 MeV bombarding energy. The angular distributions were analyzed with exact finite-range distorted wave Born approximation calculations assuming a direct α-cluster transfer. Extracted spectroscopic strengths leading to low-lying levels of 28Si and relative spectroscopic strengths between transitions to 28Si and 30Si ground states are consistent with those previously obtained by several α-transfer reactions. Many strongly populated levels have been observed at E x ≳ 10 MeV for 28Si. A marked similarity was found between the deuteron spectrum and the 24Mg(α, α) 24Mg excitation function in this excitation energy region. A brief comparison of the present α-transfer results with previous two-nucleon transfer data leading to 28,30Si is also presented.

στ+strength in nuclei

The $\ensuremath{\sigma}{\ensuremath{\tau}}_{+}$ strength function is studied with the quasiparticle random phase approximation. The residual interaction modifies the pairing theory strength function in two ways. The particle-hole interaction reduces the overall strength by about a factor of 2, without shifting strength between different levels. The particle-particle interaction does not affect the overall strength, but shifts part of the strength from the lowest excitation to a higher excitation energy region. By comparing the theory with the observed $log(\mathrm{ft})$ values of the ${\ensuremath{\beta}}^{+}$ decay in medium heavy nuclei, we find that an additional quenching is required for the $\ensuremath{\sigma}{\ensuremath{\tau}}_{+}$ mode, similar in magnitude to the additional quenching present in other isovector spin-flip transitions. Finally, we predict that there is a large concentration of the $\ensuremath{\sigma}{\ensuremath{\tau}}_{+}$ strength at higher excitation energy which cannot be observed by the ${\ensuremath{\beta}}^{+}$ decay.NUCLEAR STRUCTURE $\ensuremath{\sigma}{\ensuremath{\tau}}_{+}$ states in odd-odd mass nuclei between $A=100\ensuremath{-}150$. QP-RPA calculation with zero-range interaction. Calculated $log(\mathrm{ft})$.

Charged‐particle stopping in crystals

AbstractThe energy loss by a charged particle moving along a given path in the crystal lattice is calculated in terms of quantum perturbation theory. In calculating the contribution of excitations with lowenergy transfer, the sum rule is used. The highly‐excited states are described in a quasi‐classical approximation. The results obtained with these approximations coincide in the intermediate region of excitation energy. The results obtained allow the electrons of the core and the valence electrons to be treated from a unified standpoint. The averaging of the obtained expression for ΔE(b) over the random impact‐parameteri distribution leads to the Lindhard‐Scharff formula. The calculated dependence of the mean relatve proton‐energy loss upon the dechanneling depth is compared with available experimental data.

Giant resonance excitation in theAl27(Li6,He6)Si27reaction at 93 MeV

A giant resonance in /sup 27/Si is observed in the excitation energy region of 10--18 MeV in the spin-isospin flip /sup 27/Al(/sup 6/Li, /sup 6/He)/sup 27/Si reaction. A microscopic distorted-wave Born approximation analysis using a G-matrix nucleon-nucleon force and a local energy approximation knockout exchange correction indicates multipolarities between lambda = 3 and lambda = 5 for the giant resonance.

Ground-state photoneutron reactions inN15

Photoneutron angular distributions were measured by time-of-flight techniques for the reaction $^{15}\mathrm{N}(\ensuremath{\gamma}, {n}_{0})^{14}\mathrm{N}$ over the region of excitation energy from 15 to 25 MeV. Ground state cross sections were obtained by stepping the bremsstrahlung end point over the energy region of interest in 2 MeV intervals. By fitting the spectral data to a series of Legendre polynomials, angular distribution coefficients were extracted and interpreted on the basis of a simple single particle model. It appears that a large fraction of the photoabsorption strength leading to decays via the ground state channel is due to the formation of ${J}^{\ensuremath{\pi}}={\frac{3}{2}}^{+}$, $T=\frac{1}{2}$ states in $^{15}\mathrm{N}$ which decay by $d$-wave neutron emission. The data support an approximation of purely electric dipole absorption in the region measured. Some small amount of $s$-wave neutron emission interfering with the dominant ${p}_{\frac{1}{2}}\ensuremath{\rightarrow}{d}_{\frac{3}{2}}$ transition is consistent with an observed value for the $\frac{{a}_{2}}{{a}_{0}}$ coefficient of -0.7\ifmmode\pm\else\textpm\fi{}0.2. The ($\ensuremath{\gamma}, {n}_{0}$) cross section integrated between threshold and 30 MeV is estimated to represent about one-third of the total strength in the neutron channel. A state identified at 17.3 MeV is consistent in energy and composition with a theoretical prediction based on a shell model calculation using a residual interaction with a Soper mixture of exchange forces.NUCLEAR REACTIONS $^{15}\mathrm{N}(\ensuremath{\gamma}, {n}_{0})$, ${E}_{\ensuremath{\gamma}}=15\ensuremath{-}25$ MeV; measured differential cross sections as function of angle; extracted Legendre coefficients; estimated $\frac{s}{d}$ matrix-element ratio in dipole approximation.