E1 Strength Distributions Research Articles

Giant resonances in unstable oxygen isotopes

Electromagnetic and nuclear breakup of the neutron-rich Oxygen isotopes ranging from A = 17 to A = 22 is studied experimentally in reactions at energies around 600 MeV/u. The beams were produced in fragmentation reactions and separated by the GSI Fragment Separator FRS. By measuring the four-momenta of all decay products after inelastic scattering and neutron decay of the projectile, the excitation energy is determined. From the differential cross sections dσ dE ∗ for electromagnetic excitation, the E1-strength distributions can be deduced. For 18,20,22O, low-lying dipole strength is observed, exhausting about 5% of the Thomas Reiche Kuhn sumrule for energies up to 5 MeV above the continuum threshold.

Continuum excitations in6He

The three-body breakup 6He→4He+n+n is studied experimentally, using a secondary 6He ion beam of 240 MeV/nucleon incident on carbon and lead targets. Integrated cross sections for one- and two-neutron knockout and differential cross sections dσ/dE* and dσ/dϑ for inelastic nuclear or electromagnetic excitations into the 6He continuum are presented. The E1-strength distribution is deduced from electromagnetic cross sections and is found to exhaust (10±2)% of the energy-weighted Thomas-Reiche-Kuhn sum rule or (40±8)% of the cluster sum rule for excitation energies below 5 MeV. Both the energy-weighted and non-energy-weighted dipole cluster sum rules are almost exhausted integrating the strength up to 10 MeV, a fact from which the root-mean-square distance between the α core and the two valence neutrons of rα−2n=(3.36±0.39) fm is derived. The known Iπ=2+ (1.80 MeV) resonance in 6He is observed in nuclear inelastic scattering; model-dependent values of the quadrupole deformation parameter δ2=(1.7±0.3) fm or B(E2,0+→2+)=(3.2±0.6)e2 fm4 are derived. No clear signature could be obtained for predicted higher-lying 2+ resonances, but low-lying continuum strength of multipolarity other than dipole, likely of monopole and quadrupole multipolarity, is indicated by the data. Two-body correlations in the decaying 4He+n+n system are investigated. The astrophysical relevance of the data with regard to the two-neutron capture process 4He(2n,γ)6He is briefly discussed.Received 27 October 1998DOI:https://doi.org/10.1103/PhysRevC.59.1252©1999 American Physical Society

Dipole strength distribution in doubly even deformed nuclei

The M1 and E1 strength distributions in the energy range 3-12 MeV are calculated in random-phase approximation for the deformed nuclei 154Sm, 168Er, 178Hf and 238U. It is found that the orbital motion, although providing on the whole a modest contribution to the M1 strength, plays a significant role in shaping the M1 spectra because of the destructive interference between orbital and spin amplitudes. Strong E1 transitions also occur in the same energy range. Their total strength in the energy range 3.6-7.6 MeV is about four times larger than the M1 strength. Because of these highly intense E1 transitions, the total dipole strength distribution computed as a sum of the M1 and E1 strengths is considerably different from the spectra of the M1 transitions alone.

Dipole γ-ray transition rates in 238U

M1 and E1 transition rates from the ground to excited states and between excited states in 238U are calculated within the quasiparticle-phonon nuclear model with the wave functions consisting of one- and two-phonon terms. We show that there are relatively large M1 transitions between 2+ states in the low-energy region. The fragmentation of the one-phonon states strongly affects M1 and E1 strength distributions. The correlation takes place between E1 and E3 transition strengths. We show that there are fast E1 and M1 transitions between large components of the wave functions differing by an octupole or quadrupole phonon.

Invariant-mass spectroscopy of 10Li and 11Li

Break-up of secondary 11Li ion beams (280 MeV/nucleon) on C and Pb targets into 9Li and neutrons is studied experimentally. Cross sections and neutron multiplicity distributions are obtained, characterizing different reaction mechanisms. Invariant-mass spectroscopy for 11Li and 10Li is performed. The E1 strength distribution, deduced from electromagnetic excitation of 11Li up to an excitation energy of 4 MeV comprises ∼8% of the Thomas-Reiche-Kuhn energy-weighted sumrule strength. Two low-lying resonance-like structures are observed for 10Li at decay energies of 0.21(5) and 0.62(10) MeV, the former one carrying 26(10)% of the strength and likely to be associated with an s-wave neutron decay. A strong di-neutron correlation in 11Li can be discarded. Calculations in a quasi-particle RPA approach are compared with the experimental results for 10Li and 11Li.

Correlation between E1 and E3 transition strengths and influence of the fragmentation of one-phonon states on the E1 strength distribution in well-deformed nuclei

The E1 and E3 transition rates from ground to excited states with Kpi =0- and 1-, and between excited states are calculated within the quasiparticle-phonon nuclear model in several doubly even well-deformed nuclei. The correlation between E1 and E3 transition strengths is observed. The calculated summed B(E1) strength (Ex<or=4 MeV) for the Kpi =0- levels is three times as large as for the Kpi =1- levels. We show that the fragmentation of the one-phonon states above 2.5 MeV strongly affects the E1 strength distribution.

Giant resonances in a neutron rich nucleus

To understand the response of very neutron rich nuclei to external fields, we study the E1 and E2 giant resonances in 22O using a microscopic model i.e. the time-dependent density-matrix theory (TDDM). TDDM is an extension of the time-dependent Hartree-Fock theory to include two-body correlations and gives us a framework to study both the positions and widths of the giant resonances. It is found that the E1 and E2 strength distributions quite differ from those in stable nuclei due to the existence of excess neutrons.

Radiative strength in the compound nucleus 157Gd.

The distribution of E1 and M1 strength in $^{157}\mathrm{Gd}$ compound nucleus has been studied. An investigation of capture of 2 and 24 keV neutrons in a $^{156}\mathrm{Gd}$ target resulted in resonance-averaged intensities of primary gamma rays between 3.7 and 6.4 MeV. From these intensities the gamma-ray strength functions have been derived for E1 and M1 radiation. We compare several formulations of strength functions to these resonance capture and/or photoabsorption data. Further we use these prescriptions in calculations of the total average radiation width, radiative capture cross sections, and gamma-ray spectra, and compare them to available experimental information. By analyzing these results strong evidence was found for an E1 strength function which is based on a generalized Lorentzian, enhanced compared to spherical nuclei, with an energy-dependent spreading width and a nonzero limit as the energy tends to zero. For M1 radiation the giant resonance spin-flip mode is favored.

Complete spectroscopy of51,52V via the50,51V(n, ?) reactions

The gamma-ray spectra emitted after thermal neutron capture in highly enriched50V andnatV targets have been studied using in-pile targets at the ILL high flux reactor and pair and germanium detectors. The neutron binding energies in51V and52V were determined to beBn(51V)=11051.11(17) keV andBn(52V)=7311.22(26) keV. The thermal neutron capture cross-section in50V was measured to be 21−2+4 b. From 724 lines attributed to51V, 330 transitions, comprising 90% of theγ-ray flux, were placed into the level scheme. Fifty-nine primary dipole transitions from the 11/2+ or 13/2+ capture states in51V were established from which the E1 strength distribution was deduced. The energy scaling of these primary transitions was found to follow the E1-giant dipole resonance dominance. Many new levels were established; a number of states proposed in previous (d,p) and (n,γ) work were confirmed from their primary population and decaying secondary radiations. The density of levels in the high spin (9/2≦I≦15/2) region was parametrized with the Fermi gas model. The spin distributions of51V were analyzed and a spin cut-off parameter σ=2.8 (3) was deduced. — A nearly complete level scheme of52V up to 3.5 MeV excitation and similar results concerning the level density and the primary γ-ray spectrum were obtained in the51V (n, γ)52V study.

(e,e'f) Coincidence experiments on 235U and 238U

Coincidence experiments for (e, e'f) on the actinide nuclei 235U and 238U have been performed at the Mainz Microtron (MAMI A) concentrating on three subjects: multipole strength distributions and form factors for the lowest multipolarities, the mass split in the fission decay of various giant multipole resonances, and the separation of near barrier fission channels. Data were taken at four values of momentum transfer ( q eff ≈ 0.20, 0.28, 0.53, and 0.71 fm −1 for 238U, q eff ≈ 0.20, 0.44, 0.57, and 0.71 fm −1 for 235U) for excitation energies ω = 4–22 MeV. The fission fragments have been detected using the Giessen PPAC-Ball. A model-independent multipole analysis yields both form factors and strength distributions for E1, E2/E0, and E3 excitations. The extracted E1 strength distributions in the fission channel are in good agreement with recent photofission data. The extracted E2/E0 strength distributions in the fission channel show two distinct resonant structures centered at ω ≈= 10 and 14 MeV, attributed to concentrated collective E2 and E0 strength, respectively. Up to 12 MeV for 238U (19±2)% and for 235U (39±2)% of the isoscalar E2 energy-weighted sum rule is exhausted. The extracted form factors can be described within a hydrodynamical model by use of parameters c tr c 0 = 1.2 and 1.0 for E1 and E2, respectively. The mass split in the fission decay of giant multipole resonances has been studied via the measurement of time-of-flight difference spectra of both fission fragments. A model-independent multipole decomposition yields the fission width ratios Γ S/Γ A as functions of excitation energy for separated multipoles (E1, E2/E0, and E3). An increased symmetric contribution is evident for the isoscalar giant quadrupole resonance as compared to giant E1 and E3 resonances. Complete in- and out-of-plane fission fragment angular correlations for 238U have been measured for the two lowest values of momentum transfer. A combined analysis with the extracted E1 and E2/E0 multipole strength distributions yields strength distributions for 6 near-barrier fission channels ( J π , K)=(0 +,0), (1 −,0), (1 −, 1), (2 +,0), (2 +, 1), and (2 +, 2) up to 8 MeV. An estimation of the respective fission barriers deduced from the increase of the fission cross sections is given.

Electric spin resonances in light nuclei

Very recent pion inelastic scattering experiments at LAMPF have revealed the existence of strong spin-flip E1 resonances in the vicinity of the GDR in several light nuclei. We present here the results of shell-model calculations of S = 0 and S = 1 E1 strength distributions which offer a broad theoretical context for the discussion of electric spin excitations. Our results for 16O and 40Ca corroborate the LAMPF data and indicate that a major fraction of the spin-flip strength still lies above the GDR.

The distribution of E1 strength below particle threshold studied by resonant scattering of gamma rays

A procedure is presented to determine total photoabsorption cross sections σ t by resonant scattering of γ-rays. It is shown that σ t follows along the GDR lorentzian line down to 3.5 MeV. Indications for nonstatistical deviations from the lorentzian line are observed.

Electroexcitation of 208Pb, distribution of electric dipole and quadrupole strength and fragmentation of the isoscalar quadrupole giant resonance

Low momentum transfer, high-resolution inelastic electron scattering on 208Pb has been used to study the distribution of E1 and E2 strength in the region of excitation energy E x = 8−12MeV. The E1 and E2 strength is very fragmented and the EWSR strength amounts to (10 +7 −6)% and (29 +11 −8)% in the investigated energy region, respectively. The E2 strength found is less than most current theoretical predictions but agrees qualitatively with a 1p - 1h +2p - 2h model calculation presented in this paper. The E2 strength is also smaller than what is known from hadron scattering and the shape of the strength distribution is also markedly different in electron and hadron scattering.

Isospin splitting of giant E1 excitations in the N = 82 nucleus 142Nd

The distribution of E1 strength in N = 82 nuclei has been investigated by proton-capture reactions. For 142 Nd, the reaction 141 Pr(p, γ 0 ) 142 Nd reveals the “usual” 1 − , T < giant resonance at E x ≈ 15 MeV with a width of several MeV. Superposed on this resonance are several narrow 1 − , T > analogue resonances, which interfere in a characteristic manner with the broad T < resonance. As observed for the N = 50 nuclei a major portion of the E1, T > strength is pushed up into the high-lying resonances. For 140 Ce, preliminary observations on 139 La(p, γ 0 ) 140 Ce have confirmed the relative weakness of the reaction.