Photon-scattering Experiments Research Articles

Photoexcitation of Ge76

The dipole strength of the nuclide $^{76}$Ge was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.8 and 12.3 MeV at the $\gamma$ELBE facility. We identified 210 levels up to an excitation energy of 9.4 MeV and assigned spin $J$ = 1 to most of them. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical $\gamma$-ray cascades. The photoabsorption cross section up to the neutron-separation energy was determined and is compared with predictions of the statistical reaction model. The derived photon strength function is compared with results of experiments using other reactions.

Electric and magnetic dipole strength in Zn66

The dipole strength of the $N=28$ closed-shell nuclide $^{54}\mathrm{Fe}$ was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of kinetic energies of 7.5 and 13.9 MeV at the $\ensuremath{\gamma}\mathrm{ELBE}$ facility as well as using quasimonoenergetic and linearly polarized photon beams of 26 different energies within the range from 5.5 to 11.4 MeV at the $\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$ facility. About 100 $J=1$ states were newly identified, out of them 19 with ${1}^{+}$ and 30 with ${1}^{\ensuremath{-}}$ assignments. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical $\ensuremath{\gamma}$-ray cascades. As a result, the photoabsorption cross section up to the neutron-separation energy was determined and compared with predictions of the statistical reaction model. The experimental $M1$ strengths from resolved ${1}^{+}$ states are compared with results of large-scale shell-model calculations.

Dipole response of Rb87 and its impact on the Rb86(n,γ)Rb87 cross section

Background: Detailed information on the low-lying dipole response in atomic nuclei along isotonic or isotopic chains is well suited to systematically investigate the structure and evolution of the pygmy dipole resonance (PDR). Moreover, the dipole strength below and around the neutron separation energy Sn has impact on statistical model calculations for nucleosynthesis processes. Purpose: The photon strength function (PSF) of Rb87, which is directly connected to the photoabsorption cross section, is a crucial input for statistical model calculations constraining the Maxwellian-averaged cross section (MACS) of the neutron capture of the unstable s-process branching-point nucleus Rb86. Within this work, the photoabsorption cross section is investigated. Methods: The photoabsorption cross section of the N=50 nucleus Rb87 was determined from photon-scattering experiments via the nuclear resonance fluorescence (NRF) technique. Bremsstrahlung beams at the γELBE facility in conjunction with monoenergetic photon beams at the HIγS facility were used to determine the integrated cross sections Is of isolated states as well as the averaged cross section as function of the excitation energy. Decays to the ground state were disentangled from decays to first low-lying excited states. Statistical and experimental approaches for the γ-decay properties at various excitation energies were applied. The linearly polarized photon beams at HIγS provide information on the ratio of electric and magnetic type of radiation. Results: Within this work, more than 200 ground-state decays and associated levels in Rb87 were identified. Moreover, transitions below the sensitivity limit of the state-by-state analysis were taken into account via a statistical approach from the bremsstrahlung data as well as model-independently from the HIγS data. The photoabsorption cross sections at various excitation energies were determined. The dipole response between 6 and 10 MeV of Rb87 is in agreement with assuming contributions of electric multipolarity, only. Conclusions: The photoabsorption cross section of Rb87 does not contradict with the trend of decreasing E1 strength with increasing proton number along the N=50 isotonic chain but might also be associated with a constant trend. The experimental γ decay at various excitation energies of the HIγS data supports the statistical approach but does not provide a stringent proof due to the limited sensitivity in the decay channels. The additional E1 strength observed in the present experiments significantly enhances the MACSs compared only to recent microscopic HFB+QRPA (Hartree-Fock-Bogoliubov plus quasiparticle random-phase approximation) calculations using the D1M interaction. Moreover, theoretical estimations provided by the KADoNiS project could be significantly improved.6 MoreReceived 18 June 2020Revised 24 August 2020Accepted 28 September 2020DOI:https://doi.org/10.1103/PhysRevC.102.044327©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsEnergy levels & level densitiesNuclear astrophysicsPhotonuclear reactionsRadiative captureProperties59 ≤ A ≤ 89Nuclear Physics

Electric and magnetic dipole strength in Fe54

The dipole strength of the $N=28$ closed-shell nuclide $^{54}\mathrm{Fe}$ was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of kinetic energies of 7.5 and 13.9 MeV at the $\ensuremath{\gamma}\mathrm{ELBE}$ facility as well as using quasimonoenergetic and linearly polarized photon beams of 26 different energies within the range from 5.5 to 11.4 MeV at the $\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$ facility. About 100 $J=1$ states were newly identified, out of them 19 with ${1}^{+}$ and 30 with ${1}^{\ensuremath{-}}$ assignments. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical $\ensuremath{\gamma}$-ray cascades. As a result, the photoabsorption cross section up to the neutron-separation energy was determined and compared with predictions of the statistical reaction model. The experimental $M1$ strengths from resolved ${1}^{+}$ states are compared with results of large-scale shell-model calculations.

Firm spin and parity assignments for high-lying, low-spin levels in stable Si isotopes

A natural silicon target was investigated in a ^{text {nat}}Si(mathbf {gamma },gamma ^{prime }) photon-scattering experiment with fully linearly-polarised, quasi-monochromatic mathbf {gamma } rays in the entrance channel. The mean photon energies used were langle {E}_{gamma }rangle = 9.33, 9.77, 10.17, 10.55, 10.93, and 11.37 MeV, and the relative energy spread (full width at half maximum) of the incident beam was varDelta E_{gamma }/langle {E}_{gamma }rangle approx 3.5–4 %. The observed angular distributions for the ground-state decay allow firm spin and parity assignments for several levels of the stable even-even silicon isotopes.

Probing the E2 properties of the scissors mode with real photons

The E2/M1 multipole mixing ratio δ1→2 of the 1+ sc → 2+ 1 γ-ray transition of 156Gd and 164Dy has been measured using the linearly polarized photon beams of the HIγS facility. The employed method of photonscattering experiments in combination with polarized, quasi-monochromatic beams and a dedicated detector setup is highly sensitive to the electric quadrupole-decay properties of the scissors mode.

Dipole strength in 80Se below the neutron-separation energy for the nuclear transmutation of 79Se

The γ-ray strength function (γSF) in 80 Se is an important parameter to estimate the neutron-capture cross section of 79 Se which is one of the long-lived fission products (LLFPs). Until now, the γSF method was applied for 80 Se only above the neutron-separation energy (S n ) and the evaluated 79 Se(n,γ) cross section has an instability caused by the GSF below S n . We studied the dipole-strength distribution of 80 Se in a photon-scattering experiment using bremsstrahlung produced by an electron beam of an energy of 11.5 MeV at the linear accelerator ELBE at HZDR. The present photoabsorption cross section of 80 Se was combined with results of (γ,n) experiments and are compared with predictions usinmg the TALYS code. We also estimated the 79 Se(n,γ) cross sections and compare them with TALYS predictionms and earlier work by other groups.

E1 and M1 strength functions at low energy

We report photon-scattering experiments using bremsstrahlung at the γELBE facility of Helmholtz-Zentrum Dresden-Rossendorf and using quasi-monoenergetic, polarized γ beams at the HIγS facility of the Triangle Universities Nuclear Laboratory in Durham. To deduce the photoabsorption cross sections at high excitation energy and high level density, unresolved strength in the quasicontinuum of nuclear states has been taken into account. In the analysis of the spectra measured by using bremsstrahlung at γELBE, we perform simulations of statistical γ-ray cascades using the code γDEX to estimate intensities of inelastic transitions to low-lying excited states. Simulated average branching ratios are compared with model-independent branching ratios obtained from spectra measured by using monoenergetic γ beams at HIγS. E1 strength in the energy region of the pygmy dipole resonance is discussed in nuclei around mass 90 and in xenon isotopes. M1 strength in the region of the spin-flip resonance is also considered for xenon isotopes. The dipole strength function of 74 Ge deduced from γELBE experiments is compared with the one obtained from experiments at the Oslo Cyclotron Laboratory. The low-energy upbend seen in the Oslo data is interpreted as M1 strength on the basis of shell-model calculations.

Dipole strength inSe80forsprocess and nuclear transmutation ofSe79

The dipole strength distribution of $^{80}\mathrm{Se}$ was studied in a photon-scattering experiment by using bremsstrahlung produced with an electron beam of energy 11.5 MeV at the linear accelerator ELBE. We identified $180\phantom{\rule{0.28em}{0ex}}\ensuremath{\gamma}$ transitions up to an energy of 9.6 MeV, and analyzed the strength in the quasicontinuum of the spectrum. Simulations of statistical $\ensuremath{\gamma}$-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section below the neutron-separation energy derived in this way was combined with the photoabsorption cross section obtained from an earlier $(\ensuremath{\gamma},n)$ experiment and used as an input for the calculation of $^{79}\mathrm{Se}(n,\ensuremath{\gamma})$ reaction rates on the basis of the statistical reaction model.

Dipole strength distribution ofGe74

The dipole strength distribution of $^{74}\mathrm{Ge}$ was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.0 and 12.1 MeV at the linear accelerator ELBE. We identified 94 levels with spin $J$ = 1 up to an excitation energy of 8.9 MeV and analyzed the strength in the quasicontinuum of states. Simulations of statistical $\ensuremath{\gamma}$-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section below the neutron-separation energy derived in this way is combined with the photoabsorption cross section obtained from an earlier $(\ensuremath{\gamma},n)$ experiment and compared with phenomenological approximations.

Pygmy dipole strength in86Kr and systematics ofN=50isotones

The dipole strength of the $N=50$ nucleus ${}^{86}$Kr was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.9 and 11.2 MeV delivered by the linear accelerator ELBE as well as using quasimonoenergetic and linearly polarized $\ensuremath{\gamma}$ rays of 10 energies within the range from 4.7 to 9.3 MeV delivered by the HI$\ensuremath{\gamma}$S facility. A high-pressure gas target was used. We identified 39 levels up to an excitation energy of 10.1 MeV. Simulations of $\ensuremath{\gamma}$-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section derived in this way up to the neutron-separation energy is combined with the photoabsorption cross section obtained from a $(\ensuremath{\gamma},n)$ experiment at HI$\ensuremath{\gamma}$S. The enhanced $E1$ strength found in the range from 6 to 10 MeV is compared with the ones in the $N$ $=$ 50 isotones ${}^{88}$Sr, ${}^{90}$Zr, and ${}^{92}$Mo and with predictions of calculations within the quasiparticle-phonon model.

Dipole strength distribution in56Fe

Electromagnetic dipole transitions in ${}^{56}$Fe were measured in photon-scattering experiments with a linearly polarized photon beam. The parity quantum numbers of the excited dipole states were determined by the intensity asymmetry of resonantly scattered $\ensuremath{\gamma}$ rays with respect to the polarization plane of the incident photon beam. While the summed magnetic dipole ($M1$) strength was determined as $\ensuremath{\Sigma}B(M1)\ensuremath{\uparrow}=3.52(17)$ ${\ensuremath{\mu}}_{N}^{2}$ at excitation energies between 7 and 10 MeV, the summed electric dipole ($E1$) strength below 10 MeV was obtained as $\ensuremath{\Sigma}B(E1)\ensuremath{\uparrow}=78.0(15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ${e}^{2}$ fm${}^{2}$. The observed $M1$ strength was compared with shell-model predictions in the $pf$ shell using the GXPF1J and KB3G effective interactions. In addition, the $E1$ strength was compared with random-phase approximation calculations with the Skyrme interaction.

Fine Structure of the GiantM1Resonance inZr90

The M1 excitations in the nuclide 90Zr have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 J(π)=1+ states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) μ(N)(2). The result for the total M1 strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

Photon scattering off 94Zr and 96Zr

Photon-scattering experiments off the nuclei 94Zr and 96Zr were performed at the 10-MeV bremsstrahlung facility DHIPS at the super-conducting electron linear accelerator S-DALINAC at Darmstadt. The dipole excitation-strength distributions were determined in an energy range between 2 and 8 MeV, including the 1− quadrupole-octupole coupled Two-Phonon-State and parts of the Pygmy Dipole Resonance. In total 74 transitions in 96 Zr were observed and for most of the excited states the spin quantum number could be firmly assigned to J = 1. Calculations within the Quasiparticle Phonon Model show a good agreement.

Fine structure of the giant M1 resonance in 90Zr

M1 excitations in the 90Zr nucleus have been studied in a photon-scattering experiment with monoenergetic and 100% linearly-polarized beams from 7 to 11 MeV. More than 40 Jπ = 1+ states were identified for the first time from the observed ground-state transitions revealing the fine structure of the giant M1 resonance with center of gravity of 9 MeV and sum strength of 3.8(3) μ2N. The results of the present work are compared with predictions from the shell model and the quasi-particle random-phase approximation.

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.

Isospin Character of the Pygmy Dipole Resonance inSn124

The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (α, α'γ) coincidence method at Eα=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (α, α'γ) as well as in (γ, γ') reactions and a group of states at higher energies which is only excited in (γ, γ') reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.

E1strength inPb208within the shell model

The dipole response of the doubly magic nuclide $^{208}\mathrm{Pb}$ was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 9.0 and 15.0 MeV. The present $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ data combined with $(\ensuremath{\gamma},n)$ data from the literature are compared with results of shell-model calculations and calculations using a quasiparticle random-phase approximation. The shell-model calculations including (2p-2h) excitations describe the experimental $E1$ strength well and reproduce the spreading of the giant dipole resonance by applying a small smearing width only.

Enhanced electric dipole strength below particle-threshold as a consequence of nuclear deformation

Photoabsorption cross sections ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ up to the neutron-separation energy ${S}_{n}$ were measured for the stable even-mass isotopes $^{92\ensuremath{-}100}\mathrm{Mo}$ in photon-scattering experiments. The photon-scattering data were analyzed in a novel way by taking into account the intensity of unresolved levels at high excitation energy and high level density. Simulations of $\ensuremath{\gamma}$-ray cascades were performed to estimate the intensity distribution of inelastic transitions to low-lying levels and, hence, to deduce intensities and branching ratios of the ground-state transitions needed for the determination of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$. The present $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{'})$ data can be combined for the first time with $(\ensuremath{\gamma},n)$ data, which allows us to obtain ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ in the energy range from about 4 MeV up to the giant dipole resonance for a series of isotopes. The ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ values below ${S}_{n}$ increase with the number of neutrons above the neutron shell closure at $N=50$. Calculations using a quasiparticle random-phase approximation in a deformed Woods-Saxon potential describe this effect as a consequence of the increasing nuclear deformation.

Multipole mixing ratios of transitions inB11

The mixing ratios of $M1$ and $E2$ radiation for transitions in $^{11}\mathrm{B}$ have been determined by measuring the azimuthal asymmetry of the radiation emitted from levels populated by resonant absorption of polarized photons. The photon-scattering experiments were carried out at the Free-Electron Laser Laboratory at Duke University using nearly monoenergetic and linearly polarized photon beams. The mixing ratios were deduced from a comparison of the measured azimuthal asymmetries with calculations for the angular distributions of mixed transitions.