Giant Resonance Energy Region Research Articles

Isospin asymmetry in the continuum of the A = 14 mirror nuclei

We study the isospin asymmetry in the isoscalar (IS) excitations in the mirror nuclei 14O and 14C by using the Hartree–Fock(HF)+random phase approximation (RPA) linear response function theory with a Skyrme interaction to take into account the continuum effect properly. The asymmetry in the IS monopole, dipole responses is pointed out in the continuum near the particle threshold with respect to the excitation energy and the sum rule strength. On the other hand, no clear sign of the asymmetry is found in the giant resonance (GR) region. In the quadrupole case, the calculated strengths of the mirror nuclei show almost the same energy dependence from the threshold to the GR region. It is found that the transition densities of the monopole response show an extended halo structure near the threshold, while those of the GR region show a typical radial dependence of the compressional collective mode without any halo effect. Contrary to the transition densities of the monopole response, those of quadrupole response do not show any sign of the extended feature of wavefunctions neither near the threshold nor the GR energy region. Calculated strength distributions of the IS multipole states are compared with recent experimental data obtained by the multipole decomposition analysis of α inelastic scattering on 14O.

High-energy photon beam production with laser-Compton backscattering

We have produced a beam of high-energy gamma-rays by Compton backscattering of 1064 nm laser photons from 1 GeV electrons circulating in a storage ring. Measuring the energy spectra of the backscattering photons with an HPGe detector, we have found that the maximum energy of 17.6 MeV and the measured energy spectra show agreement with the simulation calculations, while the detected photon yields were measured at about 4×10 3, 2×10 3 and 3×10 2 photons s −1 mA −1 W −1 for the 20, 10 and 2 mmφ collimators, respectively. The photon energy widths from the collimators correspond to 6.6– 17.6 MeV , 12.4– 17.6 MeV and 17.3– 17.6 MeV , respectively. By using these photons, we measured the total nuclear photoabsorption cross-sections in the E1 giant resonance energy region for 197 Au using the attenuation method and we have demonstrated that the photon attenuation method will be a useful tool for studying photonuclear reactions.

Meson-exchange currents in electromagnetic transitions of the 4He system

In the framework of the refined resonating-group model (RRGM) observables of the photo- and electrodisintegration of 4He into p- and n-channels and those of the corresponding radiative-capture reactions in the giant-resonance energy region are calculated explicitly taking into account meson-exchange currents. The most recent experimental photo- and electrodisintegration cross sections as well as the radiative-capture analyzing powers are well described. Meson-exchange currents are found to make large contributions to the dominant E1 and the M1 transition. Large discrepancies to older calculations are explained.

89Y

Angular distribution of the /sup 89/Y(e,p/sub 0/) reaction has been measured in the giant resonance energy region at 12 laboratory angles ranging from 30/sup 0/ to 140/sup 0/. The obtained differential cross sections have been decomposed into E1 and E2 components using a resonance model. The E1 and E2 components of the /sup 89/Y(..gamma..,p/sub 0/) cross section were estimated to exhaust 2.9% and 1.7% of the E1 and E2 sums, respectively. The derived excitation functions for E1 and E2 are well explained by a direct-semidirect model. The result confirms the isovector giant quadrupole excitation in /sup 89/Y.

Surprisal Analysis of the63,65Cu (e,p0) Reactions

Surprisal analysis is applied to photoreaction. The statistical model of Hauser-Feshbach is extended to analyze the 63,65 Cu(γ, p 0 ) reactions. A unified understanding of the measured cross sections in the giant resonance energy region, which has yet to be achieved, is attempted.

The 63,65Cu(e, p 0) reactions in the giant resonance region

Angular distributions of the 63,65Cu(e, p 0) reactions have been measured in the giant resonance energy region. The resulting differential cross sections have been decomposed into E1 and E2 components by a resonance model. The excitation functions for the two isotopes are significantly different, clearly reflecting threshold effects. The transition strengths of the low- and high-energy parts of the cross sections are reproduced by the statistical model and the direct-semidirect model, respectively, and the overall fit to the data is obtained by the sum of these two results.

Electron-Nucleus Non-Integrated Sum Rules and the Projection-Operator Method

Non-integrated sum rules in terms of time-dependent correlation functions are discussed. A statistical treatment of the time-dependent correlation functions for nuclei is suggested and a projection method clue to Mori is applied to the solution of an equation-of-motion for the transition operators. The complete, nuclear response function can be obtained by assum­ ing a non-linerrr random force. The differential cross sections at fixed momentum transfer can be written as the sum of a linear and a random part. § I. Introduction Experimental research on electrodisintegration of nuclei covers both the giant resonance energy region and, at much larger energies, the quasi-elastic scattering region. In the first region the electro-excitation is treated as excitation of collective quasidiscrete states of various multipolarity. Recently the existence of \'arious multipole giant resonances other than El, postulated for a long time, has been clemon:;trated experimentally from electron and hadron scattering, at excitation en­ ergies ol 10 MeV and above.n.~) All these experiments indicate the existence at these high excitation energies of collective states, which are modes in which an appreciable fraction of the nucleons move together. In the quasi-elastic peak region a single particle approach is pursued. The electron scattering takes place as it would on a free individual nucleon. If the nucleons were free, this peak would be sharp and would occur at an energy loss of q 2/2lv1 for momentum transfer q and nucleon mass 111. The nuclear binding shifts the position of the peak. The bind­ ing also broadens the peak, due to the momentum distribution of the bound nucleons. Berthot and Isabelle 3) have found the existence of a structure in the e- 12C quasi­ elastic peak, at constant momentum transfer, which indicates that the single nucleon scattering picture of the quasi-elastic scattering is not adequate. On the other hand the dependence of the structure on the momentum transfer q does not seem to be of the collective type. Different models are used to describe the two different regions. However electroexcitation of resonances and quasi-elastic scattering are basically the same process and in principle it is impossible to separate them. At the moment a comprehensive theory of electrodisintegration of the nucleus describing

A method of determining photonucleon cross sections for separate residual nuclei states utilizing continuum gamma-ray sources

An experimental set-up for simultaneous measurement of photoprotons and deexcitation gamma rays in the giant resonance energy region is used for evaluation of photoparticle cross sections leading to different levels of residual nuclei. The system consists of a scattering chamber with four silicon surface-barrier detectors, allowing measurement of photoproton spectra at 8 angles between 30° and 150°, inclusive, and a Ge(Li) coaxial detector assembly for analysing gamma rays from excited residual nuclei states. A least-squares method is adopted for the calculation of (γ, p) cross sections. Two types of data are used in the calculation: photoproton spectra of different bremsstrahlung end- point energies, and the deexcitation gamma-ray spectra. The latter enable selection of those states in residual nuclei which are strongly populated in the reaction, and consequently reduce the number of free parameters in the least-squares analysis. Typical cross sections are presented to illustrate the effectiveness of the method.

O16(γ,n)O15Cross Section from Threshold to 65 MeV

The ${\mathrm{O}}^{16}(\ensuremath{\gamma},n){\mathrm{O}}^{15}$ cross section has been measured using least-structure analysis of photonuclear yield functions for energy to 65 MeV. In the giant-resonance energy region considerable structure is reported, in good agreement with structure reported using other methods. The resolution of the least-structure method is comparable to resolution obtained by other methods, and in certain energy regions is superior. The cross section extends to energy above the giant resonance, exhibiting prominent structure to the highest energy measured. Absolute cross sections are reported in good agreement with values obtained elsewhere from yield functions, but approximately 20% higher than measurements made using monochromatic photons.

Study of the inverse photodeuteron reaction in 11B and 16O in the giant resonance energy region

The radiative capture of deuterons by 9Be and 14N has been studied in the energy range 0.5 MeV < E d < 5.5 MeV using direct observation of the ground state gamma transition of about 20 MeV energy. After a short description of the experimental method the results of the measurements are given in the form of excitation curves and angular distributions. For the 9Be(d, γ 0) 11B reaction the excitation curve shows no structure with a total peak cross section of about 8 μb. The results may be interpreted by direct E 1 capture besides the asymmetry of the angular distributions. For the 14N(d, γ 0) 16 O reaction, where the isospin selection rule forbids the E 1 transition, the excitation curve shows a peak at 22.7 MeV excitation energy with a total peak cross section of about 6 μb. The angular distribution at this energy has a predominantly dipole characteristic but also displays asymmetry. The peak cannot be identified with any peak of the giant E 1 resonance of 16O. There are also indications for peaks at 22.2 and 24.5 MeV excitation energy. The possible interpretations for the 14N(d, γ 0) 16O cross section are given.

Direct Observation of the Optical Anisotropy of the Holmium Nucleus

The optical anisotropy of the holmium nucleus in the giant-resonance energy region has been shown to exist by measuring the yield of photoneutrons as a function of the orientation of the nucleus with respect to a bremsstrahlung beam direction. The nuclei were aligned by the Bleaney method using a continuously operating ${\mathrm{He}}^{3}$ refrigerator to cool a single crystal of holmium ethyl sulfate to 0.29\ifmmode^\circ\else\textdegree\fi{}K. The orientation effects observed could be explained if the absorption cross section for holmium was made up primarily of a component given by the dynamic collective theory, but also included a small scalar component (about 15% of the total) which had no orientation effects associated with it. In fitting the data, a reanalysis of previous data on the holmium absorption cross section was made.

Photoneutron Reaction of S32

The photoneutron cross section of S/sup 32/ was observed up to 22 Mev with good energy resolution, together with the photoneutron spectrum using a stilbene scintillation spectrometer. Fine structures were observed in either of the two measurements. Taking into account the threshold energy and the excitation energies of the residual nucleus, these structures were shown to be consistently related with each other. Experimental results were discussed using the statistical model calculation, assuming that the emitted particles were only p-wave and f-wave particles. The integrated cross section ratio for S/sup 32/ ( gamma ,n) to P/sup 31/ ( gamma ,n) reaction and the same ratio for S/sup 32/ ( gamma ,p) to S/sup 32/ ( gamma ,n) reaction were well explained by the calculations. The difference between the energy spectrum of the photoprotons from P/sup 31/ and that from Si/sup 28/ also seemed to be well explained qualitatively. In the appendix, the contributions of the f-wave protons for the energy spectrum of photoprotons were shown. The photodisintegration of nuclei in these mass number region was concluded to occur mainly through the compound process in the giant resonance energy region. (auth)