Inelastic Electron Scattering, Fine Structure of M1 Giant Resonances and Gamow-Teller States

Richter, A.

doi:10.1088/0031-8949/1983/t5/010

Cited by 20 publications

(5 citation statements)

References 33 publications

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“…Information about energy-dependent dipole strength functions can be obtained from data on multi-step gammadecays following n-capture [17] or direct reactions [12], from inelastic electron scattering [18] or elastic photon scattering [14,15,19]. For the region above the n-threshold the electromagnetic strength in a very large number of stable nuclei has been experimentally determined by observing the neutrons emitted after the excitation by quasimonochromatic photons [5,20].…”

Section: Nuclear Resonance Fluorescencementioning

confidence: 99%

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Pygmy dipole strength close to particle-separation energies --The case of the Mo isotopes

et al. 2006

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Abstract. The distribution of electromagnetic dipole strength in 92, 98, 100 Mo has been investigated by photon scattering using bremsstrahlung from the new ELBE facility. The experimental data for wellseparated nuclear resonances indicate a transition from a regular to a chaotic behaviour above 4 MeV of excitation energy. As the strength distributions follow a Porter-Thomas distribution much of the dipole strength is found in weak and in unresolved resonances appearing as fluctuating cross section. An analysis of this quasi-continuum -here applied to nuclear resonance fluorescence in a novel way-delivers dipole strength functions, which are combining smoothly to those obtained from (γ, n) data. Enhancements at 6.5 MeV and at ∼ 9 MeV are linked to the pygmy dipole resonances postulated to occur in heavy nuclei.PACS. 21.10.Pc Single-particle levels and strength functions -24.10.Lx Monte Carlo simulations (including hadron and parton cascades and string breaking models) -25.20.Dc Photon absorption and scattering -26.50.+x Nuclear physics aspects of novae, supernovae, and other explosive environments Dipole strength in heavy nucleiThe response of nuclei to dipole radiation is of special importance for the synthesis of the chemical elements in the cosmos: particle thresholds may be crossed in hot or explosive scenarios leading to the production of new nuclides from previously formed heavier ones by dissociation in the thermal photon bath. This is likely to be the main path for the generation of the approximately 30-40 neutrondeficient nuclides which cannot be produced in neutron capture reactions [1]. For the understanding and modelling of this so-called p-process the dipole strength function up to and near the particle thresholds has to be known accurately [2]. As shown previously [3], details of the dipole strength (now in n-rich nuclei) may as well have large consequences for the r-process path and also s-process branchings are influenced by nuclear excitations [4] induced by thermal photons.The experimental knowledge [5] on dipole strength is reasonably well established for many heavy and medium mass nuclei in the region of the giant dipole resonance (GDR) by (γ, xn) studies, which often also cover the region directly above the neutron threshold S n . At lower energies three features have been discussed to be of importance for processes in high-temperature cosmic environments: a) the fall-off [6,7,8,9] of the E1-strength on the lowenergy slope of the GDR; a e-mail: e.grosse@fz-rossendorf.de b) the E1-strength between the ground-state (gs) and low energy excitations and its proper extension [10,11,12] into the regime a); c) the occurrence of additional pygmy-resonances, [3,13,14,15], which are assumed to be not as broad as the GDR, but wider as compared to the average level distance D -thus forming an intermediate structure enclosing many levels. Their low energy may well enhance their contribution to photo-dissociation processes in spite of their relatively low strength as compared to the GDR.In principle, also...

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Section: Nuclear Resonance Fluorescencementioning

confidence: 99%

“…This is why an average level density in its dependence on the excitation energy can only be determined from a fluctuation analysis on the basis of PorterThomas statistics [18]. Similarly, the dipole strength in a certain energy interval has to be obtained by integrating the complete nrf-spectra -i.e.…”

Section: Level Densities and Fluctuating Cross Sectionsmentioning

confidence: 99%

Pygmy dipole strength close to particle-separation energies --The case of the Mo isotopes

et al. 2006

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“…The resultant 2M1 strengths supply an independent measurement of the Ml quenching factors deduced from recent (e,e f ) and (p,p f ) experiments. 3 The 0+ first excited states 4 of 40 Ca (E 0 = 335 MeV, r 1/2 = 2.1 ns) and 90 Zr (#0=1.76 MeV, Ty 2 = 62 ns) were populated via known (p,p') resonances at ^ = 5.08 MeV and ^ = 7.08 MeV, respectively, with use of a pulsed beam (0.5 ns width) supplied by the Emperor accelerator of the Max-Planck-Institut. Inelastic protons were detected at backward angles in four surface-barrier detectors, each subtending a solid angle of 0.17 sr; they were mounted together with the metallic targets ( 40 Ca: 0.3 mg/cm 2 , 99.97%; 90 Zr: 0.7 mg/cm 2 , 97%) in the center of the "crystal ball" (CB) detector.…”

mentioning

confidence: 99%

Double Gamma Decay inCa40andZr
Schirmer
¹
,
Habs
²
,
Kroth
³

et al. 1984
Phys. Rev. Lett.
43
0
18
0
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The rare double gamma decay of the first excited 0 + state in 40 Ca and 90 Zr has been measured with a segmented 4TT Nal detector system, which allows suppression of the perturbing background due to positron annihilation in flight. In both cases the directional correlation of the two photons is found to be asymmetric around 90°, which is explained by an interference of 2EI and 2M1 transitions. The deduced Ml quenching factors agree with those from (e,e') and (p,p') measurements. PACS numbers: 23.20.En, 27.40, + z, 27.60. +j Many attempts have been made to observe the double gamma decay in nuclei such as 16 0, 40 Ca, and 90 Zr: As these nuclei have a ground and first excited state with spin and parity 0 + , the first excited state can only decay by internal conversion and pair production or by the simultaneous emission of two y rays, each with a continuous energy spectrum but summing up to the O^ -> 0+ transition energy. As a result of the small 2y branching ratio ( = 10~4), these early experiments suffered from insufficient statistics and background problems. In this Letter we present the first unambiguous observation and detailed investigation of the 2y decay mode in nuclei, which was performed by use of the Heidelberg-Darmstadt "crystal ball," 2 a 4TT y-ray detector system comprising 162 individual Nal(Tl) modules. The surprising result of our measurements on 40 Ca and 90 Zr is that the nuclear 2y decay proceeds not only via 2EI but also via equally strong 2M1 transitions. The resultant 2M1 strengths supply an independent measurement of the Ml quenching factors deduced from recent (e,e f ) and (p,p f ) experiments. 3 The 0+ first excited states 4 of 40 Ca (E 0 = 335 MeV, r 1/2 = 2.1 ns) and 90 Zr (#0=1.76 MeV, Ty 2 = 62 ns) were populated via known (p,p') resonances at ^ = 5.08 MeV and ^ = 7.08 MeV, respectively, with use of a pulsed beam (0.5 ns width) supplied by the Emperor accelerator of the Max-Planck-Institut. Inelastic protons were detected at backward angles in four surface-barrier detectors, each subtending a solid angle of 0.17 sr; they were mounted together with the metallic targets ( 40 Ca: 0.3 mg/cm 2 , 99.97%; 90 Zr: 0.7 mg/cm 2 , 97%) in the center of the "crystal ball" (CB) detector. Special care has been taken to suppress the perturbing y background stemming from the decay of the O2" state via internal pair conversion and the subsequent positron annihilation in flight (PAF). For these events the y-ray energies E yX ,E yl and their relative angle 0 12 are kinematically related bywhereas for the two-quantum decay all relative angles are allowed. Since the angle resolution of the CB is ±9°, the disturbing PAF events with E y i + E y2 = E e + + 2m e c 2 =£0 can De identified by means of Eq.(1) as long as they occur in the CB center. We therefore surrounded the target and the silicon detectors by a small Lucite box of 5-mm wall thickness to ensure that the positrons were either stopped within 5 cm from the CB center or at least sufficiently degraded in energy. In the data analysis we accepted onl...

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“…Information about energy dependent dipole strength functions can be obtained from data on multi-step gammadecays following n-capture [17] or direct reactions [12], from inelastic electron scattering [18] or elastic photon scattering [14,15,19]. For the region above the n-threshold the electromagnetic strength in a very large number of stable nuclei has been experimentally determined by observing the neutrons emitted after the excitation by quasimonochromatic photons [5,20].…”

Section: Nuclear Resonance Fluorescencementioning

confidence: 99%

“…The high level density in combination with Porter-Thomas fluctuations cause a large portion of the strength to appear in many weak transitions which are likely to be missed experimentally. This is why an average level density in its dependence on the excitation energy can only be determined from a fluctuation analysis on the basis of Porter-Thomas statistics [18]. Similarly, the dipole strength in a certain energy interval has to be obtained by integrating the complete nrf spectra -i.e.…”

Section: Level Densities and Fluctuating Cross Sectionsmentioning

confidence: 99%

Pygmy dipole strength close to particle-separation energies—The case of the Mo isotopes

Rusev¹,

Grosse²,

Erhard³

et al.

The 2nd International Conference on Nuclear Physics in Astrophysics

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Abstract. The distribution of electromagnetic dipole strength in 92, 98, 100 Mo has been investigated by photon scattering using bremsstrahlung from the new ELBE facility. The experimental data for well separated nuclear resonances indicate a transition from a regular to a chaotic behaviour above 4 MeV of excitation energy. As the strength distributions follow a Porter-Thomas distribution much of the dipole strength is found in weak and in unresolved resonances appearing as fluctuating cross section. An analysis of this quasi-continuum -here applied to nuclear resonance fluorescence in a novel way -delivers dipole strength functions, which are combining smoothly to those obtained from (γ, n)-data. Enhancements at 6.5 MeV and at ∼ 9 MeV are linked to the pygmy dipole resonances postulated to occur in heavy nuclei.PACS. 21.10. .+k Dipole strength in heavy nucleiThe response of nuclei to dipole radiation is of special importance for the synthesis of the chemical elements in the cosmos: Particle thresholds may be crossed in hot or explosive scenarios leading to the production of new nuclides from previously formed heavier ones by dissociation in the thermal photon bath. This is likely to be the main path for the generation of the approximately 30 -40 neutrondeficient nuclides which cannot be produced in neutron capture reactions [1]. For the understanding and modelling of this so-called p-process the dipole strength function up to and near the particle thresholds has to be known accurately [2]. As shown previously [3], details of the dipole strength (now in n-rich nuclei) may as well have large consequences for the r-process path and also s-process branchings are influenced by nuclear excitations [4] induced by thermal photons. The experimental knowledge [5] on dipole strength is reasonably well established for many heavy and medium mass nuclei in the region of the giant dipole resonance (GDR) by (γ, xn) studies, which often also cover the region directly above the neutron threshold S n . At lower energies three features have been discussed to be of importance for processes in high-temperature cosmic environments: a) the fall-off [6-9] of the E1-strength on the low-energy slope of the GDR; b) the E1-strength between the ground-state (gs) and low energy excitations and its proper extension [10-12] into the regime (a); c) the occurrence of additional pygmy-resonances, [3,[13][14][15], which are assumed to be not as broad as the GDR, but wider as compared to the average level distance D -thus forming an intermediate structure enclosing many levels. Their low energy may well enhance their contribution to photo-dissociation processes in spite of their relatively low strength as compared to the GDR. In principle, also M1-transitions contribute to the dipole strength, but the average M1 strength is typically 1-2 orders of magnitude smaller as compared to E1, and they are frequently [1-3] not taken into account. A further approximation has to be introduced to estimate dipole strengths for transitions not connected to the g...

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Inelastic Electron Scattering, Fine Structure of M1 Giant Resonances and Gamow-Teller States

Cited by 20 publications

References 33 publications

Pygmy dipole strength close to particle-separation energies --The case of the Mo isotopes

Pygmy dipole strength close to particle-separation energies --The case of the Mo isotopes

Double Gamma Decay inCa40andZr
Schirmer
¹
,
Habs
²
,
Kroth
³

et al. 1984
Phys. Rev. Lett.
43
0
18
0
View full text Add to dashboard Cite

Pygmy dipole strength close to particle-separation energies—The case of the Mo isotopes

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Inelastic Electron Scattering, Fine Structure of M1 Giant Resonances and Gamow-Teller States

Cited by 20 publications

References 33 publications

Pygmy dipole strength close to particle-separation energies --The case of the Mo isotopes

Pygmy dipole strength close to particle-separation energies --The case of the Mo isotopes

Double Gamma Decay inCa40andZrSchirmer1, Habs2, Kroth3 et al. 1984Phys. Rev. Lett.430180View full textAdd to dashboardCite

Pygmy dipole strength close to particle-separation energies—The case of the Mo isotopes

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Resources

About

Double Gamma Decay inCa40andZr
Schirmer
¹
,
Habs
²
,
Kroth
³

et al. 1984
Phys. Rev. Lett.
43
0
18
0
View full text Add to dashboard Cite