(e,e′n) Coincidence Studies of the Giant Multipole Resonances ofPb

Abstract

The isoscalar monopole and quadrupole giant multipole resonances of 208 Pb have been studied in the excitation region of 9 to 16 MeV by coincident electron scattering. Concentrations of strength have been found at 10.4 and 14.2 MeV, in reasonable agreement with the predictions of mean-field theory. The measurements reported are the first nuclear-structure investigation carried out by use of the (e,e'n) reaction. Giant multipole resonances are the most important manifestations of collective behavior in atomic nuclei. Our knowledge concerning these fundamental modes of excitation is still rather primitive (as compared, for example, to our knowledge of the corresponding bound excitations), despite extensive theoretical and experimental research efforts over the last thirty years. 1,2 Perhaps the most significant reason for this unsatisfactory situation has been the lack of a probe of sufficient versatility and precision. Photonuclear reactions are well suited for the study of dipole excitations, but they can neither excite higher multipoles easily nor study the spatial characteristics of the excitation matrix elements. Hadronic probes are limited in accuracy because of their complicated reaction mechanisms and their lack of multipole selectivity. Inclusive electron scattering, the probe most often used for high-precision studies of nuclear structure, is of limited use for the study of continuum excitations because of the larger "background" due to the elasticscattering radiative tail.The recent development of cw electron accelerators has provided a powerful new probe for the study of giant resonances: coincident electron scattering. The coincidence requirement removes the contribution of the elastic radiative tail, allowing the study of the giant resonances with the same precision and versatility routinely available in (e,e') studies of discrete, low-energy excitations.Giant multipole resonances, being collective excitations, are best studied in heavy nuclei. The substantial Coulomb barrier that characterizes these nuclei inhibits charged-particle decay, making (e,e'n) the reaction channel of choice. However, the detection of low-energy (0.5-10 MeV) neutrons in the hostile environment of electron-scattering halls presents formidable technical problems. As a consequence, (e,e'n) experiments have not been feasible at low-duty-factor electron-accelerator facilities. We report in this Letter the development of an (e.e'n) facility and its use for the study of the giant multipole resonances in 208 Pb. 208 Pb, the heaviest doubly closed-shell nucleus, offers the best case available for testing our understanding of collective motion. Its excitation spectrum at low energies has been carefully investigated both theoretically and experimentally. Mean-field theory, in the form of density-dependent Hartree-Fock calculations, provides a satisfactory description of the ground-state properties of 208 Pb (and other doubly closed-shell nuclei). 3 ' 4 It has been suggested that the dynamic behavior of these nuclei might be adequat...

Isospin character of transitions to bound states inPb204,206,208Hören1, Auble2, Beene3 et al. 1991Phys. Rev. C22320View full textAdd to dashboardCite

“…The M"/M~d educed for Pb [16,17] and " Sn [18] are in excellent agreement with the values expected for a pure isoscalar transition to the GQR. Furthermore, the Mz value deduced from an (e, e'n ) measurement [20] on Pb also agrees well with the heavy-ion results.…”

Giant Resonances in the Doubly Magic NucleusC48afrom the (e,Strauch1, Neumann-Cosel2, Rangacharyulu3 et al. 2000Phys. Rev. Lett.465340View full textAdd to dashboardCite

“…The coincident detection of neutrons in the hostile environment of an electron accelerator represents a considerable experimental task, and only a few exploratory investigations have been performed [12][13][14][15][16][17]. The present results have been obtained with a new setup for ͑e, e 0 n͒ reactions developed at the S-DALINAC.…”

“…The latter value represents an unknown mixture of monopole and quadrupole parts. At the photon point the cross section corresponds to B͑E2͒ 1 25 16p B͑E0͒, which can be derived exactly in plane wave Born approximation but should also approximately hold in distorted wave Born approximation [12]. Thus, assuming, e.g., equal exhaustion of the ISGMR and ISGQR, the number given above would translate to 46͑6͒% of the respective EWSR's.…”

Isoscalar quadrupole giant resonance in208Pb: A statistical analysis of high-resolution inelastic electron scattering spectra

“…therein), both the question of the actual amount of depletion and of the spreading of E2 strength in the region of the GQR needs a precise answer. Furthermore, the situation with respect to the possible discrepancy of GQR strength in single arm (p, p'), (3He, 3He') and (e, c() reactions on one side and (e, e') reactions on the other side turned worse when in a 2~ e'n) coincidence experiment performed at Illinois [12][13][14] it was found that the integrated transition strength observed between Ex = 9-12.5 MeV exhausted 65% of the A T= 0, E2 EWSR, a value essentially in agreement with the inelastic hadron scattering, but in clear disagreement with the (e, e') results.…”