O16(γ,p)

Abstract

The O(y, po) reaction has been studied with linearly polarized bremsstrahlung photons in and below the giant E1 resonance. The parity of the absorbed radiation was determined from the observed azimuthal asymmetry of the emitted protons. Combined with unpolarized measurements the polarized results determine the proton decay amplitudes of the M1 resonance at E"=16. 2 MeV in O. The shape of the unpolarized O(y, p3) angular distribution in the giant E1 resonance was derived from the measured analyzing power. NUCLEAR REACTIONS ' O(y, p), E =15 -25 MeV; measured analyzing power 8 =90' linearly polarized bremsstrahlung; 0 dipole levels deduced m", 16.2 MeV1+ resonance deduced po decay amplitudes; 0 GEDR deduced p3 angular distribution. Investigations of photonuclear reactions with linearly polarized photons will become possible on a large scale in the near future. A monoenergetic almost completely polarized photon beam is available now at Frascati. ' The new generation of high duty factor electron accelerators presently under construction allow tagging the linearly polarized off-axis bremsstrahlung. Also the synchrotron radiation from the planned large electron positron storage ring LEP will be highly polarized and could be used for nuclear physics experiments. In this Communication we show that studies of photonuclear reactions with linearly polarized photons provide additional information about the excited nuclear states and their decay: (i) The parity of the absorbed radiation is determined in an unambiguous and completely model independent way. (ii) The decay amplitudes of overlapping resonances lith different parities can be separated.(iii) The (y, particle)-angular distribution for a dipole transition of known parity can be derived from a polarization measurement at only one angle. To exemplify this, we report here. on a study of the 'sO(y, pp) and "O(y, p3) reactions with linearly polarized bremsstrahlung photons in and below the region of the giant electric dipole resonance (GEDR).

Recent Progress on Nuclear Magnetic Dipole Excitations

“…The 16.2-MeV I + state in 160 could also be observed in an experiment with polarized bremsstrahlung (49), and it was shown that the Ci>, particle) reaction provides new information about decay amplitudes of overlapping unbound states of different parity and/or different multipolarity, infor mation not obtainable from unpolarized measurements. The measured 1 60 (y, p) spectrum is depicted in Figure 15 results of the inelastic electron scattering experiment (75) (1.12 ± 0.07 f1�), but in addition to the ground-state decay width, the partial decay widths to other states could be measured in NRF (77).…”

“…A breakthrough for NRF experiments with bremsstrahlung was the development of the linearly polarized bremsstrahlung beam facility at the University of Giessen 65-MeV electron linear accelerator (11,49). With this experimental arrangement it was possible for the fi rst time to determine parities of highly excited dipole states close to the particle threshold in an NRF measurement and to study systematically the spin-flip M I resonance, which is located around 10 MeV.…”

“…Another possibility is to change, by steering coils, the angle of incidence of the electron beam on the bremsstrahlung radiator target and to select the off-axis angle by a sub sequent fixed collimator. When using proton detectors (Si surface barrier detectors or telescopes), the energy dependence of the degree of polarization can be determined more easily (49). Furthermore such an arrangement, as installed at the Giessen polarized bremsstrahlung facility (11, 49), offers the advantage of a fi xed reaction target position, which allows a fully symmetric setup of four detectors (up, down, left, right).…”

Parity of Bound Dipole States inPb208

“…It was extrapolated below the 6-MeV detection threshold of the polarimeter by dashed lines using the known energy dependence of the photon polarization. 15 The width of these lines gives the statistical accuracy of the 2 H(y pol ,/>) measurement. The error bars (one standard deviation) show the measured asymmetries for the strongest transitions.…”

Linearly Polarized Quasi-monochromatic Photon Beams From Compton Backscattering Of Laser Lights At ETL

“…Polarized photon beams [ 1,2] have been interested in the field of the nuclear physics, because the real photon has a high spin selectivity and ambiguous parity assignment has been done [3]. Almost completely linearly polarized photon beam was generated by Compton backscattering of linearly polarized laser photons in several facilities [4][5][6], because of the small spin-flip amplitude in the Compton scattering at the forward direction.…”