Direct measurement of theHe4(γ,p0<

Abstract

A direct measurement of the He(y, po) reaction was performed using 34 MeV end-point bremsstrahlung photons. Photoprotons were simultaneously detected at nine angles. The magnitude of the deduced cross section is at variance with the results from recent experiments but is in perfect agreement with the previously recommended values. Consequently, as yet no definite conclusion can be drawn about the validity of charge symmetry of the nuclear force.The question of the absolute magnitude of the "He(y, po) reaction continues to raise a lot of attention and debate. This reaction is of particular importance since a direct comparison of the (y,p ) and (y, n ) cross sections for the self-conjugate nucleus He yields valuable information [1] concerning the concept of charge symmetry of the nuclear force, which is considered to be one of its basic characteristics. If this property is valid, the ratio R of these two cross sections should be close to unity in the giant dipole resonance (GDR) region [2). Deviations from this value mainly arise from Coulomb e8'ects, but realistic calculations raise the value of R only to about 1.1 [2,3].However, as yet the experimental situation is not clear.In 1983, a balanced review by Calarco et al. [2,4] of all available experimental total cross section data for both the (y, p) and (y, n ) reactions yielded surprising results.In this evaluation much more credibility was given to the (p, y) capture data as compared to the direct (y, p) values, mainly because the latter ones were obtained with a bremsstrahlung beam (continuous spectrum) and only the proton was detected. Calarco et aI. concluded that the ratio R showed a maximum around 25 MeV, equaling a value of 1.7+0.2, its explanation requiring an extremely large amount of isospin mixing between the excited states of He and thus inconsistent with charge symmetry. Of particular interest here is the fact that Calarco recommends a (y,p ) cross section which reaches 1.85+0.12 mb at 26 MeV, gradually decreasing to about 1.3 mb at 34 MeV. The magnitude and behavior of such cross section could be reasonably well theoretically described by Wachter et al. [3]. However, these same authors concluded that, relying on a selected set of (y, n) data, the di8'erences between the cross sections for both mirror channels did not require any isospin violating nuclear force. Nevertheless, the (y, p ) cross section as recommended by Calarco was not confirmed by subsequent experiments. Bernabei et al. [5] reported a direct measurement of the absolute total "He(y, p) cross section in the 28.6 -58.1 MeV energy interval, using a quasimonoenergetic photon fiux and a nearly 4m. proton detector. These (y,p) cross section results had about the same magnitude as Calarco's suggested (y, n) cross section, leading to a mean value of R =1.01+0.06. A measurement [6] of the inverse H(p, y) He process in the energy region corresponding to photon energies between 21.3 and 31.1 MeV confirmed the latter data set. These new experimental (y,p) cross sections could be extremely well re...

Simultaneous measurement of the (γ,p)/(γ,n) cross-section ratio inHe4as a test of charge symmetry

“…[I] were determined from in ditidual measurements of the (y,p) and (y, n) channels separately. Since this review, three new measurements of the proton channel have been reported [6][7][8]. The data from [6,7] are more consistent with the recommended photoneutron cross section, while [8] reports a cross section nearly twice as large, in agreement with the recommended proton cross section.…”

“…Since this review, three new measurements of the proton channel have been reported [6][7][8]. The data from [6,7] are more consistent with the recommended photoneutron cross section, while [8] reports a cross section nearly twice as large, in agreement with the recommended proton cross section. The only new published neutron data are from a capture reaction [9] which finds a peak cross-section value that is consistent with the recommended neutron value.…”

“…[I] (which give the large value of R"), (b) disagrees with the sum of the recent photoproton data [6,7] and the recommended photoneutron values (where R" is closer to unity), (c) agrees with the sum of Ref. [8] and the recommended neutron cross section, and (d) agrees with the sum of the latest (n, y) measurement [9] and the recommended photoproton data.…”

Simultaneous measurement of the photodisintegration ofHe4in the giant dipole resonance region

“…The 4 He(γ,p) 3 H cross section, σ(γ, p), was measured by detecting the protons by means of a NE213 liquid scintillator [11] and/or a Si(Li) detector array [12]. Note that the latest result by Hoorebeke et al using 34 MeV endpoint bremsstrahlung photons [12] is larger than the data by Bernabei et al [11] using a monochromatic photon beam by about 40% at around 30 MeV. The difference of these two data sets, however, becomes smaller with increasing the γ-ray energy, and they agree with each other at 33 MeV within the experimental uncertainty.…”

Study of the isospin response of theHe4continuum using theHeRaue1, Bland2, Carman3 et al. 1995Phys. Rev. C2110View full textAdd to dashboardCite

“…However, a measurement [15] of photon elastic scattering from He has constrained the total photoabsorption cross section, finding it to be larger than the sum of the FB o(y,p) measurements and the CBD then the sum of o(y,p) and cr(y, n) should give the total photoabsorption cross section. The situation has become even more controversial since yet another remeasurement [9] of cr(y, p) was found to be consistent with the original CBD values.…”