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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...

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The total deuteron photodisintegration cross section has been determined in an alternative way, based on the absolute measurement of the 90' cross section and, to a lesser extent, the 0' and 180' cross sections. The accuracy of the deduced total cross sections amounts to 3%, including systematic errors. The agreement of our data with the results from recent theoretical calculations is very good.PACS number(s): 25.20. -x, 27.10.+h For a number of reasons, the study of the characteristics of the deuteron photodisintegration process has continuously attracted a lot of attention, from both the experimental and the theoretical side. More specifically, this reaction has been considered as a fundamental testing ground for the details of the nucleon-nucleon interaction. At present, the correspondence between the experimental photodisintegration data and the results from conventional theoretical approaches (including mesonexchange currents, isobaric configurations, and relativistic spin-orbit terms) seems to have reached a satisfying level. In particular, the magnitude of the total photoabsorption cross section in the low-energy region (here arbitrarily defined as the interval between the reaction threshold and, say, 25 MeV) can be extremely well described by theory. This agreement is accentuated by the high accuracy (3 -5% total uncertainty, i.e. , the sum of statistical and systematic errors) of the results from the total absorption measurements [1,2]. Such accuracy is necessary as the predictions from the various theoretical calculations are in accordance with each other within the 2%%uo level.Results from direct H(y, p) measurements [3] have a somewhat larger uncertainty (about 7%), while the data deduced from the neutron-proton radiative capture experiments [4 -9] generally show a 5 -10% total error. A balanced review of all data in the relevant energy range has been given in Refs. [2] and [8], while the general status of the theoretical as well as the experimental situation for the deuteron photodisintegration is extensively described in a recent review by Arenhovel and Sanzone [10]. However, there also exists an indirect way to determine the total photodisintegration cross section with good accuracy. This is based on an adequate knowledge of the Now at Master Foods, +d sin Ocos0+e sin 0, with 0 the c.m. proton emission angle; the so-called "Partovi coefficients" a up to e contain the information concerning the e.m. multipolarity of the involved transition(s). Consequently, one obtains (0') = a +c, do o (2) (180')=a -c, do (3) (90') =a +b +e, der (4) while the total absorption photodisintegration cross section O. T reads cr r = 4m (a + , ' b + , ', e) . --Alternately, the latter can be written as (5) cr r = 4ir -a + -( 90') -e 1 2 do, 2 3The knowledge of o. T now solely depends on the magnitude of the various terms entering the above expression; in principle, these should all be taken from experiment.In previous papers [12], we have described our results for the forward, backward, and 90' deuteron photodisintegra...

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