Photoneutron Cross-Section Resonances inO16

Abstract

The cross section for the reaction 0 16 (y,w)0 15 has been measured from threshold energy to 60 MeV using bremsstrahlung from the Iowa State University 70-MeV synchrotron. The O 15 radioactivity yield curve was measured and the relative cross section extracted using least-structure analysis. 1 " 3 The resulting relative cross section, the result of least-structure analysis of the average of three yield curves, is shown in Fig. 1. The structure appearing above the giant resonance has appeared in essentially all of a very large number of analyses of individual and averaged yield curves. The errors in least-structure results are highly correlated and vary smoothly over the energy range; consequently error flags have been shown only at a few points. The horizontal error flags above the resonances do not represent imprecision in the resonance energy, but instead they represent the extent to which least-structure analysis would spread an infinitely narrow resonance in the cross section at that energy. This spreading is determined by the full width at half-maximum of a "resolution function" resulting from least-structure smoothing, and it is directly correlated with the average error of the yield function analyzed. Consequently, resolution of resonances above the giant resonance can be improved by improving data precision.Three activation curves were measured in each of two separate experiments in which different samples, beam geometries, and energy control systems were used. In the first experiment data were taken and analyzed in 0. 5-MeV increments, while in the second experiment data were taken and analyzed in 0.483-MeV increments.Radioactivity was measured by detecting the annihilation radiation from the 123-second positron activity of O 15 in a 3x3-in. Nal(Tl) scintillation crystal coupled to an EMI 953IB photomultiplier tube. Samples were bombarded for 3 min, one minute was allowed to elapse during which samples were removed from the synchrotron and placed in a standard counting geometry, and the radioactivity was counted for 3 min. Above 20 MeV, counting rates were typically 500 000 counts per minute. Stability of the counting system was held to ±0. 25% during the experiments by frequent checks with a Na 22 standard source.Samples were cylinders of boric acid 2.8 cm long compressed to a density of 1.4 g/cm 3 . The T "T O^y.nJO 19 CROSS SECTION FROM AVERAGED YIELD DATA .-^-^Ni 30 35 40 ENERGY MEV 50 55 60 FIG. 1. Relative cross section for the reaction 0 16 (y,w)0 15 calculated from the average of three radioactivity yield curves by the least-structure method. 250

Praseodymium-141 Photoneutron Cross Section to 65 MeV

“…20 During the past several years repeated measurements of the O u (y,n)O u reaction cross section from threshold energy to 65 MeV have been made using bremsstrahlung photons from the Iowa State University synchro tron. [21][22][23][24] Each measurement has revealed appreciable structured cross section above the giant resonance and improved measurement techniques have made possible the resolution of essentially all known structure within the giant resonance. 24 In addition to the wide energy-range measurements, data were taken on Pr 141 in 0.125-MeV increments, the yield curves just spanning the giant-resonance region.…”

Photodisintegration of Calcium-40

“…The least structure technique for conversion of the experimental yield into cross section (27, 28) was developed in 1902 and has been used since then with good results (29). This method enables one to Investigate the region above the giant resonance because the cross section solutions are smoothed, but at the expense of experimental resolution.…”