Photofission of Heavy Nuclei at Energies up to 4 GeV

Abstract

Total photofission cross sections for 238 U, 235 U, 233 U, 237 Np, 232 Th, and nat Pb have been measured simultaneously, using tagged photons in the energy range Eγ =0. 17-3.84 GeV. This was the first experiment performed using the Photon Tagging Facility in Hall B at Jefferson Lab. Our results show that the photofission cross section for 238 U relative to that for 237 Np is about 80%, implying the presence of important processes that compete with fission. We also observe that the relative photofission cross sections do not depend strongly on the incident photon energy over this entire energy range. If we assume that for 237 Np the photofission probability is equal to unity, we observe a significant shadowing effect, starting below 1.5 GeV.PACS numbers: 25.85. Jg, 25.20.Dc It has long been assumed [1,2] that the photofission cross section σ γF for the uranium and transuranic isotopes should exhaust the total nuclear photoabsorption cross section σ γA for incident photon energies above 50 MeV, so that the fission probability of these nuclei at these energies was generally assumed to be unity. If this were the case, the photofission method would provide a very convenient way to determine the total photoabsorption cross section. This interesting quantity [3], not easily determined by other methods for heavy nuclei, would yield information on the intrinsic properties of nucleons inside the nuclear medium, as well as on the hadronic nature of the photon.One might assume the total photoabsorption cross section per nucleon σ γA /A to be independent of A. In the ∆-resonance region, this has been shown to be plausible by various measurements for several complex nuclei ranging from Li to U, leading to the concept of the "universal curve" [4]. For several nuclei, recent measurements have been performed up to 2.6 GeV [5]. If we define the photofissility W F of a nucleus as the probability that this nucleus undergoes fission, then we can express W F as the ratio σ γF /σ γA . A constant σ γA /A implies that, for two nuclei, the ratio of their photofissilities is given by the ratio of their photofission cross sections per nucleon σ γF /A.The assumption W F ≃ 1 had to be reconsidered after a Novosibirsk group [6] reported that the fission probability for 237 Np is about 30% higher than that for 238 U in the photon energy range 60-240 MeV. Subsequent measurements performed at the Saskatchewan Accelerator Laboratory (SAL) [7] corroborated these results for the same energy range. We now observe this trend to be true up to 4 GeV.This experiment was the first measurement performed in Hall B at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) using the photon-tagging facility [8]. An electron beam is passed through a thin radiator to produce bremsstrahlung. The residual energy of an electron after bremsstrahlung emission is deduced from its measured position in the focal plane of the tagger bending magnet. The two-layer focal-plane detector array has an energy resolution of 0.1% provided by the highly segmented ...

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