Abstract
Photo-nuclear reactions were investigated using a high power table-top laser. The laser system at the University of Jena (I ∼ 3–5×1019 W cm-2) produced hard bremsstrahlung photons (kT∼2.9 MeV) via a laser–gas interaction which served to induce (γ, p) and (γ, n) reactions in Mg, Ti, Zn and Mo isotopes. Several (γ, p) decay channels were identified using nuclear activation analysis to determine their integral reaction yields. As the laser-generated bremsstrahlung spectra stretches over the energy regime dominated by the giant dipole resonance (GDR), these yield measurements were used in conjunction with theoretical estimates of the resonance energies Eres and their widths Γres to derive the integral reaction cross-section σint(γ,p) for 25Mn, 48, 49Ti, 68Zn and 97, 98Mo isotopes for the first time. This study enabled the determination of the previously unknown cross-section ratios for these isotopes. The experiments were supported by extensive model calculations (Empire) and the results were compared to the Thomas–Reiche–Kuhn (TRK) dipole sum rule as well as to the experimental data in neighboring isotopes and good agreement was observed. The Coulomb barrier and the neutron excess strongly influence the ratios for increasing target proton and neutron numbers.
Highlights
On a natural uranium target in 2000 by a single 1020 W cm−2 laser pulse from the Vulcan facility at the Rutherford Appleton Laboratory [14] and by a similar experiment at the Lawrence Livermore National Laboratory [15]
This problem has been highlighted by the International Atomic Energy Agency (IAEA) which points out that the shortfall in current knowledge hinders any progress in the development of transport simulation codes [23] and dose calculations for photon therapy [24]
The present study demonstrates the use of high power lasers in this regard and contributes substantial new information on photo-nuclear physics research
Summary
Since the initial experiments demonstrating the creation of relativistic electrons from the interaction of intense laser fields with matter [33, 34], advances in laser technology have seen a rapid increase with regard to the maximum electron energy. In order to convert the forward peaked relativistic electrons emerging from the laser– gas interactions into hard bremsstrahlung photons that can trigger nuclear reactions (Eγ 6– 8 MeV), a 4.1 mm thick tantalum radiator of dimensions 40 × 40 mm was placed in the vacuum chamber ( p 3.5 × 10−2 mbar) around 10 cm behind the interaction zone as shown in figure 1 For this specific set-up and identical gas jet pressure and contrast ratio conditions, the beam divergence of the highly energetic electrons was measured to be less than 10 mrad in a previous experiment [12]. After re-normalization with the isotopic abundance in the target, the integral yield of the photo-induced residual radioactive nuclei N0 present at time t0 was determined
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