Photo-neutron cross-sections of GDR response in the heavy-nucleus ¹⁶⁹Tm

We are constructing a cross-section calibration standard for photo-neutron reactions, by obtaining high-precision

(γ,1n) and (γ,2n) cross sections. Photo-absorption reactions are when a nucleus absorbs electromagnetic radiation

of energies around 10-20 MeV, exciting the isovector giant dipole resonance (GDR) response in the nucleus. Most

heavy nuclei decay by emitting neutrons which are the experimental signature. However, neutron detection is

difficult since they are uncharged and there is sometimes one or two emitted in the reaction meaning that the

efficiency can be hard to quantify. Our measurement uses ¹⁶⁹Tm as a target since its (γ,1n) and (γ,2n) reactions

result in unstable nuclei with well-understood decays with characteristic γ rays: ¹⁶⁸Tm and ¹⁶⁷Tm, respectively.

The photo-activation measurement was carried out at the γELBE Bremsstrahlung-creation facility in Dresden,

Germany. Both (γ,1n) and (γ,2n) reactions were observed using irradiations at multiple different electron energies,

meaning (γ,1n) and (γ,2n) cross-sections can be determined. Once we extract the cross-sections, this nucleus

will be used as a calibration standard for other photo-absorption measurements. The preliminary results of our

analysis will be presented at the APS DNP meeting.