Letter to the Editor
Deexcitation via unbound states in the radiative capture of 14 MeV neutrons
F. Cvelbar
; Institute “Jožef Stefan”, Ljubljana and University of Ljubljana
A. Hudoklin
; Institute “Jožef Stefan”, Ljubljana and University of Ljubljana
M. V. Mihailović
; Institute “Jožef Stefan”, Ljubljana and University of Ljubljana
M. Potokar
; Institute “Jožef Stefan”, Ljubljana and University of Ljubljana
Abstract
Comparing the cross section far the 14 MeV neutron capture measured by the activation technique (σ_act) with the integrated cross sections (σ_int) obtained from our measurements of prompt y-ray spectra, we found that in light and medium weight nuclei (aluminium, phosphorus, vanadium and manganese) the compound states deexcite in the first step almost completely to the bound states of final nuclei. For three cases of heavier nuclei (cooper, indium and iodine) this comparison showed that the deexcitation via unbound states is several times more intense than the deexcitation leading directly to the bound states.
The activation cross section includes the deexcitation of compound system via bound states as well as via unbound ones, while the integrated cross section covers the events leading in the first transition directly to the bound states. The integrated cross section is obtained by the integration of prompt y-ray spectra in the energy interval from En to En + Bn where E is the neutron energy and Bn is the binding energy of the last neutron in the ground state of the final nuclei. Thus σ_act is expected to exceed σ_int but the difference should be small as the probability for ( dipole) Y-deexcitation is proportional to the E3Y and the cascade deexcitation of unbound states is reduced due to the open channels for particle emission.
Until recently the activation cross sections were reported only and the contribution of the deexitation of the compound system via unbound states has not been known. Theoretical calculations tried to explain the 14 MeV neutron capture process by taking into account only the deexcitation leading in the first step to the bound states. Results were compared either with the experimental y-ray spectra (few recent cases) 1) or with the available data on σ_act 2). In order to estimate the contribution of the deexcitation via unbound states to the activation cross-section, we measured σ_int for seven elements with known σ_int by the experimental technique described elsewhere 3). Gamma-rays from spherical samples placed around the neutron source were registered by a scintillation telescope pair spectrometer. Due to the nearly spherical geometry spectra obtained were already integrated over solid angle 4π.
Keywords
Hrčak ID:
317749
URI
Publication date:
5.4.1970.
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