Investigation of (n,2n) Reactions at High Energy Neutron Facility

1. NATIONAL TECHNICAL UNIVERSITY OF ATHENS and NATIONAL CENTER for SCIENTIFIC RESEARCH “DEMOKRITOS” Investigation of (n,2n) reactions using the high energy neutron facility at NCSR “Demokritos” R. Vlastou1, A. Kalamara1, M. Kokkoris1, M. Serris2, N. Patronis3 S. Harissopulos4, M. Axiotis4, A. Lagoyannis4 1 Department of Physics, National Technical University of Athens, 157 80 Athens, Greece, 2 Hellenic Army Academy, 16673 Vari, Athens, Greece 3 Department of Physics, University of Ioannina, 45110 Ioannina, Greece, 4 Institute of Nuclear Physics, NCSR "Demokritos", 153 10 Aghia Paraskevi, Greece 25th Symposium of the Hellenic Nuclear Physics Society, NCSR “Demokritos”, June 2016

2. The High Energy Neutron beam at “Demokritos” is produced via the 3H(d,n)4He reaction at energies ~15-21 MeV with deuteron beams ~2-4.5 MeV The Ti-tritiated target of 373 GBq activity, consists of 2.1 mg/cm2 Ti-T layer on a 1mm thick Cu backing for good heat conduction. The flange with the tritium target assembly is air cooled during the deuteron irradiation. Neutron beam flux of the order of ~105-106 n/sec·cm2. To monitor the neutron flux a BF3 detector is used, while the absolute flux of the beam is determined via the 27Al(n,α) reference reaction Cross section measurements via the activation technique After the neutron irradiation : Off-line measurements of γ-ray transitions from the residual nuclei by using HPGe detectors εr ≈ 80% and 56%

3. This new High Energy neutron facility has been characterized by using NeuSDesc and MCNP simulations as well as the multiple foil activation technique It opens a vast field of research on a variety of neutron induced reactions at an important energy range to test and improve nuclear models and investigate reaction mechanisms. The pre-equilibrium mechanism becomes important in the de-excitation of the compound nucleus Cross Section Measurements of reactions already investigated with the low energy neutron facility in the past using the activation technique (n,2n) reactions on 191Ir, 193Ir, 174Hf, 176Hf, 197Au recently measured at 15.3, 17.1 and 20.9 MeV

4. 191Ir(n,2n)190Ir reaction (N.Patronis et al. Phys.Rev.C75(2007)034607) Isomeric cross sections are of fundamental interest since they are governed by the spins of the levels involved in the compound nucleus evaporation process Statistical model calculations with code STAPRE-F have been performed by using the Generalized Superfluid Model (GSM) for the calculation of nuclear level densities in the continuum to test it in the mass region ~190 and for isomeric cross section production. The high spin isomeric state m2 is fed by a small part of the continuum which depends on the spin cut-off parameter and consequently on the moment of inertia. A reduced value of the rigid body moment of inertia is needed for a better agreement with data. The population of the isomeric state is also strongly dependent on the details of the introduced level scheme. Further measurements are needed above 15 MeV to resolve discrepancies in experimental points and theoretical predictions

5. Further measurements are planned at 19 MeV to complete the energy range which can be covered by the facility as well as theoretical calculations using the code EMPIRE

6. 197Au(n,2n)196Au reaction(A.Tsinganis et al. Phys.Rev.C83(2010)024609) Statistical model calculations were carried out with the use of three different codes STAPRE-F, EMPIRE2.19 and TALYS1.2. which were also compared in their implementation of the Generalized Superfluid Model (GSM) for the calculation of nuclear level densities in the continuum. The σg+m1 cross section was easily reproduced by the calculations, while for σm2, the theoretical results could only reproduce the general trend of the experimental data, with the distribution shifted at higher energies. Several tests were performed to improve the theoretical predictions which reveal the importance of the level scheme of the residual nucleus and the limitations of the nuclear codes to embed high spin discrete states in the continuum which would increase the feeding of the isomeric state. Further measurements are needed above 15 MeV to resolve discrepancies in experimental points and theoretical models

7. Further measurements are planned at 19 MeV to complete the energy range we can cover as well as theoretical calculations using the code EMPIRE

8. 174,176Hf (n,2n) reactions (M.Serris et al. Phys.Rev. C86(2012)034602). Natural Hf consists of 6 isotopes 174, 176, 177, 178, 179, 180Hf and three of them produce long lived residual nuclei and can thus be studied by using the neutron activation technique Studies of neutron induced reactions on Hf are of considerable importance for practical applications in nuclear technology: a) due to its high thermal neutron absorption cross section, Hf is used for reactor control rods in nuclear submarines b) n-induced reactions on W and Ta in reactor materials could lead to long lived isomeric states of Hf isotopes with rather harmful γ-ray production. Experimental data available only at ~14MeV The 176Hf(n,2n) reaction has an energy threshold of ~8.2MeV Opens with En >~9MeV While the contaminant 174Hf(n,γ) reaction has no threshold Opens with parasitic low energy neutrons

9. However, In the high energy region, the 176Hf(n,2n)175Hf reaction is also contaminated by the 177Hf(n,3n)175Hf reaction. Using the cross section from ENDF, the expected number of counts from the (n,3n) reaction has been deduced to correct for this contribution

10. Further measurements are planned at 19 MeV to complete the energy range which can be covered by the facility Theoretical calculations will be performed using the code EMPIRE

11. Thank you For your attention