Precise studies of the three-nucleon force

Few-nucleon systems are studied at energies below the pion production threshold with the purpose of  precise testing the state-of-the-art nuclear interaction potentials. In the case of three-nucleon system, theoretical calculations predict significant effects of dynamics beyond the pairwise interaction between nucleons: a so-called three-nucleon force (3 Nucleon Force, 3NF). Experiments conducted with the use of large acceptance (nearly 4π) detection systems provide data for the deuteron breakup reaction in collision with a proton in various kinematic configurations of the final state. A reach set of data for differential cross sections is a basis for determining the effect of 3NF, testing approaches to include Coulomb repulsion between protons into theoretical calculations and tracing relativistic effects. Measurements of this reaction using the WASA@COSY detector were carried out in the upper range of deuteron beam energies of interest, 150-200 MeV/nucleon. The first set of results shows that even at such high energies cross-section for configurations in which protons fly “close together” (with a small relative momentum) is dominated by Coulomb repulsion between protons. In other kinematic regions, we observe the effects of 3NF, but there are also cases where all the theoretical predictions, regardless of the model, underestimate the experimental data. Relativistic calculations, so far not including 3NF in the potential, do not improve the description either. The data measured with BINA@KVI detector at the beam energy of 80 MeV/nucleon, indicate discrepancies of similar nature and in similar phase space regions, while previous measurements at the beam energy of 65 MeV/nucleon were very well described by calculations taking into account both the 3NF and the Coulomb repulsion. The source of this discrepancy remains a puzzle: are the current 3NF models imperfect or does the difference stem from neglecting relativistic effects? The progress of theoretical calculations and the continuation of research in the intermediate energy range using the BINA detector at CCB can help in solving the problem.

B.Kłos (M. Berłowski, I.Ciepał, E.Czerwiński, L.Jarczyk, B. Kamys,  St.Kistryn, W.Krzemień, P.Kulessa, A.Kupść, A.Magiera, P.Moskal, W.Parol, D.Pszczel, K.Pysz, M.Skurzok, J.Smyrski, J.Stepaniak, E.Stephan, A.Szczurek, A.Trzciński, A.Wrońska, J.Zabierowski, M.J.Zieliński, P.Żuprański, J.Golak, A.Kozela R.Skibiński, I.Skwira-Chalot, A.Wilczek, H.Witała),  WASA@COSY collaboration at al.

Three-nucleon dynamics in dp breakup collisions using the WASA detector at COSY-Jülich

Physical Review C 101, 044001 (2020)

https://doi.org/10.1103/PhysRevC.101.044001

W.Parol, A.Kozela( K.Bodek, J.Golak, St.Kistryn, B.Kłos, J.Kuboś, P. Kulessa, A. Łobejko, A.Magiera, R.Skibiński, I.Skwira-Chalot, E.Stephan, D.Rozpędzik, A.Wilczek, H.Witała, B.Włoch, A.Wrońska, J.Zejma) et al.

Measurement of differential cross sections for deuteron-proton breakup reaction at 160 MeV

arXiv:2004.02651

Three-body forces in carbon and oxygen nuclei

To test the predictive power of ab initio nuclear structure models, the lifetime of the second 2+ state in neutron-rich 20O, τ(22+)=150(-30,+80)fs, and an estimate for the lifetime of the second 2+ state in 16C have been obtained, for the first time. The results were achieved via a novel Monte Carlo technique that allowed us to measure nuclear state lifetimes in the tens to hundreds femtosecond range by analyzing the Doppler-shifted gamma-transition line shapes of nuclei measured by  AGATA+PARIS+VAMOS setup. The experimental lifetimes agree with predictions of ab initio calculations using  two- and three-nucleon interactions for 20O and 16C. The present measurement shows the power of observables provided by electromagnetic transitions, determined with high-precision gamma spectroscopy, in assessing the quality of first-principles nuclear structure calculations.

M.Ciemała, (B.Fornal, A.Maj, P.Bednarczyk, N.Cieplicka-Orynczak, J.Grębosz, Ł.W.Iskra, M.Kicinska-Habior, M.Kmiecik, M.Lewitowicz, K.Mazurek, M.Matejska-Minda, P.Napiorkowski, M.Rejmund, B.Wasilewska, M.Zieblinski) et al.

Testing ab initio nuclear structure in neutron-rich nuclei: Lifetime measurements of second 2+ state in 16C and 20O

Physical Review C 101, 021303(R) (2020)
https://doi.org/10.1103/PhysRevC.101.021303

News in the theory of nuclear fission

One of the key observables of nuclear fission is fragment mass distribution. Theoretical description of the potential energy surface and inertia parameters allows predicting fragment mass distributions of each particular nuclide. We have investigated 96 isotopes of the elements from Pt (Z = 78) do Ra (Z = 88). Calculations were performed in the three-dimensional collective model based on the energies from the macroscopic-microscopic model with effective Fourier parametrization of nuclear shape. Experimental data are well reproduced when the neck rupture probability depends on the neck thickness.

Krzysztof Pomorski, Artur Dobrowolski, Rui Han, Bożena Nerlo-Pomorska, Michał Warda, Zhigang Xiao, Yongjing Chen, Lilie Liu, and Jun-Long Tian

Mass yields of fission fragments of Pt to Ra isotopes

Physical Review C 101, 064602 (2020)
https://doi.org/10.1103/PhysRevC.101.064602