p-process nucleosynthesis via proton-capture reactions in thermonuclear supernovae explosions

Model calculations within the framework of the so-called γ process show an underproduction of the p nucleus with the highest isotopic abundace 92Mo. This discrepancy can be narrowed by taking into account the alternative production site of a type Ia supernova explosion. Here, the nucleus 92Mo can be produced by a sequence of proton-capture reactions. The amount of 92Mo nuclei produced via this reaction chain is most sensitive to the reactions 90Zr(p,γ) and 91Nb(p,γ). Both rates have to be investigated experimentally to study the impact of this nucleosynthesis aspect on the long-standing 92Mo-problem. We have already measured the proton-capture reaction on 90Zr using high-resolution in-beam γ-ray spectroscopy. In this contribution, we will present our preliminary results of the total cross sections as well as the partial cross sections. Furthermore, we plan to measure the 91Nb(p,γ) reaction soon. Due to the radioactive target material, the 91Nb nuclei have to be produced prior to the experiment. The current status of this production will be presented in this contribution.


Introduction
The elements heavier than iron are mainly created by neutron-capture reactions in the s and r processes [1,2]. Nevertheless, between 30 and 35 isotopes, the so-called p nuclei, are bypassed by the reaction paths of both processes [3]. Their existence can be explained by several processes summarized as p process [4][5][6][7][8]. The relative abundances of most p nuclei can be explained by the so-called γ process [9]. Nevertheless, the γ process cannot account for the high isotopic abundance of the neutron-magic p nucleus 92 Mo of I % = 14.84% [10]. Different network calculations predict a production of 92 Mo by more than one order of magnitude below the expected yield. Hence, an additional production mechanism is needed. A chain of radiative proton-capture reactions may contribute to the stable and long-lived neutron-magic N = 50 nuclei during a supernova of type Ia [11,12] [13]. These values could be confirmed by a 4π in-beam summing crystal measurement by A. Spyrou et al. [14]. Nevertheless, a measurement using γ-ray spectroscopy by C. E. Laird et al. disagrees with the two other data sets for the total cross section [15]. In order to solve this discrepancy, we measured this reaction again using high-resolution in-beam γ-ray spectroscopy. This method allows to investigate the total cross section as well as partial cross sections. To date, partial cross sections of 90 Zr(p,γ) are not published at all. The experiment was performed at the Institut für Kernphysik of the Universität zu Köln, Germany, where we used HORUS with a target chamber optimised for nuclear astrophysics experiments [16]. Thirteen high-purity germanium (HPGe) detectors were placed at five angles relative to the beam axis in order to determine the angular distribution of the prompt γ rays of the excited reaction products. The angular distribution of the γ transitions are mandatory to obtain the number of γ rays emitted in the solid angle for each transition. Since 91 Nb has an isomer at 104.62 keV [17], the sum of all ground state transitions (except the transition between isomer and groundstate) and of all transitions terminating in the isomer yield the total cross section. Furthermore, the depopulation of the so-called entry state can be investigated in order to obtain partial cross sections. More details about this experimental method can be found in Ref. [18]. The 90 Zr(p,γ) reaction was investigated for energies between E p = 3.6 MeV and 5.1 MeV. A measurement as low in energy as E p = 2.5 MeV is planned in near future. We used isotopically enriched material consisting of 97.65(10)% 90 Zr. Besides other zirconium isotopes, 0.96(5)% 91 Zr is included. The two reactions 90 Zr(p,γ) and 91 Zr(p,n) both produce 91 Nb above a proton energy of about 2 MeV. The contribution of both reactions can be disentangled by performing measurements with another target with different enrichments (5.99(10)% in 90 Zr and 89.20(10)% in 91 Zr) at the same energies. Details about this procedure can be found in the contribution of P. Erbacher in this volume. Figure 1 shows our preliminary results for the total cross section in comparison with the cross section data by Refs. [14,15] [19,20]. The cross section predictions are sensitive to the γ-ray strength function and the proton+nucleus optical model potential. Hence, the cross section predictions were calculated with all variations available in TALYS for this nuclear physics input. Our experimental results agree very well with the theoretical predictions using the default settings of the TALYS code. These calculations use the proton+nucleus optical model potential of Ref. [21] and the γ-ray strength function of Ref. [22]. Besides the total cross sections, we have analysed partial cross sections as well. Up to date, we have obtained partial cross sections for the transitions terminating in the ground state and three excited states. As an example, the partial cross section to the ground state is depicted in Fig.  2. It is compared to TALYS calculations using the default settings [21,22].

Production of 91 Nb
The 91 Nb(p,γ) reaction might play an important role for the production of the p nucleus 92 Mo. It is possible to perform this experiment using standard kinematics due to the long half-life of the isotope 91 Nb (t 1/2 = 680 a). In a first step, the target material has to be produced. An amount of 10 16 nuclei should be sufficient to perform the experiment taking into account the proton-beam intensity and detection system under construction at FRANZ, Frankfurt a. M., Germany [29]. One way to produce 91 Nb is the bombardment of 92 Mo with protons. Three reactions produce 91 Nb either directly or via β-decay of the reaction products: 92 Mo(p,2p), 92 Mo(p,pn), and 92 Mo(p,2n).
In order to determine the number of produced 91 Nb after irradiation, the induced radioactive decays have to be observed. 91 Nb has an isomer at an energy of 104.62 keV  [21], whereas calculations using the potential of Ref. [23] are depicted as dashed line. The different colours label the γ-ray strength function used for the calculations: lightgreen [22], blue [24,25], cyan [26], magenta [27], and red [28].  with a half-life of t 1/2 = 60.86 d. Its decay can be investigated by detecting two γ rays with energies of 104.62 keV and 1204.67 keV [17]. After the decay of the ground state, no γ rays are emitted. Therefore, only X-rays can to be detected in order to determine the number of 91 Nb nuclei.
A thick target with a high enrichment in 92 Mo is needed for the production of 10 16 91 Nb nuclei. The number of produced nuclei cannot be determined via γ-ray spectroscopy of the X-rays. Only a small fraction of the X-rays can escape from the thick target due to selfabsorption. Since no experimental data is available for the energy-dependent production cross sections of 91 Nb, we decided to measure them prior to the target production runs.

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The measurement of the production cross sections was performed at the cyclotron of the Physikalisch-Technische Bundesanstalt at Braunschweig, Germany. Naturally composed targets were bombared with protons with energies of E p = 15, 16, 17, 18, and 19 MeV. Two activation runs were performed for each energy: a short activation of about 15 minutes and a long activation of about 8 h. The duration of the short activation is in the order of the halflife of 91 Mo and is used to determine the 92 Mo(p,pn) 91 Mo cross section. The long activations were used to determine the total yield of 91 Nb via x-ray detection for the ground state and γ-ray detection for the isomer.
The experiment was performed in July 2014, hence, the spectroscopy is still in progress at the Institut für Angewandte Physik at Frankfurt a. M., Germany. Estimates based on the first spectra show that a sufficient production of 91 Nb via proton-induced reactions on 92 Mo is possible [30].