The (n, α) reaction in the s-process branching point 59 Ni

Transcription

1 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH Proposal to the ISOLDE and Neutron Time-of-Flight Committee The (n, α) reaction in the s-process branching point 59 Ni January 5, 2012 CERN-INTC / INTC-P /01/2012 C. Weiss 1,2, C. Guerrero 2, E. Griesmayer 3, J. Andrzejewski 4, G. Badurek 1, E. Chiaveri 5, R. Dressler 6, S. Ganesan 7, E. Jericha 1, F. Kaeppeler 8, P. Koehler 9, C. Lederer 10, H. Leeb 1, J. Marganiec 4, A. Pavlik 10, J. Perkowski 4, R. Reifarth 11, T. Rauscher 12, D. Schumann 6, G. Tagliente 13, V. Vlachoudis 2 1 Vienna University of Technology, Vienna, Austria 2 CERN, Geneva, Switzerland 3 CIVIDEC Instrumentation GmbH, Vienna, Austria 4 Uniwersytet Lodzki, Lodz, Poland 5 CEA, Saclay, France 6 PSI, Villigen, Switzerland 7 former BARC, Mumbai, India 8 Karlsruhe Institute of Technology, Karlsruhe, Germany 9 Oak Ridge National Laboratory (ORNL), Oak Ridge, USA 10 University of Vienna, Vienna, Austria 11 Goethe University of Frankfurt, Frankfurt, Germany 12 University of Basel, Basel, Switzerland 13 Instituto Nazionale di Fisica Nucleare, Bari, Italy Spokesperson: C. Weiss (christina.weiss@cern.ch), C. Guerrero (carlos.guerrero@cern.ch) Technical coordinator: E. Berthoumieux (eric.berthoumieux@cern.ch) 1

2 Abstract: We propose to measure the 59 Ni(n, α) 56 Fe cross section at the neutron time of flight (n TOF) facility with a dedicated chemical vapor deposition (CVD) diamond detector. The (n, α) reaction in the radioactive 59 Ni is of relevance in nuclear astrophysics as it can be seen as a first branching point in the astrophysical s-process. Its relevance in nuclear technology is especially related to material embrittlement in stainless steel. There is a strong discrepancy between available experimental data and the evaluated nuclear data files for this isotope. The aim of the measurement is to clarify this disagreement. The clear energy separation of the reaction products of neutron induced reactions in 59 Ni makes it a very suitable candidate for a first cross section measurement with the CVD diamond detector, which should serve in the future for similar measurements at n TOF. Requested protons: 1x10 18 protons on target 2

3 1 Introduction and Motivation Neutron induced charged particle reactions, (n, cp) reactions, are important in various fields, amongst others nuclear astrophysics, nuclear technology and medical physics. At n TOF a dedicated prototype detector for (n, cp) cross section measurements, using the CVD diamond technology, has been developed and tested in 2011 [1]. After an upgrade of the detector setup, the first cross section measurement for a qualified isotope is proposed here. Due to its open (n, cp) reaction channel at low neutron energies, the radioactive isotope 59 Ni is of relevance in nuclear astrophysics [2] as well as in nuclear technology [3]. 59 Ni decays with a half life of years into stable 59 Co through almost pure e capture, emitting predominantly 7 kev X-rays or Auger e with a maximum energy of 6.07 kev. In Astrophysics, neutron-rich heavy elements with A 56 are produced through neutron capture processes in various different stellar environments. One classifies the rapid neutron capture process (r-process), which takes place in explosive stellar environments, and the slow neutron capture process (s-process) during He burning episodes of stellar evolution, both starting at the dominant seed abundance of 56 Fe. Contrary to the r-process, the path of the s-process in the chart of nuclides evolves along the valley of stability via successive neutron capture processes and subsequent β -decays. Unlike in most isotopes, the (n, α) and (n, p) channels in 59 Ni are open in the neutron energy region of the s-process, hence these reactions are competing with neutron capture. Accordingly, 59 Ni can be considered as a first branching point in the s-process path. As indicated in the flow diagram of figure 1, (n, α) reactions on 59 Ni have a recycling effect, which weakens the role of 58 Ni as a secondary s-process seed. Figure 1: Competing reactions at the 59 Ni branching point. Consecutive capture reactions are indicated by grey, the competing (n, p) and (n, α) reactions by red and green arrows, respectively. The relevance of 59 Ni to nuclear technology is related to radiation induced damage of structural materials. In nuclear reactor environments, alloys containing Ni, namely stainless steel, are used for their high strength and corrosion resistance. In such environments material embrittlement has to be kept carefully under control. 3

4 One of the sources for irradiation induced material damage is helium deposition. The two-step reaction 58 Ni(n, γ) 59 Ni(n, α) 56 Fe contributes significantly to the He production in environments with a strong thermal neutron component [4]. If the structural material is exposed to a neutron field with a wide energy spectrum, this reaction becomes even more important because of the presence of a large resonance at 203 ev, which seems visible in all open reaction channels at this neutron energy, but its strength is not well known for each of the channels. Such an exposure has to be especially considered for fusion reactor installations like ITER, where steel is used as structural material near to the core in the blanket modules [5]. The competing 58 Ni(n, γ) 59 Ni(n, p) 59 Co reaction has to be considered likewise when investigating radiation induced material damage, caused by the displacements in the material lattice caused by the emitted p. The 58 Ni(n, γ) reaction cross section has been measured at n TOF in 2011 [6], and information about the 59 Ni(n, p) cross section will be available through the proposed measurement. 2 Status of 59 Ni(n, cp) cross sections In the 70ies, four different measurements [7, 8, 9, 10] of the 59 Ni(n, α) cross section have been performed at thermal energies, yielding highly discrepant values from 11.4 barn to 22.3 barn. The available experimental data beyond the thermal energy region correspond to various Figure 2: Evaluated nuclear data for the 59 Ni(n, α) 56 Fe and the 59 Ni(n, tot) cross section in comparison with experimental data from Harvey et al. [11]. measurements performed by Harvey et al. at ORELA in 1975 [7], which are the basis of the present evaluations [12]. One of the data sets [11], indicated as 59 Ni(n, tot) cross section, seems to follow the data published in [7] for the 59 Ni(n, α) cross section at the big 203 ev resonance. These data show the presence of an additional dominant resonance 4

5 around 130 ev, and one at 340 ev, which are not found in the mentioned reference to the experiment. Whether the experimental data are not reliable or the resonances are actually coming from the 59 Ni(n, α) reaction is not clear. In figure 2 the evaluated nuclear data for the 59 Ni(n, α) reaction of the ENDF/B-VII.0, JENDL-4.0, and ENDF/B-VI.8 libraries, which predict the strength of the 203 ev resonance in this channel different by at most 10%, are compared to the experimental data set [11]. The (n, p) cross section is taken from the ENDF/B-VII.0 database. The total cross section of 59 Ni, also taken from the ENDF/B-VII.0 library, is plotted in the same graph to show that the resonance structure, apparent in the experimental data, is not considered in the evaluated cross sections of any open channel in this energy region. Because of these inconsistencies it is important to clarify the presence of additional resonances in this energy range with a dedicated measurement, with the eventual goal of updating the evaluated nuclear data, which serves as an input in model calculations in nuclear astrophysics and in risk assessment simulations for nuclear reactor facilities. The discrepancies become even more apparent beyond this energy region, see figure 3, proving that the cross sections for this isotope are not well known. The energy region up to 100 kev is especially important to clarify the role of 59 Ni in the astrophysical s-process. Figure 3: Evaluated nuclear data for the 59 Ni(n, α) 56 Fe and the 59 Ni(n, tot) cross section in comparison with experimental data from Harvey et al. [11]. In addition to the scientific motivation, we propose this measurement also for technological reasons. The upgraded CVD diamond detector (see details below) has to be qualified for future cross section measurements at n TOF, in particular for (n, cp) reactions. In this respect, the possibilities for particle discrimination are of high interest. The 59 Ni(n, α) 56 Fe reaction has a positive reaction Q-value of MeV. The energy of the outgoing α particles is E α = MeV. The competing (n, p) reaction, which has an approximately 10 times smaller calculated reaction cross section at the 203 ev resonance, 5

6 yields protons with energies of E p = MeV. The total energy of the γ cascade following a capture reaction in 59 Ni is E γ = MeV. This very clear energy separation of the reaction products qualifies 59 Ni as a suitable isotope for a measurement of different ejectiles with the CVD diamond detector and will serve to investigate the limits of the particle discrimination that can be reached. 3 Experimental Setup The proposed measurement will be performed using a polycrystalline (pcvd) diamond detector 250 µm in thickness. The active detector area will be 28 x 28 mm 2, which allows one to use about 95% of the neutron fluence available at n TOF, when the collimator of 0.9 mm radius (capture setup at n TOF) is in place. The reduced thickness of the diamond detector in comparison with the one used for the prototype detector (500 µm) will have two advantages. 1. The ambient and neutron induced backgrounds will be reduced by 50%. 2. The diamond material in use with a charge collection distance of 200 µm and more promises electrical properties close to a single-crystal CVD diamond, which has been proven to have excellent spectroscopic [13, 14] and particle discrimination [15] properties. In contrast to the measurement performed with the prototype detector, the proposed experimental setup will be under vacuum in order to minimize the energy loss of the outgoing particles on their way to the detector. At the Paul Scherrer Institute [16] two multi-purpose samples of 3 cm radius will be produced out of an existing 59 Ni transmission sample. The total available mass is 6 mg of 59 Ni, in the form of Ni 2 O 3. The samples will be positioned at either side of the diamond detector in close geometry, thus covering a maximum solid angle. Because of geometrical constrains, only about 2 mg of the total available mass of 59 Ni will actually contribute to this measurement. 4 Objectives and beam time request The goals of the proposed measurement will be to measure the integral of the dominant resonance at 203 ev for the (n, α) reaction with an accuracy of less than 2%; to investigate the cause of the resonance structure in the experimental data of Harvey et al. [11]; to identify resonances in the kev region of astrophysical interest and clarify the discrepancies shown in figure 3; to characterize the response of the upgraded CVD diamond detector to the products of the 59 Ni(n, α) and 59 Ni(n, p) reactions; 6

7 to test the qualification of CVD diamond detectors for future (n, cp) cross section measurements. The details of the requested proton budget are listed in table 1. The calculation corresponds to the measurement of 2 mg of 59 Ni, taking the ENDF/B-VII.0 data for the (n, α) cross section as reference. In addition, the background level will have to be determined and the neutron fluence will have to be monitored. Table 1: Number of protons requested for the measurement of the 59 Ni(n, α) cross section. Measurement Purpose # of protons 59 Ni (n, α) cross section and 5x10 17 detector response measurement Sample-out In-beam background 3x B Neutron fluence monitoring 2x10 17 Beam-off Background from sample activity 0 Total 1x10 18 The expected counts for the (n, α) and (n, p) reactions in the dominant resonance at 203 ev are about 10 5 and 10 4, respectively. Above 400 ev a prediction is not reliable as the cross sections are unclear. 5 Conclusions The measurement of the 59 Ni(n, α) 56 Fe cross section at the n TOF facility with a dedicated CVD diamond detector is proposed here. It is a next step of the development of a CVD diamond prototype detector at n TOF, which has been upgraded to obtain lower backgrounds and higher efficiency. This new measurement of the 59 Ni(n, α) cross section, essential for applications in the field of nuclear technology as well as nuclear astrophysics, shall clarify the inconsistencies between experimental data and evaluated cross sections. Summary of requested protons: 1x10 18 protons on target. 7

8 References [1] C.Weiss, V.Vlachoudis et al.; Reference measurement of the high energy (n,α) cross section of 10 B with a CVD diamond detector for the prospect of future applications, CERN-INTC , INTC-I-125 (2010). [2] M.Pignatari, et al.; The weak s-process in massive stars and its dependence on the neutron capture cross sections, The astrophysical journal Vol. 710, (2010). [3] OECD Nuclear Energy Agency; Comparison Calculations for an Accelerator-driven Minor Actinide Burner, NEA/NSC/DOC (2001)13. [4] V. Gopalakrishnan, R.V. Nandedkar, S. Ganesan; Comparison of calculated helium production in stainless steel due to neutron irradiation with experiment, Journal of Nuclear Materials 228, (1996). [5] M.J.Loughlin; ITER - Activation of Blanket Module, IDM Number: ITER D 2DK3WQ (2008). [6] J.L.Tain, et al.; The role of Fe and Ni for s-process nucleosynthesis in the early Universe and for innovative nuclear technologies, Cern-INTC , INTC/P [7] J.A.Harvey, J.Halperin, N.W.Hill, S.Raman, R.L.Macklin; (n, α), (n, p), (n, γ), and total neutron-cross-section measurements on 59Ni, Int. Conf. on Interact. of Neutr. with Nuclei, Lowell 1976, Vol.1, p.143 (1976) USA. [8] J.McDonald, N.G.Sjoestrand; Measurements of thermal neutron cross-sections for He production in Ni-59, Atomkernenergie, Vol.27, Issue.2, p.112 (1976) Germany. [9] R.D.Werner, D.C.Santry; Measured Thermal-Neutron Cross Section for the 59Ni(n, α)56fe Reaction, Nuclear Science and Engineering, Vol.56, p.98 (1975) USA. [10] H.M.Eiland, G.J.Kirouac; Measurements of the Ni-59(n, α) cross section for thermal neutrons, Nuclear Science and Engineering, Vol.53, p.1 (1974) USA. [11] [12] [13] E.Griesmayer et al.; High-Resolution Energy and Intensity Measurement with CVD Diamond at REX-ISOLDE, CERN BE-Note [14] M.Pillon et al.; Experimental response functions of a single-crystal diamond detector for MeV neutrons, NIMA 640 (2011) [15] C.Weiss, E.Griesmayer, A.Zimbal; Spectroscopic measurement of 14 MeV neutrons with CVD diamond detectors at PTB Braunschweig, CERN note under preparation. [16] 8