Measurement of the Gd-160(n, gamma) cross section at n_TOF and its medical implications

Measurement of the 160Gd(n, γ) cross section at n_TOF and its medical implications M. Mastromarco1,2, A. Manna3,4, F. García-Infantes5, S. Amaducci6, A. Mengoni7,3, C. Massimi3,4, G. Tagliente1, S. Cristallo8,9, O. Aberle10, V. Alcayne11, S. Altieri12,13, J. Andrzejewski14, V. Babiano-Suarez15, M. Bacak10, J. Balibrea15, C. Beltrami12, S. Bennett16, A. P. Bernardes10, E. Berthoumieux17, R. Beyer18, M. Boromiza19, D. Bosnar20, M. Caamaño21, F. Calviño22, M. Calviani10, D. Cano-Ott11, A. Casanovas22, F. Cerutti10, G. Cescutti23,24, E. Chiaveri10,16, P. Colombetti25,26, N. Colonna1, P. Console Camprini3,7, G. Cortés22, M. A. Cortés-Giraldo27, L. Cosentino6, S. Dellmann28, M. Di Castro10, S. Di Maria29, M. Diakaki30, M. Dietz31, C. Domingo-Pardo15, R. Dressler32, E. Dupont17, I. Durán21, Z. Eleme33, S. Fargier10, B. Fernández27, B. Fernández-Domínguez21, P. Finocchiaro6, S. Fiore7,34, V. Furman35, A. Gawlik-Rami˛ega14, G. Gervino25,26, S. Gilardoni10, E. González-Romero11, C. Guerrero27, F. Gunsing17, C. Gustavino34, J. Heyse36, W. Hillman16, D. G. Jenkins37, E. Jericha38, A. Junghans18, Y. Kadi10, K. Kaperoni30, G. Kaur17, A. Kimura39, I. Knapová40, M. Kokkoris30, Y. Kopatch35, M. Krtiˇ cka40, N. Kyritsis30, I. Ladarescu15, C. Lederer-Woods41, J. Lerendegui-Marco15, G. Lerner10, T. Martínez11, A. Masi10, P. Mastinu42, E. A. Maugeri32, A. Mazzone1,43, E. Mendoza11, V. Michalopoulou30, P. M. Milazzo23, R. Mucciola8,44, F. Murtas 45 , E. Musacchio- Gonzalez42, A. Musumarra46,47, A. Negret19, A. Pérez de Rada11, P. Pérez-Maroto27, N. Patronis33, J. A. Pavón- Rodríguez27, M. G. Pellegriti46, J. Perkowski14, C. Petrone19, E. Pirovano31, J. Plaza11, S. Pomp48, I. Porras5, J. Praena5, J. M. Quesada27, R. Reifarth28, D. Rochman32, Y. Romanets29, C. Rubbia10, A. Sánchez11, M. Sabaté-Gilarte10, P. Schillebeeckx36, D. Schumann32, A. Sekhar16, A. G. Smith16, N. V. Sosnin41, M. E. Stamati33, A. Sturniolo25, A. Tarifeño-Saldivia15, D. Tarrío48, P. Torres-Sánchez5, E. Vagena33, S. Valenta40, V. Variale1, P. Vaz29, G. Vecchio6, D. Vescovi28, V. Vlachoudis10, R. Vlastou30, A. Wallner18, P. J. Woods41, T. Wright16, R. Zarrella3,4, P. Žugec20, and the n_TOF Collaboration 1Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy 2Dipartimento Interateneo di Fisica, Università degli Studi di Bari, Italy 3Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 4Dipartimento di Fisica e Astronomia, Università di Bologna, Italy 5University of Granada, Spain 6INFN Laboratori Nazionali del Sud, Catania, Italy 7Agenzia nazionale per le nuove tecnologie (ENEA), Italy 8Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Italy 9Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Teramo, Italy 10European Organization for Nuclear Research (CERN), Switzerland 11Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain 12Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Italy 13Department of Physics, University of Pavia, Italy 14University of Lodz, Poland 15Instituto de Física Corpuscular, CSIC - Universidad de Valencia, Spain 16University of Manchester, United Kingdom 17CEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France 18Helmholtz-Zentrum Dresden-Rossendorf, Germany 19Horia Hulubei National Institute of Physics and Nuclear Engineering, Romania 20Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia 21University of Santiago de Compostela, Spain 22Universitat Politècnica de Catalunya, Spain 23Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy 24Department of Physics, University of Trieste, Italy 25Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Italy 26Department of Physics, University of Torino, Italy 27Universidad de Sevilla, Spain 28Goethe University Frankfurt, Germany 29Instituto Superior Técnico, Lisbon, Portugal 30National Technical University of Athens, Greece 31Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany EPJ Web of Conferences , 09002 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409002 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http ://creativecommons.org/licenses/by/4.0/). s 32Paul Scherrer Institut (PSI), Villigen, Switzerland 33University of Ioannina, Greece 34Istituto Nazionale di Fisica Nucleare, Sezione di Roma1, Roma, Italy 35Joint Institute for Nuclear Research (JINR), Dubna, Russia 36European Commission, Joint Research Centre (JRC), Geel, Belgium 37University of York, United Kingdom 38TU Wien, Atominstitut, Stadionallee 2, 1020 Wien, Austria 39Japan Atomic Energy Agency (JAEA), Tokai-Mura, Japan 40Charles University, Prague, Czech Republic 41School of Physics and Astronomy, University of Edinburgh, United Kingdom 42INFN Laboratori Nazionali di Legnaro, Italy 43Consiglio Nazionale delle Ricerche, Bari, Italy 44Dipartimento di Fisica e Geologia, Università di Perugia, Italy 45INFN Laboratori Nazionali di Frascati, Italy 46Istituto Nazionale di Fisica Nucleare, Sezione di Catania, Italy 47Department of Physics and Astronomy, University of Catania, Italy 48Uppsala University, Sweden Abstract. Neutron-capture reactions on gadolinium isotopes play an important role in several fields of physics, in particular in nuclear Astrophysics for the understanding of the nucleosynthesis of heavy elements (beyond iron) in stars via the s- and r-processes [1] and in nuclear technology. Another important application of gadolinium is linked to the production of terbium, that offers a set of clinically interesting isotopes for theranostics, characterized by complementary physical decay characteristics. In particular, the low-energy β−emitter terbium-161 is very similar to lutetium-177 in terms of half-life (6.89 d), β−energy and chemical properties. Being a significant emitter of conversion/Auger electrons, greater therapeutic effect can therefore be expected in comparison to Lu-177 [2, 3]. For this reason, in the last decade, the study of the neutron capture reaction 160Gd(n,γ)161Gd and the subsequent β−decay in terbium-161 is getting particular attention. As the nuclear data on the Gd-160 neutron capture reaction are quite scarce and inconsistent, a new measurement of the capture cross section of Gd-160 at the CERN neutron Time-Of-Flight facilty was performed in order to provide high resolution, high-accuracy data on this important reaction, in the energy range from thermal to hundreds of keV. In this contribution, the preliminary results of the n_TOF measurement are presented. 1 Introduction Terbium, member of the lanthanides row, offers a poker of clinically interesting isotopes for radionuclide therapy with complementary physical decay characteristics; among these, the low-energy β−emitter Tb-161 is very similar to Lu-177 regarding the half-life (6.89 d), β-energy and chemical properties. Being a significant emitter of conversion/Auger electrons, greater therapeutic effect can therefore be expected in comparison to Lu-177 [2, 3]. Moreover, it also emits low-energy photons that are useful for diagnostic applications. Among the possible ways of producing 161Tb, the neutron capture reaction on 160Gd followed by the β-decay of 161Gd (T1/2=3.66 m) in 161Tb, is poorly explored. As of today, despite of its importance only a few experimental data sets exist at thermal neutron energy (25.3 meV) and in the Unresolved Resonance Region (URR), both characterized by discrepancies larger than 50%. In the Resolved Resonance Region (RRR) experimental data are completely missing. Discrepancies of the same order are reflected in the evaluations and main nuclear data libraries (ENDF/B-VIII.0, JEFF-3.3 and JENDL-4.0). Accordingly, large discrepancies are present for the values of Maxwellian Averaged Cross Section (MACS) retrieved from Karlsruhe Astrophysical Database of Nucleosynthesis in Stars (KADoNiS) database [4]. This unsatisfactory situation for medical and astrophysical applications triggered for a new measurement of the neutron capture cross-section of Gd-160 at the CERN neutron Time of Flight facility n_TOF. Thanks to the high neutron energy resolution of EAR-1 and to the very high neutron flux in EAR-2, capture data were collected from thermal neutron energy up to a few hundreds of keV. In the present work the sample characteristics as well as the experimental setup in both areas followed by the preliminary results are shown. 2 Samples The gadolinium sample, supplied by the Institut Luaue- Langevin (ILL) in Grenoble (France), consisted of 317 mg GdO2in powdered form enchiched to 98.10% in 160Gd. The main characteristic of the sample is the very low presence of the odd-isotopes Gd-155 and Gd-157 whose very high cross section [5] would affect the measurement in the low neutron energy region. The detailed sample composition is reported in table 1. The very low presence of odd-A isotopes 155,157Gd is the result of 54 days of irradiation at the ILL thermal reactor. The sample was enclosed in a 2- cm-diametr PEEK capsule, 2.1 mm-thick and closed by a thin Kapton foil. A post-measurement X-ray Spectroscopy indicated negligible sample inhomogeneity. In addition, a 197Au disk, a dummy-sample (i.e. a PEEK capsule without gadolinium oxide) and an emptysample were used in order to normalize the data and estimate the background. All samples were glued onto a 6- µm-thick Mylar foil attached to an aluminium ring of 5 cm diameter as illustrated in figure 1 for the 160Gd sample. EPJ Web of Conferences , 09002 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409002 2 Table 1. Gd-160 enrichment: sample isotopic composition after 54 days irradiatian at the high neutron flux reactor of ILL resulted in Gd-155 and Gd-157 at trace levels. Isotope % Gd-152 3.80 ×10−5 Gd-153 3.90 ×10−10 Gd-154 0.02 Gd-155 3.30 ×10−5 Gd-156 0.59 Gd-157 4.20 ×10−6 Gd-158 1.29 Gd-160 98.10 Figure 1. The Gd-160 sample in the PEEK container enclosed by the Kapton foil; the dummy-sample was made with the same procedure. 3 The experimental setup In order to extract the cross section in the neutron energy range spanning from thermal up to the Unresolved Resonances Region (URR), the neutron capture reaction measurement on 160Gd was performed in both experimental areas of the neutron time-of-flight facility n_TOF at CERN. At n_TOF neutrons are produced by spallation process on a lead target by 20 GeV/c protons coming from the CERN Proton Synchrotron. The initially fast neutrons are moderated and transported to the two experimental areas at the end of the flight paths 185 and 19 meter long, respectively. In both areas different arrays of deuterated benzene scintillators, namely C6D6[6] and sTED [7] have been used. These detetection systems are characterized by low neutron-sensitivity and low γ-detection efficiency. The total-energy detection principle associated with the pulse height weighting technique (PHWT) ensures the proportionality between the detection efficiency and the corresponding energy of γ-rays coming from the cascade; see [8–10] for details. Measurements started at the experimental area EAR- 1. The incident proton-pulse coming from the Proton Syncrotron produces in EAR-1 an instantaneus flux of ≃ 106neutrons/cm2/proton-pulse spanning from thermal energies up to the GeV energy region. The main characteristics of the EAR-1 neutron flux is the high energy resolution ∆En/En.3.0 x 10−3for En≤100 keV [11], which makes it particularly suitable for the study of resonant structures at high definition. As previously mentioned, the experimental setup consists of four deuterated benzene liquid scintillation detectors (C6D6) mounted at 125◦relative to the neutron beam direction and at ≃10 cm upstream the sam- Figure 2. Experimental setup in EAR-1. Detectors placed at 125◦relative the neutron direction. Figure 3. Experimental setup in EAR-2. Eight sTED scintillators are placed on the same plane at ≃5 cm from the sample, while the C6D6are placed below at 125◦relative to the vertical neutron direction. ples position in order to minimize the effects of anisotropic emission (p-wave) of γcascades (see figure 2). To extract informations on the 160Gd(n, γ)161Gd reaction at energies below 1 eV, measurements were also performed at the high-flux experimental area (EAR-2) placed in the vertical direction at 19 m above the spallation target. Here, an enhanced signal/background ratio is ensured by shorter distance of EAR-2 from the lead target, the neutron beam has a 30 times higher flux (&107 neutrons/cm2/proton-pulse) than in EAR-1 although with a lower resolution (∆En/En=4.0 ×10−3at 1 eV, ∆En/En =4.0 ×10−2at 1 MeV) [12]. The detection setup in EAR-2 consisted of eight so-called Segmented Total Energy Detectors (sTED) and two C6D6. The sTED detectors, have been recently developed for (n, γ) measurements in EAR2; the smaller and segmented scintillation volume compared to conventional C6D6detectors, reduce pile-up effects at high counting rate and the gain shift linked to the prompt γ-flash arising from the spallation process. The sTED scintillators, are placed at ≃5.0 cm from the sample in a plane perpendicular to the neutron beam direction as shown in figure 3; see [13] for details. For each sample, data were collected in both experimental areas for a total number of protons available of 1.5×1018. In the following the preliminary results are shown. 4 Results A thorough analysis would require to use the total energy principle by combining the detection system with the Pulse Height Weighting Technique (PHWT) [8, 9]. Neverthe- EPJ Web of Conferences , 09002 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409002 3 1− 10 1 10 2 10 3 10 4 10 5 10 Neutron Energy (eV) 4− 10 3− 10 2− 10 Counts/E/bunch Gd-160 Dummy Empty Figure 4. Counts per neutron-bunch as a function of the neutron energy for Gd-160 compared to the dummy and the empty sample in the whole neutron energy range. 1− 10 1 10 2 10 3 10 4 10 5 10 Neutron Energy (eV) 4− 10 3− 10 2− 10 Counts/E/bunch Gd-160 Dummy Empty Figure 5. Count per neutron-bunch as a function of the neutron energy for Gd-160 compared to the dummy and the empty sample in the RRR; on average in the RRR Gd-160 related counts are 10% larger than dummy ones. less, preliminary unweighted count distributions provide a good idea of the data-quality collected. Figures 4, 5 and 6 show the counts distribution in the energy range of interest, measured in EAR-1. As expected the main background component arises from the PEEK capsule (see figures 4 and 5) whose contribution can be estimated with the dummy-sample measurement. Figure 6 shows the count rate for the Au and the empty in the range of Au saturated resonance at 4.9 eV; here, the main background source, estimated by the empty-sample is the beam-related background, although this has a quite negligible effect. The count-rate for energies below 1 eV measured in EAR-2, is shown in figure 7. Also here the PEEK related background represents the largest contribution. Altogether, the count-rate related to neutron capture on Gd- 160 is more than 15% larger compared to the main background. 5 Conclusions The 160Gd(n,γ)161Gd reaction has been measured in the neutron energy range from thermal to a few hundreds of keV in both n_TOF experimental areas. Albeit an accurate analysis requires the use of PHWT, preliminary 1 10 Neutron Energy (eV) 4− 10 3− 10 2− 10 1− 10 Counts/E/bunch Au-197 Empty Figure 6. Au and empty count distribution in the range of the 197Au satured resonance at 4.9 eV. 2− 10 1− 10 1 Neutron Energy (eV) 2− 10 1− 10 Counts/E/bunch Gd-160 Dummy Empty Figure 7. Count distribution of sample measured in EAR-2 for neutron energies below 1.0 eV. results show good-data quality and an acceptable signal/background ratio. For a precise determination of the cross section, the implementation of the setup geometry, the samples size and composition in Monte Carlo simulations will allow the precise determination of the Weighting Functions for the PHWT. We estimate that Resonance Shape Analysis (RSA) with the bayesian SAMMY code will provide resonance kernels with an accurancy better than 5%. References [1] F. Käppeler et al., Rev. of Mod. Phys 83, 157 (2011) [2] C. Müller et al., Jour. of Nucl. Med. 53, 1951 (2012) [3] S. 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Lerendegui-Marco et al.,New perspectives for neutron capture measurements in the upgraded CERN n_TOF Facility, ND2022 Conference, EPJ Web of Conferences EPJ Web of Conferences , 09002 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409002 5