First high resolution measurement of neutron capture resonances in Yb-176 at the n TOF CERN facility

First high resolution measurement of neutron capture resonances in 176Yb at the n_TOF CERN facility. F. García-Infantes1,2, J. Praena1,2, A. Casanovas3, M. Mastromarco4,5, O. Aberle2, V. Alcayne6, S. Altieri7,8, S. Amaducci9, H. Amar Es-Sghir1, J. Andrzejewski10, V. Babiano-Suarez3, M. Bacak2, J. Balibrea3, S. Bennett11, A. P. Bernardes2, E. Berthoumieux12, D. Bosnar13, M. Busso14,15, M. Caamaño16, F. Calviño17, M. Calviani2, D. Cano-Ott6, D. M. Castelluccio18,19, F. Cerutti2, G. Cescutti20,21, S. Chasapoglou22, E. Chiaveri2,11, P. Colombetti23,24, N. Colonna4, P. C. Console Camprini18,19, G. Cortés17, M. A. Cortés-Giraldo25, L. Cosentino9, S. Cristallo14,26, M. Di Castro2, D. Diacono4, M. Diakaki22, M. Dietz27, C. Domingo-Pardo3, R. Dressler28, E. Dupont12, I. Durán16, Z. Eleme29, S. Fargier2, B. Fernández-Domínguez16, P. Finocchiaro9, S. Fiore18,30, V. Furman31, A. Gawlik-Ramięga10, G. Gervino23,24, S. Gilardoni2, E. González-Romero6, C. Guerrero25, F. Gunsing12, C. Gustavino30, J. Heyse32, D. G. Jenkins33, E. Jericha34, A. Junghans35, Y. Kadi2, T. Katabuchi36, I. Knapová37, M. Kokkoris22, Y. Kopatch31, M. Krtička37, D. Kurtulgil38, I. Ladarescu3, C. Lederer-Woods39, J. Lerendegui-Marco3, G. Lerner2, A. Manna19,40, T. Martínez6, A. Masi2, C. Massimi19,40, P. Mastinu41, F. Matteucci20,21, E. A. Maugeri28, A. Mazzone4,42, E. Mendoza6, A. Mengoni18,19, V. Michalopoulou22,2, P. M. Milazzo20, R. Mucciola14,15, F. Murtas 43, E. Musacchio-Gonzalez41, A. Musumarra44,45, A. Negret46, A. Oprea46, P. Pérez-Maroto25, N. Patronis29, J. A. Pavón-Rodríguez25,2, M. G. Pellegriti44, J. Perkowski10, C. Petrone46, L. Piersanti14,26, E. Pirovano27, S. Pomp47, I. Porras1, N. Protti7,8, J. M. Quesada25, T. Rauscher48, R. Reifarth38, D. Rochman28, Y. Romanets49, F. Romano44, C. Rubbia2, A. Sánchez6, M. Sabaté-Gilarte2, P. Schillebeeckx32, D. Schumann28, A. Sekhar11, A. G. Smith11, N. V. Sosnin39, M. Spelta19,40, M. E. Stamati29,2, G. Tagliente4, A. Tarifeño-Saldivia3, D. Tarrío47, N. Terranova18,43, P. Torres-Sánchez1, S. Urlass35,2, S. Valenta37, V. Variale4, P. Vaz49, D. Vescovi38, V. Vlachoudis2, R. Vlastou22, A. Wallner50, P. J. Woods39, T. Wright11, P. Žugec13 and The n_TOF Collaboration (www.cern.ch/ntof) 1University of Granada, Spain 2European Organization for Nuclear Research (CERN), Switzerland 3Instituto de Física Corpuscular, CSIC - Universidad de Valencia, Spain 4Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy 5Dipartimento Interateneo di Fisica, Università degli Studi di Bari, Italy 6Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain 7Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Italy 8Department of Physics, University of Pavia, Italy 9INFN Laboratori Nazionali del Sud, Catania, Italy 10University of Lodz, Poland 11University of Manchester, United Kingdom 12CEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France 13Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia 14Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Italy 15Dipartimento di Fisica e Geologia, Università di Perugia, Italy 16University of Santiago de Compostela, Spain 17Universitat Politècnica de Catalunya, Spain 18Agenzia nazionale per le nuove tecnologie (ENEA), Italy 19Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 20Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy 21Department of Physics, University of Trieste, Italy 22National Technical University of Athens, Greece 23Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Italy 24Department of Physics, University of Torino, Italy 25Universidad de Sevilla, Spain 26Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Teramo, Italy 27Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany 28Paul Scherrer Institut (PSI), Villigen, Switzerland 29University of Ioannina, Greece 30Istituto Nazionale di Fisica Nucleare, Sezione di Roma1, Roma, Italy 31Joint Institute for Nuclear Research (JINR), Dubna, Russia 32European Commission, Joint Research Centre (JRC), Geel, Belgium 33University of York, United Kingdom 34TU Wien, Atominstitut, Stadionallee 2, 1020 Wien, Austria EPJ Web of Conferences , 09001 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409001 © 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 35Helmholtz-Zentrum Dresden-Rossendorf, Germany 36Tokyo Institute of Technology, Japan 37Charles University, Prague, Czech Republic 38Goethe University Frankfurt, Germany 39School of Physics and Astronomy, University of Edinburgh, United Kingdom 40Dipartimento di Fisica e Astronomia, Università di Bologna, Italy 41INFN Laboratori Nazionali di Legnaro, Italy 42Consiglio Nazionale delle Ricerche, Bari, Italy 43INFN Laboratori Nazionali di Frascati, Italy 44Istituto Nazionale di Fisica Nucleare, Sezione di Catania, Italy 45Department of Physics and Astronomy, University of Catania, Italy 46Horia Hulubei National Institute of Physics and Nuclear Engineering, Romania 47Uppsala University, Sweden 48Department of Physics, University of Basel, Switzerland 49Instituto Superior Técnico, Lisbon, Portugal 50Australian National University, Canberra, Australia Abstract. Several international agencies recommend the study of new routes and new facilities for producing radioisotopes with application to nuclear medicine. 177Lu is a versatile radioisotope used for therapy and diagnosis (theranostics) of cancer with good success in neuroendocrine tumours that is being studied to be applied to a wider range of tumours. 177Lu is produced in few nuclear reactors mainly by the neutron capture on 176Lu. However, it could be produced at high-intensity acceleratorbased neutron facilities. The energy of the neutrons in accelerator-based neutron facilities is higher than in thermal reactors. Thus, experimental data on the 176Yb(n,γ) cross-section in the eV and keV region are mandatory to calculate accurately the production of 177Yb, which beta decays to 177Lu. At present, there are not experimental data available from thermal to 3 keV of the 176Yb(n,γ) cross-section. In addition, there is no data in the resolved resonance region (RRR). This contribution shows the first results of the 176Yb capture measurement performed at the n_TOF facility at CERN. 1 Introduction Nuclear medicine has proven to be a much needed medical specialty in order to diagnose and treat several diseases, among them, cardiovascular diseases and cancer, the first and the second causes of mortality worldwide, respectively [1]. Diagnosis represents 90% of the procedures in nuclear medicine and the most used radioisotope is the 99Mo/99mTc which is produced in nuclear reactors [2]. For this reason, the so-called “world technetium crisis” in 2009 was a first alarm to find out new procedures and facilities to produce radioisotopes for nuclear medicine [3,4]. In addition, in the last decades more than 3000 new radioactive isotopes have been discovered in different nuclear physics facilities. Many of them could have properties that make them useful for nuclear medicine and potentially they could have better properties for diagnosis and therapy than the conventional ones. In this framework, several international agencies and committees recommend the study of new routes for producing radioisotopes with application to nuclear medicine [2,3,5,6]. This has been specially pushed in the last years with the development and the availability of high-intensity accelerators and new installations. In addition, these new installations can provide several radioisotopes quantities at regional level. CERN’s MEDICIS at Isolde facility is an excellent example [7]. Besides the charged-particle facilities also acceleratorbased neutron sources are being considered for radioisotope production. In this context, IFMIF-DONES (International Fusion Material Irradiation Facility - Demonstration Neutron Source) is one of the possible facilities to be used for this. It is an ESFRI (European Strategy Forum on Research Infrastructures), and the European city host is Granada (Spain) [8]. Its main objective is the irradiation of materials for fusion reactor technology. IFMIF-DONES has already considered the production of radioisotopes as one of the main complementary applications of the facility [9]. Indeed, the design of the building includes an experimental hall for such applications. Furthermore, 177Lu (t1/2≈6.65 d) is one the of most important emergent radioisotopes [10]. It is used for theranostics (therapy and diagnosis), with good success in gastroenteropancreatic neuroendocrine tumours [11]. Currently, 177Lu is under study for several other tumours with good results [12]. At present, 177Lu is only produced in nuclear reactors through two production routes: the direct route, 176Lu(n,γ)177,177mLu; and the indirect route, 176Yb(n,γ)177Yb (t1/2≈1.9 h) →177Lu + 177mLu [13]. At present, the most used production route is the direct route, 176Lu(n,γ). Although the cross-section of the direct route is higher, several advantages in the indirect route have been pointed out: i) The specific activity is four times higher [13]. About 100% of the theoretical specific activity can be achieved [14,15]. ii) The contaminants that remain in the final quantity of the material are much lower. iii) The undesirable 177mLu (t1/2≈160 d) is produced in the direct route, 0.05%, whereas in the indirect route it is less than 10-5 % [16]. 177mLu creates important problems, EPJ Web of Conferences , 09001 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409001 2 the urine of the patient must be treated as radioactive waste, and the patient receives an undesirable dose. These properties have a direct impact on the quality of the diagnosis and the therapy. The higher specific activity allows a much better tumour uptake; thus, the dose deliver to tumour for the same activity is much higher in case of the indirect route, and in addition, the quality of imaging of the tumour is much better [15,16]. In order to investigate the feasibility of producing 177Lu in neutron irradiation facilities by the indirect route, it is crucial to have accurate nuclear data of the reaction 176Yb(n,γ). However, at present there is data only from one measurement by Wisshak et al [17], from 8 to 100 keV energy range. None of the resonances of the reaction were measured in this experiment. In addition, discrepancies are found with other experiments carried out by means of integrated cross-section in this energy range. There are several important facts that indicate a highresolution measurement is needed. The most relevant are: i) There is no data from thermal to 3 keV. ii) There is no high-resolution measurement above 3keV of the 176Yb(n,γ) cross-section. However, resonances have been detected in transmission experiments [18]. Regarding nuclear data libraries, ENDF/B-VIII.0 [19] and JEFF-3.3 [20] use different upper limits for the resolved resonance region (RRR), 5 and 50 keV, respectively. Therefore, the measurement presented here will provide, for the first time, high resolution data of the 176Yb(n,γ) cross section from thermal up to 100 keV, covering the full range of the RRR. 2 Experiment at the n_TOF (CERN) facility The neutron capture cross section of 176Yb has been measured by means of the Time-of-Flight technique at the neutron Time-of-Flight (n_TOF) facility at CERN. At n_TOF, pulsed neutron beams are produced by spallation of proton beams from CERN PS accelerator into a massive lead target. Experiments are performed in two Experimental AReas (EAR): EAR1 has very high resolution in energy due to its 185 m flighpath, whereas EAR2, which is only 20 m away from the target, has worse resolution but much higher flux. In the CERN’s Second Long Shutdown (2019-2020), the facility has gone through a major upgrade, consisting of the installation of a new spallation target designed to fully optimise the features of both beam lines [23]. In 2021, a full commissioning campaign was carried out in order to characterize the neutron flux of both EARs with the new target [25,26]. Aiming at the highest possible in resolution, the experiment was performed in EAR1. The 176Yb(n,γ) reaction was measured by employing four custom-made C6D6 liquid scintillation detectors, optimized for an extremely low neutron sensitivy. The yield of the reaction is determined by applying the Total Energy Detection (TED) [24,27,28]. Fig. 1. Experimental Set-Up with four custom C6D6 scintillation detectors, and the 176Yb2O3 enriched sample in position. These detectors were positioned at 125° to the beam position to reduce the in-beam gamma ray background and minimize the effects of the primary radiation angular distribution [29], Fig. 1 shows the experimental set-up. In addition, a SiMon detector, cosisting in an array of four silicon detectors facing a thin enriched lithium fluoride foil, was used to monitor the neutron flux during the experiment. The sample used for the experiment consisted in a 176Yb2O3 enriched to 99.43% purity, with a weight of 1.5976 g. The powder was pressed into a quartz capsule with internal diamater and thickness of 19 and 2 mm, respectively. The sample and the quartz capsule are shown in Fig. 2. Fig. 2. The left image shows the 176Yb2O3 enriched sample put in place, and the right image shows the quartz capsule put in place. The measurement was carried out at the end of the 2021 campaign at n_TOF. In addition to the 176Yb2O3 sample, additional ancillary measurements were performed: a measurement on a gold sample, whose capture cross section is accurately known, was performed for absolute yield normalization by applying the saturated resonance technique [30]; an empty 176Yb quartz capsule was used to measure its contribution to the background, and lead and carbon samples were empoyed to estimate the background due to scattering of neutrons and in beam gamma rays in the 176Yb sample. In addition, calibration runs with radioctive sources were perfomed on a weekly basis to ensure an accurate amplitude-to-energy calibration throughout all the measurement. The total number of counts as a function of neutron energy with the ytterbium sample are shown in Fig. 3, for EPJ Web of Conferences , 09001 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409001 3 one detector. Also, beam-related background to the quartz capsule (Dummy Sample), beam-related background (Empty) and no-beam-related background (BeamOFF) are also shown. Fig. 3. Total counting rate per pulse of 176Yb (5000 bins/decade) and contribution of the different background components (500 bins/decade). Background contribution of scattered neutrons and in-beam γ-rays not included yet. As previously mentioned, in order to determine correctly the capture yield with C6D6 detectors it is necessary to apply the TED technique. For this, two conditions must be fulfilled: i) no more than one γ-ray is detected per capture cascade, and ii) the detection efficiency is proportional to the γ-ray energy. Fulfilling these two conditions, we obtain that the efficiency is proportional to the energy of the cascade, and therefore, independent of the de-excitation path. It means: εC=αEC=α(Sn+En), (1) where where Sn is the neutron separation energy of the resulting nucleus and En is the energy of the incident neutron. The first condition can be easily achieved by using small volume and low Z gamma ray detectors, like the C6D6 detectors used for this measurement. In order to fulfil the second condition we applied the Pulse Height Weighting Technique (PHWT), which has been validated to perform measurements at n_TOF [27]. Fig. 4. The blue line represents the weighted counts for the Ytterbium sample, the red line represent the unweighted counts for the Ytterbium sample. The unweighted count has been normalized to the weighted counts at the first ytterbium resonance. In this technique, each count detected is weighted according to its amplitude by a factor, given by a Weighting Function (WF). To calculate the WF it is necessary to obtain the response function of the C6D6 detectors to monoenergetic g rays in the energy range of interest, which is accomplished by employing detailed MC simulations of the full detection setup [31]. Fig 4 shows a comparison between the weighted and the unweighted counts per pulse spectra measured with the 176Yb sample, in the neutron energy range between 100 and 1000 eV. No background subtraction has been applied. The non-weighted counts has been normalized with respect to the weighted counts, at the first resonance (~98 eV, according to resonances reported in ENDF). 3 Results After applying the PWHT, the experimental yield Yexp can be expressed as: Yexp(En)=fNorm·fCorr.·{Cw(En)-Bw(En)}/{Φn(En)·(Sn+En)} (2) where Cw are the weighted sample counts and Bw are the weighted background counts, (Sn+En) is the full energy of the capture γ-ray cascade, Φn is the neutron fluence intersecting the sample, fNorm is a normalization factor and fCorr is a correction factor. As mentioned above, the evaluation of the flux is ongoing, therefore a preliminary “flux” has been employed for the present work. Fig. 5 shows the 176Yb(n, γ) reaction yield in the RRR interval from 0.1 to 1 keV. Fig. 5. The top plot shows the preliminary yield of the reaction from 100 eV to 450 eV, where the first ytterbium resonances are found. The bottom plot shows the preliminary yield of the reaction from 450 eV to 1 keV. EPJ Web of Conferences , 09001 (2023)284 ND2022 https://doi.org/10.1051/epjconf/202328409001 4 4 Conclusions 176Yb capture cross section is required for an in-depth study of 177Lu production, via the indirect route, in future high-intensity accelerators as IFMIF-DONES. The resonances region is of great interest for this purpose, with the corresponding set-up it will be possible to maximize the production in that energy region. However, the available experimental data only covers the neutron energies at thermal and in the range from ~5 to ~200 keV, respetively. The present work will allow for a high-resolution measurement of the cross section from thermal up to several keV, covering the full range of the RRR. Finally, an in-depth study we will be able to extend our analysis into the URR. In summary, a high-resolution measurement of the 176Yb(n,g) cross section was succesfully performed at n_TOF EAR1 in 2021. The first preliminary experimental results presented here show that an accurate analysis of the resonace reigion will be possible. The next steps towards the determination of the capture yield will include the calculation of several experimental correction factors, and an in-depth study of the capture gray cascades of 176Yb+n. In parallel, the evaluation of the neutron flux of EAR1 is currently ongoing [25]. Finally, in the last step of the analysis the capture cross section will be extracted from the experimental yield by employing the R-Matrix bayesian code SAMMY [32]. Acknowledgments We acknowledge to Richard Henkelmann (ITG Company) and Ulli Koester (ILL) for the 176Yb2O3 sample. F.G.I aknowledges the CERN doctoral student programme. This work was partial financial supported from the Spanish Ministerio de Ciencia e Innovación (Proyectos de I+D+i: PID2020-117969RB-I00), and Junta de Andalucía projects P20-00665 and B-FQM-156- UGR20. 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