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The n_TOF Collaboration (www.cern.ch/ntof)

# Measurement and analysis of the 246Cm and 248Cm neutron capture cross-
sections at the EAR2 of the n_TOF facility at CERN

V. Alcayne  Centro de Investigaciones Energéticas Medioambientales y
Tecnológicas (CIEMAT), Spain  A. Kimura  Japan Atomic Energy Agency (JAEA),
Tokai-Mura, Japan  E. Mendoza  Centro de Investigaciones Energéticas
Medioambientales y Tecnológicas (CIEMAT), Spain  D. Cano-Ott  Centro de
Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain
O. Aberle  European Organization for Nuclear Research (CERN), Switzerland  F.
Álvarez-Velarde  Centro de Investigaciones Energéticas Medioambientales y
Tecnológicas (CIEMAT), Spain  S. Amaducci  INFN Laboratori Nazionali del Sud,
Catania, Italy  Dipartimento di Fisica e Astronomia, Università di Catania,
Italy  J. Andrzejewski  University of Lodz, Poland  L. Audouin  Institut de
Physique Nucléaire, CNRS-IN2P3, Univ. Paris-Sud, Université Paris-Saclay,
F-91406 Orsay Cedex, France  V. Bécares  Centro de Investigaciones Energéticas
Medioambientales y Tecnológicas (CIEMAT), Spain  V. Babiano-Suarez  Instituto
de Física Corpuscular, CSIC - Universidad de Valencia, Spain  M. Bacak
European Organization for Nuclear Research (CERN), Switzerland  TU Wien,
Atominstitut, Stadionallee 2, 1020 Wien, Austria  CEA Irfu, Université Paris-
Saclay, F-91191 Gif-sur-Yvette, France  M. Barbagallo  European Organization
for Nuclear Research (CERN), Switzerland  Istituto Nazionale di Fisica
Nucleare, Sezione di Bari, Italy  F. Bečvář  Charles University, Prague, Czech
Republic  G. Bellia  INFN Laboratori Nazionali del Sud, Catania, Italy
Dipartimento di Fisica e Astronomia, Università di Catania, Italy  E.
Berthoumieux  CEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette,
France  J. Billowes  University of Manchester, United Kingdom  D. Bosnar
Department of Physics, Faculty of Science, University of Zagreb, Zagreb,
Croatia  A. Brown  University of York, United Kingdom  M. Busso  Istituto
Nazionale di Fisica Nucleare, Sezione di Perugia, Italy  Dipartimento di
Fisica e Geologia, Università di Perugia, Italy  M. Caamaño  University of
Santiago de Compostela, Spain  L. Caballero-Ontanaya  Instituto de Física
Corpuscular, CSIC - Universidad de Valencia, Spain  F. Calviño  Universitat
Politècnica de Catalunya, Spain  M. Calviani  European Organization for
Nuclear Research (CERN), Switzerland  A. Casanovas  Universitat Politècnica de
Catalunya, Spain  F. Cerutti  European Organization for Nuclear Research
(CERN), Switzerland  Y. H. Chen  Institut de Physique Nucléaire, CNRS-IN2P3,
Univ. Paris-Sud, Université Paris-Saclay, F-91406 Orsay Cedex, France  E.
Chiaveri  European Organization for Nuclear Research (CERN), Switzerland
University of Manchester, United Kingdom  Universidad de Sevilla, Spain  N.
Colonna  Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy  G.
Cortés  Universitat Politècnica de Catalunya, Spain  M. A. Cortés-Giraldo
Universidad de Sevilla, Spain  L. Cosentino  INFN Laboratori Nazionali del
Sud, Catania, Italy  S. Cristallo  Istituto Nazionale di Fisica Nucleare,
Sezione di Perugia, Italy  Istituto Nazionale di Astrofisica - Osservatorio
Astronomico di Teramo, Italy  L. A. Damone  Istituto Nazionale di Fisica
Nucleare, Sezione di Bari, Italy  Dipartimento Interateneo di Fisica,
Università degli Studi di Bari, Italy  M. Diakaki  National Technical
University of Athens, Greece  European Organization for Nuclear Research
(CERN), Switzerland  M. Dietz  School of Physics and Astronomy, University of
Edinburgh, United Kingdom  C. Domingo-Pardo  Instituto de Física Corpuscular,
CSIC - Universidad de Valencia, Spain  R. Dressler  Paul Scherrer Institut
(PSI), Villigen, Switzerland  E. Dupont  CEA Irfu, Université Paris-Saclay,
F-91191 Gif-sur-Yvette, France  I. Durán  University of Santiago de
Compostela, Spain  Z. Eleme  University of Ioannina, Greece  B. Fernández-
Domínguez  University of Santiago de Compostela, Spain  A. Ferrari  European
Organization for Nuclear Research (CERN), Switzerland  P. Finocchiaro  INFN
Laboratori Nazionali del Sud, Catania, Italy  V. Furman  Affiliated with an
institute or an international laboratory covered by a cooperation agreement
with CERN  K. Göbel  Goethe University Frankfurt, Germany  R. Garg  School of
Physics and Astronomy, University of Edinburgh, United Kingdom  A. Gawlik-
Ramiega  University of Lodz, Poland  S. Gilardoni  European Organization for
Nuclear Research (CERN), Switzerland  T. Glodariu† Horia Hulubei National
Institute of Physics and Nuclear Engineering, Romania  I. F. Gonçalves
Instituto Superior Técnico, Lisbon, Portugal  E. González-Romero  Centro de
Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain
C. Guerrero  Universidad de Sevilla, Spain  F. Gunsing  CEA Irfu, Université
Paris-Saclay, F-91191 Gif-sur-Yvette, France  H. Harada  Japan Atomic Energy
Agency (JAEA), Tokai-Mura, Japan  S. Heinitz  Paul Scherrer Institut (PSI),
Villigen, Switzerland  J. Heyse  European Commission, Joint Research Centre
(JRC), Geel, Belgium  D. G. Jenkins  University of York, United Kingdom  E.
Jericha  TU Wien, Atominstitut, Stadionallee 2, 1020 Wien, Austria  F.
Käppeler† Karlsruhe Institute of Technology, Campus North, IKP, 76021
Karlsruhe, Germany  Y. Kadi  European Organization for Nuclear Research
(CERN), Switzerland  N. Kivel  Paul Scherrer Institut (PSI), Villigen,
Switzerland  M. Kokkoris  National Technical University of Athens, Greece  Y.
Kopatch  Joint Institute for Nuclear Research (JINR), Dubna, Russia  M.
Krtička  Charles University, Prague, Czech Republic  D. Kurtulgil  Goethe
University Frankfurt, Germany  I. Ladarescu  Instituto de Física Corpuscular,
CSIC - Universidad de Valencia, Spain  C. Lederer-Woods  School of Physics and
Astronomy, University of Edinburgh, United Kingdom  H. Leeb  TU Wien,
Atominstitut, Stadionallee 2, 1020 Wien, Austria  J. Lerendegui-Marco
Universidad de Sevilla, Spain  S. Lo Meo  Agenzia nazionale per le nuove
tecnologie (ENEA), Bologna, Italy  Istituto Nazionale di Fisica Nucleare,
Sezione di Bologna, Italy  S. J. Lonsdale  School of Physics and Astronomy,
University of Edinburgh, United Kingdom  D. Macina  European Organization for
Nuclear Research (CERN), Switzerland  A. Manna  Istituto Nazionale di Fisica
Nucleare, Sezione di Bologna, Italy  Dipartimento di Fisica e Astronomia,
Università di Bologna, Italy  T. Martínez  Centro de Investigaciones
Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain  A. Masi  European
Organization for Nuclear Research (CERN), Switzerland  C. Massimi  Istituto
Nazionale di Fisica Nucleare, Sezione di Bologna, Italy  Dipartimento di
Fisica e Astronomia, Università di Bologna, Italy  P. Mastinu  Istituto
Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy  M. Mastromarco
European Organization for Nuclear Research (CERN), Switzerland  F. Matteucci
Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy  Dipartimento
di Astronomia, Università di Trieste, Italy  E. A. Maugeri  Paul Scherrer
Institut (PSI), Villigen, Switzerland  A. Mazzone  Istituto Nazionale di
Fisica Nucleare, Sezione di Bari, Italy  Consiglio Nazionale delle Ricerche,
Bari, Italy  A. Mengoni  Agenzia nazionale per le nuove tecnologie (ENEA),
Bologna, Italy  V. Michalopoulou  National Technical University of Athens,
Greece  P. M. Milazzo  Istituto Nazionale di Fisica Nucleare, Sezione di
Trieste, Italy  F. Mingrone  European Organization for Nuclear Research
(CERN), Switzerland  A. Musumarra  INFN Laboratori Nazionali del Sud, Catania,
Italy  Dipartimento di Fisica e Astronomia, Università di Catania, Italy  A.
Negret  Horia Hulubei National Institute of Physics and Nuclear Engineering,
Romania  R. Nolte  Physikalisch-Technische Bundesanstalt (PTB), Bundesallee
100, 38116 Braunschweig, Germany  F. Ogállar  University of Granada, Spain  A.
Oprea  Horia Hulubei National Institute of Physics and Nuclear Engineering,
Romania  N. Patronis  University of Ioannina, Greece  A. Pavlik  University of
Vienna, Faculty of Physics, Vienna, Austria  A. Pérez de Rada  Centro de
Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain
J. Perkowski  University of Lodz, Poland  L. Persanti  Istituto Nazionale di
Fisica Nucleare, Sezione di Bari, Italy  Istituto Nazionale di Fisica
Nucleare, Sezione di Perugia, Italy  Istituto Nazionale di Astrofisica -
Osservatorio Astronomico di Teramo, Italy  I. Porras  University of Granada,
Spain  J. Praena  University of Granada, Spain  J. M. Quesada  Universidad de
Sevilla, Spain  D. Radeck  Physikalisch-Technische Bundesanstalt (PTB),
Bundesallee 100, 38116 Braunschweig, Germany  D. Ramos-Doval  Institut de
Physique Nucléaire, CNRS-IN2P3, Univ. Paris-Sud, Université Paris-Saclay,
F-91406 Orsay Cedex, France  T. Rauscher  Department of Physics, University of
Basel, Switzerland  Centre for Astrophysics Research, University of
Hertfordshire, United Kingdom  R. Reifarth  Goethe University Frankfurt,
Germany  D. Rochman  Paul Scherrer Institut (PSI), Villigen, Switzerland  Y.
Romanets  Instituto Superior Técnico, Lisbon, Portugal  C. Rubbia  European
Organization for Nuclear Research (CERN), Switzerland  M. Sabaté-Gilarte
European Organization for Nuclear Research (CERN), Switzerland  Universidad de
Sevilla, Spain  A. Saxena  Bhabha Atomic Research Centre (BARC), India  P.
Schillebeeckx  European Commission, Joint Research Centre (JRC), Geel, Belgium
D. Schumann  Paul Scherrer Institut (PSI), Villigen, Switzerland  A. G. Smith
University of Manchester, United Kingdom  N. V. Sosnin  University of
Manchester, United Kingdom  A. Stamatopoulos  National Technical University of
Athens, Greece  G. Tagliente  Istituto Nazionale di Fisica Nucleare, Sezione
di Bari, Italy  J. L. Tain  Instituto de Física Corpuscular, CSIC -
Universidad de Valencia, Spain  T. Talip  Paul Scherrer Institut (PSI),
Villigen, Switzerland  A. Tarifeño-Saldivia  Universitat Politècnica de
Catalunya, Spain  L. Tassan-Got  European Organization for Nuclear Research
(CERN), Switzerland  National Technical University of Athens, Greece  Institut
de Physique Nucléaire, CNRS-IN2P3, Univ. Paris-Sud, Université Paris-Saclay,
F-91406 Orsay Cedex, France  P. Torres-Sánchez  University of Granada, Spain
A. Tsinganis  European Organization for Nuclear Research (CERN), Switzerland
J. Ulrich  Paul Scherrer Institut (PSI), Villigen, Switzerland  S. Urlass
European Organization for Nuclear Research (CERN), Switzerland  Helmholtz-
Zentrum Dresden-Rossendorf, Germany  S. Valenta  Charles University, Prague,
Czech Republic  G. Vannini  Istituto Nazionale di Fisica Nucleare, Sezione di
Bologna, Italy  Dipartimento di Fisica e Astronomia, Università di Bologna,
Italy  V. Variale  Istituto Nazionale di Fisica Nucleare, Sezione di Bari,
Italy  P. Vaz  Instituto Superior Técnico, Lisbon, Portugal  A. Ventura
Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy  V.
Vlachoudis  European Organization for Nuclear Research (CERN), Switzerland  R.
Vlastou  National Technical University of Athens, Greece  A. Wallner
Australian National University, Canberra, Australia  P. J. Woods  School of
Physics and Astronomy, University of Edinburgh, United Kingdom  T. Wright
University of Manchester, United Kingdom  P. Žugec  Department of Physics,
Faculty of Science, University of Zagreb, Zagreb, Croatia

###### Abstract

The 246Cm(n,$\gamma$) and 248Cm(n,$\gamma$) cross-sections have been measured
at the Experimental Area 2 (EAR2) of the n_TOF facility at CERN with three
C6D6 detectors. This measurement is part of a collective effort to improve the
capture cross-section data for Minor Actinides (MAs), which are required to
estimate the production and transmutation rates of these isotopes in light
water reactors and innovative reactor systems. In particular, the neutron
capture in 246Cm and 248Cm open the path for the formation of other Cm
isotopes and heavier elements such as Bk and Cf and the knowledge of
(n,$\gamma$) cross-sections of these Cm isotopes plays an important role in
the transport, transmutation and storage of the spent nuclear fuel. The
reactions 246Cm(n,$\gamma$) and 248Cm(n,$\gamma$) have been the two first
capture measurements analyzed at n_TOF EAR2. Until this experiment and two
recent measurements performed at J-PARC, there was only one set of data of the
capture cross-sections of 246Cm and 248Cm, that was obtained in 1969 in an
underground nuclear explosion experiment. In the measurement at n_TOF a total
of 13 resonances of 246Cm between 4 and 400 eV and 5 of 248Cm between 7 and
100 eV have been identified and fitted. The radiative kernels obtained for
246Cm are compatible with JENDL-5, but some of them are not with JENDL-4,
which has been adopted by JEFF-3.3 and ENDF/B-VIII.0. The radiative kernels
obtained for the first three 248Cm resonances are compatible with JENDL-5,
however, the other two are not compatible with any other evaluation and are
20% and 60% larger than JENDL-5.

##  I Introduction

Accurate neutron capture cross-section data for Minor Actinides (MAs) are
required for accurate neutronic calculations of nuclear systems (light water
reactors, fast reactors, and accelerator-driven systems) and for determining
quantities relevant to the transport, transmutation, and storage of the
nuclear fuel, such as the radiotoxicity, the decay heat, or the neutron
emission rates. In particular, the capture cross-sections of 246Cm and 248Cm
are important for determining the content of these isotopes in the spent
nuclear fuel and because they open the path for the formation of heavier Cm
isotopes and elements such as Bk and Cf. In addition, 246Cm is important since
it is one of the main neutron emitters after 300 years of cooling down
Aliberti  _et al._ (2006).

The neutron capture cross-sections of 246Cm and 248Cm (246,248Cm) have been
measured at n_TOF Experimental Area 2 (EAR2) Weiss  _et al._ (2015); Sabaté-
Gilarte  _et al._ (2017a) with three C6D6 detectors Plag  _et al._ (2003) and
a dedicated sample. In addition to the 246,248Cm sample, another one was
measured in the same experimental campaign at n_TOF to obtain the capture
cross-section of 244Cm Alcayne  _et al._ (2019, 2020, 2023). In this case, the
measurement was performed in two areas of the n_TOF facility: Experimental
Area 1 (EAR1) Guerrero  _et al._ (2013) and EAR2. The final results of the
244Cm measurements will be published in a future article. The samples used in
the experiments were provided by Japan Atomic Energy Agency (JAEA), which used
material from the same batch to perform another two capture measurements with
the ANNRI setup at J-PARC Kimura  _et al._ (2012); Kawase  _et al._ (2021).
The only other previous capture measurements of these isotopes were performed
in 1969 with the neutrons produced by an underground nuclear explosion Moore
and Keyworth (1971).

In this paper, the experimental setup is described in Sec. II, the procedure
for obtaining the experimental capture yields is presented in Sec. III, the
analysis of the yields for retrieving the capture cross-sections is detailed
in Sec. IV and the conclusions are presented in Sec. V.

##  II Experimental setup

###  II.1 n_TOF EAR2

Neutrons at n_TOF are produced by spallation reactions with a 20 GeV/c pulsed
proton beam that impinges on a lead block. The pulses have a nominal intensity
of $7\times 10^{12}$ protons and a time spread of 7 ns Root Mean Square (RMS)
Guerrero  _et al._ (2013). There are two experimental areas at n_TOF: the EAR1
located at $\sim$185 m Guerrero  _et al._ (2013) in the horizontal direction,
and the EAR2 Weiss  _et al._ (2015), located at $\sim$20 m in the vertical
direction.

The neutron fluence in EAR2 is $\sim$40 times larger than in EAR1 Sabaté-
Gilarte  _et al._ (2017a). In addition, EAR2 is $\sim$10 times closer to the
target than EAR1, so the signal-to-background ratio produced by the natural
radioactivity of the samples is improved by more than two orders of magnitude
compared to EAR1. These features have made possible new measurements with
shorter half-lives isotopes (i.e. highly radioactive), smaller capture cross-
sections and/or smaller samples Barbagallo  _et al._ (2016); Sabaté-Gilarte
_et al._ (2017b); Damone  _et al._ (2018); Stamatopoulos  _et al._ (2020).

###  II.2 Detection setup

The prompt $\gamma$-rays emitted in the (n,$\gamma$) reactions were detected
with three BICRON detectors filled with C6D6 liquid Plag  _et al._ (2003),
whose front face is located at 5 cm from the center of the sample. A general
view of the experimental setup is shown in the top panel of Fig. 1. C6D6
detectors have been used since 2006 for many capture measurements at the n_TOF
facility Marrone  _et al._ (2006); Domingo-Pardo  _et al._ (2006); Casanovas
_et al._ (2020), including also measurements of actinides Gunsing  _et al._
(2012); Fraval  _et al._ (2014); Mingrone  _et al._ (2017); Mastromarco  _et
al._ (2017); Lerendegui-Marco  _et al._ (2018). The main advantages of these
detectors are the low neutron sensitivity and good time resolution, with
pulses of $\sim$10 ns Full Width Half Maximum.

Figure 1: At the top panel, a picture of the experimental setup used in the
measurement consisting of three C6D6 detectors. In the bottom panel, the
geometry implemented in Geant4 Agostinelli  _et al._ (2003) to simulate the
detection system response.

Two additional detectors have been used to monitor the neutron beam. A wall
current monitor detector measures the proton current in each pulse before
impinging on the lead block. The other detector is a silicon monitor (SiMon2)
Cosentino and others. (2015) used to measure the neutron fluence at EAR2. It
consists of four silicon pad detectors with a surface of $3\times 3$ cm2 and a
thickness of 300 $\mu$m. The four detectors are placed outside the beam
looking to an in-beam 6LiF foil for detecting the products of the 6Li(n,t)
reaction. The cross-section for this reaction is standard from thermal to 1
MeV Carlson  _et al._ (2009).

The signals from all the individual detectors have been recorded by the n_TOF
Digital Acquisition System (DAQ) U. Abbondanno  _et al._ (2005), based on
SPDevices ADQ412DC-3G cards with 1 GHz sampling rate and 14-bit resolution.
The data from the silicon monitor and the C6D6 detectors are analyzed using a
pulse shape routine to extract the amplitude and time of the signals Žugec
(2016); C. Guerrero  _et al._ (2008). This information and the detector
numbers have been stored in ROOT files Brun and Rademakers (1997) for their
posterior analysis.

###  II.3 Cm sample

The Cm sample used in the experiment was acquired by the JAEA from the Russia
Research Institute of Atomic Research in 2007. The sample consists of a Cm
oxide pellet of dimensions 2.5 mm (radius) x 0.5 mm (height) enclosed in an
aluminum casing of dimensions 4.5 mm (radius) x 1.2 mm (height). Pellets
created with the same batch material were used in the Kimura et al. Kimura
_et al._ (2012) and Kawase et al. Kawase  _et al._ (2021) experiments.

The pellet contains approximately 1.1 mg of 246Cm and 0.2 mg of 248Cm. The
absolute masses of the different isotopes are not accurately known. However,
their relative fractions are precisely determined since molar isotopic
enrichment was well-established in 2010 by thermal ionization mass
spectrometry and $\alpha$-particle spectrometry using a small amount of the
same batch used to prepare the samples Kimura  _et al._ (2012). The mole
fractions of the isotopes at the time when the n_TOF measurement was performed
(summer 2017) are presented in Table 1. These values have been obtained from
the isotopic evolution of the values measured in 2010, taking into account the
radioactive decays.

Isotope | Mole fraction (%)
---|---
240Pu |  9.2 $\pm$ 0.2
243Am |  1.2 $\pm$ 0.2
244Cm | 20.1 $\pm$ 0.4
245Cm |  1.0 $\pm$ 0.3
246Cm | 57.0 $\pm$ 1.3
247Cm |  2.8 $\pm$ 0.4
248Cm |  8.7 $\pm$ 0.2
Table 1: Isotopic abundances in the sample, at the time of the measurement at
n_TOF (summer 2017).

As can be seen in Table 1, there is also a significant amount of 244Cm in the
sample. As mentioned above, the results for this isotope will be presented in
a future work, which would also include measurements performed with another
sample performed at EAR1 and EAR2 Alcayne  _et al._ (2019, 2020, 2023).

Together with Cm isotopes, the sample contains also 240Pu, which is the
daughter of 244Cm (T1/2 (244Cm): 18.11 years Chechev (2006)). As explained in
detail in Section IV.1.1 240Pu is used for the normalization of the capture
cross-sections of 246Cm and 248Cm.

In order to reduce the in-beam material the Cm sample was placed in the center
of the beam using a ring made with aluminum, Mylar and Kapton (Fig. 2).

Figure 2: Picture including the Cm sample (Al case), the Kapton strip
(Kapton), the Mylar foil (Mylar) and the outside aluminum ring (Al ring). The
Al ring inner diameter of 7 cm is considerably larger than the beam diameter,
which is less than 6 cm.

##  III Data analysis

###  III.1 Data Reduction

The energy calibration and energy resolution of each C6D6 detector were
determined by comparing the detector response to several $\gamma$-ray sources
obtained experimentally and via Monte Carlo simulations. The detailed geometry
implemented in Geant4 Agostinelli  _et al._ (2003) for the simulations is
shown in Fig. 1. After the calibration and energy resolution determination,
the detector response to the six $\gamma$-ray sources is accurately reproduced
with the simulations as shown in Fig. 3. The calibration procedure was
repeated every week during the 3 months of the experiment to monitor and
correct the small gain shifts (less than 7% along the entire experiment).

Figure 3: Simulated detector responses (dashed lines with different colors)
compared to measured spectra from calibration sources (in black).

After a careful analysis, we noticed that the detectors exhibit changes in
gain as a function of the time-of-flight (i.e. neutron energy ($E_{n}$)),
requiring a correction of the energy calibration. More details concerning this
effect are provided in Alcayne  _et al._ (2024). We attribute the origin of
the gain shifts to the effect of the EAR2 particle flash (consisting of
relativistic charged particles, high-energy neutrons, and prompt $\gamma$-rays
with very short time-of-flight, less than 1 $\mu s$) on the C6D6
photomultipliers. The gain of the detectors suddenly changes just after the
particle flash and it takes about 10 $\mu s$ to recover the values obtained
with the calibration sources. These 10 $\mu s$ corresponds, approximately, to
time-of-flight neutron energies of $\sim$0.02 eV.

The gain shift effect has been characterized for each detector as a function
of the neutron energy using an 88Y $\gamma$-ray source placed close to the
detectors in measurements with the neutron beam, as shown in Fig. 4. The
obtained shift values have been then fitted with a logarithmic function, shown
in Fig. 4, which was used to correct the energy calibration of each detector
as a function of the neutron energy. As described in detail in references
Alcayne  _et al._ (2024); Alcayne (2022) these gain shifts can not be
attributed to pile-up effects.

Figure 4: Experimental gain shift for each detector (Exp.) obtained with the
88Y measurement as a function of the neutron energy, i.e. time-of-flight. The
dashed lines are the fits of each detector response with a logarithmic
function.

The DAQ system used at n_TOF records all the signals. However, the analysis
routine may fail when fitting two consecutive pulses located very close one to
another. To correct for these pile-up effects, the paralyzable method
described in Knoll (2000) has been used, with a constant dead-time ($\tau$) of
20 ns. The maximum corrections applied to the yields due to the pile-up
effects are smaller than 2%.

###  III.2 Background

The background is defined as the events detected in the C6D6 detectors
originated from reactions other than (n,$\gamma$) in 246Cm or 248Cm. The
background can be divided into various components:

  1. (i)

Beam-off background produced by the activity of the sample. As presented in
Fig 5, the beam-off background is considerably low compared to other
background sources, due to the high instantaneous fluence of the EAR2.

  2. (ii)

Beam-on background determined with a Dummy sample identical to the one of Cm
but without any actinide (Fig. 5). The characterization of the complete
background not related to reactions on the actinides is obtained with this
measurement. The subtraction of this background is one of the main sources of
uncertainty in this experiment. To address this, we meticulously examined
regions between resonances, where actinide influence on the time-of-flight
spectrum is minimal. We ensured compatibility of counts between the dummy and
sample after subtracting the beam-off background in these regions. The
resulting average scaling factor of 1.0035(40) was utilized to scale the
background appropriately.

Figure 5: Top panel: Measured counting rate (0.12 $<$ Edep $<$ 6.0 MeV) as a
function of the neutron energy for the Cm sample (Total), together with the
beam-on and beam-off backgrounds (see the text for details). The units are
counts per pulse and per unit lethargy ($\Delta$LnE, which means that the bin
contents have been divided by the natural logarithm of the ratio between the
upper and lower bin limits). Bottom panel: measured deposited energy spectra
for the first resonance of 246Cm, between 4.2 and 4.35 eV, together with the
beam-on and beam-off backgrounds.

  3. (iii)

Regarding the background due to the interaction of the neutron beam with the
actinides in the sample the different reactions have been separated:

    1. (a)

Capture: in most cases, the resonances of 240Pu, 243Am, 244Cm, 245Cm, and
247Cm present in the sample (see Tab. 1), are sufficiently separated from
those of 246Cm and 248Cm, so they do not have any significant impact to the
analysis of the Resonance Parameters (RP) in the Resolved Resonance Region
(RRR). In any case, uncertainties in the RP parameters due to the presence of
these resonances are negligible.

    2. (b)

Elastic scattering: this component is negligible in the analyzed region, due
to the low neutron sensitivity of the detectors Plag  _et al._ (2003) and the
capture to elastic scattering cross-section ratio of the actinides in the
sample.

    3. (c)

Fission: the background due to fission reactions has been subtracted from the
capture yield in the resonance analysis, as described in Sec. IV. This
background component has been determined from the fission detection efficiency
($\varepsilon_{fis}$) and the fission cross-section present in the JENDL-4.0
evaluation. The JEFF-3.3 JEF (2017), JENDL-5 Iwamoto  _et al._ (2023) and
ENDF/B-VIII.0 Brown  _et al._ (2018) libraries have adopted the JENDL-4.0 for
the fission of these isotopes. The value of $\varepsilon_{fis}$ has been
determined by comparing the experimental yield for the first (2.94 eV) and
second (3.17 eV) resonances of 247Cm, which has a significant fission reaction
channel, and the evaluated cross-section from JENDL-4.0. It has been then
assumed that the $\varepsilon_{fis}$ values are the same for all the Cm
isotopes. The fit to the two 247Cm resonances to obtain $\varepsilon_{fis}$ is
presented in Fig. 6. The derived value of $\varepsilon_{fis}$ is 0.085(22),
which is about three times larger than the capture detection efficiency of the
actinides of the measurement (Table 3).

Figure 6: Experimental yield (Exp) of the Cm sample together with the partial
contribution of the different isotopes (243Am, 246Cm and 247Cm) calculated
with JENDL-4.0. The contribution of the 247Cm fission yield has been re-scaled
to fit the experimental data, thus obtaining $\varepsilon_{fis}$. The black
line (Sum) is the sum of the contributions of the different yields.

The uncertainty in the determination of the fission detection efficiency (25%)
has been estimated by considering 20 % uncertainty due to the 247Cm fission
cross-section in JENDL-4.0, a 15% uncertainty due to the uncertainty in the
abundance of 247Cm in the sample (Table 1), and a 5% uncertainty due to
counting statistics.

###  III.3 Pulse Height Weighting Technique (PHWT)

The efficiency to detect a capture cascade ($\varepsilon_{c}$) depends in
principle on the $\gamma$-cascade deexcitation pattern that may also depend on
the neutron energy. To avoid this dependency, the spectra from C6D6 detectors
have been traditionally analyzed with the Pulse Height Weighting Technique
(PHWT) Macklin and Gibbons (1967); Abbondanno  _et al._ (2004); E. Mendoza
_et al._ (2023). That is mainly based on the fulfillment of these two
conditions:

  1. (i)

The individual detector $\gamma$-ray efficiency ($\varepsilon_{\gamma}$) has
to be much smaller than one ($\varepsilon_{\gamma}$ $\ll$ 1), so that at most
one of the $\gamma$-rays of the cascade is detected in each detector.

  2. (ii)

$\varepsilon_{\gamma}$ has to be proportional to the energy of the
$\gamma$-ray ($E_{\gamma}$), $\varepsilon_{\gamma}$= $k\cdot E_{\gamma}$.

Assuming these conditions, $\varepsilon_{c}$ becomes almost proportional to
the sum energy of the $\gamma$-rays of the cascade ($E_{c}$), so it is
independent of the deexcitation pattern:

$\varepsilon_{c}=1-\prod_{j}(1-\varepsilon_{\gamma
j})\approx\sum_{j}\varepsilon_{\gamma j}\approx k\cdot E_{c}$ (1)

In this expression the $j$ index loops through all the $\gamma$-rays of the
cascade. For the actinides measured in this experiment, the neutron separation
energies ($S_{n}$) are of the order of 6 MeV, which is more than four orders
of magnitude larger than the maximum incident neutron analyzed energy of
$\sim$400 eV. Therefore, the sum energy of the resulting capture cascade can
be approximated as the neutron separation energy of the isotope
($E_{c}=S_{n}+E_{n}\cong S_{n}$).

The detector itself does not satisfy condition (ii) of the PHWT. Therefore, a
mathematical manipulation of the detector response is performed to establish a
proportional relationship between the detection efficiency and the energy of
the $\gamma$-rays. The counts registered at each deposited energy are weighted
by a factor depending on its energy (pulse height), given by the so-called
Weighting Function (WF).

Following the same procedure as in Abbondanno  _et al._ (2004), the WF for the
detection setup was calculated by simulating the detector response to 30
monoenergetic $\gamma$-rays from 0.1 to 10 MeV using Geant4. For practical
purposes, the WF was assumed to have the polynomial dependence (of 5th degree)
on the deposited energy and its final version was obtained from fitting
individual points with such a polynomial. Applying the obtained WF to the
detector response, the proportionality condition between the weighted counts
and Eγ is satisfied with an RMS deviation better than 0.5% for $\gamma$-rays
with $E_{\gamma}$ between 0.1 and 10 MeV.

###  III.4 Corrections to the PHWT

In practice, there are different experimental effects, which may disturb the
straightforward application of the PHWT technique. In our case, they are:

  1. (i)

The 0.12 MeV threshold set in each detector, which makes condition (ii) of the
PHWT technique unfulfilled for low energy $\gamma$-rays.

  2. (ii)

Effect of $\gamma$-ray summing, produced when two or more $\gamma$-rays are
detected in one detector simultaneously.

  3. (iii)

The internal electron conversion, leading to the emission of non-detectable
electrons instead of $\gamma$-rays.

To account for the deviations produced by these effects, correction factors
(FPHWT) are calculated for each isotope by performing Monte Carlo simulations
of the detection of (n,$\gamma$) cascades. The (n,$\gamma$) cascades of 240Pu,
246Cm and 248Cm necessary to determine the FPHWT have been obtained with the
NuDEX code E. Mendoza  _et al._ (2023). This code generates the full level
scheme of the nucleus, together with the branching ratios of each level, to
compute the (n,$\gamma$) cascades using models of photon strength functions
(PSFs) and level density (LD). The data are taken from the RIPL-3 Capote  _et
al._ (2009) and ENSDF ENS  databases, among others.

The (n,$\gamma$) cascades for 240Pu and 246Cm have been firstly obtained with
the default PSF and LD values given by NuDEX. As can be seen in Fig. 7 the
simulation of the detector response with the default NuDEX cascades does not
exactly reproduce the experimental spectra obtained for this reaction.
Therefore adjusted PSF and LD have been obtained and used for the cascades of
240Pu and 246Cm. The adjusted cascades of 240Pu have been obtained reproducing
the experimental data obtained with the Total Absorption Calorimeter (TAC)
Guerrero  _et al._ (2009), the process is described in detail in Mendoza  _et
al._ (2020); Alcayne (2022). The (n,$\gamma$) cascades of 246Cm have been
obtained by fitting the PSF and LD to the deposited energy spectrum obtained
for the first resonance at 4.3 eV. As shown in Fig. 7, the adjusted cascades
for 240Pu and 246Cm reproduce the experimental data significantly more
accurately than the default cascades generated by NuDEX.

Figure 7: Comparison of experimental and simulated deposited energy spectra of
240Pu(n,$\gamma$) (0.9 $<E_{n}<$ 1.1 eV)(top) and 246Cm(n,$\gamma$) (4.2
$<E_{n}<$ 4.35 eV) (bottom). The experimental spectra (Exp) are compared with
the simulated spectra obtained with the default NuDEX cascades (Default NuDEX)
and with the cascades obtained after adjusting the PSF parameters to reproduce
the experimental spectra (Adjusted). This plot shows data only for one of the
detectors, hereafter the same procedure would be followed for simplicity.

Unfortunately, we were unable to extract any spectrum for 248 due to the lack
of statistics, so the default NuDEX cascades are used for this isotope. As
shown in Fig. 8, the shapes of the simulated deposited energy spectra of the
three cascades are similar.

Figure 8: Comparison between the simulated deposited energy spectra in the
detector due to 240Pu(n,$\gamma$), 246Cm(n,$\gamma$) and 248Cm (n,$\gamma$)
cascades. These cascades were used in the analysis.

The adopted (n,$\gamma$) cascades have been used to calculate the FPHWT
parameters for each isotope. It could be that for other resonances of the same
isotope, we had different cascades. However, according to the values obtained
in Table 3 of E. Mendoza  _et al._ (2023) the change in FPHWT for other
resonances are of the order of 0.5% for all the nuclei of this work.

The yields of Cm have been normalized to the strongest resonance of 240Pu at
1.056 eV. In consequence, the calculated Cm cross-sections will depend not on
the individual FPHWT values, but on the FPHWT(246Cm)/FPHWT(240Pu) and
FPHWT(248Cm)/FPHWT(240Pu) ratios. Table 2 shows the FPHWT values and their
ratios for different thresholds. In the analysis a threshold of 0.12 MeV is
used, however, there could be some uncertainty in the exact threshold value
used due to the detector calibration. Nevertheless, the differences in the
ratios for detection thresholds varying between 0.10 and 0.15 MeV are less
than 0.3%.

| 0.1 MeV | 0.12 MeV | 0.15 MeV | 0.30 MeV
---|---|---|---|---
240Pu | 1.0925(4) | 1.0977(4) | 1.1056(4) | 1.1505(4)
246Cm | 1.1641(4) | 1.1705(4) | 1.1805(4) | 1.2334(4)
248Cm | 1.0831(4) | 1.0886(4) | 1.0971(4) | 1.1446(4)
240Pu/246Cm | 0.9385(5) | 0.9378(5) | 0.9365(5) | 0.9327(5)
240Pu/248Cm | 1.0087(5) | 1.0084(5) | 1.0078(5) | 1.0052(5)
Table 2: FPHWT values and ratios between them for different detection
thresholds, and the ratios between them. The uncertainties in the table are
due to counting statistics in the Monte Carlo simulations.

In practice, it is necesary to propagate the uncertainties in the cascades to
the FPHWT values. If we compare the FPHWT(246Cm)/FPHWT(240Pu) ratio obtained
with the fitted cascades and with the default NuDEX cascade the difference is
lower than 1.5% Alcayne (2022). Taking this into account, in addition to the
uncertainties in the Geant4 simulations, we have estimated the uncertainty in
the FPHWT(246Cm)/FPHWT(240Pu) ratio at 1% and in the FPHWT(248Cm)/FPHWT(240Pu)
ratio at 2%. The second is higher due to not having adjusted the cascades of
248Cm.

###  III.5 Capture yield

The theoretical neutron capture yield of an isotope $i$, $Y_{i}(E_{n})$, is
defined as the fraction of incident neutrons that induce (n,$\gamma$) reaction
in the sample in that isotope. The yield is related to the capture cross-
section of the isotope ($\sigma_{\gamma i}$) with:

$Y_{i}(E_{n})=F_{m}(E_{n})(1-e^{-n\cdot\sigma_{tot}(E_{n})})\cdot\frac{A_{i}\cdot\sigma_{\gamma
i}(E_{n})}{\sigma_{tot}(E_{n})}$ (2)

where $n$ is the sample areal density, $A_{i}$ is the atom fraction of that
isotope, $\sigma_{tot}$ is the sum of the total cross-sections of all the
isotopes present in the sample, weighted by their isotopic abundances, and
$F_{m}$ is a factor to correct for the multiple interactions. The calculation
of $F_{m}$ is described in Sec. IV. This theoretical yield is then compared
with the experimental yield to obtain the capture cross-section of the
isotope. The experimental yield can be determined after applying the PHWT,
described in Section III.3, as follows:

$Y_{PHWT,exp,i}=F_{PHWT,i}\frac{C_{w}-B_{w}}{S_{n,i}\cdot\phi_{n}}$ (3)

where $C_{w}$ is the total weighted counting rate, $B_{w}$ is the background
weighted counting rate, $S_{n,i}$ is the total energy of the cascade for the
isotope $i$ and $\phi_{n}$ is the number of neutrons impinging on the sample
per unit time.

As a consequence of the pulse-height weighting procedure, the statistical
fluctuations obtained in the weighted yield are larger than with the standard
counting technique, i.e. the uncertainties due to counting statistics are
larger. It was then proposed in Lerendegui-Marco  _et al._ (2018); Lerendegui-
Marco (2018); E. Mendoza  _et al._ (2023) to obtain the yield without
weighting the counts and then to normalize this yield to the weighted yield
obtained with the PHWT. By doing so the uncertainties can be reduced. To use
this approach, the efficiency to detect the cascades ($\varepsilon_{c,i}$)
must be equal for all the measured resonances in the respective isotope. The
actinides of the sample (240Pu, 246Cm and 248Cm) are 0+ heavy nuclei with more
than a million levels below the neutron separation energy. Therefore, as
calculated in E. Mendoza  _et al._ (2023), the relative variation in detection
efficiency between each resonance is to be at most $\sim$1% for these
isotopes. This 1% relative uncertainty in efficiency is significantly smaller
than the increase of the counting statistics uncertainty due to the pulse-
height weighting procedure. We thus analyzed the data without the application
of pulse-height weights, then the yield is calculated as:

$Y_{exp,i}=\frac{C-B}{\varepsilon_{c,i}\cdot\phi_{n}}$ (4)

where $C$ is the total counting rate and $B$ is the background counting rate.
The counting rate was calculated with an energy deposited threshold of 0.12
MeV and an upper limit of 6 MeV.

The number of neutrons impinging on the sample per unit time ($\phi_{n}$) has
been calculated multiplying the total fluence by the fraction of neutrons
intercepted by the sample (Beam Intersection Factor, BIF). The BIF is not
expected to change significantly in the energy range of this measurement. The
energy dependence of the total fluence during the experiment has been obtained
with the SiMon2 neutron detector Cosentino and others. (2015). This fluence is
in excellent agreement with the evaluated fluence Sabaté-Gilarte  _et al._
(2017a), which was obtained with a combination of various measurements with
different detectors in 2015 and 2016. Therefore, the SiMon2 fluence has been
used for the analysis with a 1% uncertainty in its energy dependence. The BIF
of the Cm sample has been obtained by using the saturated resonance method
Macklin  _et al._ (1979); A. Borella  _et al._ (2007) with the first resonance
of 197Au at 4.9 eV. For this purpose, a gold sample with the same radius as
the Cm pellet has been measured obtaining a BIF of 0.0314(7). For the time-to-
energy conversion, the time-of-flight effective distance was calculated to
reproduce the energies of the resonances of 197Au in the JEFF-3.3 library
until 400 eV. The distance obtained was 19.425(1) m.

The detection efficiency ($\varepsilon_{c,i}$) has to be calculated to
determine the non-weighted yield ($Y_{exp,i}$). The efficiency values of each
isotope have been calculated normalizing to the weighted yield by comparing
the yield integrals of the resonances with weight ($Y_{PHWT,exp,i}$) and
without ($Y_{exp,i}$). The capture efficiency values obtained doing the
weighted average over all the resonances for each isotope are presented in
Table 3.

|  240Pu |  246Cm |  248Cm
---|---|---|---
$\varepsilon_{c}$ | 0.03023(2) | 0.02898(18) | 0.02685(50)
Table 3: Efficiency to detect the capture cascades for one detector. The
uncertainties in the table are only due to counting statistics.

The areas of the resonances obtained with the weighted yield
($Y_{PHWT,exp,i}$) and non-weighted yield ($Y_{exp,i}$) are compatible as
presented in Fig. 9.

Figure 9: Ratio between the areas of the resonances of the yields obtained
with and without the PHWT (i.e. from Eqs. 3 and Eq. 4) for 246Cm and 248Cm
resonances. The uncertainties of the data are only due to counting statistics.

###  III.6 Uncertainties

The uncertainties considered in the yield calculation, in addition to those
due to counting statistics, are the following:

  1. 1.

Uncertainty in the normalization. The capture yield has been normalized to the
first resonance of 240Pu at 1.056 eV, as described in Sec. IV.1.1. The
uncertainties associated with the normalization procedure are presented in
Table 4. In addition to these uncertainties, the 2.75% uncertainty in the
capture cross-section of 240Pu at $\sim$1 eV in JEFF-3.3 should be considered.

Uncertainty |  246Cm |  248Cm
---|---|---
Abundance 240Pu/246-248Cm in the sample | 2.7 | 3.3
$\mathrm{F_{PHWT,^{246-248}Cm}/F_{PHWT,^{240}Pu}}$ | 1.0 | 2.0
$\mathrm{\varepsilon_{c,^{240}Pu}/\varepsilon_{c,^{246-248}Cm}}$ | 1.1 | 2.0
Total quadratic sum | 3.0 | 4.3
Table 4: Uncertainties (in %) in the normalization. In the last row, the
uncertainties are added quadratically to obtain the total normalization
uncertainty.

  2. 2.

Uncertainty in the gain shift correction of the energy calibration. The
uncertainty in the correction has been estimated in 2-4%, depending on the
energy range. This uncertainty is propagated to a different value in each
resonance that goes from 2 to 6%.

  3. 3.

Uncertainty in the shape of the neutron fluence, which is 1%.

  4. 4.

Uncertainty in the beam-on background subtraction. The uncertainty due to
systematic effects in the determination of the background has been estimated
to be 0.4%. The propagation of this uncertainty is presented in Figs. 12 and
14.

  5. 5.

Uncertainty in the fission background produced by the actinides in the sample.
The estimated uncertainty in the determination of the fission detection
efficiency is 25%. The propagation of this uncertainty to the resonance
depends on the ratio of the capture and fission cross-sections and goes from 2
to 15%.

##  IV Resonance analysis

###  IV.1 Methodology

The capture yields in the range of 1-400 eV (where individual resonances can
be well resolved in our experiment) have been analyzed with the SAMMY code
Larsson (2006) using the Reich-Moore approximation to the R-matrix theory
Frohner (2000). The different effects of the experimental yield have been
taken into account with models implemented in the SAMMY code: multiple
interactions, Doppler broadening, and resolution broadening.

The resolution broadening is a consequence of the fact that neutrons arriving
at the sample at a certain time do not have all the same energy. The time-to-
energy distribution of the neutrons is given by the so-called Resolution
Function (RF). At the EAR2, contrary to the EAR1 Lorusso  _et al._ (2004), the
RF depends on the sample position and its dimensions Vlachoudis  _et al._
(2021). This is due to the non-optimized geometry of the spallation target.

The RF at n_TOF is normally calculated using simulations performed with FLUKA
Ahdida  _et al._ (2022); Lorusso  _et al._ (2004). In fact, at EAR2 this
method works for samples covering all the neutron beam Vlachoudis  _et al._
(2021). However, as shown in Fig. 10, the code is not able to reproduce the
shape of the yield for small samples. For this reason, the RF for the Cm
campaign has been obtained by performing an adjustment. The parameters have
been fitted to match the well-established resonances of 197Au measured in the
range from 4 to 400 eV for a sample with the same dimensions (2.5 mm radius)
as the Cm sample Alcayne (2022). Fig. 10 shows that this RF reproduces the
capture yield much more accurately than the RF obtained from Monte Carlo
calculations with FLUKA. The resonance of Au presented in Fig. 10 has a
neutron energy of 78.27 eV and in the plot, the center of the resonance is at
around 77.8 eV, this shift is due to the fact that the RF apart from changing
the shape of the resonances also shifts its position. The uncertainties in the
obtained RF have been quantified and propagated to the resonance analysis
Alcayne (2022).

Figure 10: Experimental 197Au capture yield near the 78.3 eV (Exp) for a 2.5
mm radius sample compared with two theoretical yields. Both have been obtained
with the resonance parameters of JEFF-3.3 with two different RF. The first
yield has been calculated with the RF obtained from Monte Carlo calculations
(JEFF-3.3 MC), and the second (JEFF-3.3-Adj) uses the RF obtained after the
adjustment. In the bottom panel of the figure, the residuals (Res) defined as
the differences between the experimental data points and the theoretical
JEFF-3.3 yield divided by the statistical uncertainties of the data points are
plotted.

At n_TOF EAR2 due to the considerable uncertainty in the RF, we are mostly
sensitive to the area of the resonances and not to their shape. For this
reason in the fit, we fitted only the neutron width ($\Gamma_{n}$), which for
the resonances of the experiment is usually the most sensitive to the
resonance area. Therefore for radiation and fission widths, respectively
$\Gamma_{\gamma}$ and $\Gamma_{f}$, the values from JENDL-4.0 have been taken.
The libraries JEFF-3.3 and ENDF/B-VIII.0 report the same RP for these
isotopes. The energy of the resonances ($E_{0}$) has been also obtained on the
fits.

All the resonances have been assumed to be s-wave with $J^{\pi}=1/2^{+}$ since
no $p$-wave resonances should be observable in our experiment below 400 eV.
The resonances of 240Pu and 244Cm have been also analyzed, but the results
will be reported in a stand-alone publication, together with the results
obtained from other measured Cm samples Alcayne  _et al._ (2023). In the SAMMY
analysis, the isotopes 27Al and 16O, also mixed with Cm in the sample, have
been included to properly account for their contributions to the self-
shielding and multiple interactions corrections. The detection efficiencies
for these two isotopes have been set to zero, as their contributions have
already been subtracted with the dummy sample. The results of the fits are
shown in Figs. 16, 17, 18, 19 and 20.

The background resulting from fission reactions has been calculated using the
fission detection efficiency ($\varepsilon_{f}$) and the fission yield of each
isotope. As the fission yields depend on the RP, in particular in the fitted
parameters E0 and $\Gamma_{n}$, a recursive approach has been employed. This
method involves initially subtracting the fission background estimated using
the resonance parameters from the previous fitting iteration, followed by
fitting the resonance parameters (E0 and $\Gamma_{n}$). These two steps are
iteratively repeated until convergence is reached.

####  IV.1.1 Normalization to 240Pu

As already mentioned, the capture yield, and thus the capture cross-sections
of 246Cm and 248Cm, have been normalized to the first resonance of 240Pu at
1.056 eV using the JEFF-3.3 resonance parameters. The experimental yield and
the SAMMY fit of the Pu resonance are presented in Fig.11. The uncertainties
in the normalization, described in Table 4, are 3% and 4.3% for 246Cm and
248Cm, respectively. The mass of 240Pu in the sample which has been obtained
from the fit is 0.159(4) mg (the uncertainty does not account for the
uncertainty in the JEFF-3.3 cross-section).

Figure 11: 240Pu experimental capture yield (Exp) in the energy range of the
strongest resonance compared to the yield obtained with the JEFF-3.3 resonance
parameters (JEFF-3.3) used to perform the normalization. The background (BKG)
is also included in the plot. In the bottom panel, the residuals are
presented.

###  IV.2 Uncertainties in the resonance parameters

The uncertainties in the yield have been propagated to the E0 and $\Gamma_{n}$
resonance parameters. In particular, the uncertainties due to counting
statistics in the experimental yield have been propagated by the SAMMY code.
However, the uncertainties due to systematic effects in the yield, summarized
in Section III.6, have been propagated to the parameters with the method
described in the following paragraphs.

The different uncertainties have been divided into two groups, those that are
considered to be fully uncorrelated between the fitted $\Gamma_{n}$ values of
the resonances and those that are considered fully correlated. The
uncertainties due to counting statistics, the resolution function, the fluence
shape and the gain shifts of the deposited energy calibration have been
considered as fully uncorrelated. On the other hand, the uncertainties due to
the fission background, the beam-on background and the normalization have been
considered as fully correlated.

Each of the uncertainties due to systematic effects have been propagated to E0
and $\Gamma_{n}$ parameters by using two additional yields. If the nominal
yield is described with a set of data points (Ej,Yexp,j), with Ej the neutron
energy and Yexp,j the capture yield at the point $j$, then the two new yields
have been constructed as (Ej,Y${}_{exp,j}+\sigma_{k,j}$) and
(Ej,Y${}_{exp,j}-\sigma_{k,j}$), where $\sigma_{k,j}$ is the yield uncertainty
due to the $k$-type of uncertainty at point $j$. The fitting process has been
repeated for both yields, and two new sets of $E_{0}$ and $\Gamma_{n}$
parameters have been obtained for each $k$-type of uncertainty and for each
resonance. The uncertainties in the fitted quantities have been obtained as
half of the differences, in absolute value, between the two values obtained in
the fits. The obtained uncertainties are given in Tables 5 and 7, also in the
tables the fully uncorrelated (U) and fully correlated (C) uncertainties were
summed in squares.

###  IV.3 Resonance parameters of 246Cm

A total of 13 resonances of 246Cm have been fit, from 4 to 400 eV, as
presented in Table 5. The total uncertainties in the calculation of the
$\Gamma_{n}$ parameters for the first four resonances are 4-6% mainly due to
the uncertainty in the normalization. The uncertainties at higher energies are
dominated by the subtraction of the beam-on background and by counting
statistics. The contribution of individual sources to the uncertainty in
$\Gamma_{n}$ is presented in Fig. 12.

E0 | $\Gamma\mathrm{{}_{n}}$ |  $\Gamma\mathrm{{}_{n}}$ uncertainty (meV) | Rk (meV)
---|---|---|---
Sta | Fis | Dummy | Gain | RF | Norm | Sum | Sum | Total | Total | Value | Uncer.
(eV) | (meV) | (U) | (C) | (C) | (U) | (U) | (C) | (C) | (U) | (%)
4.3129(2) | 0.324 | 0.001 | 0.009 | 0.001 | 0.003 | 0.005 | 0.009 | 0.013 | 0.006 | 0.015 | 4.5 | 0.315 | 0.016
15.310(1) | 0.541 | 0.006 | 0.009 | 0.009 | 0.004 | 0.015 | 0.015 | 0.020 | 0.017 | 0.026 | 4.8 | 0.524 | 0.029
84.576(5) | 23.4 | 0.4 | 0.8 | 0.5 | 0.3 | 0.4 | 0.7 | 1.1 | 0.7 | 1.3 | 5.6 | 13.82 | 0.52
91.945(8) | 13.6 | 0.3 | 0.2 | 0.4 | 0.1 | 0.3 | 0.4 | 0.6 | 0.5 | 0.7 | 5.5 | 9.72 | 0.43
158.41(2) | 39.9 | 2.1 | 1.5 | 2.9 | 0.6 | 0.6 | 1.1 | 3.5 | 2.2 | 4.1 | 10 | 18.39 | 0.93
193.85(5) | 17.7 | 1.4 | 0.1 | 1.6 | 0.2 | 0.1 | 0.5 | 1.7 | 1.4 | 2.2 | 13 | 11.7 | 1.0
232.5(2) | 4.3 | 1.0 | 0.1 | 1.9 | 0.2 | 0.1 | 0.1 | 1.9 | 1.0 | 2.2 | 51 | 3.8 | 1.7
250.8(1) | 18.4 | 2.2 | 0.3 | 2.7 | 0.3 | 0.7 | 0.5 | 2.8 | 2.3 | 3.6 | 20 | 11.9 | 1.6
278.1(1) | 31.6 | 4.2 | 1.9 | 4.9 | 0.7 | 1.4 | 0.9 | 5.3 | 4.4 | 6.9 | 22 | 16.2 | 1.9
287.9(2) | 17.0 | 2.7 | 0.3 | 3.3 | 0.2 | 0.9 | 0.5 | 3.4 | 2.9 | 4.5 | 26 | 11.3 | 2.0
306.8(2) | 49.1 | 8.5 | 0.5 | 11.2 | 2.6 | 2.4 | 1.4 | 11.3 | 9.1 | 14.5 | 30 | 20.3 | 2.5
316.6(1) | 24.2 | 4.6 | 0.3 | 5.9 | 1.2 | 0.9 | 0.7 | 5.9 | 4.8 | 7.6 | 31 | 14.2 | 2.6
381.1(2) | 59.9 | 18.5 | 2.9 | 28.0 | 1.6 | 3.6 | 1.7 | 28.2 | 18.9 | 34.0 | 57 | 21.9 | 4.6
Table 5: $\Gamma_{n}$ and $E_{0}$ parameters of 246Cm obtained in this work.
The different uncertainties for $\Gamma_{n}$ presented in the table are due to
counting statistics (Stat), subtraction of the background produced by the
fission events (Fis), subtraction of the beam-on background (Dummy),
correction for the gain shifts of the energy calibration (Gain), determination
of the RF (RF) and normalization (Norm). The uncertainty in the capture cross-
section of 240Pu (2.75%, according to JEFF-3.3) has not been included in the
table. The different uncertainties are considered to be fully correlated (C)
or fully uncorrelated (U) between the resonances. The quadratic sum of the
different uncertainties has been also included. In addition, the radiative
kernels with their uncertainties are also in the table. Figure 12: Total
uncertainties (in percentage) of the $\Gamma\mathrm{{}_{n}}$ parameters of
246Cm obtained in this work (Total). The uncertainties due to counting
statistics (Stat), due to the subtraction of the beam-on background (Dummy)
and the rest of the uncertainties (Other) are also presented for comparison.

In the fits the $\Gamma_{n}$ and the energy of the resonances have been fit
whereas $\Gamma_{\gamma}$ and $\Gamma_{f}$ were fixed. In order to compare our
results with previous measurements obtained with different $\Gamma_{\gamma}$
and $\Gamma_{f}$ parameters the radiative kernels
(R${}_{k}=g_{J}\Gamma_{\gamma}\Gamma_{n}/\Gamma$) which are proportional to
the area of the resonances have been calculated and compared in Fig. 13. The
JENDL-4.0 values are obtained from the Maslov et al. evaluation Maslov  _et
al._ (1996). This evaluation, performed in 1996, is based on transmission
measurements R. E Coté  _et al._ (1964); J. R. Berreth  _et al._ (1972); R. W.
Benjamin  _et al._ (1974); Belanova  _et al._ (1975) and on the capture
measurement by Moore et al. Moore and Keyworth (1971). Details of all the
measurements are in Table 6. The JEFF-3.3 and ENDF/B-VIII.0 Brown  _et al._
(2018) libraries have adopted the JENDL-4.0 values for this isotope. There are
two other recent capture measurements by Kimura et al. Kimura  _et al._ (2012)
and Kawase et al. Kawase  _et al._ (2021) performed at J-PARC with germanium
detectors Kin  _et al._ (2009). The latest JENDL-5 evaluation Iwamoto  _et
al._ (2023) directly used the resonance parameters of the Kawase et al.
measurement.

Experiment | Type | Energy range
---|---|---
246Cm |  248Cm
Cote et al. (1964) R. E Coté  _et al._ (1964) | Transm. | 4-27 eV | -
Moore et al. (1971) Moore and Keyworth (1971) | Capture | 80-380 eV | 26 eV
Berreth et al. (1972) J. R. Berreth  _et al._ (1972) | Transm. | 4-16 eV | -
Benjamin et al. (1972) R. W. Benjamin  _et al._ (1974) | Transm. | 4-160 eV | 7-3000 eV
Belanova et al. (1975) Belanova  _et al._ (1975) | Transm. | 4-160 eV | 7-100 eV
Kimura et al. (2012) Kimura  _et al._ (2012) | Capture | 4-16 eV | 7-27 eV
Kawase et al. (2021) Kawase  _et al._ (2021) | Capture | 4-550 eV | 7-100 eV
Table 6: Transmission (Transm.) and capture 246Cm and 248Cm measurements
available in EXFOR in the RRR. Some of the measured yields extend up to higher
energies, but the resonance parameters are only reported up to the energies
presented in the table.

Figure 13: Ratio between the 246Cm Rk values obtained in different
experiments, including this work, and JENDL-4.0.

Discrepancies have been found between the results obtained in this measurement
and some of the previous ones. The observed resonance at 306.8 eV, assigned to
246Cm, was not present in JENDL-4. This assignment is based on the measurement
with another Cm sample that contains a different ratio of 244Cm to 246Cm, more
information on this sample can be found in Alcayne  _et al._ (2019, 2020,
2023). In addition, this resonance is too big to be associated with 248Cm.
This resonance was also observed in the recent measurement by Kawase et al.
with a neutron width of 34.6(31) meV, the value obtained in our measurement is
49(11) meV.

Furthermore, the resonance-like structure of 246Cm at 360.8 eV was not
properly fitted with a radiative width of 34.7 meV, this value has been used
to fit the rest of the resonances of 246Cm above 20 eV because these values
were reported in JENDL-4.0. In order to correctly fit this resonance we had to
use a value of at least 60 meV for $\Gamma_{\gamma}$. The reported radiative
widths of all resonances were always between 25 and 35 meV and only
fluctuations of a few percent for these values are expected. For this reason,
we think that this structure is a doublet with a separation between the
resonance of 2-3 eV. In the previous measurement by Moore et al. the resonance
was also considered as a doublet. In the Kawase et al. measurement, the
resonance is fitted with a radiative width of 34.7 meV.

In addition, in the JENDL-4.0 library the RP for three weak resonances at
32.95, 47 and 131 eV were given. As shown in Fig. 19, our data are not
compatible with the existence of the resonances with the areas given. The
neutron width allowed by our data would need to be smaller by a factor of 3 to
10 depending on the resonance. Note that these resonances were included in the
Maslov et al. evaluation without being observed in any previous measurement.

The Rk values obtained for the first three resonances at 4.31, 15.31 and 84.58
eV are compatible with the previous measurements and with JENDL-4.0 and
JENDL-5. The remaining resonances are within one standard deviation not
compatible with JENDL-4.0, but are compatible with the Kawase et al.
experiment, and thus with JENDL-5. For neutron energies below 160 eV, n_TOF
provided Rk values with uncertainties smaller than 5%, which are similar to
those of Kawase et al. and considerably smaller than in the previous capture
and transmission measurements. The uncertainties at higher energies are
between 10 and 20% for all but one resonance.

###  IV.4 Resonance parameters of 248Cm

A total of 5 resonances of 248Cm have been observed and fitted in the analysis
from 7 to 100 eV, as presented in Table 7. The uncertainties in the
calculation of the $\Gamma_{n}$ parameters for the first two resonances are
below 10% and for the other three are below 50%, as shown in Fig. 14.

Figure 14: Total uncertainties of the $\Gamma\mathrm{{}_{n}}$ parameters of
248Cm obtained in this work in percentage (Total). The uncertainties due to
counting statistics (Stat), due to the subtraction of the beam-on background
(Dummy) and the rest of the uncertainties (Other) are also presented for
comparison.

The JENDL-4.0, JEFF-3.3, ENDF/B-VIII.0 and JENDL-5 libraries adopt the same
evaluation for this isotope, which is the one performed by Kikuchi et al.
Kikuchi and Nakagawa (1984). This evaluation considers only the transmission
data of Benjamin et al. R. W. Benjamin  _et al._ (1974) and the capture data
of Moore et al. Moore and Keyworth (1971).

The values of the Rk obtained for the first three resonances are compatible
with the previous measurements and evaluations (Fig. 15). However, the values
obtained for the other two resonances at 76 and 98.8 eV are 20% and 65% larger
than the Kikuchi et al. evaluation and are not compatible within one sigma
with any other previous measurement. The uncertainties in the Rk for the 248Cm
resonances in this work are below 15%.

Figure 15: Ratio between the 248Cm Rk values obtained in different experiments, including this work, and JENDL-4.0. E0 | $\Gamma\mathrm{{}_{n}}$ |  $\Gamma\mathrm{{}_{n}}$ uncertainty (meV) | Rk (meV)
---|---|---|---
Sta | Fis | Dummy | Gain | RF | Nor | Sum | Sum | Total | Total | Value | Uncer.
(eV) | (meV) | (U) | (C) | (C) | (U) | (U) | (C) | (C) | (U) | (%)
7.246(1) | 1.94 | 0.01 | 0.05 | 0.02 | 0.02 | 0.11 | 0.08 | 0.10 | 0.11 | 0.15 | 7.8 | 1.79 | 0.14
26.90(1) | 18.2 | 0.3 | 0.2 | 0.5 | 0.2 | 1.0 | 0.8 | 0.9 | 1.0 | 1.4 | 7.7 | 11.56 | 0.60
35.01(1) | 9.8 | 0.3 | 1.2 | 0.5 | 0.3 | 1.0 | 0.4 | 1.3 | 1.1 | 1.8 | 18 | 7.4 | 1.0
76.07(2) | 268 | 59 | 61 | 46 | 7 | 31 | 11 | 77 | 67 | 102 | 36 | 23.37 | 0.91
98.84(4) | 371 | 85 | 23 | 142 | 10 | 3 | 16 | 145 | 86 | 169 | 45 | 36.1 | 1.6
Table 7: Same as Table 5 but for the 248Cm resonances.

##  V Summary and conclusions

The capture cross-sections of 246Cm and 248Cm have been measured at the EAR2
of the n_TOF facility with three C6D6 detectors. This work is the first
analysis of a capture measurement at n_TOF EAR2, profiting from the high
instantaneous fluence in this area to measure a sample with a very low mass
($\sim$1 mg of 246Cm and $\sim$0.2 mg of 248Cm ) and high radioactivity
($\sim$2 GBq). A total of 13 resonances of 246Cm between 4 and 400 eV and 5
resonances of 248Cm between 7 and 100 eV have been observed and analyzed. The
radiative kernels of 246Cm have been obtained with an uncertainty of less than
5% at energies below 160 eV. At higher energies, the uncertainties are between
10 and 20% for the majority of the resonances. For 248Cm, the uncertainties in
radiative kernels are around 10% as a result of the increase of the
uncertainties due to counting statistics. The radiative kernels of the first
three resonances of 246Cm obtained in this work are compatible with all the
previous measurements and evaluations. For the remaining 246Cm resonances, the
radiative kernels are only compatible within one sigma with the recent
measurement by Kawase et al. In the case of 248Cm also the radiative kernels
of the first three resonances are compatible with all previous measurements
and evaluations, for the other two resonances the radiative kernels obtained
are considerably higher than evaluations and previous experiments.

###### Acknowledgements.

This work was supported in part by the I+D+i grant PGC2018-096717-B-C21 funded
by MCIN/AEI/10.13039/501100011033, by project PID2021-123100NB-I00 funded by
MCIN/AEI/10.13039/501100011033/FEDER, UE, by project PCI2022-135037-2 funded
by MCIN/AEI/10.13039/501100011033/ and European Union NextGenerationEU/PRTR,
by the European Commission H2020 Framework Programme project SANDA (Grant
agreement ID: 847552), Croatian Science Foundation project IP-2022-10-3878 and
by funding agencies of the n_TOF participating institutions.

## APPENDIX: FITS OF THE RESONANCES

Figure 16: Experimental capture yield measured in EAR2 for the Cm sample
compared with the fitted yields. The experimental capture yield (Exp) includes
the uncertainties due to counting statistics only. The green, blue and cyan
lines correspond to the capture yield for each isotope. The orange line (BKG)
corresponds to the background due to fission and capture events in the
actinides. The red line (Sum) corresponds to the sum of all the capture and
fission yields. It is important to notice that the resonances in the figures
are shifted in energy due to the RF, see Section IV.1 for more details.

Figure 17: Experimental 246Cm capture yields close to the first six resonances
(black) compared with the yield obtained with the fit (green) and with the
JENDL-4.0 data (red). The evaluation in JENDL-4.0 has also been used in
JEFF-3.3 and ENDF/B-VIII.0. In blue, the calculation of the background due to
the other actinides and in pink the fission background. It is important to
notice that the resonances in the figures are shifted in energy due to the RF,
see Section IV.1 for more details.

Figure 18: Same as Fig. 17 but for the resonances between 230 and 400 eV.

Figure 19: Same as Fig. 17 but for the resonances at 32.95, 47 and 131 eV.

Figure 20: Same as Fig. 17 but for the resonances of 248Cm.

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