Improved STEREO simulation with a new gamma ray spectrum of excited gadolinium isotopes using FIFRELIN Guillaume Pronost Kamioka Observatory, ICRR, University of Tokyo Colloquium, 2020 May 20th STEREO experiment I STEREO is a collaboration between 4 French laboratories (CEA/IRFU, IN2P3/LPSC, IN2P3/LAPP, ILL) and MPIK (Germany). I Very short baseline reactor neutrino experiment (10m from reactor), looking at sterile neutrino oscillation. I Liquid scintillator detector, loaded with Gd, installed near the ILL research reactor. I Data-taking started in 2016, so far the collaboration published analysis of ∼ 2 years of data taking: 179 days-on, and 235 days-off. Colloquium, 2020 May 20th 2/21 [email protected] Reactor neutrino anomaly I Sterile neutrino hypothesis in reactor neutrino was revived by the paper \The Reactor Antineutrino Anomaly" (Mention 2011) I Paper was based on a new estimation of the reactor neutrino flux prediction by Mueller et al. (Mueller 2011). Mention 2011 I The anomaly indicates a deficit of detected neutrino compared to the prediction. I It was so far confirmed by most of the recent reactor neutrino experiment. I Hypothesis to explain this anomaly are the existence of sterile neutrino oscillation or issue with the reactor D. Lhuillier, Moriond 2019 neutrino prediction. Colloquium, 2020 May 20th 3/21 [email protected] Status of Sterile neutrino search I Sterile neutrino hypothesis is disfavored by several results (Planck, Daya Bay, RENO, NEOS, etc.) I Last results of STEREO are excluding a large part of the allowed space. arXiv:1912.06582 Colloquium, 2020 May 20th 4/21 [email protected] STEREO detector I The STEREO detector is a liquid scintillator detector divided in 6 Target cells and one γ-catcher. Journal of Instrumentation 13, 07 (2018): P07009 I The Target's Liquid scintillator is loaded with Gd (concentration 0:2 wt.%, mean neutron capture time ∼ 18 µsec). I PMT are separated from LS by a thick acrylic buffer to have homogeneous light and to reduce PMT induced radioactivity BG. I Multiple Target cell should allow to see sterile oscillation in the detector. Colloquium, 2020 May 20th 5/21 [email protected] Gd gamma cascade knowledge I Precise knowledge of Gd-deexcitation process (gamma multiplicity, single energies of the gamma) is critical for small and/or segmented detectors. ! Risk of high energy gammas escaping is higher Hagiwara et al., PTEP 2019, 023D01, 29pp. I Current knowledge is not enough: I DANSS reported large uncertainties on the Gd-n signal and states: \It is very difficult to simulate reliably the low energy part of the n-Gd capture signal since there are many cascade decay chains with unknown probabilities. Our MC does not describe well the lower energy part of the spectrum and hence we do not use it for the comparison." (Phys. Lett. B7 87, 56 (2018).) I In Daya-Bay 1230 days results, they indicated they used 4 models of the Gd-n γ emission ( Phys. Rev. D 100, 052004 (2019) ) Colloquium, 2020 May 20th 6/21 [email protected] FIFRELIN code (simplified view) I FIFRELIN is a MC simulation code for nuclear fission simulation developped by the CEA (France) I It is based on experimental nuclear fission data and predict the unknown observable with some models I FIFRELIN also simulates the deexcitation of the fission fragments This is what is used here for this paper. I Above Sn (threshold for neutron emission), the neutron emission is simulated step-by-step following temperature dependent neutron spectrum for the primary fragments (following Weisskopf evaporation theory) I Then γ emission of the secondary fragments is simulated with O. LITAIZE, P(ND)2 -2 Perspectives on Nuclear Data for Hauser-Feschbach method (next slides) the Next Decade, 14-17 october, 2014, BIII, France Colloquium, 2020 May 20th 7/21 [email protected] Deexcitation process: Hauser-Feschbach method I In Hauser-Feschbach method, the fragments deexcitation is performed using nuclear levels defined as follow for γ: I Below ERIPL, experimentally determined nuclear levels from RIPL-3 database (level E, spins, parities and all partial widths). I Above ERIPL and below Elimit missing levels are sampled from model until reaching the theoritical level density ρ 4 −1 I Abe Elimit (corresponding to 5 × 10 MeV ) levels are gathered into bins I The γ partial width between level a and b are computed from gamma XL strength function Sγ (γ): ρ is the level density function, L is the multipolarity, and X is the type (electric or magnetic). F. Be˘cv´ar https://doi.org/10.1016/S0168-9002(98)00787-6 (eq. 2) I For neutron emission, the neutron partial widths are computed with neutron transmission coefficent from an optical model calculation Colloquium, 2020 May 20th 8/21 [email protected] Deexcitation process: Hauser-Feschbach method O. LITAIZE, P(ND)2 -2 Perspectives on Nuclear Data for the Next Decade, 14-17 october, 2014, BIII, France Colloquium, 2020 May 20th 9/21 [email protected] Deexcitation process: F. Be˘cv´ar'salgorithm I One issue for γ deexcitation simulation is the difficulty of simulating γ cascade taking into account Porter-Thomas statistical fluctuations for level not well known. (see C. E. Porter and R. G. Thomas Phys. Rev. 104, 483 (1956)) I These fluctuations are not considered in standard Hauser-Feschbach method, Becvar proposed to include them with the following modification: . Above Ecrit (ERIPL), unknown set of level energies representing a random discretization of an a priori known level density formula. For level above Ecrit γ (and electron) partial width are random quantity defined by eq (2) taking into account the Porter-Thomas fluctuations: yXL are random values drawn independently from N (0; 1) I Each cascade start from a single well defined initial level with a known excitation energy, spin and parity and the next level is determined randomly following the partial radiation widht. Colloquium, 2020 May 20th 10/21 [email protected] Deexcitation process: R`egniern/γ emission I In case of the calculation of the neutron/γ deexcitation from nuclei with high spin values, there is a numerical limitation of Hauser-Feschbach method due to the approximation assumed in the structure of the nuclei above Ecrit. (This is similar to the γ emission issue without Porter-Thomas fluctuations) I It can be solved by Fluctuating Structure Properties (FSP) method which consists in applying the same approach than Be˘cv´arbut to neutron partial width See D. Regnier et al. Comput. Phys. Commun. 201, 19 (2016) https://doi.org/10.1016/j.cpc.2015.12.007 Colloquium, 2020 May 20th 11/21 [email protected] Gd case in FIFRELIN: Nuclear levels I In the paper, the level density is defined following the Composite Gilbert Cameron Model: CTM stands for Constant Temperature Model, and FGM for Fermi Gas Model I EM is defined as the energy where \the CTM and the FGM nuclear level densities match along with their derivatives" Colloquium, 2020 May 20th 12/21 [email protected] Gd case in FIFRELIN: Transitions (1/2) I In case of Gd-n, after a thermal neutron capture 156Gd* and 158Gd* are close to Sn. Neutron emission are unlikely according to the paper. The following is only about γ transition. I Electric dipole transitions are described by Enhanced Generalized Lorentzian Model (EGLO), which allows better agreement between the model and the experimental data at low energy: E1 transitions for 144Nd R. Capote et al., Nucl. Data Sheets 110, 3107 (2009). Colloquium, 2020 May 20th 13/21 [email protected] Gd case in FIFRELIN: Transitions (2/2) I In [15] (R. Capote et al.) we can find: φM1 is the shape of the γ strength functions. φM1 can be constant or extracted from SLO. I In [15] E2 transitions are best described by SLO shape. However, [15] says others (M2, E3 and M3) are best described by constant strength functions. Colloquium, 2020 May 20th 14/21 [email protected] STEREO simulation I STEREO simulation is based on Geant4. I The neutron transportation is handled by NeutronHP, with interaction cross-section coming from ENDF/B-VII.1 (default for Geant4 since 2012) I Standard deexcitation processes of Geant4 are by-passed, using instead either: - GLG4sim - This FIFRELIN simulation Colloquium, 2020 May 20th 15/21 [email protected] Gd simulation from GLG4sim and from FIFRELIN I Good agreement on γ multiplicity for natural Gd (∼ 4 with \few percents difference”) I 15% of the γ above 3:5 MeV with FIFRELIN (7% for GLG4sim) I Due to the compactness and the segmentation of the STEREO detector these high energy γ are very important (mean free path in LS for 5 MeV γ is ∼40cm, corresponding to the size of the detector) I Conversion e− in 70% of the cascades but < 1% above 200 keV Colloquium, 2020 May 20th 16/21 [email protected] STEREO calibration Data/MC comparison (1/2) Colloquium, 2020 May 20th 17/21 [email protected] STEREO calibration Data/MC comparison (2/2) I KS test was performed on the tail from 3 to 7 MeV shows 11% agreement with FIFRELIN and more the 5 standards deviation for GLG4sim. I Non-linearity effects are calibrated and taken into account in the STEREO simulation. This includes quenching, as well as the fact energy scale is anchored by 54Mn decay (834.848 MeV γ) (this explains why the Gd-n peak is around 8.5 MeV) Quenching effect with different γ-ray source 10.1088/1748-0221/13/07/P07009 Colloquium, 2020 May 20th 18/21 [email protected] Data/MC agreement I Better agreement on the Gd-fraction and IBD efficiency with FIFRELIN: Colloquium, 2020 May 20th 19/21 [email protected] Summary I This article presents a new simulation of Gd-n induced cascade.