Nuclear Inst. and Methods in Physics Research, A 977 (2020) 164306 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima Correlating the fissile mass of standard uranium samples with delayed gamma rays from fission products Fabiana Rossi a,∗ , Tatjana Bogucarska b , Mitsuo Koizumi a , Hee-Jae Lee a , Bent Pedersen b , Douglas Chase Rodriguez a , Tohn Takahashi a , Giovanni Varasano b a Integrated Support Centre for Nuclear Nonproliferation and Nuclear Security, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki-ken 319-1118, Japan b European Commission, Joint Research Center, Directorate for Nuclear Safety and Security, Nuclear Security Unit, Via Enrico Fermi 2749, 21027 Ispra (VA), Italy ARTICLE INFO Keywords: Delayed gamma-ray spectroscopy Fissile mass Non-destructive assay Safeguards verification ABSTRACT The Japan Atomic Energy Agency and the European Commission Joint Research Centre are collaborating to develop delayed gamma-ray spectroscopy (DGS) for nuclear materials for safeguards verification in reprocessing plants. In this paper, we describe DGS interrogation using the Pulsed Neutron Interrogation Test Assembly with standard samples of different 235 U enrichments. By analyzing gamma-ray spectra, we reveal a linear correlation between the sample mass and both the total counts above 3.3 MeV and the peak counts of specific high-energy gamma-ray. We were able to observe, qualify and quantify specific gamma rays peak down to the depleted uranium (0.5 g 235 U) mass sample. Based on this, we demonstrate that our technique is able to estimate the total fissile mass with a statistical uncertainty <2% when taking into account self-shielding and gamma self-absorption corrections. Using integrated counts above 3.3 MeV we were able to reduce the mass-dependent bias for the higher enrichments (∼3 to 4%) to <4%. 1. Introduction The verification of nuclear materials is a key task in nuclear safe- guards [1,2], particularly for reprocessing plants [3] where uranium and plutonium can be found in multiple forms. Passive non-destructive assay (NDA) techniques are currently used to verify low-radioactivity nuclear material. However, these techniques are not applicable to high- radioactivity nuclear material (HRNM) due to the intense neutron and gamma-ray emissions that overwhelm the passive signature in the low energy range [4,5]. For reprocessing plants, hybrid K-edge densitom- etry (HKED) is applied to determine the U and Pu elemental masses. The isotope dilution mass spectrometry (IDMS) DA method is used to quantify the fissionable isotopic composition in the sample. However, IDMS is time-consuming, produces waste, and requires the use of spikes (certified tracers containing U and Pu) that have limited availability. To avoid the drawbacks of DA, the Japan Atomic Energy Agency (JAEA) is collaborating with the Joint Research Centre (JRC) of the Eu- ropean Commission (EC) to develop a set of NDA techniques to quantify nuclear material [6]. Of these, delayed gamma-ray spectroscopy (DGS) is considered useful to quantify the ratio of fissile nuclides (e.g. 235 U, 239 Pu, and 241 Pu) in samples through analysis of high-energy gamma rays from short-lived fission products [7]. DGS consists of an irradiation phase in which an external neutron source is used to induce fission in a sample, followed by a measurement ∗ Corresponding author. E-mail address: rossi.fabiana@jaea.go.jp (F. Rossi). phase in which the gamma rays are collected. This sequence is repeated for several cycles to achieve statistical significance [8,9]. Due to the presence of long-lived fission products emitting strong gamma rays in the low energy range (e.g. 137 Cs at 662 keV), which mask the gamma- ray signatures of short-lived fission products below ∼2 MeV, we here focused DGS on a high energy range (≥3 MeV). However, the total fissile mass in the sample is needed for verifi- cation purposes [1,2]. Similar to passive techniques, the total fissile mass can be assessed with differential die-away analysis (DDA) that evaluates the fission neutrons over an extended time [10]. As presented in previous studies [7,11–13], the detection of delayed gamma rays with energies above 3 MeV after the induction of fission in nuclear material is possible, even for small quantities of the material. In these studies a hint of correlation between the sample mass and the gamma- ray counts were observed, therefore we tried here to improve upon these results. Thus, the challenge was to use high-energy gamma rays for the fissile mass evaluation. This will open the possibility to integrate and combine DGS and DDA for the fissile mass evaluation in safeguard verifications, implementing a double double-check measurement using different signature and reducing results uncertainty. Moreover, combin- ing these with HKED for the total elemental mass it will be possible to verify the 235 U enrichment. https://doi.org/10.1016/j.nima.2020.164306 Received 18 February 2020; Received in revised form 10 June 2020; Accepted 25 June 2020 Available online 27 June 2020 0168-9002/© 2020 Elsevier B.V. All rights reserved.