Β-delayed Neutrons Research Articles

OpenMC(TD): Current status and next development stages

This work presents the current state and the next stages in the development of the modified code OpenMC(TD), a modified version of the Monte Carlo code OpenMC which includes the time dependence related to the emission of β-delayed neutrons, but instead of simulating these neutrons using the traditional 6-group structure, features an implementation where individual precursors decay. This code is in active development, but already shows promising results. The upcoming development stages for OpenMC(TD) have two complimentary aspects, one related to the Monte Carlo code and another related to experimental measurements at a research nuclear reactor. Regarding the code, it is planned to implement the branchless collision variance reduction technique, while to validate the simulations, transient experiments will be performed at the RECH-1 research nuclear reactor.

Fissile mass estimation using β-delayed γ-rays and neutrons from fast and epithermal induced fissions

A potential method for mass estimation of shielded special nuclear materials (SNM) is proposed. The method uses fast and epithermal neutrons, generated by an external pulsed neutron source, to induce fission in SNM. The subsequent β-delayed γ-rays and neutrons, emitted by the fission products, are measured simultaneously by two 3He slab detectors and a coaxial high purity germanium (HPGe) detector, located around the tested material. Measuring simultaneously both, β-delayed γ-rays and neutrons measurement, offers improved sensitivity for fissile materials detecting.Laboratory examination of the method was performed in the Pulsed Neutron Interrogation Test Assembly (PUNITA), using CBNM (Central Bureau for Nuclear Measurements) samples of Uranium.

An advanced uranium ore grade estimation method based on photofission driven by an e-LINAC

A method for evaluating the grade of uranium (U) ores through the analysis of β-delayed neutrons following (γ, fission) is discussed. By measuring β-delayed fission neutrons stimulated from 238U, the U-Ra disequilibrium problem, which limits the accuracy of traditional grade estimation methods that rely on measuring radioactivity, is addressed. In this research, 6 MV bremsstrahlung photons, delivered by a 6 MeV electron linear accelerator (e-LINAC), were used to irradiate U ore samples, and a moderator surrounded thermal neutron detector was designed to register the emitted neutrons after each X-ray pulse. Although the prompt fission neutrons were discarded due to the interference caused by the photoneutrons emitted from the matrix nuclides in the analytical environment, an excellent linear relationship was observed between the counts of β-delayed neutrons and U ore grades, demonstrating that this method is promising for the in-situ analysis of U ores.

Uncertainty quantification works relevant to fission yields and decay data

In the present paper, firstly, we review our previous works on uncertainty quantification (UQ) of reactor physics parameters. This consists of (1) development of numerical tools based on the depletion perturbation theory (DPT), (2) linearity of reactor physics parameters to nuclear data, (3) UQ of decay heat and its reduction, and (4) correlation between decay heat and β-delayed neutrons emission. Secondly, we show results of extensive calculations about UQ on decay heat with several different numerical conditions by the DPT-based capability of a reactor physics code system CBZ.

Muon capture on light isotopes in Double Chooz

Using the Double Chooz reactor neutrino detector, we have measured the products of µ − capture on 12C, 13C, 14N and 16O. Over a period of 490 days, we collected 2.3 × 106 stopping cosmic µ −, of which 1.8 × 105 captured on these nuclei in the inner detector. The resulting isotopes were tagged using prompt neutron emission (when applicable), the subsequent beta decays, and, in some cases, β-delayed neutrons. Production of these βn isotopes, primarily 9Li, which are backgrounds, was found at a significance of 5.5σ. The probability of 9Li production per capture on natC is (2.4 ± 0.9(stat) ± 0.1(syst)) × 10−4. We have made the most precise measurement of the rate of 12C(µ −, ν)12B to date, of nuclear captures. By tagging excited states emitting gammas, the ground state transition rate to 12B is found to be .

Scaling beta-delayed neutron measurements to large detector areas

We explore the performance of a cargo screening system that consists of two large-sized composite scintillation detectors and a high-energy neutron interrogation source by modeling and simulation. The goal of the system is to measure β-delayed neutron emission from an illicit special nuclear material by use of active interrogation. This task is challenging because the β-delayed neutron yield is small in comparison with the yield of the prompt fission secondary products, β-delayed neutrons are emitted with relatively low energies, and high neutron and gamma backgrounds are typically present. Detectors used to measure delayed neutron emission must exhibit high intrinsic efficiency and cover a large solid angle, which also makes them sensitive to background neutron radiation. We present a case study where we attempt to detect the presence of 5 kg-scale quantities of 235U in a standard air-filled cargo container using 14 MeV neutrons as a probe. We find that by using a total measurement time of ∼11.6 s and a dose equivalent of ∼1.7 mrem, the presence of 235U can be detected with false positive and false negative probabilities that are both no larger than 0.1%.

Nuclear structure properties and stellar weak rates for 76Se: Unblocking of the Gamow Teller strength

At finite temperatures (≥ 107K), 76Se is abundant in the core of massive stars and electron capture on 76Se has a consequential role to play in the dynamics of core-collapse. The present work may be classified into two main categories. In the first phase we study the nuclear structure properties of 76Se using the interacting boson model-1 (IBM-1). The IBM-1 investigations include the energy levels, B(E2) values and the prediction of the geometry. We performed the extended consistent-Q formalism (ECQF) calculation and later the triaxial formalism calculation (constructed by adding the cubic term to the ECQF). The geometry of 76Se can be envisioned within the formalism of the potential energy surface based on the classical limit of IBM-1 model. In the second phase, we reconfirm the unblocking of the Gamow–Teller (GT) strength in 76Se (a test case for nuclei having N>40 and Z<40). Using the deformed pn-QRPA model we calculate GT transitions, stellar electron capture cross section (within the limit of low momentum transfer) and stellar weak rates for 76Se. The distinguishing feature of our calculation is a state-by-state evaluation of stellar weak rates in a fully microscopic fashion. Results are compared with experimental data and previous calculations. The calculated GT distribution fulfills the Ikeda sum rule. Rates for β-delayed neutrons and emission probabilities are also calculated. Our study suggests that at high stellar temperatures and low densities, the β+-decay on 76Se should not be neglected and needs to be taken into consideration along with electron capture rates for simulation of presupernova evolution of massive stars.

Conceptual design of a hybrid neutron-gamma detector for study of β-delayed neutrons at the RIB facility of RIKEN

The conceptual design of the BRIKEN neutron detector at the radioactive ion beam factory (RIBF) of the RIKEN Nishina Center is reported. The BRIKEN setup is a complex system aimed at detecting heavy-ion implants, β particles, γ rays and β-delayed neutrons. The whole setup includes the Advanced Implantation Detection Array (AIDA), two HPGe Clover detectors and up to 166 3He-filled counters embedded in a high-density polyethylene moderator. The design is quite complex due to the large number and different types of 3He-tubes involved and the additional constraints introduced by the ancillary detectors for charged particles and γ rays. This article reports on a novel methodology developed for the conceptual design and optimisation of the 3He-counter array, aiming for the best possible performance in terms of neutron detection. The algorithm is based on a geometric representation of two selected detector parameters of merit, namely, the average neutron detection efficiency and the efficiency flatness as a function of a reduced number of geometric variables. The response of the neutron detector is obtained from a systematic Monte Carlo simulation implemented in GEANT4. The robustness of the algorithm allowed us to design a versatile detection system, which operated in hybrid mode includes the full neutron counter and two clover detectors for high-precision gamma spectroscopy. In addition, the system can be reconfigured into a compact mode by removing the clover detectors and re-arranging the 3He tubes in order to maximize the neutron detection performance. Both operation modes shows a rather flat and high average efficiency. In summary, we have designed a system which shows an average efficiency for hybrid mode (3He tubes + clovers) of 68.6% and 64% for neutron energies up to 1 and 5 MeV, respectively. For compact mode (only 3He tubes), the average efficiency is 75.7% and 71% for neutron energies up to 1 and 5 MeV, respectively. The performance of the BRIKEN detection system has been also quantified by means of Monte Carlo simulations with different neutron energy distributions.

Measurement of the heaviestβ-delayed 2-neutron emitter:136Sb

The β-delayed neutron emission probability, Pn , of very exotic nuclei is crucial for the understanding of nuclear structure properties of many isotopes and astrophysical processes such as the rapid neutron capture process (r -process). In addition β-delayed neutrons are important in a nuclear power reactor operated in a prompt sub-critical, delayed critical condition, as they contribute to the decay heat inducing fission reactions after a shut down. The study of neutron-rich isotopes and the measurement of β-delayed one-neutron emitters (β1n) is possible thanks to the Rare Isotope Beam (RIB) facilities, where radioactive beams allow the production of exotic nuclei of interest, which can be studied and analyzed using specific detection systems. This contribution reports two recent measurements of β-delayed neutron emitters which allowed the determination of half-lives and the neutron branching ratio of isotopes in the mass region above A = 200 and N > 126, and a second experiment which confirmed 136 Sb as the heaviest double neutron emitter (β2n) measured so far.

MONSTER: a time of flight spectrometer for β-delayed neutron emission measurements

The knowledge of the β-decay properties of nuclei contributes decisively to our understanding of nuclear phenomena: the β-delayed neutron emission of neutron rich nuclei plays an important role in the nucleosynthesis r-process and constitutes a probe for nuclear structure of very neutron rich nuclei providing information about the high energy part of the full beta strength (Sβ) function. In addition, β-delayed neutrons are essential for the control and safety of nuclear reactors. In order to determine the neutron energy spectra and emission probabilities from neutron precursors a MOdular Neutron time-of-flight SpectromeTER (MONSTER) has been proposed for the DESPEC experiment at the future FAIR facility. The design of MONSTER and status of its construction are reported in this work.

Nuclear astrophysics with neutrons

Neutrons play a crucial role in astrophysics during the heavy element nucleosynthesis. The largest fraction of isotopes heavier than iron is produced by neutron capture processes on short (r process) and long timescales (s process). During the ``slow neutron capture process'' (s process) heavier elements are produced by successive captures of in-situ produced neutrons from the reactions 13C(α,n)16O and 22Ne(α,n)25Mg (with densities of 106−1010 cm−3) in the interior of stars and following β-decays. With this scenario the reaction path runs along the valley of stability up to 209Bi and produces about 50% of the solar abundances of the heavy elements. Important nuclear physics parameters for s-process nucleosynthesis are neutron capture cross sections (for En = 0.3–300 keV, corresponding to stellar temperatures between kT= 8 and 90 keV) and β-decay half-lives. Neutron capture measurements can be performed via activation in a quasi-stellar neutron spectrum utilizing several (p,n) reactions, or by the time-of-flight technique. The ``rapid neutron capture process'' (r process) is responsible for the remaining 50% of the solar abundances. Here neutrons with densities of 1020−1030 cm−3 are captured on a very fast timescale (ms) during a Core Collapse Supernova in a region close to the forming neutron star. The r-process nuclei are thus very short-lived, neutron-rich isotopes up to the actinides, which can only be produced and investigated at large-scale radioactive-beam facilities. Here the most important nuclear physics parameters are masses, half-lives, and at later stages also β-delayed neutrons. This paper will summarize the role of neutrons in nuclear astrophysics and give a short overview about the related astrophysics programs at the GSI Helmholtz research center and the FRANZ facility in Germany.

Performances of a [formula omitted]-delayed neutron detection array at Peking University

A β-delayed neutron detection array composed of a neutron sphere and two neutron walls was constructed in the State Key Laboratory of Nuclear Physics and Technology at Peking University. Recently the performances of this detection array were largely improved and tested with a Co60 source, cosmic rays and C16 and N17 radioactive beams. The Tyvek 1056D paper and silicone grease were chosen for the reflection and coupling materials, respectively. For the neutron sphere with large detection solid angle (30% of 4π steradian), the intrinsic efficiency is about 14.1% at a neutron energy of 1MeV and the detection threshold is about 350keV; for the neutron walls with flexible setup, these values are 36.5% and 200keV, respectively. The combined array of neutron sphere and neutron walls has successfully been applied to measure the β-delayed neutrons emitted from neutron-rich unstable nuclei.

The nuclear car wash: A system to detect nuclear weapons in commercial cargo shipments

A concept for detecting the presence of special nuclear material ( 235U or 239Pu) concealed in intermodal cargo containers has been developed, studied, and recent performance results are described. It is based on interrogation with a pulsed beam of 3–7 MeV neutrons that produce fission events and subsequent detection of their β-delayed neutron emission or β-delayed high-energy γ-radiation reveals the presence of fissionable material. Fission product β-delayed γ-rays above 3 MeV are nearly 10 times more abundant than β-delayed neutrons and are distinct from natural radioactivity and from nearly all of the induced activity in a normal cargo. Detector backgrounds and potential interferences with the fission signature radiation have been identified and quantified. Their impact on detection sensitivity is relatively minor and can be addressed readily. Components of a simple laboratory prototype have been assembled, tested with the most challenging cargo threat scenarios, and results compared to computer simulations. Preliminary results will be presented.

Preliminary results utilizing high-energy fission product γ-rays to detect fissionable material in cargo

A concept for detecting the presence of special nuclear material (235U or 239Pu) concealed in intermodal cargo containers is described. It is based on interrogation with a pulsed beam of 7MeV neutrons that produce fission events and their β-delayed neutron emission or β-delayed high-energy γ radiation between beam pulses provide the detection signature. Fission product β-delayed γ-rays above 3MeV are nearly 10 times more abundant than β-delayed neutrons and are distinct from natural radioactivity and from nearly all of the induced activity in a normal cargo. Detector backgrounds and potential interferences with the fission signature radiation have been identified and quantified.

Signatures of fissile materials: high-energy γ rays following fission

The spectral and temporal dependence of β-delayed γ rays in the range E⩾3.0 MeV from thermal–neutron-induced fission are shown to be characteristic signatures of 235 U and 239 Pu . For the interrogation of large sea-going cargo containers filled with hydrogenous materials, β-delayed γ rays can offer an increase in sensitivity for the detection of these and other fissile materials by as much as 10 3–10 4 compared to the detection of β-delayed neutrons.

Production of neutron-rich surface-ionized nuclides at PARRNe

Yields of neutron-rich isotopes produced by fast neutron induced fission of 238U were measured at the ISOL set-up Production d’Atomes Radioactifs Riches en Neutrons. A surface ion source was used to ionize selectively elements with low ionization potentials. In order to observe also the most n-rich isotopes, the identification was achieved by a combined measurement of β- and γ-rays and β-delayed neutrons. The flux of fast neutrons inducing fission was generated by a 1 μA beam of 26 MeV deuterons stopped in a graphite converter. The target was a standard ISOLDE type 238UC X /graphite (50 g/cm 2) heated by a tantalum oven to about 2050 °C. A large variety of radioactive isotopes could be determined quantitatively. Production rates measured for Rb and Cs isotopes are presented here.

Quantum Barrier Penetration Studies with Oriented Nuclei: Proton and Neutron Emission from Exotic Isotopes

We report a programme of novel experiments involving direct proton emission and β-delayed proton and neutron emission for exotic nuclei oriented at low temperatures. Full modelling of the quantum tunneling and angular distribution has been made for the first time for these processes. Their study has application in many fields of natural science. The study of barrier effects involving angular momentum and non-spherical deformation can be approached more directly via single-particle emission than via alpha decay, which suffers from complications relating to preformation of the alpha particle. Using the new formalism, predictions are presented for angular distributions of β-delayed neutrons from oriented 137I and of direct proton emission from oriented 147Tm.

Super-strong β-decay of 11Li into 11Be ∗ (18 MeV) state: a probe for halo p-wave component?

An experimental study of β-delayed charged particles (H, He, Be), neutrons and γ-rays that follow the 11Li β-decay, clearly indicates a β-feeding to an exited state at 18.1 MeV in 11Be with a large Gamow-Teller strength, B GT ≥ 1.0. The large strength of this β-transition implies a broad overlap of the 11Be ∗ state at 18.1 MeV and the ground state of 11Li. The deduced width of this resonance is ≅ 1 MeV. A strong suppression of the branch from 18.1 MeV state into the 10Be+n channel is observed. This branch, with the largest decay energy, has a partial width less than 1% of the total width. A centrifugal barrier consideration fails to explain this effect. A possible consequence of the observed effect for the 11Li structure is that the admixture of (p 1 2 ) 2 configuration in 11Li might be small.

A modular 4π counter for β-delayed neutrons

We report on the construction of a detector array for low-energy β-delayed neutrons consisting of sixteen NE 213 scintillation counters. Three pulse-shape discrimination techniques were compared, with special emphasis on the discrimination of neutrons from γ-rays in the energy region down to 50 keVee (keV electron equivalent). The efficiency as a function of neutron energy was obtained by comparing the precisely known neutron spectrum from the spontaneous fission of 252Cf and the neutron distribution measured in coincidence with the fission events. The neutron energy was determined with the time-of-flight (TOF) technique. In order to examine the light output function L( E p) for recoil protons, a careful energy calibration was done by fitting the Compton edges from standard γ-ray sources. Neutron efficiencies were also calculated with a Monte Carlo code. Agreement between measured and calculated efficiency curves was observed for two differently shaped detectors and neutron flight paths of 1 and 3 m. This correspondence justifies the efficiency calculation for the 4π array with the source in the center. For neutron energies above 800 keV an error of less than 5% in detection efficiency can be reached.

β-Delayed neutrons from very neutron-rich sodium and magnesium isotopes

β-delayed neutron yields from 29–34Na are measured with a high-efficiency liquid-scintillator neutron detector. P n values and half-lives are deduced for 29–34Na and 31–34Mg descendants.