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80 years of experimental photo-fission research
Abstract
High energy photons, or \(\gamma \)-rays, were among the very first probes used to induce fission. Their significant impact in this field is due to particular properties of the \(\gamma \)-rays, such as the lack of a Coulomb barrier and the low, well-defined angular momentum transfer, but also to the variety of \(\gamma \)-ray sources developed over the years. This variety, going from simple but intense bremsstrahlung beams, through complex virtual photon excitations, to high resolution monochromatic sources of several types, gave rise to extensive photo-fission research programs. The review presents the evolution over more than 80 years of the methodology and instrumentation used in photo-fission experiments. The most important developments in fundamental and applied science are summarized and discussed. The main improvements necessary for the progression of this field into the age of nuclear photonics are outlined.
... Photo-fission research was reviewed recently [58]. A basic experimental technique in the field is the study of FPYs. ...
Nuclear photonics is a emerging field of science which combines research with new generation \(\gamma \)-ray sources based on traditional and laser-based electron accelerators. Here, we discuss isomeric studies carried out with \(\gamma \)-ray beams having either continuous, or quasi-monochromatic photon spectra. In experiments with high-power lasers, intense secondary radiation sources are generated. Such experiments explore isomer population or de-excitation in plasma environments. Laser-accelerated particle beams with high charge also facilitate high-energy nuclear excitations, leading to efficient production of nuclear isomers with higher lying isomeric state (reaching MeV energy level) within very-short time scales. The population of isomeric states is a sensitive diagnostic method for characterization of the laser-driven secondary radiation sources.
This paper addresses some of the of open problems in photonuclear physics which await to be resolved using high-brilliance γ-ray beams, such as precise measurements of total or partial cross sections of photonuclear reactions related to astroparticle physics and nuclear astrophysics. The readiness for such measurements at ELI-NP, as well as the state-of-the-art instrumentation which is available are discussed. The possibility to utilize γ-beams with orbital angular momentum in photonuclear experiments is addressed, too.
The Variable Energy Gamma facility of the Extreme Light Infrastructure-Nuclear Physics center will deliver a gamma beam generated by the Compton scattering of laser and electron beams. It will have the highest spectral density and the lowest bandwidth available worldwide. A suite of several experimental setups is being developed for a wide range of research programs in fundamental and applied nuclear science driven by gamma beams. The proposed design concept for this facility is outlined. A study of the gamma beam properties at its source, as well as those after collimation, is presented. The impact of the variation in the parameters of the electron and laser beams on the quality of the gamma beam is analyzed and discussed.
A specific objective of the recent IAEA Coordinated Research Project on Photonuclear Data and Photon Strength Functions (Code F41032; Duration 2016-2019) has been to measure photonuclear cross-section data where needed, for unexplored nuclei and cases of discrepant existing data. A dedicated experimental campaign has been conducted at the laser Compton-scattering γ-ray source of the NewSUBARU synchrotron radiation facility of SPring8, Japan, where photoneutron reactions for 11 nuclei from ⁹Be to ²⁰⁹Bi have been investigated in the Giant Dipole Resonance energy region. The measurements followed the development of a flat-effi ciency moderated neutron detection array and the associated neutron-multiplicity sorting techniques. The IAEA CRP campaign has been followed at NewSUBARU by new measurements of photofission and photoneutron reactions on ²³⁸U and ²³²Th in the energy range of 5.87 MeV – 20.14 MeV. The neutron-multiplicity sorting of high-multiplicity fission neutron coincidence events has been performed using a dedicated energy dependent, multiple firing statistical treatment. The photoneutron (γ, xn) and photofission (γ, fxn) reactions have been discriminated by considering a Gaussian distribution of prompt-fission-neutrons (PFN) multiplicities predicted by the evaporation theory. We here provide preliminary experimental (γ, n), (γ, 2n) and (γ, F) cross sections, average energies of PFN and of photoneutrons emitted in (γ, n) and (γ, 2n) reactions, as well as the mean number of PFN per fission act. The new ²³⁸U cross sections are compared with recent statistical-model calculations performed with the EMPIRE code on existing data.
The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 2,143 new measurements from 709 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Among the 120 reviews are many that are new or heavily revised, including a new review on Machine Learning, and one on Spectroscopy of Light Meson Resonances.
The Review is divided into two volumes. Volume 1 includes the Summary Tables and 97 review articles. Volume 2 consists of the Particle Listings and contains also 23 reviews that address specific aspects of the data presented in the Listings.
The complete Review (both volumes) is published online on the website of the Particle Data Group (pdg.lbl.gov) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is available in print, as a web version optimized for use on phones, and as an Android app.
Fission product yields (FPYs) are a uniquely sensitive probe of the fission process, with well established dependence on the species of nucleus undergoing fission, its excitation energy and spin. Thus FPYs are well suited for testing Bohr’s hypothesis in the context of nuclear fission, which states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using FPYs, we have performed a new highprecision test of the combined effects of the entrance channel, spin and parity on the fission process from two of the most commonly used particles to induce fission neutrons and photons. The ²³⁹ Pu(n,f) reaction at E n = 4.6 MeV and the ²⁴⁰ Pu(γ,f) reaction at E γ = 11.2 MeV were used to produce a ²⁴⁰ Pu ∗ compound nucleus with the same excitation energy. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL’s FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ -ray Source (HI γ S) facility. The FPYs from these two reactions are compared quantitatively for the first time.
The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 3,324 new measurements from 878 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Among the 120 reviews are many that are new or heavily revised, including a new review on High Energy Soft QCD and Diffraction and one on the Determination of CKM Angles from B Hadrons.
The Review is divided into two volumes. Volume 1 includes the Summary Tables and 98 review articles. Volume 2 consists of the Particle Listings and contains also 22 reviews that address specific aspects of the data presented in the Listings.
The complete Review (both volumes) is published online on the website of the Particle Data Group (pdg.lbl.gov) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is available in print and as a web version optimized for use on phones as well as an Android app.
There has been much recent interest in nuclear fission, due in part to a new appreciation of its relevance to astrophysics, stability of superheavy elements, and fundamental theory of neutrino interactions. At the same time, there have been important developments on a conceptual and computational level for the theory. The promising new theoretical avenues were the subject of a workshop held at the University of York in October 2019; this report summarises its findings and recommendations.
The Gamma Factory initiative proposes to develop novel research tools at CERN by producing, accelerating, and storing highly relativistic, partially stripped ion beams in the SPS and LHC storage rings. By exciting the electronic degrees of freedom of the stored ions with lasers, high‐energy narrow‐band photon beams will be produced by properly collimating the secondary radiation that is peaked in the direction of ions' propagation. Their intensities, up to 10¹⁷ photons per second, will be several orders of magnitude higher than those of the presently operating light sources in the particularly interesting γ–ray energy domain reaching up to 400 MeV. This article reviews opportunities that may be afforded by utilizing the primary beams for spectroscopy of partially stripped ions circulating in the storage ring, as well as the atomic‐physics opportunities made possible by the use of the secondary high‐energy photon beams. The Gamma Factory will enable ground‐breaking experiments in spectroscopy and novel ways of testing fundamental symmetries of nature.
The production cross sections of neutron-rich fission residues in reactions induced by 238U projectiles at 950A MeV impinging on Pb and Be targets were investigated at the Fragment Separator (FRS) at GSI. These two targets allowed us to investigate fission processes induced by two reaction mechanisms, Coulomb- and nuclear-excitations, and to study the role of these mechanisms in the neutron excess of the final fragments.
Composite materials consisting of ⁶Li scintillator particles in an organic matrix can enable thermal neutron detectors with excellent rejection of gamma-ray backgrounds. The efficiency of transporting scintillation light through such a composite is critical to the detector performance. This optical raytracing study of a composite thermal neutron detector quantifies the various sources of scintillation light loss and identifies favorable photomultiplier tube (PMT) readout schemes. The composite material consisted of scintillator cubes within an organic matrix shaped as a right cylinder. The cylinder surface was surrounded by an optical reflector, and the light was detected by PMTs attached to the cylinder end faces. A reflector in direct contact with the composite caused 53% loss of scintillation light. This loss was reduced 8-fold by creating an air gap between the composite and the reflector to allow a fraction of the scintillation light to propagate by total internal reflection. Replacing a liquid mineral oil matrix with a solid acrylic matrix decreased the light transport efficiency by only ∼10% for the benefit of creating an all-solid-state device. The light propagation loss was found to scale exponentially with the distance between the scintillation event and the PMT along the cylinder main axis. This enabled a PMT readout scheme that corrects for light propagation loss on an event-by-event basis and achieved a 4.0% energy resolution that approached Poisson-limited performance. These results demonstrate that composite materials can enable practical thermal neutron detectors for a wide range of nuclear non-proliferation and safeguard applications.
An overview is given on some of the main advances in experimental methods, experimental results and theoretical models and ideas of the last years in the field of nuclear fission.
New approaches extended the availability of fissioning systems for experimental studies of nuclear fission considerably and provided a full identification of all fission products in A and Z for the first time.
In particular, the transition from symmetric to asymmetric fission around
226Th and some unexpected structure in the mass distributions in the fission of systems around Z = 80 to 84 as well as an extended systematics of the odd-even effect in fission fragment Z distributions have been measured [A. N. Andreyev et al., Rep. Progr. Phys. 81 (2018) 016301].
Three classes of model descriptions of fission presently appear to be the most promising or the most successful ones:
Self-consistent quantum-mechanical models fully consider the quantum-mechanical features of the fission process. Intense efforts are presently made to develop suitable theoretical tools [N. Schunck, L. M. Robledo, Rep. Prog. Phys. 79 (2016) 116301] for modeling the non-equilibrium, large-amplitude collective motion leading to fission.
Stochastic models provide a fully developed technical framework. The main features of the fission-fragment mass distribution are well reproduced from mercury to fermium and beyond [P. M\"oller, J. Randrup, Phys. Rev. C 91 (2015) 044316]. However, the limited computer resources still impose restrictions, for example on the number of collective coordinates and on an elaborate description of the fission dynamics.
In an alternative semi-empirical approach [K.-H. Schmidt et al., Nucl. Data Sheets 131 (2016) 107], considerable progress in describing the fission observables has been achieved by combining several theoretical ideas, which are essentially well known.
This approach exploits
(i) the topological properties of a continuous function in multidimensional space,
(ii) the separability of the influences of fragment shells and macroscopic properties of the compound nucleus,
(iii) the properties of a quantum oscillator coupled to the heat bath of other nuclear degrees of freedom,
(iv) an early freeze-out of collective motion, and
(v) the application of statistical mechanics for describing the thermalization of intrinsic excitations in the nascent fragments.
This new approach reveals a high degree of regularity and allows calculating
high-quality data that are relevant for nuclear technology without
specific adjustment to empirical data of individual systems.
This review paper describes the destructive and non-destructive measurements implemented or under development at CEA, in view to perform the most complete radioactive waste characterization. First, high-energy photon imaging (radiography, tomography) brings essential information on the waste packages, such as density, position and shape of the waste inside the container and in the possible binder, quality of coating and blocking matrices, presence of internal shields or structures, presence of cracks, voids, or other defects in the container or in the matrix, liquids or other forbidden materials, etc. Radiological assessment is then performed using a series of non-destructive techniques such as gamma-ray spectroscopy, which allows characterizing a wide range of radioactive and nuclear materials, passive neutron coincidence counting and active neutron interrogation with the differential die-away technique, or active photon interrogation with high-energy photons (photofission), to measure nuclear materials. Prompt gamma neutron activation analysis (PGNAA) can also be employed to detect toxic chemicals or elements which can greatly influence the above measurements, such as neutron moderators or absorbers. Digital auto-radiography can also be used to detect alpha and beta contaminated waste. These non-destructive assessments can be completed by gas measurements, to quantify the radioactive and radiolysis gas releases, and by destructive examinations such as coring homogeneous waste packages or cutting the heterogeneous ones, in view to perform visual examination and a series of physical, chemical, and radiochemical analyses on samples. These last allow for instance to check the mechanical and containment properties of the package envelop, or of the waste binder, to measure toxic chemicals, to assess the activity of long-lived radionuclides or pure beta emitters, to determine the isotopic composition of nuclear materials, etc.
A principle of new nondestructive assay (NDA) technique based on the photofission reaction ratio (PFRR) has been developed, which is aimed at measuring the isotopic composition of nuclear fuel materials without relying on their self-generated neutron information. The feasibility of the PFRR method was validated in the ²³⁵U-²³⁸U system previously. However, applicability of the PFRR method in the other material is not yet validated. In this study, applicability of PFRR method to the ²³²Th-²³³U system was confirmed, PFRR method showed good reproducibility of predicted value of ²³²Th and ²³³U isotopic composition.
Fission-fragment mass distributions were measured for U237–240, Np239–242, and Pu241–244 populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction O18+U238 at the Japan Atomic Energy Agency tandem facility. Among them, the data for U240 and Np240,241,242 were observed for the first time. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the highest measured energy in contrast to expectations of predominantly symmetric fission due to the washing out of nuclear shell effects. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission.
In the last two decades, through technological, experimental and theoretical advances, the situation in experimental fission studies has changed dramatically. With the use of advanced production and detection techniques both much more detailed and precise information can now be obtained for the traditional regions of fission research and, crucially, new regions of nuclei have become routinely accessible for fission studies.
This work first of all reviews the recent developments in experimental fission techniques, in particular the resurgence of transfer-induced fission reactions with light and heavy ions, the emerging use of inverse-kinematic approaches, both at Coulomb and relativistic energies, and of fission studies with radioactive beams.
The emphasis on the fission-fragment mass and charge distributions will be made in this work, though some of the other fission observables, such as prompt neutron and γ-ray emission will also be reviewed.
A particular attention will be given to the low-energy fission in the so far scarcely explored nuclei in the very neutron-deficient lead region. They recently became the focus for several complementary experimental studies, such as β-delayed fission with radioactive beams at ISOLDE(CERN), Coulex-induced fission of relativistic secondary beams at FRS(GSI), and several prompt fusion-fission studies. The synergy of these approaches allows a unique insight in the new region of asymmetric fission around <sup>180</sup>Hg, recently discovered at ISOLDE. Recent extensive theoretical efforts in this region will also be outlined.
The unprecedented high-quality data for fission fragments, completely identified in Z and A , by means of reactions in inverse kinematics at FRS(GSI) and VAMOS(GANIL) will be also reviewed. These experiments explored an extended range of mercury-to-californium elements, spanning from the neutron-deficient to neutron-rich nuclides, and covering both asymmetric, symmetric and transitional fission regions.
Some aspects of heavy-ion induced fusion-fission and quasifission reactions will be also discussed, which reveal their dynamical features, such as the fission time scale. The crucial role of the multi-chance fission, probed by means of multinucleon-transfer induced fission reactions, will be highlighted.
The review will conclude with the discussion of the new experimental fission facilities which are presently being brought into operation, along with promising 'next-generation' fission approaches, which might become available within the next decade.
SOFIA (Studies On Fission with Aladin) is a novel experimental programme, dedicated to accurate measurements of fission-fragment isotopic yields. The set-up allows to fully identify, in nuclear charge and mass, both fission fragments in coincidence for the whole fission-fragment range. It was installed at the GSI facility, to benefit from the relativistic heavy-ion beams available there, and thus to use inverse kinematics. This paper reports on fission yields obtained in electromagnetic-induced fission of 238U.
We successfully performed electron scattering off unstable nuclei which were produced online from the photofission of uranium. The target Cs137 ions were trapped with a new target-forming technique that makes a high-density stationary target from a small number of ions by confining them in an electron storage ring. After developments of target generation and transportation systems and the beam stacking method to increase the ion beam intensity up to approximately 2×107 ions per pulse beam, an average luminosity of 0.9×1026 cm−2 s−1 was achieved for Cs137. The obtained angular distribution of elastically scattered electrons is consistent with a calculation. This success marks the realization of the anticipated femtoscope which clarifies the structures of exotic and short-lived unstable nuclei.
Mean values of the number of protons and neutrons of the primary fission fragments at scission are determined for the asymmetric fission of 16 fissioning isotopes, from Ac219 up to Np238. Our results confirm that the main asymmetric fission mode around the heavier uranium isotopes is indeed characterized by an average atomic number around 〈ZH〉=54 in the heavy fission fragments. However, they also unambiguously show a stabilization effect in the light fission fragments around 〈NL〉=52–54 in the neutron-deficient thorium and actinium isotopes. This is a clear signature that these deformed proton and neutron shell closures around 54 play a major role in the nuclear fission process. The evolution along the thorium chain shows that the neutron shell appears to be dominant in the asymmetric fission of the lighter thorium isotopes, in contrast to the heavier thorium isotopes for which the stabilization originates from the proton shell.
In the frame of a long-term research program on the characterization of large radioactive waste packages by photofission, the Nuclear Measurement Laboratory of CEA IRESNE has measured cumulative yields of ²³⁹Pu, ²³⁵U and ²³⁸U photofission products by using a Bremsstrahlung photon beam produced by a 17.5 MeV linear electron accelerator. A characterization of the energy of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium, zinc and zirconium. The contribution of neutron fission in the different samples has also been estimated by MCNP simulations in order to assess as precisely as possible the photofission yields. Finally, 26 cumulative photofission product yields are reported for ²³⁹Pu, 28 for ²³⁸U and 26 for ²³⁵U, with half-lives ranging from 14 min to more than 3 days, some of them being not recorded so far in the literature. Among these reported photofission product yields, 18 have been measured for all 3 actinides, which can thus be used for their discrimination. A differentiation criterion based on delayed gamma-ray ratios has been established to determine the most efficient photofission product couples to estimate the enrichment of a ²³⁵U/²³⁸U mixture or the fissile fraction (²³⁵U+²³⁹Pu)/actinide mass in a mixture of uranium and plutonium.
Atomic nuclei are quantum many-body systems of protons and neutrons held together by strong nuclear forces. Under the proper conditions, nuclei can break into two (sometimes three) fragments which will subsequently decay by emitting particles. This phenomenon is called nuclear fission. Since different fission events may produce different fragmentations, the end-products of all fissions that occurred in a small chemical sample of matter comprise hundreds of different isotopes, including α particles, together with a large number of emitted neutrons, photons, electrons and antineutrinos. The extraordinary complexity of this process, which happens at length scales of the order of a femtometer, mostly takes less than a femtosecond but is not entirely over until all the lingering β decays have completed – which can take years – is a fascinating window into the physics of atomic nuclei. While fission may be more naturally known in the context of its technological applications, it also plays a crucial role in the synthesis of heavy elements in astrophysical environments. In both cases, simulations are needed for the many systems or energies inaccessible to experiments in the laboratory. In this context, the level of accuracy and precision required poses formidable challenges to nuclear theory. The goal of this article is to provide a comprehensive overview of the theoretical methods employed in the description of nuclear fission.
Past studies validated the feasibility of the photofission reaction ratio (PFRR) method using both Gaussian and bremsstrahlung photons to estimate the isotopic composition of nuclear fuel materials without relying on their self-generated neutron information. However, the current PFRR method cannot solve a multinuclide system with more than two nuclides because the instability of the inverse matrix increases with the addition of the number of nuclides. Thus, this research proposes a numerical method for solving the simultaneous equations of a three-nuclide system onto PFRR to estimate the isotopic composition of nuclides. The results show good reproducibility with all cases maintained within a 10% isotopic composition difference except cases 6 and 7 of the first two photon energy combination schemes with maximum composition differences of 15.6% and 13.9% for 10% actual composition, respectively. A 20% actual composition of case 5 for the second photon energy combination scheme has a deviation of 10.6%, which is slightly larger than the 10% composition difference too. Out of three photon energy combination schemes, 6 MeV – 6.5 MeV – 11 MeV has the highest coefficient of determination for all three nuclides and the smallest deviation of below 10% composition difference. Random sampling with normal distribution was performed on the loss to photofission particles from MCNP with 200 sets for each 10 cases on the 6 MeV – 7 MeV – 11 MeV photon energy combination to study the stochastic errors. The isotopic compositions were calculated with the same numerical method, and the difference between the estimated and actual compositions that resulted were fitted with R. The fitting results show good agreement within 91.5% confidence intervals.
The Gamma Factory (GF) is an ambitious proposal, currently explored within the CERN Physics Beyond Colliders program, for a source of photons with energies up to ≈400 MeV and photon fluxes (up to ≈1017 photons s−1) exceeding those of the currently available gamma sources by orders of magnitude. The high‐energy (secondary) photons are produced via resonant scattering of the primary laser photons by highly relativistic partially‐stripped ions circulating in the accelerator. The secondary photons are emitted in a narrow cone and the energy of the beam can be monochromatized, down to 10−3–10−6 level, via collimation, at the expense of the photon flux. This paper surveys the new opportunities that may be afforded by the GF in nuclear physics and related fields. The Gamma Factory is an ambitious proposal for a source of photons with energies up to hundreds of MeV and photon fluxes exceeding those of the currently available gamma sources by orders of magnitude. The paper surveys the new opportunities in nuclear physics and related fields that may be afforded by the this facility.
Here we present the ELI Gamma Above Neutron Threshold setup recently implemented at the Extreme Light Infrastructure – Nuclear Physics. The main goals of the detector array are presented, as well as the mechanical design and the performance of the individual detectors. In addition, we have performed Geant4 simulations for realistic beam conditions. Finally, we show the performance of the array as a whole and outline the analysis procedure for complex experimental measurements by reproducing the angular correlations of neutrons following spontaneous fission of a ²⁵²Cf source.
A Rapid Belt-driven Irradiated Target Transfer System, named RABITTS, was developed for use at the Triangle Universities Nuclear Laboratory. This system allows for cyclic activation with neutron or photon beams, and measurement of reaction products using γ-ray spectroscopy. Both a 1 meter and 10 meter transfer system have been developed with transit times as low as 0.4 and 1.0 s, respectively. The systems are deployed at the tandem accelerator laboratory for use with monoenergetic neutron beams, and at the High-Intensity γ-ray Source facility for activation using photon beams. A detailed characterization of the systems’ performance and sensitivity is presented. In order to produce the highest accuracy cross-section data, a model for calculating corrections to cyclic activation with variable beam flux is developed and presented. We have commissioned these systems by measuring 197mAu, where we report a measured half-life of 7.73±0.05 s and a ¹⁹⁷Au(n,n′)197mAu isomer production cross of 628±28 mb at neutron energy En=2.0MeV. In addition, we measured 90mZr, where we report a measured half-life of 799.7±8.0ms and a ⁹⁰Zr(n,n′)90mZr isomer production cross of 180±12 mb at En=4.6MeV. These measured half-lives are in excellent agreement with the evaluated values and the cross-section measurements are performed at previously unmeasured incident neutron energies.
A very intense γ beam of the Gamma Factory facility proposed at CERN can be used to generate radioactive ion beams (RIBs) with high production yields and study the structure of exotic neutron-rich nuclei. The radioactive nuclides are generated via photo-fission in several actinide targets and thermalized in high-purity cryogenic helium, filling a gas cell which is enclosing the targets. Electric fields are used to extract heavy ions and form RIBs which can be send to various selection and measurement stations. Estimates for the production and extraction yields of exotic neutron-rich nuclei with such a setup are provided. A study of the impact of space charge, build-up inside the gas cell, on the extraction properties is presented and it is demonstrated that the beam needs to be chopped for achieving optimal extraction yields.
Low-energy fission of 234,235 U and 237,238 Np radioactive beams, provided by the Fragment Separator (FRS) of the GSI Helmholtzzentrum für Schwerionenforschung facility (GSI), has been studied using the Reactions with Relativistic Radioactive Beams / Studies on Fission with Aladin (R3B/SOFIA) setup. The latter allows us, on an event-by-event basis, to simultaneously identify, in terms of their mass and atomic numbers, the fissioning nucleus in coincidence with both fission fragments after prompt-neutron emission. This article reports new results on elemental, isotonic, isobaric, and isotopic yields. Moreover, the high accuracy of our data allowed us to study in detail proton even-odd staggering, from elemental yields; neutron excess, from isotopic yields; and total prompt-neutron multiplicity, from the difference of masses of the fissioning nucleus and fission fragments. These results are then compared to previous experimental data in order to probe how these fission observables change as function of the excitation energy and atomic and neutron numbers of the compound nucleus.
Nuclear reactions induced by photons play a vital role for very different aspects of basic research and applications in physics. They are a key ingredient for the synthesis of nuclei in the Universe and provide, due to the selectivity and the model-independence of the reaction mechanism, an extremely valuable probe for researchers. The penetrability of photons in the MeV energy range makes them, in addition, an ideal tool for meeting various societal challenges. The last two decades saw a rapid development of advanced photon sources and detection methods for photonuclear reaction products. Bremsstrahlung and quasi-monoenergetic photon beams with unprecedented intensity and quality combined with state-of-the-art detector technology paved the way for new scientific discoveries and technological applications.
This review focuses on a comprehensive overview of the most important developments since the turn of the millenium restricted to the energy range between atomic and hadronic degrees of freedom. This includes a description of the formalism of photonuclear reactions below and above the particle-separation threshold. The most important techniques used to generate photon beams in the MeV energy range are presented along with selected facilities and instrumentation for diagnostics and for the analysis of photonuclear reactions. The power of photons to probe the atomic nucleus is exemplified in a number of selected examples from fundamental and applied science. New developments, facilities, and ideas promise a vivid future for photonuclear physics.
Shanghai laser electron gamma source (SLEGS) is an energy-variable gamma-ray source developed based on the inverse Compton scattering of 10640 nm photons from a 100 W CO2 laser on 3.5 GeV electrons from the Shanghai Light Source. It produces gamma-rays in the energy range of 0.4–21.7 MeV with a flux of 105–107 photons/s. A collimator system was designed to produce quasimonochromatic gamma-ray beams with a best possible bandwidth of 1.1–4.4% and filter out the substandard gamma-rays. The collimator system composed of coarse and fine collimators, constitutes a critical component of the SLEGS. By Geant4 simulations, we show that the collimator system is effective in tuning beam bandwidths and regular dodecagonal beam spots at the target position.
The existing photofission product yield data sets are often based on nuclear models, and limited experimental measurements of these fission product yields have been performed. These experiments resulted in measured cumulative fission product yields (CFPYs) for ²³⁸U and ²³²Th for the bremsstrahlung X-ray endpoint energies of 8, 14, and 20 MeV. The half-lives of the reported fission products range from 1.07–40.8 s. This work is motivated by a demonstrated need for novel isotope production methods for nuclear forensics and improved nuclear data to support the nonproliferation community. A high-purity germanium detector and a pneumatic transfer system were employed for measurements of short-lived fission products, collecting data between cycles of accelerator irradiation and measurement.
In the frame of a long-term research program on the characterization of large radioactive waste packages by photofission, the Nuclear Measurement Laboratory of CEA IRESNE, France, has measured cumulative photofission yields of ²³⁵U and ²³⁸U short-lived and long-lived fission products by using a Bremsstrahlung photon beam produced by a 16 MeV electron linear accelerator (LINAC). To this aim, a characterization of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium and zirconium. The residual neutron flux exiting the LINAC head (lead collimator, borated polyethylene and cadmium shield) has also been characterized by neutron activation analysis with indium samples to estimate the contribution of photoneutron fissions in the uranium samples used to assess the photofission yields. Finally, 49 fission product yields are reported for ²³⁸U and 26 for ²³⁵U, with half-lives ranging from 64 s to more than 3 days, some of them not recorded so far in the literature Some photofission products cumulative yields show significant differences between ²³⁵U and ²³⁸U, which confirms the possibility of an isotopic discrimination method based on delayed gamma-ray ratios analysis for radioactive waste characterization.
The applicability of the photofission reaction ratio (PFRR) method to identify high-enriched uranium is studied by switching the photon source from the Gaussian spectrum to a bremsstrahlung spectrum. The combination of 6 and 11 MeV Gaussian photon energy was used in previous study for applying PFRR method because the photofission cross sections between ²³⁵U and ²³⁸U at these two energies differ significantly. A parametric study was conducted with the incident electron energy 7.0 MeV and the result showed that these electrons generate similar photofission reactions as that of 6.0 MeV Gaussian photons, while 13.5 MeV electrons is found to give even better results for PFRR considering the measurement noise contributed by multiple neutron emission, (γ, 2n) reaction. Bremsstrahlung photons were injected into a 1 mm thick uranium metal target with varying uranium enrichment. The PFRR increases linearly with uranium enrichment, and is found to provide a higher sensitivity than the Gaussian photons owing to higher total photofission reactions.
Comprehensive calculations of photon-induced reactions on U233–238 targets for incident photon energies from 3 up to 30 MeV are undertaken with the statistical model code empire-3.2 Malta. Results are compared with the experimental data from EXFOR and with the current evaluations. The differences and the similarities between the models and parameters used in calculations of photon- and neutron-induced reactions on the same nuclei are discussed with focus on fission. The role of the extended optical model for fission that includes partial damping in the continuum in improving the description of the measured data is pointed out.
The existing CERN accelerator infrastructure is world unique and its research capacity should be fully exploited. In the coming decade its principal modus operandi will be focused on producing intense proton beams, accelerating and colliding them at the Large Hadron Collider (LHC) with the highest achievable luminosity. This activity should, in our view, be complemented by new initiatives and their feasibility studies targeted on re-using the existing CERN accelerator complex in novel ways that were not conceived when the machines were designed. They should provide attractive, ready-to-implement research options for the forthcoming paradigm-shift phase of the CERN research. This paper presents one of the case studies of the Gamma Factory initiative (Krasny, 0000) – a proposal of a new operation scheme of ion beams in the CERN accelerator complex. Its goal is to extend the scope and precision of the LHC-based research by complementing the proton–proton collision programme with the high-luminosity nucleus–nucleus one. Its numerous physics highlights include studies of the exclusive Higgs-boson production in photon–photon collisions and precision measurements of the electroweak (EW) parameters. There are two principal ways to increase the LHC luminosity which do not require an upgrade of the CERN injectors: (1) modification of the beam-collision optics and (2) reduction of the transverse emittance of the colliding beams. The former scheme is employed by the ongoing high-luminosity (HL-LHC) project. The latter one, applicable only to ion beams, is proposed in this paper. It is based on laser cooling of bunches of partially stripped ions at the SPS flat-top energy. For isoscalar calcium beams, which fulfil the present beam-operation constrains and which are particularly attractive for the EW physics, the transverse beam emittance can be reduced by a factor of 5 within the 8 seconds long cooling phase. The predicted nucleon–nucleon luminosity of LNN=4.2×1034s⁻¹cm⁻² for collisions of the cooled calcium beams at the LHC top energy is comparable to the levelled luminosity for the HL-LHC proton–proton collisions, but with reduced pile-up background. The scheme proposed in this paper, if confirmed by the future Gamma Factory proof-of-principle experiment, could be implemented at CERN with minor infrastructure investments.
Taking benefit of the R3B=SOFIA setup to measure the mass and the nuclear charge of both fission fragments in coincidence with the total prompt-neutron multiplicity, the scission configurations are inferred along the thorium chain, from the asymmetric fission in the heavier isotopes to the symmetric fission in the neutron-deficient thorium. Against all expectations, the symmetric scission in the light thorium isotopes shows a compact configuration, which is in total contrast to what is known in the fission of the heavier thorium isotopes and heavier actinides. This new main symmetric scission mode is characterized by a significant drop in deformation energy of the fission fragments of about 19 MeV, compared to the well-known symmetric scission in the uranium-plutonium region.
Photo-induced reaction cross section data are of importance for a variety of current or emerging applications, such as radiation shielding design and radiation transport analyses, calculations of absorbed dose in the human body during radiotherapy, physics and technology of fission reactors (influence of photo-reactions on neutron balance) and fusion reactors (plasma diagnostics and shielding), activation analyses, safeguards and inspection technologies, nuclear waste transmutation, medical isotope production and astrophysical applications.
To address these data needs the IAEA Photonuclear Data library was produced in 1999, containing evaluated photo-induced cross sections and neutron spectra for 164 nuclides which were deemed relevant for the applications.
Since the release of the IAEA Photonuclear Data Library however, new experimental data as well as new methods to assess the reliability of experimental cross sections have become available. Theoretical models and input parameters used to evaluate photo-induced reactions have improved significantly over the years. In addition, new measurements of partial photoneutron cross sections using mono-energetic photon beams and advanced neutron detection systems have been performed allowing for the validation of the evaluations and assessments of the experimental data. Furthermore, technological advances have led to the construction of new and more powerful gamma-beam facilities, therefore new data needs are emerging.
We report our coordinated efforts to address these data needs and present the results of the new up-to-date evaluations included in the new updated IAEA Photonuclear Data Library consisting of 219 nuclides. The new library includes 188 new evaluations produced by the CRP evaluators, and one evaluation taken from the JENDL/PD-2016 library, while 20 evaluations were retained from the previous 1999 IAEA Photonuclear Data Library. In most of the cases, the photon energy goes up to 200 MeV. A total of 55 nuclides are new in this library reflecting the progress in measurements but also the developing data needs. In this paper we discuss the new assessment method and make recommendations to the user community in cases where the experimental data are discrepant and the assessments disagree. In addition, in the absence of experimental data, we present model predictions for photo-induced reaction cross section on nuclides of potential interest to medical radioisotope production.
A new method is presented to measure the emission angle of fission fragments in a double Frisch-grid ionisation chamber which is independent of the measured fragment energy. The emission axis measurement is dependent only on the relative timing of the charge-integrated signals from a triply radially-segmented anode, the geometry of the detector and the drift velocity of liberated electrons. The energy independence means that corrections for Frisch-grid inefficiency and gain matching of pre-amplifiers are not required. An isotropic ²⁵²Cf(sf) source has been used to test this method by comparing experimental data to a Monte-Carlo calculation of the angular efficiency of the detector. Agreement is found between the simulated and measured efficiencies. The method has also been compared to one of the established polar-angle measurement methods, where agreement is once again found.
The Ionization by Radial Electron Neat Adaptation (IRENA) ion source has been designed to operate under extreme radiation conditions. Based on the electron beam generated plasma concept, the ion source is specifically adapted for thick target exploitation under intense irradiation. A validation prototype has been already designed and tested offline. The design of a new optimized prototype for online difficult beams production with ISOL facilities will be presented and discussed. In particular, simulation activities for thermionic emission, ions confinement and extraction will be presented and results discussed.
Background: High-accuracy and self-consistent fission product yield (FPY) data are needed to advance microscopic/macroscopic descriptions of the nuclear fission process, to improve the predictive power of phenomenological models, and for applications in nuclear energy, nuclear forensics, and homeland security.
The inverse kinematics technique, applied to radioactive beams and combined to the Coulomb excitation method, is a powerful tool to study low-energy fission. A novel experimental set-up was developped within the R3B/SOFIA (Reactions with Relativistic Radioactive Beams / Studies On FIssion with Aladin) collaboration to identify in mass and atomic numbers both fission fragments in coincidence. These new data provide, for the first time, elemental, isobaric and isotonic yields for the fission along the thorium isotopic chain. Results are also compared to previous measurements using, either the same reaction mechanism, or thermal-neutron induced fission. This latter comparison permits to probe the influence of the excitation energy in the fission process.
Low Temperature Nuclear Orientation (LTNO) experiments can probe magnetic properties of nuclei. The presence of PolarEx at the ALTO facility allows to perform these kind of on-line experiments with neutron rich beams. This paper presents the formalism of the LTNO technique and the set-up of PolarEx. It focuses on the analysis process for multipolarity mixing ratio extraction.
Producing intense radioactive beams, in particular those consisting of short-lived isotopes requires the control of the release efficiency. The released fractions of 11 elements were measured on 14 samples that were characterized by various physicochemical analyses in a correlated paper (Part 1). A multivariate statistical approach, using the principal component analysis, was performed to highlight the impact of the microstructure on the release properties. Samples that best release fission products consist of grains and aggregates with small size and display a high porosity distributed on small diameter pores. They were obtained applying a mixing of ground uranium dioxide and carbon nanotubes powders leading to homogeneous uranium carbide samples with a porous nanostructure. A modelling under on-line ALTO conditions was carried out using the FLUKA code to compare the yields released by an optimized and a conventional target.
A study of photofission of 232Th and 238U was performed by using quasimonoenergetic, linearly polarized γ-ray beams from the High Intensity γ-ray Source at Triangle Universities Nuclear Laboratory. The prompt-photofission neutron polarization asymmetries, neutron multiplicities, and the photofission cross sections were measured in the near-barrier energy range of 4.3 to 6.0 MeV. This data set constitutes the lowest energy measurements of those observables to date using quasimonoenergetic photons. Large polarization asymmetries are observed in both nuclei, consistent with the E1 excitation as observed by another measurement of this kind made in a higher-energy range. Previous experimental evidence of a deep third minimum in the 238U fission barrier has been identified as an accelerator-induced background.
Performance of the ELIGANT-GN array being developed at ELI-NP was evaluated using dedicated GEANT4 simulation code. The array is designed to consist of 17 LaBr3:Ce and 17 CeBr3 detectors as well as 33 BC501A and 29 ⁶Li-glass detectors. The energy and time responses of the detectors were studied giving information on gamma and neutron absolute detection efficiencies of the array. A planned day-one experiment regarding the gamma and neutron decay from GDR in ²⁰⁸Pb is discussed and, based on simulations, the data analysis procedure is proposed. The detection efficiencies as well as the count rates estimations are presented.
Microscopic methods and tools to describe nuclear dynamics have considerably been improved in the past few years. They are based on the time-dependent Hartree–Fock (TDHF) theory and its extensions to include pairing correlations and quantum fluctuations. The TDHF theory is the lowest level of approximation of a range of methods to solve the quantum many-body problem, showing its universality to describe many-fermion dynamics at the mean-field level. The range of applications of TDHF to describe realistic systems allowing for detailed comparisons with experiment has considerably increased. For instance, TDHF is now commonly used to investigate fusion, multi-nucleon transfer and quasi-fission reactions. Thanks to the inclusion of pairing correlations, it has also recently led to breakthroughs in our description of the saddle to scission evolution, and, in particular, the non-adiabatic effects near scission. Beyond mean-field approaches such as the time-dependent random-phase approximation (TDRPA) and stochastic mean-field methods have reached the point where they can be used for realistic applications. We review recent progresses in both techniques and applications to heavy-ion collision and fission.
Background: High-accuracy data are needed to advance microscopic descriptions of the nuclear fission process, to improve the predictive precision of phenomenological models, and for applications in nuclear energy and homeland security. Purpose: The main goal of this work is to provide high-accuracy cross-section data for photofission of U235, U238, and Pu239 in the energy range from the fission threshold to the high-energy tail of the giant dipole resonance. These new data should contribute significantly to the reduction of the systematic uncertainty in evaluated photofission cross-section databases. Method: Cross-section ratios for photofission of the "big three" actinide nuclei were measured using a quasimonoenergetic photon beam and dual-fission chambers. The measurements were performed at the HIγS facility using a photon beam produced from Compton backscattering of free-electron laser light. The dual-fission chamber enabled simultaneous counting of fission events from two targets. This method allows for cross-section ratio measurements with very small systematic errors. The largest source of systematic error in the determination of the cross-section ratios is the uncertainty in the ratio of the target thicknesses. Results: We report photon-induced fission cross-section ratios for σ[U235(γ,f)/U238(γ,f)], σ[U235(γ,f)/Pu239(γ,f)], and σ[U238(γ,f)/Pu239(γ,f)] at photon energies from 9.0-17.0 MeV. More than 20 data points were measured for each ratio, and the systematic uncertainties are less than 2%. Conclusion: The present photofission cross-section ratio data sets are compared to ratios computed from previous measurements, to the corresponding neutron-induced fission cross-section ratios, and to ratios computed from evaluated databases. The data obtained in this work for σ[U235(γ,f)/U238(γ,f)] are consistent with the existing data, while for σ[U235(γ,f)/Pu239(γ,f)] and σ[U238(γ,f)/Pu239(γ,f)] the present data are systematically lower.
Despite longstanding speculation concerning the harnessing of atomic energy, the prospect of doing so was not a foregone conclusion upon the discovery of fission in late 1938. Many aspects of the new phenomenon such as secondary neutrons, cross‐sections, and energetics had to be investigated before the feasibility of nuclear reactors or explosives could be established. Here, experimental and theoretical developments in the 18 months following the discovery of fission are summarized; these develeopments laid the groundwork for wartime nuclear programs. Despite longstanding speculation concerning the harnessing of atomic energy, the prospect of doing so was not a foregone conclusion upon the discovery of fission in late 1938. Experimental and theoretical developments in the 18 months following the discovery of fission are summarized.
The cumulative yields of various fission products within the 77–153 mass regions in the 2.5-GeV bremsstrahlung-induced fission of Th232 have been determined by using the recoil catcher and an off-line γ-ray spectrometric technique at the Pohang Accelerator Laboratory, Korea. The mass-yield distributions were obtained from the cumulative yields after charge-distribution corrections. The peak-to-valley (P/V) ratio, the average value of light mass (〈AL〉) and heavy mass (〈AH〉), and the average postfission number of neutrons (〈v〉expt) were obtained from the mass yield of the Th232(γ,f) reaction. The present and literature data in the Th232(γ,f) reaction were compared with the similar data in the U238(γ,f) reaction at various excitation energies to examine the role of potential energy surface and the effect of standard I and standard II asymmetric modes of fission. It was found that (i) even at the bremsstrahlung end-point energy of 2.5 GeV, the mass-yield distribution in the Th232(γ,f) reaction is triple humped, unlike U238(γ,f) reaction, where it is double humped. (ii) The peak-to-valley (P/V) ratio decreases with the increase of excitation energies. However, the P/V ratio of the Th232(γ,f) reaction is always lower than that of the U238(γ,f) reaction due to the presence of a third peak in the former. (iii) In both the Th232(γ,f) and U238(γ,f) reactions, the nuclear structure effect almost vanishes at the bremsstrahlung end-point energies of 2.5–3.5 GeV.
An empirical parametrization for the production cross sections of U238 photofission fragments at low energies (Eγ<30 MeV) is developed. This parametrization, GIFU238, consists of three parts, namely, the photofission cross section, the mass yield, and the isobaric charge distribution. The photofission cross section is parametrized to reproduce measured data over a wide energy range. The mass yield distribution is described by the energy-dependent multimodal fission model. The isobaric charge distribution is improved, relative to other parametrizations, to describe many experimental data sets over a broad mass range. Comparisons with different measured U238 photofission data sets at various energies reveal that GIFU238 is in very good agreement with measured elemental and mass yields and can accurately reproduce experimental isotopic yields. Production cross sections (yields) of photofission fragments calculated by this parametrization indicate that many neutron-rich nuclei approaching the r-process path can be accessed via photofission of U238 at radioactive-beam facilities.