Photofission Products Research Articles

STRUCTURE OF MASS DISTRIBUTIONS OF PHOTOFISSION PRODUCT YIELDS OF 238U AT 17.5 MeV BREMSSTRAHLUNG ENERGY

The values of 29 relative yields of products belonging to 26 mass chains of photofission of 238U were obtained at a maximum bremsstrahlung energy of 17.5 MeV (near the second chance fission threshold). The stimulation of the 238U photofission reaction was performed at the electron accelerator of the Institute of Electron Physics of the National Academy of Sciences of Ukraine, the M-30 microtron. The GEANT4 toolkit was used to model the components of the bremsstrahlung spectrum. The presence of a fine structure in the mass distribution of heavy product yields was found to be localized in the mass range 133-134, 138-139, and 143-144. A multi gaussian model (superposition of three Gaussians) was used to theoretically describe the structure. The calculated values of product yields using the GEF code describe in general terms and predict the fine structure of the mass distribution of photofission products of 238U.

Photofission delayed gamma-ray measurements in a large cemented radioactive waste drum during LINAC irradiation

Nuclear materials characterization in 870 L cemented waste drums is challenging due to significant neutron and gamma ray attenuation. Among the existing non-destructive approaches, photon active interrogation is of particular interest. A LINAC accelerates electrons to energies greater than 10 MeV, leading to highly penetrating interrogating bremsstrahlung photons that can reach nuclear materials in the depth of the 870 L waste drum and allow the detection of delayed gamma rays emitted by photofission products. Because the interrogating photon and neutron fluxes are extremely high during irradiation, measurements with high purity germanium detectors are usually performed after a long irradiation time, for example by moving the detector or the waste drum. Therefore, such measurements are only sensitive to delayed gamma rays associated to photofission products having sufficient long half-life of at least tens of seconds, which cuts a valuable signal coming from fission products with a shorter half-life. In this paper, we report gamma rays measurements of very short lived photofission products using a 3 in × 3 in cylindrical LaBr3(Ce) fast scintillation detector. A depleted uranium sample was irradiated by a pulsed bremsstrahlung source with maximum energy 17.5 MeV in a macro pulsing mode (1 s irradiation, 2 s cooling). This allowed to acquire a strong delayed gamma signal within a time window of 2 s.A clear signature of nuclear materials was observed even in the depth of a concrete mock-up, with an excellent signal-to-noise ratio beyond 3.5 MeV where the background of nonnuclear activated materials is negligible. These results are compared to a semi analytical model that qualitatively agrees but underestimates the measurements by a constant factor 1.7, probably caused by a wrong beam intensity normalization. Indeed, in this work the LINAC pulse time structure (macro pulsing) was different from its nominal working point used to characterize the beam (usual pulsing mode). Since it allows direct calculations of the detector response to delayed gamma-ray photofission measurements, this model is a fast calculation alternative to time-consuming Monte Carlo simulations, in view of further studies that will follow this feasibility demonstration.

ISOTOPIC IDENTIFICATION OF PHOTOFISSED NUCLEAR MATERIALS IN STAINLESS STEEL CONTAINERS USING DELAYED GAMMA-RAYS

The results of isotope identification of nuclear materials (232Th, 235U, 238U, 239Pu) packed in hermetic stainless steel containers by their stimulated delayed gamma radiation are presented. The ratios of the values of the time dependences of the intensity of gamma radiation of the light and heavy product of their photofission were used for the analysis. Gamma radiation from photofission products 89Rb (1031.9; 1248.1 keV) and 138Cs (1009.8; 1436.8; 2218.0; 2639.6 keV) was used to differentiate nuclear materials. The photofission reaction the samples of nuclear materials was stimulated on an electron accelerator, an M-30 microtron, at maximum bremsstrahlung energy of 12.5 MeV. The possibility of carrying out isotopic identification of nuclear materials using the following sets of ratios of the values of the time dependences of the gamma-line intensity has been demonstrated: X(1031.9)/X(2639.6), X(1248.1)/X(1009.8), X(1248.1)/X(1436.8), X(1248.1)/X(2218.0), X(1248.1)/X(2639.6).

New measurements of cumulative photofission yields of 239Pu, 235U and 238U with a 17.5 MeV Bremsstrahlung photon beam and progress toward actinide differentiation

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.

PRODUCT YIELDS FOR THE PHOTOFISSION OF 239Pu WITH BREMSTRAHLUNG AT 17.5 MeV BOUNDARY ENERGY

The values of relative cumulative yields of 12 products (85mKr, 91mY, 92Sr, 97Zr, 99Mo, 105Ru, 133I, 134I, 135I, 138Cs, 139Ba, 142La, 143Ce) of the 239Pu photofission was measured at a maximum bremsstrahlung energy of 17.5 MeV (av-erage excitation energy ~ 12.03 MeV). 239Pu photofission reaction was stimulated on the electron accelerator of the Institute of Electron Physics NAS of Ukraine – M-30 microtron to simulate the spectra of bremsstrahlung’s photons, secondary electrons, and photoneutrons that hit the 239Pu target, the GEANT4 code was used. The input of accom-panying nuclear reactions to the yield of 239Pu photofission products for the given experimental parameters was also evaluating. The obtained experimental data of the yields of products 239Pu photofission were compared with the program codes GEF and Talys1.9.5 simulations.

Short-lived photofission product yields from [formula omitted] and [formula omitted] at Bremsstrahlung X-ray endpoint energies of 8, 14, and 20 MeV for nuclear forensics isotope production applications

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.

Measurement of cumulative photofission yields of 235U and 238U with a 16 MeV Bremsstrahlung photon beam

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.

Structure of Mass-Yield Distributions of 232Th Photofission Product by Bremsstrahlung at Energy 17.5 MeV

Relevance. One of the most promising areas for studying the fission process is to investigate its features under the action of photon radiation, since the interaction of gamma quanta with the nucleus is completely electromagnetic with well-known characteristics. Information on the yields of 232Th nuclear photofission products is of particular interest from the standpoint of experimental and theoretical studies. The nucleus of this element is located on the border between pre-actinides and light actinides. Purpose. The purpose of this study is to experimentally investigate the structure of the mass distribution of yields of 232Th photofission products at a bremsstrahlung energy of 17.5 MeV (energy close to the threshold of the first-chance fission, where experimental data are not available). Methods. 232Th photofission response was simulated on the electron accelerator of the Institute of Electron Physics NAS of Ukraine – M-30 microtron. The bremsstrahlung spectrum was modelled for the case of electron interaction (E=17.5 MeV) with a tantalum converter (1 mm) using the GEANT4 code 10.7. Yields of 232Th photofission products were measured by gamma-ray spectrometry. The yields of 232Th photofission products were modelled using the GEF 2020 / 1.1 and Talys 1.95 codes. Results. The value of cumulative yields of 23 products (85mKr, 88Kr, 88Rb, 89Rb, 91Sr, 92Sr, 94Y, 95Zr, 97Nb, 99Mo, 101Tc, 131I, 132Te, 133I, 134Te, 135I, 138Cs, 139Ba, 140Ba, 141Ce, 142La, 143Ce, 146Ce) belonging to 22 isobaric mass chains (light: 85; 88; 89; 91; 92; 94; 95; 97; 99; 101, heavy: 131; 132; 133; 134; 135; 138; 139; 140; 141 ; 142; 143; 146 fragments) of the 232Th photofission was measured at a maximum bremsstrahlung energy of 17.5 MeV (average excitation energy ~ 11.3 MeV). The resulting mass distribution of heavy fragments indicates the presence of increased yields of products localized around mass 133-134, 138-139, and 143-144, which is associated with the influence of such a nuclear structure as the proximity of closed nuclear shells and the even-odd effect. Conclusions. The measurement results indicate the presence of a fine structure in the resulting mass distribution of 232Th photofission product yields, which is manifested in increased yields of products localised in the mass regions 133-134, 138-139, and 143-144. The obtained theoretical output values calculated using the GEF 2020 / 1.1 and Talys 1.95 codes describe in general terms and predict the fine structure of the mass distribution of 232Th photofission products

Localization of nuclear materials in large concrete radioactive waste packages using photofission delayed gamma rays

The characterization of radioactive waste packages is mandatory for their transport, interim storage and final disposal. In this framework, the Nuclear Measurement Laboratory of CEA DES IRESNE Institute, at Cadarache, France, uses a high-energy electron linear accelerator (LINAC) to produce an interrogating bremsstrahlung beam with endpoint energies ranging from 9 to 21 MeV to perform X-ray imaging and high-energy photon interrogation on large concrete packages. In particular, highenergy photon beam induces photofission reactions in both fissile (235U, 239Pu, 241Pu) and fertile (238U, 240Pu, 232Th, etc.) actinides possibly present in the radioactive waste. In order to assess their mass, we use delayed gamma rays emitted by their photofission products, which are measured with a 50 % relative efficiency High-Purity Germanium (HPGe) detector. Actinide differentiation, which is important for the fissile mass estimation, is based on the ratios of gamma rays emitted by different photofission products and requires appropriate corrections for the gamma attenuation in concrete. To this aim, we report here a localization method of point-like nuclear materials in the concrete matrix, based on the differential attenuation of several gamma rays emitted by a same photofission product. We use here the 1435.9 and 2639.6 keV lines of 138Cs, with both experimental data and MCNP numerical simulations to determine the (r,θ) coordinates of nuclear materials. Then, the depth inside the concrete matrix, which is determined with a precision of a few percent, mainly depending on counting statistics on 1435.9 and 2639.6 keV net peak areas, is used to correct for the different gamma ratios used in the actinide identification method. Experimental tests with uranium samples have been performed to validate the localization method.

Simulation the Yields of Actinide Nuclei Photofission Products as Sources of Delayed Gamma Radiation for the Needs of Analyzing their Isotopic Composition

Simulation the Yields of Actinide Nuclei Photofission Products as Sources of Delayed Gamma Radiation for the Needs of Analyzing their Isotopic Composition

Fission product yield measurements using monoenergetic photon beams

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.Purpose: In a collaboration between the Triangle Universities Nuclear Laboratory (TUNL), Los Alamos National Laboratory (LANL), and Lawrence Livermore National Laboratory (LLNL), the dependence of a number of cumulative FPYs on the incoming neutron energy has been measured and unexpected energy dependencies of certain fission products have been reported [M. E. Gooden, Nucl. Data Sheets 131, 319 (2016)]. To investigate whether this observation is unique to neutron-induced fission, a program has been initiated to measure FPYs in photon-induced fission.Method: The photon-induced FPYs were measured by a combination of fission counting using a specially designed dual-fission chamber and $\ensuremath{\gamma}$-ray counting. The measurements were carried out with a monoenergetic photon beam at the $\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$ facility. Gamma-ray counting of the activated targets was performed with well-shielded high-purity germanium (HPGe) detectors over a period of two months after irradiation to properly identify the decay history of fission products.Results: We report on our photofission product yield measurements on $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, and $^{239}\mathrm{Pu}$ using a monoenergetic photon beam of ${E}_{\ensuremath{\gamma}}=13\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. More than 40 fission products were uniquely identified, and their yield values were computed. The use of the fission chamber with post-activation measurements has provided absolute fission product yield data with minimal uncertainties.Conclusion: The photon-induced cumulative fission product yields of $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, and $^{239}\mathrm{Pu}$ are compared with previous photon- and neutron-induced fission measurements. In the near future data will be obtained at lower and higher photon energies.

Revision of forensics-relevant nuclear data in Sb127 β decay

With a half-life of 3.85(5) d, the fission product $^{127}\mathrm{Sb}$ is potentially useful for the characterization of fallout debris that would be collected for forensic analysis following a hypothetical nuclear event. The $\ensuremath{\beta}$ decay of $^{127}\mathrm{Sb}$ has many characteristic $\ensuremath{\gamma}$-ray lines, however, the majority of decay strength is concentrated in the three strongest lines near 686, 473, and 784 keV, based on low-precision spectroscopic measurements from decades ago. In a new measurement of $^{127}\mathrm{Sb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay, obtained from radiochemical separations of U photofission products, we show that the energies of the 686 and 784 lines are actually shifted lower by 0.8 and 1.1 keV, respectively. Such large discrepancies in the nuclear data inhibit accurate isotopic analyses and have important implications, for example, on the assessment of fission debris that might be collected in the days following a nuclear event. Therefore, a new evaluation of $^{127}\mathrm{Sb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay and appropriate revisions to the various nuclear databases are warranted. This paper reports a new set of $\ensuremath{\gamma}$-ray energies and relative intensities corresponding to $^{127}\mathrm{Sb}$ decay.

Simulation the yields of actinide nuclei photofission products induced by bremsstrahlung of electron accelerators

An approach that allows to adapt the GEF- code to simulate the yields of the products of the photofission of actinide nuclei is proposed. A simulation of the bremsstrahlung spectrum of the microtron M-30 at electron energies of 8,5 ÷ 20 MeV was carried out. The calculations of the excitation energy have been made for the fissioning nuclei 232 Th, 235 U, 238 U, 237 Np, 239 Pu, 240 Pu, 242 Pu, 241 Am. The modeling of photofission product yields of nuclei 232 Th, 235 U, 238 U, 237 Np, 239 Pu, 240 Pu, 242 Pu, 241 Am with the use of GEF-code was carried out.

Simulation the yields of actinide nuclei photofission products induced by bremsstrahlung of electron accelerators

Simulation the yields of actinide nuclei photofission products induced by bremsstrahlung of electron accelerators

Exploratory study of fission product yield determination from photofission of Pu239 at 11 MeV with monoenergetic photons

Measurements of fission product yields play an important role for the understanding of fundamental aspects of the fission process. Recently, neutron-induced fission product-yield data of $^{239}\mathrm{Pu}$ at energies below 4 MeV revealed an unexpected energy dependence of certain fission fragments. In order to investigate whether this observation is prerogative to neutron-induced fission, a program has been initiated to measure fission product yields in photoinduced fission. Here we report on the first ever photofission product yield measurement with monoenergetic photons produced by Compton back-scattering of FEL photons. The experiment was performed at the High-Intensity Gamma-ray Source at Triangle Universities Nuclear Laboratory on $^{239}\mathrm{Pu}$ at ${E}_{\ensuremath{\gamma}}=11$ MeV. In this exploratory study the yield of eight fission products ranging from $^{91}\mathrm{Sr}$ to $^{143}\mathrm{Ce}$ has been obtained.

Fission product yield measurements using monoenergetic photon beams

Measurements of fission products yields (FPYs) are an important source of information on the fission process. During the past couple of years, a TUNL-LANL-LLNL collaboration has provided data on the FPYs from quasi monoenergetic neutron-induced fission on 235 U, 238 U, and 239 Pu and has revealed an unexpected energy dependence of both asymmetric fission fragments at energies below 4 MeV. This peculiar FPY energy dependence was more pronounced in neutron-induced fission of 239 Pu. In an effort to understand and compare the effect of the incoming probe on the FPY distribution, we have carried out monoenergetic photon-induced fission experiments on the same 235 U, 238 U, and 239 Pu targets. Monoenergetic photon beams of E γ = 13.0 MeV were provided by the HIγS facility, the world's most intense γ-ray source. In order to determine the total number of fission events, a dual-fission chamber was used during the irradiation. These irradiated samples were counted at the TUNL's low-background γ-ray counting facility using high efficient HPGe detectors over a period of 10 weeks. Here we report on our first ever photofission product yield measurements obtained with monoenegetic photon beams. These results are compared with neutron-induced FPY data.

Photofission product yields of 238U and 239Pu with 22-MeV bremsstrahlung

In homeland security and nuclear safeguards applications, non-destructive techniques to identify and quantify special nuclear materials are in great demand. Although nuclear materials naturally emit characteristic radiation (e.g. neutrons, γ-rays), their intensity and energy are normally low. Furthermore, such radiation could be intentionally shielded with ease or buried in high-level background. Active interrogation techniques based on photofission have been identified as effective assay approaches to address this issue. In designing such assay systems, nuclear data, like photofission product yields, plays a crucial role. Although fission yields for neutron-induced reactions have been well studied and readily available in various nuclear databases, data on photofission product yields is rather scarce. This poses a great challenge to the application of photofission techniques. In this work, short-lived high-energy delayed γ-rays from photofission of 238U were measured in between linac pulses. In addition, a list-mode system was developed to measure relatively long-lived delayed γ-rays from photofission of 238U and 239Pu after the irradiation. Time and energy information of each γ-ray event were simultaneously recorded by this system. Cumulative photofission product yields were then determined using the measured delayed γ-ray spectra.

Energy dependence of exotic nuclei production cross sections by photofission reaction in GDR range

Photofission of actinides is studied in the region of nuclear excitation energies that covers the entire giant dipole resonance (GDR) region. The mass distributions of 238U photofission fragments have been explored theoretically for eight different endpoint bremsstrahlung energies from 11.5 to 67.7 MeV, which correspond to average photon energies of 9.09 to 15.90 MeV. Among these energies, 29.1 MeV corresponds to the average photon energy of 13.7 ± 0.3 MeV, which coincides with the GDR peak for 238U photofission. The integrated yield of 238U photofission, as well as charge distribution of photofission products, is calculated and its role in producing nuclei and their neutron-richness is investigated.

Yields of neutron-rich nuclei by actinide photofission in the giant dipole resonance region

Photofission of actinides is studied in the region of nuclear excitation energies that covers the entire giant dipole resonance (GDR) region. A comparative analysis of the behavior of the symmetric and asymmetric modes of photon induced fission as a function of the average excitation energy of the fissioning nucleus is performed. The mass distributions of $^{238}$U photofission fragments are obtained at the endpoint bremsstrahlung energy of 29.1 MeV which corresponds to mean photon energy of 13.7$\pm$0.3 MeV that coincides with GDR peak for $^{238}$U photofission. The integrated yield of $^{238}$U photofission as well as charge distribution of photofission products are calculated and its role in the production of neutron-rich nuclei and their exoticity is explored.

New Experimental Results on the Cumulative Yields From Thermal Fission of $^{235} {\rm U}$ and $^{239}{\rm Pu}$ and From Photofission of $^{235} {\rm U}$ and $^{238} {\rm U}$ Induced by Bremsstrahlung

The yields of fission products are one of the main characteristics of the fission process. In the field of nuclear waste package characterization, using Photon Activation Analysis (PAA), these yields are needed in order to optimize a technique enabling the identification of actinides ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">235</sup> U, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">238</sup> U, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">239</sup> Pu), based on the detection of delayed gamma-rays. As the lack of data in the field of photofission is strongly penalizing for the tuning of this technique, we designed several measurement campaigns in order to determine the yields of various photofission products. The experiments were based on the detection of delayed gamma-rays and delayed neutrons emitted during the same measurement. The feasibility of this technique was first verified in the context of active neutron interrogation, by comparing experimental results for the thermal fission of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">235</sup> U and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">239</sup> Pu with reference values provided by several recent databases (ENDFB 6.8, JEFF 3.1). The method was then applied to active photon interrogation, in order to obtain the yields of nuclides formed by the photofission of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">235</sup> U and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">238</sup> U. This paper presents the experimental results obtained with these measurements.