Heavy Fission Fragments Research Articles

Total excitation energy distribution for neutron-induced fission and photo-fission of uranium isotopes

The fission fragment total excitation energy, TXE(A), is investigated for neutron-induced fission of uranium isotopes using three different methods. Different methods for calculations of the TXE(A) produced by the decay of low excited systems are analyzed and their results are compared with the available TXE values. The calculated TXE values have been compared with the results of other studies for the neutron induced fission of 233U, 235U as well as the photo-fission of 238U. The scission point method for evaluating TXE has been modified by comparing the available and calculated results. TXE(A) is evaluated for other uranium isotopes using three methods. It is found that TXE values in the neutron induced fission of 233U, 235U and the photo-fission of 238U are greater than the calculated TXE values of other uranium isotopes. The TXE values of light uranium isotopes decrease with increasing their mass numbers, while the TXE values of heavy uranium isotopes increase as their mass numbers increase. Also, the calculated TXE values for all isotopes increase sharply near the symmetric region, which is due to the formation of unstable fission fragment. On the other hand, a slight increase in neutron multiplicity and TXE values can be observed in heavy fission fragments (Ah>155), which could be due to the neutron excess. However, the sharp increase in neutron multiplicity and TXE values in some studies for the heavy fission fragments (Ah>155) is due to a missing correction in the data analysis.

Total kinetic, excitation energy and neutron multiplicity distribution of californium isotopes

The mean total kinetic energy as a function of fission fragments, <TKE>(A), is calculated for neutron-induced fission of 247,249,251,253,255Cf, as well as for spontaneous fission of 248,250,252,254Cf using the scission point model. A comparative analysis is performed between the calculated results and existing experimental data. Investigation into the behavior of <TKE>(A) is conducted, particularly focusing on intermediate and heavy actinides. While nuclear and Coulomb energy evaluations suffice for estimating <TKE> values in light actinides, this approximation proves inadequate for evaluating TKE values for intermediate and heavy actinides. Additionally, an evaluation is made of the total excitation energy as a function of fission fragments, <TXE>(A), concerning spontaneous fission in 247−254Cf. It is found that TXE values sharply increase near the symmetric region across all isotopes and exhibit a gradual increase in heavy fission fragments. Furthermore, an exploration of the total neutron multiplicity as a function of fission fragments, νT(A), is conducted for 252Cf fission using two methods. The νT(A) results obtained through these methods are compared with experimental data, and νT(A) is estimated for unmeasured californium isotopes. The average neutron emission per fission event, ν¯(Ai), is then calculated for spontaneous fission of 251Cf and 253Cf, yielding values of 3.46 and 3.59, respectively.

Machine learning methods for fission product identification from Bragg curves

A fission time projection chamber (fission-TPC) was developed to provide precise neutron-induced fission measurements for several major actinides. As fission fragments lose energy in one of the gas volumes of the fission-TPC, energy loss information is captured and may be used to determine fission product yields as the stopping power of an ion is dependent on the atomic number. The work presented here demonstrates the ability to apply machine learning techniques for Bragg curve classification. A set of one million energy loss curves for 24 different fission-fragment elements was generated using common stopping power software. A ResNet architecture optimized for 1D data was used to train, test, and validate a model for light and heavy fission fragments using the simulated data. The resultant classification accuracy for the light and heavy fragments indicates that this could be a viable method for elemental classification of data from the fission-TPC.

Charge polarization calculated with a microscopic model for the fission fragments of U-236

We investigated theoretical methods to estimate the charge polarization of the fission fragments. The method is based on the microscopic mean-field model with the nucleon degree of freedom, and is represented in the three-dimensional coordinate space for dealing with any nuclear deformation. We employed the Skyrme effective interaction for the calculations and uranium-236 as a target nucleus. We reported static and dynamic methods in which the potential energy surface of the fissioning nucleus concerning the quadrupole and octupole deformations are used. Although the static method deduced the charge polarization which indicates the nuclei with the magic number and the octupole deformation, the quantities are not consistent with the existing data library (Wahl’s systematics). The dynamic method with the iso-energy initial condition on the potential energy surface showed the finite charge polarization of light and heavy fission fragments which is consistent with the data library. Furthermore, it indicates that the charge polarization has energy dependence of the initial states of the dynamic method.

Fragmentation analysis of 88Mo* compound nucleus in view of different decay mechanisms

In reference to the experimental data, the decay mechanism of 88Mo* compound system formed in 48Ti+40Ca reaction is investigated at three beam energies (Ebeam = 300, 450, and 600 MeV) using the collective clusterization approach of Dynamical Cluster decay Model (DCM). The calculations are done for spherical choice of fragmentation and with the inclusion of quadrupole (β2) deformations having “optimum” orientations. According to the experimental evidence 88Mo* decays via Fusion-Evaporation (FE) and Fusion-Fission (FF) processes, thus the decay cross-sections of this hot and rotating compound system are calculated for both channels. In FF decay mode, the explicit contribution of Intermediate Mass Fragments (IMF), Heavy Mass Fragments (HMF) and fission fragments (symmetric/asymmetric) is detected within DCM framework. The calculated FE and FF decay cross-sections find nice agreement with the available experimental data. Experimentally, it has been observed that the total contribution of FE and FF decay cross-sections is less than the total reaction cross-sections possibly due to the presence of some nCN component such as deep inelastic collisions (DIC), which generally contributes above critical angular momentum (ℓcr). The possibility of DIC contribution can be addressed as a future assignment in view of diminishing pocket of interaction potential above ℓcr.

Fission fragments and energy spectra of primary electrons in a fissioning plasma

Some isotopes like 3He, 23BUF6 gases irradiated by thermal neutron flux may produce partially ionized plasma with the presence of fast particles like protons, tritium or heavy fission fragments. If a sample with 3He gas sets in thermal neutron flux, the helium atoms may born highly energetic particles: protons and tritium nuclei, which create in their own turn primary electrons and their other successive generations.In the present paper Boltzmann kinetic equation for plasma, created by volume source of ionization is developed and solved for special cases, like presence of external electric field, strong enough to cause Townsend ionization. The ways of possible existing of highly energetic electrons accelerated by strong electric field energy also thoroughly discussed. Statistical approach based on the Monte Carlo technique including definition of self-consistent electric field formed by boundary conditions and nuclear induced plasma internal properties coupled with direct Boltzmann formalism also was developed and discussed. Thus the outbreak or the time explosion of the energy distributions of all fast particle including primary and secondary electrons are calculated in the programming complex as a function of time from 10 ps to 10 ns and for different neutron flux pulses and neutrons homogeneous spatial distribution. Non-linear problems such as an electron–ion recombination and successive generation of secondary electrons and electron queues are solved by Monte Carlo technique and comprehensively described. Detailed calculations show that so called runaway electrons are not appearing in nuclear induced plasma and whatever strong external electric field applied, there is no way to supply them relativistic energy and create so called runaway electrons. The appearance of electrons with an energy around MeV is possible when electrons (beta electrons) are born within nucleus due to internuclear transformations of up and down quarks affected by neutrinos oscillations. It takes place in nuclear fission process provoked by neutrinos and antineutrinos Reines et al (1960 Phys. Rev. 117 159). Time dependent spectra of primary electrons as well as their successive generations are calculated and compared with existing experimental data and developed for nuclear induced Boltzmann kinetic equation solutions. It is also should be noted that the bremsstrahlung radiation or braking radiation can not be treated as a type of force (friction force). It is clear that the transformation of kinetic energy into electromagnetic energy is a unique phenomenon and if it exists while fast electron is stopping in the medium, then there should be the reversed process, when electron gains its kinetic energy from surrounding electric field.

Fine structure effect among heavy-ion induced fission fragments at near and above barrier energies

The fission decay analysis of 201Bi*, 206Po*, 212Rn*, 216Ra*, 227Pa* and 228U* nuclei produced in 19F-induced reactions at near and above barrier energies is carried out within the framework of dynamical cluster-decay model (DCM) based on quantum mechanical fragmentation theory (QMFT). The interplay between two modes of fission, i.e. symmetric (SF) and asymmetric (aSF), in the fission fragment mass distributions is investigated. For the present set of calculations the SF to aSF peak ratio is less than unity ($ \frac{P_{SF}}{P_{aSF}} < 1$), which in turn suggests the dominance of asymmetric fission for above mentioned compound nuclei. The role of shell corrections and deformations of fragments in the fission dynamics is duly addressed. The calculated fission cross-sections show nice agreement with the experimental data, except for few energies above the Coulomb barrier. This disagreement is associated with the possible presence of non-compound nucleus (nCN) fission as higher energies are more prone to the nCN process. The study of fission fragment mass distributions is further extended to the isotopic analysis of above mentioned pre-actinide and actinide nuclei at common centre-of-mass energy $ E_{c.m.} \approx 80$ MeV near the Coulomb barrier. It is observed that the lighter isotopes exhibit symmetric fission distribution, whereas heavier ones prefer to decay via asymmetric path. A transition between SF and aSF occurs around fissioning nuclei with $ N/Z \approx 1.4$, showing the triple humped mass distribution suggesting comparable contribution of symmetric and asymmetric fission. Finally, the most energetic asymmetric light ($ A_L$) and heavy ($ A_H$) fission fragments are identified for a long isotopic chain of pre-actinide and actinide nuclei. Interestingly, the heavier asymmetric fission fragments are located near a proton number around $ Z=50$ for the pre-actinide region and $ Z=54$ for actinide nuclei.

Impact of pear-shaped fission fragments on mass-asymmetric fission in actinides.

Nuclear fission of heavy (actinide) nuclei results predominantly in asymmetric mass splits1. Without quantum shell effects, which can give extra binding energy to their mass-asymmetric shapes, these nuclei would fission symmetrically. The strongest shell effects appear in spherical nuclei, such as the spherical 'doubly magic' (that is, both its atomic and neutron numbers are 'magic' numbers) nucleus 132Sn, which contains 50 protons and 82 neutrons. However, a systematic study of fission2 has shown that heavy fission fragments have atomic numbers distributed around Z=52 to Z=56, indicating that the strong shell effects in 132Sn are not the only factor affecting actinide fission. Reconciling the strong spherical shell effects at Z=50 with the different Z values of fission fragments observed in nature has been a longstanding puzzle3. Here we show that the final mass asymmetry of the fragments is also determined by the extra stability provided by octupole (pear-shaped) deformations, which have been recently confirmed experimentally around 144Ba (Z=56)4,5, one of very few nuclei with shell-stabilized octupole deformation6. Using a quantum many-body model of superfluid fission dynamics7, we find that heavy fission fragments are produced predominantly with 52 to 56 protons, which is associated with substantial octupole deformation acquired on the way to fission. These octupole shapes, which favour asymmetric fission, are induced by deformed shells at Z=52 and Z=56. By contrast, spherical magic nuclei are very resistant to octupole deformation, which hinders their production as fission fragments. These findings may explain surprising observations of asymmetric fission in nuclei lighter than lead8.

Fission fragment velocity distribution measurement using time of flight technique

Two large area position sensitive Multi Wire Proportional Counters (MWPCs) have been developed for the detection of Fission Fragments (FFs). The detectors were characterized using FFs from 252Cf source to obtain position information by employing delay-line read out method and the resolution is about 1 mm in both X and Y directions. Velocity distribution of the FFs produced from spontaneous fission of 252Cf has been measured employing a time-of-flight (TOF) technique, using a barium fluoride (BaF2) detector and a MWPC. The most probable velocities obtained for the heavy and light fragment groups are 1.035 ± 0.003 and 1.378 ± 0.004 cm/ns, respectively. The energy dependence of fragment velocity distribution was also studied after degrading the energy of the fragments by using Mylar foils. It is observed that the width of the distribution for the heavy fission fragments decreases with the reduction in the energies, but for light fragments it does not show any significant dependence on the energy.

Spontaneous fission of the end product in α-decay chain of recoiled superheavy nucleus: A theoretical study

The temperature-dependent preformed cluster model [PCM[Formula: see text]] is employed to extend our recent work [Niyti, G. Sawhney, M. K. Sharma and R. K. Gupta, Phys. Rev. C 91 (2015) 054606] on [Formula: see text]-decay chains of various isotopes of [Formula: see text]–118 superheavy nuclei (SHN), to spontaneous fissioning nuclei [Formula: see text]Lr, [Formula: see text]Rf, [Formula: see text]Db, [Formula: see text]Rg, and [Formula: see text]Cn occurring as end products of these [Formula: see text]-decay chains. The behavior of fragment mass distribution and competitive emergence of the dominant decay mode, i.e., the [Formula: see text]-emission versus spontaneous fission (SF), are studied for identifying the most probable heavy fission fragments, along with the estimation of SF half-life times T[Formula: see text] and total kinetic energy (TKE) of the above noted isotopes of [Formula: see text]–112 nuclei decaying via the SF process. The mass distributions of chosen nuclei are clearly symmetric, independent of mass and temperature. The most preferred decay fragment is found to lie in the neighborhood of doubly magic shell closures of [Formula: see text] and [Formula: see text], with largest preformation factor [Formula: see text]. In addition, a comparative study of the “hot compact” and “cold elongated” configurations of [Formula: see text]-deformed and [Formula: see text]-oriented nuclei indicates significantly different behaviors of the two mass fragmentation yields, favoring “hot compact” configuration.

Tagging fast neutrons from a 252Cf fission-fragment source

Coincidence and time-of-flight measurement techniques are employed to tag fission neutrons emitted from a 252Cf source sealed on one side with a very thin layer of Au. The source is positioned within a gaseous 4He scintillator detector. Together with α particles, both light and heavy fission fragments pass through the thin layer of Au and are detected. The fragments enable the corresponding fission neutrons, which are detected in a NE-213 liquid-scintillator detector, to be tagged. The resulting continuous polychromatic beam of tagged neutrons has an energy dependence that agrees qualitatively with expectations. We anticipate that this technique will provide a cost-effective means for the characterization of neutron-detector efficiency in the energy range 1–6MeV.

Collinear cluster tri-partition: Kinematics constraints and stability of collinearity

A new mode of nuclear fission has been proposed by the FOBOS collaboration, called Collinear Cluster Tri-partition (CCT), suggesting that three heavy fission fragments can be emitted perfectly collinearly in low-energy fission. It is surprising that CCT escaped observation for so long given the relatively high reported yield, of roughly 0.5% relative to binary fission. These claims call for an independent verification with a different experimental technique. Verification experiments based on direct observation of CCT fragments with fission fragment spectrometers require guidance with respect to the allowed kinetic energy range, which we present in this paper. We discuss corresponding model calculations which, if CCT is found in such verification experiments, could indicate how the breakups proceed. We also study the intrinsic stability of collinearity. Three different decay models are used, which span together the timescales of three-body fission. These models are used to calculate the possible kinetic energy ranges of CCT fragments in 235U(n,f) and 252Cf(sf). We use semi-classical trajectory calculations with a Monte-Carlo method to study the intrinsic stability of collinearity. CCT has a high net Q-value, but in a sequential decay, the intermediate steps are energetically and geometrically unfavorable or even forbidden. Moreover, perfect collinearity is extremely unstable, and broken by the slightest perturbation. According to our results, the central fragment would be very difficult to detect due to its low kinetic energy, raising the question of why previous experiments could not detect a missing-mass signature corresponding to CCT. We find that a realization of CCT would require an unphysical fine-tuning of the initial conditions. Our results enable independent experimental verification and encourage further critical theoretical studies of CCT.

Experimental programme on absolute fission fragment yields with the lohengrin spectrometer: New optical and statistical methodologies

The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. The mass and isotopic yields of the fission fragments have a direct influence on the amount of neutron poisons that limit the fuel burnup and on the evaluation of the residual power of the reactor after shutdown. The fission yields of the plutonium nuclides are also mandatory for the studies on the fuel multi-recycling. In order to significantly improve the precision of nuclear data, more and more fundamental fission models are used in the evaluation processing. Therefore, tests of fission models become a central issue to achieve a coherent libraries of nuclear data. In this framework, an important investigation in the experimental limits of facilities is required to provide complete sets of data with their coherent variance-covariance matrices. In the past with the LOHENGRIN spectrometer of the ILL, priority has been given for the studies in the light fission fragment mass range. The LPSC in collaboration with ILL and CEA has developed a measurement program on symmetric and heavy mass fission fragment distributions. The combination of measurements with ionisation chamber and Ge detectors is necessary to describe precisely the heavy fission fragment region in mass and charge. Recently, new measurements of fission yields and kinetic energy distributions, with different fissioning systems, were performed with this facility. The focus has been done on the selfnormalization of the data to provide new absolute measurements, independently of any libraries, and the experimental covariance matrix. To reach precise measurements, a new experimental procedure was developed along with a new analysis method based on metadata. Because of the complex correction scheme from count rates to yields, a classical propagation uncertainty is not possible. The new analysis path gives the mean value of mass yields, its probability density function and the associated experimental variance-covariance matrices. All this information is a first step to bring nuclear data into statistical tests of the underlying hypothesis of fission models.

Two neutron correlations in photo-fission

A large body of experimental work has established the strong kinematical correlation between fission fragments and fission neutrons. Here, we report on the progress of investigations of the potential for strong two neutron correlations arising from the nearly back-to-back nature of the two fission fragments that emit these neutrons in the photo-fission process. In initial measurements, a pulsed electron linear accelerator was used to generate bremsstrahlung photons that impinged upon an actinide target, and the energy and opening angle distributions of coincident neutrons were measured using a large acceptance neutron detector array. A planned comprehensive set of measurements of two neutron correlations in the photo-fission of actinides is expected to shed light on several fundamental aspects of the fission process including the multiplicity distributions associated with the light and heavy fission fragments, the nuclear temperatures of the fission fragments, and the mass distribution of the fission fragments as a function of energy released. In addition to these measurements providing important nuclear data, the unique kinematics of fission and the resulting two neutron correlations have the potential to be the basis for a new tool to detect fissionable materials. A key technical challenge of this program arises from the need to perform coincidence measurements with a low duty factor, pulsed electron accelerator. This has motivated the construction of a large acceptance neutron detector array, and the development of data analysis techniques to directly measure uncorrelated two neutron backgrounds.

Modgil-Virk Formulation of Single Activation Energy Model of Radiation Damage Annealing in SSNTDs: A Critical Appraisal

Passage of heavy ions produces radiation-damage trails known as latent tracks in a variety of solid-state nuclear-track detectors (SSNTDs). These tracks are made visible in an optical microscope by a simple process known as chemical etching. It is a well-known fact that latent tracks are radiation damage trails in SSNTDs, which can be annealed by thermal heating. Modgil-Virk formulation of single-activation-energy model of radiation damage annealing was proposed as an empirical approach for explaining the thermal fading of nuclear tracks in SSNTDs. The empirical formulation of this model is based on track annealing data collected from both isothermal and isochronal experiments performed on different types of SSNTDs using a variety of heavy ion beams and fission fragments. The main objective of this empirical model was to resolve some contradictions of variable activation energy derived by using Arrhenius plots to study annealing in mineral SSNTDs. Some equivalent versions of the Modgil-Virk model have been proposed but the concept of single activation energy is vindicated in all empirical formulations. The model always yields a unique value of activation energy independent of the nature of the ion beam used and the degree of annealing. The anisotropy of the mineral SSNTDs is revealed by variation in activation energy along different crystal planes and even with different orientations of the ion beam on the same plane. Some recent experiments are a pointer to the successful exploitation of this model for future cosmic-rays studies using SSNTDs.

Probabilities for the emission of light particles and their energy and angular distributions for true quaternary nuclear fission

On the basis of quantum-mechanical fission theory, the features of true quaternary nuclear fission are studied by treating this fission process as a sequence of three processes following one another in the course of time. The first two processes are the escape of the first and then the second of the two light particles emitted from the neck of a fissioning nucleus because of a nonadiabatic character of the collective deformation motion of this nucleus. Finally, the third process is the separation of the fissioning nucleus into two rather heavy fission fragments. The differences that arise in the emission probabilities and in the angular and energy distributions upon going over from the first emitted to the second emitted prescission third and fourth particles are analyzed by invoking experimental data on the spontaneous and thermalneutron-induced fission of nuclei, and it is shown that these differences are caused by the changes both in the geometric configuration of the fissioning nucleus and in the shell structure of its neck after the first prescission particle is emitted from it.

Partition between the fission fragments of the excitation energy and of the neutron multiplicity at scission in low-energy fission

The partition between the light ($L$) and the heavy ($H$) fission fragments of the excitation energy available at scission is studied in the framework of the sudden approximation, i.e., under the assumption that the neck rupture and the absorption of the neck pieces by the fragments happen infinitely fast. We are dealing with a sudden transition between two different nuclear configurations (${\ensuremath{\alpha}}_{i}\ensuremath{\rightarrow}{\ensuremath{\alpha}}_{f}$) and we only need to know the two sets of neutron eigenstates involved. The accent in the present work is put on the dependence of this share of energy on the mass asymmetry ${A}_{L}/{A}_{H}$ of the primary fission fragments during the low-energy fission of ${}^{236}$U. In particular, for every fragment mass $A$ we estimate the scission neutron multiplicity ${\ensuremath{\nu}}_{\mathrm{sc}}$, the average energy cost for their release $\ensuremath{\langle}{E}_{\ensuremath{\nu}\mathrm{sc}}\ensuremath{\rangle}$, the primary fragments' excitation energy ${E}_{\mathrm{sc}}^{*}$, and the corresponding temperature ${T}_{\mathrm{sc}}$. The results are analyzed separately for each value of $\ensuremath{\Omega}$ (the projection of the angular momentum on the symmetry axis). As general trends, a decrease of ${E}_{\mathrm{sc}}^{*}$ (${T}_{\mathrm{sc}}$) and an increase of ${\ensuremath{\nu}}_{\mathrm{sc}}$ ($\ensuremath{\langle}{E}_{\ensuremath{\nu}\mathrm{sc}}\ensuremath{\rangle}$) with increasing $A$ were observed.

SOME CHARACTERISTICS AND APPLICATIONS OF PARALLEL PLATE AVALANCHE DETECTORS (A REVIEW)

Parallel plate avalanche detectors (PPADs) are new gas detectors and are being extensively used to detect heavy ions, fission fragments and minimum ionizing particles. Due to their excellent timing characteristics and low cost, they are being widely used in different areas of science and technology, particularly as start and stop detectors in different experiments. These detectors are ideal for coincidence measurements and for photofission studies. Different characteristics; construction operation, performance, time resolution, energy resolution and experimental techniques based on PPADs are reviewed in this paper.

Introducing the fission–fusion reaction process: using a laser-accelerated Th beam to produce neutron-rich nuclei towards the N=126 waiting point of the r-process

We propose to produce neutron-rich nuclei in the range of the astrophysical r-process around the waiting point N=126 by fissioning a dense laser-accelerated thorium ion bunch in a thorium target (covered by a CH2 layer), where the light fission fragments of the beam fuse with the light fission fragments of the target. Via the 'hole-boring' mode of laser Radiation Pressure Acceleration using a high-intensity, short pulse laser, very efficiently bunches of 232Th with solid-state density can be generated from a Th layer, placed beneath a deuterated polyethylene foil, both forming the production target. Th ions laser-accelerated to about 7 MeV/u will pass through a thin CH2 layer placed in front of a thicker second Th foil closely behind the production target and disintegrate into light and heavy fission fragments. In addition, light ions (d,C) from the CD2 production target will be accelerated as well to about 7 MeV/u, inducing the fission process of 232Th also in the second Th layer. The laser-accelerated ion bunches with solid-state density, which are about 10^14 times more dense than classically accelerated ion bunches, allow for a high probability that generated fission products can fuse again. In contrast to classical radioactive beam facilities, where intense but low-density radioactive beams are merged with stable targets, the novel fission-fusion process draws on the fusion between neutron-rich, short-lived, light fission fragments both from beam and target. The high ion beam density may lead to a strong collective modification of the stopping power in the target, leading to significant range enhancement. Using a high-intensity laser as envisaged for the ELI-Nuclear Physics project in Bucharest (ELI-NP), estimates promise a fusion yield of about 10^3 ions per laser pulse in the mass range of A=180-190, thus enabling to approach the r-process waiting point at N=126.

Entropy Driven Excitation Energy Sorting in Superfluid Fission Dynamics

It is shown that the constant-temperature behavior of nuclei in the superfluid regime leads to an energy-sorting process if two nuclei are in thermal contact, as is the case in the fission process. This effect explains why an increase of the initial excitation energy leads an increase of the number of emitted neutrons from the heavy fission fragment, only. The observed essentially complete energy sorting may be seen as a new counterintuitive manifestation of quantum-mechanical properties of microscopic systems.