Bremsstrahlung Photon Spectrum Research Articles

Evaluation of delayed neutron yields and time spectra from photofission of 238U induced by Bremsstrahlung photons below 9 MeV

Few data exist regarding the yields and time spectra of delayed neutrons from the photofission reaction on actinides. In this paper, we report the characteristics of both six-group and eight-group delayed neutron time models for the photofission of 238U induced by a Bremsstrahlung photon spectrum with an end-point energy at 9 MeV. This study was conducted using a 9 MeV linear electron accelerator housed at the SAPHIR platform located at CEA Paris-Saclay, France. A reference sample of depleted uranium was positioned inside a neutron detection block, previously designed and optimized by Monte Carlo simulation, made of high-density polyethylene covered by cadmium and including ten one meter-long 3He proportional counters. Based on the consistent set of half-lives proposed by Spriggs et al. as early as 1999 for neutroninduced fission, our description in eight delayed neutron groups for 238U at 9 MeV is the first published for photofission. These new results bring valuable information to the scarce data available in the literature and represent useful inputs for the development of photofission-based nuclear measurement systems as they meet a significant interest in the fields of nuclear data, radioactive waste package characterization and for security-related applications.

Bremsstrahlung photons from stopping in heavy-ion collisions

We examine the spectrum of bremsstrahlung photons that results from the stopping of the initial net charge distributions in ultrarelativistic nucleus-nucleus collisions at the CERN Large Hadron Collier (LHC). This effect has escaped detection so far since it becomes sizable only at very low transverse momentum and at sufficiently forward rapidity. We argue that it may be within reach of the next-generation LHC heavy-ion detector ALICE-3 that is currently under study, and we comment on the physics motivation for measuring it.

Stripping of the NaI (Tl) detector response function for continuous energy photon spectrum by SVD approach

We have tried to obtain the spectrum of internal bremsstrahlung photons which are released simultaneously with beta particles and neutrinos from the 90Sr-90Y source in secular equilibrium. Detector parameters of the 5.08 cm × 5.08 cm NaI(Tl) detector are determined using standard calibration sources. A detector response function is generated for a single-energy gamma photon using detector parameters and Monte Carlo method. The Singular Value Decomposition (SVD) method is used for stripping of the detector response function effects from the raw experimental spectrum. The stripped experimental data are compared with KUB theory and a different stripping method that we used our former work on the same isotope. There is a much better agreement between experimental and theoretical findings comparing the former experimental data.

Algorithms and Analyses for Joint Spectral Image Reconstruction in Y-90 Bremsstrahlung SPECT

Quantitative yttrium-90 (Y-90) SPECT imaging is challenging due to the nature of Y-90, an almost pure beta emitter that is associated with a continuous spectrum of bremsstrahlung photons that have a relatively low yield. This paper proposes joint spectral reconstruction (JSR), a novel bremsstrahlung SPECT reconstruction method that uses multiple narrow acquisition windows with accurate multi-band forward modeling to cover a wide range of the energy spectrum. Theoretical analyses using Fisher information and Monte-Carlo (MC) simulation with a digital phantom show that the proposed JSR model with multiple acquisition windows has better performance in terms of covariance (precision) than previous methods using multi-band forward modeling with a single acquisition window, or using a single-band forward modeling with a single acquisition window. We also propose an energy-window subset (ES) algorithm for JSR to achieve fast empirical convergence and maximum-likelihood based initialization for all reconstruction methods to improve quantification accuracy in early iterations. For both MC simulation with a digital phantom and experimental study with a physical multi-sphere phantom, our proposed JSR-ES, a fast algorithm for JSR with ES, yielded higher recovery coefficients (RCs) on hot spheres over all iterations and sphere sizes than all the other evaluated methods, due to fast empirical convergence. In experimental study, for the smallest hot sphere (diameter 1.6cm), at the 20th iteration the increase in RCs with JSR-ES was 66 and 31% compared with single wide and narrow band forward models, respectively. JSR-ES also yielded lower residual count error (RCE) on a cold sphere over all iterations than other methods for MC simulation with known scatter, but led to greater RCE compared with single narrow band forward model at higher iterations for experimental study when using estimated scatter.

Low-energy bremsstrahlung photon in relativistic nucleon+nucleon collisions

We study the production of bremsstrahlung photons in relativistic $\mathrm{nucleon}+\mathrm{nucleon}$ collisions by introducing a deceleration time of electromagnetic currents. It is found that the bremsstrahlung photon spectrum at low energy does not depend on the deceleration time, but solely on the amount of reduced electromagnetic current in collision. On the other hand, the photon spectrum becomes soft with increasing deceleration time. We also find that the bremsstrahlung photon spectrum in $p+n$ collisions is quite different from that in $p+p$ collisions at low energy.

Development of a Method for Measurement of Photonuclear-Reaction Cross Sections with the Aid of Simulated Quasimonoenergetic-Photon Spectra

A procedure for measuring photonuclear-reaction cross sections by employing the method of quasimonochromatization of the bremsstrahlung photon spectrum is developed. This procedure permits determining reaction cross sections via measuring the reaction yield at three fixed values of the electron energy. The procedure is equivalent to measuring the cross section in question for quasimonochromatic photons. Bremsstrahlung-photon spectra are obtained using state-of-the-art simulation means involving the GEANT4 code as applied to a specific experiment with allowance for its geometry, target thicknesses, electron-beam parameters, etc. The results obtained by simulating quasimonochromatic distributions of bremsstrahlung photons are presented for experiments at the LUE-8-5 linear electron accelerator of Institute for Nuclear Research (Moscow, Russian Academy of Sciences). These results are found to be in good agreement with the experimental energy distribution of the cross section for the reaction 111Cd(γ, γ') in the energy range between 5 and 8 MeV.

GEANT 4 simulation of 99Mo photonuclear production in nanoparticles

GEANT 4 Monte-Carlo simulation toolkit is used to study the kinematic recoil method of 99Mo photonuclear production. Simulation for bremsstrahlung photon spectrum with maximum photon energy 30MeV showed that for MoO3 nanoparticle escape fraction decreases from 0.24 to 0.08 when nanoparticle size increases from 20nm to 80nm. For the natural molybdenum and pure 100Mo we obtained the lower values: from 0.17 to 0.05. The generation of accompanying molybdenum nuclei is significantly lower for pure 100Mo and is about 3.6 nuclei per single 99Mo nucleus, while natural molybdenum nanoparticle produce about 48 accompanying nuclei. Also, we have shown that for high-energy photons escape fraction of 99Mo decreases, while production of unwanted molybdenum isotopes is significantly higher.

Mixed Source Interrogation of Steel Shielded Special Nuclear Material Using an Intense Pulsed Source

This paper explores the benefits of using a mixed photon and neutron radiation source for active detection of special nuclear material. More than fifty irradiations were performed using an 8 MV electron accelerator employing and induction voltage adder (IVA). The experiments used a high atomic number converter to produce a Bremsstrahlung photon spectrum which was then used to create a neutron source via a nuclear interaction with heavy water (deuterium oxide, D2O). This mixed particle source was used to irradiate a depleted uranium (DU) sample, inducing fission in the sample. Several thicknesses of steel shielding were tested in order to compare the performance of the mixed photon and neutron source to a Bremsstrahlung-only source. An array of detectors were fielded to record both photons and neutrons emitted by the fission reactions. A correlation between steel shielding and a detection figure-of-merit can be seen in all cases where the Bremsstrahlung-only source was used. The same relationship for the mixed photon-neutron source is less consistent. The data collected from the fielded detectors is compared to MCNP6 calculations and good agreement is found.

A dosimetric study of Beta induced bremsstrahlung in bone

A dosimetric study of beta-induced bremsstrahlung in bone is important in the field of radiation protection. The beta-induced bremsstrahlung spectra produced by 113 pure beta nuclides in the bone are computed. The spectral shapes are primarily responsible for variations in the shapes by depth-dose distributions. They are intended to provide a quick and convenient reference for spectral shapes and to give an indication of the wide variation in these shapes. The computed bremsstrahlung spectrum is used in the evaluation of bremsstrahlung dose in bone. The evaluated beta bremsstrahlung dose as a function of distance for the studied nuclides is also presented. The beta bremsstrahlung dose decreases with the increase in distance. Present work also estimates the dosimetric parameters of bremsstrahlung such as yield, intensity and dose rate of bremsstrahlung by various pure beta nuclides in tissues of human skeleton such as cortical bone, red marrow, yellow marrow, spongiosa and cartilage. The yield, intensity and dose rate of bremsstrahlung in the cortical bone are higher than that of red marrow, yellow marrow, spongiosa and cartilage. Hence cortical bone is more beta/bremsstrahlung radiosensitive than that of other tissues of human skeleton. The estimated bremsstrahlung efficiency, intensity, photon spectra and photon track-length distributions determine the quality and quantity of the radiation. Precise estimation of this source term is very important in planning for radiotherapy and diagnosis.

A collimator optimization method for quantitative imaging: application to Y-90 bremsstrahlung SPECT.

Post-therapy quantitative 90Y bremsstrahlung single photon emission computed tomography (SPECT) has shown great potential to provide reliable activity estimates, which are essential for dose verification. Typically 90Y imaging is performed with high- or medium-energy collimators. However, the energy spectrum of 90Y bremsstrahlung photons is substantially different than typical for these collimators. In addition, dosimetry requires quantitative images, and collimators are not typically optimized for such tasks. Optimizing a collimator for 90Y imaging is both novel and potentially important. Conventional optimization methods are not appropriate for 90Y bremsstrahlung photons, which have a continuous and broad energy distribution. In this work, the authors developed a parallel-hole collimator optimization method for quantitative tasks that is particularly applicable to radionuclides with complex emission energy spectra. The authors applied the proposed method to develop an optimal collimator for quantitative 90Y bremsstrahlung SPECT in the context of microsphere radioembolization. To account for the effects of the collimator on both the bias and the variance of the activity estimates, the authors used the root mean squared error (RMSE) of the volume of interest activity estimates as the figure of merit (FOM). In the FOM, the bias due to the null space of the image formation process was taken in account. The RMSE was weighted by the inverse mass to reflect the application to dosimetry; for a different application, more relevant weighting could easily be adopted. The authors proposed a parameterization for the collimator that facilitates the incorporation of the important factors (geometric sensitivity, geometric resolution, and septal penetration fraction) determining collimator performance, while keeping the number of free parameters describing the collimator small (i.e., two parameters). To make the optimization results for quantitative 90Y bremsstrahlung SPECT more general, the authors simulated multiple tumors of various sizes in the liver. The authors realistically simulated human anatomy using a digital phantom and the image formation process using a previously validated and computationally efficient method for modeling the image-degrading effects including object scatter, attenuation, and the full collimator-detector response (CDR). The scatter kernels and CDR function tables used in the modeling method were generated using a previously validated Monte Carlo simulation code. The hole length, hole diameter, and septal thickness of the obtained optimal collimator were 84, 3.5, and 1.4 mm, respectively. Compared to a commercial high-energy general-purpose collimator, the optimal collimator improved the resolution and FOM by 27% and 18%, respectively. The proposed collimator optimization method may be useful for improving quantitative SPECT imaging for radionuclides with complex energy spectra. The obtained optimal collimator provided a substantial improvement in quantitative performance for the microsphere radioembolization task considered.

Photon Signals from Quarkyonic Matter

We calculate the bremsstrahlung photon spectrum emitted from dynamically evolving "quarkyonic" matter and compare this spectrum with that of a high chemical potential quark-gluon plasma as well as to a hadron gas. We find that the transverse momentum distribution and the harmonic coefficient is markedly different in the three cases. The transverse momentum distribution of quarkyonic matter can be fit with an exponential but is markedly steeper than the distribution expected for the quark-gluon plasma or a hadron gas, even at the lower temperatures expected in the critical point region. The quarkyonic elliptic flow coefficient fluctuates randomly from event to event and within the same event at different transverse momenta. The latter effect, which can be explained by the shape of quark wave functions within quarkyonic matter, might be considered as a quarkyonic matter signature, provided that the initial temperature is low enough that the quarkyonic regime dominates over deconfinement effects and the reaction-plane flow can be separated from the fluctuating component.

Terrestrial gamma ray flashes with energies up to 100 MeV produced by nonequilibrium acceleration of electrons in lightning

Using Monte Carlo models simulating energetic electrons in inhomogeneous electric field and the transport of energetic photons in the Earth's atmosphere, we show that the spectrum of bremsstrahlung photons generated by nonequilibrium energetic electrons produced during stepping of lightning leaders can deviate from the typical spectrum of relativistic runaway electron avalanches (RREA) developing in weak homogeneous electric fields. This deviation is especially pronounced in the high‐energy tail of the electron energy distribution for lightning leaders possessing high voltages (several hundreds of megavolts) and extremely high fields around their tips. The photon spectrum obtained accurately reproduces the recently discovered high‐energy tail (up to 100 MeV) of terrestrial gamma‐ray flashes (TGFs). This analysis provides the first direct evidence that TGFs are produced by lightning and not over large distances in weak thunderstorm electric fields.

Uncertainty and sensitivity analysis of the effect of the mean energy and FWHM of the initial electron fluence on the Bremsstrahlung photon spectra of linear accelerators

A calculation of the correct dose in radiation therapy requires an accurate description of the radiation source because uncertainties in characterization of the linac photon spectrum are propagated through the dose calculations. Unfortunately, detailed knowledge of the initial electron beam parameters is not readily available, and many researchers adjust the initial electron fluence values by trial-and-error methods. The main goal of this work was to develop a methodology to characterize the fluence of initial electrons before they hit the tungsten target of an Elekta Precise medical linear accelerator. To this end, we used a Monte Carlo technique to analyze the influence of the characteristics of the initial electron beam on the distribution of absorbed dose from a 6MV linac photon beam in a water phantom. The technique is based on calculations with Software for Uncertainty and Sensitivity Analysis (SUSA) and Monte Carlo simulations with the MCNP5 transport code. The free parameters used in the SUSA calculations were the mean energy and full-width-at-half-maximum (FWHM) of the initial electron distribution. A total of 93 combinations of these parameters gave initial electron fluence configurations. The electron spectra thus obtained were used in a simulation of the electron transport through the target of the linear accelerator, which produced different photon (Bremsstrahlung) spectra. The simulated photon spectra were compared with the 6-MV photon spectrum provided by the linac manufacturer (Elekta). This comparison revealed how the mean energy and FWHM of the initial electron fluence affect the spectrum of the generated photons. This study has made it possible to fine-tune the examined electron beam parameters to obtain the resulted absorbed doses with acceptable accuracy (error <1%).

Development of High-Power Laser Based Nuclear Applications

We present a novel nuclear activation-based method for the investigation of high-energy photons and electron dynamics within the laser-plasma interaction zone. This method is based on high density activation targets which are a pseudoalloy of several selected isotopes with different photo-neutron disintegration reaction thresholds. The gamma decay spectrum emitted by the activated target is used for the reconstruction of the bremsstrahlung spectrum generated by the electrons. This allows for the reconstruction of the spectrum of bremsstrahlung photons without any anticipated fit procedures. Furthermore, the characterization of the electrons in the interaction zone is accessible immediately.The consolidated findings about the interaction mechanisms could be used to realize, control and characterize laser driven particle generation, such as a pulsed neutron source for nuclear and material sciences using special target designs and materials in a pseudoalloic compound of isotopes. An additional application is the laser assisted nuclear transmutation to produce short-lived isotopes with activities suitable for medical diagnostics and therapy.

Bremsstrahlung emission accompanying α-decay of deformed nuclei

A fully quantum model describing the angular distribution of the bremsstrahlung photon emission accompanying the α-decay of a deformed nucleus is presented for the first time. An analytical dependence of the bremsstrahlung probability on the quadrupole deformation parameter β 2 of the decaying nucleus has been found. Such a model and the detailed formalism allow us to calculate the stable spectra of the bremsstrahlung emission probability for different values of the angle between the direction of the α-particle motion and the symmetry axis of the decaying nucleus. We present the results of this new model by the calculation of the photon emission probability accompanying the α-decay of the 226Ra nucleus which has the nuclear deformation parameter β 2 = 0.151 . The results show by a clear way the role and the influence of the nuclear deformation of the decaying nucleus on the bremsstrahlung photon spectrum.

Evaluation of quantitative 90Y SPECT based on experimental phantom studies

In SPECT imaging of pure beta emitters, such as 90Y, the acquired spectrum is very complex, which increases the demands on the imaging protocol and the reconstruction. In this work, we have evaluated the quantitative accuracy of bremsstrahlung SPECT with focus on the reconstruction algorithm including model-based attenuation, scatter and collimator–detector response (CDR) compensations. The scatter and CDR compensation methods require pre-calculated point-spread functions, which were generated with the SIMIND MC program. The SIMIND program is dedicated for simulation of scintillation camera imaging and only handles photons. The aim of this work was therefore twofold. The first aim was to implement simulation of bremsstrahlung imaging into the SIMIND code and to validate simulations against experimental measurements. The second was to investigate the quality of bremsstrahlung SPECT imaging and to evaluate the possibility of quantifying the activity in differently shaped sources. In addition, a feasibility test was performed on a patient that underwent treatment with 90Y-Ibritumomab tiuxetan (Zevalin®). The MCNPX MC program was used to generate bremsstrahlung photon spectra which were used as source input in the SIMIND program. The obtained bremsstrahlung spectra were separately validated by experimental measurement using a HPGe detector. Validation of the SIMIND generated images was done by a comparison to gamma camera measurements of a syringe containing 90Y. Results showed a slight deviation between simulations and measurements in image regions outside the source, but the agreement was sufficient for the purpose of generating scatter and CDR kernels. For the bremsstrahlung SPECT experiment, the RSD torso phantom with 90Y in the liver insert was measured with and without background activities. Projection data were obtained using a GE VH/Hawkeye system. Image reconstruction was performed by using the OSEM algorithm with and without different combinations of model-based attenuation, scatter and CDR compensations. The reconstructed images were then evaluated in terms of the accuracy of the total activity estimate in the liver insert. It was found that the activity in a large source such as the liver was estimated with a bias of around −70%, when no compensations were included in the reconstruction, whereas when compensations were included the bias obtained was between −10 and 16%. It is concluded that although the 90Y bremsstrahlung spectrum is continuous with no pronounced peak and the count rate is low, it is possible to achieve reasonably accurate activity estimates from bremsstrahlung SPECT images if proper compensations are applied in the reconstruction. This conclusion was also confirmed by the patient study.

Numerical evaluation of nuclide analysis of I129, Sr90, and Cs137 using bremsstrahlung high energy x ray generated by ultrashort pulse laser

We analyzed numerically the feasibility of nuclide analysis of spent fuels using an ultraintense, ultrashort laser. The (γ,n) reaction yields for each nuclide such as I129, Sr90, and Cs137 were calculated using the bremsstrahlung spectrum and the nuclear reaction cross sections. The dependence of the (γ,n) reaction yield on the irradiation time was derived. We also calculated the dependence of the nuclide ratio on the bremsstrahlung photon temperature. In order to investigate the influence of electron temperature and target thickness on the bremsstrahlung photon spectrum, the calculations were performed using the Monte Carlo code GEANT4. When gold was used as bremsstrahlung converter target at a temperature of the incident electrons of 30MeV, the target thickness providing maximum nuclear reaction yield was found to be 5mm.

A theoretical model for the production of Ac-225 for cancer therapy by photon-induced transmutation of Ra-226

Radium needles that were once implanted into tumours as a cancer treatment are now obsolete and constitute a radioactive waste problem, as their half-life is 1600 years. We are investigating the reduction of radium by transmutation on a small scale by bombarding Ra-226 with high-energy photons from a medical linear accelerator (linac) to produce Ra-225, which subsequently decays to Ac-225, which can be used as a generator to produce Bi-213 for use in ‘targeted alpha therapy’ for cancer. This paper examines the possibility of producing Ac-225 with a linac using an accurate theoretical model in which the bremsstrahlung photon spectrum at 18 MV linac electron energy is convoluted with the corresponding photonuclear cross sections of Ra-226. The total integrated yield can then be obtained and is compared with a computer simulation. This study shows that at 18 MV, the photonuclear reaction on Ra-226 can produce low activities of Ac-225 with a linac. However, a high power linac with high current, pulse length and frequency is needed to produce practical amounts of Ac-225 and a useful reduction of Ra-226.

The kinetic effects of electron beam precipitation and resulting hard X-ray intensity in solar flares

The numerical solutions of the time-dependent kinetic Fokker-Planck equation are presented for fast electrons in- jected from the solar corona into a flaring atmosphere and precipitating with energy and pitch-angle diffusion into a loop with a converging magnetic field. The electrons are assumed to lose their energy in Coulomb collisions with particles of the partially ionised ambient plasma and Ohmic heating owing to the electric field induced by the precipitating beam. The electric field induced by a precipitating electron beam is found to cause a return current beam, which comes back to the source in the corona with a wide pitch angle distribution. The return current is assumed to arise from the ambient plasma and from beam electrons scattered into negative pitch-angles. Energy and pitch-angle distributions of precipitating and return current electron beams at various atmospheric depths are presented along with the precipitating beam abundances, energy fluxes and resulting hard X-ray bremsstrahlung (photon) spectra for electron beams with power law energy spectra with spectral indices of 3, 5, 7 and initial energy fluxes of 10 8 ,1 0 10 ,1 0 12 erg cm −2 s −1 . Energy distributions of the return current cover energies lower than 60 keV for weaker soft beams and increase to 65 keV for moderate soft beams, or to 70-75 keV for more intense and hard beams. The maxima are at 30 keV for weaker soft beams and are shifted towards higher energies, up to 50 keV, for harder and more intense beams. As a result, the photon spectra of hard X-ray emission emitted from a flaring atmosphere are found to have a broken power-law (elbow-type) shape with a higher energy part retaining the spectral index δhigh associated with the electron beam's initial index and varying slightly with the beam parameters. However, the lower energy part of the X-ray photon spectra shows a much smaller increase or even a substantial decrease of its spectral index δlow for more intense or harder beams. These simulated broken power-law photon spectra produced by precipitating electron beams with a single spectral index agree reasonably well with the photon energy spectra from the flares of 20 and 23 July, 2002 observed by the RHESSI payload.

Determination of Electron Flux Spectra in a Solar Flare with an Augmented Regularization Method: Application to Rhessi Data

Kontar et al. (2004) have shown how to recover mean source electron spectra \(\bar F(E)\) in solar flares through a physical constraint regularization analysis of the bremsstrahlung photon spectra I(ε) that they produce. They emphasize the use of non-square inversion techniques, and preconditioning combined with physical properties of the spectra to achieve the most meaningful solution to the problem. Higher-order regularization techniques may be used to generate \(\bar F(E)\) forms with certain desirable properties (e.g., higher-order derivatives). They further note that such analysis may be used to infer properties of the electron energy spectra at energies well above the maximum photon energy observed. In this paper we apply these techniques to data from a solar flare observed by RHESSI on 26 February, 2002. Results using different orders of regularization are presented and compared for various time intervals. Clear evidence is presented for a change in the value of the high-energy cutoff in the mean source electron spectrum with time. We also show how the construction of the injected electron spectrum F0(E0) (assuming that Coulomb collisions in a cold target dominate the electron transport) is facilitated by the use of higher-order regularization methods.