Photoneutron Yield Research Articles

PHOTON AND PHOTONEUTRON OUTPUTS SIMULATION FROM A TANTALUM CONVERTER AT THE M-30 MICROTRON

Reliable information about the outputs of bremsstrahlung photons and secondary photoneutrons from converters on electron accelerators with mandatory consideration of their technical characteristics is necessary for optimizing the process of sample irradiation and predicting the results of experimental studies. The results of modeling the de- pendence of photon and photoneutron yields and their ratios on the thickness of the tantalum converter (thickness – 0.04; 0.4; 1.0; 4.0; 10.0 mm) are presented at fixed energies of the initial electron beam – 10.0, 12.5, 14.5, 17.5, and 20 MeV. Calculations were carried out for the electron accelerator of the National Academy of Sciences of Ukraine, the M-30 microtron, taking into account its technical characteristics (electron discharge node). The GEANT4 toolkit was used for computer simulation.

TOPAS simulation of photoneutrons in radiotherapy: accuracy and speed with variance reduction

Objective. We provide optimal particle split numbers for speeding up TOPAS Monte Carlo simulations of linear accelerator (linac) treatment heads while maintaining accuracy. In addition, we provide a new TOPAS physics module for simulating photoneutron production and transport. Approach. TOPAS simulation of a Siemens Oncor linac was used to determine the optimal number of splits for directional bremsstrahlung splitting as a function of the field size for 6 MV and 18 MV x-ray beams. The linac simulation was validated against published data of lateral dose profiles and percentage depth-dose curves (PDD) for the largest square field (40 cm side). In separate simulations, neutron particle split and the custom TOPAS physics module was used to generate and transport photoneutrons, called ‘TsPhotoNeutron’. Verification of accuracy was performed by comparing simulations with published measurements of: (1) neutron yields as a function of beam energy for thick targets of Al, Cu, Ta, W, Pb and concrete; and (2) photoneutron energy spectrum at 40 cm laterally from the isocenter of the Oncor linac from an 18 MV beam with closed jaws and MLC. Main results. The optimal number of splits obtained for directional bremsstrahlung splitting enhanced the computational efficiency by two orders of magnitude. The efficiency decreased with increasing beam energy and field size. Calculated lateral profiles in the central region agreed within 1 mm/2% from measured data, PDD curves within 1 mm/1%. For the TOPAS physics module, at a split number of 146, the efficiency of computing photoneutron yields was enhanced by a factor of 27.6, whereas it improved the accuracy over existing Geant4 physics modules. Significance. This work provides simulation parameters and a new TOPAS physics module to improve the efficiency and accuracy of TOPAS simulations that involve photonuclear processes occurring in high-Z materials found in linac components, patient devices, and treatment rooms, as well as to explore new therapeutic modalities such as very-high energy electron therapy.

Photon energy dependence of photoneutron production from heavy targets

The double differential cross-sections (DDXs) of photoneutron production via the photonuclear reaction on tantalum, tungsten, and bismuth for 13 and 17 MeV linearly polarized photon beams were measured using the time-of-flight method at the NewSUBARU-BL01 facility. Polarized photons were obtained by the Laser Compton backscattering (LCS) technique. Two distinct components were observed on the spectra: the lowenergy component up to 4 MeV and the high-energy above 4 MeV. The angular distribution of the low-energy component was isotropic, whereas the high-energy was distributed anisotropically and influenced by the angle between direction of photon polarization and neutron emission, especially for 17 MeV photon energy. These phenomena were similar to those of 197Au target observed in the previous studies. The low-energy neutron distributions at 13 and 17 MeV photon energies were almost identical for all three targets. The DDX energy integration was calculated and compared among the three targets for two-photon energies. Given the horizontal polarization (plane parallel to the x-axis) of the incident photons, the emission angles of 90° on the x-axis and y-axis recorded the maximum and minimum photoneutron yield, respectively. The differences between these two positions were minor for 181Ta and natW at 13 MeV photon energy and noticeable for other cases. An underestimation of the TENDL nuclear data library was observed on the photoneutron DDXs compared to the experimental results for 181Ta and 209Bi.

Comparison of the TRIPOLI-4®, DIANE, and MCNP6 Monte Carlo Codes on the Barber & George Benchmark for Photonuclear Reactions

In view of their key role in radiation shielding and nuclear instrumentation applications, photonuclear reactions are receiving growing attention. In this work, we compare the results of the Monte Carlo codes TRIPOLI4®, DIANE, and MCNP® with respect to the Barber and George (B&G) benchmark, with the aim of assessing the accuracy of both nuclear data and particle transport codes for the simulation of photonuclear reactions. We compute the photoneutron yield resulting from the Bremsstrahlung radiation induced by a monoenergetic electron beam (10.5 to 35.5 MeV) impinging on C, Al, Cu, Pb, Ta, and U material targets. The simulation specifications closely follow those of the B&G experiment. For all codes, the reference nuclear data libraries are ENDF/B-VII.1 for neutron transport and photonuclear reactions and EPDL97/EEDL97 for photon/electron transport. Comparisons of the simulation results show an overall agreement between the codes and experimental data and in-between codes, despite some discrepancies. In order to investigate these effects, we performed a sensitivity analysis by tallying the photon production in addition to neutron production by replacing the electron source with a pure photon source (to single out the impact of electron transport) and by replacing the ENDF/B-VII.1 library with the IAEA/PD-2019. The major contribution to the observed discrepancies is found to be related to the electromagnetic shower models used for coupled electron-photon transport in Monte Carlo codes.

Measurement of neutron yield for a medical linear accelerator below 10 MV.

The recent trend toward 10MV for volumetric radiotherapy treatment such as volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), and stereotactic ablative body radiotherapy (SABR) introduces photoneutron production, with implications for non-therapeutic patient dose and additional shielding requirements for treatment room design. The sharply nonlinear drop-off in photoneutron production below 10MV to negligible at 6MV has scarcely been characterized quantitatively, yet can elucidate important practical insights. To measure photoneutron yields in a medical linac at 8MV, which may strike a reasonable balance between usefully increased beam penetration and dose rate as compared to 6MV while reducing photoneutron production which is present at 10MV. A Varian iX linear accelerator undergoing decommissioning at our clinic was made to operate over a range of photon energies between 6 and 15MV by calibrating the bending magnet and adjusting other beam generation parameters. Neutron dose within the treatment room was measured using an Anderson-Braun type detector over a continuum of intermediate energies. The photoneutron production for energies below 10MV was measured, adding to data that is otherwise scarce in the literature. Our results are consistent with previously published results for neutron yield. We found that the photoneutron production at 8MV was about 1/10 of the value at 10MV, and about 10 times higher than detector background at 6MV. Photoneutron production drops off below 10MV, but is still present at 8MV. An 8MV beam is more penetrating than a 6MV beam, and may offer a suitable tradeoff for modern radiotherapy techniques such as VMAT, SRS, and SABR. Further studies are needed to better understand the impact on treatment plan quality between 8 and 10MV beams considering the benefits to facility requirements and non-therapeutic patient dose.

Measurement of Photoneutron Yields Using the RPI LINAC and Assessment of Evaluated Photoneutron Data for Tantalum and Beryllium

A new experiment configuration was designed, developed, and implemented to measure photoneutron yields using the Rensselaer Polytechnic Institute (RPI) electron linear accelerator (LINAC) at the RPI Gaerttner LINAC Center. The experiment configuration includes a new target assembly that converts the LINAC electron beam into a high energy bremsstrahlung photon flux incident upon a sample material of interest. The photons excite nuclei in the sample of interest, which can subsequently emit neutrons (photoneutrons). The photoneutrons emitted in the direction of the detector system travel through a series of collimated vacuum pipes before reaching a pair of proton-recoil high-energy neutron detectors. The signals generated by the neutron detectors are processed using a digital data acquisition system and subsequently analyzed to determine the energy-dependent photoneutron yield from the sample of interest. The new experiment configuration was used to perform proof-of-concept experiments to measure the photoneutron yields from samples of tantalum and beryllium. The measured results were then compared against the results from Monte Carlo simulations of the detailed experiment configurations to perform preliminary assessments of evaluated photoneutron data libraries.

Improvement of the Bayesian neural network to study the photoneutron yield cross sections

Improvement of the Bayesian neural network to study the photoneutron yield cross sections

Linac natW target close-in geometry photoneutron study by nuclear track distribution function

During electron beam stopping on natW target in a linear accelerator, photonuclear (γ, xn) reactions occur; the tungsten converter provides a non-negligible neutron yield with an energy spectrum that significantly depends on surrounding mass nuclei. Reduction of the neutron radiation field is convenient to limit the side-effects that accompany the tumor or cancer radiotherapy. A close-in irradiation geometry is proposed to improve therapy effectiveness. The convenience of the proposed experimental arrangement is assessed using Monte Carlo simulation and experimental results based on nuclear track-etch methodology. Photoneutron yield for two energy groups (thermal and epithermal) are determined experimentally via boron (98%) converter and cadmium-filter employing a passive detector (poly allyl di-glicol carbonate polimer). Etched track diameter histograms are described by distribution functions to determine the ratio between thermal and higher energy neutrons. New insights are given into therapy beam quality and radiotherapy dose delivery based on bar histograms unfolding.

Calculation of core neutronic parameters in electron accelerator driven subcritical TRIGA reactor

Abstract Investigating the dependence of reactor core neutronic parameters on the accelerator related parameters such as source multiplication coefficient (K s ), beam profile and electron beam energy (E e ), have a significant impact on the design of future ADS reactors and the cost of their construction. Using the MCNPX code, core neutronic parameters were calculated for three eigenvalues levels (k s ) such as axial and radial distributions of neutron flux, effective multiplication coefficient (K eff), photoneutron yield (Y n/e ), net neutron multiplication (M), energy gain (G), energy constant gain (G 0), power peaking factor (P max/P ave), and importance of neutron source (φ*) for the tungsten target in the core of TRIGA reactor which was driven by an electron accelerator. According to the results, when K s increases safety margin and the required accelerator current (I e ) decrease, but the parameters G and φ* increase. Also, using the parabolic spatial distribution for the electron source instead of the uniform spatial distribution, the parameters Y n/e , G and M increase. In addition, by increasing E e from 100 to 500 MeV, Y n/e and G are improved, and I e and accelerating power (p acc) decrease. Therefore, these results suggest that the dependence of reactor core neutronic parameters on accelerator related parameters is necessary to optimize the design cost of an ADS reactor.

Photoneutron production in heavy water reactor fuel lattice from accelerator-driven bremsstrahlung

The coupling of low-energy electron linear accelerators (eLINACs) to a large heavy water reactor is proposed to create an accelerator-driven photoneutron source (ADS). Photoneutron yields of 1012 pn/s per kW of beam power can be achieved in the ADS-CANDU concept where the wide fuel channel spacing of heavy water reactors represents a near-optimal geometry for conversion of accelerator-driven bremsstrahlung into photoneutrons in the heavy water moderator with minimal parasitic attenuation of photons in the fuel channels. Twenty MeV electron beam energy is most efficient at producing photoneutrons despite having the largest fuel channel shielding effect (28% attenuation compared to an infinite heavy water medium). The majority of photoneutrons are produced during source-photon first flights, so the spatial distribution and emission spectra of the ADS in the secondary (γ,n) converter are correlated to the doubly differential angle and energy distribution of the bremsstrahlung emitted from the (e-,γ) converter. Compton-scattered photons and tertiary bremsstrahlung originating from the electron–positron pair and recoil electron secondary particles are important contributors to the photoneutron yield of higher energy eLINACs systems. Photonuclear data for the deuterium photoneutron reaction cross section and secondary electron transport and tertiary bremsstrahlung production physics implemented in Monte-Carlo radiation transport codes can dominate ADS simulation results and are the same magnitude as the physical phenomena such as the fuel channel shielding effect.

New target with low photoneutron yield for LINAC radiotherapy applications

This paper presents three new materials composed of TiXV0,035XCuX-1,035 (X = 2, 4 and 6%), is proposed as Linear Accelerator target. Its response to electron beam based on photoneutron production, is assessed by MC simulation and nuclear track-etch methodology. The outcome is compared to a tungsten target irradiated by energetic 16 MeV electron beam. Photoneutron yield, of two energy groups (thermal and epithermal) were determined via converter matter 10B (98%) and Cd-filter by PADC-track density comparison. The multi-metal Ti2V0,07Cu97.93 target related to therapy beam quality, resulted advantageous in comparison to that provided by W-target, commonly used in the LINAC.

Photonuclear production of medical isotopes 62,64Cu using intense laser-plasma electron source

62,64Cu are radioisotopes of medical interest that can be used for positron emission tomography (PET) imaging. Moreover, 64Cu has β− decay characteristics that allow for targeted radiotherapy of cancer. In the present work, a novel approach to experimentally demonstrate the production of 62,64Cu isotopes from photonuclear reactions is proposed in which large-current laser-based electron (e−) beams are generated from the interaction between sub-petawatt laser pulses and near-critical-density plasmas. According to simulations, at a laser intensity of 3.4 × 1021 W/cm2, a dense e− beam with a total charge of 100 nC can be produced, and this in turn produces bremsstrahlung radiation of the order of 1010 photons per laser shot, in the region of the giant dipole resonance. The bremsstrahlung radiation is guided to a natural Cu target, triggering photonuclear reactions to produce the medical isotopes 62,64Cu. An optimal target geometry is employed to maximize the photoneutron yield, and 62,64Cu with appropriate activities of 0.18 GBq and 0.06 GBq are obtained for irradiation times equal to their respective half-lives multiplied by three. The detection of the characteristic energy for the nuclear transitions of 62, 64Cu is also studied. The results of our calculations support the prospect of producing PET isotopes with gigabecquerel-level activity (equivalent to the required patient dose) using upcoming high-intensity laser facilities.

Characterization of photo-neutrons produced by 150 MeV and 1 GeV electrons impinging on high Z-metallic targets for neutron resonance spectroscopy

Monte Carlo calculations have been performed using MCNP code to study the generation, angular distribution and energy spectrum of photo-neutrons for 1 GeV and 150 MeV electron beam energies impinging on different thickness of Tungsten, Tantalum and Lead targets. It is noticed that the photo-neutron yield increases as the target thickness increases then saturates beyond an optimized thickness of the target. Moreover, the photo-neutron yield shows significant increase as the electron energy increases. At the optimized thickness, the angular distribution of photo-neutrons is found almost isotropic for 150 MeV electrons and anisotropic for 1 GeV electrons. Further, by increasing the electron energy and/or the target thickness the angular distribution is found to be forward peaked. The energy spectrum of photo-neutrons can be well described by a Maxwellian distribution for both electron energies. Such calculations can help in developing a photo-neutron source based time of flight facility (TOF) for elemental and isotopic identification via neutron resonance spectroscopy. Photo-neutron yields, angular distribution, mean energy, energy spectrum and nuclear temperature for 1 GeV and 150 MeV electron energies and different target materials are presented.

МОДЕЛИРОВАНИЕ ИСТОЧНИКА НЕЙТРОНОВ НА ОСНОВЕ УСКОРИТЕЛЕЙ ЭЛЕКТРОНОВ В GEANT4

Monte Carlo calculations with geant4 code provides a convenient tool for studying the parameters of neutron sources. The purpose of this work was simulation of the 30 to 140 MeV electron beam transport through a tungsten target with the geant4 program code. In addition, the neutrons yield produced in the target from the electron beam at the IREN JINR facility was studied. The geant4 program code correctly describes the neutron photoproduction process during the primary electron beam interactions with a tungsten target. To verify the cross sections in geant4 at high photon energies, the cross sections in the TALYS program code were calculated. Used cross sections are in good agreement with previous experimental data. The obtained photoneutron yield dependences from the electron energy for a target simulating an IREN facility are from 7.0·10-3 at 30 MeV to 4.2·10-2 neutrons per electron at 200 MeV.

Microscopic calculation of delayed-photoneutron production in D2O using Geant4

In this work we apply the Geant4 toolkit and recent fission yield data to calculate the photoneutron yield from the thermal fission of 233,235U and 239Pu within a quasi-infinite bath of D2O. The simulation results were fit to a series of exponential terms enabling a direct comparison to historical “11 group” photoneutron data and the application for use in kinetics analyses. Large discrepancies were observed comparing the simulation results to the recommended 235U photoneutron yields (Laughton, 1998) within the shortest (<100 s) and longest (>105 s) half-life groups. These results raise concerns on the validity of the yields derived from experimental data. A detailed discussion on the origin of the historical data is presented as well as explanations for the observed disagreement.

The effect of the flattening filter on photoneutron production at 10MV in the Varian TrueBeam linear accelerator.

Neutrons are an unavoidable by-product of high-energy radiation therapy treatments that deliver unwanted nontarget dose to patients. Use of flattening-filter-free (FFF) photon beams has been shown to significantly reduce photoneutron production per monitor unit (MU) of dose delivered. The purpose of this investigation was to characterize the photoneutron production of the 10MV and 10MV FFF beams of the Varian TrueBeamTM linear accelerator. Neutron fluence spectra were measured using a Nested Neutron SpectrometerTM (NNS, Detec Inc., Gatineau, Canada). The ratios of neutron fluence and ambient dose equivalent for the 10MV FFF beam relative to the 10MV beam, dubbed FF-ratios (FFF/FF), were used to characterize the difference between the two beams. FF-ratios were compared under the following three conditions (a) per MU, at various locations in the treatment room, (b) per MU, with the linac jaws opened and closed, and (c) per electron striking the bremsstrahlung target, as opposed to per MU, at one location with the jaws closed. On average, the neutron fluence for the 10MV FFF beam was 37% lower per MU than the 10MV beam (FF-ratio=0.63). The FF-ratio in neutron fluence and ambient dose equivalent did not vary by much between different locations within the treatment room. However, the FF-ratio in neutron ambient dose equivalent was reduced significantly when the linac jaws were opened compared to closed, which implies that the jaws contribute more to the photoneutron spectrum of the 10MV FFF beam than to the 10MV beam. Finally, it was found that the 10MV FFF beam produces more photoneutrons per electron striking the bremsstrahlung target than the 10MV beam (FF-ratio=2.56). The photoneutron fluence per MU produced by the 10MV FFF beam is 37% lower than the 10MV beam of a Varian TrueBeam linac. Accordingly, a reduction in neutron dose received by patients is achieved through use of the unflattened beam, provided that treatment plans for each beam require approximately the same number of MU. It was found to be instructive to compare the photoneutron yield per source electron between the two beams as it helped provide an understanding of the physics underlying photoneutron production in both beams.

Quantification of the validity of simulations based on Geant4 and FLUKA for photo-nuclear interactions in the high energy range

Photo-nuclear interactions are relevant in many research fields of both fundamental and applied physics and, for this reason, accurate Monte Carlo simulations of photo-nuclear interactions can provide a valuable and indispensable support in a wide range of applications (i.e from the optimisation of photo-neutron source target to the dosimetric estimation in high energy accelerator, etc). Unfortunately, few experimental photo-nuclear data are available above 100 MeV, so that, in the high energy range (from hundreds of MeV up to GeV scale), the code predictions are based on physical models. The aim of this work is to compare the predictions of relevant observables involving photon-nuclear interaction modelling, obtained with GEANT4 and FLUKA , to experimental data (if available), in order to assess the code estimation reliability, over a wide energy range. In particular, the comparison of the estimated photo-neutron yields and energy spectra with the experimental results of the n@BTF experiment (carried out at the Beam Test Facility of DaΦne collider, in Frascati, Italy) is here reported and discussed. Moreover, the preliminary results of the comparison of the cross sections used in the codes with the“evaluated’ data recommended by the IAEA are also presented for some selected cases (W, Pb, Zn).

Estimation of photoneutron yield in linear accelerator with different collimation systems by Geant4 and MCNPX simulation codes

At present, the bremsstrahlung photon beams produced by linear accelerators are the most commonly employed method of radiotherapy for tumor treatments. A photoneutron source based on three different energies (6, 10 and 15 MeV) of a linac electron beam was designed by means of Geant4 and Monte Carlo N-Particle eXtended (MCNPX) simulation codes. To obtain maximum neutron yield, two arrangements for the photo neutron convertor were studied: (a) without a collimator, and (b) placement of the convertor after the collimator. The maximum photon intensities in tungsten were 0.73, 1.24 and 2.07 photon/e at 6, 10 and 15 MeV, respectively. There was no considerable increase in the photon fluence spectra from 6 to 15 MeV at the optimum thickness between 0.8 mm and 2 mm of tungsten. The optimum dimensions of the collimator were determined to be a length of 140 mm with an aperture of 5 mm × 70 mm for iron in a slit shape. According to the neutron yield, the best thickness obtained for the studied materials was 30 mm. The number of neutrons generated in BeO achieved the maximum value at 6 MeV, unlike that in Be, where the highest number of neutrons was observed at 15 MeV. Statistical uncertainty in all simulations was less than 0.3% and 0.05% for MCNPX and the standard electromagnetic (EM) physics packages of Geant4, respectively. Differences among spectra in various regions are due to various cross-section and stopping power data and different simulations of the physics processes.

Design and Simulation of Photoneutron Source by MCNPX Monte Carlo Code for Boron Neutron Capture Therapy

Introduction Electron linear accelerator (LINAC) can be used for neutron production in Boron Neutron Capture Therapy (BNCT). BNCT is an external radiotherapeutic method for the treatment of some cancers. In this study, Varian 2300 C/D LINAC was simulated as an electron accelerator-based photoneutron source to provide a suitable neutron flux for BNCT. Materials and Methods Photoneutron sources were simulated, using MCNPX Monte Carlo code. In this study, a 20 MeV LINAC was utilized for electron-photon reactions. After the evaluation of cross-sections and threshold energies, lead (Pb), uranium (U) and beryllium deuteride (BeD2)were selected as photoneutron sources. Results According to the simulation results, optimized photoneutron sources with a compact volume and photoneutron yields of 107, 108 and 109 (n.cm-2.s-1) were obtained for Pb, U and BeD2 composites. Also, photoneutrons increased by using enriched U (10-60%) as an electron accelerator-based photoneutron source. Conclusion Optimized photoneutron sources were obtained with compact sizes of 107, 108 and 109 (n.cm-2.s-1), respectively. These fluxs can be applied for BNCT by decelerating fast neutrons and using a suitable beam-shaping assembly, surrounding electron-photon and photoneutron sources.

Design and parameter optimization of a small-scale electron-based ADS for radioactive waste transmutation

This paper presents the design and feasibility of an electron-LINAC-based small-scale system (ADS) for nuclear waste transmutation. FLUKA simulations have been performed to evaluate the photoneutron yield in high-Z metallic targets such as silver, tungsten, lead, tantalum and uranium irradiated by electron beams of 20–200MeV. The parameters involved in the photoneutron production mechanism including electron beam energy, target material and target shape have been investigated in order to obtain maximum photoneutron production. The neutron reflectors of the ADS, in particular, beryllium, lead and beryllium oxide (BeO) with various thicknesses have been studied. The results show that a combination of an internal reflector of Pb with a thickness of 3cm and an external reflector of BeO with a thickness of 10cm improves the fluence rate. The photoneutron energy spectrum, photoneutron fluence distribution and heat deposition in the electron target have also been presented. At incident electron beam energy of 155MeV, a neutron source of ∼ 4.6 × 1010 (n/cm2/s/mA) has been achieved, which is highly applicable for using in nuclear waste transmutation. The designed ADS has the ability to transmute ∼ 1.5 × 1022 (atoms/y/mA). The obtained results are promising and could lead to the development of a small-scale ADS based on electron LINAC for radioactive waste transmutation and for numerous applications when employed as a photoneutron source.