Photoneutron Spectra Research Articles

Simulating the head of a TrueBeam linear particle accelerator and calculating the photoneutron spectrum on the central axis of a 10-MV photon using particle and heavy-ion transport system code.

Photon energy is higher than the (γ,n) threshold, allowing it to interact with the nuclei of materials with high z properties and liberate fast neutrons. This represents a potentially harmful source of radiation for humans and the environment. This study validated the Monte Carlo simulation, using the particle and heavy-ion transport code system (PHITS) on a TrueBeam 10-MV linear particle accelerator's head shielding model and then used this PHITS code to simulate a photo-neutron spectrum for the transport of the beam. The results showed that, when comparing the simulated to measured PDD and crosslines, 100% of the γ-indexes were<1 (γ3%/3mm) for both simulations, for both phase-space data source and a mono energy source. Neutron spectra were recorded in all parts of the TrueBeam's head, as well as photon neutron spectra at three points on the beamline.

Experimental study of photoneutron spectra from tantalum, tungsten, and bismuth targets for 16.6 MeV polarized photons

ABSTRACT The double differential cross-sections (DDXs) of photoneutron production via the photonuclear reaction on tantalum, tungsten, and bismuth for 16.6 MeV linearly polarized photon beam 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 low-energy 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 affected by the polarized incident photons. These distributions were similar to the previous studies on the 197Au target. The low-energy component's data were fitted with the Maxwellian function. According to the fitting results, the slopes of low-energy neutrons’ distribution for natTa and natW are similar and steeper than that of 209Bi. The anisotropy of the high-energy component can be expressed as a function of cos 2 Θ , where Θ is the angle between the direction of the photon polarization and neutron emission.The DDX energy integrations of the high-energy component were calculated and compared between the three targets. These integration values on natTa and natW were comparable and around 1.5 times smaller than those on 209Bi, regardless of the detection angles.

GEANT4 Simulation of Photoneutron Spectrum from Medical Linear Accelerator

Photons with energy totaling more than 10 MeV provide efficient treatment for deeply seated tumors but interact with the nuclei of high-Z materials constituting a head of the linac. These interactions result in photoneutrons that deliver an additional out-of-field dose to the patient, which increases the risk of radiation-induced cancer. Monte Carlo simulation is an accurate strategy for estimating the effective photoneutron dose for a patient. In the current study, the possibility of using GEANT4 to calculate the photoneutron spectrum from the medical linac was investigated. The free-in-air photoneutron spectrum from a head of the linac was simulated using the NeutronHP experimental package. Validation of the simulated model was carried out based on a comparison of simulated and measured percentage depth–dose curves from photons in the water phantom. The obtained photoneutron spectrum was compared with the previously measured spectrum at the Varian Thilogy linac. GEANT4 may improve the accuracy of calculations of the effective dose based on photoneutrons. However, the simulated model should be improved and optimized. In the future, this model may constitute a physical basis for the prediction of the risk of radiation-induced cancer at our clinical center.

Spectroscopic analysis of photoneutrons in intense [formula omitted]-ray background

CR-39 SSNTD is used to measure the photoneutron spectrum produced by a medical linear accelerator in an intense γ-ray background. The spectroscopic resolution and the neutron detection threshold have been improved by introducing the event selection criteria, based on the track diameter-brightness correlation. The CR-39 detector’s efficiency is determined by adapting the 1H(n,el) cross section from the ENDF/B-VIII.0 evaluations. The measured spectrum was reproduced through Talys-1.96 calculations by implementing the Gogny-HFB microscopic level density model.

Target mass dependence of photoneutron spectrum for 16.6 MeV photons on medium-heavy mass targets

We measured neutron energy spectra for the photonuclear reactions of 16.6 MeV linearly polarized photons on Pb, Au, Sn, Cu, Fe and Ti. The spectra for each target show dependence on the target mass, as well as on the angle between the directions of neutron emission and polarization. The components show angular dependence were extracted from the spectra for each target and compare with the excited levels of residual nuclei. Based on the data, we discuss a plan for further experiments to clearly observe the features of the spectra.

Evaluation of neutron spectra and different fluences inside a radiotherapy room with a modified simple geometry of LINAC head

The photoneutron spectra and ambient dose equivalent were determined in a radiotherapy room with a 15 MV linear accelerator using Monte Carlo simulation. A modified simple geometry of the accelerator’s head was modeled by different simulations. Then, by putting it in a radiotherapy room, the photoneutron spectra at different distances from the isocenter and in the maze were determined. The source term spectrum is due to evaporation, and knock-on neutrons. In the presence of the walls, as the distance from isocenter is increased, evaporation and knock-on neutrons are reduced; however, epithermal and thermal neutrons tend to remain constant. The walls also play an influential role in increasing the photoneutron ambient dose equivalent. Subsequently, by placing this modified accelerator’s head in the radiotherapy room, the coefficients of scattered and thermal fluences were also estimated.

Evaluation of the effectiveness of steel for shielding photoneutrons produced in medical linear accelerators: A Monte Carlo particle transport study

PurposeTo confirm the effectiveness of steel as a material for shielding photoneutrons produced in medical linear accelerators and report values for neutron first and second tenth value layer to be used as radiation protection quantities. MethodsMonte Carlo code Geant4 was used to simulate transport of photoneutrons through primary barriers containing layers of concrete and steel. The photoneutron spectrum generated in high energy modern accelerator head components was simulated and projected to barriers of different thicknesses of steel including a control case of no barrier. To derive the neutron tenth value layer (TVLn), absorbed dose was evaluated at 1 cm depth in a water phantom outside the barrier. The fluence and energy spectrum of neutrons leaking outside the barrier were also calculated. The neutron source model was used to quantitatively assess the neutron shielding for a newly designed bunker. ResultsFor an 18 MV photon beam produced by a linear accelerator, the first and second TVLn, for steel as a neutron shielding material were determined to be 15.7–16.1 cm and 28.5–32.4 cm, respectively, depending on the vendor. The lateral distribution of neutrons outside steel barriers is investigated. Also, we show the relative efficiency of steel for fast and epithermal neutrons and determine the neutron energy spectrum outside a steel barrier. ConclusionThis paper agrees with previous studies for Pu-Be sources, and we would advocate using these values for the purposes of high energy linear accelerator bunker design.

Photoneutron spectrometry by novel multi-directional spherical neutron spectrometry system

Neutron spectrometry in science and technology applications in general and accurate exotic photoneutron (PN) dosimetry of cancer patients undergoing high-dose high-energy X-rays therapy in medical accelerators in particular is of vital need. In this study, a novel passive multi-directional multi-detector neutron spectrometry system was developed and home-made using 6 polycarbonate/10B detectors on 6 sides of polyethylene (PE) cubes used bare and also embedded at center of PE spheres of 8 different diameters. The system provided well-resolved unfolded directional PN spectra showing thermal and fast PN peaks of 6 sides and mean spectrum in 5 field sizes at isocenter and other locations in 18 MV Siemens ONCOR medical linear accelerator bunker. The neutron spectrometry system developed has unique characteristics such as being simple, efficient, low cost, practical, and insensitive to low-LET radiation with well-resolved directional and mean spectra easily applicable in medicine, health, environment, science and technology in developing and developed laboratories.

Photoneutron reaction kinematics and error of commonly used approximations

An exact closed-form classical equation and a transcendental relativistic equation for the energy-angle relationship of the neutrons produced from the photoneutron reaction are derived in the laboratory coordinate system. Additional formulas are derived to describe the restriction on scattering angle in the double-valued photoneutron energy regime for incident photon energies near the kinematic threshold energy of the target nucleus. The full range of photoneutron reaction independent variables (target nucleus mass number, neutron separation energy, and photon energy) encountered in practical photoneutron production applications are explored to give the applicable range of classical kinematics and other approximations in the calculation of the photoneutron energy-angle relationship. A six-way comparison study of classical, relativistic, and approximation equations presented in this work and those implemented in radiation transport codes is performed. The well-known and widely-used approximations presented without derivation by Wattenberg and Hanson in the late 1940s are only applicable to energy spread of neutrons produced from gamma-ray photoneutron sources, the intended application of the approximations. The Wattenberg and Hanson approximations are inappropriate for general purpose radiation transport simulation and give unphysical negative photoneutron energies for incident photon energies near the kinematic threshold. Recent versions of the MCNP code appear to use erroneous equations related to inelastic neutron scattering to describe photoneutron kinematics resulting in the overestimation of photoneutron energy. Simulated photoneutron spectra are significantly hardened and cannot be used in contemporary nuclear science applications such as characterization of accelerator-driven photoneutron sources. The Monte-Carlo code developer and user communities need to exhaustively review, correct, document, verify, and validate photoneutron physics implementations in the codes that are in common use.

Investigation of Neutron Contamination of Flattening Filter and Flattening Filter-Free 10-MV Photon Beams in Elekta InfinityTM Accelerator

Introduction: This study aimed to measure the neutron contamination of flattening filter (FF) and flattening filter-free (FFF) 10-MV photon beams delivered by the Elekta InfinityTM accelerator. Material and Methods: The photoneutron spectrum produced by the Linac head was evaluated using a Monte Carlo (MC) simulation. The geometry and composition of the head Linac material were modelled based on information obtained from the manufacturer. In this simulation, MC N-Particle Transport Code software (MCNP6) was utilized to model the Linac head and simulate the particle transport. Evaluation of neutron contamination was carried out for the Linac with FF and without it (i.e., FFF). In this regard, the FFF beam was built by removing the FF from the Linac components. The scoring plane, as the neutron spectra calculation area for FF and FFF beams, was placed 99 cm from the target. Results: The neutron type produced by the head Linac Elekta InfinityTM 10-MV photon mode was mostly thermal and fast. Although there were differences in the neutron intensity of FF and FFF beams, the type of neutrons produced by these two modes had the same energy. Based on the photoneutron reaction energy threshold, it can be concluded that the neutrons produced from the head Linac were the result of photoneutron interactions of high-energy photons with molybdenum-96 and tungsten-184 isotopes. Conclusion: The photoneutron quantity did not change for FF and FFF beams; however, a larger quantity of neutrons was produced in the FF beam.

Investigation of LINAC Structural Effects on Photoneutron Specified Parameters Using FLUKA code

Introduction: The utilization of high-energy photons in the medical linear accelerator can lead to photoneutron production. This study aimed to evaluate the effect of the physical components of the head, including flattening filter (FF) andmultileaf collimator (MLC), as well as the dependence of therapeutic field size on the photoneutron spectrum, dose, and flux. Material and Methods: The present study reported the simulation of the fundamental linac head components of the Varian Clinac 2100 performing in X-ray mode with 18 MV energy by the FLUKA code. The percentage depth dose and lateral dose profile were measured using a PTW thimble chamber to ensure the simulation reliability. Results: Photoneutron spectrum analysis indicated that neutrons with highest relative biological effectiveness were delivered to the phantom surface, and opening the field from 0×0 to 40×40 shifted the spectrum by 24.545% to the higher energies. The target and the vicinity parts played the most prominent roles in neutron contamination. The relationship between the field size and the photoneutron dose was non-linear, and it reached a peak of 20×20 . Although using small fields formed by the MLC contribute to a lower dose compared to those shaped by the jaws, MLC-equipped machines result in 21.98% higher dose. Moreover, the flattening filter removal unexpectedly increased the isocenter photoneutron dose by 11.63%. This undesirable dose can be up to 2.54 mSv/Gy for the reference field at the isocenter while the out-of-field dose is about 0.5 mSv/Gy for most of the field dimensions. Conclusion: As a result, it is critical to consider this unwanted absorbed dose, which is seriously influenced by the implemented therapeutic conditions.

Analysis of Photo Neutron Production From the Patient During Radiotherapy Treatment using CR 39 SSNTD

Most radiation treatments for curing cancer are completely non invasive. Usually patients will undergo radiation treatment to relieve symptoms and pain. The research focuses the various processes involved in generating photo neutron spectrum and dose proceeding from accelerator for treatment. The electrons are stimulated to high energies through a linear tube. These high-energy electron beams similarly treats shallow malignant growths. It tends to strike an objective to create x-ray beams for deep-seated tumor treatments. Hydrogen thyratrons are the direct current power supply for the modulator that has a pulse-forming network with switch tube. This results in a flat-topped high voltage pulse from the modulator area to DC pulse for a few microseconds of the period.Photo neutron spectrum and dose calculated. 15 MV lead to photo neutron production from many of the elements including Tungsten, Lead, Tantalum Nitrogen, Phosphorus and so on. Particularly 207Pb has photo neutron threshold as low as 6.74 MeV and 182W has threshold 8.04 MeV. Moreover, Nitrogen and Phosphorus have a lesser threshold for photo-proton emission at 7.55 MeV and 7.3 MeV respectively. Considering the radiation-weighting factor for neutrons, it is obvious that, the neutron dose considered in the treatment planning process, and we have to incorporate some mechanism in the radiotherapy treatment delivery for neutron free photons.

Analysis of Photo Neutron Dose to Patient from a 15 MV Linear Accelerator

Medical Linear Accelerator produces high-intensity bremsstrahlung beam, with typical energy ranges from 4 to 18MV for treatment purposes. The materials including patient exposed to this high energy spectrum lead to a photonuclear reaction in them. In these Photon energy domains, photo neutron production (n, p, and γ, n) is the major reaction channel. This is important as Carbon, Oxygen, Hydrogen, and Nitrogen are the predominant elements in a typical human being and having low photon neutron threshold. Apart from this, the equipment involved in a radiation treatment including Head, Couch, immobilization accessories, etc contributes to photo neutron production. The present study was recorded and analyzed the photo neutron spectra due to 15 MV medical linear accelerator using CR-39 Solid State Nuclear Track Detector and image processing software.

Research on Photo Neutron Dose to Patient by (n, p) Reaction from a 15 MV Linear Accelerator

Introduction: Photoneutron produced during radiotherapy treatment was measured and the possibility for the radiation yield by the (n, p) reaction within the body was calculated. Materials and Methods: Photoneutron spectrum was measured using Columbia Resin (CR 39) film from a Siemens Primus Plus Linear accelerator and the possibility for the radiation yield by the (n, p) reaction within the body was calculated during radiotherapy treatment. Results--- The photoneutron spectrum was measured with CR 39 SSNTD and automatic track counting software. The interaction cross section for the (n,p) reaction for each energy and element were determined and radiation yield was calculated.For hydrogen, there is no (n,p) reaction. The (n,p) reaction is absent in C12for neutron of energy below 13.5 MeV. Oxygen has (n,p) reaction above 10 MeV. Nitrogen has a low threshold and there is no threshold for potassium and calcium. Conclusion--- From the result, it is clear that (n, p) reactions are significant and even though the dose from these reactions is low, the impact will be severe due to the high LET nature of proton. The data can be used to minimize the photoneutron production during the radiotherapy treatment in the future. The outcome of the work indicates the importance of photoneutron transport and dose deposition to be included in the treatment planning algorithm

The plastic tray effect on the spectrum of photoneutrons prudused by 18 MV medical linac using the Monte Carlo method

Introduction: One of the important problems of using high energy linacs is the production of photoneutrons. High energy medical linacs produce secondary neutrons. These phtoneutrons increase patient dose Materials and Methods: In this study, the effect of plastic trays on spectrum of photoneutrons produced by 18MV linac is investigated using the MCNP4C code. To determine the photoneutron spectrum a simplified linac head simulated. In order to contribute the all components of the spectra, an isotropic neutron source was considered with an energy spectrum derived from the relevant equation. Source defined at the center of a thick walled sphere of tungsten. Two kinds of trays, PMMA and Polyethylene, each with 1.5 cm thickness defined just under the aperture of simplified linac head. A typical treatment room simulated with the walls made of standard concrete. Neutron detectors defined as spheres of 10 cm diameter. One of the detectors defined just under the linac head and the other at the adjacent of the bucker door. Three situations were considered, one without tray and two others with trays. Simulation carried out by running the computer to produce 100 millions of particles for each case. F4 tally was used. Results: The results show that spectrum of photoneutrons differs considerably for open field and shielded fields.

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.

AMBIENT DOSE EQUIVALENT RECEIVED AT 15 MV LINAC ENVIRONMENTS DUE TO PHOTONEUTRONS CONTAMINATION.

Mapping of the photoneutron spectrum and flux performed at several positions over the treatment table of a 15 MV medical linac. Neutron activation technique has been applied to obtain the neutron spectra in the mixed γ -n field. Thick foils have been used due to the low neutron flux and the measurements were unfolded using the MINUIT minimizer. GEANT4 code simulations have been performed to reproduce the photoneutrons' spectrum and estimate the flux along the treatment table. The mixed radiation field around medical linear accelerator was evaluated and the variation of the ambient dose equivalent over the treatment table was estimated. Photoneutrons' ambient dose equivalent inside the irradiation field estimated as 0.56 ± 0.05 while the peripheral as 0.42 ± 0.04 mSv Gy-1. The effective dose inside the irradiation field is ranged between 0.004 and 0.049 while the peripheral up to 0.035 mSv Gy-1.

The $\gamma$ γ -ray angular distribution in fast neutron inelastic scattering from iron

The angular distribution of $ \gamma$ -rays emitted after inelastic scattering of fast neutrons from iron was determined at the n ELBE neutron time-of-flight facility. An iron sample of natural isotopic composition was irradiated by a continuous photo-neutron spectrum in the energy range from about 0.1 up to 10 MeV. The de-excitation $\gamma$ -rays of the four lowest excited states of 56Fe and the first excited state of 54Fe were detected using a setup of five high-purity germanium (HPGe) detectors and five LaBr3 scintillation detectors positioned around the sample at $ 30^\circ$ , $55^{\circ}$ , $90^{\circ}$ , $ 125^{\circ}$ and $150^{\circ}$ with respect to the incoming neutron beam. The resulting angular distributions were fitted by Legendre polynomials up to 4th order and the angular distribution coefficients $a_{2}$ and $ a_{4}$ were extracted. The angular distribution coefficients of three transitions in 56Fe are reported here for the first time. The results are applied to a previous measurement of the inelastic scattering cross section determined using a single HPGe detector positioned at $125^{\circ}$ . Using the updated $\gamma$ -ray angular distribution, the previous cross section results are in good agreement with reference data.

MCNP5 evaluation of photoneutron production from the Alexandria University 15 MV Elekta Precise medical LINAC

Photoneutron production, and the dose equivalent, in the head assembly of the 15 MV Elekta Precise medical linac; operating in the faculty of Medicine at Alexandria University were estimated with the MCNP5 code. Photoneutron spectra were calculated in air and inside a water phantom to different depths as a function of the radiation field sizes. The maximum neutron fluence is 3.346×10−9 n/cm2-e for a 30×30 cm2 field size to 2–4 cm-depth in the phantom. The dose equivalent due to fast neutron increases as the field size increases, being a maximum of 0.912 ± 0.05 mSv/Gy at depth between 2 and 4 cm in the water phantom for 40×40 cm2 field size. Photoneutron fluence and dose equivalent are larger to 100 cm from the isocenter than to 35 cm from the treatment room wall.

Estimation of photoneutron dosimetric characteristics in tissues/organs using an improved simple model of linac head

Neutrons as a one of the by-products of high-energy photons in radiotherapy increase the patient's risks and could cause secondary cancers. Therefore, a new corrected simplified model for linac head was introduced to calculate equivalent (H) and absorbed (D) doses in different tissues/organs of a phantom model. The photoneutron spectrum calculated with this model was in agreement with the spectrum obtained by using a detailed linac´s head model having all components. Besides, an anthropomorphic phantom was irradiated under different gantry angles of the corrected simplified model aiming to emulate 15, 18, 20, and 25MV Siemens linacs. The results indicated that tissues which were within the treatment field received more dose than others. Furthermore, tissues which were in the vicinity of each other and the same depth in the phantom nearly received the same doses. Finally, fatal secondary cancer risk was also studied.