High-energy Electron Linear Accelerators Research Articles

Characterization of the X-ray spectrum of a linear accelerator

The Nuclear Measurement Laboratory (LMN) at CEA Cadarache in France uses a high-energy electron linear accelerator, LINAC (9-21 MeV), to characterize nuclear waste drums. It enables to explore new examination modalities, such as active photon interrogation or dualenergy CT to scan large concrete objects with diameters up to 140 cm. These techniques require precise awareness of the photon spectrum emitted by the LINAC. However, direct measure of this photon energy spectrum cannot be achieved because of the accelerator pulses causing detector saturation. During the last few years, a large number of indirect methods has been developed. From an experimental point of view, the simplest indirect method for spectrum estimation method is ransmission measurements. Because it can be set up easily and accurately using an ionization chamber as well as an appropriate screen. The obtained transmission curve depends on the photon energy spectrum, which can be estimated using inverse models. In this paper, we present the development of a numerical model to determine the energy spectrum from an attenuation curve via transmission measurements which combines two types of inverse models: a continue model and a discrete model. We validate this tool using a test spectrum and its transmission curve obtained via Monte-Carlo simulation. This qualification allowed us to determine its sensitivity (signal-to-noise ratio, SNR) in order to have a good convergence. We show that if the SNR is less than 4%, we have a good estimation of the photon energy spectrum. Then, it was experimentally tested with a transmission curve obtained at the laboratory.

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

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

Thermal analysis at target for neutron generation in fissile assay

A target was designed to produce intense neutrons of 1012 neutrons/sec in order to analyze the fissile content by interrogation in the Lead Slowing Down Spectrometer (LSDS) system. A high energy and current electron linear accelerator was used in neutron production by bremsstrahlung radiation. However, the interaction by electron and radiation in the target generates large amounts of heat. Several coolants were simulated in the target and the heat removal capacity was performed. The thermal property of coolant in the target was calculated using the CFX code. The heat distribution by the cooling method was simulated and the forced convection method was used to calculate the temperature distribution on the different coolants and flow rate. Helium shows a good cooling property, but it needs high flow rate with high pressure. Air may be a good candidate coolant if the limit of neutron activation is satisfied in the circulating system. When the coolants were used in the target, each metal plate of the target was kept under the melting temperature. For the estimation of the coolant performance, the neutron influence by the coolant was examined as well. The material property was also evaluated for various target cases. The displacement per atom (DPA) analysis of the target was performed for a life time operation of the LSDS system. For the target container material (HT-9), the optimum thickness was obtained from the sensitivity simulation.

国产医用高能电子直线加速器的质量保证与维护

国产医用高能电子直线加速器的质量保证与维护

Influence of head cover on the neutron dose equivalent in Monte Carlo simulations of high energy medical linear accelerator

Neutron contamination of radiotherapeutic photon beam occurs when energies higher than 10 MeV are used in radiotherapy. To correctly assess the neutron doses that medical personnel and patients receive, it is highly important to know the spectra of the produced photoneutrons. One of the most common ways to determine such spectrum is to perform Monte Carlo simulations of the accelerator. Major issue in the Monte Carlo modelling is that the manufacturers often does not provide full specifications of the accelerators head, so some parts of the head are omitted from the simulation. Within this paper we present a model that includes head cover compared to the one where it is omitted, as it can often be found in the references. Neutron fluxes, spectra, mean energies and place of origin are compared in isocenter, at the point 1 m above target and the point 1 m aside from the target, in both models. In all the considered planes the flux change was found to be more than 20 %, with a significantly change in neutron energy, what is also important in neutron dosimetry. Ignoring the head cover in the Monte Carlo modelling of the high energy electron linear accelerators in radiotherapy, will introduce a large uncertainty of neutron doses assessing a patient, or a medical professional.

The influence of field size and off-axis distance on photoneutron spectra of the 18 MV Siemens Oncor linear accelerator beam

At present, high energy electron linear accelerators (LINACs) producing photons with energies higher than 10 MeV have a wide use in radiotherapy (RT). However, in these beams fast neutrons could be generated, which results in undesired contamination of the therapeutic beams. These neutrons affect the shielding requirements in RT rooms and also increase the out-of-field radiation dose to patients. The neutron flux becomes even more important when high numbers of monitor units are used, as in the intensity modulated radiotherapy. Herein, to evaluate the exposure of patients and medical personnel, it is important to determine the full radiation field correctly. A model of the dual photon beam medical LINAC, Siemens ONCOR, used at the University Hospital Centre of Osijek was built using the MCNP611 code. We tuned the model according to measured photon percentage depth dose curves and profiles. Only 18 MV photon beams were modeled. The dependence of neutron dose equivalent and energy spectrum on field size and off-axis distance in the patient plane was analyzed. The neutron source strength (Q) defined as a number of neutrons coming from the head of the treatment unit per x-ray dose (Gy) delivered at the isocenter was calculated and found to be 1.12 × 1012 neutrons per photon Gy at isocenter. The simulation showed that the neutron flux increases with increasing field size but field size has almost no effect on the shape of neutron dose profiles. The calculated neutron dose equivalent of different field sizes was between 1 and 3 mSv per photon Gy at isocenter. The mean energy changed from 0.21 MeV to 0.63 MeV with collimator opening from 0 × 0 cm2 to 40 × 40 cm2. At the 50 cm off-axis the change was less pronounced. According to the results, it is reasonable to conclude that the neutron dose equivalent to the patient is proportional to the photon beam-on time as suggested before. Since the beam-on time is much higher when advanced radiotherapy techniques are used to fulfill high conformity demands, this makes the neutron flux determination even more important. We also showed that the neutron energy in the patient plane significantly changes with field size. This can introduce significant uncertainty in dosimetry of neutrons due to strong dependence of the neutron detector response on the neutron energy in the interval 0.1–5 MeV.

Estimation of neutron and gamma radiation doses inside the concrete shield wall for 10 and 15 MV medical linear accelerators

High-energy electron linear accelerators (linacs) are used in radiotherapy for tumor treatment. By operating at electron energies above the thresholds of the (γ, n) photonuclear reaction, undesirable photoneutrons and radioisotopes are generated by high-energy x-rays. These photonuclear reactions occur on the linac accelerator head components, which are made from various materials. In these reactions, one can assume that secondary neutrons are dispersed all over the treatment rooms. The present work observed ambient dose equivalent of photon and neutron, and number of etch pits of wide-energy range Neupit produced by thermal and fast neutrons inside a model of iron shielding wall. The neutron dose around the surface of the shielding wall was detected less than three orders of magnitude of photons at 10 MV linac. The total neutron dose in a 15 MV linac was about one order higher than that in the 10 MV linac. In addition, the 15 MV linac incident photons clearly generated secondary neutrons inside the iron shielding wall.

Neutron Spectrometry With a Passive Bonner Sphere System Around a Medical LINAC and Evaluation of the Associated Unfolding Uncertainties

It is generally known that the use of high-energy electron linear accelerators (LINACs) in radiotherapy medical treatments may generate secondary neutrons, mainly via photonuclear (gamma, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> ) giant dipole resonance reactions of incident photons with all the heavy materials present inside the gantry and along the beam line. A detailed knowledge (i.e., fluence energy distribution) of this parasite radiation, which is approximately isotropic and not confined within the primary LINAC beam field, would be of great interest to estimate the associated radiological risk for the patient and the working staff. It has been shown, in this study, that our recently developed passive Bonner sphere system, using pure gold activation foils as central detectors, is well adapted to measure neutron spectra at pulsed and intense mixed n-gamma fields with high-energy photon component. This system was used to characterize the neutron field around a new generation medical electron LINAC. Two measurement positions (isocenter and maze entrance) inside the treatment room of this facility, with the machine operating in Bremsstrahlung photon mode, were chosen. The obtained specific <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">198</sup> Au saturation activities were processed by means of the NUBAY unfolding code, which performs a Bayesian estimation of a parameterized spectrum, to derive the final neutron spectra. Another unfolding method (MAXED), based on the maximum entropy principle and which may depend to some extent on considered the initial guess or default spectrum, was also applied to check the robustness of the NUBAY solutions as well as to carry out a sensitivity analysis to confirm their stability and to corroborate the associated uncertainties on their output results. Also presented are the obtained integral quantities, in terms of neutron fluence and ambient dose equivalent rates normalized to the primary LINAC photon dose, together with an estimation of their associated uncertainties due to the unfolding code used.

Readout System for Cost-Effective Radiation Monitoring System

Bremsstrahlung gamma radiation and photoneutrons are predominantly produced during the operation of high-energy electron linear accelerators. Therefore, generated radiation could cause radiation damage of electronic devices installed in the accelerator tunnel. A real-time radiation monitoring system RadMon has been designed to monitor both types of radiation at crucial spots inside a free electron laser FLASH. Several different versions of readout systems were designed and tested to choose the most suitable architecture for the cost-effective readout of the radiation monitoring system RadMon.

Radiation measurement in the environment of FLASH using passive dosimeters

Sophisticated electronic devices comprising sensitive microelectronic components have been installed in the close proximity of the 720 MeV superconducting electron linear accelerator (linac) driving the FLASH (Free Electron Laser in Hamburg), presently in operation at DESY in Hamburg. Microelectronic chips are inherently vulnerable to ionizing radiation, usually generated during routine operation of high-energy particle accelerator facilities like the FLASH. Hence, in order to assess the radiation effect on microelectronic chips and to develop suitable mitigation strategy, it becomes imperative to characterize the radiation field in the FLASH environment. We have evaluated the neutron and gamma energy (spectra) and dose distributions at critical locations in the FLASH tunnel using superheated emulsion (bubble) detectors, GaAs light emitting diodes (LED), LiF-thermoluminescence dosimeters (TLD) and radiochromic (Gafchromic EBT) films. This paper highlights the application of passive dosimeters for an accurate analysis of the radiation field produced by high-energy electron linear accelerators.

The application of a SRAM chip as a novel neutron detector

High-density, fast digital devices, like field programmable gate arrays (FPGAs), microcontrollers, and static random access memories (SRAMs), can be produced by nanotechnology. New technologies allow the design of fast and powerful devices; however, the decreasing dimensions create new problems. Even at ground level, cosmic ray particles arriving from outer space can affect digital devices and provoke single-event effects (SEEs) due to the smaller sensitive volume (SV). In general, for decreasing feature size of memory cells the expected critical charge decreases and the expected sensitivity to radiation increases. High-density SRAM chips were used to design a fast response, highly sensitive neutron detector. We have conducted experiments with SRAMs at the DESY Research Centre in Hamburg, Germany. Memory contents (number of SEU) were recorded as a function of neutron expose time. The chips were exposed to a neutron field from an americium-beryllium neutron source (241AmBe). The second experiment was accomplished in the 450 MeV electron Linac (Linac II) tunnel. Another batch of SRAMs was irradiated with 60Co gamma rays to a dose of about 60 Gy, and no SEU was registered. This shows that gamma radiation has no substantial effect on the production of SEU in the SRAM. The proposed detector could be ideal for the detection of pulsed neutron radiation produced by high-energy electron linear accelerators and synchrotron facilities, which are currently in operation and planned for the near future.

Femtosecond electron bunches from an RF-gun

Sub-picosecond electron pulses become a tool of increasing importance to study dynamics at an atomic level. Such electron pulses can be used directly or be converted into intense coherent far infrared radiation or equally short X-ray pulses. In principle, sub-picosecond electron pulses can be obtained in large, high-energy electron linear accelerator systems by repeatedly applying an energy slew and magnetic compression . Another process is the production of short electron pulses at low energies from an RF-gun with a thermionic cathode together with a bunch compressing α -magnet. In this paper, we present a systematic analysis of capabilities and limits of sub-picosecond electron pulses from such a source. We discuss particular parameter choices as well as the impact of geometric and electric specifications on the 6-dimensional phase space electron distribution . Numerical beam simulations with the computer code PARMELA are performed including effects and limitations due to space charge forces. While the production of femtosecond electron bunches is of primary concern, we also consider the preservation of such short bunches along a beam transport line.

Femtosecond electron bunches from an RF-gun

Sub-picosecond electron pulses become a tool of increasing importance to study dynamics at an atomic level. Such electron pulses can be used directly or be converted into intense coherent far infrared radiation or equally short X-ray pulses. In principle, sub-picosecond electron pulses can be obtained in large, high-energy electron linear accelerator systems by repeatedly applying an energy slew and magnetic compression. Another process is the production of short electron pulses at low energies from an RF-gun with a thermionic cathode together with a bunch compressing α-magnet. In this paper, we present a systematic analysis of capabilities and limits of sub-picosecond electron pulses from such a source. We discuss particular parameter choices as well as the impact of geometric and electric specifications on the 6-dimensional phase space electron distribution. Numerical beam simulations with the computer code PARMELA are performed including effects and limitations due to space charge forces. While the production of femtosecond electron bunches is of primary concern, we also consider the preservation of such short bunches along a beam transport line.

Reduction of food intake following X-ray irradiation of rats—involvement of visceral afferent nerves

Radiotherapy for malignant tumours often elicits anorexia or loss of appetite as an adverse effect. However, the mechanism for this is poorly understood. The present study was undertaken to investigate if visceral afferents are responsible for reduction of food intake following X-ray irradiation. Rats were exposed bilaterally to X-ray (10 MV) irradiation with total doses of 1.5, 3 and 6 Gy, using a high-energy electron linear accelerator at a dose rate of 4.9 Gy min −1. X-ray irradiation of the whole body, abdomen or head with doses of 1.5, 3 and 6 Gy reduced food intake in a dose-dependent manner. The reduction of food intake after X-ray irradiation of the whole body or abdomen was significantly greater than when only the head was irradiated. Reduction of food intake was observed for the first 4 days after 6 Gy X-ray irradiation of the abdomen, while it was observed only on the first day after the same 6-Gy irradiation of the abdomen in animals whose small-diameter afferents were ablated by capsaicin pre-treatment. These results suggested that the abdominal afferent nerves at least contribute to the reduction of food intake observed on second to fourth days after 6-Gy abdominal irradiation. Taken together, the present evidence suggests that the reduction of food intake following X-ray irradiation of the whole body or the abdomen is partly mediated via abdominal afferent nerves. Moreover, the results of X-ray irradiation to the head suggest that X-ray irradiation directly influences the central nervous system to reduce food intake.

The maximum electron energy achievable in conventional resonant linear accelerators

It is shown that, in resonant high-energy electron linear accelerators, undulator radiation should be observed, which is emitted by off-axis beam particles interacting with the transverse field of nonsynchronous spatial harmonics of the fundamental axisymmetric mode of a periodic accelerating structure. The mean power emitted by an individual electron is proportional to the squared distance between the electron and the accelerator axis and to the squared electron energy. This circumstance may limit the maximum energy to which offaxis beam electrons can be accelerated.

Response of Electrochemical Sensors to Ionizing Radiation in High-Temperature Aqueous Environments

Abstract A high-energy electron linear accelerator (LINAC) generated intense electromagnetic bremsstrahlung that approximated the gamma dose rate in the core of a boiling water nuclear reactor (BWR). Experiments with polymer-free platinum, pH, and silver/silver chloride electrodes at BWR operating temperatures in a stainless steel (SS) autoclave showed reversible, stable voltage offsets that depended on water chemistry and radiation intensity. These changes occurred slowly, and the potentials required several minutes to attain a steady state. Even with a remote reference, it is not possible to determine unequivocally the magnitude and direction of the shift for a specific electrode, but these measurements are most consistent with substantial (100 mV) offsets for silver/silver chloride electrodes, and relatively stable operation for zirconia-membrane pH electrodes. Radiation had no significant effect on high-temperature conductivity. Chemical interactions with radiolytic species offer the simplest explanat...

Measurement of Neutrons from High Energy Electron Linear Accelerators

Abstract Electron linear accelerators (linacs) above 10 MeV are capable of generating significant neutron fluxes, particularly when operating in the X ray mode. This paper describes a prototype passive, integrating system for neutron measurements, based on the plastic etch-pit technique, and the methodology used to determine neutron leakage from linacs. Results are presented from measurements on several machines, and are compared with those obtained using an established gold activation technique. Close agreement was observed. The results also indicate that neutron leakage may be a function of machine age and state of adjustment.

Measurement of Neutrons from High Energy Electron Linear Accelerators

Electron linear accelerators (linacs) above 10 MeV are capable of generating significant neutron fluxes, particularly when operating in the X ray mode. This paper describes a prototype passive, integrating system for neutron measurements, based on the plastic etch-pit technique, and the methodology used to determine neutron leakage from linacs. Results are presented from measurements on several machines, and are compared with those obtained using an established gold activation technique. Close agreement was observed. The results also indicate that neutron leakage may be a function of machine age and state of adjustment.

Neutron Contamination From Medical Electron Accelerators

This report was produced by a scientific committee of eight members, chaired by Richard C. McCall. It is a reference manual dealing with the problem of neutron production that results from the operation of high-energy electron linear accelerators used in radiation therapy. As radiation therapy becomes more sophisticated, in many hospitals in the United States and abroad the trend is to higher energy machines. Accelerators operated below 10 meV do not represent a major hazard; however, as accelerators operate in the 10 to 50 meV range, neutron production may be seen. The source of neutron production in the treatment machine and the methods of protection and measurement are exquisitely detailed in this book. There is an extensive reference list to the source information. This pamphlet goes into details concerning facilities design, including wall thickness, shielding requirements, machine placement, and maze design. Information on hazards, patient risks, and operating personnel risks

Gamma ray and π0 production from hydrogen and complex nuclei by 270–375 MeV gamma rays

Measurements are reported on the differential cross section for secondary gamma ray production on hydrogen and nuclear targets at 90° in the laboratory through the interactions of primary gamma rays in the energy range 270–375 MeV. A difference method using bremsstrahlung beams at different end point energies was employed. The gamma ray detector was a high resolution Nal(Tl) spectrometer and time of flight discrimination was used to reject neutrons. For incident gamma rays in the range 270–375 MeV the first pion nucleon resonance is strongly excited and the observed gamma ray yields are consistent with those expected due to an admixture of coherent and incoherent π0 photoproduction in the resonance region. The experiment marks the first successful use of a large Nal(Tl) crystals as a gamma ray spectrometer at a high energy electron linear accelerator, despite the low duty cycle and its accompanying difficulties for such detectors.