Characterization and optimization of EBT2 radiochromic films dosimetry system for precise measurements of output factors in small fields used in radiotherapy

The accurate determination of absorbed dose in small photon beams, especially for stereotactic radiation therapy, is a difficult task with commercially available detectors. As these small fields are characterized by high dose gradients, a lack of lateral particle equilibrium and a variation of energy spectra with beam sizes, a dosimeter with high resolution, tissue-equivalence and high precision is required. The new radiochromic film EBT2, which meets these criteria, was fully characterized in Institut de Radioprotection et Sûreté Nucléaire (IRSN) for this application. This type of film was tested with the reading system EPSON Dual Lens Perfection V700 flatbed scanner in transmission mode. Warm-up effects of the scanner were studied as well as the influence of the scanner light. Uniformity of unirradiated and irradiated EBT2 films in terms of pixel value was found to be respectively 0.3% (1 SD) and 0.5% (1 SD). An original, accurate and efficient radiochromic film dosimetry protocol was established. The overall uncertainty for dose measurement with EBT2 films using this protocol was estimated at less than 2% (1 SD). Encouraging measurements of output factors were performed on a Novalis system.     

Collimation of the circulating beam in the U-70 synchrotron by means of the reflection of particles in crystals with axial orientation

The possibilities of the extraction and collimation of a circulating beam by a new method due to the reflection of particles in crystals with axial orientation were experimentally investigated in the Fall 2010 run at the U-70 synchrotron. Such crystals have positive features, because the axial potential is five times larger than the planar potential. It has been shown that the collimation efficiency can reach 90% due to axial effects in the crystal. Losses of the circulating beam on a collimator have been reduced by several times; this makes it possible to suppress the muon jet near the steel collimator of the circulating beam.     

OHTE: A Helical Pinch with Pitch Reversal

The OHTE configuration is obtained by surrounding a reversed field pinch (RFP) with a stellarator-like helical winding whose pitch is chosen to enhance field line pitch reversal. The helical coil current needed to form a separatrix boundary is calculated analytically for a simple plasma model, which gives results in close agreement with a numerical two-dimensional (2-D) MHD equilibrium code. Basic properties of the field line transform, which is predominantly in the axial or toroidal direction, are investigated. The 2-D equilibrium code is used to investigate the effects of current profiles and high beta, and the OHTE is compared with the RFP. These calculations show that the helical winding can significantly enlarge the parameter space for interchangestable finite-? equilibria with pitch reversal while avoiding axial current reversal.     

Experimental study of the possibility to increase the pitch factor of an intense relativistic helical electron beam

We present the results of experiments on the formation of a high-quality relativistic helical electron beam (HEB) in a magnetron-injection gun. It is shown that suppression of parasitic excitation of microwaves in the input part of the transportation channel allows eliminating high-voltage breakdowns in the gun and achieving greater beam compression. A modulation of the electron beam current at the frequency of longitudinal electron oscillations between the cathode and the magnetic mirror in the trap, which is related to the instability of the helical electron beam, has been observed for the first time. The modulation depth can reach tens percent. Pickup of reflected electrons by a special diaphragm makes it possible to increase the achievable pitch factor, eliminate the beam-current modulation and, as a result, form although with a current loss on the diaphragm, an HEB with record-breaking values of the pitch factor, which exceed 2. For a moderate HEB compression, when the portion of reflected electrons is relatively low, their pickup by the diaphragm allows one to form a beam in which the total energy of the transverse motion of electrons conserves despite the loss of part of the current. After the optimization, a beam with an electron energy of 300 keV, a current of 100 A, a pitch factor of 1.5, and a velocity spread of 20% is obtained for a 15% loss of the current on the diaphragm. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 855–863, October 2008.     

Energy response of the new EBT2 radiochromic film to x-ray radiation

Gafchromic EBT2, Radiochromic film is assessed for its change in optical density response to x-ray radiation over a broad energy range, from low energy kilovoltage to megavoltage x-rays. A small energy dependence was found with variations in the change in optical density when scanned in the red component of a desktop scanner light source per unit dose of 6.5% from 50 kVp to 10 MV. This produces a slightly smaller and thus even more energy independent film than its predecessor, EBT film whose response varied by 7.7% over the same energy range. The energy response peaked at 100 kVp with a 5% over response compared to 6 MV x-rays and the minimum response found at both 50 kVp and 250 kVp being a 1.5% under response. It should be noted that the shape of the energy dependence response curve increases from 50 kVp up to 100 kVp followed by a decrease through to higher energies whilst the original EBT was found to increase in response from 50 kVp through to 10 MV. A reflected net optical density change of 0.215 ± 0.006 OD for the first Gray of radiation was found for EBT2 analysed in reflection mode at 6 MV x-ray energy. The minimal energy dependence of the EBT2 film provides further enhancement compared to EBT for its accuracy with respect to spectral changes in the beam to measure beams such as IMRT where complex field and multileaf collimator configurations exist causing small spectral changes to occur over the treatment field or at depth where spectral changes also occur.     

Atom lithography without laser cooling

Using direct-write atom lithography, Fe nanolines are deposited with a pitch of 186 nm, a full width at half maximum (FWHM) of 50 nm, and a height of up to 6 nm. These values are achieved by relying on geometrical collimation of the atomic beam, thus without using laser collimation techniques. This opens the way for applying direct-write atom lithography to a wide variety of elements.     

Effect of pump spectra and axial mode separation on the single longitudinal mode performance in end pumped solid-state lasers with semi-monolithic gain medium

We present a theoretical model to study the effect of the pump spectrum and axial mode separation on the single longitudinal mode operation of a laser with a homogeneously broadened semi-monolithic gain medium of short absorption depth. The characterizing parameter is the ratio of the pump power up to which single longitudinal mode is possible to the pump power at the lasing threshold and is denoted by r_max. A numerical study using a Gaussian-shaped pump spectrum reveals that, for a small value of the axial mode separation and for a crystal with short absorption depth at the pump wavelength, the value of r_max reduces significantly with increase in the FWHM of the pump spectrum and thereby results in degradation of the single longitudinal mode performance of the system. However, for large value of the axial mode separation, the SLM performance was found to be nearly independent of the variations in the spectral bandwidth of the pump beam, location of its peak emission wavelength and its polarization.     

Excitation of a cylindrical cavity by a helical current and an axial electron beam current

The explicit expressions (in the Vainshtein and Markov forms) are derived for the excitation of a cylindrical cavity with perfectly conducting walls and with impedance end faces. Excitation of a cylindrical cavity and a cylindrical waveguide with a preset nonuniform axial electron-beam current and a helical current with a variable pitch, which is excited by a concentrated voltage source and is loaded by a preset pointlike matched load, is considered. For the helical current, the integro-differential equation is formulated. The traveling-wave tube (TWT) is simulated in the preset beam current approximation taking into account the nonuniform winding of the spiral coil, nonuniform electron beam, and losses.     

Optimum design of aspheric collimation lenses for optical antenna system

Three kinds of aspheric collimation lenses for optical antenna have been design by optimization. The aspheric cylinder collimation lenses with aspheric surfaces (such as elliptic, hyperbolic and parabolic marginal profiles) have been researched for the semiconductor laser beam with the characteristic of dot emitting source. Based on genetic algorithm and the optimization toolbox of MTLAB, the divergence angle has been optimized. The collimation divergence angle is less than 115 μrad has been measured by laser beam analyzer. This optimum design laser beam collimation lenses as a pre-collimation system can be used for optical antenna system. And it can be widely used in modern space laser communication.     

Collimation testing using axial intensity

We proposed a new method of measuring the degree of collimation of laser beam using axial intensity information near paraxial focus. Preceding methods for collimation testing are mainly either based on self-imaging or interferometric techniques. The new method is to employ the diffraction behavior of noncollimated wave in circular aperture diaphragm. The principle of the proposed method and experiment results are presented. Due to simplicity of the method and its low cost, it is a promising method for checking the collimation of laser beam.     

加绕变升角宽频带椭圆螺旋天线

 椭圆螺旋天线的结构比传统的圆柱螺旋天线更加适合于卫星通信的终端,然而由于其圆极化特性变差而限制了其广泛应用。把改善轴向模螺旋天线辐射特性的两种方法结合起来应用于椭圆螺旋天线,提出了带有附加螺旋线的变升角轴向模椭圆螺旋天线。以一个5圈、短轴长轴之比为0.65的椭圆螺旋天线为例,用FEKO软件对其辐射特性进行了仿真。结果表明,这种方法提高了椭圆螺旋天线的方向性系数,同时具有较好的圆极化特性。在2.2~4.2 GHz频率范围内,最高增益为12.86 dB,比单绕均匀升角的椭圆螺旋天线提高了1.2 dB。     

Experimental study of a powerful magnetron-injection gun for relativistic gyrodevices

We show that it is possible to increase significantly (up to one-half of the Langmuir value) the current of the electron beam in a magnetron-injection gun operated in the nonadiabatic regime of formation of a relativistic helical electron beam if the velocity spread is small. The obtained beam has a current of 200 A, a velocity spread of about 10%, a pitch factor of about 1, and an electron energy of 340 keV. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radio.zika, Vol. 49, No. 8, pp. 680–685, August 2006.     

94GHz缓变结构回旋管设计与数值模拟研究

通过分析广义传输线理论中的模式耦合系数,优化设计了一支94GHz光滑缓变结构回旋管,当电子注电压50kV,电流6A,横纵速度比1.4,工作磁场3.548 5T时,在频率94.099GHz处得到了41%互作用效率,约120kW的功率输出;与折变结构回旋管相比,缓变结构回旋管中的工作模式纯度提高约27dB,注波互作用效率提高约7%。基于自洽非线性理论计算的互作用效率与PIC模拟结果有较好的一致性。     

Electron-optical system for a large-orbit gyrotron

An electron-optical system generating a rectilinear or helical 250 keV/4 A/10 μs electron beam with a high compression factor is developed. For the former beam, a compression factor as high as 4400 and a current density of 25 kA/cm₂ are achieved. In the process of forming the helical beam, the electrons rotating about the system’s axis (paraxial beam) acquire an initial velocity in a transverse magnetic field produced by a kicker. Their pitch factor is increased to a desired (operating) value in an adiabatically growing magnetic field. In tentative experiments with the helical beam in a large-orbit gyrotron, generation was obtained at the second cyclotron harmonic (223 GHz).     

Experiments on the formation of an intense helical electron beam under conditions of picking-up of the electrons reflected from the magnetic mirror

We propose a method of increasing the pitch factor and decreasing the spread of rotational velocities of helical electron beams (HEBs) formed in nonadiabatic magnetron-injection guns of gyroresonant devices. The method is based on the effect of a special diaphragm mounted at the starting point of the transport channel on the process of formation of the laminar electron beam. The diaphragm located at one of the trajectory minima has such a diameter that it cannot be bent around by electrons with minimum rotational velocities. Such electrons land on it, whereas the remaining electrons pass further, moving in an increasing magnetic field. Then the electrons with the maximum rotational velocities reflect from the magnetic mirror adiabatically and land on the other side of the same diaphragm. Thus, the electron beam in the cavity contains electrons with a smaller resulting spread of rotational velocities. In the region of the HEB formation, the accumulation of the space charge of reflected electrons is eliminated, and the shielding of the electric field at the cathode is reduced, which eventually leads to an increase in the HEB pitch factor. Using such a diaphragm in the regime of current limitation by the space charge, the HEB with a high pitch factor (about 1.4) and a velocity spread acceptable for gyrotron applications (lower than 30%) was formed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 9, pp. 785–792, September 2007.     

Steady low shear rate cholesteric flow normal to the helical axis

Steady cholesteric flow at low shear rate normal to the helical axis is studied analytically for shear flow and plane Poiseuille flow on the basis of Leslie’s continuum theory. For general asymmetric solutions the angle made by the director at the sample centre with the primary flow is found to profoundly affect the oscillations of the apparent viscosity with pitch for pitches of the order of the sample thickness. The velocity and orientation profiles are also found to change drastically. These considerations may be important in flow experiments on long pitch cholesterics.     

Analysis of phantom centering positioning on image noise and radiation dose in axial scan type of brain CT

ABSTRACT

This study examined the dose and image quality according to the position change of a human phantom in a CT scan. This study used an MDCT 128 Slice CT Scanner instrument. An axial scan was performed with a 16 cm CTDI phantom of a human phantom, and the dose was measured using a pencil chamber meter. The phantom was scanned 10 cm above and below the isocenter and 15 cm above the right and left. The position of the phantom is indicated by C-0 in the isocenter position, S-10 in the upper 10 cm, I-10 in the lower 10 cm, R-15 in the right 15 cm, and L-15 in the left 15 cm. The test was performed 30 times using the brain CT protocol to calculate the dose and the dose width product (DWP). The acquired images were analyzed using the ImageJ program. Statistical analysis was performed using SPSS with one-way ANOVA (p < .05). The mean DWP values of the CT scanner were C-0 31.97 mGy·cm, S-10 24.52 mGy·cm, I-10 24.28 mGy· cm, R-15 17.95 mGy·cm, and L15 17.6 mGy·cm. Compared to the isocenter (C-0), the DLP values measured at each site were 23.3% for S-10, 24% for I-10, 43.8% for R-15, and 44.9% for L15. A significant difference in the one-way ANOVA statistical process was observed (p>0.05). C-0 was measured to be 7.42 HU, S-10 7.87 HU, I-10 8.4 HU, R-15 117 HU, and L-15 13.6 HU for evaluating the image quality. Compared to C-0, S-10 was 5.39%, I-10 was 13.2%, R-15 was 57.6%, and L-15 was 83.2%. The PSNR for S-10, I-10, R-15, and L-15 was 17.37, 17.5, 16.62, and 16.37 dB, respectively. A good quality image can be obtained by positioning the subject precisely in the isocenter in the axial scan, if possible, because the irradiated dose to the subject is low, which can lead to an increase in noise in image reconstruction.     

螺旋锥束计算机断层成像倾斜扇束反投影滤波局部重建算法

螺旋锥束计算机断层成像(CT)作为常用的临床诊断工具,如何尽可能地减少其辐射剂量是热点研究领域之一.局部成像利用准直器减小射线直照区域,能够有效降低CT辐射剂量.然而,局部成像会造成投影数据横向截断,产生局部重建问题.现有螺旋反投影滤波(BPF)算法只能实现局部曲面重建,难以实现局部体区域重建.在圆轨迹扇束BPF算法的基础上,通过加权修正和坐标扩展,提出了螺旋锥束CT倾斜扇束反投影滤波(TFB-BPF)重建算法.该算法把重建区域按层划分,对各层构建倾斜的扇形束几何,并使用经过加权修正的TFB-BPF算法逐层进行重建.该算法最大的优势是滤波线(与原始螺旋锥束BPF算法中PI线等价)在二维平面内选择,算法更加简洁高效,并且能够应用于局部体区域的重建.实验结果表明,算法能够有效重建物体局部体区域,并且重建图像质量较好,没有明显的截断伪影.     

Heavy ions in RHIC — How to get to higher luminosities?

Luminosity mainly depends on the beam intensity of the circulating beams and the beam profiles at the interaction points. By either increasing the total beam current or decreasing its size the instantaneous luminosity can be increased. In addition any method to slow down beam growth and debunching processes will further improve the luminosity which can be integrated over a store. Optimizing beam cleaning procedures such as collimation or abort gap cleaning will further increase the amount of usable luminosity for experiments. This paper will give a brief introduction to some accelerator basics of interest and the luminosity as a function of beam parameters. Limitations to the achievable integrated luminosity as well as future upgrades to overcome these limitations are presented.     

Effect of impurity on the cholesteric-smectic-A transition

The divergence of the helical pitch near the cholesteric-smectic-A transition has been measured in cholesteryl nonanoate doped with controlled amounts of cholesteryl chloride. The apparent critical exponent v is found to increase sensitively but smoothly with chloride concentration. While the physical origin is unclear, the results indicate the value of v in pure cholesteryl nonanoate is nonclassical.