High energy X-ray computed tomography for industrial applications

A high energy X-ray computed tomography (CT) system with an electron linear accelerator was developed to image cross-sections of large-scale and high-density materials. An electron linear accelerator is used as the X-ray source. The maximum X-ray energy is 12 MeV, and the average energy is around 4 MeV. The intensity of the X-ray fan beam passing through the test object is measured by a 15-channel detector array. CWO (CdWO₄) scintillators and photodiodes are used as the X-ray detectors. The crosstalk noise due to scattering of X-ray photons by adjacent detectors is reduced to less than 1.6% by installing tungsten shields between the scintillators. Extra channels are used to compensate for base line shift of the circuits. These techniques allowed attainment of a dynamic range of more than 85 dB and a noise level comparable to the signal amplitude of X-rays transmitted in a 420-mm thick iron block. A spatial resolution of 0.8 mm was confirmed with an iron test piece 200 mm in diameter     

Development of precise energy spectrum measurement technique for high-power pulsed X-ray sources for industrial use

It is difficult to obtain an accurate energy spectrum by measurement for X-rays which are produced by linear accelerators because so many photons are emitted within a very short period and the photons have energies in a wide range. In order to solve the problem, we developed a new technique utilizing the Bayesian estimation method combined with attenuation curve measurement using a step-shaped attenuation material. We experimentally confirmed the validity of the presented technique for an X-ray computed tomography (CT) system. By using the accurate energy information of emitted X-rays obtained by the developed technique, the quality of non-destructive inspection images can be expected to be improved for industrial X-ray radiography, X-ray CT and so on, using high-power X-ray sources.     

Compton scatter and X-ray crosstalk and the use of very thinintercrystal septa in high-resolution PET detectors

To improve spatial resolution, positron emission tomography (PET) systems are being developed with finer detector elements. Unfortunately, using a smaller crystal size increases intercrystal Compton scatter and X-ray escape crosstalk, causing positioning errors that can lead to degradation of image contrast. We investigated the use of extremely thin (<300 μm) lead strips for passive shielding of this intercrystal crosstalk. Using annihilation gamma rays and small (2- and 3-mm wide) bismuth germanate (BGO) crystal detectors in coincidence, crosstalk studies were performed with either two small adjacent crystals [(one-dimensional) (1-D)] or one crystal inside a volume of BGO [(two-dimensional) (2-D)]. The fraction of Compton scattered events from one crystal into an adjacent one was reduced, on average, by a factor of 3.2 (2.2) in the 1-D experiment and by a factor of 3.0 (2.1) in 2-D one, with a 300 (150)-μm-thick lead strip in between the crystals and a 300-700-keV energy window in both crystals. We could not measure a reduction in bismuth X-ray crosstalk with the use of lead septa due to the production of lead X-rays of similar energy. The full-width at half-maximum (FWHM) of the coincident point-spread function (CPSF) was not significantly different for the 1- and 2-D studies, with or without the different septa in place. However, the FWTM was roughly 20% smaller with the 300-μm lead shielding in place. These results indicate that intercrystal crosstalk does not affect the positioning resolution at FWHM, but does affect the tails of the CPSF. Thus, without introducing any additional dead area, an insertion of very thin lead strips can reduce the extent of positioning errors. Reducing the intercrystal crosstalk in a high-resolution PET detector array could potentially improve tomographic image contrast in situations where intercrystal crosstalk plays a significant role in event mispositioning     

Evaluation of K X-ray escape and crosstalk in CdTe detectors andmulti-channel detectors

The simple structure of CdTe semiconductor detectors facilitates their downsizing, and their possible application to radiographic sensors has been studied. The escape of K X-rays from these detectors increases with reduction of their dimensions and affects the measurements of X- and gamma-ray spectra. K X-rays also produce crosstalk in multi-channel detectors with adjacent channels. Therefore, K X-rays which escape from the detector elements degrade both the precision of energy spectra and spatial resolution. The ratios of escape peak integrated counts to total photon counts for various sizes of CdTe single detectors were calculated for gamma rays using the Monte Carlo method. Also, escape and crosstalk ratios were simulated for the CdTe multi-channel detectors. The theoretical results were tested experimentally for 59.54-keV gamma rays from a ²⁴¹Am radioactive source. Results showed that escape ratios for single detectors were strongly dependent on element size and thickness. The escape and crosstalk ratios increased with closer channel pitch. Our calculated results showed a good agreement with the experimental data. The calculations made it clear that K X-rays which escaped to neighboring channels induced crosstalk more frequently at smaller channel pitch in multi-channel detectors. A radiation shielding grid which blocked incident photons between the boundary channels was also tested by experiment and by calculation. It was effective in reducing the probability of escape and crosstalk     

Two-colored Laser Circulation System for Monochromatic Tunable Hard X-ray Source

A two-colored laser pulse circulation system for a monochromatic tunable hard X-ray source via laser electron Compton scattering is investigated. The demonstration system of the X-ray source is under construction at the University of Tokyo. It consists of the X-band (11.424 GHz) electron linear accelerator and two Nd: YAG laser systems. The main advantage of this system is a monochromatic tunable hard X-ray. It is calculated that the X-ray intensity will be about 10⁸ photons/s. In order to enhance the X-ray intensity for medical applications such as dual energy X-ray CT, a two-colored laser pulse circulation system has been designed. The laser pulse circulation experiment without an electron beam has been carried out by using a Nd: YAG laser fundamental wave (50 mJ) and a second harmonics wave (25 mJ). The result shows that the X-ray intensity can be enhanced by a factor of 10 times higher (i.e., up to 10⁹ photons/s). This work is a part of the JST (Japan Science and Technology Agency) project. The entire X-ray source system is a part of a larger national project on the development of an advanced compact medical accelerator sponsored by the NIRS (National Institute for Radiological Science). The University of Tokyo and KEK are responsible for the X-ray source.     

Some characteristics of X-ray imaging for energy region of over 100 keV using plastic scintillation fiber array

In this work, characteristics of using PSFs （plastic scintillation fibers） coupled with CCD （charge-coupled devices ） to build area detectors for high energy X-ray imaging are studied with a Monte Carlo simulation, which cover an energy range of a few hundred keV to about 20 MeV. It was found that the efficiency of PSF in detecting X-ray with energy above a few hundred keV is low. We can use large incident flux to increase the output signal to noise ratio （SNR）. The performance can also be improved by coating PSF with X-ray absorption layers and the MTF of the system is presented. By optimizing the absorption layer thickness, the crosstalk of the area detector built with PSF decreases.     

Response function estimation with fine energy bins for the energy-resolved computed tomography using a transXend detector

A new ‘transXend’ detector system has been developed for energy-resolved computed tomography (CT). It consists of several segmented detectors that measure X-rays as electric currents. Response functions of segmented detectors are estimated using component materials of a body under inspection to unfold X-ray spectra. To avoid material-dependent measurements, response functions inherent to segmented detectors are evaluated by Monte Carlo calculations. CT was performed for a phantom consisting of five resins and was analyzed by the estimated response functions. The linear attenuation coefficients for the five resins have excellent agreement with database values.     

Pinhole imaging to observe spatial jitters of a triple-pulse X-ray source on the Dragon-Ⅱ LIA

In high-energy flash radiography,scattered photons degrade the acquiring image,which limits the resolving power of interfaces and density of dense object.The application of large anti-scatter grid can reduce the scattered photons remarkably,but this requires a stable source position in order to reduce the loss of signal photons in the grid structure.The pinhole imaging technique is applied to observe spatial jitters of a triple-pulse radiographic source of a linear induction accelerator.Numerical simulations are conducted to analyze the imaging performance with the same or close parameters of the pinhole object and experimental alignment.Experiments are carried out to observe spatial jitters of the source between different measurements.Deviations of the source position between different pulses are measured in each experiment.     

Experimental response function of NaI(Tl) scintillation detector for gamma photons and tomographic measurements for defect detection

The response function of gamma detector is an important factor for spectrum analysis because some photons and secondary electrons may escape the detector volume before fully depositing their energy, of course destroys the ideal delta function response. An inverse matrix approach, for unfolding of observed pulse-height distribution to a true photon spectrum, is used for construction of experimental response function by formulating a 40 × 40 matrix with bin mesh (E^1/2) of 0.025 (MeV)^1/2 for the present measurements. A tomographic scanner system, operating in a non-destructive and non-invasive way, is also presented for inspection of density variation in any object. The incoherent scattered intensity of 662 keV gamma photons, obtained by unfolding (deconvolution) the experimental pulse-height distribution of NaI(Tl) scintillation detector, provides the desired information. The method is quite sensitive, for showing inclusion of medium Z (atomic number) material (iron) in low Z material (aluminium) and detecting a void of ∼2 mm in size for iron block, to investigate the inhomogeneities in the object. Also, the grey scale images (using “MATLAB”) are shown to visualise the presence of defects/inclusion in metal samples.     

Luminescence Emission Properties of $({rm Lu},{rm Y})_{2}{rm SiO}_{5}$:Ce (LYSO:Ce) and $({rm Lu},{rm Y}){rm AlO}_{3}$:Ce (LuYAP:Ce) Single Crystal Scintillators Under Medical Imaging Conditions

LYSO:Ce and LuYAP:Ce are single crystal non-hygroscopic scintillators of high density, high light yield and short decay time, which have been successfully used in small animal PET imagers. In the present study, the luminescence emission properties of (Lu_0.9, Y_0.1)₂SiO₅:Ce (LYSO:Ce) and (Lu_0.7, Y_0.3)AIO₃:Ce (LuYAP:Ce) crystals were investigated for use in X-ray medical imaging. Both crystals had dimensions of 2 times 2 times 8 mm³, with all surfaces polished. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) and the detector optical gain (DOG) (emitted light photons per incident x-ray photon) in a wide range of X-ray energies employed in mammography (22-49 kVp) and in general X-ray imaging (50-140 kVp). Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The emission spectrum under X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility to various optical photon detectors incorporated in medical imaging detectors. Optical characteristics such as transmission and absorption spectra were investigated in addition to the scintillation properties. The light emission performance of the two scintillation materials studied was found adequately high for X-ray imaging.     

Application of gamma ray computed tomography to non-destructive testing

A gamma ray computed tomography (ψ-ray CT) system has been developed for applications to large specimens of high density materials.The system features a wide fan beam of ψ-rays, extendable up to 20 degrees, 20 channel ψ-ray scintillation detectors using bismuth germanate oxide with a counting rate being up to 10⁵ per second for reducing the mechanical scan time. Performance of the ψ-ray CT system is confirmed by measuring a test piece and a stepping motor (diameter: 5 cm; main material component: iron), for instance. It can represent these cross-sectional images with a spatial resolution of 1.5 mm. About 10 min are required to obtain these images using a 100 mCi ¹³⁷Cs source.     

Readout electronics for a high-resolution soft X-ray spectrometer based on silicon drift detector

The readout electronics for a prototype soft X-ray spectrometer based on silicon drift detector (SDD),for precisely measuring the energy and arrival time of X-ray photons is presented in this paper.The system mainly consists of two parts,i.e.,an analog electronics section (including a pre-amplifier,a signal shaper and filter,a constant fraction timing circuit,and a peak hold circuit) and a digital electronics section (including an ADC and a TDC).Test results with X-ray sources show that an energy dynamic range of 1-10 keV with an integral nonlinearity of less than 0.1％ can be achieved,and the energy resolution is better than 160 eV@5.9 keV FWHM.Using a waveform generator,test results also indicate that time resolution of the electronics system is about 3.7 ns,which is much less than the transit time spread of SDD (＜100 ns) and satisfies the requirements of future applications.     

Energy spectra measurements of X-ray emission from electron interaction in a dense plasma focus device

Electrons generated during a pinch implosion in a hollow anode Mather-like plasma focus device (PF) are considered as a possible X-ray source via the impinging of those particles on medium and high-Z targets. A usual PF device has been slightly modified to optimise the X-ray production and their measurements by means of a suitable and non-invasive spectrometer. This ensemble allows measurements of X-rays generated booth by electrons turning back to the anode and by target collision of the so-called relativistic electron beam. The spectrum of the emitted photons is evaluated by using a differential absorption based technique. The X-ray spectrometer consists of a stack of LiF dosimeters which act both as detectors and filters to give curves of attenuated intensities. Finally, the energy distribution is calculated from such attenuation curves using an iterative procedure based on spectral algebra formalism.     

A gamma-ray diffraction instrument for high-intensity Mössbauer sources

A Mössbauer gamma-ray diffraction instrument has been developed which utilizes high intensity sources produced by neutron irradiation. Most of the early work has used 70 Ci, 5.1 d ¹⁸³Ta sources produced by double neutron capture which yield 12 × 10¹⁰ photons/s for the 46.5 keV Mössbauer transition in ¹⁸³W. A dewar is enclosed in the shielding cask which allows sources to be cooled to 77 K. Samples are located approximately midway between source and detector, which are separated by 155 cm. Collimators in front of the sample and detector commonly limit the beam to 3 mm wide by 25 mm high. A 27% minimum is seen in the velocity spectrum for the 46.5 keV photons scattered from the 200 Bragg reflection in LiF taken in transmission. Enriched, room temperature absorbers mounted on a rotor can be Doppler-shifted up to 17 meV. Q-space resolution of 0.01 Å⁻¹ is significantly better than that reported by other groups using Mössbauer scattering. When a microfoil internal conversion electron (MICE) detector is used, the instrument has the potential for directly measuring the energy transferred by the inelastically scattered photons, especially for low energy excitations in solids which require high energy resolution.     

电子辐照加速器屏蔽墙中预埋管道倾角对辐射防护性能影响的蒙特卡罗计算

电子辐照装置屏蔽墙中总存在一些较大口径的管道如辐照电缆传输孔道,这些管道的存在势必会降低屏蔽体的局部屏蔽能力。为定量评估管道对屏蔽效果的影响,本文以3 MeV电子辐照加速器为例,采用蒙特卡罗MCNP程序分别模拟计算了电子打靶以后产生的能谱中1 MeV以下和1 MeV以上不同单能光子入射同一屏蔽体时,管道以不同角度穿透屏蔽墙时管道出口处及屏蔽墙外辐射场分布;并比较了在不同墙厚下这两个能段对管道出口处辐射场的影响程度。计算结果表明:不同单能光子入射屏蔽体,在管道穿墙角度≥45°时,管道出口处辐射场变化不再明显,对于1 MeV以下的低能光子在墙外的透射辐射场基本可忽略,对于1 MeV以上的能量光子,其墙外光子透射辐射场随角度增大影响显著;随着墙厚增加,低能光子衰减越显著,高能光子在管道穿墙夹角设计中越占据了主导地位。     

Energy and Angular Responses of the Criticality Accident Detector using a Plastic Scintillator

The Japan Atomic Energy Agency (JAEA), Nuclear Fuel Cycle Engineering Laboratories, operates a spent fuel reprocessing plant and MOX (Plutonium-Uranium Mixed Oxide) fuel fabrication plants. Criticality accident detectors have been installed in these facilities. The detector, the Toshiba RD120, is composed of a plastic scintillator coupled to a photomultiplier tube, and an operational amplifier. The alarm triggering point is set to 1.0–3.6 mGy.h^?1 in photon dose rate to detect the minimum accident of concern. However, a plastic scintillator is principally sensitive not only to primary photons but also to neutrons by secondary photons and heavy charged particles produced in the detector itself. The authors calculated energy and angular responses of the RD120 criticality accident detector to photons and neutrons using Monte Carlo computer codes. The response to primary photons was evaluated with the MCNP-4B and EGS4 calculations, and photon and X-ray irradiation experiments. The response to neutrons that produce secondary photons and heavy charged particles from neutron interactions was computed using the MCNP-4B and SCINFUL, respectively. As a result, reliable response functions were obtained. These results will be a great help in reassessing the coverage area and in determining the appropriate triggering dose rate level in criticality accidents.     

NaI:TI与LaBr_3:Ce~(3+)闪烁体探测器能量分辨的GEANT4模拟

闪烁体探测器γ能谱测量中,闪烁光子的产生、传输及光电器件收集、倍增均对能谱的分辨有巨大影响。为了精确模拟闪烁体探测器的能量分辨,采用蒙特卡洛模拟软件GEANT4建立了一套模型,分别对Na I(TI),LaBr_3:0.5%Ce~(3+)闪烁体探测器测量能量为662 ke V的γ射线能谱进行模拟,获得的结果与实验测试结果能够很好的符合,验证了建立模型合理性与可靠性,为闪烁体的设计提供了一套更精确的开发工具。     

X射线工业CT散射修正

X射线工业CT（ICT）依赖准直器压低散射光子存在实际困难和不足。为解决这一问题，需采用相应的修正软件加以校正。文章从理想的模型出发，近似推导透射式ICT中散射X光子的理论计算公式，并把计算结果与实验结果进行比较。比较结果表明，两者吻合较好。据此，文章给出了散射光子软件修正的可行性方案。      

Energy distributions of multiple backscattered photons in materials

Backscattering of gamma photons from a material is of fundamental importance in radiation shielding,industrial and medical applications, radiation dosimetry,and non-destructive testing. In Compton scattering, incident photons undergo multiple scatterings within the material(target) before exiting. Gamma photons continue to soften in energy as the number of scatterings increases in a thick target; in other words, the energy of gamma photons decreases as the scatterings increase in case of a thick target and results in the generation of singly and multiply scattered events. In this work, the energy distribution of backscattered gamma photons with backscattering intensity and energy probabilities were calculated by using the Monte Carlo method for metallic, biological, and shielding materials with various thicknesses of slab geometry. The materials under study were targeted with gamma photons of 0.279, 0.662, 1.250, and 2.100 Me V energies. In addition, the energy distributions of multiply scattered gamma photons were studied for materials with infinite geometry.The results are presented and discussed in detail by comparing with other Monte Carlo calculations.     

Multiple Scattering Streaming of Gamma-Rays through Cylindrical Ducts

A semi-analytic Monte Carlo calculation has been applied to the duct streaming problem. The first positions of collision of photons are selected systematically in a certain volume surrounding the duct, and further scattering positions determined by the random walks of the photons. The expectation value that the photons arrive at the duct exit from their scattering point are integrated over the volume.

The multiple scattering doses through ducts surrounded by a lead and water shield are presented. The ratio between scattered and unscattered dose is about 40% for duct in water when E ₀=1 MeV and L/R ₀=10; the ratio decreases with increase of E ₀ and L/R ₀. It also decreases with increasing atomic numbers of the shield materials. A large part of the scattered dose consists of once scattered photons. The statistical deviation of the calculated dose is about 5% for 6,000–8,000 histories, when the relative dose attenuation is of the order of 10^-6.

The buildup factors of γ-rays from a point isotropic source for a slab shield are also obtained using the above method; the results agree well with those by the moments method.