基于蒙特卡罗方法的NaI探测器效率刻度及其测量轫致辐射实验

为了将NaI探测器更好地应用到轫致辐射谱测量工作中,对一套NaI探测器做了研究:利用~(137)Cs,~(60)Co等同位素γ源,结合蒙特卡罗方法,得到全能峰效率的模拟值与实验测量值符合得较好;利用蒙特卡罗N粒子编码模拟NaI对不同能量光子的响应,得到了该探测器对光子的能量响应,并将获得的能量响应用于轫致辐射的解谱工作,解谱结果与原始谱符合得很好;将该探测器应用到强流电子束打靶轫致辐射测量实验中,对轫致辐射在NaI探测器中的响应做了初步测量.     

γ射线和PET用晶体单元相互作用特性的研究

γ射线与晶体单元的相互作用特性对于PET探测器的研制有着重要的参考意义. 通过实验测得了不同能量的γ射线在BGO, CsI, NaI 3种闪烁晶体中的探测效率、 能量分辨率等相互作用特性参数, 并与蒙特卡洛模拟结果进行了比较. 结果给出了最适合PET探测器的晶体单元. It’s important for development of PET detector that the interaction effect of γ rays and scintillator units are studied. The detection efficiencies and energy resolutions of γ rays for BGO, CsI and NaI crystal detector units are measured. Geant4 Monte Carlo simulations are compared with the experimental results. The results of the best detection units for PET detector are presented.     

闪烁光子传输引起的图象压缩

在小型γ相机中,基于γ射线与NaI(T1)晶体的相互作用及其产生的闪烁光子的传输特性,用GEANT3软件包建立的一个蒙特卡罗模拟程序,研究闪烁光子在位置灵敏光电倍增管光阴极上的空间分布,结果表明用重心读出法得到的图象明显从周围向中心压缩.本文讨论了这种压缩的物理机制,并提出了消除压缩效应的方法.     

Geometrical model approximation for ionization cross sections of atoms and ions in ion-atom collisions

The applicability of the geometrical model (GM) proposed in [14, 15] to calculate the ionization probability and cross section of atoms and ions during collisions with ions, including structural ions, was studied at high velocities of partners. The GM represents a version of the classical impulse approximation. It was shown that the GM in all cases under consideration allows estimation of the ionization probability and total corss section of an atom or an ion during collisions with ions with deviations of no more than 50% from the results of the classical Monte Carlo method. GM calculations are rather simple in computation respect and limit the ionization probability by unity at arbitrary impact parameters.     

Quantification of cerebral blood flow in nonhuman primates using arterial spin labeling and a two-compartment model

Noninvasive absolute quantification of cerebral blood flow (CBF) with high spatial resolution is still a challenging task. Arterial spin labeling (ASL) is a promising magnetic resonance imaging (MRI) method for accurate perfusion quantification. However, modeling of ASL data is far from being standardized and has not been investigated in great detail. In this study, two-compartment modeling of monkey ASL data in three physiological conditions (baseline, sensory activated and globally elevated CBF) is reported. Absolute perfusion and arterial transit times were derived for gray matter (GM) and white matter (WM) separately. The uncertainties of the model's result were determined by Monte Carlo simulations. The fitted CBF values for GM were 133 ml/min/100 ml at baseline condition, 165 ml/min/100 ml during visual stimulation and 234 ml/min/100 ml for globally elevated CBF after intravenous injection of acetazolamide. The ratio of GM to WM CBF was 2.5 at baseline and was found to decrease to 1.6 after application of acetazolamide. The corresponding arterial transit times decreased from 742 to 607 ms in GM and from 985 to 875 ms in WM. Monte Carlo simulations showed that absolute CBF values can be determined with an error of 11-15%, while the arterial transit time values have a coefficient of variation of 25-31%. With an alternative acquisition scheme, the precision of the arterial transit times can be improved significantly. The CBF values in the occipital lobe of the monkey brain quantified with ASL are higher than previously reported in positron emission tomography studies.     

A method to enhance coincidence time resolution with applications for medical imaging systems (TOF/PET)

A coincidence timing spectrometer was assembled using NaI(Tl) inorganic and BC418 type organic (plastic) scintillation detectors. The constant fraction timing method was used. Coincidence time resolution values of the detectors, which have great importance in TOF/PET measurements, were obtained separately. The resolutions were enhanced using a different method in a start-stop coincidence spectrometer: the signals from the start detectors were delayed. The results from the FLUKA Monte Carlo simulation code agreed well with the experimental results.     

血液中Na活化放射性测量的蒙特卡罗模拟

针对超临界事故中人体受到中子辐照后感生的24Na活度测量,采用MCNP软件建立蒙特卡罗模拟模型,分别模拟不同类型NaI探测器对24Na衰变的两条射线全能峰的探测效率和塑料闪烁体探测器对24Na衰变的射线总计数的探测器效率。模拟结果表明:井型NaI探测器与圆柱型相比,24Na衰变的1.38 MeV和2.76 MeV 射线全能峰探测效率分别提高了4.30倍和4.11倍;塑料闪烁体探测器对24Na衰变的射线的探测效率是NaI探测器对24Na 射线的探测效率的1.72倍;同时粗略计算了探测器计数与人体所受中子辐照剂量之间的关系。     

Impact of measurement approach on the quality of gamma scanning density profile in a tray type lab-scale column

This article presents a study for investigating impact of the measurement approach on the quality of gamma scanning density profile in tray type columns using experimental and computational evaluations. Experimental density profiles from the total and the photopeak count measurements, as two approaches in gamma ray column scanning technique, has been compared with the computational density profile from Monte Carlo simulation results. We used a laboratory distillation column of 51 cm diameter as an illustrative example for this investigation. ¹³⁷Cs was used as a gamma ray source with the activity of 296 MBq (8 mCi), with a NaI(Tl) detector. MCNP4C Monte Carlo code has been used for simulations. The quality of the density profile in the photopeak count approach is relatively within 155–204% better than that of the total count approach for experimental results. The same comparison for simulation results leads to a relative difference within 100–135% for the density profile.     

Investigation of NaI(Tl)-Crystal Having Anomalous Inhomogeneity of Light Yield Spatial Distribution

An experiment with a layer-by-layer irradiation of the large volume NaI(Tl)-crystal has been performed. The inhomogeneity of the light yield spatial distribution along the crystal’s axis was revealed. A new method of accounting for the revealed inhomogeneity in the processing of the gamma-ray spectra registered using a crystal with nonuniform response is proposed. This method is quite universal: the conventional stages of the detector investigation (the calibration with standard gamma-ray sources and the inhomogeneity estimation using a layer-by-layer irradiation) are complemented by another one stage with computing of the absorbed energy distribution using Monte Carlo simulation. Finally, based on the results of these stages, the combined broadening function is formed for calculation of the response function.     

基于MCPT解法器库的专用辐射探测模拟软件研发与应用北大核心CSCD

甲状腺内^(131)I放射性活度与辐射探测结果的比例关系与甲状腺几何尺寸、探测距离等因素相关,是估算甲状腺内^(131)I含量与其可能造成的辐照损伤的关键参数。基于MCPT辐射输运数值模拟算法器库开发了用于开展NaI探测器伽马辐射测量模拟的应用程序,进而建立了多组具有不同容积的甲状腺型容器和不同探测距离的物理模型,最终通过蒙特卡罗数值计算得到了不同测量状态下探测器的探测效率。在甲状腺型容器与探测器距离较远时,数值模拟给出的结果与理论计算结果一致,证明此应用程序可用于定量分析NaI的探测效率。数值模拟结果表明,小距离模型的结果受甲状腺样容器的大小和距离的显著影响,模拟给出的探测效率表为开展深入细致的实验研究奠定了基础。     

Energy and intensity distributions of 279 keV multiply scattered photons in bronze — an inverse response matrix approach

An inverse response matrix converts the observed pulse-height distribution of a NaI(Tl) scintillation detector to a true photon spectrum. This also results in extraction of intensity and energy distributions of multiply scattered events originating from interactions of 279 keV photons with thick targets of bronze. The observed pulse-height distributions are a composite of singly and multiply scattered events in addition to bremmstrahlung originating from slowing down of Compton and photo-electrons in thick targets. To evaluate the contribution of multiply scattered events, the spectrum of singly scattered events contributing to inelastic Compton peak is reconstructed analytically. The optimum thickness (saturation depth), at which the number of multiply scattered events saturate, has been evaluated in different energy bin meshes chosen for scintillation detector response unfolding. Monte Carlo calculations based upon the package developed by Bauer and Pattison (Compton scattering experiments at the HMI (1981), HMI-B 364, pp. 1–106) supports the present experimental results.      

Experimental and simulated validation of the energy dependence of saturation thickness of multiple scattered gamma rays

Saturation thickness for multiple scattering gamma rays from multiple sources has been measured experimentally and simulated using the Monte Carlo N-Particle (MCNP) Code. Experimental measurements were performed using a collimated beam of gamma-rays from ⁵⁷Co, ²⁰³Hg, ¹³³Ba, ²²Na, ¹³⁷Cs, ⁶⁵Zn and ⁶⁰Co sources. The gamma rays were directed at rectangular aluminium targets of varying thickness. A NaI (Tl) scintillation detector placed at a backscattering angle of 180° was used to detect the scattered photons. The measured and calculated saturation thickness increases with increasing energy of incident gamma-rays. Experimental and simulated values are compared and are in good agreement.     

Benchmark measurements and simulations of dose perturbations due to metallic spheres in proton beams

Monte Carlo simulations are increasingly used for dose calculations in proton therapy due to its inherent accuracy. However, dosimetric deviations have been found using Monte Carlo code when high density materials are present in the proton beamline. The purpose of this work was to quantify the magnitude of dose perturbation caused by metal objects. We did this by comparing measurements and Monte Carlo predictions of dose perturbations caused by the presence of small metal spheres in several clinical proton therapy beams as functions of proton beam range and drift space. Monte Carlo codes MCNPX, GEANT4 and Fast Dose Calculator (FDC) were used. Generally good agreement was found between measurements and Monte Carlo predictions, with the average difference within 5% and maximum difference within 17%. The modification of multiple Coulomb scattering model in MCNPX code yielded improvement in accuracy and provided the best overall agreement with measurements. Our results confirmed that Monte Carlo codes are well suited for predicting multiple Coulomb scattering in proton therapy beams when short drift spaces are involved.     

Experimental evaluation of effective atomic number of composite materials using back-scattering of gamma photons

A method has been presented for calculation of effective atomic number (Z_eff) of composite materials, by using back-scattering of 662?keV gamma photons obtained from a ¹³⁷Cs mono-energetic radioactive source. The present technique is a non-destructive approach, and is employed to evaluate Z_eff of different composite materials, by interacting gamma photons with semi-infinite material in a back-scattering geometry, using a 3^″?×?3^″ NaI(Tl) scintillation detector. The present work is undertaken to study the effect of target thickness on intensity distribution of gamma photons which are multiply back-scattered from targets (pure elements) and composites (mixtures of different elements). The intensity of multiply back-scattered events increases with increasing target thickness and finally saturates. The saturation thickness for multiply back-scattered events is used to assign a number (Z_eff) for multi-element materials. Response function of the 3^″?×?3^″ NaI(Tl) scintillation detector is applied on observed pulse-height distribution to include the contribution of partially absorbed photons. The reduced value of signal-to-noise ratio interprets the increase in multiply back-scattered data of a response corrected spectrum. Data obtained from Monte Carlo simulations and literature also support the present experimental results.     

边界条件对生物组织漫射光时域模拟计算的影响

采用蒙特卡洛方法与漫射近似相结合的杂合算法,在不同的边界条件下(零边界条件、外延边界条件、部分流边界条件),对板状生物组织模型的漫射光的时域分布进行了计算,计算结果表明:采用外延边界条件,在保证计算精度的基础上,和单纯的蒙特卡洛方法相比,较大地提高了运算速度.     

Auxiliary field diffusion Monte Carlo calculation of nuclei with A < or = 40 with tensor interactions

We calculate the ground-state energy of (4)He, (8)He, (16)O, and (40)Ca using the auxiliary field diffusion Monte Carlo method in the fixed-phase approximation and the Argonne v(6)' interaction which includes a tensor force. Comparison of our light nuclei results to those of Green's function Monte Carlo calculations shows the accuracy of our method for both open and closed-shell nuclei. We also apply it to (16)O and (40)Ca to show that quantum Monte Carlo methods are now applicable to larger nuclei.     

蒙特卡罗模拟在非平衡动态系统中的应用

非平衡系统几乎存在于自然和人造系统的各个层面上,它以非零连续的流量为特征。完全非对称简单排他过程被认为是对这类系统建模和模拟的一个范例。对蒙特卡罗方法如何模拟该类系统进行了介绍。分析了通过蒙特卡罗模拟观察到的一些有趣的物理现象如自发性对称破缺、有限尺寸效应和跳跃过程。非对称的低-低密度相破缺与系统的有限尺寸效应密切相关,建议开展更细致的蒙特卡罗模拟以进一步加深对仍处于争论中的非平衡系统有限尺寸效应的认识。     

低能电子在固体表面背散射系数的直接Monte Carlo方法模拟

应用单次碰撞的直接Monte Carlo方法计算能量范围从100 eV~10 keV的电子在固体Al,Si,Au表面的背散射系数,其中低能电子在固体中的弹性散射和非弹性散射截面分别应用Mott散射截面和Born近似下的广义振子强度计算模型得到.通过与压缩历史Monte Carlo方法的模拟计算结果及实验值的比较,结果表明,对于100 eV~10 keV范围的低能区电子,采用直接方法计算得到的电子背散射系数与实验值符合较好,直接方法比压缩历史方法更适合于能量在10 keV以下的电子输运计算.     

A boundary-based net-exchange Monte Carlo method for absorbing and scattering thick media

A boundary-based net-exchange Monte Carlo method was introduced (JQSRT 74 (2001) 563) that allows to bypass the difficulties encountered by standard Monte Carlo algorithms in the limit of optically thick absorption (and/or for quasi-isothermal configurations). With the present paper, this method is extended to scattering media. Developments are fully 3D, but illustrations are presented for plane parallel configuration. Compared to standard Monte Carlo algorithms, convergence qualities have been improved over a wide range of absorption and scattering optical thicknesses. The proposed algorithm still encounters a convergence difficulty in the case of optically thick, highly scattering media.     

Coupling constant renormalization due to instantons in the O(3) non-linear sigma model

The renormalized lattice coupling constant for the O(3) non-linear sigma model is calculated, including instanton effects, and the correlation length estimated. The results are in good agreement with the Monte Carlo simulation of Shenker and Tobochnik. The topological charge density is also discussed in the light of recent Monte Carlo simulations.