250~600 kV X射线石墨空腔电离室壁修正因子及物理常数的蒙卡模拟北大核心CSCD

石墨空腔电离室由于对较高能量段有较好的能量响应,广泛应用于137Cs(有效能量为662keV)、60Co(有效能量为1250keV)以及平均能量为350keV的192Irγ射线空气比释动能的测量。运用EGSnrc程序模拟自制石墨空腔电离室在250~600kV窄谱辐射质下量值复现过程中的壁修正因子及物理常数。结果显示,石墨与空气的阻止本领比、空气与石墨的质能吸收系数之比、韧致辐射份额常数均与平均能量间有比较明显的关系可循,壁修正因子随着X射线能量的增加,则会有先变大后减小的趋势。蒙特卡洛模拟的结果为建立250~600kVX射线空气比释动能国家基准、实现量值复现提供了技术数据,达到了复现结果测量不确定度的预期要求。     

~(60)Coγ射线参考辐射标准装置

实验室建立了满足国家量值溯源和传递要求的60Coγ射线参考辐射标准装置,主要用于治疗水平剂量计和防护仪表的检定与校准以及相关研究工作。文章首先介绍了60Coγ射线参考辐射标准装置的结构组成、物理原理及采用的技术路线;其次利用标准剂量仪测量不同源探距下的空气比释动能率,并测量源探距为1 500 mm处辐射场的均匀区大小,实验结果表明该辐照装置辐射场的空气比释动能率很好地符合平方反比定律,辐射场均匀区尺寸大于绝大多数剂量计的尺寸,该标准装置能够很好地满足应用需求。     

~(60)Coγ辐射的空气比释动能及水吸收剂量的NIM-IAEA双边比对

在中国计量科学研究院(NIM)~(60)Coγ基准实验室,将~(60)Coγ辐射的空气比释动能及水吸收剂量基准值传递到国际原子能机构(IAEA)的FC65-G-2869电离室,给出了此电离室的~(60)Coγ辐射的空气比释动能和水吸收剂量的校准因子,并且比较了这2个物理量的校准因子与IAEA提供的相关结果的差异。实验结果表明,NIM给出的FC65-G-2869电离室的~(60)Coγ辐射的空气比释动能和水吸收剂量的校准因子相对IAEA提供的结果分别偏差0.5%和0.1%,比对结果达到了相关要求。为NIM参与国际计量局(BIPM)组织的正在进行中的一轮~(60)Coγ辐射的空气比释动能及水吸收剂量的关键比对作了前期准备。     

高剂量率近距离192Ir治疗源的井型电离室仪器的校准

针对后装治疗用192Ir放射源的参考空气比释动能率的量值急需进行溯源。参考国际通例,由指型电离室(PTW-30013)在60Co γ射线和250kV X射线下的空气比释动能校准因子推导得出192Ir γ射线的空气比释动能校准因子,从而测定192Ir放射源参考空气比释动能率标准值,进而完成对井型电离室的参考空气比释动能率校准。通过不确定度评定得出:192Ir放射源的参考空气比释动能率的不确定度为3.6%,井型电离室校准因子的不确定度为3.8%。     

低能X射线空气比释动能国际关键比对

为促进我国在低能X射线空气比释动能计量领域内的量值统一并达到国际等效,中国计量科学研究院(NIM)于2018年采用平板自由空气基准电离室复现低能X射线空气比释动能,并通过传递电离室Radcal-RC6M在国际计量局(BIPM)低能X射线辐射场中开展了间接比对,其比对结果在合成标准不确定度为0.37%范围内与国际计量局等效一致,该比对结果已纳入国际计量局的关键比对数据库(KCDB)。     

用于放疗量传的照射量、空气比释动能和水吸收剂量量值体系比较

用照射量和空气比释动能校准的电离室进行剂量测量时,依据IAEA TRS 277报告,需要经历四级量值转换过程,不确定度也较大,但目前仍是国内使用的量值体系.我国正在建立60Co γ射线以及高能光子下的水吸收剂量基准装置并进行国际比对,之后将拥有水吸收剂量的量值复现的能力.在60Co γ射线参考辐射场和加速器高能X射线辐射场下,使用NE2571和NE2570/1A、PTW TW30013和PTW UNIDOS两套电离室剂量仪,分别按照277和398报告的要求计算并比较2种方法计算出的水吸收剂量值,从而验证了277报告和398报告的一致性.     

γ射线空气比释动能测量不确定度分析

基于空气比释动能测量的Bragg-Gray空腔理论,通过采用石墨空腔电离室对γ射线空气比释动能率进行测量,建立了测量数学模型,考虑到空气比释动能测量过程中的各影响量及其修正,采用实验测量或蒙卡模拟计算的方法得到各修正因子并分析不确定度,得到γ射线空气比释动能测量的扩展不确定度约为0.5%,其中主要是由壁效应修正和平均电离功的不确定度贡献,因此有必要开展相应的研究,以提高空气比释动能的测量水平。     

防护水平X射线电离室的校准及不确定度分析

防护水平电离室剂量计是辐射防护的主要计量器具,需要在窄谱参考辐射质下进行检定和校准。利用EGSnrc软件模拟了参考辐射质X射线能谱,分析得到的能谱分辨率和平均能量与ISO 4037-1推荐值的最大偏差分别为7.1％和1.04％,均满足规范要求。依托60~250kV X射线空气比释动能国家基准装置,在窄谱系列参考辐射质下完成了距离X光机1m处参考点的空气比释动能量值复现;然后通过替代法对两个传递电离室进行校准并完成量值传递;最后利用传递电离室复现的2.25m处的空气比释动能率对PTW-32002球形电离室进行校准,获得相应的校准因子,校准因子相对扩展不确定度为2.2%(k=2)。     

X射线环境辐射监测仪器校准方法的研究

环境辐射监测仪表作为微弱放射性监测计量器具,它的量值溯源是一个急需解决的问题。由于环境辐射剂量率低且电离信号微弱,故通常采用大体积电离室或者充压电离室进行测量。依托60~250kV X射线空气比释动能基准电离室,在完成重过滤窄谱X射线辐射质空气比释动能绝对测量的基础上,通过逐级替代法完成大体积环境辐射监测仪器的校准。测量结果不确定度为5.6%(k=2),实现环境水平X射线空气比释动能测量量值溯源,为国内环境辐射监测仪器在低剂量率水平的性能评价提供计量保障。     

自制球形空腔电离室的性能测试

为建立和完善250～600 kV段X射线空气比释动能基准,自主研制了一系列石墨空腔电离室作为基准电离室,以球形空腔电离室为例,在参考辐射场上对其性能进行了相关测试,包括电离室的饱和特性、重复性以及漏电情况.结果显示电离室的工作电压为600V,重复性好于0.1％,漏电流值小于30fA,本套电离室的性能符合相关要求.     

Physical parameters and correction factors for ionization chambers for absolute measurement of air kerma in γ-ray fields

Values of physical parameters and correction factors essential for the absolute measurement of air kerma in ¹³⁷Cs and ⁶⁰Co γ-ray fields were obtained using an EGS5 program for spherical, cylindrical and pancake ionization chambers. The mean mass collision stopping power ratio for graphite and air, was found to vary depending on the cutoff energy of electrons employed in calculation. The ratio between the energies deposited in cavity air due to Compton electrons emitted from the air and those from the graphite wall increases as the chamber size is increased. It also increases as the γ-ray energy is reduced and is equal to 0.09 for ¹³⁷Cs γ-rays in a spherical ionization chamber of cavity diameter 12 cm. Correction factors for γ-ray attenuation in chamber walls and those for the contribution of scattered γ-rays to chamber responses were obtained separately. The wall correction factor, which is equal to the product of these two factors, is close to unity for pancake chambers.     

X射线石墨空腔电离室壁修正因子的蒙特卡罗模拟

在250～600 kV X射线宽谱系列辐射质下,运用EGSnrc蒙特卡罗程序模拟计算3种不同结构石墨空腔电离室的壁修正因子.根据辐射质和光机参数使用BEAMnrc程序包模拟8组规范下辐射质的X射线注量谱,将模拟得到的相空间文件作为X射线源,运用EGSnrcMP程序包中的cavity程序代码模拟计算得到石墨空腔电离室的壁修正因子.模拟结果表明:对于相同体积的球型电离室,收集极长的电离室其壁修正因子更大;对于收集极长度相同电离室,球柱球型电离室比球型电离室的壁修正因子数值大.并且当X射线能量增加时,电离室的壁修正因子都有减小的趋势.     

Simulation studies on a prototype ionisation chamber for measurement of personal dose equivalent, Hp(10)

A prototype ionisation chamber for direct measurement of the personal dose equivalent, Hp(10), similar to the one developed by the Physikalisch-Technische Bundesantalt (PTB), was designed and constructed by the Metrological Laboratory of Ionizing Radiation (LMRI) of Nuclear and Technological Institute (ITN). Tests already performed have shown that the behaviour of this chamber is very similar to the PTB chamber, mainly the energy dependence for the X-ray radiation qualities of the ISO 4037-1 narrow series N-30, N-40, N-60, N-80, N-100 and N-120 and also for gamma radiation of 137Cs and 60Co. However, the results obtained also show a dependence on the energy and angles of incident radiation and a low magnitude of the electrical response of the ionisation chamber. In order to optimise the performance of the chamber, the LMRI initiated numerical simulation of this ionisation chamber by Monte Carlo method using the MCNPX code.     

Graphitization of polyacrylonitrile carbon fibers and graphite irradiated by γ rays

To investigate the effect of γ-ray radiation on the microstructure of carbon fibers (CF) and graphite, the carbon fibers and graphite were irradiated by ⁶⁰Co source at room temperature. X-ray diffraction results indicate that the interlayer spacing d₀₀₂ of CF and graphite decreased after irradiation. The intensity of (002) peak in CF decreased while the peak of the (002) plane in graphite becomes sharper after irradiation. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy determines that γ-ray irradiation slightly improves the carbon content of CF surface layer. Compton scattering effect and heating caused by γ-ray are proposed to be responsible for the graphitization of CF and graphite.     

Re-evaluation of correction factors of a primary standard graphite calorimeter in 60Co gamma ray beams as a basis for the appointment of the BEV absorbed dose rate to water reference value

The graphite calorimeter of the Federal Office of Metrology and Surveying (BEV-Bundesamt für Eich- und Vermessungswesen) was established in the 1980s as the primary standard for the absorbed dose to water for (60)Co gamma ray beams. To maintain the primary standard at an international level the graphite calorimeter and its corresponding components had to undergo a refurbishment and modernisation process. The correction factors of the graphite calorimeter were re-evaluated with Monte Carlo and experimental methods to obtain improved values. These are the correction for the effect of the gaps (1.0061), the scaling correction (0.9998), the correction for the difference in air attenuation (0.9971) and the corrections for the effective measurement depths in the graphite phantom for the graphite calorimeter (0.9886) and the CC01-105 ionisation chamber (0.9913). Consequently, it was necessary to change the reference value for the absorbed dose rate to water of the (60)Co teletherapy unit used for the calibration of secondary standard dosemeters.     

A Monte Carlo calculation of detector design for position emission tomography

A Monte Carlo computer simulation of γ-ray detection in NaI and BGO has been performed with a view toward designing a new type of camera for positron emission tomography (PET). The intrinsic spatial resolution of large continuous detectors employing the principle of the Anger camera has been obtained using a comprehensive computer code. Some results applicable to detectors based on discrete small crystals are discussed also.     

β射线剂量仪校准因子不确定度分析

为准确评估β射线剂量仪校准因子的不确定度,将经过溯源的薄窗电离室放置在β射线辐射场的测量点处进行测量得到剂量率参考值,然后采用替代法将β射线剂量仪放在同一位置进行测量得到指示值,通过二者的比值得到β射线剂量仪的校准因子,并对校准因子的不确定度进行评定。不确定度的来源主要包括参考辐射场的均匀性、替代法测量过程中位置的一致性、显示值的统计标准偏差等。结果表明在校准过程中,通过提高参考辐射场的均匀性和参考位置的一致性可以改善校准因子的不确定度。     

球形石墨空腔电离室壁修正因子模拟计算

采用蒙特卡罗方法对球形石墨空腔电离室壁修正因子Kwall进行模拟计算,开展光子能量、壁厚、石墨密度对Kwall的影响研究。计算结果显示对同一体积空腔电离室,Kwall随着能量的变化而发生变化,在低能区域Kwall值急剧减小;Kwall随壁厚增加近线性增加;同时,在模拟计算不确定度大于0.2%时,石墨材料的密度对于Kwall的影响可以忽略。这些结果表明,对于石墨空腔电离室壁修正因子高精度的计算,需要考虑光子能量、壁厚以及石墨密度的影响。     

~(137)Cs γ射线空气比释动能基准石墨空腔电离室的研制

根据Bragg-Gray理论研制了圆柱形石墨空腔电离室,用于~(137)Cs空气比释动能基准的建立。通过理论计算灵敏体积内部电场分布,优化了电离室壁和收集极之间接地保护的结构设计,在此基础上,对研制的电离室的饱和曲线、本底电流以及稳定性等电学性能进行了测试,其结果表明完全达到设计要求。对电离室的各项修正因子进行了理论计算和实验测量,实现了~(137)Cs空气比释动能的量值复现,其合成标准不确定度为0.25%。