• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Investigating the minimum detectable activity concentration and contributing factors in airborne gamma-ray spectrometry

    2021-11-13 01:30:56YiGuKunSunLiangQuanGeYuanDongLiQingXianZhangXuanGuanWanChangLaiZhongXiangLinXiaoZhongHan
    Nuclear Science and Techniques 2021年10期

    Yi Gu · Kun Sun · Liang-Quan Ge· Yuan-Dong Li · Qing-Xian Zhang·Xuan Guan· Wan-Chang Lai · Zhong-Xiang Lin · Xiao-Zhong Han

    Abstract In this study, the theory of minimum detectable activity concentration (MDAC) for airborne gamma-ray spectrometry (AGS) was derived, and the relationship between the MDAC and the intrinsic efficiency of a scintillation counter, volume, and energy resolution of scintillation crystals, and flight altitude of an aircraft was investigated. To verify this theory, experimental devices based on NaI and CeBr3 scintillation counters were prepared, and the potassium, uranium, and thorium contents in calibration pads obtained via the stripping ratio method and theory were compared. The MDACs of AGS under different conditions were calculated and analyzed using the proposed theory and the Monte Carlo method. The relative errors found via a comparison of the experimental and theoretical results were less than 4%. The theory of MDAC can guide the work of AGS in probing areas with low radioactivity.

    Keywords Airborne gamma-ray spectrometry (AGS) ·Minimum detectable activity concentration (MDAC) ·Sensitivity

    1 Introduction

    The minimum detectable activity concentration(MDAC) is the lowest activity concentration (Bq/g) that can be reliably detected under certain measurement conditions [1] and is an important technical specification of airborne gamma-ray spectrometry (AGS) [2]. Both the IAEA guidance and Chinese industry standards specify that before using AGS, parameters such as the detection sensitivity to the ground and the MDAC of AGS should be determined to ensure the accuracy of measurement results[3, 4].

    Numerous studies have been conducted to determine the factors that influence the MDAC in AGS. Roy Po¨lla¨nen et al. studied the relationship between the minimum detectable activity and flight altitude. The minimum detectable activity of a detector for137Cs and60Co sources was obtained from data measured at different flight altitudes [5]. Casanovas et al. researched the MDAC of NaI and LaBr3for131I and137Cs sources [6]. Tang et al. calculated the MDAC of high-purity germanium (HPGe) and LaBr3for a variety of artificial radionuclides using the Monte Carlo (MC) method and investigated the effects of source term size, flight altitude, and gamma-ray energy on MDAC [7, 8]. Ni et al. researched the effects of flight altitude, gamma-ray energy, background level, and other factors on MDAC using extensive measured data [9]. The above results showed that MDAC positively correlates with flight altitude, gamma-ray energy, and background level.Under the same measurement conditions, MDACs of different detectors also varied. The above studies used experimental or simulated data to analyze the relationships between MDAC and the aforementioned factors. In addition, the theory for MDAC indicates that MDAC is influenced by the absolute detection efficiency, background level, and emission probability of gamma photons [1].However, no theory indicates the relationship between MDAC and intrinsic efficiency,gamma-ray energy,energy resolution, total volume, and flight altitude.

    The authors began with the basic principle of AGS,then derived a theory for MDAC considering detection sensitivity to ground, and finally analyzed the relationship between MDAC and the above factors based on this theory.Using the Geant4 software,the intrinsic efficiencies of NaI and CeBr3scintillation counters with different volumes were simulated for different energies. These intrinsic efficiencies were calculated using MDAC theory. Experimental devices with a NaI scintillation counter (one 10 cm × 10 cm × 40 cm) and a CeBr3scintillation counter (six φ4.5 cm × 5 cm) were established. Based on the elemental content of the calibration pads obtained using the experimental device, the accuracy of the theory was evaluated. Finally, the MDAC of AGS was calculated at different intrinsic efficiencies, energy resolution levels,total volumes, and flight altitudes, and the experimental devices were used for field flight experiments. Accurate evaluation of the MDAC of AGS and in-depth research on the factors influencing MDAC can guide the work of AGS in measuring low radioactivity areas.

    2 Theory derivation

    2.1 Detection sensitivity of AGS to ground

    The detection sensitivity to the ground(Sen,cps/(μg/g))describes the degree of change in the net counts of fullenergy peaks np(cps) in the gamma-ray spectrum caused by a change in radioactive elements C (μg/g) in a geological body, and it is given by [10]:

    where dC is the change in the radioactive element content C in a geological body, and dnpis the change in the net count of the full-energy peaks npfor this radioactive element. It is assumed that the radioactive substance in a geological body is uniformly distributed. The density of soil is given by ρ(g/cm3),V is volume(cm3),m is mass(g),the content of certain radioactive elements is given by C(g/g); A is specific activity (Bq/g), T1/2is half-life (s), Aγis atomic weight (g/mol), λ is decay constant (s-1); NAis Avogadro constant, number of gamma rays with energy E released by each decay of a radioactive element is given as pγ(pcs), linear attenuation coefficient of gamma rays with energy E is given by μ0(m-1) for soil, and linear attenuation coefficient of gamma rays with energy E is μ1(m-1)for air. Additionally, the number of gamma rays with energy E in a volume of soil is represented by NV(cps/cm3).In the spherical coordinate system,the solid angle of a gamma-ray spectrometer is given by 2θ0, Spis crosssectional area of the scintillation crystal(m2),H represents the flight altitude of AGS(m),R is detection radius for the ground(m)(R = H × tanθ0),and the intrinsic efficiency of the gamma-ray spectrometer is given by εinp.Therefore,the net count rate of the full-energy peak (np) for gamma rays with energy E in a gamma-ray spectrometer is expressed as[10, 11]:

    2.2 MDAC of AGS

    The activity concentration of a radionuclide that releases gamma rays with energy E is given by AE(Bq/g). In the instrumental spectrum, AEis derived from the net count rate of the full-energy peak nINPand the detection sensitivity to the ground, as:

    In the instrumental spectra,the net peak count is equal to the full-energy gross count peak minus the background count. Figure 1 shows the peak patterns of the full-energy peaks in the instrumental spectrum with different energy resolutions.

    Fig. 1 (Color online) Full-energy peaks with energy E in the instrumental spectra of scintillation crystals with different energy resolutions.The red line is the full-energy peak with energy resolution η1, the blue line is the full-energy peak with energy resolution η2,η1 <η2, nINP is the net peak count, and nB is the background count

    It is assumed that the shape of the full-energy peak is described by a Gaussian function(the standard deviation of the Gaussian function is σ) [1]. Meanwhile, the background of the instrumental spectrum is described by a linear function. According to the instrumental spectrum,the left and right channels of the range of interest are expressed as iLand iR,while the counts of the left and right channel are given by fLand fR, respectively, the peak channel is iE, and the count of the peak channel is fE. The background count nBcan be written as:

    From Eq. (9), with increases in the intrinsic efficiency and cross-sectional area of scintillation crystals,the MDAC value decreases, and with increases in the flight altitude and energy resolution values of the scintillation counter,the MDAC value increases. The MDAC of AGS can be optimized by reducing the flight altitude and the value of the energy resolution and increasing the intrinsic efficiency and the cross-sectional area of the scintillation crystals.

    3 Simulation analysis

    3.1 Model of an AGS detector

    The scintillation counter is the core unit of AGS. It mainly includes scintillation crystals, photomultiplier tubes, and associated electronics modules. The types of scintillation crystals that can be used in AGS are NaI,CeBr3,and LaBr3[5-8,12-14].138La in LaBr3scintillation crystals is radioactive, and its characteristic peak at 1.44 MeV overlaps with the full-energy peak of40K at 1.46 MeV. In the radioactivity exploration work using AGS,40K was the object isotope. Therefore, we constructed two experimental devices: a scintillation counter based on CeBr3scintillation crystals (referred to as the CeBr3scintillation counter) and a scintillation counter based on NaI (Tl) scintillation crystals (referred to as the NaI scintillation counter). All scintillation counters used the automatic spectrum stabilization method of a241Am source [15]. The automatic spectrum stabilization method of the241Am source is a dynamic peak correction technique that uses the full-energy peak for the 59.5 keV gamma rays emitted from the Am source as a reference peak and employs the algorithm built into the energy spectrometer to correct the peak position. Sketches of the scintillation counters are shown in Fig. 2.

    Fig. 2 (Color online) Structural sketches of the CeBr3 and NaI scintillation counters. a Integrated circuits; b foam; c photomultiplier tubes;d carbon fiber sheet; e scintillation crystal

    The CeBr3scintillation counter was equipped with six φ4.5 cm × 5 cm CeBr3scintillation crystals, and the output spectrum is the synthetic spectrum of the six scintillation crystals(the synthesis is linearly cumulative).CeBr3scintillation crystals were produced by the Beijing Glass Research Institute,and the energy resolution of the crystals was 4% (at gamma-ray energy of 0.662 MeV). The NaI scintillation counter was equipped with one 10 cm × 10 cm × 40 cm scintillation crystal. NaI scintillation crystals were produced by Saint-Gobain Crystals, and the energy resolution of the crystals was 8.7% (at gamma-ray energy of 0.662 MeV). The box of the scintillation counter was made of a carbon fiber sheet (thickness of 0.15 cm). The spaces between the scintillation crystals were filled with foam (density of 0.95 g/cm3). Moreover, the scintillation counter was hung below an unmanned aerial vehicle.

    3.2 MC simulation

    The MC technique is a powerful tool for simulating particle transport. In this study, the detection efficiency of AGS was calculated using the Geant4 [16].

    Owing to limitations in the manufacturing process and the production costs of scintillation crystals, the maximum volume of a single crystal has an upper limit. At present,the largest volume sizes of CeBr3and NaI scintillation crystals available on the market are φ7.62 cm × 7.62 cm and 10 cm × 10 cm × 40 cm, respectively. Numerous research results and application examples have shown that the combination of multiple scintillation crystal arrays is effective for increasing the volume of scintillation counters[2, 17, 18]. Thus, we increased the volume of scintillation crystals using arrays. Table 1 lists information about different scintillation counters, including crystal type, singlecrystal size, crystal number, and total volume. We simulated the intrinsic efficiency of scintillation counters with different volumes (as shown in Table 1).

    The intrinsic detection efficiency is the ratio of the number of particles identified by the detector to the number of particles injected into the detector.When simulating the intrinsic efficiency, the physical structure of the detector presented in Sect. 3.1 was used to model the detector.

    For CeBr3scintillation crystals, an Al-Mg alloy was used as the material of the crystal shell (thickness of 0.2 cm, density of 2.7 g/cm3). The scintillation crystals were placed in a rectangular pattern and equally spaced.The distance between adjacent scintillation crystals was 0.8 cm, and the distance of the outermost crystal from the carbon fiber plate was 0.87 cm.

    For NaI scintillation crystals, the material of the crystal shell was stainless steel(density of 7.8 g/cm3).For crystals with dimensions of 10 cm × 10 cm × 40 cm, the thickness of the stainless steel was 0.05 cm. The distance between adjacent scintillation crystals was 2 cm, and the distance of the outermost crystal from the carbon fiber plate was 1 cm. For crystals with dimensions of φ7.62 cm ×7.62 cm, the thickness of the stainless steel was 0.2 cm.The pattern used for the placement of the scintillation crystals was the same as that used for the CeBr3crystals.

    The number of particles identified by the detector is the sum of the full-energy peak in the energy spectrum. The number of particles injected into the detector was the sum of primary particles injected into the bottom and sides of the detector box. The number of initial particles sampled was set to 1 × 107. The shape of the radioactive source is round platform (radius of 110 cm, height of 60 cm). The energy was set to a single value (1.46, 1.76, and 2.61 MeV).Air was used as the filling medium outside the detector box.

    3.3 Simulation result

    Using Geant4 software, the intrinsic efficiencies of NaI and CeBr3for gamma-ray energies of 1.46, 1.76, and 2.61 MeV were simulated for different total volumes(Table 2).

    As shown in Table 2,the intrinsic efficiency varies with the total volume. The larger the volume, the higher the intrinsic efficiency. The intrinsic efficiency is difficult to derive from theory and is independent of the measurementconditions.Therefore,the effective detection efficiency εeff(% · m2) is defined as εeff= εinp× Sp. This indicates that the effective detection efficiency is the product of the intrinsic efficiency and cross-sectional area of the scintillation crystal (m2). For example, the effective detection efficiency of six φ4.5 cm × 4.5 cm CeBr3scintillation crystals for 1.46-MeV gamma rays is 0.047% m2. Using the data presented in Tables 1 and 2,the effective detection efficiencies of the two scintillation counters were calculated for different energies at different volumes.

    Table 1 Information on different scintillation counters, including crystal type, single-crystal size, number of crystals, and total volume

    Table 2 Simulation results for the intrinsic detection efficiency of two scintillation counters (at 1.46, 1.76, and 2.61 MeV) at different total volumes (%)

    In the exploration of radioactivity via AGS, the elemental contents of K, U, and Th in the ground are mainly obtained using the total energy peak count rates of40K(1.46 MeV),214Bi(1.76 MeV),and208Tl(2.61 MeV).The volume of six φ 4.5 cm × 5 cm CeBr3scintillation crystals used to count K is taken as an example. It is assumed that the atmospheric pressure is one standard atmosphere,air temperature is 20 °C, density of air is 1.205 kg/m3[2],density of the geological body is 2.3 kg/m3, line reduction factor of the geological body for a 1.46-MeV gamma-ray is 12.3 /m, line reduction factor of the air for a 1.46-MeV gamma-ray is 0.0129/m,number of gamma rays produced in each decay of40K is 0.11, half-life of40K is 4.01 × 1016s,abundance of40K in the geological body is 0.014%,and background count rate is 4.47 cps.Calculated from Eq. (8), at the ground surface, the MDAC of the CeBr3scintillation counter at40K is 52.69. Based on the above parameters, the MDACs of CeBr3at40K were calculated for changes in the effective detection efficiency,energy resolution, and flight altitude (Table 3).

    According to the results in Table 3 for40K, if the effective detection efficiency is doubled or the flight altitude is reduced by approximately half,the MDAC value of AGS will be reduced by approximately half.Improving the energy resolution can also improve the MDAC of AGS.

    4 Experiment and result analyses

    4.1 Experimental verification

    The physical experimental validation was mainly based on actual measurements of the calibration pads using the experimental device. The theory and the stripping ratio method were employed to calculate the elemental content in the calibration pads, and the calculated results were compared with the actual contents to determine relative errors and test the theory. The calibration pads are located in Luojiang County, Sichuan Province, and belong to the nuclear industry radiation testing and protection equipmentmeasurement and certification station. Stripping ratios are recommended in the IAEA guidelines and Chinese industry standards [3, 4].

    Table 3 MDACs (Bq/g) of AGS for 40 K for various effective detection efficiencies(%m 2),energy resolutions(%),and flight altitudes (m)

    The calibration pads include the background pad,K pad,U pad,Th pad,and mixture pad.Figure 3 shows the spectra measured using the CeBr3scintillation counter for the mixture pad. The measured spectra are in the range of 0.41-2.81 MeV.

    Fig. 3 (Color online) Spectra of mixed calibration sample obtained using the scintillation counters

    Table 4 shows that the relative errors between the actual contents and the contents obtained using the stripping ratios were less than 5%, and the relative errors between the actual contents and the contents obtained using Eq. (5)were less than 4%. The comparison results demonstrate that calculations using Eq. (5) are reliable.

    4.2 Relationship between MDAC and different factors

    The MDACs were calculated for different effective detection efficiencies under the same conditions and were normalized. As shown by the black line in Fig. 4, thecalculated values were in accordance with the theoretical curve, and the relative errors between the measured and calculated values were 3.8% (CeBr3) and 4% (NaI).Because the volume and energy resolution of NaI in the experiment were different from those of CeBr3, the values of MDAC measured with NaI were normalized to equate the volume and energy resolution of NaI to those of CeBr3.The effective detection efficiency was calculated as the product of the intrinsic efficiency and the cross-sectional area of the scintillation crystal. Intrinsic efficiency is an intrinsic parameter of the detector itself. Therefore, when designing a detector, it is important to select scintillation crystals with a large total volume and high intrinsic efficiency.

    The MDACs were calculated for the same conditions but with different energy resolutions, and the calculated values were normalized.As shown by the red line in Fig. 4,the calculated values were in accordance with the theoretical curve, and the relative errors between the measured and calculated values were 3.8% (CeBr3) and 6.8% (NaI).Similarly, the MDAC of NaI was normalized. The MDAC value can be optimized by increasing the energy resolution(decreasing the η value),which is an intrinsic parameter of the detector itself. The energy resolution is influenced by the type of scintillation crystal and the design of the circuit module. Therefore, when designing the detector, it is important to choose a scintillation crystal with a low energy resolution and optimize the circuit module.

    In research on the effect of flight altitude on MDAC,the MDAC value at 100 m was used as a baseline value and to normalize the other MDAC values. As shown by the blue line in Fig. 4, the calculated values are in accordance with the theoretical curve.The AGS system was hung below an unmanned aerial vehicle,and the gamma-ray spectrum was measured on the ground surface to obtain background data at altitudes in the range of 50-100 m. The relative errors between the measured and calculated values of MDAC were all less than 5% (i.e., 4.65%, 4.65%, -2.41%, 4.65%,and 2.35%). The MDAC value increased as the flight altitude increased. When using AGS to measure the gamma-ray spectrum on the ground surface, the flight altitude should be minimized to optimize the MDAC of the AGS.

    4.3 Experiments on-field applications

    The field application experimental area was located in a prospective metallogenic area in Jiangxi Province, China.The experimental area is 2.2 km2.The experimental device was the NaI scintillation counter (see Sect. 3.1 for details of the experimental devices), which was installed on the bottom of an F-120 single-rotor unmanned aerial vehicle.The flight altitude and flight speed of the unmanned aerial vehicle were 80 m and 10 m/s, respectively. The AGS sampling time was set to 1 s. There are 1,960 sets of measured data, including geodetic coordinates, GPS heights, radar heights, and net peak areas of the K, U, and Th elemental full-energy peaks. The data for the net peak areas were highly corrected. Hence, only the measurement result for 1.46 MeV is shown here.

    According to the statistics for data measured with the NaI scintillation counter, the activity concentration of elemental K had the mean value of 110.1 Bq/g,variance of 11.88, maximum value of 153.0 Bq/g, minimum value of 73.9 Bq/g, and median value of 110.1 Bq/g. The MDAC measured with the NaI scintillation counter at 80 m for energy of 1.46 MeV was calculated as 70.1 Bq/g. All activity concentration values in the data measured by the NaI scintillation counter were greater than the MDAC values. A contour plot of the activity concentrations of elemental K distribution measured by the NaI scintillation counter in the experimental area is displayed in Fig. 5.

    As shown in Fig. 5, the results measured by the NaI scintillation counter show that the distribution of activity concentrations with high, medium, and low values has a certain regular character.There were no significantly highvalue areas in the experimental area, and two relatively high-value areas occurred in the western and central-eastern parts of the experimental area; medium-value areas were located in the central, central-eastern, and northern parts of the experimental area, and their areas were large and connected; while low-value areas were mainly concentrated in the southern part of the experimental area and occurred to a certain extent in the eastern part of the experimental area. The experimental data from field applications demonstrate the possibility of obtaining a reliable MDAC for the detector using a theoretical formula.

    5 Conclusion

    MDAC is an important technical specification for AGS,and clarification of the influencing factors of MDAC can provide theoretical guidance for the design of AGS systems. Although some studies have shown that MDAC is influenced by factors such as intrinsic efficiency, energy resolution,total volume,and flight altitude,there is still no unified theory that directly and concretely describes the functional relationships between MDAC and these influencing factors. Therefore, we investigated the factors influencing MDAC, derived a theory that incorporated the above parameters, and verified the accuracy of the theory.The verification results indicate that the relative errors between the results calculated from the derived theoretical formula and the actual values were less than 4%. When designing an AGS system, it is recommended that reduced flight altitude and scintillation counter high intrinsic detection efficiencies and excellent energy resolution, and large total volumes should be used to reduce the MDAC value of an AGS system. The proposed theory of MDAC can guide the work of AGS in measuring areas of low radioactivity.

    Author contributionsAll authors contributed to the study’s conception and design.Material preparation,data collection,and analysis were performed by Yi Gu,Kun Sun,Liang-Quan Ge,Yuan-Dong Li,Qing-Xian Zhang, Xuan Guan, Wan-Chang Lai, Zhong-Xiang Lin,and Xiao-Zhong Han.The first draft of the manuscript was written by Yi Gu and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

    成人漫画全彩无遮挡| 亚洲国产日韩一区二区| 久久99精品国语久久久| 国产亚洲最大av| 国产精品久久久久成人av| 国产精品久久久久久久久免| 国产成人freesex在线| 亚洲精品久久久久久婷婷小说| 国产一区亚洲一区在线观看| 在线 av 中文字幕| 国产日韩欧美在线精品| 99国产综合亚洲精品| 国产欧美日韩综合在线一区二区| 91在线精品国自产拍蜜月| 99热这里只有是精品在线观看| 精品视频人人做人人爽| 成人毛片a级毛片在线播放| 成人国产av品久久久| 啦啦啦啦在线视频资源| 欧美人与善性xxx| 亚洲精品视频女| 又粗又硬又长又爽又黄的视频| 性色avwww在线观看| 免费黄网站久久成人精品| 精品少妇黑人巨大在线播放| 五月伊人婷婷丁香| 久久人人爽av亚洲精品天堂| 午夜福利影视在线免费观看| 99久久精品国产国产毛片| 美女cb高潮喷水在线观看| 国产精品一区www在线观看| 99热6这里只有精品| 少妇 在线观看| 久久国内精品自在自线图片| 简卡轻食公司| 国产精品久久久久久精品电影小说| 高清毛片免费看| 国产成人精品福利久久| 菩萨蛮人人尽说江南好唐韦庄| 亚洲精品久久午夜乱码| 精品99又大又爽又粗少妇毛片| 高清视频免费观看一区二区| 五月伊人婷婷丁香| 国产av国产精品国产| 超碰97精品在线观看| 尾随美女入室| 69精品国产乱码久久久| 校园人妻丝袜中文字幕| 内地一区二区视频在线| 九九在线视频观看精品| 国产国拍精品亚洲av在线观看| 五月天丁香电影| 精品久久久噜噜| 成年美女黄网站色视频大全免费 | 大又大粗又爽又黄少妇毛片口| 午夜福利在线观看免费完整高清在| 桃花免费在线播放| 亚洲av免费高清在线观看| 国产高清三级在线| 制服丝袜香蕉在线| 啦啦啦在线观看免费高清www| 美女cb高潮喷水在线观看| 少妇精品久久久久久久| 日韩av不卡免费在线播放| 夫妻午夜视频| 久久久久国产精品人妻一区二区| 在线观看一区二区三区激情| 人妻一区二区av| 日韩一本色道免费dvd| 国产精品三级大全| 999精品在线视频| 99re6热这里在线精品视频| 街头女战士在线观看网站| 亚洲国产精品一区三区| 中文字幕制服av| 少妇高潮的动态图| 爱豆传媒免费全集在线观看| 国产 精品1| 少妇被粗大的猛进出69影院 | 少妇人妻 视频| 热re99久久国产66热| 十八禁高潮呻吟视频| 最近中文字幕2019免费版| 国产精品秋霞免费鲁丝片| 日日摸夜夜添夜夜爱| 久久久久久久国产电影| 国产成人午夜福利电影在线观看| 亚洲久久久国产精品| 人人妻人人澡人人爽人人夜夜| 亚洲,欧美,日韩| 日日摸夜夜添夜夜添av毛片| 久久精品国产亚洲av涩爱| 性高湖久久久久久久久免费观看| 欧美三级亚洲精品| 久久女婷五月综合色啪小说| 黑丝袜美女国产一区| 国产成人精品无人区| 国产黄频视频在线观看| 国产欧美日韩综合在线一区二区| 国产亚洲欧美精品永久| 中文字幕最新亚洲高清| 国产成人av激情在线播放 | 自线自在国产av| 久久精品国产鲁丝片午夜精品| 日韩一区二区视频免费看| 99国产精品免费福利视频| 天堂中文最新版在线下载| 中文精品一卡2卡3卡4更新| 日本欧美国产在线视频| 亚洲性久久影院| 观看av在线不卡| 中文字幕av电影在线播放| 老熟女久久久| 中文天堂在线官网| 在线观看美女被高潮喷水网站| 啦啦啦啦在线视频资源| 熟女电影av网| 在线观看国产h片| 欧美亚洲日本最大视频资源| 免费高清在线观看日韩| 男女边吃奶边做爰视频| 欧美成人精品欧美一级黄| 国产精品一区二区三区四区免费观看| 在线观看三级黄色| 妹子高潮喷水视频| 精品国产露脸久久av麻豆| 国产精品人妻久久久影院| 久久久久网色| 国产精品99久久久久久久久| 日韩大片免费观看网站| a级毛色黄片| 久久精品国产亚洲av涩爱| 99热国产这里只有精品6| 丝袜美足系列| 少妇精品久久久久久久| 日本与韩国留学比较| 日日爽夜夜爽网站| 大又大粗又爽又黄少妇毛片口| 午夜日本视频在线| 啦啦啦视频在线资源免费观看| 国产亚洲精品第一综合不卡 | 黑人猛操日本美女一级片| 99热这里只有是精品在线观看| 亚洲国产日韩一区二区| 亚洲综合色网址| 少妇人妻精品综合一区二区| 黄色一级大片看看| 精品久久久久久电影网| 嘟嘟电影网在线观看| 看十八女毛片水多多多| 久久久精品94久久精品| 女的被弄到高潮叫床怎么办| 寂寞人妻少妇视频99o| √禁漫天堂资源中文www| 韩国高清视频一区二区三区| 国产成人精品福利久久| 激情五月婷婷亚洲| 香蕉精品网在线| 免费观看av网站的网址| 色视频在线一区二区三区| 中文字幕人妻熟人妻熟丝袜美| 边亲边吃奶的免费视频| 欧美日韩视频高清一区二区三区二| 亚洲人与动物交配视频| 麻豆乱淫一区二区| 精品久久久噜噜| 五月开心婷婷网| av网站免费在线观看视频| 欧美亚洲 丝袜 人妻 在线| 少妇丰满av| 中文字幕最新亚洲高清| 亚洲综合精品二区| 亚洲国产色片| 国产精品一二三区在线看| 精品亚洲乱码少妇综合久久| 亚洲国产精品999| 亚洲精品久久久久久婷婷小说| 久久国产亚洲av麻豆专区| 亚洲国产毛片av蜜桃av| 成人手机av| 高清不卡的av网站| 精品人妻在线不人妻| 欧美bdsm另类| 秋霞在线观看毛片| 大话2 男鬼变身卡| 亚洲色图综合在线观看| 大又大粗又爽又黄少妇毛片口| 午夜av观看不卡| 性色avwww在线观看| 久久久欧美国产精品| 观看av在线不卡| 精品一区二区三区视频在线| 欧美激情极品国产一区二区三区 | 成人无遮挡网站| 日本欧美视频一区| 伦理电影免费视频| 中文字幕亚洲精品专区| 亚洲成色77777| 免费播放大片免费观看视频在线观看| 99热这里只有是精品在线观看| 久久女婷五月综合色啪小说| 天美传媒精品一区二区| 秋霞伦理黄片| 日韩三级伦理在线观看| 久久精品久久久久久久性| 成人亚洲精品一区在线观看| 欧美国产精品一级二级三级| 在线观看人妻少妇| 欧美bdsm另类| 热re99久久精品国产66热6| 伦理电影大哥的女人| 一本—道久久a久久精品蜜桃钙片| 秋霞在线观看毛片| 国产成人精品久久久久久| 99九九线精品视频在线观看视频| 在线观看免费视频网站a站| 五月伊人婷婷丁香| 国产精品一二三区在线看| 你懂的网址亚洲精品在线观看| 18禁在线播放成人免费| 久久久国产欧美日韩av| 亚洲欧美成人综合另类久久久| 久久精品夜色国产| 国产高清不卡午夜福利| 亚洲精品久久午夜乱码| 日韩在线高清观看一区二区三区| 777米奇影视久久| 亚洲精品456在线播放app| 精品熟女少妇av免费看| 久久午夜福利片| 久久毛片免费看一区二区三区| 精品久久久久久电影网| 中文字幕亚洲精品专区| 亚洲精品亚洲一区二区| 国精品久久久久久国模美| 欧美3d第一页| 亚洲,欧美,日韩| av免费观看日本| 成人漫画全彩无遮挡| 最近2019中文字幕mv第一页| 亚洲三级黄色毛片| 国产精品一区二区在线观看99| www.色视频.com| 久久久久久久精品精品| 水蜜桃什么品种好| 成人午夜精彩视频在线观看| 狠狠婷婷综合久久久久久88av| av福利片在线| 9色porny在线观看| 香蕉精品网在线| 久久女婷五月综合色啪小说| 久久久久精品久久久久真实原创| 国产精品女同一区二区软件| 国产69精品久久久久777片| 高清视频免费观看一区二区| 又粗又硬又长又爽又黄的视频| 久久久久久久国产电影| 国产免费一级a男人的天堂| 久久国产亚洲av麻豆专区| 成人午夜精彩视频在线观看| 精品午夜福利在线看| 建设人人有责人人尽责人人享有的| 国产av精品麻豆| 丰满迷人的少妇在线观看| 欧美精品高潮呻吟av久久| 国产一区二区三区av在线| 婷婷色综合www| 十八禁网站网址无遮挡| a级毛片免费高清观看在线播放| 十八禁高潮呻吟视频| 大香蕉久久成人网| 久久久久国产精品人妻一区二区| 国产免费一级a男人的天堂| 久久午夜福利片| 精品人妻熟女毛片av久久网站| 免费久久久久久久精品成人欧美视频 | 免费不卡的大黄色大毛片视频在线观看| 国产精品久久久久久久久免| 制服诱惑二区| 欧美3d第一页| 成人手机av| 亚洲人与动物交配视频| 美女大奶头黄色视频| 大片免费播放器 马上看| 亚洲性久久影院| 多毛熟女@视频| 亚洲精品一区蜜桃| 日韩不卡一区二区三区视频在线| 中文字幕免费在线视频6| 国产精品偷伦视频观看了| 黄色视频在线播放观看不卡| 国产免费一级a男人的天堂| 嫩草影院入口| 女人久久www免费人成看片| 久久ye,这里只有精品| 日本91视频免费播放| av福利片在线| 日本黄色片子视频| 男女啪啪激烈高潮av片| av女优亚洲男人天堂| 欧美激情极品国产一区二区三区 | 欧美 亚洲 国产 日韩一| 国产成人freesex在线| 黑人猛操日本美女一级片| 精品久久久噜噜| 十分钟在线观看高清视频www| 国产爽快片一区二区三区| 只有这里有精品99| 久久久久精品性色| 国产成人精品无人区| 欧美日韩国产mv在线观看视频| 欧美人与善性xxx| 欧美另类一区| 久热这里只有精品99| 18禁裸乳无遮挡动漫免费视频| 三上悠亚av全集在线观看| 制服诱惑二区| 美女国产高潮福利片在线看| 国产成人精品久久久久久| 好男人视频免费观看在线| 国产在线免费精品| 久久97久久精品| 美女视频免费永久观看网站| 亚洲国产日韩一区二区| 国产一区二区在线观看日韩| 国产精品一区二区在线不卡| 久久久精品区二区三区| 久久精品国产亚洲av涩爱| 午夜av观看不卡| 国产在视频线精品| 日韩一区二区视频免费看| 观看av在线不卡| 日韩av免费高清视频| 三级国产精品片| 夫妻性生交免费视频一级片| √禁漫天堂资源中文www| 18禁动态无遮挡网站| 你懂的网址亚洲精品在线观看| 久久精品久久精品一区二区三区| 欧美激情国产日韩精品一区| 久久久精品94久久精品| 亚洲国产成人一精品久久久| 免费观看在线日韩| √禁漫天堂资源中文www| 亚洲国产成人一精品久久久| 亚洲第一区二区三区不卡| 国产精品欧美亚洲77777| 狂野欧美白嫩少妇大欣赏| 欧美变态另类bdsm刘玥| 少妇丰满av| 亚洲国产精品成人久久小说| 伦精品一区二区三区| 午夜福利在线观看免费完整高清在| 亚洲精品色激情综合| 香蕉精品网在线| 日本与韩国留学比较| 99热这里只有是精品在线观看| 久久久午夜欧美精品| 少妇 在线观看| 国产成人精品无人区| 国产老妇伦熟女老妇高清| 久久人妻熟女aⅴ| 免费观看的影片在线观看| 免费观看av网站的网址| 男人爽女人下面视频在线观看| 我的老师免费观看完整版| 中文字幕精品免费在线观看视频 | 亚洲av.av天堂| 国产日韩欧美亚洲二区| 97超视频在线观看视频| 日韩视频在线欧美| 男女边吃奶边做爰视频| 熟女av电影| 久久人人爽av亚洲精品天堂| 岛国毛片在线播放| 午夜91福利影院| 精品国产露脸久久av麻豆| 人妻人人澡人人爽人人| 国产免费福利视频在线观看| 丝袜脚勾引网站| 日韩一本色道免费dvd| 最近的中文字幕免费完整| 亚洲av国产av综合av卡| 韩国av在线不卡| 国产黄色免费在线视频| 日本黄色片子视频| 欧美日韩精品成人综合77777| 亚洲精品国产av蜜桃| av福利片在线| 亚洲精品乱码久久久v下载方式| 视频区图区小说| 久久久久久人妻| 日本vs欧美在线观看视频| 久久精品国产鲁丝片午夜精品| 久久久久久伊人网av| 熟妇人妻不卡中文字幕| 草草在线视频免费看| av线在线观看网站| 色婷婷av一区二区三区视频| 国产色爽女视频免费观看| 97超视频在线观看视频| 久久狼人影院| 日韩强制内射视频| 欧美+日韩+精品| 中文字幕av电影在线播放| 国产精品一区二区在线观看99| 精品久久久精品久久久| 成人毛片60女人毛片免费| 黄色视频在线播放观看不卡| 国产精品成人在线| 中文字幕亚洲精品专区| 女人精品久久久久毛片| 精品一区二区三卡| 成人18禁高潮啪啪吃奶动态图 | 精品久久国产蜜桃| 人人妻人人爽人人添夜夜欢视频| 美女福利国产在线| 久久久久久久久久人人人人人人| 日韩免费高清中文字幕av| 插逼视频在线观看| a 毛片基地| 只有这里有精品99| 内地一区二区视频在线| 精品久久蜜臀av无| 蜜桃在线观看..| 久久99热这里只频精品6学生| 成人免费观看视频高清| 大香蕉97超碰在线| 亚洲精品aⅴ在线观看| 秋霞在线观看毛片| 国产午夜精品久久久久久一区二区三区| 亚洲精品中文字幕在线视频| 国产黄频视频在线观看| 欧美 亚洲 国产 日韩一| a级片在线免费高清观看视频| 色婷婷av一区二区三区视频| 性色av一级| 亚洲激情五月婷婷啪啪| 大香蕉97超碰在线| 精品一品国产午夜福利视频| 国产精品 国内视频| 丝袜美足系列| 日韩在线高清观看一区二区三区| 大码成人一级视频| 亚洲不卡免费看| 超色免费av| 中文乱码字字幕精品一区二区三区| 99国产综合亚洲精品| 国产免费视频播放在线视频| 蜜桃在线观看..| 欧美另类一区| 秋霞伦理黄片| 亚洲国产精品一区三区| 国产精品一区二区三区四区免费观看| 久久婷婷青草| 国产成人免费观看mmmm| av黄色大香蕉| 欧美少妇被猛烈插入视频| 一个人看视频在线观看www免费| 美女内射精品一级片tv| 欧美精品一区二区大全| 欧美亚洲日本最大视频资源| 色吧在线观看| 欧美变态另类bdsm刘玥| 黄色毛片三级朝国网站| 欧美日韩一区二区视频在线观看视频在线| 少妇人妻精品综合一区二区| 日韩精品有码人妻一区| 国产成人精品一,二区| 搡老乐熟女国产| 亚洲中文av在线| 亚洲欧美清纯卡通| 国产精品国产三级专区第一集| 亚洲精品国产av蜜桃| 爱豆传媒免费全集在线观看| 国产精品不卡视频一区二区| 亚洲婷婷狠狠爱综合网| a级毛片在线看网站| 亚洲国产欧美日韩在线播放| 日韩三级伦理在线观看| 亚洲国产精品999| 亚洲婷婷狠狠爱综合网| 久久免费观看电影| 久久久久久久大尺度免费视频| 国产精品国产三级国产av玫瑰| av视频免费观看在线观看| 国产av国产精品国产| 亚洲第一区二区三区不卡| 日韩一区二区三区影片| 一区二区av电影网| 两个人免费观看高清视频| 人人妻人人添人人爽欧美一区卜| 欧美日韩一区二区视频在线观看视频在线| 国产日韩欧美视频二区| 亚洲综合色惰| 国产又色又爽无遮挡免| 午夜福利视频精品| 久久热精品热| 18禁观看日本| 国产亚洲午夜精品一区二区久久| 国产在线免费精品| a 毛片基地| 久久精品国产自在天天线| 久久国产精品男人的天堂亚洲 | 亚洲综合色网址| 亚洲欧美色中文字幕在线| 国产成人免费观看mmmm| 久久久久久久久久久丰满| 亚洲美女黄色视频免费看| 中文乱码字字幕精品一区二区三区| 久久久久久久久久人人人人人人| 国模一区二区三区四区视频| 夫妻性生交免费视频一级片| 亚洲,一卡二卡三卡| 久久久久久久久久成人| 18禁在线播放成人免费| a级片在线免费高清观看视频| 伦理电影免费视频| 久久久精品免费免费高清| 少妇的逼好多水| 久久这里有精品视频免费| 欧美另类一区| 老熟女久久久| 久久久久久久久大av| 大又大粗又爽又黄少妇毛片口| 最近中文字幕高清免费大全6| 亚洲欧美一区二区三区黑人 | 精品少妇久久久久久888优播| a级片在线免费高清观看视频| 国产精品一区二区在线不卡| 尾随美女入室| 午夜久久久在线观看| 久久狼人影院| 亚洲国产精品999| 久久人人爽av亚洲精品天堂| 国产精品99久久久久久久久| 在线观看美女被高潮喷水网站| 蜜桃在线观看..| 青春草亚洲视频在线观看| 两个人免费观看高清视频| 亚洲欧洲精品一区二区精品久久久 | 国产伦理片在线播放av一区| 亚洲,一卡二卡三卡| 亚洲av欧美aⅴ国产| 内地一区二区视频在线| 伦理电影免费视频| 熟女电影av网| 国产一级毛片在线| 国产男女内射视频| 女人久久www免费人成看片| 国产黄色视频一区二区在线观看| 日韩制服骚丝袜av| 熟女av电影| 国产亚洲精品久久久com| 亚洲色图 男人天堂 中文字幕 | 国产在线视频一区二区| 亚洲欧美中文字幕日韩二区| 草草在线视频免费看| 日韩在线高清观看一区二区三区| 美女cb高潮喷水在线观看| 亚洲精品日韩av片在线观看| 亚洲丝袜综合中文字幕| 一区二区三区免费毛片| 大香蕉久久成人网| 欧美97在线视频| 高清不卡的av网站| 午夜日本视频在线| 国产色爽女视频免费观看| xxxhd国产人妻xxx| 精品人妻偷拍中文字幕| 乱人伦中国视频| 亚洲情色 制服丝袜| 制服丝袜香蕉在线| 亚洲人成网站在线播| 久久ye,这里只有精品| 狠狠婷婷综合久久久久久88av| 如何舔出高潮| a级毛片免费高清观看在线播放| 人体艺术视频欧美日本| 久久精品久久久久久久性| 午夜激情久久久久久久| 国产成人精品在线电影| 国产69精品久久久久777片| 大片电影免费在线观看免费| 大香蕉久久网| 少妇 在线观看| 免费不卡的大黄色大毛片视频在线观看| 欧美日本中文国产一区发布| 国产黄色视频一区二区在线观看| 日日撸夜夜添| 亚洲精品美女久久av网站| 51国产日韩欧美| 日日撸夜夜添| 成年人免费黄色播放视频| 免费黄色在线免费观看| 国产欧美另类精品又又久久亚洲欧美| 大香蕉久久网| 国产精品一二三区在线看| 久久人人爽人人爽人人片va| 亚洲人成77777在线视频| 亚洲成人一二三区av| 夫妻午夜视频| av.在线天堂| 熟妇人妻不卡中文字幕| 18禁在线播放成人免费| 三级国产精品欧美在线观看| 久久久久久久大尺度免费视频| 欧美另类一区| 成人免费观看视频高清| 51国产日韩欧美| 成人国产av品久久久| 18禁裸乳无遮挡动漫免费视频| 岛国毛片在线播放| 国产精品一二三区在线看| 九色成人免费人妻av|