伴随粒子法γ能谱本底测量技术

为了解决伴随粒子法瞬发能谱实验中随机本底的影响,通过实验研究了标记中子和非标记中子与物质作用产生的瞬发信号和随机本底信号。对不同情况随机本底谱测量进行了分析,提出了小尺度样品和大尺度样品本底测量方式。研究表明:对小尺度样品,可以通过去掉样品,选择与有样品测量的谱相同的时间窗进行本底测量;对于大尺度样品,可以通过在随机本底范围选取时间窗进行本底测量。     

氮气低温除氡技术与极低水平氡测量方法研究

高纯低本底氮气在低本底实验中有重要作用,对氮气的放射性氡本底纯化技术和低本底氡测量方法进行研究非常重要。利用低温物理吸附技术,对氮气中的氡进行纯化,同时建立了静电收集结合低温富集法的氡测量装置, 能够测量氮气中10 μBq/m3的氡本底。利用氡测量装置测量了普通氮气中和纯化氮气中的氡,结果表明,纯化系统可以将氡本底为20 mBq/m3的氮气降低至(25.2±5) μBq/m3,即可将氡本底降低到纯化前的0.125%。研究还发现,纯化能力与氮气中氡本底相关,在氮气中氡本底为几十mBq/m3到几十μBq/m3范围内,随着氡本底的下降,纯化能力也随之下降。主要原因是,随着氮气中氡的分压降低,吸附能力下降。另外,在极低本底下由系统本底及泄露引入的氡污染影响显著增大。     

X射线管激发X荧光光谱连续本底扣除方法研究

X射线管是目前X射线荧光光谱分析中最常采用的激发源,它所产生的原级谱成为了X荧光光谱中本底成分的主要来源,在对这种光谱进行进一步的分析处理之前需要对其本底进行扣除,对本底估计的准确性直接影响后续处理步骤的效果。对射线管激发X荧光光谱的成分进行了分析,针对其本底特点构造了一种本底强度的估计方法,并根据实测谱线构建了理论测试谱线以便对光谱处理算法的效果进行评价。该方法利用测得X射线荧光光谱中不包含特征峰的谱段对X射线管原级谱造成的本底成分进行估计,使用只包含连续本底的谱段对整个测量谱段进行插值,从而避免了谱线特征峰重叠或对半高宽估计不当时所产生的影响。利用构建的测试光谱对SNIP法、傅里叶变换法和本文的本底估计方法的使用效果进行了比较,使用该方法估计的本底与理论本底更加接近。结果表明使用的方法对X射线管激发的X荧光光谱的本底估计准确,可以采用这种方法对连续本底进行扣除,在对实际测得的X射线荧光光谱的本底扣除中取得了较好的应用效果。     

天然中子本底剂量水平的测定

本文介绍了一种高灵敏度中子监测器,它对天然中子本底的计数率约每分钟20次计数。实验测定了几个基本特性:脉冲幅度分辨率、灵敏度和方向性。用该监测器在一年的时间内连续测量了北京玉泉路地区的天然中子本底的剂量水平,给出了年平均剂量当量值。与日本KEK和CERN测得的数据进行了比较。     

大空腔探测系统的Am-Li中子优化屏蔽

为了减少Am-Li中子本底对高浓铀部件质量主动多重性测量的影响,对大空腔探测系统(NPLNMC)Am-Li中子本底的优化屏蔽进行了模拟研究,提出了一个基于高密度聚乙烯为中子屏蔽体的优化方案。通过对比模拟结果与屏蔽前实验测量结果,发现屏蔽使Am-Li中子本底探测效率明显降低,从原来的15.77%降为屏蔽后的1.94%,大约降低了87.7%;而屏蔽对裂变中子计数的影响却相对较小,只比屏蔽前降低约2.4%。本底中子计数的降低明显提高了系统对铀部件质量测量的灵敏度,在3000s测量时间内,其质量测量下限从原来的大约6.4kg下降到屏蔽后的2.6kg;同时,屏蔽后的NPL-NMC系统在相同测量条件下,铀部件质量测量准确性提高50%以上。     

氘氚源中子穿过聚乙烯样品泄漏中子谱的测量与模拟

采用飞行时间技术测量了氘氚（D-T）源中子穿过不同厚度板状聚乙烯样品后40°方向的泄漏中子时间到达谱,样品的长和宽均为100 cm, 厚度分别为4.5, 9, 18和27 cm。 本底谱测量采用了无样本底测量和无样堵孔本底测量2种方案, 利用MCNP-4C程序模拟了相同实验条件下的泄漏中子时间到达谱, 模拟过程中考虑了源中子的能谱与角分布、脉冲中子束宽度、 探测器的效率以及样品的有效面积。通过比较发现, 采用无样测量谱作为本底时,计算值/实验值（C/E）值大于1, 并且随着样品厚度的增加而偏离1;而本底谱采用无样堵孔测量谱时, C/E小于1, 并且随着样品厚度的增加而接近1。通过对两套本底谱的分析, 并结合蒙特卡罗模拟, 计算求得了相应样品厚度下比较接近实际的本底谱,采用该模拟计算本底谱后,C/E值有了明显的改善。The neutron leakage spectrum were measured at 40° by time of flight method for polyethylene slabs with the thicknesses of 4.5, 9,18 and 27 cm,respectively. The experimental results were compared with the MCNP-4C simulations, which carefully considered many effects, such as the angle and energy distributions of the source neutrons,the width of the beam pulse,the detection efficiency and the effective measured area. The sample out and collimator filled spectra were measured as the background, and the results showed that C/E values were larger than 1 when using the sample out background spectra,but smaller than 1 when using the collimator filled background spectra. Combine these two different background spectra, a new method was used to calculate the background spectrum,and the results have a better agreement with the simulations.     

PbS量子点光致荧光寿命的实验测量与确定

通过测量PbS量子点的吸收谱、时间分辨的荧光谱以及透射电子显微镜图,确定了不同粒径、不同温度、不同本底PbS量子点的光致荧光寿命,得到了描述第一吸收峰波长随量子点粒径变化的经验公式。结果表明,荧光寿命强关联于量子点粒径,可用负指数经验公式表达;荧光寿命弱依赖于温度;本底材料由于表面极化效应对荧光寿命也有影响。     

探测器束流相关本底的实验研究

介绍了在北京正负电子对撞机上进行的束流相关本底实验研究,并与计算结果进行了比较.通过实验获得了束流相关本底的特征;并用相应的软件(TURTLE和GEANT3)对束流–气体相互作用本底进行了细致的模拟.实验和模拟结果的比较表明,现有的软件工具模拟在4倍以内是可靠的,从而为正在设计建造的新一代北京正负电子对撞机和北京谱仪的本底模拟奠定了基础.     

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介绍了EAST低杂波天线相位采集控制系统中两个主要部分的相位本底的校准技术。一是由相控多结波导阵天线与等离子体相切弧面引入的相位本底的静态校准,二是天线移动时软波导伸缩引起的可变相位本底的动态校准。低杂波天线相位本底校准技术为相位反馈控制系统提供了准确的相位基点,为实现预设相位与实调相位的一致性奠定了基础。     

北京谱仪BES辐射本底水平的测定

北京谱仪BES辐射本底水平是由大厅内的辐射水平决定的.降低厅内的辐射水平是减少BES的辐射本底和提高BES探测效率和“信噪比”的重要途径.文中着重研究了谱仪厅内辐射本底的强度,来源及特点,并探讨了减少辐射的方法.     

钢中残量元素As,Sn,Sb的X射线荧光光谱分析

本文介绍了用日本理学3530XRF光谱仪扫描道测定钢中痕量元素As、Sn、Sb。由于短波强大的背景破坏了检测线的线性,通过用DATAFLEX 181B背景扣除的数据处理,消除了这些影响,成功地校准了曲线。方法简单;快速;XRFS结果与化学分析一致,该法已用于炼钢的过程控制和原材料分析。     

Double-quantum filtered rotational-echo double resonance

The homonuclear scalar coupling of a directly bonded 13C-13C pair has been used to create a double-quantum filter (DQF) to remove the natural-abundance 13C background in 13C{15N} rotational-echo double-resonance (REDOR) experiments. The DQF scalar and REDOR dipolar evolution periods are coincident which is important for sensitivity in the event of weak 13C-15N dipolar coupling. Calculated and observed 13C{15N} DQF-REDOR dephasings were in agreement for a test sample of mixed recrystallized labeled alanines. Glycine metabolism in a single uniform-15N soybean leaf labeled for 6 min by 13CO2 was measured quantitatively by 13C{15N} DQF-REDOR with no background interferences.     

Exclusive measurement of coherent η-photoproduction from the deuteron

Coherent photoproduction of η-mesons from the deuteron has been measured from threshold up to E _γ≈ 750 MeV using the photon spectrometer TAPS at the tagged photon facility at the Mainz microtron MAMI. For the first time, differential coherent cross-sections have been deduced from the coincident detection of the η-meson and the recoil deuteron. A missing-energy analysis was used for the suppression of background events so that a very clean identification of coherent η-photoproduction was achieved. The resulting cross-sections agree with previous experimental results except for angles around 90^° in the γd c.m. system where they are smaller. They are compared to various model calculations. Received: 5 April 2001 / Accepted: 3 September 2001     

寻找BATSEγ暴的TeV能区伴随γ暴

利用西藏ASγ实验三期阵列的重建数据，对25个BATSEγ暴的TeV能区伴随γ暴进行了符合寻找。在BATSEγ暴方向的90％误差范围内，找出在给定的小天区和时间间隔内出现的显著性较高的TeV事例团，并采用“等天顶角方法”来估计背景。发现少量事例团对背景有明显超出，考虑试验次数后，其超出还不足以认定为γ暴。通过Monte Carlo模拟给出了95％置信水平下流强上限的估计值为7．1×10^-9photons／（cm^2·s）。Searching for TeV burst-like events coincident with the BATSE GRBs data was made by using the ASγ （Tibet-Ⅲ） data. In the period we analysed, there were 25 BATSE GRBs in the field view of Tibet. A search region was defined by the BATSE 90% confidence level positioning error. A GRB candidate was chosen as a shower cluster appearing in a given small sky window and a given time interval. An equi-zenith-angle method was used to estimate the background. No significant TeV GRBs were detected. The flux upper limit at the 95% confidence level was estimated to be about 7.1×10^-9 γ（cm^2·s） by Monte Carlo simulation.     

Measurements ofn−p correlations in the reaction of relativistic neon with uranium

We report a preliminary measurement of coincident neutron-proton pairs emitted at 45° in the interaction of 400, 530, and 650 MeV/A neon beams incident on uranium. Charged particles were identified by time of flight and momentum, as determined in a magnetic spectrometer. Neutral particles were detected using a thick plastic scintillator, and their time of flight was measured between an entrance scintillator, triggered by a charged particle, and the neutron detector. The scatter plots and contour plots of neutron momentum vs. proton momentum appear to show a slight correlation ridge above an uncorrelated background. The projections of this plane on then-p momentum difference axis are essentially flat, showing a one standard deviation enhancement for each of the three beams energies. At each beam energy, the calculated momentum correlation function for the neutron-proton pairs is enhanced near zero neutron-proton momentum difference by approximately one standard deviation over the expected value for no correlation. This enhancement is expected to occur as a consequence of the attractive final state interaction between the neutron and proton (i.e., virtual or “singlet” deuterons). The implications of these measurements are discussed.     

氟化氩离子准分子真空紫外辐射

利用激光等离子体软Ｘ射线为激励源，首次观察到离子性氟化氩准分子在１２５ｎｍ附近的真空紫外辐射，其实验结果与理论计算值符合很好。     

Production of electron-positron pairs and photon emission by an electron in an axial-vector background field

The possibility of radiative effects induced by fermion interaction with a constant axial-vector background field is studied within an extended standard model. The production of electron-positron pairs by photons and photon emission by electrons and positrons are examined. The rates of these processes are calculated within the Furry picture. It is shown that the rates obtained in this way depend strongly on the polarization states of the particles involved. As a result, ultrarelativistic particles predominantly occupy specific spin states—namely, charged particles occur in states where the sign of the helicity is coincident with the sign of the effective potential, while photons have polarization of sign opposite to the sign of the effective potential. This leads to spatial asymmetries, which may give rise to observable phenomena in astrophysical and cosmological investigations.     

Poynting flux impulses and the ionospheric switching of geomagnetic substorms

Observations were made of impulse events in Poynting flux calculated from electric and magnetic disturbances encountered by the Polar satellite when on high-latitude field-lines in the magnetotail. These were found to be coincident within±6 min with impulsive spikes in cosmic radio background absorption in the D region of the ionosphere as detected by the Imaging Riometer for Ionospheric Studies riometer in Finland. They were also coincident with substorm onset at the same geomagnetic latitude as determined by a change of gradient in International Monitor for Auroral Geomagnetic Effects’ X-component magnetograms. The interpretation of the observations was that magnetospheric compression waves from the geomagnetic equator region of the magnetotail were coupling to progressively initiate field-guided Alfvén shear waves towards higher geomagnetic latitudes over a large volume of the magnetosphere. The study suggested that they were then able either directly or indirectly to ionise the D region of the ionosphere and in the process to cut deep electrically conducting channels between the magnetosphere and the ionosphere through which currents could flow and initiate the characteristic signature of geomagnetic substorms in ground magnetograms.     

Transmission mode time-reversal super-resolution imaging

The theory of time-reversal super-resolution imaging of point targets embedded in a reciprocal background medium [A. J. Devaney, "Super-resolution imaging using time-reversal and MUSIC," J. Acoust. Soc. Am. (to be published)] is generalized to the case where the transmitter and receiver sensor arrays need not be coincident and for cases where the background medium can be nonreciprocal. The new theory developed herein is based on the singular value decomposition of the generalized multistatic data matrix of the sensor system rather than the standard eigenvector/eigenvalue decomposition of the time-reversal matrix as was employed in the above-mentioned work and other treatments of time-reversal imaging [Prada, Thomas, and Fink, "The iterative time reversal process: Analysis of the convergence," J. Acoust. Soc. Am. 97, 62 (1995); Prada et al., "Decomposition of the time reversal operator: Detection and selective focusing on two scatterers," J. Acoust. Soc. Am. 99, 2067 (1996)]. A generalized multiple signal classification (MUSIC) algorithm is derived that allows super-resolution imaging of both well-resolved and non-well-resolved point targets from arbitrary sensor array geometries. MUSIC exploits the orthogonal nature of the scatterer and noise subspaces defined by the singular vectors of the multistatic data matrix to form scatterer images. The time-reversal/MUSIC algorithm is tested and validated in two computer simulations of offset vertical seismic profiling where the sensor sources are aligned along the earth's surface and the receiver array is aligned along a subsurface borehole. All results demonstrate the high contrast, high resolution imaging capabilities of this new algorithm combination when compared with "classical" backpropagation or field focusing. Above and beyond the application of seismo-acoustic imaging, the time-reversal super-resolution theory has applications in ocean acoustics for target location, and ultrasonic nondestructive evaluation of parts.     

数值模拟输运问题的一种破开算子方法

本文通过破开算子方法,把二维输运问题的控制方程破开为对流问题和扩散问题。在任意四边形网格的离散下,用特征线法解对流问题,并采用伽辽金加权余量法,从而有效地减少插值所引起的数值阻尼,提高计算精度。用有限单元法和迭代计算格式解扩散问题。由于采用了辛普生积分公式,在每个时间步长都不需要求逆矩律,节省了计算时间。算例表明,本文数值模拟结果与精确的理论解吻合较好。