Preparation of a Pu standard source: I. Isotope dilution alpha-ray spectrometry used for standardization

A ²³⁸Pu source was prepared by electrodeposition method. For the standardization of the source, isotope dilution alpha-ray spectrometry (IDAS) was investigated: The content of ²³⁸Pu in a sample solution was determined both by IDAS and by isotope dilution mass spectrometry (IDMS) using a standard reference material of ²³⁹Pu as a spike. The estimated uncertainty in the determination of ²³⁸Pu is 0.13% for IDAS, and the factor of IDAS/IDMS is calculated to be 1.0000±0.0015.     

Functional equation for the fading correction of imaging plates

Several experiments were performed to investigate the effects of temperature on the fading of the commercial imaging plate, IP (BAS-UR). The fading characteristics were measured under controlled temperatures between 0°C and 60°C after irradiation with ²³⁸U alpha-ray and ⁶⁰Co gamma-ray sources. Applying Arrhenius' equation to the experimental results, we calculated a universal functional equation that includes two variables: elapsed time (t) and temperature (K). The photo-stimulated luminescence (PSL) calculated by this equation showed good agreement with the results of our experimental ones between 0°C and 40°C. From this equation, the activation energies of the fading are estimated to be about 0.9 eV for both ²³⁸U alpha-ray and ⁶⁰Co gamma-ray sources.     

Artificial neural network modelling of uncertainty in gamma-ray spectrometry

An artificial neural network (ANN) model for the prediction of measuring uncertainties in gamma-ray spectrometry was developed and optimized. A three-layer feed-forward ANN with back-propagation learning algorithm was used to model uncertainties of measurement of activity levels of eight radionuclides (²²⁶Ra, ²³⁸U, ²³⁵U, ⁴⁰K, ²³²Th, ¹³⁴Cs, ¹³⁷Cs and ⁷Be) in soil samples as a function of measurement time. It was shown that the neural network provides useful data even from small experimental databases. The performance of the optimized neural network was found to be very good, with correlation coefficients (R²) between measured and predicted uncertainties ranging from 0.9050 to 0.9915. The correlation coefficients did not significantly deteriorate when the network was tested on samples with greatly different uranium-to-thorium (²³⁸U/²³²Th) ratios. The differences between measured and predicted uncertainties were not influenced by the absolute values of uncertainties of measured radionuclide activities. Once the ANN is trained, it could be employed in analyzing soil samples regardless of the ²³⁸U/²³²Th ratio. It was concluded that a considerable saving in time could be obtained using the trained neural network model for predicting the measurement times needed to attain the desired statistical accuracy.     

First application of calorimetric low-temperature detectors in accelerator mass spectrometry

For the first time, calorimetric low-temperature detectors were applied in accelerator mass spectrometry, a well-known method for determination of very small isotope ratios with high sensitivity. The aim of the experiment was to determine with high accuracy the isotope ratio of ²³⁶U/²³⁸U for several samples of natural uranium, ²³⁶U being known as a sensitive monitor for neutron flux. Measurements were performed at the VERA tandem accelerator at Vienna, Austria. The detectors consist of sapphire absorbers and superconducting transition edge thermometers operated at T≈ 1.5 K. The relative energy resolution obtained for 17.39 MeV ²³⁸U is ΔE/E=4–9×10⁻³, depending on the experimental conditions. This performance enabled to substantially reduce background from neighbouring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of ²³⁶U/²³⁸U=6.5×10⁻¹² could be obtained, representing the smallest ²³⁶U/²³⁸U ratio measured until now.     

电沉积228Th面源的特性

采用电沉积法制备了一枚228Th面源。通过α能谱分析,观察了²²⁸Th面源中²²⁴Ra与²²⁸Th达到放射性平衡的过程,确定该源的电沉积回收率为54%。将该²²⁸Th面源置于钍射气室内,测量钍射气的体积活度,以确定其钍射气析出率。结果显示,228Th面源钍射气析出率具有良好的稳定性、复现性,且受温湿度影响较小。     

A cryosystem for the detection of alpha and spontaneous-fission activities in volatile species

A system for the on-line detection of very volatile and spontaneous-fission activities produced in nuclear reactions has been developed. The reaction products are continuously transported by a gas-jet from the target area to the detector unit. Volatile species are condensed on a solar cell kept at low temperature ( 40 K) by a cryogenic pump. An annular surface barrier detector is placed in front of the solar cell. Fission fragments and their energies are recorded by coincidence signals from both detectors, whereas -particle spectra are measured with the surface barrier detector. The background is less than one coincident event per day, thus allowing measurements at low production rates. A half-life region down to a few seconds can be covered as is shown with short-lived radon isotopes. The system was used in searches for very volatile superheavy elements in the reactions ⁴⁸Ca+²⁴⁸Cm and ²³⁸U+²³⁸U.     

富集10B的Ce:Li6Lu(10BO3)3晶体生长及其闪烁性能

为了提高中子探测效率, 以富集¹⁰B的H₃¹⁰BO₃为原料, 通过提拉法生长了富集¹⁰B的Ce:Li₆Lu(¹⁰BO₃)₃晶体。X射线激发发射光谱测试表明: 其发光峰位于360~480 nm, 属于Ce³⁺离子典型的5d - 4f跃迁发光, 其闪烁发光效率为BGO晶体的3.9倍。在350 nm紫外光和¹³⁷Cs所发出的662 keV的γ射线激发下测得的衰减时间分别为21.0 ns 和31.7 ns, 在¹³⁷Cs辐射源激发下所测得的相对光输出是CsI(Tl)晶体的20%, 能量分辨率为9.7%。在慢化²⁵²Cf中子源激发下可以观测到明显的中子全能峰, 其能量分辨率为33%。上述研究结果表明, Ce:Li₆Lu(¹⁰BO₃)₃晶体具有较高的闪烁效率、快的衰减时间和良好的中子探测效率, 是一种具有应用前景的中子探测用闪烁晶体。     

Concepts of UCN sources for the FRM-II

Three concepts for sources of ultra-cold neutrons (UCN) for the reactor FRM-II at Garching near Munich are studied: one, Mini-D₂, is a source with 170 cm³ of solid deuterium in the beam tube SR4 and the second one a large solid-deuterium source (volume about 30 dm³), mounted in the beam tube SR5 as an advanced cold source with a number of neutron guides. The third one, Mark 3000, uses superfluid ⁴He at a cold-neutron guide. A UCN density of up to 7×10⁴ cm⁻³ may possibly be achieved in the storage volumes of Mini-D₂ yielding more than 10⁹ UCN for extraction to an attached experimental setup. The usable UCN flux at the periphery of the large deuterium source is predicted to be 2×10⁷ cm⁻² s⁻¹. Mark 3000, finally, is expected to yield a UCN density of about 10⁵ cm⁻³.     

Optical properties of rare earth ion (Nd, Er and Tb)-doped alumina films prepared by the sol–gel method

Rare earth ion (Nd³⁺, Er³⁺ and Tb³⁺)-doped alumina films were prepared by the sol–gel method using aqueous alumina sol. The effects of dopant concentration and treatment temperature on the optical properties, absorption and emission were examined for the doped films. Alumina films prepared by this method gave a high dopant concentration (0–15 mol% per alumina). Significant concentration quenching did not occur in this concentration range. The emissions from ⁵D₃ and ⁵D₄ of Tb³⁺-doped film reflected sensitively a matrix environment around Tb³⁺ ions. Er³⁺- and Nd³⁺-doped alumina films resonantly excited by cw Ti–sapphire laser (800 nm) showed upconversion emission at room temperature. The former gave 548 nm (⁴S_3/2→⁴I_15/2) and 640 nm (⁴F_9/2→⁴I_11/2) signals, and the latter 640 nm (⁴G_7/2→⁴I_11/2), which were dependent on alumina.     

A coincidence system for radionuclide standardization using surface barrier detectors

A system for the standardization of alpha-gamma or electron-X radionuclide emitters is described. The system consists of one or two surface barrier detectors for alpha or electron detection which are coupled to thin-window NaI(Tl) crystals suitable for low-energy X- or gamma-ray detection. The performance of the system has been verified by the standardization of ²⁴¹Am, ¹³⁷Cs and ¹⁰⁹Cd solutions. The activity has been obtained using the extrapolation method applied to the 4π -γ and 2π e-X coincidence techniques. The surface barrier detection efficiency was varied by placing absorbers over the radioactive source or by changing the source-to-detector distance. The results were compared to those obtained using conventional absolute systems based on gas-flow and pressurized 4π proportional counters, or using radioactive solutions standardized in international comparisons sponsored by the Bureau International des Poids et Mesures, France. The expected and measured activities agreed within the experimental uncertainties, which were: 0.2% for ²⁴¹Am, 0.7% for ¹³⁷Cs and 0.6% for ¹⁰⁹Cd.     

Spot F positron source electro-deposited on a graphite rod

2.2 GBq of ¹⁸F (half-life 110 min) was effectively deposited on a graphite electrode of diameter 3 or 5 mm. The fraction of the electro-deposited ¹⁸F was as high as 97% of the total ¹⁸F produced in the solution. The fraction of the positrons emitted out of the electrode was 44% of the total β⁺. Thus, as much as 36% of the β⁺ from the produced ¹⁸F is available for the source of slow positron beams.     

A fission-fragment-sensitive target for X-ray spectroscopy in neutron-induced fission

A fission-fragment-sensitive detector built for low-energy photon spectroscopy applications at the WNR “white” neutron source at Los Alamos is described. The detector consists of eight layers of thin photovoltaic cells, onto which 1 mg/cm² of pure ²³⁸U is deposited. The detector serves as an active target to select fission events from background and other reaction channels. The fairly small thickness of the detector with respect to transmission of 20–50 keV photons permits the measurement of prompt fission-fragment X-rays. Results with the GEANIE photon spectrometer are presented.     

Bi/Tm共掺TiO2-BaO-SiO2-Ga2O3玻璃光谱特性与能量转换机理研究

用高温熔融法制备了Bi、Tm、Bi/Tm掺杂TiO₂-BaO-SiO₂-Ga₂O₃玻璃系统。在808 nm激光激发下, 与Tm单掺杂玻璃相比, Bi/Tm共掺玻璃中Tm³⁺的³H₄→³F₄跃迁荧光(~1485 nm)得到了显著的增强, 而Tm³⁺的³F₄→³H₆跃迁荧光(~1810 nm)减弱。在980 nm激光激发下, Tm单掺玻璃中没有观察到Tm离子的特征发光, 而在Bi/Tm共掺玻璃中观察到Tm³⁺的³F₄→³H₆跃迁荧光(~1810 nm)。这是由于在808和980 nm激光二极管(LD)各自激发下, Bi/Tm共掺玻璃中活性Bi离子的近红外发光能量传递给Tm³⁺, 分别产生³F₄→³H₄与³H₆→³H₅跃迁所致。采用Inokuti-Hirayama模型, 分析了该玻璃体系中Bi→Tm的能量传递机理。结果表明, Bi→Tm的能量传递属于电偶极–偶极相互作用。     

Ni2+掺杂尖晶石相透明微晶玻璃制备及光学性能

实验制备了Ni²⁺掺杂的ZnO-MgO-Al₂O₃-SiO₂(ZMAS)体系的透明微晶玻璃, 研究了微晶玻璃的超宽带发光现象。热分析结果表明样品的玻璃化转变温度(T_g)和析晶峰温度(T_c)分别为754℃和948℃。采用X射线粉末衍射分析了两种热处理制度对玻璃的晶体形核、晶体生长及物相变化的影响, 结果表明: 采用阶梯温度热处理制度可以得到Ni²⁺掺杂的尖晶石相透明微晶玻璃。紫外-可见吸收光谱和荧光光谱测试表明制备的Ni²⁺掺杂ZMAS微晶玻璃中Ni²⁺以四配位和六配位共同存在于尖晶石相中, 红外荧光中心位于1324 nm的样品荧光半高宽达490 nm。样品的超宽带荧光主要是由于微晶玻璃中六配位Ni²⁺在ZnAl₂O₄和MgAl₂O₄形成的尖晶石相固溶体晶体场中的³T_2g(³F)→³A_2g(³F) 能级跃迁。实验结果表明, 制备的微晶玻璃在超宽带光纤放大器等光子器件中具有潜在的应用前景。     

Thermal neutron cross-section and resonance integral for Dy(n,γ)Dy reaction

The thermal neutron cross-section (σ₀) and the resonance integral (I₀) of the reaction ¹⁶⁴Dy(n,γ)¹⁶⁵Dy were measured by the activation method, using ⁵⁵Mn(n,γ)⁵⁵Mn monitor reaction as a single comparator. The diluted MnO₂ and Dy₂O₃ powder samples within and without a cylindrical Cd shield case were irradiated in an isotropic neutron field obtained from the ²⁴¹Am–Be neutron sources, moderated with paraffin wax. The γ-ray spectra from the irradiated samples were measured by high-resolution γ-ray spectrometry with a calibrated n-type Ge detector. The necessary correction factors for γ-ray attenuation, thermal neutron and resonance neutron self-shielding effects and epithermal neutron spectrum shape factor () were taken into account in the determinations. The thermal neutron cross-section for ¹⁶⁴Dy(n,γ)¹⁶⁵Dy reaction studied has been determined to be 2672±104 b at 0.025 eV. This result has been obtained relative to the reference thermal neutron cross-section value of 13.3±0.1 b for the ⁵⁵Mn(n,γ)⁵⁶Mn reaction. For the thermal neutron cross-section, most of the experimental data and evaluated one in ENDF/B-VI, in general, are in good agreement with the present result. The resonance integral has also been measured relative to the reference value of 14.0±0.3 b for the ⁵⁵Mn(n,γ)⁵⁶Mn monitor reaction using a 1/E¹⁺ epithermal neutron spectrum of the ²⁴¹Am–Be neutron source. By defining Cd cut-off energy 0.55 eV, the resonance integral obtained was 527±89 b. The existing experimental and evaluated data for the resonance integral are distributed from 335 to 820 b. The present resonance integral value agrees with some previously reported values, 520 b by Holden, 505 b by Simonits et al. and 575±100 b by Heft, within the limits of error.     

Eu掺杂KNN陶瓷的制备及可调性发光研究

稀土离子掺杂铁电陶瓷是一类新型光致变色材料, 在光开关、光信息存储等领域具有潜在应用价值。本研究采用水热法制备了(K_0.5 Na_0.5)_1-xEu_xNbO₃(KNN:xEu)前驱体粉体, 随后利用高温烧结得到对应陶瓷样品。在465 nm激发下, 观察到615 nm处有强的红色发光, 对应于Eu ³⁺的 ⁵D₀→ ⁷F₂跃迁。通过紫外光照射, KNN:Eu陶瓷从乳白色变为深灰色。随后经过200 ℃加热10 min, 着色陶瓷又变回到初始颜色, 显示出良好的光致变色行为。紫外照射和反复加热循环可以有效调控该陶瓷的发光强度。且经过多次循环之后, 发光强度没有明显衰减。在紫外光照射下, KNN:0.06Eu陶瓷发光强度的可调比(ΔR_t)高达83.9%, 说明发光具有良好的可调性。进而结合发光中心和色心之间的能量转移, 对KNN:Eu陶瓷的光致变色和发光机理进行了解释。     

CeO2修饰Mn-Fe-O复合材料及其NH3-SCR脱硝催化性能

氨选择性催化还原(NH3-SCR)技术需要进一步研发在相对较低温度(<300℃)下具有良好催化活性、高稳定性及环境友好的脱硝催化材料。本工作采用草酸共沉淀法制备Mn-Fe-O催化材料,并对其进行不同含量CeO2修饰,用于低温NH3-SCR脱硝催化反应。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、氮气吸附-脱附、X射线光电子能谱(XPS)、程序升温还原或脱附(H2-TPR、NH3-TPD)等手段对催化剂进行了表征。催化结果表明,在相同反应条件下适量CeO2修饰后的Mn-Fe-O样品比纯Mn-Fe-O表现出更优异的NH3-SCR脱硝催化性能,在80℃时NO转化率在95%以上,且具有较高的N2选择性。CeO2修饰提高了Mn-Fe-O氧化物表面的Fe^3+、Mn^3+和Mn4+含量及表面酸性位点数量,从而有助于NH3的吸附及催化反应的进行,并且Fe^2+/Fe^3+、Mn^2+/Mn^3+/Mn^4+以及Ce^3+/Ce^4+电子对之间的相互氧化还原反应提高了催化剂的氧化还原能力及稳定性。     

A large solid-angle array for heavy ions from APEX

A large solid-angle array of Low Pressure Multi-Wire Proportional Counters (LPMWPCs) has been constructed as part of the ATLAS Positron Experiment (APEX). Eight three-element trapezoidal counters span 360° in φ and 20–68° in laboratory θ. Angle sensitivity in θ is provided by a transmission-line delay cathode, while the 24-fold segmentation provides angle sensitivity in φ. Details are given on the design of the counters and associated hardware. Performance of the counters is described, based on tests with scattered heavy ions ranging from ⁵⁸Ni to ²³⁸U.     

Density Determination of a Small Isotopically Enriched Silicon Single Crystal

For a new determination of the Avogadro constant using small crystals of isotopically enriched silicon, the density of a sample was determined at Physikalisch-Technische Bundesanstalt. The sample has a mass of 58 g and a ²⁸Si enrichment of about 99.98%. Its density was compared by the pressure-of-flotation method to the density of a large hollow transfer standard that was manufactured from natural silicon, to have the same density. The flotation measurement yielded a relative density difference of 0.64(10) 10^-6. The density of the transfer standard was then measured by hydrostatic weighing, which is traceable to a primary density standard. Thus, the density of the small ²⁸Si sample was determined to be 2320.08031(40) kg/m ³, i.e., with a relative standard uncertainty of 0.17 10^-6     

Yb3+-Tm3+共掺BiOBr纳米晶的近红外上转换发光特性研究

采用水热法合成了Yb³⁺-Tm³⁺共掺BiOBr纳米晶, 研究了其上转换发光性能。在980 nm光激发下, 样品中Tm³⁺离子实现了³H₄→³H₆、¹G₄→³F₄和¹G₄→³H₆跃迁, 进而发出强烈的近红外光(801 nm)和较弱的红光(655 nm)与蓝光(485 nm)。探讨了样品的上转换发光机理, 上转换发光强度与激发功率的关系表明在980 nm激发下Tm³⁺的蓝光和红光发射为三光子过程, 而近红外发光为双光子过程。随着Yb³⁺浓度增加, 近红外发光显著增强, 近红外光与蓝光(I_{801 nm}/I_{485 nm})的发光强度比高达71.4。研究结果表明, Yb³⁺-Tm³⁺共掺BiOBr纳米晶在生物荧光标记领域具有潜在的应用前景。