用激光计数放射性氪原子

原子阱痕量分析是一种基于激光的新方法,能够对数微升氪气中丰度低至10^-14的⁸⁵Kr和⁸¹Kr原子进行计数测量．介绍了在合肥建成的痕量放射性氪同位素测量系统,包括ATTA装置、水中溶解气提取和氪气分离提纯的样品处理设备．利用该系统对国内多处大气中⁸⁵Kr浓度的测量结果为1.3～1.6 Bq/m³,与文献中北半球大气本底值相吻合．对华南和华北的几个浅层和深层地下水样品进行了⁸⁵Kr和⁸¹Kr测年,示范了该系统的应用．     

原子阱,氪-81,撒哈拉的地下水

自放射性碳年龄测定法在1949年首次在实验中得以证实以来,长寿命的由宇宙射线产生的放射性核素痕量分析这一领域,无论是在分析技术还是在可利用的核素种类上,都取得了长足的进步.它所产生的影响已经渗透到广泛的科学和技术领域.文章介绍了一种新的痕量分析方法——原子阱痕量分析(atomtraptraceanalysis,ATTA).这种方法已经用于分析环境样品中的氪-81(半衰期为23万年;同位素含量约为1×10-12)和氪-85(半衰期为10.8年;同位素含量约为2×10-11)核素.氪-81主要由宇宙射线产生于大气层上部.它是理想的测定年代范围从5万年到100万年的水和冰样品的放射性核素.另一方面,氪-85是铀-235和钋-239的核裂变产物.对氪-85的同位素含量测定可以用于监督国际核公约的执行情况.在ATTA中,一个由激光光束和磁场共同产生的原子阱可以有选择地捕陷特定核素的原子.被囚禁的原子可以通过观察其荧光来进行探测.作为ATTA技术的第一次实际应用,一个国际合作小组确定了撒哈拉沙漠地区Nubian含水土层中地下水的寿命.利用ATTA对地下水寿命的成功测定,标志着放射性氪年龄测定法在地球科学中的广泛应用已经具备了可行性.     

提高在线同位素分离器分离效率的实验研究

描述了兰州在线同位素分离器的最新进展.利用FEBIAD离子源(受迫电子引起的弧放电源)开展了中、低能在线实验.在线同位素分离器的质量分辨本领达到500—700;Kr和Xe的离线分离总效率分别达到3.1%和4.5%;¹⁰⁴In,¹⁰⁴Ag,¹⁰⁴Gd的在线分离总效率达到1%—2%;Hg的在线分离总效率达到5%.成功地实现了对In,Ag,Gd,Hg,Rn,At等一系列元素的同位素分离.     

新丰中子同位素:186Hf

在兰州重离子加速器(FDUL)上,用60MeV/μ ¹⁸O离子轰击厚天然钨靶,通过多核子转移反应产生¹⁸⁶Hf. 使用放射化学分离技术从钨和反应产物混和物中分离出铪,并由HPGe探测器测量铪样品的活性.观测到了¹⁸⁶Hf的子体¹⁸⁶Ta的737,5keVY射线的增长、衰变行为,结果表明,本实验首次合成并鉴别了新丰中子同位素¹⁸⁶Hf,测得它的半衰期为(2.6±1.2)min.     

通过捕获原子认识全球的地下水

利用放射性氪同位素进行定年的梦想始于1969年,瑞士伯尔尼大学的Loosli和Oeschger首先发现了空气中存在81Kr.这种半衰期为23万年的核素由宇宙射线产生,在大气中分布十分均匀,同位素丰度约6×10-13.小部分氪气会溶于水或被冻在冰里,其中的81Kr由于衰变将简单地随着时间而减少,因此它对测定10万到100万年间水或冰的年代十分理想.而此年代范围超出了14C(半衰期为5700年)所能应用的范围.     

Kr同位素的形状共存

用Woods-Saxon推转壳模型对缺中子Kr同位素^70,72,74,76Kr进行了TRS计算. 结果表明, ^70,72Kr的基态为扁椭球, 同时存在着长椭球形变. ^74,76Kr的基态为长椭球, 并与扁椭球形变共存. 另外, 对⁷²Kr的推转计算结果表明, 随着ω的增大, 它的基态由扁椭球变成了长椭球.     

利用加速器质谱计测量10Be形成截面

加速器质谱计具有很高的灵敏度,能测量样品中含量极微的杂质同位素.¹⁰Be是一种宇宙成因核,在地学、环境科学和宇宙学等领域有广泛的应用,它为纯β衰变核,端点能量低且半衰期很长.采用测量剩余核放射性方法确定¹⁰Be形成截面很困难.本文报道利用加速器质谱计技术测量¹⁰Be反应总截面的方法和结果,并对应用前景作了讨论.     

Ba同位素的截面测量

用60MeV/u¹⁸O离子轰击天然铀靶,通过²³⁸U(¹⁸O,X)反应产生钡的放射性同位素.使用放射化学分离方法从被照射过的铀靶中分离出Ba,用HPGeγ射线探测器测量Ba样品的γ射线谱,经对所得时间累计谱的处理和分析得到了Ba同位素的产生截面,并观察到Ba同位素分布中的双峰分布现象.     

182Hf实验室参考标准的研制与加速器质谱测量

介绍了用于HI-13串列加速器质谱(AMS)测量的¹⁸²Hf实验室参考标准的研制. 通过¹⁸⁰Hf的连续热中子俘获产生¹⁸²Hf, 用热电离质谱对初始参考标准的同位素比进行了精确测量. 取一定量的初始参考标准, 加入已知量的稳定同位素(¹⁸⁰Hf)作为稀释剂, 通过逐级稀释的方法, 最终得到同位素比从10^－6—10^－10的一系列参考标准. 并对该系列参考标准进行了AMS测量, 结果显示¹⁸²Hf/¹⁸⁰Hf的测量值与标称值呈良好的线性关系, 说明该AMS系统可以准确测量同位素比在10^－6—10^－10范围内的样品.     

Z=105质量数为259的新同位素的合成和鉴别

利用120MeV的²²Ne离子束轰击²⁴¹Am靶,通过²⁴¹Am(²²Ne,4n)²⁵⁹Db反应合成了一个Z=105,质量数为259的新同位素.反应产物是用氦喷嘴技术和转动轮装置传输收集的.借助一系列金硅面垒探测器探测到了反应产物及其子核的α衰变.新同位素的原子序数Z和质量数A是借助该同位素和已知的²⁵⁵Lr核之间的遗传关系得到了确定的鉴别.新同位素²⁵⁹Db的测量半衰期为(0.51±0.16)s;它的α粒子能量为9.47MeV.由本实验导出的²⁵⁹Db的Qα值同理论预言结果能够较好地符合.     

Temperature dependence of the disorientation cross section σ 1 2/(1) in rubidium-rare gas collisions

The cross sections for the collision induced transfer²P_1/2m_j=1/2 → m_j = ?1/2 optically polarized Rb-vapour has been measured for collisions with He, Kr and Xe in the temperature range 30 °C → ～ 400 °C. The measurements were done by observing the degree of polarization in the fluorescent light emitted by the Rb-vapour. To avoid hyperfine structure effects, the nuclear spin was decoupled with a magnetic field. Calculations are presented in the temperature range 20 K → 1000 K using a semiclassical treatment and the Pascale Vandeplanque potentials. In the case of helium theoretical and experimental results are at variance, whereas for krypton and xenon satisfactory agreement is obtained. For the heavy rare gases the importance of trajectory effects, notably orbiting, becomes apparent.     

Hyperpolarized 83Kr MRI of lungs

Hyperpolarized (hp) ⁸³Kr (spin I = 9/2) is a promising gas-phase contrast agent that displays sensitivity to the surface chemistry, surface-to-volume ratio, and surface temperature of the surrounding environment. This proof-of-principle study demonstrates the feasibility of ex vivo hp ⁸³Kr magnetic resonance imaging (MRI) of lungs using natural abundance krypton gas (11.5% ⁸³Kr) and excised, but otherwise intact, rat lungs located within a custom designed ventilation chamber. Experiments comparing the ⁸³Kr MR signal intensity from lungs to that arising from a balloon with no internal structure inflated to the same volume with krypton gas mixture suggest that most of the observed signal originated from the alveoli and not merely the conducting airways. The ⁸³Kr longitudinal relaxation times in the rat lungs ranged from 0.7 to 3.7 s but were reproducible for a given lung. Although the source of these variations was not explored in this work, hp ⁸³Kr T₁ differences may ultimately lead to a novel form of MRI contrast in lungs. The currently obtained 1200-fold signal enhancement for hp ⁸³Kr at 9.4 T field strength is found to be 180 times below the theoretical upper limit.     

Hyperfine structure induced isotopic effects in krypton resonance ionization mass spectrometry

A time-of-flight mass spectrometer combined with a pulsed laser system has been used for determination of krypton isotope ratios of air samples containing 10⁶-10⁷ krypton atoms. A three-color, doubly resonant ionization scheme employing a 116.48 nm transition from the ground into the first 4s²4p⁵(²P°_1/2)5s J = 1 excitation level has been applied. The magnitude of a hyperfine splitting of this level has been estimated for ⁸¹Kr, ⁸³Kr and ⁸⁵Kr isotopes. Hyperfine structure induced isotopic effects have been investigated under different saturation conditions and found to be negligible when the first and the second transitions are strongly saturated.     

New stage of search for 2K(2ν) capture of 78Kr

A technique to search for 2K capture of ⁷⁸Kr with a large low-background proportional counter filled with a sample of krypton enriched in ⁷⁸Kr up to 99.8% at a pressure of 4.51 atm is described in this paper. The results of first measurements are presented. An analysis of data collected for 159 h yielded a new limit to the half-life of ⁷⁸Kr with regard to 2K capture (T _1/2 ≥ 1.5 × 10²¹ yr (90% C.L.)). The sensitivity of the facility to the process for one year of measurement was evaluated to be S = 1.0 × 10²² yr (90% C.L.). The text was submitted by the authors in English.     

Kinetic studies of Kr2F1 in electron-beam excited mixtures

Different mixtures containing atomic krypton and fluorinated compounds (Kr-NF₃, Kr-SF₆) and atomic krypton, argon and fluorinated compounds (Ar-Kr-NF₃, Ar-Kr-SF₆, Ar-Kr-F₂) were excited with intense e-beam pulses. The fluorescence produced by Kr₂F¹ was studied using time resolved spectroscopic techniques. Rate constants for the production and quenching of Kr₂F¹ were deduced from the measurements. In all the mixtures used, at krypton partial pressure higher than about 100 mbar, direct production from Kr¹₂ is considered as the most important formation channel of Kr₂F¹.     

Transition probabilities in Kr II and Kr III spectra

On the basis of the relative line intensity ratio (RLIR) method, transition probability values of the spontaneous emission (Einsteins A values) of 14 transitions in the singly (Kr II) and 7 transitions in doubly (Kr III) ionized krypton spectra have been obtained relatively to the reference A values related to the 435.548 nm Kr II and 324.569 nm Kr III, the most intensive transitions in the Kr II and Kr III spectra. Our Kr III transition probability values are the first data obtained experimentally using the RLIR method. A linear, low-pressure, pulsed arc operated in krypton discharge was used as an optically thin plasma source at a 17 000 K electron temperature and 1.65 x 10²³ m^-3 electron density. Our experimental relative A values are compared with previous experimental and theoretical data.Received: 16 June 2003, Published online: 16 September 2003PACS: 52.70.Kz Optical (ultraviolet, visible, infrared) measurements - 32.70.Cs Oscillator strengths, lifetimes, transition moments - 32.70.Fw Absolute and relative intensities     

High-resolution diode-laser spectroscopy on a fast beam of metastable atoms for detecting very rare krypton isotopes

A fast krypton ion beam with an energy of 10 keV is transferred through a mass filter and neutralized in a Rb- (or Cs-) vapour charge exchange cell. The emerging beam of metastable Kr atoms of a selected Kr isotope is collinearly irradiated with a tunable cw GaAlAs diode laser at 811 nm. The spectrum of the 1s ₅–2p ₉ transition covers 10 GHz and consists of one line each for the five stable isotopes of Kr with even mass number and 15 hyperfine structure lines of⁸³Kr. The individual lines are recorded by detecting the fluorescence signal perpendicular to the beams. Photons are guided to the PM tube by diffuse reflection from a high reflectance thermoplastics light collector. Up to 20 photons per atom are emitted by cycling between the two states during the flight time of 1.2 µs through the detector. The observed absorption linewidth of 100 MHz is a combination of laser linewidth and energy uncertainty in the fast atomic beam. In order to achieve maximum isotope selectivity the beam energy has to be adjusted in such a way that the Doppler shifted lines of all isotopes form an optimum pattern for detecting the very rare isotopes⁸¹Kr and⁸⁵Kr in natural environmental samples.     

Wide-band exciplex halogen lamps operating on inert gas mixtures with chlorine and Freon-12 molecules

The results of analysis of the spectral characteristics of short-wave radiation sources operating on transitions in argon, krypton, and xenon monohalogenides, as well as chlorine molecules, excited by a longitudinal low-pressure glow discharge are considered. Radiation emitted by ArCl^*, KrCl^*, XeCl^*, Cl ₂ ^** , and Cl ₂ ^* molecules in a spectral range of 170–350 nm is optimized using complex working mixtures of Ar-Kr-(Xe)-Cl₂ in the lamps. The average radiation power of the lamps ranges from 1 to 10 W for an efficiency of ≤25%. Optimization of wide-band lamps on transitions in chlorine molecules and the decay products of Freon-12 molecules (CF₂Cl₂) is carried out on mixtures of helium with chlorine and Freon-12 molecules. This makes it possible to develop lamps emitting in a spectral range of 140–270 nm and containing no costly inert gases (Xe or Kr) in their working mixtures. Exciplex halogen lamps with a wide-band emission spectrum in the VUV-UV range can be used in spectrometers as radiation sources in experiments with absorption and in high-energy chemistry, ecology, and medicine.