加速器质谱测量

分类号密级UDC学位论文加速器质谱测量32Si的方法研究龚杰指导教师姓名：何明（研究员、博士、中国原子能科学研究院）姜山（副所长、研究员、硕士、中国原子能科学研究院）申请学位级别硕士专业名称粒子与原子核物理论文提交日期2010年月日论文答辩日期2010年月日学位授予单位和日期中国原子能科学研究院2010年7月答辩委员会主席评阅人二○一○年六月中国原子能科学研究院硕士研究生学位论文加速器质谱测量32Si的方法研究龚杰指导老师：何明研究员姜山研究员专业：粒子与原子核物理研究方向：加速器质谱学学位级别：硕士二○一○年六月Study on the Measurement of Silicon-32 with Accelerator Mass SpectrometryByGong JieP.O.Box:275(50) Beijing 102413Email: gongjie001@Supervisor: Prof. He MingProf. Jiang Shan China Institute of Atomic EnergyJune 2010摘要硅（Si）是一种类金属元素，在地壳中的含量仅次于氧，广泛存在于各种岩石、砂砾、尘土之中，是构成地壳的基础元素之一。

自然界中的32Si是通过初级或次级宇宙射线与空气中的Ar通过散裂反应以很小的反应截面产生，是硅的23种同位素中唯一的长寿命放射性同位素。

32Si的半衰期约为140a，介于44Ti(49a)和14C(5730a)之间，在100~1000a尺度的测年和硅的生物地球化学循环研究中具有极其重要的应用价值。

利用32Si可以开展地下水年龄的测量、冰川的年龄和流动速度的测量、地下水混合与流动过程及海水混合作用的研究、大气环流及海洋中Si的生物化学循环以及海洋硅质沉积层的年龄测量、测定年轻沉积物的沉积速率、估算陨石在宇宙射线中的暴露时间等等多方面的工作。

但是32Si的大规模应用受到诸多条件的限制：1.由于32Si的天然产生率非常低，在自然界的同位素丰度比非常低（32Si/Si <10-14），因此大量的32Si样品难以获得；2.以前的32Si的测量多采用衰变计数法，该方法需要大量的样品，且测量灵敏度低，周期长，样品制备烦琐；3.目前32Si的半衰期值尚无公认的精确值，目前公布的32Si的半衰期存在较大偏差，这在很大程度上阻碍了32Si的应用。

加速器质谱分析技术（AMS: Accelerator Mass Spectrometry）具有样品用量少、测量时间快、测量灵敏度高等优点。

利用AMS技术测量32Si可克服以上提到的应用限制因素：1.AMS测量灵敏度高，需要的样品量少，仅需SiO2样品1mg 即可完成32Si的测量,可直接对自然样品进行测量；2样品制备简便，测量时间短(1小时)；3.AMS高灵敏测量32Si的方法可为32Si半衰期的精确测量提供可靠的技术支撑。

因此AMS方法是测量32Si最具潜力的方法。

国外目前已对AMS测量32Si的方法进行过一些研究，建立了多种测量方法。

中国原子能科学研究院拥有高能串列加速器，大型Q3D磁谱仪等诸多便利条件，非常有利于进行32Si的测量。

为此本工作旨在建立基于中国原子能院AMS 系统的32Si测量方法，为其广泛应用研究奠定基础。

为了建立32Si的AMS测量研究，本工作主要开展了以下两方面的研究工作：1.32Si实验室标准样品的研制：要开展32Si的测量，首先需要制备32Si的实验室标准和适用于AMS测量的化学流程，为此开展了以下研究工作：（1）32Si生成反应的计算设计：调研各种可能生成32Si的反应道，通过各种途径查找其反应截面数据，对各种反应道进行综合分析，结合国内的实验条件，最终确定了利用原子能院游泳池堆反应堆辐照31P(n,γ)32P(n,p)32Si生产32Si的方案；根据反应堆的中子通量及中子热快比计算了32Si的产额随照射时间的变化，确定了样品的照射时间和需要的冷却时间。

（2）32Si的生产：根据前面的计算设计，我们以Mg2P2O7为样品形式将31P 在游泳池堆照射了250小时，理论计算得到照射后样品中32Si与31P原子个数比可达1.3×10-12，（3）32Si样品的制备：建立了从辐照样品中的分离32Si的化学流程，在样品经过冷却后，制备出32Si/Si在10-12的实验室标准样品。

在此工作中通过对放化分离流程和稀释流程的改进，建立了自己的简便易操作的样品处理流程。

2. AMS测量32Si方法的建立：32Si测量最主要的干扰是同量异位素32S的非常强烈的干扰，因此要建立高灵敏的32Si测量方法最主要的工作就是如何排除和鉴别32S，为此开展了以下工作：（1）32Si引出形式的选择：通过系统实验研究，比较各种不同样品形式和不同引出形式下的束流情况，最终确定了测量32Si所需最佳的样品形式SiO2+Fe (1:5) 和离子引出形式Si-，提高了Si的引出效率，有利于提高灵敏度。

（2）32S排除方法建立：建立了独特且非常有效的排除同量异位素干扰的方法。

针对Q3D磁谱仪高动量分辨的特点，选用厚度均匀性非常好的Si3N4膜，利用32Si和32S过膜后的剩余能量的不同，通过Q3D进行鉴别，最终建立了 E-Q3D排除32S干扰的方法，极大地压低了32S的干扰，对32S的压低因子达106。

（3）32Si测量与鉴别：通过利用自行设计建立的多阳极电离室成功实现了对32Si和32S的进一步鉴别，对32S的压底因子达到106。

在此过程建立了精确寻找32Si焦面位置的方法，使得在没有高含量样品标定焦面位置的情况下可以准确地对低含量样品进行测量。

实验成功地实现了32Si实验室标准样品的测定，对空白样品的测量结果表明测量灵敏度(32Si/Si)好于1⨯10-14，达到了世界先进水平。

该方法具有极强的排除32S的干扰的能力，其对32S的压低因子达到了世界最好水平。

本工作在以下方面有创新：1.样品的化学处理流程方面，建立了新的辐照样品化学分离流程和高含量样品稀释流程，并加入去S步骤，适合于方便快速地制备AMS测量用32Si样品。

2.国际上首次利用Q3D大型磁谱仪配合多阳极电离室进行32Si和32S的排除与鉴别，成功建立了高灵敏的∆E-Q3D方法。

总之，本论文在32Si的生产、32Si样品的制备、AMS测量32Si的方法方面建立了一整套的方法，并且在多方面有所创新，为32Si的应用研究提供了方法，为下一步的32Si半衰期的精确测定奠定了基础。

关键词：32Si AMS 磁谱仪同量异位素AbstractSilicon (Si) is a kind of metalloid elements, which is widely exists in all sorts of rock, gravel and dust, and also is one of the basic elements of the Earth crust which is just behind the oxygen in content. 32Si, generated in small amount through primary or secondary cosmic-ray spallation with Ar in the atmosphere, is the only long lived radio isotope in the 23 isotopes of silicon. The half-life of 32Si is about 140a, which is between the 44Ti (49a) and 14C (5730a). It has very important application value in geochronometry in the range of 100-1000 years and the research of global biogeochemical silica cycle.32Si can be used in many researches, such as mearsurement of the age of groundwater and glacier, research on the role of mixing and flow process of groundwater and seawater, research on the biochemical cycle of Si in the atmospheric circulation, mearsurement of the age of siliceous sediments in ocean, determination of the deposition rate of shallow sendments, estimate the cosmic rays exposure age of meteorite and so on.But the large-scale application of 32Si is limited by many conditions: 1. Due to the low natural production rate of 32Si and the low natural isotope abundance ratio (32Si/Si (10-14), it is difficult to get plenty of 32Si samples. 2. The radiometry method which was mostly used to measure 32Si before needs a large mount of samples. Except that, it has a low sensitivity, a long measurement time and a complex sample preparation procedure. 3. There is still not an accurate half-life value of 32Si now. The values reported now vary in a large range. It’s a large hamper to the 32Si applications.The accelerator mass spectrometry technology is characterized as a little sample usage, a short measurement time and high sensitivity. Using the AMS technology to measure 32Si can overcome the limiting factors mentioned above: 1.AMS has high sensitivity and needs as less as 1mg sample. Thus it can measure the natural samples directly. 2. It has a simple sample preparation procedure and a short measurement time(1h). 3. The ultra-sensitive measurement method of 32Si by AMS can provide reliable technical support for the precise measuring of 32Si’s half-life. So AMS method is the most promising method in the measurement of 32Si. Up to now, foreign countries have done some researches on measuring 32Si by AMS and established many methods.There are many advantages for making research on the measurement of 32Si in China Institute of Atomic Energy, such as the high-energy tandem accelerator and the large Q3D magnetic mass spectrometry. So this work is to establish the method of measuring 32Si based on the AMS system of Chinese institute Atomic Energy and to lay the foundation for the large-scale application of 32Si.In order to establish the method for the measurement of 32Si by AMS, this work includes two main aspects:The produce of 32Si lab-standard sample:In order to measure 32Si, it’s the first step to produce our lab-standard sample and to establish suitable chemical procedures for AMS sample preparation. We have done the followwing works for this goal:(1) The calculation and design of the reactions for 32Si production: We made researches on all nuclear reaction channels which can produce32Si, searched the cross-section data in all ways, analyzed all the nuclear reaction channels, combined with the experiment conditions in our country and finally decided to make use of the swimming pool reactor to radiate 31P(n,γ)32P(n,p)32Si to produce 32Si; We calculated the relationship between 32Si atomic number and irradiation time according to the neutron flux and the ratio of thermal and fast neutron fluxes, and than fixed the need irradiation time and cooling time.(2) The 32Si production: According with the calculation results and design before, we irradiated 31P in the form of Mg2P2O7 for 250 hours in the swimming pool reactor. The atomic number ratio between 32Si and 31P can reach 1.3×10-12 in theory.(3) The preparation of 32Si sample: We established the chemical procedure for the separating 32Si from the irradiated sample, When the sample was cooled over, we produced our lab-standard sample of 32Si/Si ~ 10-12. Through the improvement of the separation procedure and dilution procedure, we established simple and easy-to-do procedures of sample chemical treatment.2. The interference in 32Si measurement mainly comes from the strong interference of 32S. It’s the most important work to suppress and distinguish 32S in the establishment of ultra-sensitive measurement for 32Si by AMS. We have done the following works to reach this aim:(1) The choosen of negative ion form: We made sure that the best form of sample was SiO2+Fe (1:5) and ion form was Si- needed in measuring 32Si, after compared beam currents among many kinds of samples and different ion formsthrough systemic ion source experiments.(2) The establishment of suppression method for 32S: We established a unique method to suppress the interference of isobaric. Because of the high momentum resolution of Q3D, we distinguished 32Si and 32S though Q3D by making use of the different residual energies between them after passing though a Si3N4 foil which has a good uniformity. Finally we established the method of suppressing 32S interference by ∆E-Q3D, which can greatly supress the interference of 32S. The suppression factor can reach 106.(3) The measurememt and distinguishment of 32Si: A multi-anode gas-filled ionization chamber which was signed and produced by ourselves was used to get a further identification on 32Si and 32S。