19.9
核嬗变
核嬗变是将一种核素转换为另一种核素。 这可能是由核的放射性衰减或核与另一个粒子的反应引起的。 第一个人造核是 1919 年在埃内斯特 · 鲁瑟福德实验室由嬗变反应产生的，这种反应是用其他核或中子轰炸一种类型的核。 鲁瑟福德用天然放射性同位素辐射的高速 α 粒子轰炸了氮 -14 原子，并观察到质子被弹…
核嬗变是一种元素 转化为另一种元素的过程，这可能通过放射性衰变、核聚变 和核裂变来实现。此外，欧内斯特·卢瑟福 证明了 氮-14 被快速移动的阿尔法粒子撞击时，会产生一个质子和一种不同的核素，几年后，帕特里克·布莱克特 将这种核素确定为氧-17。这个过程的简明符号按顺序 列出了靶核、被轰击和喷射的粒子 以及产物核。中子和阿尔法粒子是嬗变过程中 常见的轰击粒子。原子序数大于 92 的元素 称为超铀元素。这些元素是嬗变实验的常见目标，因为除了镎和钚之外，它们都是完全合成的，镎和钚也是在铀衰变链中 自然产生的。例如，镎-239 是通过 用裂变中子轰击铀-238 在专用核反应堆中 产生的。中子是电中性的，不会遇到来自原子核的静电排斥，因此裂变速度足以实现这种嬗变。放射性镎-239 随后 衰变为钚-239。在进一步实验中，用高速阿尔法粒子撞击钚-239，产生了原子序数为 96 的锔。与中子不同，阿尔法粒子必须 克服带正电的靶核 施加的静电排斥，因此需要更大的动能。值得注意的是，在卢瑟福和布莱克特实验中，像钚-239 这样的大原子核 产生的静电斥力要比像氮-14 这样的小原子核 产生的静电斥力大。粒子加速器，包括 直线加速器和回旋加速器，给带电核粒子提供了所需的高速。多级直线加速器 由一系列长度递增、极性交替的电子管 组成。振荡电势迅速改变极性，使带电粒子被每个管 交替吸引和排斥。随着管道变长，粒子加速，最终达到超过 光速 90%的速度。在回旋加速器中，交流电压 反而使粒子沿螺旋路径加速。粒子加速器甚至可以 用其他相对较大的原子核轰击原子核，例如用一束锌-70 轰击铅-208。产物超铀元素鎶-277 通过 主要衰变链生成 13 种 超铀元素，最终 生成铋-209。
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Q1: What is nuclear transmutation and how does it occur?
Nuclear transmutation is the conversion of one element into another through radioactive decay, nuclear fusion, or nuclear fission. Ernest Rutherford first demonstrated this in 1919 by bombarding nitrogen-14 with alpha particles, producing a proton and oxygen-17. Transmutation can occur naturally through radioactive decay or artificially when nuclei are bombarded with particles like neutrons or alpha particles.
Q2: Why do neutrons require less energy than alpha particles for transmutation?
Neutrons are electrically neutral and encounter no electrostatic repulsion from positively charged target nuclei, so fission speeds are sufficient for transmutation. Alpha particles, being positively charged, must overcome electrostatic repulsion and thus require greater kinetic energy. This repulsion is even stronger with larger nuclei like plutonium-239 than with smaller nuclei like nitrogen-14.
Q3: What are transuranium elements and how are they created?
Transuranium elements are synthetic elements with atomic numbers greater than 92 (uranium), except neptunium and plutonium, which also occur naturally in uranium decay chains. These elements are created through transmutation reactions in specialized nuclear reactors. For example, neptunium-239 is generated by bombarding uranium-238 with fission neutrons, which then decays into plutonium-239.
Q4: How do particle accelerators enable nuclear transmutation reactions?
Particle accelerators use magnetic and electric fields to increase the speeds of nuclear particles to levels necessary for transmutation. Linear accelerators use alternating electrical potential to accelerate particles through tubes of increasing length, reaching speeds exceeding 90% of light speed. Cyclotrons accelerate particles in a spiral path using alternating voltage, allowing bombardment of nuclei with high-energy particles.
Q5: What is the condensed notation used to represent transmutation reactions?
Condensed notation for transmutation lists, in order, the target nucleus, the bombarding particle, the ejected particle, and the product nucleus. This format provides a concise way to represent the entire reaction. For example, Rutherford's experiment with nitrogen-14 bombarded by alpha particles producing a proton and oxygen-17 follows this systematic notation.
Q6: How is plutonium-239 produced from uranium-238 in nuclear reactors?
Uranium-238 is bombarded with neutrons released during uranium-235 decay in nuclear reactors. This creates unstable uranium-239, which undergoes beta decay to form neptunium-239. Neptunium-239 subsequently undergoes radioactive decay and radiometric dating processes to form plutonium-239, which is now mostly formed as a byproduct during uranium decay.
Q7: What medical applications have resulted from nuclear transmutation?
Nuclear medicine has developed from the ability to convert atoms of one type into other types through transmutation. Radioactive isotopes of several dozen elements are now used for medical applications, with radiation from their decay used to image or treat various organs and body portions. This advancement demonstrates the practical benefits of transmutation technology beyond fundamental nuclear research.