第三章 放射性衰变及衰变方程式

❖ Half-lives vary from millions of years to fractions of seconds. While half-lives between a minute and a year are easily determined with fairly simple laboratory techniques
Chapter3 Unstable nuclei and Radioactive decay
第三章 放射性衰变及衰变方程式
杨金玲
❖ Radioactive decay is a spontaneous nuclear transformation that has been shown to be unaffected by pressure, temperature, chemical form, etc (except a few very special cases). This insensitivity to extranuclear conditions allows us to characterize radioactive nuclei by their decay period and their mode and energy of decay without regard to their physical or chemical condition.
• 人们把稳定核的中子/质子数 比进行标绘，得到通常称之为 稳定线的图，如右面的图所示。 纵坐标是中子数N，横坐标是质 子数Z。 • 位于稳定线左上方的核素是丰 中子核，它们的N/Z要比稳定核 素要求的N/Z大；
• 位于稳定线右下方的核是缺中子核，也叫丰质子核，它们的N/Z
要比稳定核素要求的N/Z小 。
稳定核中：
偶－偶核占了约60％；
剩下的40％中：
偶－奇核和奇－偶核几乎各占一半；
奇－奇核很少，只有6种：2 1
H
N 6
3
LiBiblioteka 10 5B14 7
V 50
23
180 73
Ta
• 质子数和中子数都是偶数的核最稳定。因为相同核子成对时最稳定
• 稳定核中偶－奇核和奇－偶核数目大致相等，说明质子成对和中子成对时
原子核提供了大致相同的稳定性。
放射性衰变通常由下式描述：
A B x E
能量是以放出的粒子或量子的动能形式出现的。 在放射性衰变时释放的能量称作Q值，它是由衰 变前后处于基态的核的质量差得出。
Q mc 2 (mA mB mx )c2
❖ The mode of radioactive decay is dependent upon the particular nuclide involved. The radioactive decay can be characterized by α-，β-，andγ-radiation.
对稳定的核素标绘出中子数N与质子数Z的关系，就会发现对于轻元素 (A<40)的稳定核，中子数近似的等于质子数，即N/Z=1;对于A>40, Z>20 的元素，稳定核中的中子、质子数比从1逐步增加到铋(83)元素的 N/Z=1.518。对于原子序数Z>83的元素，都是放射性衰变核素。 • 稳定核的N/Z比随着原子质量数A的增加而增加。原子核中，中子不 带电，质子带正电，原子核中的质子都要受到一种库仑斥力。随着原子 序数Z的增加，原子核中每个质子所受的库仑斥力随之而增加。要继续 保持稳定性就只能使N/Z比增大。
❖ The time dependence of radioactive decay is expressed in terms of the half-life (t1/2 ), which is the time required for one-half of the radioactive atoms in a sample to undergo decay. In practice this is the time for the measured radioactive intensity (or simply, radioactivity of a sample) to decrease to one-half of its previous value
❖ 10-18s, 1015y
❖ Radioactive decay involves a transition from a definite quantum state of the original nuclide to a definite quantum state of the product nuclide. The energy difference between the two quantum levels involved in the transition corresponds to the decay energy. This decay energy appears in the form of electromagnetic radiation and as the kinetic energy of the products, see Element and Nuclide Index for decay energies.
Alpha-decay（α衰变） is the emission of helium nuclei. Beta-decay （β衰变）is the creation and emission of either electrons or positrons, or the process of electron capture. Gamma-decay（γ跃迁） is the emission of electromagnetic radiation where the transition occurs between energy levels of the same nucleus. An additional mode of radioactive decay is that of internal conversion in which a nucleus loses its energy by interaction of the nuclear field with that of the orbital electrons, causing ionization of an electron instead of γ-ray emission. A mode of radioactive decay which is observed only in the heaviest nuclei is that of spontaneous fission（自发裂变） in which the nucleus dissociates spontaneously into two roughly equal parts.
❖ 地球上存在的各种同位素中，天然存在的约500多种。其中，稳定的约200种， 放射性的有300多种。加上人工合成的同位素，大约共有有3100多种，其中绝 大部分是放射性同位素。 ❖ 稳定同位素是指核结构不会自发的发生改变的核素；放射性同位素是不稳定核 素，它们能通过自发的发射出各种射线，转变成另外一种核素。 ❖ 衰变产生的子体核素可能是稳定的，也可能是放射性的。如果衰变后生成的子 核是放射性的核素，它们还要继续发生衰变，直到最终变成稳定的核素为止。