- 1半衰期
In science, the half-life of a radioactive substance is how long it takes for half of it to decay — to lose half of its . After one half-life, half of the original amount is still there. After another half-life, half of that remaining half is left, so a of what you started with. It does not jump to zero; it keeps halving, again and again. Different materials have wildly different half-lives: some last seconds, some last thousands of years. Scientists like this number because it is a steady clock. Though actually, the “life” in the word is not about being alive. It is about how long the substance lasts as itself. People also stretch the word in ordinary talk: the half-life of a song, a , or a news story is how long until it has lost half of its heat and most people have moved on. That second use is informal. In class and in the lab, half-life means the science sense.
在科學上，「the half-life of a radioactive substance」指的是放射性物質衰變掉一半所需要的時間。過了一個半衰期，剩下一半；再過一個半衰期，剩下的那一半又會減半，也就是原來的四分之一。它不會一下子歸零，而是不停地一半一半減少。不同物質的半衰期差很多，有的只有幾秒，有的則長達幾千年，科學家因此很喜歡用它當作穩定的時鐘。這裡的 life 不是指生命，而是指物質以原本的樣子存在多久。
口語上也有人借用，像「the half-life of a song, a rumour, or a news story」，指歌曲、謠言或新聞的熱度降到只剩一半、大家大多已經不再關注的那段時間。這是非正式的用法；在課堂和實驗室裡，half-life 指的就是科學上的意思。
From the word’s history
half- + life.
由 half-（一半）加上 life 組成。
組成From half- + life. Here “life” means how long something lasts, not a living person.
同家族的字
- lifethe second piece; in this word it means a span of time, not a living body
原始詞典資料4 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
half-lifenoun/ˈhæfˌlaɪf/
- 1
The time required for half the nuclei in a sample of an isotope to undergo radioactive decay.
- "The guy that just left—what's he got in his lungs?" / "Well, I'm not very sure. But the two best candidates are uranium-233 and plutonium-239, one or both." / "They're in his bones too, aren't they?" / "Yeah, they're both boneseekers. And they've got half-lives of one hundred sixty-two thousand and twenty-four thousand years respectively. So they stay hot for a long time."
- O₂^(•−) and OH^• are highly unstable (half-life at the level of microseconds and nanoseconds, respectively) and cannot cross membrane, while H₂O₂, though not a free radical but a ROS, is relatively stable (half-life around 1 ms) (Møller et al. 2007) and can cross membranes through aquaporins.
- 2
chemistrynatural-sciencesphysical-sciencesIn a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
- 3
medicinepharmacologysciencesThe time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
- For nonporous surfaces such as steel, in a dark and low-humidity environment, the CCP virus has an 18-hour half-life—the time required for it to decrease by half , according to the researchers' findings.
- 4
The amount of time it takes for an idea or a fashion to lose half of its influential power.
- Most books of scholarship have surprisingly short intellectual half-lives during which they make a difference"
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。