2. 外语
  3. For example, an atom of radioactive rubidium decays to form an atom of strontium (another element) by converting a neutron in it

For example, an atom of radioactive rubidium decays to form an atom of strontium (another element) by converting a neutron in it

admin2011-01-04  64
问题     For example, an atom of radioactive rubidium decays to form an atom of strontium (another element) by converting a neutron in its nucleus to a proton and releasing an electron, generating energy in the process. The radiogenic daughter products of the decay—in this case strontium atoms-—diffuse away and are lost above a certain very high temperature. So by measuring the exact proportions of rubidium and strontium atoms that are present in a mineral, researchers can work out how long it has been since the mineral cooled below that critical "blocking" temperature. The main problems with this dating method are the difficulty in finding minerals containing rubidium, the accuracy with which the proportions of rubidium and strontium are measured, and the fact that the method gives only the date when the mineral last cooled below the blocking temperature. Because the blocking temperature is very high, the method is used mainly for recrystallized (igneous or metamorphic) rocks, not for sediments—rubidium-bearing minerals in sediments simply record the age of cooling of the rocks which were eroded to form the sediments, not the age of deposition of the sediments themselves.
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答案例如，放射性的铷的一个原子衰退后，把一个中子转变成一个质子并释放出一个电子，形成一个锶(另外一种元素)原子，在这个过程中产生能量。衰退后的放射性子产品——在这里就是锶原子——会以一个极高的温度分散化解。所以测量一下现在矿物中铷原子和锶原子的确切比例，研究人员就会推导出该矿物在冷却到凝结温度以下之后又埋没了多少年。这种回溯方法具有弊端，因为很难发现含有铷元素的矿物、测量铷元素和锶元素的精确比例，而且推断出的时间只是矿物最后冷却到凝结温度之下的时间。因为凝结温度很高，这种方法只适用于再结晶岩石(火成岩或变质岩)，不适合沉积岩——含铷元素的矿物，它能记载岩石腐化成沉积物的冷却时间，而不是沉积物自己的蜕变年龄。
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