Rubidium, potassium and carbon are three common elements used to date the history of Earth. The rates of radioactive decay of these elements are absolutely regular when averaged out over a period of time; nothing is known to change them. To be useful as clocks, the elements have to be fairly common in natural minerals, unstable but decay slowly over millions of years to form recognizable "daughter" products which are preserved minerals.
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
A. They can be made into clocks
B. They are rich in content
C. Their decay is slow but regular
D. The products of their decay are the same
Rubidium, potassium and carbon are three common elements used to date the history of Earth. The rates of radioactive decay of these elements are absolutely regular when averaged out over a period of time; nothing is known to change them. To be useful as clocks, the elements have to be fairly common in natural minerals, unstable but decay slowly over millions of years to form recognizable "daughter" products which are preserved minerals.
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
A. The atom produced by the decay is above a certain point of temperature
B. The atom produced by the decay is easy to be detected at a cool temperature
C. The decay produced a neutron and an electron
D. The decay is sensitive to the changes in temperature
| By far the most common difficulty in
study is simple failure to get down to regular concentrated work. This
difficulty is much greater for those who do not work to a plan and have no
regular routine of study. Many students muddle along, doing a hit of this
subject or that, as the mood takes them, or letting their set work pile up until
the last possible moment. Few students work to a set time-table. They say that if they did construct a timetable for themselves they would not keep to it, or would have to alter it constantly, since they can never predict from one day to the next what their activities will be. No doubt some temperaments take much more kindly to a regular routine than others. There are many who shy away from the self-regimentatign of a weekly time-table, and dislike being tied clown to a definite programme of w A. the failure to keep to a routine of methodical and intensive work B. changing from one subject to another C. unwillingness to follow a systematic plan D. applying oneself to a subject only when one feels inclined [单项选择]It is a common knowledge that triangle has three angles, a square has four sides.
A. a triangle has... a square B. the triangle has... and a square having C. the triangle to have... a square to have D. a triangle had... a square [单项选择]This dictionary is a three-thousand common word collection with samples of ______ and spoken language.
A. writing B. wrote C. write D. written [填空题]One of the common things of the author’s three teachers is that they all inspired a ______ in their student for knowledge.
[填空题]A strong El Nino can last two or three years ______.
[单项选择] Passage Three
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