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Radiocarbon Dating Nowadays scientists can answer many questions about the past through a technique called radiocarbon (放射性碳), or carbon-14, dating. One key to understanding how and by something happened is to discover when it happened. Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at he University of Chicago. An atom of ordinary carbon, called carbon-12, has six protons(质子) and six neutrons (中子) in its nucleus(原子核). Carbon-14, or C-14, is a radioactive, unstable form of carbon that has two extra neutrons. It returns to a more stable form of carbon through a process called decay (衰减). This process involves the loss of he extra neutrons and energy from the nucleus. In Libby’s radiocarbon dating technique, the weak radioactive emissions (放射) from this decay process are counted by instruments such as a radiation detector and counter. The decay rate is used to determine the proportion of C-14 atoms in the sample being dated. Carbon-14 is produced in the Earth’s atmosphere when nitrogen (氮)-14, or N-14,interacts with cosmic rays (宇宙射线). Scientists believe since the Earth was formed, the mount of nitrogen in the atmosphere has remained constant. Consequently, C-14 formation is thought to occur at a constant rate. Now the ratio of C-14 to other carbon toms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50,000 years. All life on Earth is made of organic molecules (分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14 atoms to other carbon atoms in their tissues (组织). Once an organism (有机体) dies it stops taking in carbon in any form, and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases, and the ratio of C-14 to other carbon atoms goes down. In terms of radiocarbon dating, the fewer C-14 atoms in a sample, the older that sample is. The half-life of C-14 is about 25,000 years.
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第一篇 Light Night, Dark Stars Thousands of people around the globe step outside to gaze at their night sky. On a clear night, with no clouds, moonlight, or artificial lights to block the view, people can see more than 14,000 stars in the sky, says Dennis Ward, an astronomer (天文学家) with the University Corporation for Atmospheric Research (UCAR) in Boulder, Colo. But when people are surrounded by city lights, he says, they’re lucky to see 150 stars. If you’ve ever driven toward a big city at night and seen its glow from a great distance, you’ve witnessed light pollution. It occurs when light from streetlights, office buildings, signs, and other sources streams into space and illuminates (照亮) the night sky. This haze (薄雾) of light makes many stars invisible to people on Earth. Even at night, big cities like New York glow from light pollution, making stargazing difficult. Dust and particles of pollution from factories and industries worsen the effects of light pollution. "If one city has a lot more light pollution than another," Ward says, "that city will suffer the effects of light pollution on a much greater scale." Hazy skies also make it far more difficult for astronomers to do their jobs. Cities are getting larger. Suburbs are growing in once dark, rural areas. Light from all this new development is increasingly obscuring (使变模糊) the faint (微弱的) light given off by distant stars. And if scientists can’t locate these objects, they can’t learn more about them. Light pollution doesn’t only affect star visibility. It can harm wildlife too. It’s clear that artificial light can attract animals, making them go off course. There’s increasing evidence, for example, that migrating (迁徙) birds use sunsets and sunrises to help find their way, says Sydney Gauthreaux Jr., a scientist at Clemson University in South Carolina. "When light occurs at night," he says, "it has a very disruptive (破坏性的) influence." Sometimes birds fly into lighted towers, high-rises, and cables from radio and television towers. Experts estimate that millions of birds die this way every year. How does light pollution affect wildlife
Radiocarbon Dating Nowadays scientists can answer many questions about the past through a technique called radiocarbon (放射性碳), or carbon-14, dating. One key to understanding how and by something happened is to discover when it happened. Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at he University of Chicago. An atom of ordinary carbon, called carbon-12, has six protons(质子) and six neutrons (中子) in its nucleus(原子核). Carbon-14, or C-14, is a radioactive, unstable form of carbon that has two extra neutrons. It returns to a more stable form of carbon through a process called decay (衰减). This process involves the loss of he extra neutrons and energy from the nucleus. In Libby’s radiocarbon dating technique, the weak radioactive emissions (放射) from this decay process are counted by instruments such as a radiation detector and counter. The decay rate is used to determine the proportion of C-14 atoms in the sample being dated. Carbon-14 is produced in the Earth’s atmosphere when nitrogen (氮)-14, or N-14,interacts with cosmic rays (宇宙射线). Scientists believe since the Earth was formed, the mount of nitrogen in the atmosphere has remained constant. Consequently, C-14 formation is thought to occur at a constant rate. Now the ratio of C-14 to other carbon toms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50,000 years. All life on Earth is made of organic molecules (分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14 atoms to other carbon atoms in their tissues (组织). Once an organism (有机体) dies it stops taking in carbon in any form, and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases, and the ratio of C-14 to other carbon atoms goes down. In terms of radiocarbon dating, the fewer C-14 atoms in a sample, the older that sample is. Radar is used to determine the characteristics of radiocarbon.
One Good Reason to Let Smallpox Live It’s now a fair bet that we will never see the total extinction (灭绝) of the smallpox (天花) virus. When smallpox was completely got rid of in the world, scientists wanted to (51) the killer virus in the last two labs - one in the US and one in Russia. They asked: If smallpox has truly gone from the planet, (52) point was therein keeping these reserves in reality, of course, it was naive to (53) that everyone would let go of such a powerful potential weapon. Undoubtedly several nations still have (54) vials (小药瓶). And the last "official" stocks of live virus bred mistrust of the US and Russia, (55) no obvious gain. Now American researchers have (56) an animal model of the human disease, opening the (57) for tests on new treatments and vaccines (疫苗). So once again there’s a good reason to (58) the virus -just in (59) the disease puts in a reappearance. How do we (60) with the mistrust of the US and Russia (61) . Keep the virus (62) international support in a well-guarded UN laboratory that’s open to all countries. The US will object, of course, just as it (63) a multilateral (多边的) approach to just about everything. But it doesn’t mean the idea is (64) . If the virus is useful, then let’s (65) it the servant of all humanity not just a part of it.
第二篇 Shopping at Second-hand Clothing Stores When 33-year-old Pete Barth was in college, shopping at second-hand clothing stores was just something he did - "like changing the tires on his car." He looked at his budget, and decided he could save a lot of money by shopping for clothes at thrift shops. "Even new clothes are fairly disposable (用后即丢掉的) and wear out after a couple of years," Barth said. "In thrift shops, you can find some great stuff whose quality is better than new clothes." Since then, Barth, who works at a Goodwill thrift shop in the US state of Florida, has found that there are all kinds of reasons for shopping for second-hand clothing. Some people like him, shop to save money. Some shop for a crazy-looking shirt. And some hop as a means of conserving energy and helping the environment. Pat Akins, an accountant at a Florida Salvation Army (SA) (救世军) thrift shop, said hat, for her, shopping at thrift shops is a way to help the environment. "When my daughter was little, we looked at it as recycling," Akins said. "Also, why ay 30 dollars for a new coat when you can get another one for a lot less" Akins said that the SA has shops all over the US - "some as big as department stores." All of the clothes are donated (捐赠), and when they have a surplus (盈余), they’ll have "stuff a bag" specials, where customers can fill a grocery sack with clothes for only or 10 dollars. Julia Slocum, 22, points out, however, that the huge amount of second-hand clothing in the US is the result of American wastefulness. I’d say that second-hand stores are the result of our wasteful, materialistic culture," said Slocum, who works for a pro-conservation organization, the Center for a New American Dream. "Thrift shops prevent that waste from going to landfills (垃圾填埋场); they give clothing a second life, provide cheaper clothing for those who can’t afford to buy new ones and generate (生成) income for charities. They also provide a way for the wealthy and middle classes to shed (摆脱) some of the guilt for their level of consumption." The word "thrift" in paragraph 1 could be best replaced by
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