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  3. Radioactivity occurs naturally. The main source comes from natural sources in space, rocks, soil water and even the human body i

Radioactivity occurs naturally. The main source comes from natural sources in space, rocks, soil water and even the human body i

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问题    Radioactivity occurs naturally. The main source comes from natural sources in space, rocks, soil water and even the human body itself. This is called background radiation and levels vary from place to place, though the average dose is fairly constant. The radiation which is of most concern is artificial radiation which results from human activities. Sources of this include the medical use of radioactive materials, fallout and contamination from nuclear bomb tests, discharges from the nuclear industry, and the storage and dumping of radioactive waste.

   While artificial radiation accounts for a small proportion of the total , its effects can be disproportionate. Some of the radioactive materials discharged by human activity are not found in nature, such as plutonium(钚)while others which are found naturally may be discharged in different physical and chemical forms, allowing them to spread more readily into the environment, or perhaps accumulate in the food-chain.
   Many of the elements which our bodies need are produced by the nuclear industry as radioactive isotopes or variants. Some of these are released into the environment, for example iodine and carbon, two common elements used by our bodies. Our bodies do not know the difference between an element which is radioactive and one which is not. So radioactive elements can be absorbed into living tissues, bones or the bleed, where they continue to give off radiation. Radioactive strontium behaves like calcium-an essential ingredient in our bones-in our bodies. Strontium deposits in the bones send radioactivity into the bone marrow, where the blood cells are formed, pausing leukemia.
   In most cases, cell death only becomes significant when large numbers of cells are killed, and the effects of cell death therefore only become apparent at comparatively high dose levels. If a damaged cell is able to survive a radiation dose, the situation is different. In many cases the effect of the cell damage may never become apparent. A few malfunctioning cell will not significantly affect an organ where the large majority are still behaving normally.
   However, if the affected cell is a germ cell within the ovaries or testicles, the situation is different. Ionizing radiation can damage DNA, the molecule which acts as the cell’s " instruction book". If that germ cell later forms a child, all of the child’s cells will carry the same defect. The localized chemical alteration of DNA in a single cell may be expressed as an inherited abnormality in one or many future generations.
   In the same way that a somatic cell(体细胞)in body tissue is changed in such a way that it or its descendants escape the control processes which normally control cell replication, the group of cells formed may continue to have a selective advantage in growth over surrounding tissue. It may ultimately increase sufficiently in size to form a detectable cancer and in some cases cause death by spreading locally or to other parts of the body.
   While there is now broad agreement about the effects of high-level radiation, there is controversy over the long-term effect of low-level doses. This is complicated by the length of time it takes for effects to show up, the fact that the populations being studied are small and exact doses are hard to calculate.
   All that can be said is that predictions made about the effects of a given dose vary. A growing number of scientists point to evidence that there is a disproportionately high risk from low doses of radiation. Others assume a directly proportionate link between the received dose and the risk of cancer for all levels of dose, while there are some who claim that at low doses there is a disproportionately low level of risk.
Questions 56 to 60
Fill in the blanks below with information from the passage, using no more than three words for each blank.

选项
答案Background
解析 (由第一段第二、三句“The main source comes from natural sources in space，rocks，soil water and even the human body itself．This is called background radiation”可知，放射现象主要来自于自然环境如太空、岩石、土壤、水甚至是人体本身，这样的放射称为“背景放射”。因此填Background，表示“背景”。)
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本试题收录于： A类竞赛（研究生）题库大学生英语竞赛（NECCS）分类
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A类竞赛（研究生）

大学生英语竞赛（NECCS）

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