问题 单项选择题 A1/A2型题

有关特征X线的解释,错误的是()

A.高速电子与靶物质轨道电子作用的结果

B.特征X线的质与高速电子的能量有关

C.特征X线的波长由跃迁的电子能量差决定

D.靶物质原子序数较高特征X线的能量大

E.70kVp以下不产生K系特征X线

答案

参考答案:B

解析:高速运行的电子将靶物质原子中某层轨道电子击脱,形成空穴。此时,外层(高能级)轨道电子向内层(低能级)空穴跃迁,释放能量,产生X线,称为特征辐射。特征X线的波长由跃迁电子能量差决定,与高速运行电子的能量无关。高速电子的能量可决定能够击脱某壳层的电子。管电压在70kVp以下时,电子产生的动能不能把钨靶原子的K壳层电子击脱,故不能产生K系特征X线。

选择题
阅读理解

阅读理解。

     "Just five more minutes." "I'm tired!" "I can't get up." If you have ever had trouble getting up in the

morning, you are not alone. Many kids in the United States do not get enough sleep.

     Your body needs sleep-and lots of it! "Evidence (证据) shows that elementary-age children need at

least nine hours of sleep per night," said sleep expert Dr. Carl Hunt.

     Kids who do not sleep enough cannot do their best, experts say. A tired kid may be moody and act

badly. Being tired can also cause kids to forget things, have trouble learning, and have trouble playing

sports.

     Not sleeping enough can also affect kids' safety. Experts say children who are tired are more likely

to suffer injuries.

     Lots of kids lose sleep because they are busy. Many take part in after-school activities. Then they

go home, eat dinner, and do homework. Afterward, many kids watch TV, play video games, and surf

the Internet. That leaves less time for sleep.

     Sometimes you may not be able to go to bed early. But, if you get a choice whether to stay up late,

think twice. A good night's sleep can help you feel your best!

1. What is the best title for this story?[ ]

A. Kids Need More Sleep

B. Turn off the TV

C. Children Dream

D. Kids Have Busy Lives

2. According to the story, what is the main reason many kids do not get enough sleep? [ ]

A. They are afraid of the dark.

B. They are busy all day.

C. They get up early to do their homework.

D. They stay up late reading.

3. How could a good night's sleep help you feel your best? [ ]

A. You may be less moody.

B. You may have an easier time learning.

C. You may be less likely to get injured.

D. All of the above.