问题 单项选择题

在X线摄影中,X线束是以焦点作为顶点的锥形放射线束,将被照体G置于焦点与胶片之间时,因为几何投影关系,一般被照体离开焦点一定的距离a(焦-肢距),胶片离开肢体一定的距离b(肢-片距)。所以,肢体在X线胶片上的影像S要比肢体G大,是被放大了的影像,S与G之比即影像的放大率M,而且胶片离肢体越远,影像放得越大。国际放射学界公认:当照片上的半影模糊值<0.2mm时,人眼观察影像毫无模糊之感;当半影模糊值=0.2mm时,人眼观察影像开始有模糊之感。故0.2mm的半影模糊值就是模糊阈值。

放大摄影X线管焦点为0.05,允许的最大放大率为()

A.2倍

B.3倍

C.4倍

D.5倍

E.6倍

答案

参考答案:D

阅读理解

(C)

Astronaut Jim Voss has enjoyed many memorable moments in his career,including three space flights and one space walk. But he recalls with special fondness a decidedly earthbound(为地球引力所束缚的)experience in the summer of 1980 when he participated in the NASA ASEE Summer Faculty Fellowship Program. Voss, then a science teacher at West Poin,was assigned to the Marshall Space Flight Center's propulsion(推进) lab in Alabama to analyze why a hydraulic fuel pump seal (液压燃料泵的密封圈)on the space shuttle(航天飞机) was working so well when previous seals had failed. It was a seemingly tiny problem among the vast complexities of running the space program. Yet it was important to NASA because any crack in the seal could have led to destructive results for the astronauts who relied on them.

“I worked a bit with NASA engineers,” says Voss, “but I did it mostly by an analysis. I used a handheld calculator, not a computer, to do a thermodynamic(热力学的) analysis.” At the end of the summer,he,like the other NASA ASEE fellows working at Marshall,summarized his findings in a formal presentation and detailed paper. It was a valuable moment for Voss because the ASEE program gave him added understanding of  NASA,deepened his desire to fly in space,and intensified his application for astronaut status.

  It was not an easy process. Voss was actually passed over when he first applied for the astronaut program in 1978. Over the next nine years he reapplied repeatedly,and was finally accepted in 1987. Since then he has participated in three space missions. The 50 year old Army officer,who lives in Houston,is now in training for a four-month mission as a crew member on the International Space Station starting in July 2000.

 Voss says the ASEE program is wonderful for all involved. “It brings in people from the academic world and gives NASA a special property for a particular period of time. It brings some fresh eyes and fresh ideas to NASA,and establishes a link with our colleges and universities,” Voss explains. “There's an exchange of information and an exchange of perspectives that is very important.”

For the academic side,Voss says,the ASEE program also “brings institutions of higher learning more insight into new technology. We give them an opportunity to work on real world problems and take it back to the classroom.”

66. Why was the hydraulic fuel pump seal important for the space shuttle?

A) Because previous seals all failed.

B) Because it was very complex in running the space program.

C) Because great care has to be taken of the hydraulic fuel pump sealing.

D) Because any crack in the seals would cause disastrous results for the astronauts.

67.The great significance of Voss's findings lies in_________ .

A) strengthening his determination to join in space flights

B) furthering his understanding of NASA

C) consolidating his astronaut status in NASA programs

D) Both A and B

68. How many flights will Voss have finished if his four-month mission starting in July 2000 ends up successfully?

A) Three  B) Two   C) Four     D) Five

69. Which of the following is NOT TRUE according to what Voss said on the ASEE program?

A) Fresh members from the academic world participate in the program.

B) The program brings new outlooks to NASA space programs.

C) It is important for the space scientists to exchange information and perspectives.

D) American colleges and universities are a special property of NASA.

70. What does Voss want to stress in the last paragraph?

A) The technological significance of the program.

B) The educational significance of the program.

C) The philosophical significance of the program.

D) The historical significance of the program.

单项选择题