问题 计算题

(9分)某些建筑材料可产生放射性气体——氡,氡可以发生α或β衰变,如果人长期生活在氡浓度过高的环境中,那么,氡经过人的呼吸道沉积在肺部,并放出大量的射线,从而危害人体健康。原来静止的氡核(Rn)发生一次α衰变生成新核钋(Po),并放出一个能量为E0=0.09 MeV的光子。已知放出的α粒子动能为Eα=5.55 MeV;忽略放出光子的动量,但考虑其能量;1 u =931.5 MeV/ c2

①写出衰变的核反应方程;

②衰变过程中总的质量亏损为多少?(保留三位有效数字)。

答案

① RnPo+He+γ    ②0.006 16 u

题目分析:(2)①衰变方程为:RnPo+He+γ ;(2分,无γ 不扣分)

②忽略放出光子的动量,根据动量守恒定律,0=pα+pPo,(2分)

即新核钋(Po)的动量与α粒子的动量大小相等,又Ek,(1分)

可求出新核钋(Po)的动能为EPoEα. (1分)

由题意,质量亏损对应的能量以光子的能量和新核、α粒子动能形式出现衰变时释放出的总能量为ΔE=Eα+EPo+E0=Δmc2(2分)

故衰变过程中总的质量亏损是Δm==0.00616u(1分)

选择题
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       Gallery Policies

       For Visitors to National  Gallery of Art ,Washington

      Visitors must present all carried items for inspection upon entry. After inspection, all bags, backpacks, umbrellas, parcels, and other things as determined by security officers must be left at the checkrooms,

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into the galleries.

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     Additional security procedures and checks may be taken according to the decision of the Gallery.

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specifically prohibited. Tripods (三脚架) are not allowed.

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1. When people come to visit the Gallery, they should_________. 

A. leave all their carried items at the checkrooms

B. have all their carried items x-rayed at the entrance

C. take all their carried items with them without inspection

D. have all their carried-items inspected at the entrance

2. What does the Callery feel sorry for?

A. Visitors have to keep their valuable items in the checkrooms.

B. The size of visitor items allowed into the Callery is limited.

C. It cannot keep oversized visitor items due to limited space.

D. Visitor items over 17 x 26 inches must go through additional checks.

3. Parents with small children visiting the Gallery ______

A. can carry their children in soft front child carriers

B. can carry their children on their shoulders

C. can carry their children in child carriers worn on the back

D. ought to pay if they want to use pushchairs for their children

4. Visiting photographers should make sure that ________

A. pictures and videos are allowed for personal use anywhere in the Gallery

B. pictures and videos can be taken in some places for personal use

C. picture-taking and videoing are totally forbidden in the Gallery

D. tripods are allowed except in some special exhibitions