Phy 123L

Pre-Lab: Half Life of Barium – 137m Name:

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Choose the best answer. (10 pts total)

1. What does Barium-137m emit when it decays?

a. electron b. proton c. neutron d. gamma ray e. beta ray

2. In equation 2 in the lab manual, which variable represents the number of Barium-137 atom present after a time, t?

a. Nt b. N0 c. e-λt d. λ e. t

3. In equation 2 in the lab manual, which variable represents the number of Barium-137 atom present at time t = 0

a. Nt b. N0 c. e-λt d. λ e. t

4. The time interval in which the number of radioactive atoms decays to one half the original or the activity to be one half the original activity is called the _____.

a. decay constant b. half life c. radioactive decay d. probability

5. The time interval referred to in the previous question is _____ for distinct atoms or isotopes.

a. the same b. different c. Neither a or b. It’s not that simple.

6. In part B, according to the lab manual, what do you stop counting at the end of a 30 second time interval (to restart 30 seconds later) and what do you leave running continuously until your activity is about 50 counts/half minute?

a. stop the counter by pressing “manual”; leave the stopwatch running continuously

b. stop the stopwatch; leave the counter running

Questions 7-10: Suppose there are 3.0x107 radon atoms trapped in a basement. The half life of radon is t1/2 = 3.83 days.

7. What is the decay constant for radon? Give the answer in units of 1/day.

a. 0.0786 1/day b. 0.181 1/day c. 5.53 1/day

8. How many radon atoms remain after 31 days? (Hint: Use equation 2)

a. 3.0x107 atoms b. 8.2x109 atoms c. 1.1x105 atoms d. 2.6x106 atoms

Questions 9-10: The activity of a radioactive sample is the number of disintegrations per second that occur. The average activity over the time interval Δt is the magnitude (absolute value) of ΔN/Δt. So you can calculate the activity from equation 1.

9. Find the activity just after the basement is sealed to prevent further entry of radon.

N = 3.0x107 atoms

a. 2.4x106 decays/day b. 5.4x106 decays/day c. 1.7x108 decays/day

10. Find the activity 31 days later. N = answer from question 8.

a. 6.4x108 decays/day b. 2.0x104 decays/day c. 4.7x105 decays/day

CSU Pomona Updated 3/25/2009 Dr. Julie J. Nazareth