Quantum and Nuclear Physics (B) Problem Solving Mr. Klapholz Shaker Heights High School
Problem 1 An alpha particle is shot directly at a gold atom with a kinetic energy of 7.7 MeV. How close can the alpha get to the center of the nucleus?
Solution 1 Electron-Volts is not an SI unit, so let’s convert: 7.7 x 10 6 eV ( x J / 1 eV ) = 1.2 x J Initial E k of = Electrical E p at closest approach 1.2 X = kQ Q gold / R X = 9.0x10 9 1.6 x (79)(1.6x ) / R 2 R = ? R = 3.0 x m (Memorize size of nucleus ≈ m)
Problem 2 A sample of a radioactive isotope contains 1.0 x atoms and it has a half life of 6 hours. Find: a)The decay constant. b)The initial activity. c)The number of original nuclei still present after 12 hours. d)The number of original nuclei still present after 30 minutes.
Solution 2 a)We know that T ½ = 6.0 hours. How do we find the decay constant? ln(2) / = T ½ We will need S.I. units. T ½ = ? T ½ = ( 6 hours )(3600 s / hour) = s = ln(2) / T ½ = ln(2) / (21600 s) = 3.2 x s -1 This is the probability that any single nucleus will decay, in one second.
Solution 2 b) The “initial activity” is the original number of nuclei that decay per second. A = N A 0 = N 0 A 0 = [ 3.2 x s -1 ] [ 1.0 x ] A 0 = 3.2 x nuclei per second A 0 = 3.2 x Becquerel A 0 = 3.2 x Bq
Solution 2 c) 12 hours is 2 half lives. So, the number of original nuclei remaining is one-fourth of the original. N = ¼ [ 1.0 x ] N = 2.5 x nuclei
Solution 2 d) Convert 30 minutes to SI units: ( 30 minutes )( 60 s / minute ) = 1800 s The number of nuclei that are still not disintegrated is: N = N 0 e - t N = ( 1.0 x ) e -(3.2 x 10-5)(1800) = ? N = 9.4 x nuclei Check: this is more than N after 12 hours, and less than N 0.
Problem 3 The background radiation rate is 10 counts per second. Find the half life. Time / sCounts per second
Solution 3 If you subtract the background rate, you’ll see that the half life is one second. Time / s Counts per second MINUS background
Tonight’s HW: Go through the Quantum and Nuclear section in your textbook and scrutinize the “Example Questions” and solutions. Bring in your questions to tomorrow’s class.