1. Nuclear Physics Selected Topics 2  Radioactivity

2. Types of Radioactivity Three types of radiation can be emitted. Alpha particles (  ) are 4 He nuclei. Beta particles (  ) are either electrons or positrons. (A positron is the antiparticle of the electron, similar to the electron except its charge is +e.) Gamma rays (  ) are high energy photons.

3. Distinguishing Types of Radiation A radioactive beam is directed into a region with a magnetic field. The gamma particles carry no charge and they are not deflected. The alpha particles are deflected upward. The beta particles are deflected downward. (A positron would be deflected upward.)

4. Penetrating Ability of Radiation Alpha particles barely penetrate a piece of paper. Beta particles can penetrate a few mm of aluminum. Gamma rays can penetrate several cm of lead.

5. The Mathematics of Radioactive Decay - The decay constant - The decay equation - Decay curves

6. The Decay Constant The number of particles that decay in a given time is proportional to the total number of particles in a radioactive sample: λ is called the decay constant and determines the rate at which the material will decay.

7. The Decay Constant The decay rate or activity, R, of a sample is defined as the number of decays per time interval:

8. The Decay Equation The basic decay equation relates the number N of sample particles present at time t to the number N o present at time t = 0: Since R (the decay rate) is proportional to N, the decay equation may also be written as:

9. The Decay Equation Another useful form of the decay equation relates the percent activity to the initial (100%) activity:

10. Units A common unit of activity, R, is the Curie, Ci. 1 Ci = 3.7 x 10 10 decays/second. (This is the activity of one gram of radium.) The SI unit of activity is the Becquerel, Bq. 1 Bq = 1 decay / second. (Therefore, 1 Ci = 3.7 x 10 10 Bq.) The most commonly used units of activity are the mCi and the µCi.

11. Decay Curves A graph of activity vs. time may be plotted on rectangular coordinates or in the form of a semi-log graph. The value of the half-life will be found by locating the time corresponding to 50% activity. Examples of these types of graphs are on the next two slides.

12. Decay curve for Strontium-90 (half-life 28.8 y)

13. Semi-log decay curve for Strontium-90