1. Half-Life

2. Decay Rates  The number of particles decaying in a short period of time is proportional to the number of particles.  The decay constant is.  The decay rate or activity is the rate of change. Activity decreases as time increasesActivity decreases as time increases

3. Measured Activity  The SI unit of activity is the Becquerel (Bq). equals one decay/sec (s -1 )equals one decay/sec (s -1 )  The older unit is the curie (Ci). Based on the decay of 226 RaBased on the decay of 226 Ra Once activity of one gramOnce activity of one gram Now defined by BqNow defined by Bq 1 Ci = 3.7  10 10 Bq1 Ci = 3.7  10 10 Bq

4. Half-Life  The relationship for decay gives rise to an exponential relation. Decay constant is fixed for a decay reactionDecay constant is fixed for a decay reaction  Decay is usually expressed as a half-life. Time for half a sample to decayTime for half a sample to decay Half-life constant for the nucleus typeHalf-life constant for the nucleus type

5. Exponential Decay  Half-life applies to each sample separately. This is exponential decayThis is exponential decay

6. Sample Size  A source of 24 Na is marked at 1.16 MBq. Find the number of atoms of 24 Na in the sample.  Look up the half-life in a table. T = 15 hT = 15 h T = 5.4  10 4 sT = 5.4  10 4 s  The activity is given in the problem. Related to decay constant and sample size Convert to half-life

7. Specific Activity  Physical variables are often normalized to the mass. Described as “specific”Described as “specific”  Specific activity is the activity of a sample divided by the mass. Units Bq g -1 or  Ci g -1Units Bq g -1 or  Ci g -1 In solution expressed per unit volume: pCi L -1In solution expressed per unit volume: pCi L -1  For a pure radionuclide:  Normal soil has a few pCi/g  Drinking water has a recommended limit of 5 pCi/L of 226 Ra + 228 Ra.

8. Figure 29.08 14 C is a very rare isotope with a half-life of 5730 years.  In a living organism, the 14 C activity is 0.25 Bq.  When the organism dies, the 14 C exchange with the environment stops.  14 C starts decaying.  By measuring the 14 C / 12 C ratio we can estimate the age of the sample.

9. Excited States  Nuclear decay can leave a nucleus in an excited state. Many possible states may be reachedMany possible states may be reached Lifetime typically 10 -10 sLifetime typically 10 -10 s  Excess energy may be lost as a photon or electron. Single gammaSingle gamma Series of gamma emissionsSeries of gamma emissions Internal conversion betaInternal conversion beta 4.785 MeV 0.186 MeV 0 MeV 94.4%  5.5%  2.2% 3.3% 

10. Radiocarbon  Carbon-14 ( 14 C) is a radioactive isotope. Beta decayBeta decay Half-life 5730 yearsHalf-life 5730 years  Carbon-14 is produced in the atmosphere by cosmic rays. Absorbed by organismsAbsorbed by organisms Stops at deathStops at death  Fraction of 14 C to 12 C establishes the date of an object. next