Step 4. Remove sample and standard from tubes and place in separate plastic containers to measure gamma rays. [Illustration: Pulse height analyser; Sample; Standard; Gamma rays from Na-24; same container, distance, detector; Sodium iodide scintillator] Step 5. Obtain gamma-ray spectrum for sodium-24 in both sample and standard. [Illustration: (chart) Energy vs. Sample spectrum; Energy —→Standard spectrum] Step 6. Use standard to calculate 1.37 MeV gamma rays counted per minute per gram of sodium (c/m/gNa). c/m/gNa = (counts/minute measured in 1.37 peak (shaded area above))/(grams of sodium known to be in standard (step 1.)) Step 7. Use c/m/gNa and 1.37 MeV gamma rays counted per minute in sample to calculate grams of sodium in sample. grams Na in sample = (counts/minute measured in sample)/(c/m/gNa (step 6.)) Step 8. Calculate percent sodium in sample. % sodium = (grams sodium in sample (step 7.))/(weight of sample (step 1.)) × 100 THE SENSITIVITY[3] OF NEUTRON ACTIVATION ANALYSIS There are several factors that determine the sensitivity of the method. Some are variable within limits and some, like the cross section, are fixed. Time is variable to a degree, partially determined by the half-life of the nuclide created and with an upper practical limit determined by how long we want to wait for an analysis. The crucial step in the analytical procedure is the measurement of the number of radioactive atoms that were created. 1. How do we measure how many radioactive atoms are present? 2. Since there will usually be a mixture of elements in a target, and many of these will be made radioactive, how can we tell one from another? 3. Since radioactive atoms are constantly “disappearing” by radioactive decay, how do we obtain the number of atoms created from a measurement made some time after the bombardment has taken place? And what of those atoms disintegrating while others are still being created in the reactor? Radioactive atoms almost always decay by emitting negatively charged beta particles usually accompanied by gamma rays. Instruments can detect these kinds of radiation, and it is by measuring the radiation that we determine how many radioactive atoms are present. To do this we have to know the types of radiation emitted by the radioactive atoms we are trying to measure. Fortunately each kind of radioactive atom decays with a unique “pattern” scientists call a “decay scheme”. The figure on the next page shows a simplified decay scheme for manganese-56, which is produced by activation of manganese, and a diagram showing what the decay scheme means.