Part 2

In an activation analysis experiment, the analyst wants to determine the number of target atoms (N₂₃ in the above example). He can measure how long the target was in the nuclear reactor; there are ways of measuring the neutron flux, φ; and the cross section is fixed and generally known for each target nucleus. So, by measuring the number of radioactive atoms created (N₂₄), he can calculate the number of target atoms. See the figure on the next two pages.

Actually, to get the most accurate results, there are certain practical tricks he can use that increase the accuracy. Some of these will become apparent in later sections of this booklet.

The most important of these “tricks” is the use of a “standard” or “comparator”. This comparator is similar in form and composition to the sample to be measured but contains a known quantity of the element to be determined. The steps used for the analysis are simple.

1. Put the sample and comparator together into a reactor and bombard them with neutrons.

2. Remove them and measure the radioactivity produced from the sample.

3. Compare the radioactivity of the sample and the comparator and calculate the amount of the element in the sample as a proportion:

(Radioactivity in sample)/(Radioactivity in comparator) = (Quantity of element in sample)/(Quantity of element in comparator)

Neutron Activation Analysis: Detecting Sodium in a Sample of Plastic

Step 1. Weigh a sample and a standard in quartz tubes.

Step 2. Seal tubes in package for reactor irradiation.

Step 3. Bombard with neutrons for about 3 hours in a reactor.

Step 4. Remove sample and standard from tubes and place in separate plastic containers to measure gamma rays.

Step 5. Obtain gamma-ray spectrum for sodium-24 in both sample and standard.

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 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.

Until a few years ago, it was difficult to measure the number of gamma rays of a particular energy that were being emitted by a mixture of radioactive isotopes unless there were only a few such gamma rays with very different energies. Today instruments are available that can really pick them out of a complex mixture. Thus it is usually possible to “separate” with electronic instruments the radioactive element we are interested in measuring. Some of the examples below will show how this might be accomplished.

Each radioactive nuclide also has a characteristic half-life, which is a measure of how fast the radioactive atoms change (transmute) to atoms of another element. In a reactor, even while they are being produced in the target, atoms of the radioactive nuclide are decaying with the particular half-life of the nuclide. The mathematical laws that govern this process tell us that the number of atoms determines the amount of decay; i.e., the more atoms there are, the greater the amount of decay in a given period of time. (The fraction that decays in that time is constant.) As a result, the target eventually becomes “saturated”, that is, the rate of production equals the rate of decay. When the irradiation is first begun, the number of radioactive atoms increases steadily. But eventually, this rate of increase slows down until, at saturation, further irradiation no longer increases the number of radioactive atoms present in the target.