The Atomic Fingerprint: Neutron Activation Analysis

Part 1

The Atomic Fingerprint: Neutron Activation Analysis

by Bernard Keisch

CONTENTS

INTRODUCTION 4 WHAT IS NEUTRON ACTIVATION ANALYSIS? 5 THE SENSITIVITY OF NEUTRON ACTIVATION ANALYSIS 10 HOW AND WHERE TO USE IT 19 In a Physics Laboratory 19 In a Hospital 28 In a Plastics Plant 32 In a Museum 35 In a Criminology Laboratory 42 SUMMING UP: WHAT LIES AHEAD 46 APPENDIX 49 READING LIST 52 MOTION PICTURES 54

U. S. Energy Research and Development Administration Office of Public Affairs Washington, D.C. 20545

Library of Congress Catalog Card Number: 79-182556 1972

The U. S. Energy Research and Development Administration publishes a series of booklets for the general public.

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A 19th century photograph restored by neutron activation. This picture, which is in the collection of the Smithsonian Institution, was exposed to neutrons in a nuclear reactor and then placed in contact with modern photographic film. The original, which had been taken by William Henry Fox Talbot who began his career in 1834, is badly faded.

INTRODUCTION

You are a physicist investigating the properties of semiconductors, which are materials used to make transistors. The electrical properties of one specimen are not quite like the others that you’ve studied. What makes this specimen different?

You are a physician treating a patient who, because of a severe calcium deficiency, has been suffering from osteoporosis (a softening of the bones). Are you on the right track with your treatment?

You are an analytical chemist working for a plastics manufacturer. You have been asked by the plant superintendent to determine why some of the plastic coming from the plant has been discolored.

You are a curator working with the ancient coin collection in a large museum. A donor has just given the museum a group of 50 gold coins presumably about 1500 years old. Are they genuine?

You are a scientist working in the criminology laboratory of a large metropolitan city. A detective brings you a minute sample of paint taken from the clothing of a hit-and-run victim. He has a suspect whose automobile paint seems to match that sample. Can you determine his guilt or innocence?

Neutron activation analysis can be used to solve each of these problems and many more. The solutions to these particular problems are explained on pages 19-46.

The Atomic Fingerprint: Neutron Activation Analysis by Bernard Keisch

WHAT IS NEUTRON ACTIVATION ANALYSIS?

To understand neutron activation analysis, you should be acquainted with a few basic concepts. The nuclei of atoms are stable only when they contain certain numbers of neutrons and protons. The number of protons in an atom’s nucleus determines an element’s identity; the number of neutrons usually determines whether or not that atom is radioactive or nonradioactive (stable).

Thus, while all sodium atoms contain 11 protons, only those sodium atoms that contain 12 neutrons are stable. A radioactive sodium atom contains a different number of neutrons. For other elements, there may be more than one number of neutrons that results in stability; for instance, there are 10 stable atoms (isotopes) of tin, each containing a different number of neutrons in their nuclei.

The fact that nuclei can absorb additional neutrons, which, in many cases, results in the conversion of a stable nucleus to a radioactive one, makes neutron activation analysis possible. Because radioactive nuclei decay in unique ways and yield radiations that are often distinct and can be measured even in very small amounts, measurements of these radiations can determine the kind and the number of radioactive atoms that are present.

In the most common type of activation analysis, the neutron bombardment of a sample is performed in a nuclear reactor where the neutrons that strike the target atoms have been slowed down so that they have very little energy of motion. In this case, the usual reaction between the target atoms and a neutron results in the capture of the neutron and this creates a nucleus with an atomic weight of one more unit than it started with. Thus for sodium as found in nature (symbol ²³Na)

sodium-23 + a neutron → radioactive sodium-24 + gamma rays

The numbers denote the atomic weight of the atom, which is the total number of protons and neutrons in its nucleus.

In a nuclear reactor, there are many, many neutrons that can be used in this reaction; approximately 10¹² to 10¹⁴ (10¹² is a million million; 10¹⁴ is a hundred times 10¹²) pass through each square centimeter of target area every second. Not all these will strike the nuclei of sodium atoms. Of those that do, not all will be captured. A mathematical relationship that tells how many atoms of sodium-24 will be created in a cubic centimeter of the target in one second is:

N₂₄ = N₂₃φσt

where N₂₄ is the number of sodium-24 atoms created during each second in a cubic centimeter of the target; N₂₃ is the number of atoms of sodium-23 in a cubic centimeter of the target; φ is the number of neutrons crossing a square centimeter per second (called the neutron flux); t is the time in seconds that the target is in the reactor; and σ is a number that represents the probability that the conversion of sodium-23 to sodium-24 will occur. This last number is called a “cross section” and it is expressed in “barns”. One barn is equal to 10^{-24} square centimeter, which is approximately the cross-sectional area of a typical atomic nucleus.

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.

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