The Project Gutenberg eBook ofThe Atomic Fingerprint: Neutron Activation AnalysisThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: The Atomic Fingerprint: Neutron Activation AnalysisAuthor: Bernard KeischRelease date: March 5, 2015 [eBook #48406]Most recently updated: October 24, 2024Language: EnglishCredits: Produced by Stephen Hutcheson, Dave Morgan, Carol Spears,and the Online Distributed Proofreading Team athttp://www.pgdp.net*** START OF THE PROJECT GUTENBERG EBOOK THE ATOMIC FINGERPRINT: NEUTRON ACTIVATION ANALYSIS ***
This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.
Title: The Atomic Fingerprint: Neutron Activation AnalysisAuthor: Bernard KeischRelease date: March 5, 2015 [eBook #48406]Most recently updated: October 24, 2024Language: EnglishCredits: Produced by Stephen Hutcheson, Dave Morgan, Carol Spears,and the Online Distributed Proofreading Team athttp://www.pgdp.net
Title: The Atomic Fingerprint: Neutron Activation Analysis
Author: Bernard Keisch
Author: Bernard Keisch
Release date: March 5, 2015 [eBook #48406]Most recently updated: October 24, 2024
Language: English
Credits: Produced by Stephen Hutcheson, Dave Morgan, Carol Spears,and the Online Distributed Proofreading Team athttp://www.pgdp.net
*** START OF THE PROJECT GUTENBERG EBOOK THE ATOMIC FINGERPRINT: NEUTRON ACTIVATION ANALYSIS ***
byBernard Keisch
U. S. Energy Research and Development AdministrationOffice of Public AffairsWashington, D.C. 20545
Library of Congress Catalog Card Number: 79-1825561972
Photograph, spiral galaxy
The U. S. Energy Research and Development Administration publishes a series of booklets for the general public.
Please write to the following address for a title list or for information on a specific subject:
USERDA—Technical Information CenterP. O. Box 62Oak Ridge, Tennessee 37830
USERDA—Technical Information Center
P. O. Box 62
Oak Ridge, Tennessee 37830
Photograph: shelf with china
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.
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?
OR
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?
OR
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.
OR
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?
OR
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 pages19-46.
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).[1]
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 bemeasured 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[2]
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 thetarget; 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 10-24square centimeter, which is approximately the cross-sectional area of a typical atomic nucleus.
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 thefigure 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 aknownquantity 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 sampleRadioactivity in comparator=Quantity of element in sampleQuantity of element in comparator
Step 1. Weigh a sample and a standard in quartz tubes.
Piece of plastic; Standard = sodium carbonate
Step 2. Seal tubes in package for reactor irradiation.
Sealed aluminium can; Sealed quartz tubes; Sample; Standard
Step 3. Bombard with neutrons for about 3 hours in a reactor.
Neutrons
Step 4. Remove sample and standard from tubes and place in separate plastic containers to measure gamma rays.
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.
(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 samplec/m/gNa (step 6.)
Step 8. Calculate percent sodium in sample.
% sodium =grams sodium in sample (step 7.)weight of sample (step 1.)× 100
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?
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”. Thefigure on the next pageshows 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[4]also has a characteristic half-life,[5]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.
The energy of a gamma ray is equal to the energy difference between the two levels involved in the gamma-ray emission.
Anenergy level diagram. The slanted arrows indicate radioactive decay by beta-particle emission. In each case, manganese-56 decays to a certain energy level of iron-56. On the right the energy of each level is indicated. Following a beta emission to a high-energy (excited) state in iron-56, one or more gamma rays are emitted until the nucleus is de-excited to the level marked zero. The vertical arrows indicate gamma rays emitted during the de-excitation process. The energy of each gamma ray is the difference between the levels involved in the change. The numbers above the vertical arrows indicate the relative proportions of gamma rays of different energies emitted from that level.
The mathematical relationship that describes the irradiation process exactly is:
A₀ = Nφσ (1 - e-λt)
where A₀ is the radioactivity produced (disintegrations per cubic centimeter per second); N is the number of target atoms per cubic centimeter in the sample; φ is the neutron flux (neutrons per square centimeter per second); σ is the cross section for the reaction (square centimeters); λ is thedisintegration constant[6]for the radioactive atoms produced (number per second); the number “e” is the base of natural logarithms; and t is the irradiation time in seconds. Note that for short irradiation times (t very small), 1-e-λtapproximates λt, while for long irradiations (t very large), 1-e-λtapproximates 1.
Graph: Decay scheme for manganese-56
This summarizes what the decay scheme or energy level diagram shows in terms of the relative amounts of betas and gammas emitted in the decay of manganese-56. Thus, you could observe more than three times as many gamma rays having an energy of 0.847 MeV than of 1.811 MeV, etc. Note that while one, and only one, beta is emitted in the decay of one atom of manganese-56, two gammas can sometimes be emitted in one decay.
Of course, when the target is removed from the reactor, the number of radioactive atoms begins to decrease according to the characteristic half-life of the nuclide. The mathematical expression that describes the process of radioactive decay of a single nuclide is:
At= A₀e-λt
where Atis the radioactivity of an isotope at some time, t, after the end of the irradiation, and A₀ is the radioactivity at the end of the irradiation.
Fraction of saturation sodium-24 activity _vs_ Time of irradiation (hours)
The activation of sodium-23 to sodium-24, which has a half-life of 15 hours. The horizontal line marked 1.0 represents the “saturation” activity level for a sample of sodium of a certain size in a constant neutron flux. Note that after about 120 hours, the activity of the sample is within 1% of the value at saturation, which is the most active that sample will ever become at a given φ. Note also that after the first 15 hours (1 half-life) the sample is exactly half way to its value at saturation. Thus long irradiations are useful to increase the sensitivity of the analysis, but only up to a certain point.
The result of all this is that the sensitivity of an analysis depends in practice on a number of practical as well as theoretical factors:
In the next section of this booklet, there are several examples that will show you how all this works in practice. But to summarize what these factors mean in terms of sensitivity let us look at the chart in thefigure on page 18. Here all the elements are arranged in a periodic table. The sensitivities are shaded in coded ranges representing measurable quantities. They are calculated on the basis that there are no interferences, that the neutron flux is 10¹⁴ neutrons per square centimeter per second, and that we can measure 100 gamma rays per minute without much difficulty assuming a gamma-ray detector efficiency[7]of 10%. The elements labeled β yield radioisotopes that emit few or no gamma rays and can only be analyzed by neutron activation using appropriate chemical separation procedures followed by beta radioactivity measurements. Such chemical separation procedures (to remove unwanted radioactive isotopes of other elements)are also sometimes useful to improve the sensitivity of the analysis of gamma-ray emitters if necessary.
Graph: “Fraction of Sodium-24 remaining” vs. “Time of decay (hours)”
The radioactive decay curve of sodium-24. The vertical scale is notlinearbutlogarithmic. Thus, each factor of two in radioactivity occupies the same distance along the vertical axis. When two samples are being analyzed for sodium by activation analysis, they must be compared at the same time after they have been removed from the neutron flux. If this period of time is different, then a correction must be applied to one of them, based on the decay curve shown here, to allow for the difference in decay time for the two. Waiting too long after the irradiation is completed results in much poorer sensitivity for the analysis depending on the half-life of the activation product. In this case, after 2 days it takes approximately ten times as much sodium to yield the same radioactivity as it would if the sample were measured when it was fresh out of the reactor.
It is not practical to determine a few elements, shown in black squares, by activation analysis. Some others, like oxygen and nitrogen (labeled HE), can be measured by using other projectiles like fast (more energetic) neutrons, or protons or deuterons[8]produced in a device called an accelerator. Other elements, those shown in white squares, can be detected with such great sensitivity, that one can find some in almost everything. For example, if you had a cube of “pure” aluminum only 1 millimeter on a side, you could detect gold in it if there were only one atom of gold for every fifty billion atoms of aluminum.
While it isn’t often that you would want to find a gold needle in an aluminum haystack, the next section presents some practical applications. Imagine yourself as the person with the problem in these situations.
Periodic table of elements, with sensitivity code
* Th and U are radioactive but with such long half-lives thatneutron activation analysis can be used for their determination.
† µg = Microgram (one-millionth of a gram)
You are a physicist investigating the properties of semiconductors, which are materials used to make transistors. When you apply a voltage to one specimen of silicon (a semiconductor), it doesn’t behave quite like the others that you’ve studied. The electrical properties of this odd specimen are unusual and interesting and could lead to a new type of transistor. What makes this specimen different from the others? Very small amounts of impurities can cause large changes in the electrical properties of semiconductors. You would like to obtain a chemical analysis of the material, but your colleagues in chemistry tell you they would have to dissolve a good size part of your sample to analyze it and you are reluctant to give it up. How do you do it?
You decide to try neutron activation analysis. You realize you won’t be able to detect all the elements, but many of those that might affect semiconductor performance could be detected quite easily.
What will you need? A source of neutrons to activate the material and a gamma-ray spectrometer to measure the radiation from the material afterwards. This spectrometer detects and measures gamma rays and sorts them according to their energy. You find that your friend down the hall, who is a nuclear physicist, has a gamma-ray spectrometer that incorporates a lithium-drifted germanium crystal as a detectorand a pulse height analyzer. The germanium detector is a device that senses the gamma rays that enter it and gives electrical signals related to the energy of the gamma rays. It was invented only a few years ago and has a very fine resolution. That is, it can easily “pick out” gamma rays that are only slightly different in energy. For example, for gamma rays with energies of approximately 1 MeV (million electron volts), it is not unusual to distinguish between gamma rays that differ by only 2 or 3 tenths of a percent. The pulse height analyzer is an electronic device that sorts the electrical pulses from the detector according to their energy.
Gamma-ray detector
A lithium-drifted germanium-crystal gamma-ray detector. The large container is a reservoir of liquid nitrogen that keeps the detector cooled to a temperature of -196° Centigrade (321° below zero, Fahrenheit). The lead brick shield keeps out most of the gamma rays that come from naturally radioactive materials in the room. The plastic slots hold cards upon which the samples are mounted for counting. Sometimes the detector is arranged vertically and samples are placed on shelves above it.
Gamma-ray detector
What about the neutrons for the irradiation? Although there isn’t a suitable nuclear reactor[9]in your city, there is one at a university only an hour away by jet. Since it may take a few hours to get the sample to the counter after irradiation, you won’t be able to look for short-livedactivation products, i.e., those with half-lives of up to an hour. However, this will exclude only a few elements from detection.
Pulse-height analyzer
A pulse-height analyzer used for gamma-ray spectrometry. A gamma-ray spectrum is displayed on the television screen. Data is printed out automatically on the electric typewriter and also may be plotted as a graph on the paper to the left. In other systems, data may be coded onto punched paper tape as well. Such tape may be “read” by a computer that can be programmed to use the data to calculate what radioactive isotopes are present and their quantities.
Now you are ready to begin the analysis. This will be a qualitative analysis since you are merely looking for a significantly different element in that silicon crystal. How much of it is present is only of secondary interest. Therefore, if you find anything different, you will rely on an approximate calculation to tell you “how much”.
Nuclear reactor
This is called a “swimming pool” reactor because the nuclear fuel, built into metal rods, is held in a framework at the bottom of a deep pool of water. The water serves as a shield to protect workers from the radiation and also helps the reactor “go” by slowing down neutrons to make them more likely to interact with the target atoms. “Swimming pool” reactors are frequently used for neutron activation analysis and typically provide neutron fluxes of over 10¹³ (10 million million) neutrons per square centimeter per second.
Quartz capsules
These sealed quartz capsules contain samples to be irradiated in a nuclear reactor. They are about to be placed in the aluminum can, which will be sealed and positioned at the end of an aluminum pole, close to the core of a “swimming pool” reactor. Often samples are placed in plastic tubes and are carried in and out of a reactor by air pressure in a pneumatic tube system.
You carefully scrape off a small amount of material, weigh it on a sensitive balance, and put it into a short piece of pure quartz tubing. You do the same with an ordinary piece of silicon for comparison and then seal both tubes with an oxygen-gas torch. Although the tubes are both ¼ inch in diameter and about 1 inch long, the first tube is just slightly longer so you will be able to determine which is which after the irradiation.
Off it goes to the reactor in a carefully wrapped package along with instructions to irradiate the tubes for 12 hours in a neutron flux of about 10¹³ neutrons per square centimeter per second and to return them as quickly as possible after they are removed from the reactor.
The following week, the samples are delivered about 4 hours after they were removed from the reactor. Working quickly but carefully, you note that they are radioactive but easily handled by ordinary laboratory techniques. You break the quartz tubes one at a time and attach each of the two pieces of silicon to a card with self-sticking tape. Then you place each card, in turn, on a holder close to the gamma-ray detector for a period of 10 minutes. A spectrum, which is a graph of the quantity of radiation recorded in each increment of energy over the range observed for each of the samples, is plotted automatically at the end of the counting period and you may now compare the compositions of the two samples. (See thefigure on the next two pages.)
The two spectra are virtually identical except that the suspect sample has one obviously different peak in channel 157 and a somewhat smaller peak in channel 183. Referring to an energy calibration curve for the pulse height analyzer, you find that these channels correspond to 0.559 and 0.657 MeV respectively. A search of a table of nuclides, arranged by gamma-ray energy, reveals that this combination is emitted by arsenic-76, which would be the activation product for arsenic. Other data also indicate that for arsenic there should be a number of smaller peaks, including some corresponding to energies of 1.216, 1.228, 0.624, and 1.441 MeV. A closer look at the spectrum of the suspect sample reveals that these are also present.
Finally, noting that the half-life of arsenic-76 is approximately 27 hours, you wait a day and count the sample again in the same position as the previous count. A decrease in the heights of the 0.559 and 0.657 MeV peaks, by a little less than half in 24 hours, confirms that arsenic is the unusual element in this sample. It may not be the only impurity causing the peculiar behavior of this semiconductor, but it does seem a likely candidate.
Graph: “Counts in 20 minutes per 3.8 KeV channel” _vs_ “Channel Number”
The gamma-ray spectrum obtained after activation of a sample of “pure” silicon having “ordinary” properties of this type of semiconductor. Only very small quantities of various trace impurities are indicated.
Graph: “Counts in 20 minutes per 3.8 KeV channel” _vs_ “Channel number”
The gamma-ray spectrum obtained after activation of a sample of silicon having “unusual” electrical properties. While most of the spectrum is identical with that obtained from the ordinary material, there is an interesting difference.
Using the equation given onpage 12, the approximate known values for half-life, sample weight, neutron flux, and periods of irradiation and decay after irradiation, and an estimated value for the number of arsenic-76 atoms measured by the gamma-ray spectrometer, you calculate that the arsenic content of the sample is approximately 44 parts per million (ppm). (See appendix.)
With this information as a starting point, you are now ready to proceed with further research on the properties of your semiconductor, e.g., if you double the concentration of arsenic, how will that affect its properties?
You are a physician treating a patient who, because of a severe calcium deficiency, has been suffering from osteoporosis (a softening of the bones). You think you are on the right track with your treatment, but you would like to be sure in order to know whether you should continue the treatment or try something else. You would have your answer if you knew that the calcium content of his skeleton had stopped decreasing. How can you determine the amount of calcium in a living human being?
You know that the usual techniques for determining calcium in the bones are not very useful. They are either too inaccurate to show that your patient’s calcium loss has been stopped or can only be used to measure the calcium content of the bones in his extremities. The latter is not satisfactory because these few bones may not be representative of the rest of his skeleton.
Recently, however, there have been reports of neutron activation analysis of whole persons, in which the calcium content of their bones has been measured with unusually good reliability. This has been accomplished by scientists and doctors working at the University of Washington School of Medicine in Seattle.
You manage to obtain an appointment for your patient and you accompany him to the hospital for the analysis. There he is placed on a rotating platform with his head encircled by a plastic helmet and his arms and legs submerged in a water-filled plastic container. See thephotograph on the next page. The platform is located in a beam of neutrons emanating from a beryllium target 15 feet away, which is being bombarded by deuterons from a 22-MeV cyclotron. The purpose of the water is to surround the bones in that part of the subject’s skeleton with a neutron moderator equivalent to the body tissue surrounding the rest of his skeleton. (A neutron moderator slows down the neutrons and thus makes them more likely to activate the calcium in the bones.) On each side of the patient, there are two plastic containers permanently filled with a solution containing a known quantity of calcium. These serve as standards for the analysis.
The beam of neutrons is turned on for 35 to 40 seconds. It is then interrupted while platform and patient are rotated 180 degrees. The irradiation is resumed so that a uniform dose of neutrons bombards the patient from both front and back.
During the irradiation your patient receives a dose of radiation equivalent to approximately 10 ordinary chest X rays and one of the calcium isotopes in his bones (calcium-48) is activated to calcium-49. The latter has a half-life of only 8.8 minutes and so counting must begin soon after the irradiation.
Patient
A patient in position for whole body irradiation with neutrons generated by an accelerator. His arms and legs are surrounded by plexiglas containers filled with water and his head is encased in a plexiglas helmet. On either side of him are containers, which serve as standards, filled with an aqueous solution of a calcium salt. The patient is standing on a turntable that is rotated 180 degrees after half the irradiation is completed so that the dose of neutrons is uniformly distributed to the front and the back of the patient.
Patient
A patient in position for whole-body gamma-ray spectrometry. The detectors are scintillation crystals that produce pulses of light proportional in intensity to the energy of the gamma ray absorbed in the crystal. The patient is scanned from head to foot in approximately 12½ minutes at a rate that is varied to compensate for the gradual decay of the calcium-49 radioactivity during this period. Near the patient’s head are two calcium standard solutions in plexiglas containers.
The patient lies down in a padded aluminum box and, only 4 minutes after the irradiation is concluded, a ring of 4 gamma-ray scintillation detectors[10]begin to measure the gamma rays emitted by his body. These detectors, which are each 4 inches thick and 9⅜ inches in diameter, pass over his body from head to foot. This takes 12½ minutes and since the calcium-49 is decaying with a half-life of 8.8 minutes, the detectors are made to scan at a gradually decreasing rate to compensate for the reduced radioactivity during the laterparts of the counting period. Thefigure on the next pageshows the gamma-ray spectrum for the patient. Notice the peak corresponding to an energy of 3.1 MeV. Because there are small contributions to this energy peak from other activated products in the body, repeat counts are taken later (after the calcium-49 has decayed) so that these contributions can be measured and subtracted.
Twenty minutes after the irradiation period, the radioactivity of the calcium standards is measured by the same instrument. The ratio of the counts from your patient’s body to that of the standards is 0.210; this serves as an index of the calcium content of his body on this day. Because of the care taken to make the analysis repeatable, this index is probably accurate to about 1 or 2%.
Your patient’s disease usually results in a decrease of approximately 3% of the calcium in his body per year. Thus, by making the same measurement a year from now, you will be able to tell if your treatment is a success by noting that the calcium level in your patient’s bones has stopped decreasing at a dangerous rate.
You are an analytical chemist working for a company that makes plastic. It is 11:30 a.m. and you have been called by the plant superintendent because some of the plastic coming from the plant has been showing a yellowish-brown discoloration. There seem to be only a few possible reasons for it, but no easy way to tell which one is correct. One possibility is that a copper tank, in which the plastic is prepared, is somehow being corroded by excess acid in the raw material and minute quantities of dissolved copper are discoloring the plastic. You could prove that this is the cause if you could find copper in the plastic, but the plant superintendent wants the answer immediately because a few hours delay in production will jeopardize a valuable contract, and ordinary chemical analysis would take several hours. How can you quickly determine if there is copper present in the plastic?
Graph: “Counts per 12.5 minutes/50 KeV channel” _vs_ “Channel no.”
A portion of the gamma-ray spectrum obtained after neutron activation of a human body. The area in the 3.10-MeV peak, which is above the background due to sodium and chlorine activities, is a measure of the quantity of calcium in the body of the subject. A computer may make the necessary corrections due to the background (which results from overlapping of part of the other gamma-ray peaks).