,the ,atomic ,fingerprint3 ,neutron ,activation ,analysis by ,bernard ,keisch ,,contents ,,introduction #d ,,what ,,is ,,neutron ,,activation ,,analysis8 #e ,,the ,,sensitivity ,,of ,,neutron ,,activation ,,analysis #aj ,,how ,,and ,,where ,,to ,,use ,,it #ai ,in a ,physics ,laboratory #ai ,in a ,hospital #bh ,in a ,plastics ,plant #cb ,in a ,museum #ce ,in a ,criminology ,laboratory #db ,,summing ,,up3 ,,what ,,lies ,,ahead #df ,,appendix #di ,,reading ,,list #eb ,,motion ,,pictures #ed ,u4 ,s4 ,energy ,research and ,development ,administration ,office of ,public ,affairs ,washington1 ,d4,c4 #bjede ,library of ,congress ,catalog ,card ,number3 #gi-ahbeef #aigb ,illustration3 ,photograph1 spiral galaxy ,the ,u4 ,s4 ,energy ,research and ,development ,administration publishes a series of booklets for the general public4 ,please write to the following address for a title list or for information on a specific subject3 ,,userda--,technical ,information ,center ,p4 ,o4 ,box #fb ,oak ,ridge1 ,tennessee #cghcj ,illustration3 ,photograph3 shelf with china ,a #aith century photograph restored by neutron activation4 ,this picture1 which is in the collection of the ,smithsonian ,institution1 was exposed to neutrons in a nuclear reactor and then placed in contact with modern photographic film4 ,the original1 which had been taken by ,william ,henry ,fox ,talbot who began his career in #ahcd1 is badly faded4 ,,introduction ,you are a physicist investigating the properties of semiconductors1 which are materials used to make transistors4 ,the electrical properties of one specimen are not quite like the others that you've studied4 ,what makes this specimen different8 ,,or ,you are a physician treating a patient who1 because of a severe calcium deficiency1 has been suffering from osteoporosis 7a softening of the bones74 ,are you on the right track with your treatment8 ,,or ,you are an analytical chemist working for a plastics manufacturer4 ,you have been asked by the plant superintendent to determine why some of the plastic coming from the plant has been discolored4 ,,or ,you are a curator working with the ancient coin collection in a large museum4 ,a donor has just given the museum a group of #ej gold coins presumably about #aejj years old4 ,are they genuine8 ,,or ,you are a scientist working in the criminology laboratory of a large metropolitan city4 ,a detective brings you a minute sample of paint taken from the clothing of a hit-and-run victim4 ,he has a suspect whose automobile paint seems to match that sample4 ,can you determine his guilt or innocence8 ,neutron activation analysis can be used to solve each of these problems and many more4 ,the solutions to these particular problems are explained on pages #ai-df4 ,the ,atomic ,fingerprint3 ,neutron ,activation ,analysis by ,bernard ,keisch ,,what ,,is ,,neutron ,,activation ,,analysis8 ,to understand neutron activation analysis1 you should be acquainted with a few basic concepts4 ,the nuclei of atoms are stable only when they contain certain numbers of neutrons and protons4 ,the number of protons in an atom's nucleus determines an element's identity2 the number of neutrons usually determines whether or not that atom is radioactive or nonradioactive 7stable74#a ,thus1 while all sodium atoms contain #aa protons1 only those sodium atoms that contain #ab neutrons are stable4 ,a radioactive sodium atom contains a different number of neutrons4 ,for other elements1 there may be more than one number of neutrons that results in stability2 for instance1 there are #aj stable atoms 7isotopes7 of tin1 each containing a different number of neutrons in their nuclei4 ,the fact that nuclei can absorb additional neutrons1 which1 in many cases1 results in the conversion of a stable nucleus to a radioactive one1 makes neutron activation analysis possible4 ,because radioactive nuclei decay in unique ways and yield radiations that are often distinct and can be measured even in very small amounts1 measurements of these radiations can determine the kind and the number of radioactive atoms that are present4 ,in the most common type of activation analysis1 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 motion4 ,in this case1 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 with4 ,thus for sodium as found in nature 7symbol ,na7 sodium-#bc a neutron radioactive sodium-#bd gamma rays#b ,the numbers denote the atomic weight of the atom1 which is the total number of protons and neutrons in its nucleus4 ,in a nuclear reactor1 there are many1 many neutrons that can be used in this reaction2 approximately #aj to #aj 7#aj is a million million2 #aj is a hundred times #aj7 pass through each square centimeter of target area every second4 ,not all these will strike the nuclei of sodium atoms4 ,of those that do1 not all will be captured4 ,a mathematical relationship that tells how many atoms of sodium-#bd will be created in a cubic centimeter of the target in one second is3 ,n ,nt where ,n is the number of sodium-#bd atoms created during each second in a cubic centimeter of the target2 ,n is the number of atoms of sodium-#bc in a cubic centimeter of the target2 is the number of neutrons crossing a square centimeter per second 7called the neutron flux72 t is the time in seconds that the target is in the reactor2 and is a number that represents the probability that the conversion of sodium-#bc to sodium-#bd will occur4 ,this last number is called a 8cross section0 and it is expressed in 8barns04 ,one barn is equal to #aj-bd square centimeter1 which is approximately the cross-sectional area of a typical atomic nucleus4 ,in an activation analysis experiment1 the analyst wants to determine the number of target atoms 7,n in the above example74 ,he can measure how long the target was in the nuclear reactor2 there are ways of measuring the neutron flux1 2 and the cross section is fixed and generally known for each target nucleus4 ,so1 by measuring the number of radioactive atoms created 7,n71 he can calculate the number of target atoms4 ,see the figure on the next two pages4 ,actually1 to get the most accurate results1 there are certain practical tricks he can use that increase the accuracy4 ,some of these will become apparent in later sections of this booklet4 ,the most important of these 8tricks0 is the use of a 8standard0 or 8comparator04 ,this comparator is similar in form and composition to the sample to be measured but contains a known quantity of the element to be determined4 ,the steps used for the analysis are simple4 #a4 ,put the sample and comparator together into a reactor and bombard them with neutrons4 #b4 ,remove them and measure the radioactivity produced from the sample4 #c4 ,compare the radioactivity of the sample and the comparator and calculate the amount of the element in the sample as a proportion3 7,radioactivity in sample7_/7,radioactivity in comparator7 7,quantity of element in sample7_/7,quantity of element in comparator7 ,neutron ,activation ,analysis3 ,detecting ,sodium in a ,sample of ,plastic ,step #a4 ,weigh a sample and a standard in quartz tubes4 ,illustration3 ,piece of plastic2 ,standard sodium carbonate ,step #b4 ,seal tubes in package for reactor irradiation4 ,illustration3 ,sealed aluminium can2 ,sealed quartz tubes2 ,sample2 ,standard ,step #c4 ,bombard with neutrons for about #c hours in a reactor4 ,illustration3 ,neutrons ,step #d4 ,remove sample and standard from tubes and place in separate plastic containers to measure gamma rays4 ,illustration3 ,pulse height analyser2 ,sample2 ,standard2 ,gamma rays from ,na-#bd2 same container1 distance1 detector2 ,sodium iodide scintillator ,step #e4 ,obtain gamma-ray spectrum for sodium-#bd in both sample and standard4 ,illustration3 7chart7 ,energy vs4 ,sample spectrum2 ,energy --,standard spectrum ,step #f4 ,use standard to calculate #a4cg ,me,v gamma rays counted per minute per gram of sodium 7c_/m_/g,na74 c_/m_/g,na 7counts_/minute measured in #a4cg peak 7shaded area above77_/7grams of sodium known to be in standard 7step #a477 ,step #g4 ,use c_/m_/g,na and #a4cg ,me,v gamma rays counted per minute in sample to calculate grams of sodium in sample4 grams ,na in sample 7counts_/minute measured in sample7_/7c_/m_/g,na 7step #f477 ,step #h4 ,calculate percent sodium in sample4 sodium 7grams sodium in sample 7step #g477_/7weight of sample 7step #a477 #ajj ,,the ,,sensitivity#c ,,of ,,neutron ,,activation ,,analysis ,there are several factors that determine the sensitivity of the method4 ,some are variable within limits and some1 like the cross section1 are fixed4 ,time is variable to a degree1 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 analysis4 ,the crucial step in the analytical procedure is the measurement of the number of radioactive atoms that were created4 #a4 ,how do we measure how many radioactive atoms are present8 #b4 ,since there will usually be a mixture of elements in a target1 and many of these will be made radioactive1 how can we tell one from another8 #c4 ,since radioactive atoms are constantly 8disappearing0 by radioactive decay1 how do we obtain the number of atoms created from a measurement made some time after the bombardment has taken place8 ,and what of those atoms disintegrating while others are still being created in the reactor8 ,radioactive atoms almost always decay by emitting negatively charged beta particles usually accompanied by gamma rays4 ,instruments can detect these kinds of radiation1 and it is by measuring the radiation that we determine how many radioactive atoms are present4 ,to do this we have to know the types of radiation emitted by the radioactive atoms we are trying to measure4 ,fortunately each kind of radioactive atom decays with a unique 8pattern0 scientists call a 8decay scheme04 ,the figure on the next page shows a simplified decay scheme for manganese-#ef1 which is produced by activation of manganese1 and a diagram showing what the decay scheme means4 ,until a few years ago1 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 energies4 ,today instruments are available that can really pick them out of a complex mixture4 ,thus it is usually possible to 8separate0 with electronic instruments the radioactive element we are interested in measuring4 ,some of the examples below will show how this might be accomplished4 ,each radioactive nuclide#d also has a characteristic half-life1#e which is a measure of how fast the radioactive atoms change 7transmute7 to atoms of another element4 ,in a reactor1 even while they are being produced in the target1 atoms of the radioactive nuclide are decaying with the particular half-life of the nuclide4 ,the mathematical laws that govern this process tell us that the number of atoms determines the amount of decay2 i4e41 the more atoms there are1 the greater the amount of decay in a given period of time4 7,the fraction that decays in that time is constant47 ,as a result1 the target eventually becomes 8saturated01 that is1 the rate of production equals the rate of decay4 ,when the irradiation is first begun1 the number of radioactive atoms increases steadily4 ,but eventually1 this rate of increase slows down until1 at saturation1 further irradiation no longer increases the number of radioactive atoms present in the target4 ,illustration3 ,the energy of a gamma ray is equal to the energy difference between the two levels involved in the gamma-ray emission4 ,an energy level diagram4 ,the slanted arrows indicate radioactive decay by beta-particle emission4 ,in each case1 manganese-#ef decays to a certain energy level of iron-#ef4 ,on the right the energy of each level is indicated4 ,following a beta emission to a high-energy 7excited7 state in iron-#ef1 one or more gamma rays are emitted until the nucleus is de-excited to the level marked zero4 ,the vertical arrows indicate gamma rays emitted during the de-excitation process4 ,the energy of each gamma ray is the difference between the levels involved in the change4 ,the numbers above the vertical arrows indicate the relative proportions of gamma rays of different energies emitted from that level4 ,the mathematical relationship that describes the irradiation process exactly is3 ,a ,n 7#a - e-t7 where ,a is the radioactivity produced 7disintegrations per cubic centimeter per second72 ,n is the number of target atoms per cubic centimeter in the sample2 is the neutron flux 7neutrons per square centimeter per second72 is the cross section for the reaction 7square centimeters72 is the disintegration constant#f for the radioactive atoms produced 7number per second72 the number 8e0 is the base of natural logarithms2 and t is the irradiation time in seconds4 ,note that for short irradiation times 7t very small71 #a-;e-t approximates t1 while for long irradiations 7t very large71 #a-;e-t approximates #a4 ,illustration3 ,graph3 ,decay scheme for manganese-#ef ,maximum ,energy ,betas #b4hd ,me,v #ec #a4jc ,me,v #cj #j4gbj ,me,v #af #j4cj ,me,v #a ,gammas #j4hdg ,me,v #fh #a4haa ,me,v #bj #b4aaj ,me,v #aj ,other ,energies #b #b4edc ,me,v #b4feh ,me,v #b4ieg ,me,v #c4ci ,me,v 7avge #a4d gammas per beta7 ,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-#ef4 ,thus1 you could observe more than three times as many gamma rays having an energy of #j4hdg ,me,v than of #a4haa ,me,v1 etc4 ,note that while one1 and only one1 beta is emitted in the decay of one atom of manganese-#ef1 two gammas can sometimes be emitted in one decay4 ,of course1 when the target is removed from the reactor1 the number of radioactive atoms begins to decrease according to the characteristic half-life of the nuclide4 ,the mathematical expression that describes the process of radioactive decay of a single nuclide is3 ,at ,ae-t where ,at is the radioactivity of an isotope at some time1 t1 after the end of the irradiation1 and ,a is the radioactivity at the end of the irradiation4 ,illustration3 ,fraction of saturation sodium-#bd activity vs ,time of irradiation 7hours7 ,the activation of sodium-#bc to sodium-#bd1 which has a half-life of #ae hours4 ,the horizontal line marked #a4j represents the 8saturation0 activity level for a sample of sodium of a certain size in a constant neutron flux4 ,note that after about #abj hours1 the activity of the sample is within #a of the value at saturation1 which is the most active that sample will ever become at a given 4 ,note also that after the first #ae hours 7#a half-life7 the sample is exactly half way to its value at saturation4 ,thus long irradiations are useful to increase the sensitivity of the analysis1 but only up to a certain point4 ,the result of all this is that the sensitivity of an analysis depends in practice on a number of practical as well as theoretical factors3 #a4 ,the cross section of the target element4 #b4 ,the half-life of the radioactive isotope produced4 #c4 ,the time available for irradiation4 #d4 ,the flux of neutrons available for irradiation4 #e4 ,the promptness with which we can begin measuring radioactivity and the efficiency of this measurement4 #f4 ,possible interferences due to the presence of elements yielding the same radioactive elements or those yielding very similar radiations4 ,in the next section of this booklet1 there are several examples that will show you how all this works in practice4 ,but to summarize what these factors mean in terms of sensitivity let us look at the chart in the figure on page #ah4 ,here all the elements are arranged in a periodic table4 ,the sensitivities are shaded in coded ranges representing measurable quantities4 ,they are calculated on the basis that there are no interferences1 that the neutron flux is #aj neutrons per square centimeter per second1 and that we can measure #ajj gamma rays per minute without much difficulty assuming a gamma-ray detector efficiency#g of #aj4 ,the elements labeled 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 measurements4 ,such chemical separation procedures 7to remove unwanted radioactive isotopes of other elements7 are also sometimes useful to improve the sensitivity of the analysis of gamma-ray emitters if necessary4 ,illustration3 ,graph3 8,fraction of ,sodium-#bd remaining0 vs4 8,time of decay 7hours70 ,the radioactive decay curve of sodium-#bd4 ,the vertical scale is not linear but logarithmic4 ,thus1 each factor of two in radioactivity occupies the same distance along the vertical axis4 ,when two samples are being analyzed for sodium by activation analysis1 they must be compared at the same time after they have been removed from the neutron flux4 ,if this period of time is different1 then a correction must be applied to one of them1 based on the decay curve shown here1 to allow for the difference in decay time for the two4 ,waiting too long after the irradiation is completed results in much poorer sensitivity for the analysis depending on the half-life of the activation product4 ,in this case1 after #b 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 reactor4 ,it is not practical to determine a few elements1 shown in black squares1 by activation analysis4 ,some others1 like oxygen and nitrogen 7labeled ,,he71 can be measured by using other projectiles like fast 7more energetic7 neutrons1 or protons or deuterons#h produced in a device called an accelerator4 ,other elements1 those shown in white squares1 can be detected with such great sensitivity1 that one can find some in almost everything4 ,for example1 if you had a cube of 8pure0 aluminum only #a millimeter on a side1 you could detect gold in it if there were only one atom of gold for every fifty billion atoms of aluminum4 ,while it isn't often that you would want to find a gold needle in an aluminum haystack1 the next section presents some practical applications4 ,imagine yourself as the person with the problem in these situations4 ,illustration3 ,periodic table of elements1 with sensitivity code 99 ,th and ,u are radioactive but with such long half-lives that neutron activation analysis can be used for their determination4 g ,microgram 7one-millionth of a gram7 ,,how ,,and ,,where ,,to ,,use ,,it ,in a ,physics ,laboratory ,the ,problem ,you are a physicist investigating the properties of semiconductors1 which are materials used to make transistors4 ,when you apply a voltage to one specimen of silicon 7a semiconductor71 it doesn't behave quite like the others that you've studied4 ,the electrical properties of this odd specimen are unusual and interesting and could lead to a new type of transistor4 ,what makes this specimen different from the others8 ,very small amounts of impurities can cause large changes in the electrical properties of semiconductors4 ,you would like to obtain a chemical analysis of the material1 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 up4 ,how do you do it8 ,the ,solution ,you decide to try neutron activation analysis4 ,you realize you won't be able to detect all the elements1 but many of those that might affect semiconductor performance could be detected quite easily4 ,what will you need8 ,a source of neutrons to activate the material and a gamma-ray spectrometer to measure the radiation from the material afterwards4 ,this spectrometer detects and measures gamma rays and sorts them according to their energy4 ,you find that your friend down the hall1 who is a nuclear physicist1 has a gamma-ray spectrometer that incorporates a lithium-drifted germanium crystal as a detector and a pulse height analyzer4 ,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 rays4 ,it was invented only a few years ago and has a very fine resolution4 ,that is1 it can easily 8pick out0 gamma rays that are only slightly different in energy4 ,for example1 for gamma rays with energies of approximately #a ,me,v 7million electron volts71 it is not unusual to distinguish between gamma rays that differ by only #b or #c tenths of a percent4 ,the pulse height analyzer is an electronic device that sorts the electrical pulses from the detector according to their energy4 ,illustration3 ,gamma-ray detector ,a lithium-drifted germanium-crystal gamma-ray detector4 ,the large container is a reservoir of liquid nitrogen that keeps the detector cooled to a temperature of -#aif ,centigrade 7#cba below zero1 ,fahrenheit74 ,the lead brick shield keeps out most of the gamma rays that come from naturally radioactive materials in the room4 ,the plastic slots hold cards upon which the samples are mounted for counting4 ,sometimes the detector is arranged vertically and samples are placed on shelves above it4 ,illustration3 ,gamma-ray detector ,what about the neutrons for the irradiation8 ,although there isn't a suitable nuclear reactor#i in your city1 there is one at a university only an hour away by jet4 ,since it may take a few hours to get the sample to the counter after irradiation1 you won't be able to look for short-lived activation products1 i4e41 those with half-lives of up to an hour4 ,however1 this will exclude only a few elements from detection4 ,illustration3 ,pulse-height analyzer ,a pulse-height analyzer used for gamma-ray spectrometry4 ,a gamma-ray spectrum is displayed on the television screen4 ,data is printed out automatically on the electric typewriter and also may be plotted as a graph on the paper to the left4 ,in other systems1 data may be coded onto punched paper tape as well4 ,such tape may be 8read0 by a computer that can be programmed to use the data to calculate what radioactive isotopes are present and their quantities4 ,now you are ready to begin the analysis4 ,this will be a qualitative analysis since you are merely looking for a significantly different element in that silicon crystal4 ,how much of it is present is only of secondary interest4 ,therefore1 if you find anything different1 you will rely on an approximate calculation to tell you 8how much04 ,illustration3 ,nuclear reactor ,this is called a 8swimming pool0 reactor because the nuclear fuel1 built into metal rods1 is held in a framework at the bottom of a deep pool of water4 ,the water serves as a shield to protect workers from the radiation and also helps the reactor 8go0 by slowing down neutrons to make them more likely to interact with the target atoms4 8,swimming pool0 reactors are frequently used for neutron activation analysis and typically provide neutron fluxes of over #aj 7#aj million million7 neutrons per square centimeter per second4 ,illustration3 ,quartz capsules ,these sealed quartz capsules contain samples to be irradiated in a nuclear reactor4 ,they are about to be placed in the aluminum can1 which will be sealed and positioned at the end of an aluminum pole1 close to the core of a 8swimming pool0 reactor4 ,often samples are placed in plastic tubes and are carried in and out of a reactor by air pressure in a pneumatic tube system4 ,you carefully scrape off a small amount of material1 weigh it on a sensitive balance1 and put it into a short piece of pure quartz tubing4 ,you do the same with an ordinary piece of silicon for comparison and then seal both tubes with an oxygen-gas torch4 ,although the tubes are both inch in diameter and about #a inch long1 the first tube is just slightly longer so you will be able to determine which is which after the irradiation4 ,off it goes to the reactor in a carefully wrapped package along with instructions to irradiate the tubes for #ab hours in a neutron flux of about #aj neutrons per square centimeter per second and to return them as quickly as possible after they are removed from the reactor4 ,the following week1 the samples are delivered about #d hours after they were removed from the reactor4 ,working quickly but carefully1 you note that they are radioactive but easily handled by ordinary laboratory techniques4 ,you break the quartz tubes one at a time and attach each of the two pieces of silicon to a card with self-sticking tape4 ,then you place each card1 in turn1 on a holder close to the gamma-ray detector for a period of #aj minutes4 ,a spectrum1 which is a graph of the quantity of radiation recorded in each increment of energy over the range observed for each of the samples1 is plotted automatically at the end of the counting period and you may now compare the compositions of the two samples4 7,see the figure on the next two pages47 ,the two spectra are virtually identical except that the suspect sample has one obviously different peak in channel #aeg and a somewhat smaller peak in channel #ahc4 ,referring to an energy calibration curve for the pulse height analyzer1 you find that these channels correspond to #j4eei and #j4feg ,me,v respectively4 ,a search of a table of nuclides1 arranged by gamma-ray energy1 reveals that this combination is emitted by arsenic-#gf1 which would be the activation product for arsenic4 ,other data also indicate that for arsenic there should be a number of smaller peaks1 including some corresponding to energies of #a4baf1 #a4bbh1 #j4fbd1 and #a4dda ,me,v4 ,a closer look at the spectrum of the suspect sample reveals that these are also present4 ,finally1 noting that the half-life of arsenic-#gf is approximately #bg hours1 you wait a day and count the sample again in the same position as the previous count4 ,a decrease in the heights of the #j4eei and #j4feg ,me,v peaks1 by a little less than half in #bd hours1 confirms that arsenic is the unusual element in this sample4 ,it may not be the only impurity causing the peculiar behavior of this semiconductor1 but it does seem a likely candidate4 ,illustration3 ,graph3 8,counts in #bj minutes per #c4h ,ke,v channel0 vs 8,channel ,number0 ,element ,counts ,channel ,number ,gold #acjj #abe ,sodium #ijj #adj ,antimony #fej #aej ,antimony #eej #afj ,scandium #bbj #bdj ,iron #aej #cjj ,scandium #fj #caj ,cobalt #aaj #cbj ,iron #de #cej ,cobalt #ej #cfj ,sodium #ijj #cgj ,sodium #adj #dbe 7values estimated from the graph7 ,the gamma-ray spectrum obtained after activation of a sample of 8pure0 silicon having 8ordinary0 properties of this type of semiconductor4 ,only very small quantities of various trace impurities are indicated4 ,illustration3 ,graph3 8,counts in #bj minutes per #c4h ,ke,v channel0 vs 8,channel number0 ,element ,counts ,channel ,number ,gold ,sodium #j4eei ,arsenic-#ge #aejj #aej ,antimony #j4fbd ,arsenic-#ge #aij #afe #j4feg ,arsenic-#ge #ace #age ,scandium ,iron ,scandium ,cobalt #a4baf ,arsenic-#ge #ajj #ccj #a4bbh ,arsenic-#ge #fj #cce ,iron ,cobalt ,sodium #a4dd ,arsenic-#ge #be #chj ,sodium 7distinct values estimated from graph7 ,the gamma-ray spectrum obtained after activation of a sample of silicon having 8unusual0 electrical properties4 ,while most of the spectrum is identical with that obtained from the ordinary material1 there is an interesting difference4 ,using the equation given on page #ab1 the approximate known values for half-life1 sample weight1 neutron flux1 and periods of irradiation and decay after irradiation1 and an estimated value for the number of arsenic-#gf atoms measured by the gamma-ray spectrometer1 you calculate that the arsenic content of the sample is approximately #dd parts per million 7ppm74 7,see appendix47 ,with this information as a starting point1 you are now ready to proceed with further research on the properties of your semiconductor1 e4g41 if you double the concentration of arsenic1 how will that affect its properties8 ,in a ,hospital ,the ,problem ,you are a physician treating a patient who1 because of a severe calcium deficiency1 has been suffering from osteoporosis 7a softening of the bones74 ,you think you are on the right track with your treatment1 but you would like to be sure in order to know whether you should continue the treatment or try something else4 ,you would have your answer if you knew that the calcium content of his skeleton had stopped decreasing4 ,how can you determine the amount of calcium in a living human being8 ,the ,solution ,you know that the usual techniques for determining calcium in the bones are not very useful4 ,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 extremities4 ,the latter is not satisfactory because these few bones may not be representative of the rest of his skeleton4 ,recently1 however1 there have been reports of neutron activation analysis of whole persons1 in which the calcium content of their bones has been measured with unusually good reliability4 ,this has been accomplished by scientists and doctors working at the ,university of ,washington ,school of ,medicine in ,seattle4 ,you manage to obtain an appointment for your patient and you accompany him to the hospital for the analysis4 ,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 container4 ,see the photograph on the next page4 ,the platform is located in a beam of neutrons emanating from a beryllium target #ae feet away1 which is being bombarded by deuterons from a #bb-,me,v cyclotron4 ,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 skeleton4 7,a neutron moderator slows down the neutrons and thus makes them more likely to activate the calcium in the bones47 ,on each side of the patient1 there are two plastic containers permanently filled with a solution containing a known quantity of calcium4 ,these serve as standards for the analysis4 ,the beam of neutrons is turned on for #ce to #dj seconds4 ,it is then interrupted while platform and patient are rotated #ahj degrees4 ,the irradiation is resumed so that a uniform dose of neutrons bombards the patient from both front and back4 ,during the irradiation your patient receives a dose of radiation equivalent to approximately #aj ordinary chest ,x rays and one of the calcium isotopes in his bones 7calcium-#dh7 is activated to calcium-#di4 ,the latter has a half-life of only #h4h minutes and so counting must begin soon after the irradiation4 ,illustration3 ,patient ,a patient in position for whole body irradiation with neutrons generated by an accelerator4 ,his arms and legs are surrounded by plexiglas containers filled with water and his head is encased in a plexiglas helmet4 ,on either side of him are containers1 which serve as standards1 filled with an aqueous solution of a calcium salt4 ,the patient is standing on a turntable that is rotated #ahj degrees after half the irradiation is completed so that the dose of neutrons is uniformly distributed to the front and the back of the patient4 ,illustration3 ,patient ,a patient in position for whole-body gamma-ray spectrometry4 ,the detectors are scintillation crystals that produce pulses of light proportional in intensity to the energy of the gamma ray absorbed in the crystal4 ,the patient is scanned from head to foot in approximately #ab minutes at a rate that is varied to compensate for the gradual decay of the calcium-#di radioactivity during this period4 ,near the patient's head are two calcium standard solutions in plexiglas containers4 ,the patient lies down in a padded aluminum box and1 only #d minutes after the irradiation is concluded1 a ring of #d gamma-ray scintillation detectors#aj begin to measure the gamma rays emitted by his body4 ,these detectors1 which are each #d inches thick and #i inches in diameter1 pass over his body from head to foot4 ,this takes #ab minutes and since the calcium-#di is decaying with a half-life of #h4h minutes1 the detectors are made to scan at a gradually decreasing rate to compensate for the reduced radioactivity during the later parts of the counting period4 ,the figure on the next page shows the gamma-ray spectrum for the patient4 ,notice the peak corresponding to an energy of #c4a ,me,v4 ,because there are small contributions to this energy peak from other activated products in the body1 repeat counts are taken later 7after the calcium-#di has decayed7 so that these contributions can be measured and subtracted4 ,twenty minutes after the irradiation period1 the radioactivity of the calcium standards is measured by the same instrument4 ,the ratio of the counts from your patient's body to that of the standards is #j4baj2 this serves as an index of the calcium content of his body on this day4 ,because of the care taken to make the analysis repeatable1 this index is probably accurate to about #a or #b4 ,your patient's disease usually results in a decrease of approximately #c of the calcium in his body per year4 ,thus1 by making the same measurement a year from now1 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 rate4 ,in a ,plastics ,plant ,the ,problem ,you are an analytical chemist working for a company that makes plastic4 ,it is #aa3cj a4m4 and you have been called by the plant superintendent because some of the plastic coming from the plant has been showing a yellowish-brown discoloration4 ,there seem to be only a few possible reasons for it1 but no easy way to tell which one is correct4 ,one possibility is that a copper tank1 in which the plastic is prepared1 is somehow being corroded by excess acid in the raw material and minute quantities of dissolved copper are discoloring the plastic4 ,you could prove that this is the cause if you could find copper in the plastic1 but the plant superintendent wants the answer immediately because a few hours delay in production will jeopardize a valuable contract1 and ordinary chemical analysis would take several hours4 ,how can you quickly determine if there is copper present in the plastic8 ,illustration3 ,graph3 8,counts per #ab4e minutes_/#ej ,ke,v channel0 vs 8,channel no40 ,element ,counts ,channel no4 #b4ge ,me,v ,na-#bd #ejjj #aj #c4aj ,me,v ,ca-#di #cbjj #af #c4he ,me,v ,cl-#ch #ejj #cc #d4j ,me,v ,ca-#di #ajj #ci 7,values estimated from graph7 ,a portion of the gamma-ray spectrum obtained after neutron activation of a human body4 ,the area in the #c4aj-,me,v peak1 which is above the background due to sodium and chlorine activities1 is a measure of the quantity of calcium in the body of the subject4 ,a computer may make the necessary corrections due to the background 7which results from overlapping of part of the other gamma-ray peaks74 ,the ,solution ,one reason that ordinary analytical methods are so slow1 in this case1 is because the amount of copper you are looking for is so small that you would have to dissolve a large amount of plastic to get enough copper to measure4 ,you know that nearly all the plastic is carbon1 hydrogen1 and oxygen and that none of these elements are easily made radioactive when they are bombarded with low-energy neutrons4 ,you look in a table to see if copper is easily activated4 ,you find that there are two stable isotopes of copper having atomic weights of #fc and #fe4 ,each of these is easily activated1 giving radioactive isotopes1 copper-#fd and copper-#ff4 ,the latter has a half-life of about #e minutes and emits gamma rays with energies of #a4jci ,me,v1 which are easy to measure4 ,in the research building next door1 there is a small reactor that can irradiate encapsulated samples with low-energy neutrons at the rate of a million million neutrons per square centimeter per second 7#aj neutrons_/cm_/sec74 ,you calculate that if you irradiate only one tenth of a gram of the plastic for #aj minutes1 and if the plastic contains only one part of copper in one million parts of plastic1 then at the end of the irradiation the radioactive copper formed will be emitting over #djj gamma rays per second4 ,there is a pneumatic tube that can remove the irradiated sample in #bj seconds1 and you decide that it will take only a minute or two to remove the sample from its capsule and get it into a gamma-ray counter located nearby4 ,the counter is a scintillation counter that is connected to a pulse-height analyzer4 ,if you count for only #aj minutes you will detect about #ajjj gamma rays of the right energy 7allowing for the inefficiencies of the detector system74 ,this sounds like it should do the job4 ,but does the good plastic contain copper too8 ,and how much does it take to produce the discoloration8 ,you decide to use neutron activation analysis and to analyze samples of faulty plastic1 normal plastic1 and a small piece of copper foil1 which you have weighed and sealed in a small polyethylene bag as a standard4 ,your results are shown in the table below4 ,sample ,counts in #aj minutes#aa #j4a grams faulty plastic #ajj1jjj #j4a grams good plastic #a1jjj #j4a milligrams of pure copper #a1jjj1jjj ,it worked6 ,the faulty plastic contains #ajj times as much copper as the good plastic1 specifically #ajj parts per million4 7,if #j4a milligrams of pure copper gave #a1jjj1jjj counts1 then the #j4a grams of faulty plastic contains 7#ajj1jjj_/a1jjj1jjj7 #j4a milligrams or #j4ja milligrams of copper4 ,this is one ten thousandth of the weight of the plastic or #j4ja or #ajj ppm47 ,you relay the information to the plant superintendent almost before he finishes his lunch4 ,he now knows what to do and the crisis is over4 ,in a ,museum ,the ,problem ,you are a curator working with the ancient coin collection of a large museum4 ,a donor has just given the museum a group of #ej gold coins presumably about #aejj years old4 ,after months of careful study1 you have satisfied yourself that most of those coins are genuine specimens of that period4 ,judging from your experience1 you decide that a small group of five are definite forgeries4 ,however1 there are three others that you suspect are also fakes1 but you are not quite certain4 ,you know that both genuine minters and forgers often tried to save money by diluting their gold with less expensive metals such as silver and copper4 ,since the chances are slim that the forger's product has the same concentration of gold1 silver1 and copper as the genuine coins1 you realize that a chemical analysis would help you decide if the doubtful pieces were real or fake4 ,an accurate chemical analysis would require a sample of such size that the coin would be ruined as a museum specimen4 ,you need an analytical method that can be applied to an infinitesimal sample4 ,the ,solution ,you are not a scientist but you've heard about neutron activation analysis4 ,therefore1 you contact a radiochemist at a local university who is an expert in this field4 ,he decides to use a sampling technique developed by scientists at ,brookhaven ,national ,laboratory for sampling metal objects of archaeological interest4 ,you obtain from him a set of #ej quartz plates that have been ground on one side4 ,following his instructions1 you carefully scrape away a small area on the edge of each coin4 ,you then rub each freshly cleaned area across the ground surface of one plate leaving a minute streak of metal similar to a pencil mark4 ,at the scientist's laboratory1 each plate is carefully placed inside a quartz tube4 ,no attempt is made to weigh the tiny streak of metal since you wish only to compare the ratios of the metal concentrations4 ,however1 because the samples make a rather bulky package1 the scientist is concerned with the uniformity of the neutron flux that each sample will 8see04 ,he therefore also places in each tube an exactly equal weight of a gold--silver--copper alloy wire 7of known proportions7 to act as a standard neutron-flux monitor4 ,the tubes are then sealed and taken to a reactor to be irradiated for #ab hours4 ,after the samples are removed from the reactor1 the scientist carefully breaks open each of the quartz tubes and places the sample and the standard piece of wire in separate numbered plastic capsules with lids4 ,for an accurate comparison1 each capsule is prepared in the same manner4 ,about #d hours after the samples are removed from the reactor1 he begins the radioactivity measurements4 ,the sample capsules are loaded into an automatic sample-changing mechanism that places each one into an identical position above a lithium-drifted germanium detector4 7,see the chapter beginning on page #ai47 ,gamma-ray spectra are collected all day1 first from a sample1 then from its accompanying standard4 ,each count takes #b minutes1 and #c minutes are required between counts for data printout and sample changing4 ,a typical gamma-ray spectrum looks like the one in the figure on the next page4 ,notice that only gold 7gold-#aih7 and copper 7copper-#fd7 show up in this short counting time4 ,later on1 radioactivity from silver 7silver-#aajm7 can be measured using a longer counting time4 ,this can be done because while the activation products from copper and gold have relatively short half-lives 7#ab4h hours and #b4g days1 respectively71 that from silver has a half-life of #bgj days4 ,to increase the sensitivity of the analysis for silver1 the scientist repackages and re-irradiates the samples and wires for #ajj hours4 ,silver-#aajm is one of two radioactive isotopes of silver that have the same mass4 ,in this case1 one has a higher energy than the other and decays in a different way4 ,this is known as an isomeric state and it occurs for many other elements as well as for silver4 ,illustration3 ,graph3 8,counts in one minute per #c4a ,ke,v channel0 vs 8,channel0 ,element ,counts ,channel number #j4aeh ,ms,v ,au-#aii #fjj #dj #j4dab ,me,v ,au-#aih #hjj #abj #j4eaa ,me,v ,cu-#fd #ej #aej #j4fgf ,me,v ,au-#aih #dj #bje 7,values estimated from graph7 ,the spectrum obtained from a streak of metal on a quartz plate after a #c-;hour exposure to neutrons in a reactor and a #f-;hour delay before counting4 ,the activation products of gold and copper are obviously present and are easily measured in only #a minute4 ,illustration3 ,graph3 8,counts per #ajj minutes per #c4b ,ke,v channel0 vs 8,channel number0 ,energy ,counts ,channel ,number #j4dde ,me,v #bcj #acj #j4feg ,me,v #abjj #aij #j4fhg ,me,v #agj #bjj #j4gae ,me,v #bbj #bbj #j4gfj ,me,v #bcj #bbe #j4hbj ,me,v #fj #bej #j4hhd ,me,v #ejj #bfj #j4icg ,me,v #aij #bhj 7,values are estimated from graph7 ,the spectrum obtained from the same streak of metal after re-exposure to neutrons for #ajj hours and a delay of approximately #b months before counting4 ,activation products from gold and copper have decayed away and the gamma-ray spectrum of silver-#aajm is now observed4 ,in this case the sample is closer to the detector than for the earlier measurement and the measurement takes #ajj minutes4 ,two months later1 the scientist repeats the procedure of counting the samples and standards1 except that this time the plastic capsules are closer to the detector1 each count is for #ajj minutes1 and the sample changer operates for about a week4 ,a typical spectrum looks like that in the figure on page #ci4 ,the scientist can now compute ratios for the three elements in each sample and compare them with the standard1 but he decides that a computer could do it faster and with fewer errors4 ,the data collected during the two series of counts are therefore sent to a data processing center where1 in a matter of minutes1 a computer does the following for each of #ej samples3 #a4 ,finds the #j4daa-,me,v gamma-ray peak for gold-#aih4 #b4 ,determines the total counts in the peak4 #c4 ,repeats the process for the corresponding wire standard4 #d4 ,corrects the total count for the wire for the small amount of radioactive decay that occurred in the few minutes between the sample count and the standard count4 #e4 ,computes the ratio3 total count for sample_/total count for standard 7corrected7 #f4 ,repeats all the above for the #j4eaa-,me,v gamma ray for copper-#fd and 7in the longer counts7 for the #j4feh-,me,v gamma ray for silver-#aaj4 #g4 ,computes the ratios3 sample to standard 7for copper7_/sample to standard 7for gold7 and sample to standard 7for silver7_/sample to standard 7for gold74 #h4 ,tabulates and prints the ratios found in ,step #g4 ,illustration3 ,graph3 ,number of coins at indicated ratios vs4 ,copper_/gold ratio ,radioactivity ratios for #ej 8gold0 coins4 ,above are the silver to gold ratios4 ,there are two groups of genuine coins4 ,five known forgeries show considerably higher ratios than the genuine coins4 ,two of the suspect coins also show high ratios but the third1 suspect ,a1 shows a ratio that falls into one of the genuine groups4 ,below are the copper to gold ratios4 ,again there are two groups of genuine coins4 7,the same coins make up the two groups here as above47 ,the five known forgeries again show higher ratios than the genuine ones and again the same two suspects appear to be forgeries4 ,suspect ,a1 however1 shows a ratio similar to one group of the genuine specimens4 ,one therefore concludes that suspect ,a is genuine and that ,b and ,c are not4 ,for example1 suppose for sample #a there are #bj1jjj counts in the #j4dab-,me,v peak 7gold71 #aij counts in the #j4eaa-,me,v peak 7copper71 and #dej counts in the #j4fed-,me,v peak 7silver74 ,suppose also that standard #a yielded #aj1jjj1 #ejj1 and #djj counts for these three peaks 7corrected for decay71 respectively4 ,then the ratio for gold would be 7#bj1jjj_/aj1jjj7 #b4jj1 the ratio for copper would be 7#aij_/ejj7 #j4chj1 and the ratio for silver would be 7#dej_/djj7 #a4ac4 ,finally1 the activity ratio of copper to gold would be 7#j4chj_/b4jj7 #j4aij1 and the activity ratio of silver to gold would be 7#a4ac_/b4jj7 #j4efe4 ,because each sample was irradiated with an identical standard1 and counted in an identical arrangement1 the last two ratios will be the same for different samples if1 and only if1 the concentrations of gold1 silver1 and copper in those samples are in identical proportions4 ,this will be true no matter where in the reactor or for how long the irradiation took place4 ,now the scientist presents the data to you4 ,you immediately see that 7a7 the good coins fall into two groups1 one with a silver to gold activity ratio of approximately #j4ef and a copper to gold ratio of approximately #j4bj and a second group with these ratios approximately #j4ea and #j4ah2 7b7 the coins you were certain were forgeries have distinctly higher ratios ranging from #j4fj to #j4fe for silver to gold and from #j4bc to #j4cj for copper to gold2 and 7c7 of the three suspected coins1 two have ratios that fall into the range of the known forgeries1 but one1 with ratios of #j4eeb and #j4aih1 is probably genuine4 ,you present the result to the museum director in the form of a graph 7see the figure on page #da7 and a few weeks later1 #dc coins are added to the permanent exhibits of the museum1 while #g are discarded4 ,in a ,criminology ,laboratory ,the ,problem ,you are a scientist working in the criminology laboratory of a large metropolitan city4 ,a detective brings you a minute sample of paint taken from the clothing of a hit-and-run victim4 ,he has a suspect whose automobile paint seems to match that sample4 ,the suspect was found in his parked automobile1 not far from the scene of the accident4 ,he seems to fit the description given by two witnesses1 and he is extremely nervous4 ,you scrape a small sample of paint from a recently damaged area of the suspect's car1 and1 7with the aid of a microscope7 find that the pigment content seems to be the same as that taken from the victim's clothing4 ,but1 are they really from the same paint8 ,the ,solution ,you know that paint1 like almost everything else1 contains very small quantities of impurities that are present only by accident and do not affect its properties as a useful material4 ,the trace impurities1 as they are called1 will vary from batch to batch of the same paint4 ,very rarely will a match be obtained in both type and concentration of trace impurities in two samples if they are not from the same batch4 ,by measuring a sufficient number of different elements1 the probability of accidentally matching two samples can be as rare as the duplication of fingerprints in two individuals4 ,matching of trace impurities is often called a 8fingerprint0 method4 ,with neutron activation analysis1 you can obtain the 8fingerprints0 of the two samples to see if they match4 ,although this kind of evidence may be difficult to use as proof in court1 a positive match will let the detective know that he is on the right track4 ,also1 the suspect might confess if he is confronted with the evidence and realizes that he is 8caught04 ,on the other hand1 a mismatch will clear the suspect completely and the detective will know to look elsewhere for the criminal4 ,you seal each sample in a tiny polyethylene bag about inch square4 ,one sample is taken from the victim's clothing and the second1 about the same size as the first1 taken from the damaged area of the automobile4 ,in preparing these samples1 you handle all the materials with clean forceps because you realize that the most minute dirt from your fingers will be detected in the analysis4 ,the two bags are irradiated together for #a hour in a nearby reactor and #b hours later you begin counting the samples with a high-resolution1 lithium-drifted-germanium1 gamma-ray spectrometer4 ,this will give you a match 7or mismatch7 for elements that yield radioisotopes of fairly short half-life such as manganese 7#b4ef hours71 copper 7#ab4h hours71 sodium 7#ae hours71 arsenic 7#bg4g hours71 etc4 ,you plan on 8counting0 the samples again later on1 if the first counts match1 so that you can check on radioisotopes with longer half-lives such as iron 7#de days71 chromium 7#bg days71 silver 7#bgj days71 cobalt 7#e years71 etc4 ,the two gamma-ray spectra you obtain look like those in the figure on the opposite page4 ,the gamma rays from the irradiated paint taken from the victim's clothing indicate the presence of the common elements sodium1 potassium1 and copper1 but gold1 lanthanum1 and europium are also conspicuously present4 ,the gamma rays from the other sample also reveal sodium1 potassium1 and gold but in rather different proportions4 ,more striking is the absence of copper and the two rare earths1 and the presence of manganese and arsenic1 which were not indicated in the first sample4 ,the paint samples definitely do not match4 ,therefore1 you inform the detective that his suspect is innocent after all4 ,you've solved your problem1 but he still has his4 ,perhaps the same technique will provide positive proof when he finds the real culprit4 ,illustration3 ,graph3 8,counts_/#aj min per #c4c ,ke,v channel 7arbitrary scale70 vs 8,channel number0 ,element ,channel number 4#abb ,me,v ,eu 7,europium7 #ce 4#cbh ,me,v ,la 7,lanthanum7 #ie 4#cdd ,me,v ,eu #if 4#dab ,me,v ,au 7,gold7 #aaj 4#dhf ,me,v ,la #abj 4#eaa ,me,v ,cu 7,copper7 #ade 4#hae ,me,v ,la #bdj 4#hcg ,me,v ,eu #bde 4#ifa ,me,v ,eu #bij #a4cg ,me,v ,na 7,sodium7 #daj #a4ec ,me,v ,k 7,potassium7 #dee #a4fj ,me,v ,la #dge 7channel numbers estimated7 ,illustration3 ,graph3 8,counts_/#aj min per #c4c ,ke,v channel 7arbitrary scale70 vs 8,channel number0 ,element ,channel number #j4dab ,me,v ,au 7,gold7 #aaj #j4eaa ,me,v ,na #ade #j4eei ,me,v ,as 7,arsenic7 #afj #j4feg ,me,v ,as #aie #j4hdg ,me,v ,mn 7,manganese7 #bej #a4ba ,me,v ,as #cgj #a4cg ,me,v ,na 7,sodium7 #daj #a4ec ,me,v ,k 7,potassium7 #dee 7channel numbers estimated7 ,gamma-ray spectra of two samples of paint4 ,these two spectra are obviously different and1 therefore1 could not have come from the same source4 ,,summing ,,up3 ,,what ,,lies ,,ahead ,these five situations are intended to show why neutron activation analysis is used1 when it can be applied1 and how it works4 ,in the real world1 there are often many reasons why this kind of analysis is used4 ,as in the situations described here1 it may be the only workable method4 ,sometimes there may be a choice of methods1 but activation analysis is used because it has certain peculiar advantages or because it happens to be the most convenient4 ,there are other times1 however1 when other analytical methods can and should be used4 ,such situations arise when the element sought is not easily activated1 or when a satisfactory alternative method exists that is more economical or more convenient4 ,the points to remember about the use of activation analysis are that3 #a4 ,in many cases1 no elaborate sample preparation procedure is required4 #b4 ,for many elements1 it is the most sensitive analytical technique known4 ,the diversity of applications in which activation analysis is used is enormous and will probably continue to be4 ,the examples given here represent only a tiny fraction of circumstances in which the method has been used4 ,consider that it has been used successfully3 #a4 ,in the microscopic world of biology and medicine2 #b4 ,for meteorites arriving from the vast reaches of space2 #c4 ,in the production lines of consumer products2 #d4 ,for precious samples of moon rocks2 #e4 ,in the most 8down-to-earth0 business of hunting for new mineral sources2 #f4 ,for exploring the causes of ,napoleon's death nearly #aej years earlier 7see photograph on next page74 ,today1 there is virtually no field of science and technology that is untouched by this method4 ,the illustrations of procedures used in the situations described in this booklet are typical of some in use today4 ,there are many other situations that require still other techniques4 ,one of the most exciting1 which will be used with increasing frequency in the future1 involves the use of computers4 ,it has been shown that data collected by high-resolution gamma-ray spectrometers can be 8fed0 directly to a computer4 ,the computer can be programmed to identify unknown components and to determine the concentrations of elements of interest to the analyst4 ,it is entirely possible to include corrections for radioactive decay1 possible interferences from other elements present1 and many other factors4 ,it appears quite likely that the kinds of analyses described here 7as well as others7 may someday be accomplished automatically1 with far smaller chances for error and probably more economically4 ,illustration3 ,hair sample ,samples of ,napoleon's hair4 ,neutron activation analysis of these hairs revealed that he had been poisoned with arsenic4 7,he died1 however1 not from arsenic poisoning1 but from acute mercury intoxication47 ,other newer techniques that may find increased usage in the future are exemplified by the method for activation analysis of the whole human body4 ,the use of neutrons produced by nuclear machines 7such as cyclotrons or other particle accelerators7 or produced by compact1 portable isotopic sources will make neutron activation analysis even more versatile4 ,isotopic sources produce neutrons as the result of a nuclear reaction4 ,one such reaction uses alpha particles emitted by polonium-#baj 7or some other alpha emitter7 to bombard the element beryllium4 ,a different kind of isotopic source is the man-made radioisotope californium-#beb that decays by fissioning 7splitting7 spontaneously and produces neutrons in the process4 7,one milligram of californium-#beb will spontaneously produce over #aj neutrons per second47 ,while californium-#beb is quite expensive at present1 it is likely that production costs will be significantly reduced in the future4 ,with computers1 more convenient radiation sources1 and continuing improvements in the technology of gamma-ray detectors and nuclear electronics1 neutron activation analysis will become more and more a routine tool of the analyst4 ,,appendix ,calculation of arsenic concentration with no standard for comparison4 #a4 ,determination of arsenic-#gf activity produced from #a microgram of arsenic at the time it comes out of the reactor4 ,we use the equation from page #ab3 ,a ,n 7#a - e-t7 where ,n is the number of target atoms4 7,one microgram of arsenic contains 7#aj-f gram_/#ge grams per mole#ab7 #f4jb #aj atoms per mole which is #h #aj atoms of arsenic47 is the neutron flux4 7,this would be known to the reactor operator4 ,it is usually measured by inserting materials of known composition and measuring their activation4 ,in this case1 #aj neutrons per square centimeter per second47 is the activation cross section4 7,neutron cross sections have been measured and tabulated by scientists4 ,for the activation of arsenic-#ge to arsenic-#gf1 the cross section is known to be #d4b #aj-bd square centimeter47 is the disintegration constant for arsenic-#gf4 7,here1 7ln #bac_/t17in hours72 t1 the half-life for arsenic-#gf1 is #bf4f hours so 7#j4fic_/bf4f7 #j4jbf47 t is the time of the irradiation4 7,here t is #ab hours47 ,therefore3 ,a1 the activity of arsenic-#gf1 #h #aj #aj #d4b #aj-bd 7#a - e-#j4jbf #ab7 7,note3 e is a physical constant1 #b4ga7 #i #aj disintegrations per second per microgram #b4 ,determination of activity of arsenic measured in the sample and corrected back to the time of removal from the reactor4 ,we use the equation3 ,a 7,r7_/7,e ,f7 et where ,r is the measured count rate4 7,in this case1 ,r is the number of counts per second observed in the #j4eei-,me,v gamma-ray peak1 which is #ecjj counts in #bj minutes or #d4d counts per second47 ,e is the efficiency of the detector4 7,in this case1 it is the number of counts observed in the #j4eei peak for each #j4eei-,me,v gamma ray emitted by a radioactive material at the sample distance4 ,this is known for the detector being used by making other measurements and1 for the set-up used here1 is #j4jaj47 ,f is the average number of #j4eei-,me,v gamma rays emitted in each disintegration of arsenic-#gf4 7,this can be deduced from the decay scheme of arsenic-#gf4 ,see the decay scheme for manganese-#ef on page #ac4 ,in the decay of arsenic-#gf the number of #j4eei-,me,v gamma rays emitted per disintegration is approximately #j4da47 is the disintegration constant for arsenic-#gf4 7#j4jbf1 see page #di47 t is the decay time4 7,this is the number of hours from the time the sample was removed from the reactor to the time it was counted1 or #e hours47 ,therefore1 ,a1 the activity of arsenic-#gf produced in the sample at the time of removal from the reactor1 7#d4d counts per second7_/7#j4jaj #j4da7 e#j4jbf #e hours #abjj disintegrations per second #c4 ,calculation of arsenic concentration in the sample4 ,we use the equation3 ,concentration in parts per million 7,a7_/7,a ,w7 #aj where ,a and ,a were determined above and ,w is the weight of sample analyzed or #cjj micrograms 7#j4jjjc gram74 ,therefore the concentration is 7#abjj7_/7i #aj #cjj7 #aj #dd parts per million4 ,,footnotes #a,there are exceptions4 ,for a few elements there are no stable nuclei4 ,in some cases1 there are other differences that make certain atoms radioactive4 #b,these gamma rays 7called prompt gamma rays because they are instantaneously produced when the neutron is captured7 can also be used for analysis and sometimes are1 but we will not be discussing this type of analysis in this booklet4 #c,sensitivity in this case means how small an amount of an unknown element can be detected4 #d,nuclide is a general term applicable to all atomic forms of elements4 ,whereas isotopes are the various forms of a single element 7hence are a family of nuclides7 and all have the same atomic number and number of protons1 nuclides comprise all the isotopic forms of all the elements4 #e,the half-life of a radioactive nuclide is the time it takes for half the nuclei in a large sample to undergo decay4 ,note that after half of them are gone1 a second half-life period will reduce the remainder by one half1 leaving one quarter of the original number4 #f,the disintegration constant is related to the half-life1 ,t1 by the expression3 natural logarithm of #b_/;half-life1 or 7ln #b7_/7,t7 7#j4fic7_/7,t7 #g,the detector efficiency is the ratio of the number of gamma rays detected to the number emitted by the sample4 #h;,a deuteron is the nucleus of a heavy hydrogen 7deuterium7 atom and consists of one neutron and one proton4 #i,not all nuclear reactors are appropriate for this work4 ,for example1 reactors designed for electric power production do not have the means built into them for inserting and removing small samples for a 8short0 period of irradiation4 #aj;,a scintillation detector is a crystalline device1 usually sodium iodide containing a small amount of thallium1 which has the property of emitting light when energy is absorbed from nuclear radiation4 #aa;,after a #aj-minute irradiation and a #c-minute delay before counting1 corrected for decay to a common time4 #ab,one mole is the atomic weight of an atom or molecule expressed in grams1 or the weight of #f4jb #aj atoms or molecules per mole4 #acln #b is the natural logarithm of #b4 ,,reading ,,list ,general ,information ,about ,nuclear ,science ,secrets of the ,nucleus1 ,joseph ,s4 ,levinger1 ,mc,graw-,hill ,book ,company1 ,new ,york1 #aifg1 #abg pp41 #j4ej4 ,working ,with ,atoms1 ,otto ,r4 ,frisch1 ,basic ,books1 ,inc41 ,publishers1 ,new ,york1 #aife1 #if pp41 #c4ej4 ,the ,atom and ,its ,nucleus1 ,george ,gamow1 ,prentice-,hall1 ,inc41 ,englewood ,cliffs1 ,new ,jersey1 #aifa1 #aec pp41 #a4ie4 ,inside the ,nucleus1 ,irving ,adler1 ,the ,john ,day ,company1 ,inc41 ,new ,york1 #aifc1 #aib pp41 #d4ie4 ,radioisotopes and ,radiation1 ,john ,h4 ,lawrence1 ,bernard ,manowitz1 and ,benjamin ,s4 ,loeb1 ,dover ,publications1 ,inc41 ,new ,york1 #aifd1 #aca pp41 #b4ej4 ,sourcebook on ,atomic ,energy 7third edition71 ,samuel ,glasstone1 ,van ,nostrand ,reinhold ,company1 ,new ,york1 #aifg1 #hhc pp41 #ae4jj4 ,the ,semiconductor ,revolution in ,nuclear ,radiation ,counting1 ,j4 ,m4 ,hollander and ,i4 ,perlman1 ,science1 #aed3 #hd 7,october #g1 #aiff74 ,about ,activation ,analysis ,popular ,level ,neutron ,activation ,analysis1 ,vincent ,p4 ,guinn1 ,international ,science and ,technology1 ,prototype ,issue1 #gd 7#aifa74 ,distribution of ,arsenic in ,napoleon's ,hair1 ,hamilton ,smith1 ,sten ,forshufvud1 and ,anders ,wassen1 ,nature1 #aid3 #gbe 7,may #bf1 #aifb74 ,nuclear ,activation ,analysis1 ,richard ,e4 ,wainerdi and ,norman ,p4 ,du,beau1 ,science1 #aci3 #ajbg 7,march #ae1 #aifc74 ,neutron ,activation ,analysis1 ,w4 ,h4 ,wahl and ,h4 ,h4 ,kramer1 ,scientific ,american1 #fh3 #baj 7,april #aifg74 ,technical ,level ,activation ,analysis ,handbook1 ,robert ,c4 ,koch1 ,academic ,press1 ,inc41 ,new ,york1 #aifj1 #bai pp41 #h4jj4 ,neutron ,activation ,experiments in ,radiochemistry1 ,k4 ,s4 ,vorres1 ,journal of ,chemical ,education1 #cg3 #cia 7,august #aifj74 ,radioactivation ,analysis1 ,h4 ,j4 ,m4 ,bowen and ,e4 ,gibbons1 ,oxford ,university ,press1 ,london1 ,england1 #aifc1 #bie pp41 #h4jj4 ,neutron ,irradiation and ,activation ,analysis1 ,denis ,taylor1 ,van ,nostrand ,reinhold ,company1 ,new ,york1 #aifd1 #ahe pp41 #h4ie4 ,guide to ,activation ,analysis1 ,william ,a4 ,lyon 7,ed471 ,van ,nostrand ,reinhold ,company1 ,new ,york1 #aifd1 #ahf pp41 #e4ie4 ,advances in ,activation ,analysis1 ,volume #a1 ,j4 ,m4 ,a4 ,lenihan and ,s4 ,j4 ,thomson 7,eds471 ,academic ,press1 ,inc41 ,new ,york1 #aifi1 #bcc pp41 #i4ej4 ,activation ,analysis2 ,principles and ,applications1 ,j4 ,m4 ,a4 ,lenihan and ,s4 ,j4 ,thomson 7,eds471 ,academic ,press1 ,inc41 ,new ,york1 #aife1 #baa pp41 #h4ej4 ,modern ,trends in ,activation ,analysis1 ,volumes #a and #b1 ,j4 ,r4 ,de,voe and ,p4 ,d4 ,la,fleur 7,eds471 ,national ,bureau of ,standards ,special ,publication ,number #cab1 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c41 #aifi1 #bjje pp41 #h4ej4 ,pottery ,analysis by ,neutron ,activation1 ,i4 ,perlman and ,f4 ,assaro1 ,archaeometry1 #aa3 #ba 7#aifi74 ,bibliographies ,activation ,analysis3 ,a ,bibliography1 ,g4 ,j4 ,lutz1 ,r4 ,j4 ,boreni1 ,r4 ,s4 ,maddock1 and ,w4 ,w4 ,meinke 7,eds471 ,national ,bureau of ,standards ,technical ,note #dfg1 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c41 #aifi1 #h4ej4 ,forensic ,science3 ,a ,bibliography of ,activation ,analysis ,papers1 ,g4 ,j4 ,lutz 7,ed471 ,national ,bureau of ,standards ,technical ,note #eai1 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c41 #aigj1 #j4ej4 ,determination of ,light ,elements in ,metals3 ,a ,bibliography of ,activation ,analysis ,papers1 ,g4 ,j4 ,lutz 7,ed471 ,national ,bureau of ,standards ,technical ,note #ebd1 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c41 #aigj1 #j4ge4 ,pollution ,analysis3 ,a ,bibliography of the ,literature of ,activation ,analysis ,papers1 ,g4 ,j4 ,lutz 7,ed471 ,u4 ,s4 ,government ,printing ,office1 #aiga1 #j4de4 #ad-,me,v ,neutron ,generators in ,activation ,analysis3 ,a ,bibliography1 ,g4 ,j4 ,lutz 7,ed471 ,u4 ,s4 ,government ,printing ,office1 #aiga1 #a4jj4 ,oceanography3 ,a ,bibliography of ,selected ,activation ,analysis ,literature1 ,g4 ,j4 ,lutz 7,ed471 ,u4 ,s4 ,government ,printing ,office1 ,washington1 ,d4 ,c41 #aiga1 #j4ej4 ,,motion ,,pictures ,available for loan without charge from the ,,userda-,,tic ,film ,library1 ,p4 ,o4 ,box #fb1 ,oak ,ridge1 ,,tn #cghcj4 ,the ,nuclear ,witness3 ,activation ,analysis in ,crime ,investigation1 #bh minutes1 color1 #aiff4 ,this film illustrates the application of activation analysis to the investigation of criminal cases involving murder1 burglary1 and narcotics peddling4 ,nuclear ,fingerprinting of ,ancient ,pottery1 #bj minutes1 color1 #aigj4 ,animated sequences are used to explain several of the analytical techniques4 ,part of the film shows how the research is actually done in the laboratory4 ,the ,atomic ,fingerprint1 #ab minutes1 color1 #aifd4 ,the principles of neutron activation analysis are explained and the machines used in this work are shown4 ,some of its applications in crime detection1 geology and soil science1 analysis of art and archaeological objects1 oil refining1 agriculture1 electronics1 biology and medicine1 and the space sciences are illustrated4 ,neutron ,activation ,analysis1 #dj minutes1 color1 #aifd4 ,this film describes the nature1 potentialities1 and applications of neutron activation analysis4 ,the kinds of neutron sources used and the counting techniques are shown4 ,examples of applications in crime detection1 geology and geochemistry1 agriculture1 medicine1 the petroleum and chemical industries1 and the semiconductor industry are shown4 ,photo ,credits ,cover ,federal ,bureau of ,investigation #b ,smithsonian ,institution #cj #ca ,university ,hospital1 ,university of ,washington #dg ,dr4 ,sten ,forshufvud ,illustration3 ,bernard ,keisch ,the ,author ,dr4 ,bernard ,keisch received his ,b4,s4 degree from ,rensselaer ,polytechnic ,institute and his ,ph4,d4 from ,washington ,university4 ,he is now a ,senior ,fellow at the ,carnegie-,mellon ,institute of ,research at ,carnegie-,mellon ,university in ,pittsburgh4 ,he is presently engaged in a project that deals with the applications of nuclear technology to art identification4 ,this is sponsored by the ,national ,gallery of ,art and in the past has also received support from the ,u4 ,s4 ,atomic ,energy ,commission and the ,national ,science ,foundation4 ,previously he was a nuclear research chemist with the ,phillips ,petroleum ,company and senior scientist at the ,nuclear ,science and ,engineering ,corporation4 ,he has contributed articles on art authentication to a number of journals4 ,for ,,erda1 in addition to this booklet1 he has written ,the ,mysterious ,box3 ,nuclear ,science and ,art1 ,lost ,worlds3 ,nuclear ,science and ,archaeology1 and ,secrets of the ,past3 ,nuclear ,energy ,applications in ,art and ,archaeology4 ,a word about ,,erda4444 ,the mission of the ,u4 ,s4 ,energy ,research ,development ,administration 7,,erda7 is to develop all energy sources1 to make the ,nation basically self-sufficient in energy1 and to protect public health and welfare and the environment4 ,,erda programs are divided into six major categories3 ,,conservation ,,of ,,energy--,more efficient use of existing energy sources1 development of alternate fuels and engines for automobiles to reduce dependence on petroleum1 and elimination of wasteful habits of energy consumption4 ,,fossil ,,energy--,expansion of coal production and the development of technologies for converting coal to synthetic gas and liquid fuels1 improvement of oil drilling methods and of techniques for converting shale deposits to usable oil4 ,,solar1 ,,geothermal1 ,,and ,,advanced ,,energy ,,systems--,research on solar energy to heat1 cool1 and eventually electrify buildings1 on conversion of underground heat sources to gas and electricity1 and on fusion reactors for the generation of electricity4 ,,environment ,,and ,,safety--,investigation of health1 safety1 and environmental effects of the development of energy technologies1 and research on management of wastes from energy production4 ,,nuclear ,,energy--,expanding medical1 industrial and research applications and upgrading reactor technologies for the generation of electricity1 particularly using the breeder concept4 ,,national ,,security--,production and administration of nuclear materials serving both civilian and military needs4 ,,erda programs are carried out by contract and cooperation with industry1 university communities1 and other government agencies4 ,for more information1 write to ,,userda-,technical ,information ,center1 ,p4 ,o4 ,box #fb1 ,oak ,ridge1 ,tennessee #cghcj4 ,illustration3 ,,erda ,seal ,united ,states ,energy ,research and ,development ,administration ,office of ,public ,affairs ,washington1 ,d4,c4 #bjede ,transcriber's ,notes --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,where possible1 ,,utf superscript and subscript numbers are used2 some e-reader fonts may not support these characters4 --,in the text version only1 text in italics is delimited by underscores4 --,in the text version only1 superscript text is preceded by caret and delimited by brackets4 --,in the text version only1 subscripted text is preceded by underscore and delimited by brackets4 --,in the text version only1 added a brief label to each illustration2 and for graphs1 provided tabular summaries of the data where possible4