wagcmts were added t o the sample was varied from 10 t o 20 hours. X o significant differences in t’he results n-ere noted. Effect of Hydrofluoric Acid on Crucibles. There n-as a slight apparent loss in weight of the precipit a t ? due to hydrofluoric acid attack on the porcelain filter crucibles. D a t a froni 12 determinations showed t,hat on the average the crucibles lost 1.4 rng. for each determination. This loss in \%-eight appears as a small positive error of t,he order of 0.1% n h c n samples containing about 250 nig. of boric acid are being analyzed. I3ecause the precision of the recomnicntlctl procedure is approsimately i1 yo,this error is not significant. Interferences. Nitrate, perchlorxtt,, iodide, thiocyanate, chromate, (~Iiloi,atr,nitrate, and bromide form more or less insoluble salts with nit r o l l . and these substanws Ii-ould be c3sl)ected to interfere ( 3 ) . In the determination of 1iitr:ite with
nitron, Grant ( 2 ) removed the bromide interference by decomposing hydrobromic acid with chlorine water added dropwise to the boiling solution until the yellow color of bromine disappeared. Hydriodic acid lvas removed by adding an excess of potassium iodate to the neutral solution and boiling off the iodine. X t r o u s acid and chromic acid were rrmoved by reduction with hydrazine sulfate. I n the determinntion of perchlorate, Vurtheim ( 7 ) reduced nitrate and chlorate by treating the alkaline sample with Devarda’s alloy (59 parts of aluminum, 39 parts of copper, and 2 parts of zinc). These procedures for the removal of all of the commonly encountered interferences should be applicable also for the removal of interferences in borate samples prior to precipitation of the boron as nitron tetrafluolmrate.
(2) Grant, J., I n d . Chemist 8 , 160 (1932) (3) Hillehrand, W. F., Lundrll, G. E. F., Bright, H. A., Hoffman, J. I., “AppliCd>organic Analysis;’! 2nd ed., p. 157, Wiley, New l-ork, 1953. Lange, W.,Ber. 59B, 2107 (1926). Lucchesi, C. A , , Unio. Microjlrtis Pikbl. (Ann Arbor, Mich.) 13,109; Dissertation Abstr. 15, 2007 (1955). ROBS,W.J., Bleyer, A. S., Jr., White, J. C., ANAL.CHEM.29,810 (1957). Vurtheim, rl., Bee. trao. chirn. 46, 97 11927). Wimw;. ._ .-. .. C . A , , J . A m . Chem. Soc. 70, 1209 (1948). Welcher, F.,;T., “Organic Analytical Reagents, Vol. 111, Van S o s trand, Ben, York, 1948. JTherrr. E. T., ChaDin, W. H., .I. Am.”Chem. soc. 30.-16E47 ( -1908). Wberg, E., in “Handhbch der Analytischen Chemie,” 1-01. 111, p. 24, R. Freseriius, G. Jander, eds., Springer-Vrilag, Berlin, 1942. Wolszon, J. D., Hayes, J. R., Hill, W.H., h . 4 1 . . Cmni. 29, 829 (1957).
LITERATURE CITED
(1) Allen, E. T., Zies, E. G., J . An?. Crmru. SOC. 1, 739 (1918).
RECEIVED for review Xovemher 21, 1956. Accepted April 17, 195i.
Neutron Activation Cross-Section Graphs W. WAYNE MEINKE and R. S. MADDOCK Department of Chernisfry, University o f Michigan, Ann Arbor, Mich.
Revised values of all known thermal neutron activation cross sections have been plotted against the half life of the radioisotope produced. The standard error of each value is included.
A
cross-section values for thermal neutrons are important primary data for the analytical chemist becmse they indicate the relative probability with which a giren radioisotope \vi11 be formed when a substance is exposed to a source of neutrons. Thus, the detection sensitivity for an element in activation analysis as well as the yield of tracers in an irradiation depend upon these values. Unfortunately, absolute values for cross sections are not very well known. Hughes and his coworkers at Brookhaven National Laboratory hare for a nuniber of years been compiling thermal neutron cross-section values, 3s well as c.nluc.s for higher energy neutrons and first made their conipilation available in 1952 ( 2 ) . Meinke and Anderson (4) .bowed that these values for thermal energies could be summarized readily by plotting them against the half life oi the daughter radioisotope produced. This type of graph has proven useful in (iiscussions of the use of low-level CTIVAIIOP~
portable neutron sources in activation analysis (4,6 ) , as well as in considerations of the sensitivity of thiq method (3).
Recently Hughes and coworkers have revised their cross-section values ( I ) . Figures 1 and 2 give plots of new crosssection graphs using these new LLbest” values and also including the standard error quoted by Hughes for each value. Where only masimum or minimum values have been given for the cross section they are indicated by arrows on the graph. As in the previous plot the cross-section values are “atomic cross sections”-Le., the isotopic cross section (quoted by Hughes) multiplied by the natural abundance of the isotope. Figure 1 includes cross sections for all stable isotopes for which natural abundances are known. Figure 2 includes all cross sections given by Hughes for isotopes which are in themselves radioactive and which do not occur in nature. The atomic and isotopic cross sections are identical in Figure 2. An isotope that is underlined on the graph indicates that neutron activation produces a metastable daughter activity. A wavy line, on the other hand. indicates activation to the ground state of an isomeric pair. (Antimony-123 gives tivo metastable antimony-124
states and therefore has two underlines.) In most cases the metastable state decays independently by beta emission or contributes only a small amount to the activity of the ground state. I n some cases, however, the metastable state decays completely into the ground state. thus augmenting its activity. The points with an underline and a wavy line are examples of this latter case and represent the total cross-section values foy the formation of the ground state both directly and by decay of the shortlived metastable state. 4s an example, stable cobalt-59 is activated to 10.4-minute cobaltr6Om with a 16 =t3-barn cross section and a t the same time to the 5.28-year cobalt-60 ground state with a 20 f 3-barn cross section. Since all the 10.4-minute metastable state of cobalt-60 decays directly to the ground state, the 5.28-year actikity is produced with an effective cross section of 36.0 f 1.5barns. ‘These graphs contain all the ( n , y) cross-section values given by Hughes for thermal neutrons except for a few reactions where the cross sections are too small or the half lives too large to be plotted on the graphs. These values are given in Tables I and 11. From these graphs it is possible to tell a t a glance which elements will be VOL. 29, NO. 8, AUGUST 1957
1171
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Figure 1 . Atomic cross section for thermal neutron activation of isotopes found in nature vs. half life of daughter radioisotope produced
1 172
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ANALYTICAL CHEMISTRY
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Figure 2. Atomic cross section for thermal neutron activation of isotopes not found in nature radioisotope produced VOL. 2 9 ,
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NO. 8, AUGUST 1957
1173
activated n ith high sensitivity a t different irradiation tinies. Furthermore, the graphs should prove helpful in estimating the activities of one isotope relative to another for specific conditions of irradiation. It is interesting to note how poorly the absolute cross sections for most of the isotopes are knonn. Values for a few isotopes such as manganese-55 have been determined quite accurately. However, most of the cross sections for the remaining isotopes are knon n only to within 10, 20, or even 40%. Thus, it is of utmost importance when performing analyses by activation to utilize the standard analytical practice of comparison against a standard. This practice eliminates the necessity for knoiving accurate cross-section values as \\ell :is absolute values for the geometry. counting efficiency, and the like of the experimental arrangement. Whenever absolute values of the cross section must be used, it can lie expected that the work \vi11 be considerably less accurate. Comparison of Figures 1 and 2 indicates that isotopes not found in nature generally have high cross sections, very few of which are less than 1.0 barn. As more of these cross sections are determined it n-ill be interesting to note hether this is a fundamental relationship or a factor dependent a t present upon technical difficulties.
Table I.
Thermal Neutron Cross-Section Values Not Included in Figure 1
Log Half I d c oi Daughter, AZinutes 5 81 2 15
I’aieiit
Log Atomic Cross Section for Thermal S e i i t m i Activation, Balm
Standard Eiioi of Cross Section,
8 93 2 48 05
