CHARTS OF ISOTOPES AND OF DISINTEGRATION AND TRANSMUTATION REACTIONS HUGH M. SPENCER University of Virginia, Charlottesville, Virginia
PRoomss in recent years has increased the number of elements to 97, including the neutron as element of eero atomic number, and has vastly increased the number of known stable and radioactive isotopes. To date, 605 stable and radioactive isotopes (counting isobaric isotopes once only) have been studied sufficiently thoroughly to establish the values of the atomic and mass numbers with moderate certainty, their percentage abundance in the case of the stable isotopes, and the nature and period of their disintegrations in the case of the radioactive isotopes. Seaborg's Table of Isotopes (1) is an authoritative source of the pertinent data to the time of its publication. It is an important teaching and research tool. The writer has found two types of charts based upon it and later data (8) of great value in teachmg the subjects of nuclear transmutations, nuclear fissions, and natural and artificial radioactivities. These are plots of I, the isotopic number (S), against Z, the atomic number for all of the elements, and of A, the mass number, against Z for the heavier elements. The isotopic number, I, is equal to A-2Z, or the number of neutrons less the number of protons. These are shown in Figures 1and 2. Of the 605 isotopes of the 97 elements, 96 have been established with less than satisfactory certainty. The standing of the isotopes have been characterized by Seahorg according to the following letter code:
been prepared from neodymium, composed of seven stable isotopes. Incidentally the one with a half-life of approximately 200 days has also been prepared from praseodymium by the (a,n) reaction and should therefore be a51144. Many interesting facts can be read directly from Figure 1. From the definition of I = A - 22, i t is obvious that the number of neutrons, A - Z, is I Z, and the mass number is I 22. The number of neutrons equals or is greater than the number of protons, i. e., I 0, for all stable isotopes except lH1 and ;He3.' All elements of even Z, except 4Be (see footnote 4), have two or more stable isotopes. All elements of odd Z have one or two stable isotopes (potassium has three natural isotopes, but 19K40is radioactive). Beyond nitrogen, which has two stable isotopes of masses 14 and 15, the two isotopes have odd mass numbers and therefore even numbers of neutrons, two apart. The two natural isotopes of lutecium of masses 175 and 176 do not constitute an exception since the latter is radioactive. This tendency toward even numbers of neutrons is apparent also in elements of even Z. For these the most abundant stable isotopes have even numbers of neutrons. (Platinum, for which ,,Ptlg6 is the most abundant, is an exception.) The representation of the isotopes in Figure 1 is a convenient basis for discussion of the generalizations concerning abundance of the elements by Harkins (3, 4), of the region of stability, and of the type of activity by which radioactive isotopes regain ~tability.~ It is obvious. for examule.,that isoto~eswith too hieh neutron-proton ratios, i. e., too large values of I, ceach stability by the conversion of a neutron to a proton in the nucleus and expulsion of a 8- particle.
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In Figure those 'lassed as B~ ' 9 E, and have been so indicated. None of those 'lamed as have been included. Approximately 100 other artificially radioactive isotopes are known to exist. For these the half-life and type of radiation by which they decay, and, in some the energy of the are known. Although ninetenths of these are of classes D and E, the complexities of their formation from elements with a large number of stable isotopes or from nuclear fission reactions have prevented the identification of their mass numbers. Thus element 61, Xinium, which is not in Figure has six radioactive isotopes of class E and one of class F. All of these have
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This means a shift of one place to the right in Z and two places downward in I of Figure 1. Conversely, unstable isotopes with too low neutron-proton ratios may reach stability by the conversion of a proton to a neutron in the nucleus and expulsion of the positron, or P C particle, Two of the generalizations stated by F. W. ASTON,"Mass Spectra and Isotopes," 2nd Ed., E. Arnold, London, 1941, pp. zoz, 204, require ,,inor co,.,.ections. For several purposes the choice of A sgainst 2,as in Figure 2, O r of A - Z (the number of neutrons in the nucleus) against 2, as in H. S. TAYLOR AND H. A. TAYLOR, "Elementary Physical chemistry," 2nd ed., D. Van Nostrand Company, New York, 1937, PP. 45, 52, might be preferable to the I against Z chart. However, for the known isotopes the range of values of A and A - Z is 0 to 242 and 0 to 147, respectively, whererts I varies from -2 to 53.
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sense of equivalence of mass and energy must be favor In the outwardly similar K-electron capture the elec- able, and the projectile and ejected particle must penetron drawn into the nucleus reacts with a proton to trate barriers which become more effective as Z is increased. Competition between the various types of form a neutron, emission will favor one type. Thus, for example, -18+ ,H' on' @,PI, ( 4 4 , ( P A , h r ) (slow neutrons), and (up) reThus emission of a 8' particle or a K-electron capture actions occur throughout the whole range of 2, whereas (a,p) and @,a)reactions are rare for elements of high 2. decreases Z by one unit and increases I by two units. In Figure 2 values of the atomic mass number, A , are With a straight-edge in place at this slope it is easy to plotted against Z of all the known isotopes of elements see that sfiKrm,from a fission reaction, loses B- particles to form aRbss, that thii further disintegrates by P- for which Z 2 80 (except five stable and one radioparticle emission to assr", and finally to asYBD which is active isotopes of mercury for which A < 202). Disstable. Similarly s&ra finally yields &3$8. Likewise, integration processes are shown by solid h e s , transmuseKr7' and sfiKrS1 lose 8+ particles and become ssBr7aand tation processes by dotted lines. The latter are net asBrs1, respectively. The 8- activity of ,Gen, ,oSn'26, transmutation reactions involving the ejection of a 6;11e129,~ X C ' ~6pXe135, ', G ~ Nfi0Nd148, ~ ~ SON^^^^, ~ ~ ,and particle or particles. Chemical symbols are shown in ssEr171of classes A, B, A, D, A, E, E, E, and C, respec- the circles for those isotopes which have been given tively, shown in Figure 1, and the absence of 33A~77, chemical names associated with the several disintegraK I S ~sP, ~ ~ &P7, ~ , 65C~1sfi, 6111151,6111148,6111147,and tion series. The percentage natural abundance of the a~Tm'~'indicate the possibility of the future discovery stable or relatively stable isotopes of elements 80,81,82, of these isotopes or the identification of some of these 83, 90, and 92 are also given in the circles. In the case with known activities for which no mass number is at of isotopes for which branched disintegration occurs the present publi~hed.~The position of the missing iso- half-life period is given in the circles, and the pertopes would suggest that all except two of the illinium centage figures outside the circles refer to the portions isotopes are P- particle emitters and that perhaps illin- disintegrating in the manner indicated. The half-life ium of masses 147 and 149 may turn out to be stable, periods of the nonbranching radioactive isotopes are shown along the lines representing the decay. or relatively the most stable.' Recent investigations have brought to light the All but a few of the known transmutation reactions are shown in the inset of Figure 1. The first transmu- branched disintegration of four naturally radioactive tation reaction, of the (a, p) type, performed by Ruther- isotopes, which had previously been considered simple, ford in 1919, though incompletely understood a t the and have established the presence of eka-iodine and eka-cesium in the natural disintegration series. Thus time, increases I and Z by one unit each. Inspection of the chart shows that with elements of the disintegration of saAc2" proceeds by the emission of low Z, for which the naturally produced a particles are a particles to the extent of 1 per cent and yields AcK, or effective projectiles, many of the elements produced eka-cesium, a 0- particle emitter. The disintegrations are now known to be stable isotopes, although some of RaA, ThA, and AcA, in part by the lose of P- partradioactive isotopes, such as 1sA128, must have been pro- icles, yield eka-iodine of masses 218, 216, and 215 ($), duced in the early experiments. The first transmu- each of which is a-active. A fourth isotope of element tation by which it was recognized that radioactive iso- 85 has been prepared by the 8sBim ( ~ ~ 2 ggEka-IP1l %) topes were formed was reported by Curie and Joliot in reaction. This isotope decays by a-particle emission 1934. In these the (a, n) reaetion decreases I by one and K-electron capture. unit and increases Z by two units. Thus by considering the net effects of the various transmutation reactions LITERATURE CITED G.T.,Rev. Modern Phys., 16, 1 (1944). one may visualize how a given isotope might be pre- (1) SEABORG, pared. In connection with this one must consider the (2) PEREY,M., J. phys. radium, 6,28 (1945); G. T. SEABORG, Chem. Eng. New, 23,2190 (1945); H. D.S M ~ "Atomic , types of reaction which have been found possible in the Energy for Military Purposes," Princeton University range of Z concerned. The mass change in the modern Press, Princeton, 1945; B. KARLIKAND T. BERNERT, Naturwissaschafta,32,44 (1944); I.JOLIOT-CURIE, Ann. phys., 19, 107 (1944); W.JENTBCHE. F.PRANKL, AND F. a Since this paper was submitted for publication, the existence HERNEGGER, Naturuiisensshaften, 28, 315 (1940); E.P. of three of these-namely, saAs7',srCsls',and alI1"'-have been NEYAND J. H. MCQUEEN,Phys. Rev., 69,41(1946); G.T. announced by the Headquarters of the Manhattan Project in SEABORG, Science, 104, 379 (1946). Science, 103, 697 (1946). W. D., J. Am. Chem. Soc., 43,1038 (1921); M. 8. ' The absence of ,Bes which one would normally expect from (3) HARKINS, LIVINGSTON AND H. A. BETHE, Rev. Modern Phys., 9,245 the B-decay of $Lisandtheexceptionto therulethat allelements 11-937) - - .,. of even Z have two or more stable isotopes is a curious, but relatively well-understood phenomenon. (4) HARKINS, W. D., Science, 103,289 (1946) +,en
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