Wayne C. Wolsey Macalester College St. Paul, Minnesota 55105
Textbook Errors: 133
Variation of Radioactive Decay Rates
It is stated frequently in introductory chemistry texts that radioactive decav rates are invariant. Students are led to the impression, implicitly, if not explicitly, that changes in chemical form, temperature, pressure, etc. have no effect upon the half-lives of unstable nuclei. This constancy of decay is perhaps true for some particular modes of decay, but by no means is it true for all. The impression which many chemists have about the invariability of radioactive decay rates seems to be based on numerous experimentscarried out hy early workers in the field of radioactivity, in which decay rates were measured for a variety of chemical forms a t high and low pressures and temoeratures. and in the ~resenceof aoolied maenetic and elecirical fields. Any variation which the; observd was usuallv attributed to chanees in counter eeometrv. The instrumentation available inuthese early years wa~"~rohab1y not sensitive enough to detect any variation. Moreover, the measurements were usually made on the a- or &active species available a t that time. Less common modes of radioactive decay are orbital electron capture and internal conversion. Orbital electron capture frequently occurs as an alternative mode of decay simultaneously with positron emission, but occasionally it is the sole mode of decay. Internal conversion is an alternative mechanism to gamma ray emission by which an excited nucleus can lose excess enerev. Durine the internal conversion orocess. energy is transfgred from-the nucleus to anorhital e'lectwn; causine the electron to be eiected lrom theatom with k~netic energy-equal to the difference in energy between that of the equivalent gamma photon and the binding energy of the electron ejected. It would seem reasonable that anv factor which would varv the electron density near the nucleu~wouldhave a measurabie effect upon the decav rates of atoms undereoine .. .. decav. bv. either orbital electron-capture or internal conversion. One of the first workers to make this prpdiction and ohserve it experimentally was Segd ( 1 ) .He studied the decay rates of 7Be (electron capture, t112 = 53.55 da) and found a difference of about 0.3% between the decay of "Be in metallic beryllium and BeO, metallic beryllium decaying faster. This result was interpreted on the basis of an increased electron density a t the nucleus of the neutral Be atom in the metal relative to the nucleus in BeO. The effect has been confirmed for 7Be in the cases of beryllium(1I) in 0.1 N HCI and solid BezPz07 (2). Emery ( 3 , 4 ) has reviewed investigations upon other nuclei undergoing electron capture in which an alteration of decay by chemical form has been demonstrated. Other nuclei of this tvoe . . are 85Sr and 89Zr. Variation in the rate of decay for nuclei undergoing d ~ c a y by internal conversion was first observed for 9YffW2by RainSuezestionaof material suitable for this column and mest columns fnr ouhlirntion direetlv should be sent with i m a m details ~-~ .~ as posaihle, and particularly with reference to modern tcxtbooks. to \V. H. Eberhardt, School of Chemistry, Georgia lnstitutr of Terhnology, Atlanta, Georgia 30332. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the sources of errors discussed will not be cited. In order to be presented, an error must occur in at least two independent recent standard books.
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302 1 Journal of Chemical Education
bridge, Goldhaber, and Wilson (5.6).They observed that Tc in KTcO* or Tc& decavs 0.3% faster than l'c in metallic technetium. ~ i m l l a effeEts r have been reported for QomNb, "g"'Sn, lzsmTe, 1'39mTm, 193mPt, and 235mU (3, 7-9). The largest variation which has been reported (10) is that for 235mU in which an increase of 5.6% in the half-life was observed in going from the +4 oxidation state in UOz to the +6 state in UOa. Repom of this sort of variation in decay raw with chemical form have been oherved more frequently in recent yeus with the increased work in Mossbaue; spectroscopy and photoelectron spectroscopy. One might naturally wonder if themore conventional modes of decav show anv variabilitv uoon closer examination. Emery (3) s p c h a t e s thgt the fractional change in the decav rate of an alnha-active nuclide with a chanee in This skall chemical fo;m would be on the order of 1.5 X effect is probably beyond the measurement limit of modern instrumentation. The corresponding prediction for beta-active nuclei is on the order of 1 X 10-4, a much more readily detectable quantity. Bahcall(I1) has discussed an unusual type of beta-decay in which chemical bonding would be expected to play an important role. Normal beta decay is the emission of an electron with a continuous mectrum of enerev -.(uo . . to some maximum value) accompanied by an antineutrino. In some cases an electron mav. amear .. in a bound atomic level of the dauehter atom. This '.boundWelectronmay be thought to be mp;;rrd as it leaves the nucleus. althoueh Bahcall feels that d~rect creation of the electron in the bound state is more probable than caoture: The iatios of the transition probabilities, rll/I-c for the bound and continuous modes of decav have been calculated for completely ionized nuclei and are summarized in the table. We would normally not expect to encounter completely ionized nuclei, but it is predicted that these effects should be seen in the relative abundances of elements produced by nuclear processes in the interiors of stars. The large ratio of re/rc = 7 for 'MRu should mean that its decay product, Io6Rh, would bemore abundant in solar spectra than on the earth. Round beta decay has been shown tooccur for tritium and for '"Re. The latter case is of geological interest, as discussed by Bmdzinski and Conway (12). Calculatiuns show that the bare nucleus IR"Reis stable against beta decay. The energy of
Relatlve Probabllltles of Bound-State to Continuum-State Decay lor Complelely Ionized Nuclei" soto ope
r.rc
"c
0.01 0.1 09 0.7 7
12Si
83Ni g5Nb 'O%"
'72Pd "OAg 'ssEu ls7Re
'srOs 'From references110 and 1121.
0.3 2 1 0.1 1
the emitted beta particle (a "first-forbidden" transition) for 'e7He in rhenium &mpounds is less than the change in atomic binding energy; the difference in total electronic binding energy of '"Re and of the daughcer '":0s is only 15 keV. The half-life for continuous decay, tc, of 'a7He was found to he 6.6 X 101° vr usine gaseous ICsHsLReH. Brodzinski and Conwav . " co&biied thiscktinuous-decay half-life with the geologic2 (or total) half-life of 4.3 X 10'0 vr (as determined from is7~e/'s70sratios in geologically datkd molyhdenite samples) to calculate avalue of 1.2 X 10"vr for the bound-decav halflife using the relationship: lltc., llrc 11111.The transition ~robabilitvratio ra/rr - - was calculated to he 0.5 using- the relationship: r ~ /=rtc/tB. ~ In addition to chemical form having a demonstrated effect upon some types of radioactive decay, perturbations of decay rates have been reported by changes in temperature, pressure, and applied magnetic and electrical fields (3).Lowering the temperature from room temperature to 4.2OK was found to increase the decay rate of 99"T 'c by about 0.1% (13).Similarly, an increase of about 0.5% in the rate of 7Be has been reported going from 0.001-200 kbar (14). Although all of these effects
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are small, they are significant in understanding the fundamental process of radioactive decay. Literature Clted
3. p 2030 YudrarSamcr."!Edirnrs S w i . F .Cru%rr..l $ 3 Fmrn.1. T.m "AnnualRe~~cwol R andNo)n.H I ' I , A n n u a l H n i r u . l n r . P a l ~ ~ \ l t ~n181. . ( 1911.Yo, l..p I n ? 141 h e r / (; T.ln "Pldlnssnflh?lnumst~unalCen!clcncrun InnrrShrll lnnlrnll m Phnumcnaand FulurrA~>pl,,nfiw,;" Fa ? . 19'1 16 Ramhridds.K T.Cnldnsher.hl and s'lli,n.t:.l'ni%X . ,. 90.4'0 1 4 3 Ualm. R J Rr,any.n. l' a d \alndsrr.. \ I . n 'Prwecdlnl. "f the 1ntemat.rnal I" c , n * w n n o n Inner Shell Inn.m,,,m Vh,.nllmenr an I F l l L l , .\p,.lic.tl.l.".' F a . , r . F # n r . R \I .Menwn T..Pnlms.I \I andlisn 1. \ .I S.4t:s: C O N P ' ? . * . I . January. 1973. Vol. 3. p. 1806.
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(81 Cmpcr, J. A,. Hollander. J. M., and Rasmusaan. J. 0.. Phys Re". Lett., 15, 680 (19651.
19) Campbell. J. L.,Martin.B.,Sehmid-Ott, W. D.,Smend, F., andHall. U.,Phys. Re". Lett., 35,1259 119751. (10) Neve de Mauergnips, M., and Dd Marmol, P.,Phys.Left.. 49B. 428 11974). (11) Bahesll,J. N.,Phya.Re"., 124,495 (1961). 112) Brodrinak1.R.L.,andConway, D.C..Phys.Re"., 138,81368(19651. (13) Beyey.rs,D.H..andStump, R..Phys.Reu.Latt.. 1.219l1958). (14) Hensley, W. K., Ph.D. Thesis,Univ.ofRahester,197bDiasenationAbsir Int., 34, 1959-B11973).
Volume 55, Number 5, May 1978 1 303
