Philip K. Hopke State University College Fredonio, New York 14063
It is commonly stated in elementary textbooks ( 1 3 ) that the half-life, or decay constant of a radioactive isotope, is totally free of variations due to the chemical or physical environment surrounding the nucleus. Early workers tried to affect the decay constants of naturally occurring radioactive isotopes by varying the temperature between 24 and 1280"K, by applying pressures up to 2000 atm, and by varying the gravitational field by making measurements in mines and on mountaintops. Magnetic fields of up to 83,000 G were employed. These experiments all gave negative results, or the positive results could be explained as being due to changes in counting geometry or losses of volatile material (4-8). All of the nuclei studied were alpha- or beta-emitting species for which theory indicates that the only interaction with the atomic electrons should he through the Coulomb force between the emitted charged particle and the electrons. In the beta-decay process of electron capture, a proton is converted into a neutron by the capture of an orbital electron. A neutrino is emitted so the equation can he written
The rate of decay from a given atomic electron shell is directly related to the density a t the nuclear surface of electrons from that shell. For most nuclei, the energy available for the election capture is much greater than the binding energy of any of the atomic electrons. Most of the capture then takes place from inner shells where the electron density a t the nucleus is highest. The chemical interactions in the outer valence shell would not have much effect on the overall decay probability. Segre (9) predicted that for light nuclei, for which the inner shells are the valence shells, chemical effects on the decay rate might be anticipated. He proposed that 'Be he studied. A number of subsequent investigations (10-13) have shown that the decay rate for metallic beryllium is higher than the rate for BeFz or BeO, with the more electronegative fluorine atoms causing a larger change than the oxygen. More recent work (141 has been performed on "Be to include compounds which should increase the electron density. A summary of data on ?Be is given in the table. It is difficult to interpret these results with simple discussions of the bonding of Be compounds. Beryllium-7 is the only nucleus for which chemical effects on the lifetime of an electron capturing nucleus have been observed by several independent investigators. Another type of nuclear decay process for which chemical effects have been observed is the internal conversion process. If a nucleus is in an excited state following a beta-decay process or some inelastic scattering event, it can release energy in the form of electromagnetic radiation, as gamma rays. Alternatively, the nucleus can interact with the atomic electrons to deexite the nuclear state. An electron is ejected from the atom with a kinetic energy equal to the transition energy minus the atomic binding energy of the electron. The ratio of the number of elec-
Extranuclear Effects on Nuclear Decay Rates
Differences in the Decay Constant of 'Be in Different Chemical Forms
Ak
Source pair
10\eference
hes
BeO-BeF, (hexagonal) BeO-BeF. (hexaeonal) , B ~ O - B ~ F(amorphous) ; Be-BeF2 (amorphous) BeO-Be BeO-Be Be0-Be40( C H C 0 0 )6 BeO-BeBr, Be.0(CHCOO)s-BeF2 . (amorphous) BeO-Be(CsH& BeO-BeZ+(OH.) Be(CsHs)2-BeZ+(OHz)d
0.69 zt 0.03 0.609 2Z0.055
0.795 zk0.074 -0.374 zk0.077 -1.169 ~ t 0 . 1 0 6
(10,II)
(14) (14) (14)
trons to the number of photons emitted is called the internal conversion coefficient. Since the conversion process requires nuclear interaction with the atomic electrons, one may, therefore, expect to observe chemical effects on the half-lives of excited states. However, the transitions commonly encountered in nuclear spectroscopy are of such energies and multipolarities that the conversion coefficients are small, i.e., the excited states decay primarily by the emission of gamma rays. The multipolarity of the transition is the amount of angular momentum carried off by the emitted photon. For most gamma-ray transitions, the multipolarity of the transition is one or two. Furthermore, the transition energies usually exceed the K-binding energies and conversion takes place predominantly in the innermost shells, where the electrons are not strongly involved in chemical binding. The chemical effect on the half-life of a nuclear excited state becomes appreciable when the excited state is a nuclear isomer decaying via a very low-energy transition. The conversion coefficients increase sharply with decreasing transition energy and increasing multipolarity. More important, however, is the fact that if the transition does not have enough energy for conversion in the inner shells, conversion can take place only in the outer shells where chemical effects may be significant. The first study of chemical effects on internal conversion transition rates was done by Bainbridge, Goldhaber, and Wilson (15, 16) on the 142.7 keV state in 99Tc. The 142.7 keV state decays by a highly converted 2 keV isometric transition followed by a 140 keV gamma-ray. They found that A(KTc04) - X(TczS7) = 27.0 1.0 x A(TczS7). and A(Tc) - A(Tc&) = 3.1 & 1.2 x A(TczS7). Recently there have been conflicting reports of chemical effects on the decay of the 124.8 keV isomeric state in QONh.This state decays by a (2.38 0.36) keV followed by a 122.4 keV gamma. Cooper et al. (171, and Olin 118) have reported approximately 3.7% variation in the isomer half-life. However, Weirauch et al. (191, and Geiger, et al.
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(20), found no change. A further study by Smend et al. @I), also showed no chemical effects with a very high reported sensitivity. They also offer a possible explanation for these conflicting results in terms of the differences in the lattices studied. The largest variations of half-lives reported are of the decay rate of 235mU.Differences of 5.7% in half-life have been observed. This excited state lies only 73 + 5 eV above the ground state, and thus internal conversion can occur only in the outermost electron shells. Thus the chemical binding in the outer shell can have a substantial effect on the decay rate. The 235mU sources have been prepared by collecting the recoiling atoms following the alpha decay of 2 3 9 P ~The . uranium atoms have been collected and diffused into carbon films, silicon crystals, and platinum foils (22, 23). They ohserved changes in decay constants of the order of 0.1-0.3%. More complete studies were subsequently made by M. Neve de Meverguies (2426) who collected recoil atoms on a wide variety of collector foils. In these studies there seems to he a correlation between the decay rate and the free electron density within the host metal. Many of the results can he qualitatively explained (26). The electronic interaction between a metal and an impurity atom depends on the relative number of valence electrons of the metal and impurity atoms. If the effect is small, the "rigid-band" model can he applied. For this case, the valences of the metal and the impurity atoms differ by only zero or one. If the difference between valences is greater than one, the local perturhation is removed by a modification of the free-electron density in the region around the impurity atom. The impurity atom is thus screened by the free electrons. This screening is characterized by a relaxation length which is called the screening length. If the data for the group Ih metals Cu, Ag, and Au are examined, the decay constants are proportional to the nower ll(12.81 + 0.94) of the free-electron density. similar propdrtiona~itiesare observed for Groups VIII and IVh. For hieher valence metals, the denendence on atomic concentragon is weaker. The "se of these halflife changes has been proposed as a probe for surface adsorption and desorption studies (27). Chemical effects have also been noted for the decay of the 145.0 keV isomeric state in lz5Te (28). Sources of 125mTe have been incorporated into Te, TeOz, and AgzTe. The results are X(Te) - A(Ag2Te) = (2.59 i 0.18) x X(Te); X(Te02) - X(Ag2Te) = (2.23 + 0.18) x h(Te); X(Te) - X(Te02) = 0.36 + 0.17) x X(Te). The 1.64 keV state in lS3Pt was found to decay 4 i 2% faster in the metallic state than in chloride solution (29). A direct measurement of changes in the relative rates of internal conversion has been made in the case of l19Sn (30). By observation of the spectrum of emitted electrons, it was found that the 5S (valence shell) electron density a t the nucleus is approximately 30% smaller in SnOz than in white tin. This change in conversion rate of 5S electrons would only make an extremely small change in the total half-life of this level and would probably be unohservahle by the techniques used in the other studies. If chemical interactions with the outer electrons can produce such changes, it is not unreasonable to expect that changes in the physical environment of the atom might affect the electronic structure and also the rate of nuclear decay dependent on interactions with the electrons. The first physical effects were observed on the isomeric decay of 99"'Tc (31). Earlier, this same state had been found to undergo chemical effects on its decay rate (15, 16). Since technicium metal had been found to be a superconductor at 42°K (32), the possibility of changes in decay constant were investigated. Byers and Stump (31) found that the decay constant was larger in the superconducting state such that X(4.2"K, superconducting) 518
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Journal of Chemical Education
X(293-K): It was also found X(293"K) = (6.4 i 0.4) x that the application of a magnetic field sufficient to destroy the superconductivity causes the removal of gross effects; X(4.ZeK, normal) - X(293"K) = (1.3 i 0.4) x lo-& X(293-K). A larger effect of the transition to the superconducting state has been observed for 90mNbdecay in niobium metal (33). The difference in decay constant is A (normal). The sign of the given as (-19.5 + 5.5) X effect is different between technecium and niobium. This difference may obtain because of the difference in the types of the two isomeric transitions and thus there is a difference in the electronic matrix elements involved. An effect of compression on 99mTcin technecium metal has been found by K. T . Bainhridge (341. Here the compressed material has a higher decay rate; X(compressed) - X(normal) = (2.3 0.5) x X(normal) where a static pressure of 0.1 megahar was applied to the compressed sample. Calculations made by Porter and McMilIan (34) were able to reproduce this order of change after making an estimate of the compressibility of technicium metal to he 0.27 megahar-l. From this estimate and the assumption that the internal conversion coefficient varies linearly with pressure, a fractional decrease in lifetime of is calculated with the range dependent on (2-4) x the choice of the 4p hand structure. Recently, an effect of pressure on the electron capture decay rate of 7Be has been reported (35). A linear increase in the decay constant of ?Be0 was observed with increasing pressure over the range of 0 to 270 kbar. A rough calculation was able to indicate that an enhancement of the decay rate of the order of two-thirds of the value observed could he expected. In barium titanate crystals, there is ferroelectric phase transition. Various radioactive atoms have been suhstituted in the crystal at the titanium sites. The decay rates are measured a t room temperature in the ferroelectric state and above 120°C in the paraelectric state. Upon incorporation of 89Zr into BaTi03, St. Gagneux et al (36) found that A(BaTi0a (89Zr, 150'C) - X(BaTiO3 ( W r , 2 2 T ) = (8.0 + 0.3) x X(s9Zr) for the electron capture decay mode. A larger change in decay rate was observed for 99mTcincorporated in BaTiOa (37); X (paraelectric, 170") - X(ferroelectric, room temperature) = (2.6 i 0.4) x X(ferroe1ectric. room temperature). When s5Sr (38) was incorporated in these crystals, the change in decay conXP5Sr). This smaller stant was only (4.9 2.9) x magnitude is attributed to the Sr ions occupying barium ion lattice sites instead of titanium sites. A different method of inducing a high electric field was tried by mixing 99mTc in the form of NazTcC16 and KzTcCle with a powder of high dielectric constant (c 6000). This mixture was pressed into a parallel plate condenser in a layer 0.2 mm thick. T o avoid polarization, a 50 Hz oscillating field of 2 x 104 V/cm was used. When the active material was dispersed monomolecularly throughout the dielectric material, a decay constant change, AA/X of 11 + 4 x was observed (391 with the nucleus decaying faster under the influence of the electric field. It can be seen that small variations in the rate of decay of nuclear species can be produced by large changes in the physical environment. The transition to the superconducting state, high pressure, and large changes in the electric field surrounding the atom can induce fractional changes in the nuclear decay rate of the order of 10-5 to
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It is seen that the half-life of a nuclear decay process can be altered. It is important to note that for most cases the perturbation is very small, and, in the vast majority of decays, the effect is too small to be measured. There are, however, specific cases where the interactions with the valence electrons affect the rate. In these instances, mea-
surahle variations can be found. As yet these changes are not well enough understood to he utilized as an indicator of chemical interactions. It is hoped that further study will lead to such an understanding.
1141 Joh1ege.H. W..Aumann. O.C.. endBorn. H.J..Phyr. Rcu.. CZ. 1616 119701. 1151 Bainhridge,K.T., Ga1dhaber.M.. and Wilron, E.,Phys. Re". 84,1260(19611. (16) Bainbridge,K.T., Goldhsbor.M.,and Wilson. E..Phys. Rev., 90,430(19531. J. 0.. ~ h y s ~. e u Lair., . 15. 6~ 1171 cmper. J. A,. ~ o l l s ~ d eJ.r , M.. and
asm muss en.
,.*cc>
\L""",.
(la o l i n . ~ . . ~ h y s ~ eCI. u . ,111411970).
1191 Weirauch. W.. Sehmidt-Ott. W. D.. Smend. F.. and Flsmmemfeld, A,. 2. Phys.. *""
"a"
,.""a,
'"J.'a8,Li.OC,.
Literature Cited (11 Garrett. A. B.. Lippincott. W. T.. snd Verhoek. F. H., "Chemistry, A Study of Mettor." 2nd Ed., Xemr CollegePublishing. Lexington. Mars., 1972. (21 Dickenson. P. E.. Gray. H. 5.. and H o i ~ h t .Jr.. G. P.. "Chemical Principles." W. A.Benjamin. I n c . New York. 1970. 131 Barrow. G. M.. "General Chemistry." Wadwath Publishing Company. Inc.. Bolmont.Cal.. 1972. (4) Moyer. s.. and S~hweidler.E.. "Radioartiuitst;' 2nd Ed.. B. G. Teuhner. Iaibzig. 1927. (5) Kohlisuseh. K. W. F.. "Radioactivilat. Handbuch der Experimentalphynik." Val; 16. Akademirche Vorlseaeerpllachaff mbH. Leiplig, 1928. ( 6 ) -~ Rnfhs W ..., "Ranrlhwh dpr PhusiX " (Editors: Gemer. H. and Schoel. K.I. Vol. 2 2 1 , Springer, Berlin. 1933. pp. 201-214. (7) Curie. M.. and Kamerlingh Onnes. M.. LeRndivm 10.181 (19131. (81 Rulhorfonl, E.. and Pelavel. - I E.. Brie. Arroc. Advan. Sd..Rep. A. 456 11m7): Ruthorford. E.. "Collemed P a n e d Vol 2. Wiley~lnterscienee.New York. 1963. ~
(91 I101 (11) (12) (13)
~
~
.-
-
.~
Sepre, E.. Phys. Rev.. 71.274(19471. Swre. E.. and Wiegand, C.. Phys. R e v . 75,39(19491. Leinin~er.R.F..Seere. E.. and Wiegsnd, C.. Phys. R e " , 76,697119491. Kraurhsar..J.l., Wilron. E. D.. andBsinbridee, K.T.. Phys Re". 9O.610119531. Bouchez. R.. Tobailem. .I.. Robert. . I . Muxart. R.. Melbt. R.. Daudd. R., and Oaudel. P., J. P h i s Rodium 17.363119561.
(20) Goign,.J. S.. Graham,R.L.,and Johns. M. W.. Con. J. Phys.. 41.94911969). (211 Smend.F..Borchert. I.. andlsnrhoff, H . . Z Phys.. 248.326(19711. , Lerr., 17.276 11965). (22) shimizu. s..a n d ~ s z a k iH..P~YS. (23) Maraki. H.. andShimizu. S.,Phys. Rru., 14 1161 (19661. Lett.. 26B.615119681. (241 ~ e v o d e M e v w n i e r M..Php. . (251 NeuedeMevergnies. M.,Phys. Rau Lett.. 23.422 11969). . 29. n88 (1972). (261 ~ e v e d ~euergnies. e M.. mys. R ~ O Lett.. . (1972). m l N ~ Vde * ~ e v e r g n i eM.. ~ . v ~ c v u m22.463 I261 Mslliario,A,C..andBainbridge. K.T..Phys. Re". 149, 958119661. 129) Mereliur. A..Ark. Pyp,37. 427119681. 1301 Bocquet. J . P.. Chu. Y. Y., Kistnor. 0. C.. Podman. M. L.. and E m w . G. T.. Phys Rev. L e n . 17.809119661. 131) By& D. H.,andStump,R., Phys. Re". 112.77 11958). (32) Daunt, J.G., endCobhlc. J. W..Phys. Rsu. 92.607 (1953). 1111 Olin A.. and Bainhridze. K. T.. Phvs Rev.. 179.450 119691 (34) Bainhridge, K. T., toported in P&, R. A,,and MeMillan, W. G., Phys Re", 117, ,.""n\ ""C
,ad ,A"-,.
(35) Hendey, W.K.. B-tt. W.A..sndHuizenga, J.R.. S r c w e ~ 181.11N~1973). , 136) St. Gagneux, Huher, P., Lcuenherger, H., and Nyikoa, P., Hdv. Phyr A c t 4 43, 39 (371 Niahi, M.,and Shimis", S.,Phyl. R P U ,B5.321811972). (381 Nyikos. P., St. Gagncux, Huhor. P.. Kohel, H. R.. andLeuenheqcr. H., Hdu. Phya. Aclo. U.11211970). H.. St. Gagneux, Hubor. P.. Kohcl. H. R., Nyika, P.. and Seiler. H., (69) H d v . Phya. Aeto, 43.411 (19701.
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