Joseph J. law1 Louisiana State University Baton Rouge, 70803
A
Nuclear Isomers Produced by cobalt-60 Irradiation
simple method for producing and studying nuclear isomers is described in this experiment which, includiug the sample irradiation and product characterization, can be completed within a 3-hr Nuclear Chemistry laboratory session. The materials and equipment required are a W o irradiation source, a scintillation spectrometer, and several metallic foils. Krypton-83, %r, 103Rh, lllCd, I131n, 1151n, 1 7 6 L ~ , lg9Hg, and 204Pbare the naturally occurring nuclides possessing metastable levels with half-lives between 30 min and 5 hr. Because of the very small cross-section for photonuclear activation, only the formation of mmSr, 103"'Rh, lllmCd, 113mIn,115mIn,and '75"'I~uby "Co irradiation has been reported (1, 2 ) . Since the method of isotopic activation is considerably simpler than that using bremsstahlung as the photon source (3,4) and the cost of the "Co source is also much lower than that of an accelerator, it is quite feasible to use this source for a student experiment. Although the less expensive Ia7Cssource is now as popular as 6 0 C ~the , energy of the gamma quanta, 0.662 MeV, emitted from la7Csis too low for an effective activation because the threshold energies for the activation are all higher (1). Other sources like lZ4Sband '82Ta, which emit 2.09- and 1.22-MeV gammas, respectively, can be used. However, for economic reasons the most suitable source is "Go. The shape of the source is immaterial, but a good irradiation geometry is essential. A convenient way is to place a thin metallic foil of the element to be irradiated directly on the top of a disk source. Metals or compounds of Sr, Cd, and I n are recommended. The induced activity, A , of a metastable isomer is given by the well-known equation
tion correction factor, I the intensity of y-ray transitions expressed in fraction, and e/y the internal conversion coefficient. Self-absorption of I n y-rays in a foil of 0.15-mm thickness or less is negligible. Some useful data on the interesting metastable isomers, the observed integrated photopeak counting rates a t end of irradiation, and the sensitivity of this experiment are given iu t,he table. The counting rates Metastable Isomers ~somcr
o
IIalflife riiri
Energy of Inomerio Transition (IW~VI
Aotivation cross-section (u~,i
Integrated Photopeak Sensicosntin. tivity. ~ a t e orooml 1x1
Conditions sowifred in text
are based on a mass of 1 g of metal irradiated for 1 hr a t a dose rate of 106 R hr-'. Each irradiated foil of 2.9-cm diameter was placed flat at the center on top of a 7.6- X 7.6-cm NaI(T1) scintillator for gamma counting. The sensitivities for these isomers are, under the same experimental conditions, calculated on a basis of 10 cpm a t the photopeak needed for detection (5). They are expressed in terms of the mass of natural element needed. Other characteristics of these isomers, such as the intensities of y-ray transitions and the internal conversion coefficients,can be found in the literature (6). The y-ray spectra of 87mSrand of """Cd can be found in references (7) and (8), respectively. The spectrum of I13"'In and '15mIuobtained from the activation of natural I n is shown in the figure. This spectrum was obtained from a 40-min count with the same detector, PWN+a (1 e-htl)e-hl, A = M after a 1.5-hr irradiation and 5-min delay. The irradiation time was chosen to permit the identification where P is the fractional isotopic abundance, W the of the 0.393-MeV photopeak of 113mInin the presence mass of the element irradiated, N the Avogadro's No., of the 115"In isomer which appeared with a relatively 6 the gamma flux, c the activat,ion cross-section, X the high activity from the target because of its high isotopic decay constant for the metastable isomer, t~the irradiaabundance. If a semiconductor detector is used, the tion time, ta the delay time, and M the atomic weight of "amIn photopeak should be better resolved. Because of the element. A dosimetric method can be used to obthe high price of the enriched lT31nisotope it is virtually tain the flux. For the 1.17- and 1.33-MeV gamma prohibitive to use it for obtaining a spectrum which quanta emitted by W o , a dose rate of 1 R hr-' is equivalent to a flux of 4.7 X lo5and of 4.3 X lo5y ~ m - ~shows little '15mIn. Imtecium-176 and 'OaRh are unsuitable for this experisec-I, respectively. The counting rat,e for gammas, C, ment, the former because of the similarity in energy is related to the activit,y as between the ?-rays from the ground and metastable ESIA states, and the latter because of the large internal conC = 1 (47) version coefficient of the '03"Rh y-rays. Metastable isomers of shorter half-lives such as the where h' is the efficiency of detection, S the self-absorp40-sec loqmAg and the 7.2-sec ' 8 7 m Acan ~ be studied with a rapid sample-transfer system. An inexpensive homePresent address: Longwood College, Farmville, Virginia made device was described by Iddings, et al. (9). 23901.
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Volume 46,
Number I , hnuary 1969 / 49
ment serves to introduce the concept of photon activation analysis. It can also be extended to the study of radiation dosimetry. Acknowledgement
The work was support,ed jointly by the National Science Foundation and U S . Atomic Energy Commission in the Research Part,icipation Program for College Teachers at Louisiana State University, summer 1907. The author thanks Dr. F. A. Iddings, Direct,or of the Program, for his advice and continuous interest..
C H A N N E L NUMBER Gamma-roy spectrum of "amln"smln from the 'OCo mctivotion of noturd and "'In photopeok, respectively. indium. (A) x-roy; IB) and (CI, '%
I n summary, the production of radioactivity by photonuclear excitation of the ground-state is the most direct method of producing a metastable state. The induced activity can be used as a measure of the source activit,y (lo), the dose rate (If), or the amount of the irradiated element in the sample (If), when all other parameters are fixed. I n addition to the production and characterization of the nuclear isomers this experi-
50 / journal of Chemical Education
Literature Cited (1) VERES,A., Magy. Fin. Foly., 14 (2), 143 (1966). (2) VERES,A,, Aeta Phys. Amd. Sci. Hung., 16, 261 (1963). (3) LUKENS,H. R., Js., O ~ v o s ,J. W., A N D WAGNER, C. D., Inl. J . Appl. Radiat. Isotopes, 11, 30 (1961). (4) O~ovos,J. W., GUINN,V. P., LUKENS,H. R., JR.,A N D WAGNER, C. D., Nuel. Instrum. Melhods, 11, 187 (1961). ( 5 ) LUKENS, H . R., J. CHBM.EDUC.,44 ( l l ) , 668 (1967). J. M., AND PRRLMAN, I., (6) LEDRRER, C. M., HOLLANDER, "Table of Isotopes," (2nd ed.), John Wiley & Sons, Inc., New York, 1967. (7) HEATH, R. L., "Scintillation Spectrometry Gammeray Spectrum Catalogue," (2nd ed.), No. IDO-16880-2, U.S. Atomic Energy Commission, Washington, D. C., 1964, Vnl. -.. 11.
(8) VERES,A,, Int. J . Appl. Radiat. Isotopes, 14, 123 (1963). F. A., AND DECELL,R. F., Proe. Louisiana Acad. (9) IDDINGS, Set., 28, 135 (1965). M., Preprints, (10) VERES,A., PAVLICS~C, I., AND OZSGYANI, Symposium on Standardization of Radionuelides, International Atomic Energy Agency, Vienna, Oot. 1966, No. SM-7915. ~ , Isotopes and Radiation, 4 (2), 93 (1961). (11) Y O S H I H A ~ ~K., and Radiation, 4 (2), 102 (1961). (12) Y O ~ H ~ H AK., R AIsotopm ,
