Intensity of 2,505 Kev Gamma-Ray in Decay of Cobalt-60

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 1501 pp. 237~241 (April 1978). 237

Intensity of 2,505 keV Gamma-Ray in Decay of Cobalt-60

Masatoshi FUJISHIRO

Radiation Center of Osaka Prefecture*

Received July 15, 1977

The photonuclear reaction 2D(g,n) was utilized to measure the intensity of the weak 2,505 keV g-ray in the decay of 60Co. A point-like source was used to cause the reaction in heavy water, and the neutrons generated were detected with a BF3 counter. The observed intensity per decay was I(2,505) = (2.0+-0.4) x 10-8, which was smaller than previous estimates by about one order of magnitude. From the present result the reduced transition probability Bex(E4) and the partial half-life Ti/2(E4) of the crossover E4-transition between the 2,505 keV, 4+ and the ground levels in 60Ni was estimated as Be. (E4) = (7.0 +- 1.4) x 10-100e2cms and T1/2 (E4) 15 +-3 psec, respectively . KEYWORDS: cobalt 60, gamma radiation, photonuclear reactions, E4-transition, 2D(g, n) reaction, beta decay, half-life, data, reduced transition probability

2,505 keV, 4+ level, which is populated through I. INTRODUCTION the allowed b-decay of the Gamow-Teller Whether utilized as an energy standard or type, is deexcited mostly via the 1,332 keV, as an irradiation source, 60Co is one of the 2+ level emitting the well-known 1,173 and most widely used radioisotopes, and it is well 1,332 keV cascade g-rays. The 2,505 keV established that g-rays of 1,173 and 1,332 keV ray in question corresponds to the cross-g- are emitted with intensities of almost 100% over E4-transition from the 2,505 keV, 4+ following the p'-decay of the half-life of 5.26 level to the ground level, and its transition yr. However our knowledge on the intensity probability is predicted to be much smaller of the weak 2,505 keV 7.-ray is still scanty than that of the cascade E2-transition. The and uncertain. presence of the sum peak due to the 1,173 Figure 1 shows the partial decay scheme and 1,332 keV g-rays as well as the weakness of "Co relevant to the present work. The of the intensity of 2,505 keV g-ray makes it much difficult to measure the intensity di- rectly. Exact determination of the intensity of the 2,505 keV g-ray is considered important not only for comparing experiment with theory on the /-transition probability, but also for considering its influence in high-dose irradiation with an intense "Co source. This paper describes a measurement based on the photonuclear reaction T(1, n) with a point-like "Co source, and the result is compared with previous data found in literatures.

* Shinke-cho Fig. 1 Partial decay scheme of 6-Co , Sakai-shi, Osaka.

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II. METHOD has been neglected since the scattering mean free path of ~2,500 keV r-rays in heavy Figure 2 shows the experimental arrange- water is estimated to be about 24 cm. In the ment. A60Co source of about 45 Ci and of above expression I- denotes the source inten- 6 mm in diameter by 6 mm high is placed at sity (Ci) and I(Er) the intensity of the r-ray the center of a spherical flask which contains per decay of the source. From Eq. ( 1 ) we 2.1 / of heavy water, and the neutrons from can estimate 1(2,505) when C. (60Co)/Cr, (72Ga) the 2D(g, n) reaction caused by the 2,505 keV and I- (60Co)/I0(72Ga) are measured. r-ray are detected by a BF, counter. The According to the theoretical curve by Q-value of this reaction is -2,226 keV and Hulthen-Nagle(1), the cross section ratios the energy of the generated neutrons is cal- o-(2,505)/a(2,491) and a(2,508)/a(2,491) are es- culated to be 138 keV. In the next place the timated as 1.03 and 1.04, respectively. The source is replaced by a 72Ga source 60Coof intensity /, (60Co) of the 60Co source is esti- about 3 mCi to measure neutrons similarly . mated from conversion of the exposure rate measured with a standard cavity chamber, and I/0(72Ga) of the 72Ga source is measured with a Ge(Li) detector calibrated with a standard "Mn source.

MEASUREMENTS

1. Counting Rates Cn (60Co) and Cn (72Ga) of Neutrons Neutrons were measured with the apparatus shown in Fig. 2 in an irradiation room which has been made of heavy concrete of 100 cm thick. Backgrounds were measured

Fig. 2 Experimental arrangement with light water filled in the flask in place of heavy water. The contribution of natural In the latter case energies of the r-rays r-rays to the 2D(g, n) reaction in heavy water relevant to the 2D(g, n) reaction are 2,491 and could be neglected, since the counting rate 2,508 keV which differ from 2,505 keV only measured with heavy water has agreed with slightly, and accordingly energies of the that measured with light water within errors. generated neutrons may be regarded as ap- The BF, counter ND8534-60 manufactured proximately the same as that due to the by Mitsubishi Electric Inc. was put into a 2,505 keV r-ray. Consequently, the efficiency lead cylinder of 5 cm thick to be shielded of the neutron detection system and the from intense r-rays. effects of the scattered neutrons may be The results are shown in Table 1, in which thought the same for both sources. The the measured values of Cn(72Ga) have been counting rate Cn of neutrons then is propor- corrected with respect to the half-life of tional to the product of the r-ray flux 14.1 hr. For the 60Co source the average I0I(Eg) and the cross section s(Eg) of the total and background counting rates were 2D(g , n) reaction, and we obtain 17+-1 and 7+-1/hr, respectively, from which the net counting rate Cr, (60Co) of neutrons was derived as C'n(60Co) =(10+-1)/hr. The errors shown are the standard deviations of the mean values for thirty measurements. For the 72Ga source the corresponding values (1) were 1,912+-8 per 15 min and 8+-1/hr, respec- where the effect of Compton scattered r-rays tively, and accordingly

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Cn (72Ga)=(7,640+--32)/hr. meter in reading ionization currents. This exposure rate was converted to curie units Table 1 Measured results of total and using a conversion coefficient 1.32 (R/hr/Ci at background counting rates 1m), which gave the value of

I0 (60Co) =44.6+-0.7 Ci .

The conversion of exposure rate into curie is valid only for a point source, and the pre- sently converted value is believed to repre- sent the correct curie intensity since the 60Co source has point-like dimensions of 6 mmp 6 mm. x The 72Ga source was made by irradiating 0.01 g of gallium oxide of 99.99% purity in a nuclear reactor and was cooled down for about treble as long as its half-life before installation into the apparatus. A search through the literature has failed to indicate obstructive contaminants due to the possible (n, a), (n, p) and (n, 2n) reactions in the reactor. After 169.0 hr from the commencement of measurements the r -ray spectrum was measured with a 40 cc Ge(Li) detector to estimate the source intensity /, (72Ga). Figure 3 shows the resulting g-ray spectrum. Since standard sources appropriate for determining detector efficiency at around 2,500 keV were not available, the prominent 834 keV peak was used for estimating I0 (72Ga) : the 834 keV peak was compared with the 835.3 keV peak of a standard 54Mn source which has been supplied from the Radiochemical Centre, 2. Intensities (60Co) and England. The intensity of the 54Mn source I- (72Ga) of Sources at the time of measurements was 0.319 pCi The intensity /0 (60Co) of the 60Co source with +-3.7% guaranteed accuracy. The area was estimated from the exposure rate which under the peak was calculated by a computer has been measured with a standard cavity with a program which subtracts the cubic chamber(2). The distance between the source background underlying the assumed Gaussian and the chamber was 100 cm, and the both peak. In the present geometry of measure- were placed at a height of 70 cm from the ment the area of the 834 keV peak was 3,257 floor. Ionization currents were measured with +26/400 sec, while that of the 835.3 keV peak an electrometer Keithley 610C. Insertion of a was 1,396+17/400 sec, the errors being stand- 10 cm thick lead wall immediately behind the ard deviations of five measurements. There- source scarcely affected the readings 60Coof fore the intensity I- (72Ga) of the 72Ga source the meter, and accordingly the effect of the was estimated as scattered g-rays could be considered negli- I-(72Ga) =3.15+- 0.18 mCi , gible. The exposure rate thus measured was, after corrections for temperature and atmos- taking accounts of the intensity 95.5% of the pheric pressure, 58.9+-0.9 R/hr at 1 m. The 834 keV r -ray"' and the elapsed time of error shown is due to the fluctuation of the 169.0 hr. The error shown includes the statis-

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Fig. 3 p-ray spectrum of 72Ga

tical uncertainties in the peak areas, as well In the last row of Table 2 is shown the as the 3.7% uncertainty of the intensity of product Ibx d, e.g. the intensity of the g-ray the "Mn source. per decay of 72Ga. We shall adopt these intensities for our calculation. IV. ESTIMATION OF 1(2,491) AND 1(2,508) V. RESULT AND DISCUSSION The 2,491 keV g-ray corresponds to a Besides the 2,491 and 2,508 keV g-rays, transition between 3,324.9 keV, (2)- andg- several g-rays whose energies are greater 834.0 keV, 2+ levels in "Ge, the former being than 2,226 keV, the threshold energy of the fed through a b-decay of Eb =666 keV of 72Ga2D(g , n) reaction, are emitted following the ; the 2,508 keV g-ray corresponds to a p -decay of 72Ga. Table 3 shows those g-rays transition between 3,341.7 keV, (3)- andg- whose intensities are larger than 0.1% accord- 834.0 keV, 2+ levels, the former being fed ing to Ref.( 3). In the last row of Table 3 is through a b-decay of E,3=651 keV. Table 2 shown the product I(Er),s(Er), to which shows the rate Ib of these -decays and the branching ratio 5 of the 2,491 and 2,508 Table 3 72Ga g-rays which contribute keV g-ray as cited from Nuclear Data Sheets(3). to C, (72Ga) effectively

Table 2 Nuclear data on 2,491 and 2,508 keV T-rays

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neutron yields are proportional. The values The ratio Bex (E4) /Bex(E2) of the reduced of a(ET) have been read from the theoretical transition probability for the crossover E4- curve of Hulthen-Nagle(1). It is seen from transition to that for the cascade E2-transition this table that the total contribution of the is calculated from the present result as 2,515.6, 2,621.0 and 2,844.1 keV g-rays to (8.2+-1.6)x10-51 cm4. Combining this value Cn(72Ga) is 5.5% at most supposing the efficien- with Bex(E2) =8.5x10-50 e2•cm4, which is cal- cy of the neutron detection system remains culated from the partial half-life 0.3 psec of the same. Substituting all the values derived the cascade E2-transition(8), we obtain Be(E4) above into Eq. ( 1 ), the intensity 1(2,505) of =(7.0+-1.4)x10-100 e2 cm8, whereas the single- the 2,505 keV g-ray is calculated to be particle model predicts Bsp(E4) = 3.5x10-101 e2 cms. From the measured transition probabil-, I(2,505)=(2.0+-0.4)x10-8/decay, ity the partial half-life T1/2(E4) of the cross- taking account of 5.5% correction for Cn(72Ga). over E4-transition is estimated to be T1/2(E4) Table 4 shows the comparison of the present =15 +- 3 msec. result with other data found in the literature. There is an excited level at 2,626 keV with Table 4 Comparison of present P=3+ in 60Ni, and an upper limit for the result with other data decay of 60Co to this level has been esti-b- mated to be 0.002%(9). To the present author's knowledge, however, no M3-transition to the ground level has ever been reported, although the cascade 467 keV transition to the 2,159 keV, 2+ level has been detected indistinctly with a Compton suppression spectrometer(10) and by a coincidence measurement (Ref. (9)). At present, therefore, the effect of the 2,626 It is noted that the present result is smaller keV g-ray for the 2D(g,n) reaction can not than previous estimates by about one order be estimated. of magnitude, although the latter agrees barely with the value of Ref. (6) within stated ACKNOWLEDGMENT errors. The teams of both Fluharty-Deutsch The author thanks Messr. T. Tsujimoto and Morinaga-Takahashi used a bulky 60Co and K. Okamoto, Research Reactor Institute, source and detected the neutrons by the Kyoto University, for their hospitality and activation method. It is felt that, in com- help in prepairing the 72Ga source. Continu- parison with a point-like source, a bulky one ous encouragement by Dr. J. Furuta is also accompanies larger uncertainty in estimating grateful. its intensity I0 (60Co) from the measured ex- -REFERENCES- posure rate. The large error in the previous measurement by the present author (Ref. (6)) (1) MARION, J.B., et al. : "Fast Neutron Physics", Pt. 1, 28 (1960), Intersci. Publ. Inc., New York. is mainly due to the uncertainty in the cross (2) KATOH, A., et al.: Bull. Electrotech. Lab., 38, 264 section of the 9Be(g, n) reaction and to the (1974). statistical errors in the neutron counts. On (3) ALVAR, K.R. : Nucl. Data Sheet, 11, 125 (1974). the other hand, the present method has not (4) FLUHARTY, R.G., et al.: Phys. Rev., 76, 182 (1949). utilized photonuclear reaction whose absolute (5) MORINAGA, H., et al.: J. Phys. Soc. Japan, 14, 1460 (1959). ross section is known with only poorc preci- (6) FUJISHI, M. : Nucl. Sci. Eng., 52, 474 (1973). ion, and a smaller relative error has s been (7) SIEGBAHN, K.: "Beta- and Gamma-Ray Spectro- attained. According to Weisskopf's single- scopy", Chap. VII, (1955), North-Holland Publ. Co., Amsterdam. particle model, I(2,505) is roughly predicted (8) KIM, H.J. : Nucl. Data Sheet, 16, 337 (1975). o be 1.2x10-7 per decay of 60Co, twhich is (9) LOGAN, B.A., et al.: Can. J. Phys., 55, 142 (1977). lso shown in Table 4 for reference. a (10) CAMP, D.C., et al.: Phys. Rev., C, 14, 261 (1976).