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-tran- sition, 2D(g, n) reaction, beta decay, half-life, data, reduced transition prob- ability 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 irradi- ation with an intense "Co source. This paper describes a measurement based on the photo- nuclear reaction T(1, n) with a point-like "Co source, and the result is compared with pre- vious data found in literatures. * Shinke-cho Fig. 1 Partial decay scheme of 6-Co , Sakai-shi, Osaka. - 1 - 238 J. Nucl. Sci. Technol., 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 appa- ratus 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 - 2 - Vol. 15, No. 4 (Apr. 1978) 239 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 re- actor. After 169.0 hr from the commencement of measurements the r -ray spectrum was measured with a 40 cc Ge(Li) detector to esti- mate 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.