The decay of 73Ga

J. inorg,nucl.Chem.,1970,Vol.32, pp. 2483to 2488. PergamonPress. Printedin Great Britain

THE

DECAY

OF

73Ga

T. E. WARD, D. L. SWINDLE, R. J. W R I G H T and P. K. K U R O D A Department of Chemistry, University of Arkansas, Fayetteville, Ark. 72701

(Received 9 January 1970) A b s t r a c t - T h e decay of 73Ga was studied with the use of a Ge(Li) spectrometer and fl- and y-ray scintillation detectors. A new y-ray of 328 keV was found in coincidence with the previously known 740 keV y-ray. The/3 branching of 73Ga to the 0.53 sec isomeric state of 7aGe was measured to have a 1.50 _ 0.10 MeV/3 end point with an intensity of 9 ___1 per cent. A tentative decay scheme is presented. INTRODUCTION

YTHIER et al.[1] and Marquez et al.[2] in 1958 and 1959, respectively studied the decay of 7ZGa and reported results which were not consistent. The major difference was the/3 branching of 7aGa to the 0.53 sec 7aGe isomeric state whose spin parity is 1/2-. Ythier et al. [1] did not observe a/3-ray populating this state and estimated its intensity to be < 5 per cent. The conclusion drawn by the absence of such a/3 transition was that it would require the ground state spin parity of 73Ga to be 5/2-; not 3 / 2 - as for 67Ga, 69Ga, and 7'Ga. Marquez et al. [2] compared gross/3 and fly coincidences on the 310 keV y-ray region and determined that 10 per cent of the total/3 decay of 7aGa should populate the isomeric state. Ythier et al. [1] produced 7aGa through the (n, p), (n, np), and (d, an) reactions on enriched r3Ge, 74Ge, and 76Ge, respectively. Marquez et al. [2] produced 7aGa through the (y, p) reaction on enriched 74Ge. In both investigations, a chemical separation was performed, but a strong 14.1 hr riGa activity followed the purified sample. In the present study r3Ga was produced by fast neutron fission of 2az'I'h and 2aaU and radiochemically separated. The sources thus obtained were highly decontaminated from all other fission products including the 14.1 hr 72Ga activity, which is quasi-shielded by 46.5-hr 72Zn. The fission product Ga fractions could be followed from 10a to 1 cpm with only 4-91-hr 7aGa as the product, thus enabling us to obtain very clean/3- and y-ray spectra. Cross bombardment of ~aGe with fast neutrons was used for comparison of y-ray spectra of 73Ga. From the present study a new y-ray of 328 keV was observed and the/3 branching to the 1/2isomeric state of 73Ge was determined. EXPERIMENTAL

Radioactive sources. 73Ga was produced through the fast neutron (n, p) reaction on enriched rage and the (n, fission) reaction on 2aZl'h and ~sU. The 14-8 MeV neutrons were generated by the University of Arkansas' 400 KV Cockcroft-Walton linear accelerator through the well known T(D, n) 4He reaction. The 10 mg sample of 85.85 per cent enriched raGe was obtained from Union Carbide, Stable Isotope Division, Oak Ridge, Tennessee. 1. C. Ythier, R. K. Girgis, R. A. Ricci and R. Van Lieshout, Nucl. Phys. 9, 108 (1958). 2. L. Marquez, E. W. Cybulska, N. L. Costa, I. G. Almeida and J. Goldenberg, Nucl. Phys. 10, 28 (1958). 2483

2484

T.E.

WARD, D. L. S W I N D L E , R. J. W R I G H T and P. K. K U R O D A

Radiochemical procedure. The radiochemical procedure developed by Iddings [3] for the determination of radioactive G a from fission product solutions was employed in the present investigation. The radiochemical procedure consisted of a series of extractions of G a into isopropyl ether and further purification by extraction into a TTA-Benzene mixture. The sample was counted in the form of Ga(OH)3. The time required from end of bombardment to the initial count was typically 2 hr.The decontamination factor from all other fission products was estimated at > 10s. Target samples of 15-30 g of 28rI'h and ~aU (as nitrates) were required for each bombardment. No chemistry was performed on the enriched ~aGe sample. Counters. 3' spectrometry was carried out using a 3 in. × 3 in. NaI(TI) detector and a4.1 cc Ge(Li) spectrometer in conjunction with a 4096 channel Nuclear Data 3300 analyzer. For y - y coincidence measurements the 3 in. × 3 in. NaI(TI) detector and the Ge(Li) detector were used./3-`)' coincidences were carried out using a 1.5 in. diameter by 13/16 in. high cylindrical plastic detector and the NaI(TI) detector. A Canberra 800 series slow coincidence unit (1 tzs delay) was used with the N D 3300 system for coincidence measurements./3 spectra were obtained using the plastic detector.

RESULTS AND DISCUSSION

Singles Ge(Li) y-ray spectra of fission produced 73Ga radiochemically separated from irradiated z3zl'h revealed a new 328 ± 1 keV y-ray in addition to the previously known (1,2) 5 4 ± 1, 295___ 1, and 740___3 keV y-rays. In Fig. 1 is shown a typical Ge(Li) spectrum of the 295-328 keV y-ray region, r3Ga was also produced through the fast neutron (n, p) reaction on enriched 73Ge. This experiment was performed to compare the y-ray intensity ratio of the 295 and 328 keV y-rays to the ratio obtained from fission produced 73Ga. The intensity ratio remained constant, indicating that the 328 keV y-ray is associated with ~3Ga. Table 1 lists a summary of the previous investigation by Ythier et a/.[1] and Marquez et ai. [2] in addition to the results obtained in the present study. y - y coincidence measurements on the 740 _ 80 keV y-ray region was observed to have only the 328 keV y-ray in coincidence as shown in Fig. 2. The 740-+-80 keV gate was set with the NaI(TI) detector and the Ge(Li) detector observed the coincidence spectrum, fl--y coincidences on the 295-328 keV y-ray region Table 1. Radiations from r3Ga

Radiation Yl `)'2 ')'3 `)'4 fll /32 /33 134

Ythier et al. (Ref.[l]) E(keV) l*l 54___1 295±3 745-----10 1040-----15 400± 150 1190

9.3-+1 100.0 6.2--+I 1 4.5±2 95-+7%

Marquez et al. (Ref.[2]) E(keV) 1"1 53---2 310±5 740-+10 550 1300 1600T

15.0___2.2 100.0 7.8 7 83% 10%

This work E(keV) I~1 54---1 295-+1 328±1 740-+3 430T 1167T 1200-+80 1500-+100

9.1-+0.8 I00.0 17.5_+1-2 5"5--+ 1"4 4-3-+ 1-1% 9-3-+0.9% 77.4-+3.8% 9-+1%

Coincidences Ref. [1]. (295 y)(1190 /3); (295 y/(1190/3) = 1.02-+0.07 (Ein keV). Ref. [2]. Na/Nnv (310 y) = 0.90). This work: (328 `)')(740 y); Nn/N~, (295-328 y) = 0.91; (295-328 y)(1200_ 80 fl). *3' intensities re-normalized to `)'(295 keV) - 100.0 in Ref. [1] and Ref. [2]. TEnergies derived from fl-, fly-, and `)'-ray relationships. 3. G. M. Iddings, In U.S. N A S - N S rep. 3032, p. 35.

The decay of ~aGa

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revealed a 1.20_+0.08 MeV fl-ray end point./3 spectra of the fission produced raGa was observed to have a 1.50___0.10 MeV/3 end point energy as shown in Fig. 3. Comparison of/3 and/3~/coincidences on the 295-328 keV y-ray region revealed that 9__+ 1 per cent of the total fl decay of r3Ga populated the 0.53 sec isomeric state of 73Ge. Singles NaI(Tl) spectra of fission produced 73Ga was observed to have -/-rays of 54, 293-328, and 740 keV. Ythier et al. [1] reported observing a 1.04 MeV

2486

T.E.

WARD, D. L. SWINDLE, R. J. W R I G H T and P. K. K U R O D A

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Fig. 2. Gamma-gamma coincidence spectrum of the 295-328 keV y-ray region obtained with the Ge(Li)-Na(Tl) detector system. The NaI(TI) detector was gated on the 740_+ 60 keV y-ray region. Standard calibration points are marked at the top of the spectrum using the 276, 302, and 356 keV y-rays of ~aaBa.

y-ray of - 1 per cent intensity. However, Marquez et al. [2] later attributed the 1.04 MeV line to summing effects. Our results are in agreement with those of Marquez et al. [2] concerning the 1-04 MeV T-ray. The half-life of raGa was measured by gross/3 decay counting using a 2 zr endwindow methane flow proportional counter. Least-squares analysis of the/3 decay half-life of fission produced 7aGa yielded 4-91 _ 0.05 hr. 7aGa can be assigned a ground state spin parity of 3 / 2 - since the 31st odd proton of Ga should fall into the 2p a/2 single particle level. The ground state spin parity of raGe is known to be 9/2+ and the 0.53 sec isomeric state at 67.4 keV has been assigned a J~ = 1/2-[4]. The isomeric state decays by way of a 53.9 keV transition to a 13-5 keV 5/2+ level. Heymann et al. [5] studied the excited levels of raGe through the (d, p) and (p, p') reactions. A 368 keV level was assigned a spin parity of I I 2 - as determined by its angular distribution from the (d,p) stripping reaction. They also observed levels at 396 and 1141 keV from the (d, p) 4. J. P. Welker, A. W. Schardt, G. Friedlander and J. J. Howland, Jr., Phys. Rev. 92, 401 (1953). 5. G. Heymann, P. van Der Merwe and 1. J. van Heerden, Z. Phys. 218, 137 (1969).

The decay of raGa 40-0

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reaction study. The 368 and 396 keV levels were not observed in the ( p , p ' ) reaction study although a 1132 keV level was observed. Collective positive parity states are expected to be excited by the (p, p') elastic process whereas the (d, p) reaction should excite more strongly the negative parity states. Many doublets were observed in the (p, p') and (d, p) study by Heymann et al. [5]; therefore, the 1141 keV level from (d, p) and the 1132 keV level from (p, p') are not necessarily the same state. From the results of our investigation summarized in Table 1 we have constructed a tentative decay shceme as shown in Fig. 4. The 328 keV T-ray was found to be in coincidence with the 740 keV T-ray and can be assigned to depopulate the 395 keV level with the 740 keV T-ray depopulating a 1135 keV level. The 395 and 1135 keV levels could correspond to the 396 and 1141 keV levels observed in the (d, p) stripping reaction[5]. No spin parity assignments can be made in view of the fact that raGe is known [5] to have some collective nature which could hinder both/3 and T decay. The log ft value of 5.9 for the/3-ray populating the 362 keV 1/2-- level indicates that the allowed/3 transitions may be hindered. The results obtained in the present study concerning the/3 branching to the 67.4 keV 1/2- isomeric state of raGe are in agreement with the results by Marquez et al. [2]. The/3 decay of ~3Ga (J~ = 3/2-) to the isomeric state of raGe (J~ = 1/2-) has previously been classified[6] a probable allowed unfavored fl transition. Such an unfavored transition would be retarded by a factor of the order of 10-100 as compared to single particle estimates. The log ft value of 7-1 calculated for the 1.50_ 0.10 M e V / 3 transition clearly indicates that it is hindered, raGe is known 6. A. Bohr and B. R. Mottelson, Mat. Fys. Medd. Dan. Vid. Selsk. 27, 16 (1953) (3rd Edn, 1964).

2488

T . E . WARD, D. L. SWINDLE, R. J. WRIGHT and P. K. KURODA

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[5] to have a collective field assiciated with it. The negative quadrapole moment of r3Ge would indicate that it has an oblate deformation, whereas 7aGa can be assumed to have a near spherical or slightly prolate shape since 67Ga, ~gGa, and riga all have positive quadrapole moments. Therefore, in changing from a spherical or slightly prolate shape to an oblate shape, surface readjustments must accompany the particle transition and appreciably retard it. The intensity of 9.1 _+ 0.8 for the 54 keV isomeric transition would yield a conversion coefficient of 1 0 . 0 _ 0.9. The theoretical estimate for a 54 keV M2 isomeric transition is about 9.7. Of the work by Ythier et al. [ 1] and Marquez et a/.[2], if one re-normalizes their data to account for the 328 keV y-ray, one obtains intensities for the 54 keV y-ray (relative to the 295 keV y-ray) of I l _+ 2 and 18_+2.6, respectively. Acknowledgements-This work was performed under the auspices of the U. S. Atomic Energy Cbmmission contract No. At-(40-1)-3235. The authors would like to thank Mr. D. Coflield for operation of the accelerator and Mr. P. Pile for technical assistance.