The neutron-deficient yttrium isotopes 82Y, 83Y and 84Y

I. Inorg.Nucl.Chem., 1962,Vol.24, pp. 1175to 1179. PergamonPressLtd. Printedin NorthernIreland

THE N E U T R O N - D E F I C I E N T Y T T R I U M ISOTOPES s2y, say A N D s4y, V. MAXIAt, W. H. KELLY~ a n d D. J. HOREN Lawrence Radiation Laboratory, University of California, Berkeley, California (Received 5 March 1962)

Abstract---The neutron-deficient yttrium isotopes s~.y, 8ay and a4y have been produced by irradiations with the Berkeley heavy-ion linear accelerator. Where possible, identifications were made by establishing genetic relationships with known daughter or granddaughter activities. The half-life of s4y determined by direct decay is 39 ± 2 rain. By a series of timed chemical separations, the following half-lives have been established: s2y, 9 ± 3 min; say, 8 ± 2 min. No information pertaining to the radiations emitted in the decay of these yttrium isotopes, other than s4y, has been obtained. The )'-ray spectra of s4y and s3Sr are shown. EARLY attempts to study the decay of s2y revealed that its half-life was m u c h shorter t h a n the 70 m i n previously reported by CARETTO a n d WI/G. tl~ These authors also reported the half-life of s s y as 5 hr. F r o m other work at this laboratory, in which ssmSr was chemically separated from y t t r i u m at regular intervals, it was f o u n d that s s y has a 3 hr half-life. (2) The a p p a r e n t disagreement in the half-life for ssy as f o u n d by CARETTO a n d WIIG ~1) a n d KIM e t al. ~ has been resolved by the discovery o f a 5 hr isomeric state in ssy.tz) The present work was u n d e r t a k e n in order to clarify the situation pertaining to the neutron-deficient y t t r i u m isotopes with A < 84. I n this paper we report o n the identification of s2y, say a n d s4y. 1. E X P E R I M E N T A L M E T H O D The Berkeley heavy-ion linear accelerator (Hilac) was used to produce neutron-deficient yttrium isotopes by the reactions 75As(12C,xn) 87 *Y and ~gGa(leO, xn)Ss-*Y. For convenience, most of the data were obtained from bombardments of thick targets of powdered arsenic metal with about 120 MeV carbon ions, while other reactions were mainly used for corroborative purposes. The irradiation times varied from 5 min to 2 hr, depending upon the particular activities being studied. The yttrium activities reported here were also produced by spallation of natural strontium with 240 MeV protons in the Berkeley 184 in. cyclotron. To aid in the identification of 84y, a few milligrams of enriched 84Sr§ were irradiated with 5- and 15 MeV deuterons in the Crocker 60 in. cyclotron. The half-lives of 8,y and 83y were determined by establishing their genetic relationships to known strontium and rubidium isotopes by means of timed chemical separations of daughter activities. Photon spectra were examined with a 7"62 cm × 7'62 cm NaI(T1) crystal mounted on a Dumont 6363 photomultiplier coupled to a multichannel pulse-height analyser. Positron spectra were measured with a 2-5 cm diam. × 1.25 cm plastic scintillator, and gross fl-counting was done with an endwindow, flowing-methane proportional counter. * Work done under the auspices of the U.S. Atomic Energy Commission. l" Permanent address: Laboratorio di Radiochimica, Universit~i di Pavia, Pavia, Italy. :~ On leave from the Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan. § Obtained from separated Isotopes Division, Oak Ridge National Laboratory, Oak Ridge Tennessee. i1~A. A. CARETTO,Jr. and E. O. WIIG,J. Amer. Chem. Soc. 74, 5235 (1952). c2~y. E. KIM,D. J. HORENand J. M. HOLLANDER. Unpublished data (1960). tal D. J. HORENand W. H. KELLY,Bull. Amer. Phys. Soc. 7, 341 (1962); Bull. Amer. Phys. Soc. 7, 419 (1962). 1175

1176

V. MAXlA,W. H. KELLYand D. J. HoR~N

2. CHEMICAL PROCEDURES After irradiation, the arsenic targets were dissolved in hot aqua regia and boiled to dryness. The residue was dissolved in 5 ml of 0.1 N HCl from which the yttrium was extracted into a 10-ml solution of 1"5 M di-2-ethyl-hexylphosphoric acid (i.e., HDEHP) and toluene,t4) The organic phase, containing the yttrium activities, was washed several times with 0.1 N HC1, after which timed chemical separations (milks) of the daughter strontium and rubidium activities were conducted by washing with 0.1 N HCI. The "milk" intervals were varied from 5 to 17 min in different experiments. In some experiments the strontium and rubidium fractions were separated by using the standard techniques of coprecipitation of Sr(NOs)~ with Ba(NO3)~ by the addition of fuming nitric acid and chilling.TM 3. E X P E R I M E N T A L RESULTS 3"1 Yttrium-82 After an irradiation of powdered arsenic metal with 120 MeV carbon ions, the genetic relationship of s2y to its daughter S~Sr was established by a series of timed chemical separations (milkings). In these experiments yttrium was extracted into a solution of toluene and H D E H P while strontium was back-extracted into 0"1 N HC1.

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FIG. 1.---Yields of 2.0-3.0 MeV positrons of 8~Sr-S~Rb decay as a function of time of chemical separation of strontium from yttrium parent. Several days after the milking experiments were preformed, the strontium fractions were chemically isolated and measured with the scintillation spectrometer and proportional counter. From these measurements it was found that annihilation radiation, a weak 770 keV photon, and approximately 3 MeV positron branch decayed with about a 25 day half-life. These data are attributable to the decay of 82Sr-S2Rb--a~Kr, since it is known that 25 day 8~Sr decays by electron capture to 1.3 min aeRb, which.in turn decays to a2Kr with the emission of 3.1 MeV and 2.3 MeV positron branches and a weak 777 keV photon36) By counting the positrons with energies between 2 and 3 MeV, the yield of S2Sr as a function of the time of the chemical separation of strontium from yttrium was obtained. Typical results are shown in Fig. 1. From a number of such experiments, the average value obtained for the half-life of s2y is 9 + 3 min. ~4)D. F. PEPPARD,G. W. MASONand S. W. MOLINE,jr. Inorg. Nucl. Chem. 5, 141 (1957). is) D. N. SLrNDERMANand C. W. TOWNLEY,NAS-NS 3010 January (1960). ~6) Nuclear Data Sheets, National Academy of Sciences National Research Council, NRC 59-1-67 (National Research Council, Washington, D.C.)

The neutron-deficient yttrium isotopes 8.~y, say and 84y

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1178

V. MAXIA, W. H. KELLY and D. J. HOREN

3.2 Yttrium-83 The 7-ray spectra of the strontium fractions milked from yttrium contained prominent peaks at 38, 375, 511,770, 1160 and 1520 keV which decayed with a 34 hr halflife. This activity is assigned to SaSr, since the latter is known to decay with a similar half-life with the emission of a 1.2 MeV positron branch. (7) A typical ~,-ray spectrum of SSSr is shown in Fig. 2. Fig. 3 shows the yield of the 375 keV peak as a function of the time of the chemical separation of strontium from yttrium. Consideration of the data from several experiments utilizing different production modes of yttrium and different milk times leads to an average value of 8 ± 2 min for the half-life of s3y. i

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3.3 Yttrium-84 When the yttrium fraction obtained from the bombardment of arsenic with C 12 ions was chemically repurified approximately 2 hr after the irradiation, the gammaray spectrum decayed with 39 ~ 2 rain (see Fig. 4). Strontium fractions chemically separated from the yttrium activity at fixed time intervals (40 min) contained only 150 and 235 keV photons arising from the decay of sSmSr and a very weak 513 keV photon from the decay of asSt. The half-life ofaSy determined in this way is 3"0 4- 0.2 hr, in good agreement with the results of KIM et aL (~) The 39 min yttrium activity was also produced by irradiating strontium enriched in S4Sr with 15 MeV deuterons, but not with 5 MeV deuterons. These data are consistent only with the assignment of the 39 min activity to a4y. This half-life is in good agreement with that obtained by YAMAZAKI et aL ts)

c7~S. V. CASTr,mRand D. H. TEMPLETON,Phys. Rev. 88, 126 (1952). (s) T. YAMAZAKI,H. IKEGAIvlI,M, SArda, K. SArro, Y. HASHIMOTO,M. FUJIOKA,H. OHNUMA,E. TAKE~OSm, J. MATSUMOTOand A. HASHIZUME. Unpublished data; M. SAKAI. Private communication (1961).

The neutron-deficient yttrium isotopes 82y, s3y and say

1179

DISCUSSION I n this w o r k , the neutron-deficient y t t r i u m isotopes w i t h mass n u m b e r s 82, 83 a n d 84 have been synthesized a n d their half-lives f o u n d to be c o n s i d e r a b l y shorter t h a n those p r e v i o u s l y r e p o r t e d . Owing to lack o f time, no i n f o r m a t i o n has been o b t a i n e d concerning the r a d i a t i o n s emitted in the decay o f these isotopes, with the exception o f say.

Acknowledgements--Oneof us (V. M.) wishes to acknowledge the Conference Board of Associated Research Councils, Washington D.C., for a fellowship, and to thank Professor I. PERLMANfor the hospitality accorded her while a guest at the Lawrence Radiation Laboratory. We wish to thank Drs. S. S. MAR~OWITZ,R. M. DIAMONI)and J. M. HOLLANDERfor many helpful discussions. We wish also to thank Mr. J. A. HARRISfor his assistance in one of the chemical separations, and the operating crews of the Hilae and the 60 in. and 184 in. cyclotrons for providing the irradiations.