Decay of 78.4 h 89Zr

I.E.I: I 3.A [

NuclearPhysicsAll8 (1968) 538--544;~North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permissionfrom the publisher

D E C A Y O F 78.4 h Sgzr P. F. HINRICHSEN t

Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania Received 7 June 1968

Abstract: Sources of 78.4 h ssZr were produced by the say(p, n)8,Zr reaction, and the decay proper-

ties were studied with Ge(Li) detectors. The energies of previously reported ),-ray transitions were measured as E~ = 909.1 4-0.1, 1620.64-0.7 and 1713.04-0.6 keV. Two new transitions of E~ = 1656.9-4-0.7 and 1744.44-0.7 keV were also observed, the latter being assigned as a ground state transition from the [- state. The absence of any observed cascade feeding of this state leads to logft = 8.3 for this unique first-forbidden transition.

El

RADIOACTIVITY s'Zr [from s,y(p, n)]; measured Er, Iv; deduced log ft. sgy deduced levels. Ge(Li) detector.

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1. Introduction

The s 9 y nucleus has been the subject of a number of recent investigations with the (p, p') reaction 1-4), SSSr(3He ' d ) s 9 y reaction 4), 9OZr(d ' 3He ) reaction s), (n, fiT) reaction 6,7) and inelastic alpha s), deuteron 9) and electron scattering lo). A detailed study of the S9Zr and S9mZr decays using 7.6 cm × 7.6 cm NaI(T1) detectors has been reported by Van Patter and Shafroth 11), who found that 99 ~o of the 89Zr if+ decay went via the 3 + first excited state at 909 keV and 1 % via a 1.70 MeV 3,-ray from the level at 2.62 MeV. Art excess of counts on the low-energy side of the 1.70 MeV peak was assigned to a 0.12 ~ branch via a 1.62 MeV y-ray from the 2.53 MeV level. However, this analysis was complicated by the presence of the 0.51-0.91 MeV and 0.91-0.91 MeV pile-up peaks. It was, therefore, decided to confirm this weak branch and also establish accurate energy values for comparison with a recently reported (p, p ' ) m e a s u r e m e n t 1)by a Ge(Li) spectrometer study of the 89Zr decay. 2. Experimental method

The sources were produced by the s 9y(p, n)S 9Zr reaction, which has previously been shown 11) to produce radiochemically pure sources of S9Zr except for possible short-lived contaminants ot 10 min X3N and 111 min lSF. Two 500 #g/cm z yttrium foils were bombarded with i0 MeV protons, a total charge of 4000 pC being collected over a period of 40 h. For ease of handling, the foils were sandwiched between pieces t Present address: Foster Radiation Laboratory, McGill University, Montreal. 538

SgZr

539

DECAY

of scotch tape and then mounted between two 3.1 mm thick sheets of lucite to stop the positons. Two separate Ge(Li) spectrometers were used to study the decay. Initial spectra with strong sources were taken with a Ge(Li)-NaI(T1) three-crystal spectrometer with which both three-crystal pair spectra and Compton suppressed spectra can be recorded simultaneously. This spectrometer consists of a 20 cm 3 Ge(Li)) detector at the centre of a split 20.3 cm diam. by 30.5 cm long NaI(TI) annulus and has been described i

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i

egzr B'~y %

io

%

~_

w

-

x

._J bJ Z z -r n~

6

I--

o

4 o.

2 I I

0

I.I

I

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1.2

1.3

I

1.4

I

1,5

I

1.6

1.7

o

1.8

E~ Fig. 1. The s°Zr gamma-ray spectrum in the region 1.1 to 1.9 MeV, 4 d after source preparation. The only other observed transitions up to an energy of 2.9 MeV were annihilation radiation, the 909.1 keV~-ray and a weak 2614 keV peak due to ThC.

in detail previously 12). However, it has a solid angle of only 6 x 10- 3 sr and therefore, for energy measurements and detailed study Of the decay a second Ge(Li) detector of 17 cm 3 volume was used. Both detectors had energy resolution F W H M 4.6 keV at 1.33 MeV. Standard F E T preamplifiers, RC shaping amplifiers and a biased amplifier were used. The initial measurements were made with a 1024-channel ADC, while later measurements used a 4096-channel ADC. The observed spectra contained the strong annihilation radiation and 909 keV peaks as well as four peaks in the region near 1.7 MeV, see fig. 1. The fact that these peaks were due to 7-rays of energy 1.62, 1.66, 1.71 and 1.75 MeV and not secondorder effects or two escape peaks of higher-energy v-rays was confirmed by the fact that they were also observed in the three-crystal pair spectrum. The energy of the 909 keV v-ray was measured by direct comparison with the 897.98 keV v-ray of ssy. The 89Zr and 8Sy sources were adjusted to give peaks of

540

P.F. HINRICHSEN

approximately equal intensity, and sources of l arCs, 54Mn, 2°7Bi and 65Zn were also included to establish the calibration. The peaks were fitted using Gaussian graph paper la) with a typical uncertainty in peak location of _+0.1 keV. A comI0

5.0

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1.0 1: 1.5

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t/r Fig. 2. T h e intensities o f the 1.31, 1.62, 1.66, 1.71 a n d i.74 M e V ~,-rays relative to the 0.91 M e V transition as a function o f time in units o f the half-life o f 89Zr, T = 78.4 h. T h e o p e n a n d solid points refer to two separate sources a n d the error bars to statistical errors only. T h e decay c o n s t a n t for the 1.31 M e V transition is 166 h < T.~ < oc.

puter program was used to fit the calibration data to a second-order polynomial and calculate the 897.98-909 keV spacing. The energies of the calibration 7-rays were taken from the compilation o f Marion 14). Five independent measurements lead to a mean spacing AE = 11.09+0.1 keV, and hence, E~ = 909.1-t-0.1 keV. Due to the

811Zr DECAY

541

low relative intensity of the higher )'-rays, their energies were not measured by direct comparison with standard sources but relied on calibration runs made before and after each measurement. Unfortunately, gain shifts during the relatively long counting periods required to collect statistically significant data limited the accuracy of these measurements to ___0.7keV for the 1713.0 keV line. The energy spacings between these )'-rays and the 1713.0 keV transition were - 92.4 + 0.4, - 56.1 __+0.4 and 31.4 _ 0.3 keV leading to )'-ray energies of 1620.6+_0.7, 1656.9+0.7 and 1744.4__+0.7keV. These values are in good agreement with the values 1712.7___0.6 and 1621.5 keV recently reported by Heath 15,16). Care was taken to eliminate the possibility of these )'-rays being due to impurities. Background measurements contained lines due to ~°K and ThC but showed no signs of the 89Zr transitions. The decay of the source was followed for 10 d, and the intensities of these transitions relative to the 909.1 keV transition are shown in fig. 2. The relative intensities remain constant within the errors over a period in which the source activity decreased by an order of magnitude. The best values for the relative intensities were derived from the weighted mean of these measurements. The fullenergy peak efficiency of both Ge(Li) counters has been measured as a function of )'-ray energy using a 66Ga source and the relative intensities due to Camp 17). Over the region of interest, the efficiency of the 17 cm 3 counter was found to follow a power law of the form r/ = E~ 1.44 (MeV). The relative efficiency in the experimental geometry was also checked with 88y and 6°Co sources. A careful search for other possible transitions revealed only the 511-909 and 909909 pile-up peaks and a )'-ray of energy 1312.7+ 1.2 keV (fig. 1) which was not seen in the background. A possible assignment for this ),-ray is to the 2.22 MeV level which is known to decay 60 ~o via a 1.31 MeV branch to the 909 keV level and 40 ~ via a 715-1507 keV cascade 7). The first member of this cascade would be masked by the Compton distribution of the 909 keV transition, while the 1507 keV )'-peak falls between the Compton edges of the 1713 and 1744 keV transitions, see fig. 1. It was, therefore, difficult to establish the existence of this cascade, however, the upper limit on the intensity would be compatible with the expected branching. The variation of the relative intensity of the 1312.7 keV transition with time (fig. 2) indicates that this transition is however, probably due to a long-lived impurity. Furthermore, the observed intensity 1(1.31)/1(0.91) = ( 1 . 6 + 0 . 4 ) x 10 -4 (4 d after source preparation) would lead to a l o g f t of 8.5 compared with the value of approximately 13 expected for a transition to a (I) ÷ level. Specifically, this would require a negative parity for the 2.22 MeV level in disagreement with the (ct, ct') results of Alster 8). The upper limits on the intensities of transitions from the 2.53, 2.57 and 2.62 MeV levels to the ground state and 1.51 MeV level are < 5 and < 60 photons per million 89Zr decays, respectively. Four independent measurements of the relative intensity of the annihilation radiation 1(511)/I(909) lead to a mean of 0.443 _+0.002 in good agreement with the previously reported value 11). Uncertainties in the relative efficiency, however, lead to a final error of _+0.02 for the present measurement.

542

P.F. HINRICHSEN 3. R e s u l t s

The decay scheme derived from the present measurements is shown in fig. 3, and the data are compared with other recent measurements in table 1. The energy for beta decay to the ground state was taken as 2833,5+3.2 keV [ref. is)], and the no9/2+

~"1~'*°\¢=~¢=~,°'¢:' ,~,~

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Fig. 3. Decay scheme of 78.4 h " Z r .

TABLE 1 Experimental data Spin

t+ t{({)+

Level energy (keV) present a) work

909.1-4-0.1 1744.4-4-0.7

(½+, | + ) 2529.7-I-0.8 2566.0-1-0.8 (]+) 2622.1±0.8 =) e) d) e) t)

E r (keV) b)

908-4-2 1507+3 1745+3 2222+4

909 1505 1742 2217

253214 2572-1-6 2627±4

2525 2622

present work 511.0 909.1 +0.1 1507 e) 1744.4-t-0.7 715 e) 1312.7±1.2 f) 1620.6-1-0.7 1656.9+0.7 1713.0±0.6

e)

Intensity present d) work 44.3 4-2 44.2 4-0.9 100 100 <0.01 0.1274-0.004

1621.6 1712.7±0.6

<0.02 0.068-t-0.009 0.105+0.005 0.760+0.01

0.12+0.01 0.984-0.03

Ref. 1). b) Ref. =). Ref. 16). Note the values quoted in ref. 15) have been superseded by those quoted above. Ref. ix). (n, n'7) data 7). This transition is probably due to a long-lived impurity.

logfi

6.1 8.3 >8.5 7.5 7.2 6.1

89Zr DECAY

543

mograms of Lederer et al. 19) were used to calculate log f t values. The 1657 keV T-ray is assigned as a cascade transition from the recently reported 1) level at 2572 + 6 keV, while the energies of the 1621 and 1713 keV T-rays are in good agreement with the previous assignment 11) to the 2530 and 2622 keV levels. The 1744 keV y-ray is assigned as a ground state transition from the ~- level in agreement with the observation of a 1.75 MeV T-ray from this level in the (n, n'~) reaction 7). The feeding of the 1744 keV ~- level is of some interest as it is either by a unique first-forbidden electron capture transition or by cascade y-rays from higher levels. A careful search was made for possible 785, 822 and 874 keV y-rays from the 2.53, 2.57 and 2.62 MeV levels, and an upper limit could be placed on their intensity such that they would account for less than 30 ~o of the observed 1744 keV T-ray intensity. A preliminary coincidence measurement also showed no evidence for these T-rays. The transition to the 1744 keV level then has a logft = 8.3, which is typical of unique first-forbidden decays. 4. Discussion

It has been suggested by Shafroth et al. 6) that the levels of 89y can be understood in terms of the weak-coupling core-excitation model of de-Shalit 2 o). The collective 2 + and 3- states in SSSr at 1.84 MeV and 2.74 MeV are coupled to the odd p½ proton in 89y to form the 1.51 MeV ~- and 1.75 MeV ~- and the 2.22 MeV (~+) and 2.53 (k+) states, respectively. If this model is correct, the sum of the transition strengths for inelastic scattering from the coupled states in 89y should be equal to the transition strength to the core state in SSSr. The ratio of the sum of the transition strengths to the 1.51 MeV and 1.75 M e ¥ levels to that for the 1.84 MeV level in 88Sr was observed to be one-third for inelastic proton scattering 4) and one quarter in both inelastic alpha s) and electron lo) scattering measurements. Both the inelastic proton and alpha scattering data could be more convincingly explained in terms of simple shellmodel configurations for these states. Furthermore, proton pick-up studies with the 90Zr (d, 3He) reaction 5) show that the 1.51 MeV level contains at least half of the (2p~)- 1 hole strength, that the 1.75 MeV state has at least one third of the (lf~)-1 hole strength and that the dominant proton configurations in the 88Sr 2 + state at 1.84 MeV are (2p~r, 2p~ 1) and (2p~, lf~'l), which could not weakly couple to the extra (2P½) proton in 89y. This would then suggest that the ~/-decay to the 1.75 MeV state is primarily due to a ( l f l ) to (lg~) transition. For the higher levels, the situation is somewhat more complex. For proton scattering 4) the sum of the transition strengths to the 2.22 MeV and either the 2.52 MeV level or the 2.84 MeV doublet equals that to the 2.74 MeV state in 88Sr, while for inelastic alpha scattering 8) the sum of the transition strengths to the 2.22, 2.52 and 2.84 MeV groups was equal to that for the 3- state in SSSr. The transition strength to the 3- state in 88Sr for inelastic electron scattering lo) was between the sum for any two of the levels in 89y and the sum for all three. On the basis of the weakcoupling model, it is difficult to understand how three states in 89y can be involved.

544

P. F. HINRICHSEN

Recently, however, Lewis 21) has r e p o r t e d calculations in r e a s o n a b l e a g r e e m e n t with the (e, e ' ) d a t a in which the (2d~) p r o t o n state is included, thus giving two ~+ levels in 8 9 y which c a n mix a n d hence share the t r a n s i t i o n strength. T h e c o m p l e x n a t u r e o f the wave f u n c t i o n for the 2.53 keV state c o u l d a c c o u n t for the h i n d r a n c e o f the allowed t r a n s i t i o n to this level. V a n P a t t e r a n d S h a f r o t h 11) have c o m p a r e d t h e fl+ t r a n s i t i o n to the 909 k e V 9+ level with the 91M0 decay to the 3 + g r o u n d state o f 91Nb, for which the configurations s h o u l d be similar except for the a d d i t i o n o f two (lg~) p r o t o n s . T h e relative log ft values for these two transitions c a n be u n d e r s t o o d in terms o f the s 9Zr g r o u n d state wave f u n c t i o n which has a d m i x t u r e s o f b o t h (2p½)~ a n d (lg÷)02 p r o t o n configurations o f which only the latter c o n t r i b u t e s to the decay, while for 91Mo there are always (lg~) p r o t o n s available. V a n P a t t e r a n d Shafroth H ) have also suggested t h a t the 2 . 6 2 M e V state has the p r o t o n c o n f i g u r a t i o n (lg~)l(2p½)2(2p~) 2 which w o u l d explain the similarity o f the log ft values for the t r a n s i t i o n to this level a n d the 909 keV state. I t s h o u l d therefore be o f interest to l o o k for transitions in 91M0 a n a l o g o u s to those f o u n d here. I t is a pleasure to a c k n o w l e d g e stimulating discussions with Drs. D. M. V a n P a t t e r a n d S. M. S h a f r o t h a n d the help o f D. A. B l u m e n t h a l with some o f the d a t a analysis.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) I0) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)

P. F. Hinrichsen, S. M. Shafroth and D. M. Van Patter, Phys. Rev., submitted R. Van Bree and G. M. Temmer, Bull. Am. Phys. Soc. 13 (1968) 584 and private communication Yohko Awaya, J. Phys. Soc. Japan 23 (1967) 673 M. M. Stautberg, J. J. Kraushaar and B. W. Ridley, Phys. Rev. 157 (1967) 977 C. D. Kovalaski, J. S. Lilley, D. C. Shreve and N. Stein, Phys. Rev. 161 (1967) 1107 S. M. Shafroth, P. N. Trehan and D. M. Van Patter, Phys. Rev. 129 (1963) 704 P. S. Buchanan, S. C. Mather, W. E. Tucker, I. L. Morgan and E. L. Hudspeth, Phys. Rev. 158 (1967) 1041 J. Alster, D. C. Shreve and R. J. Peterson, Phys. Rev. 144 (1966) 999 E. W. Hamburger, Nucl. Phys. 39 (1962) 139 G. A. Peterson and J. Alster, Phys. Rev. 166 (1968) 1136 D. M. Van Patter and S. M. Shafroth, Nucl. Phys. 50 (1964) 113 P. F. Hinrichsen and T. Bardin, Bull. Am. Phys. Soc. 12 (1967) 462 P. Ormo, Rew Sci. Instr. 32 (1961) 1253 J. B. Marion, University of Maryland Technical Report 656 (1967) unpublished J. E. Cline and R. L. Heath, Report IDO-17222, unpublished R. L. Heath, private communication D. C. Camp, Lawrence Radiation Laboratory Report UCRL 50156 (1967) TID-4500 UC-4 unpublished J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nucl. Phys. 67 (1965) 1 C. M. Lederer, J. M. Hollander and I. Perlman, Table of Isotopes, 6th ed. (John Wiley & Son, New York, 1967) A. de-Shalit, Phys. Rev. 122 (1961) 1530 F . H . Lewis, Jr. and B. F. Gibson, Bull. Am. Phys. Soc. 13 (1968) 719