Structure in β-delayed proton spectra of N=81 precursors

Volume 178, n u m b e r 2,3

PHYSICS LETTERS B

2 October 1986

STRUCTURE IN fl-DELAYED PROTON SPECTRA OF N = 81 PRECURSORS K. S. TOTH, Y. A. ELLIS-AKOVALI Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

J. M. NITSCHKE, P. A. WILMARTH, P. K. LEMMERTZ i, D. M. MOLTZ Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA

and F. T. AVIGNONE III University of South Carolina, Columbia, SC 29208, USA Received 3 June 1986

Intense peaks, superposed on a structureless spectrum, were observed in J~ Y b fl-delayed proton decay. It is shown that the peak and statistical protons follow the decay of the s~/2 ground and h~ ~/2 isomeric states in ~5'Yb, respectively. It is concluded that the peaks, also seen in t47Dyand ]49Er decays, are due to a low density of 1/2 and 3/2 levels at ~ 4.5 MeV in the N = 82 daughters.

The recently reported [1,2] delayed-proton spectra of t a 7 D y and 149Er s h o w pronounced peak structure in contrast to those of other rare-earth delayedproton precursors (see e.g. ref. [3] ). The 147Dy spectrum, with a high-energy cutoff of ~ 4.5 MeV, is dominated by distinct peaks below 4 MeV, while the 149Er spectrum, which extends slightly beyond 6.5 MeV, has less structure. This difference was attributed [ 1] to the level densities in the fl-decay daughters: 147Tb with just one proton beyond the ( N = 82 + Z = 64) core of 146Gd would be expected to have a lower level density than 149Ho with two additional protons outside the same core. In an effort to better understand this structure and to see if it persists at still higher atomic members we investigated the next, and hitherto unknown, N=81 isotone, 151yb" Ytterbium-151 was produced in the 96Ru(58Ni, n2p) reaction by bombarding a 1.5 mg/cm 2 thick target of ruthenium, enriched in 96Ru to 96.5%, deposited onto a 2.0 mg/cm 2 thick HAVAR foil with 360 Present address: Gesellschaft fiir Schwerionenforschung, Postfach 110541, D-6100 Darmstadt 11, Fed. Rep. Germany.

150

MeV 58Ni ions from the Lawrence Berkeley Laboratory SuperHILAC. After traversing the backing the SSNi beam had a calculated energy of 250 MeV at the center of the target. Reaction products were mass separated with the OASIS on-line separator [4], collected with a programmable tape system, and transported to a position in front o f a Si particle-telescope (a 9.1 #m A E and a 702/lm E detector combination) for measuring protons. A planar hyperpure Ge detector was placed directly behind it to detect X rays. Two n-type Ge detectors were used to obtain singles and coincidence y-ray information; one of them, with a relative efficiency of 24%, faced the telescope, while the other, a 52% detector, was set one side about 4.5 cm from the radioactive source. A 1 mm thick plastic scintillator was placed in front of the 24% detector so that positrons could also be registered. With this arrangement coincidences between protons, photons, and positrons were recorded. Events in all detectors were tagged with a time signal for half-life determinations. The beam energy was chosen to optimize the production of ~ tYb, while the tape cycle time of 4 s was based on preliminary data [ 5 ] which indicated a half-life of ~ 1 s for ~51yb. 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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2 October 1986

M 1 , h I 1/2 ---~ d3/2---~ Sl/2. cascade was not observed. This 100 80 60 40 20 30

(b)

20 10

0..

~

4o

0

3o

'

(c)

2o 10

....

~

a,ll

12

4

1.0

2.0

30

40

5.0

.....

11

6,0

(d)

7,0

Proton energy (MeV)

Fig. l.Delayed protons from ~5~Ybfldecay: (a) singles spectrum, (b) spectrum in coincidence with positrons, (c) spectrum generated by subtracting part (b) from part (a), and (d) spectrum in coincidence with Tm K-X rays. Fig. l a shows the delayed-proton spectrum accumulated during a 48 h period; typical b e a m currents were about 9 × 10 ~ particles/s. The half-life o f these protons was measured to be 1.6 _+ 0.1 s. We conclude that they follow the fl decay o f the new isotope ~S~yb on the basis o f a new half-life in the isobaric chain, and coincidences with T m K-X rays and with several 15°Er y rays that have been observed [ 6] inbeam. F r o m systematics [ 7] o f levels in e v e n - Z N = 81 isotones an h~/2 isomer is expected in 15JYb at ~ 0.75 MeV above the sl/2 ground state. Direct evidence for this isomer via Yb K-X rays or the M4 -~

is consistent with the estimated isomeric decay branch o f 6 × 10 - 4 which was based on systematics [7] o f W e i s s k o p f enhancements a n d on the measured 1.6 s half-life. As discussed below, however, the delayed-proton data point to the existence o f both r decaying states in ~5~Yb. The delayed-proton spectrum o f ~Slyb, fig.la, consists o f peaks superimposed on a structureless (or statistical) spectrum. Fig. l b shows protons in coincidence with positrons. A strong e n h a n c e m e n t o f the peaks relative to the structureless portion o f the spectrum is evident together with an accompanying reduction in the higher energy proton events. Conversely, the delayed-protons coincident with T m KX rays, fig. 1d, have a statistical distribution with little indication o f structure. Fig. lc was generated by subtracting a n o r m a l i z e d spectrum similar to fig. l b from the one shown in fig. 1a, where the normalization was done through the lowest energy proton peak at 3.4 MeV. Fig. lc thus represents events in coincidence with all electron-capture (EC) decays while fig. l d selects events in coincidence only with K capture. Figs. lc and l d are similar in appearance, i.e., they are both basically structureless, they have m o r e highenergy protons than fig. lb, and, their centroid energies are 4.8 MeV while that o f fig. l b is 4.0 MeV. Coincidences with y rays show that the ~5i y b delayed protons populate the first 2 +, 3 , 5 , 4 +, and 6 + states in l S°Er (see level scheme in fig. 2). However, these excited-state decays account for only 49% o f all observed delayed protons; the r e m a i n d e r proceed directly to ground. Statistical model calculations [ 8 ] predict that 96% and 6% o f protons originating from the sj/2 and ht 1/2 151Yb levels, respectively, decay to the ground state. Therefore, most o f the ground-state protons are associated with the s~/2 level while protons populating excited states in t S°Er originate mainly from the hi 1/2 isomer. Table 1 lists the experimental distributions for proton decays to the various ~5°Er states together with percentages p r e d i c t e d for each o f the two 1s i y b levels. Calculated values for the hi ~/2 isomer are n o r m a l i z e d to the experimental feeding o f 11.5% to the 4 + level, while for the sl/2 151yb state, the calculations are n o r m a l i z e d to the experimental value o f 51.3% for the lS°Er ground state.

The y ray seen with the best statistics in eoinci151

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12

Counts/ch. ~0

I f 2 0

r~

9

//

/

11/2- 1.6s] ~ FC I - ~ t J 3/2 // ~,,/ 11/2 1.6s |

~,,,,J

_ I~,++ EC (% per 10 keV)

Counts/ch. ~ i (1)

3~f'01201078642i •~5

151v~, 70Iu

I

~6

P-7208coinc.

5"

.--~

d.)

~4 T4+~360 . - 3 ~508~_=~._ /p-j3 + colnc. 3 ";161' ~,~475 /

0 --150~ 68~.,

18.5s

~ --'--"-

11/2

151T~ 69 "~ ' '

3.8S

Fig. 2. Energy diagram for ~StYb~-delayed proton decay. Included in the figure are "theproton spectra in coincidence with positrons [see fig. lb] and with the 208 keV y ray in ~5°Er, resonances in t~Tm predicted by fl strength [1/~= ( F + + .fEe') (S/~) ] calculations for the s~/~~StYbground state, and, ~5°Erlevels seen in our proton-y-ray coincidence data. Two new y-rays, 716 and 508 keV, were observed in these coincidence measurements; we propose that they de-excitea previously unobserved 4 + level at 2.295 MeV. dence with protons is the 208 keV transition which connects the 3 - and 2 + levels. Its intensity is a good measure of delayed protons that proceed from the ~5i y b h~1/2 isomer and populate levels above the JS°Er 2 + state. The proton spectrum gated by the 208 keV ray is structureless (see fig. 2) a n d has a centroid energy equal to those o f figs. lc and ld, i.e., 4.8 MeV. This evidence shows that the statistical protons (emphasized in fig. l d ) populate excited states. The accompanying conclusion is that the peak protons ( e m p h a s i z e d in fig. l b ) proceed mostly to the 15°Er ground state. After correcting for recoil and proton separation energies the peak protons can be assigned to regions 152

2 October 1986

o f excitation energy from about 3.8 to 4.8 MeV in ~51Tm by assuming that they decay to the ~S°Er ground state. The fl+/EC ratio for 151Yb fl decay to these levels is in the range o f 3.6 to 1.9. By assuming that the statistical protons populate excited states starting at 1.6 MeV in 15°Er their distribution corresponds to fl decay to levels above 5 MeV in 151Tin where the fl+/EC ratio decreases rapidly (0.15 for a level at ~ 7 MeV, an excitation energy that matches the 4.8 MeV centroid of the statistical protons). Thus, in agreement with fig. lb, there is a factor o f approximately 10 greater positron feeding to levels associated with the peak protons. Since delayed protons from the sl 1/2 state in ~51Yb p r e d e m i n a n t l y populate the O + ground state and delayed protons from the h~ ~/2 state populate excited states in ~5°Er, we conclude that the proton peaks are associated with the s~/2 ground state o f 15~Yb while the statistical protons originate from the hi 1/2 isomer. Fig. 2 summarizes our present understanding o f the decay o f ~SJYb and 15~Ybm. The positron coincident protons are placed at an excitation energy consistent with the a s s u m p t i o n that they feed the ground state in 15°Er. Also shown are the protons observed in coincidence with the 208 keV transition. The J51Tm proton separation energy and the 15~Yb QEC are taken from the predictions o f Liran and Zeldes [9]. We show in fig. 2 the same half-life for both the hi i/2 and s l/2 states in ~51Yb. W i t h i n uncertainties a half-life o f 1.6 was measured for the peak and statistical protons, for the T m K-X rays, and for a 108 keV y ray that probably is the expected [5 ] d3/2 ~ s~/2 low-lying transition in 15iTm. Included in fig. 2 is a r a n d o m phase a p p r o x i m a tion ( R P A ) calculation [ 10] o f the beta strength function for the 15~Yb sl/2 ground state. The strengths o f i n d i v i d u a l transitions were s u m m e d in 10 keV wide bins, folded with a gaussian distribution comparable to the resolution o f the proton telescope and multiplied by the statistical rate function for EC and r + decay. The d o t t e d peaks indicate transitions to 3/2 + levels in t ~ T m while arrows p o i n t to 1.2 + levels. The calculations are sensitive to the d e f o r m a t i o n p a r a m e t e r s used. Here a quadrupole d e f o r m a t i o n e2 = - 0 . 1 5 0 was utilized, somewhat larger than the static d e f o r m a t i o n 82 = - 0 . 0 9 8 [ 11 ], to locate the calculated peaks close to the excitation energies where the proton peaks were observed. (The E2 properties

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2 October 1986

Table 1 Levels in tS°Erpopulated by delayed protons from the s~/, and ht ,,2 ~S~Ybstates. Level

Feeding (%)

energy (MeV)

jr

0.000 1.579 1.786 2.261 2.295 " 2.621 2.633 2.734

0" 2+ 3 5 4+ 6+ 7 8+

51.3+4.3 13.7--+2.3 9.9+0.9 8.6--+2.2 11.5 _+2.3 5.0-+ 1.5

Calculated Feeding (%) $1/2

h i t/2

(51.3) bl 1.3 0.9

2.3 4.6 6.8 8.4 (11.5) 8.8 2.3 1.8

a~ Level observed for the first time in this study. L,~Intensities normalized to experiment (see text).

o f transitional nuclei are generally best described by a deformation p a r a m e t e r larger than the calculated static deformation. ) Note the lowest calculated 1/2 + and 3/2 + levels; they lend support for our assignment o f the 108 keV 7 ray m e n t i o n e d earlier as being the transition between d3/2---' sl/2 single-proton states in 151Tm. The integrated fl strength yields a calculated half-life for the ~5~yb ground state o f 1.0 s in fair agreement with our measured value; the half-life predicted by the gross theory [ 12] is 0.9 s. A similar RPA analysis could not be m a d e for the h~ ~/2 level because modifications are necessary in the calculations before fl strengths can be calculated for isomeric state decays. As d e m o n s t r a t e d above, the J 5 ' y b delayed-proton spectrum is a superposition o f two spectra, one structured and the other structureless; these are associated with the fl decays of the ~5~Yb s~/2 ground and h~,/2 isomeric states, respectively. The structureless portion samples high-spin, i.e., 9/2, 11/2, and 13/2, levels in the range of 5.0--9.0 MeV excitation energy in 15~Tm, while the structured spectrum probes lowspin, i.e., 1/2 and 3/2, levels from 3.8 to 5.0 MeV. Based on the qualitative agreement between the R P A calculations and the structured spectrum we believe that the proton peaks represent strong fl-decay feeding to discrete regions of levels in '5~Tm. However, one cannot exclude the possibility that the peaks arise from large fluctuations [ 13] in fl-decay transition probabilities whose distribution follows the Port e r - T h o m a s law [ 14]. We subjected the spectrum in

coincidence with positrons (fig. l b ) to an autocorrelation analysis similar to the one discussed by Jonson et al. [ 15 ]. F r o m the calculated autocorrelation function we deduced the variance, I p / ( I p ) , to be about 0.5. If single initial ( ~Slyb s~/2 ground state) and final (~5°Er 0 ÷ ground state) levels are assumed this value for the variance suggests [ 16 ] once again a low density for 1/2 and 3/2 levels in JS'Tm at 3.8-5.0 MeV. We add that the d e c o m p o s i t i o n o f the ~5~Yb spectrum into two c o m p o n e n t s is in accord with our earlier suggestion [ 1 ] that the highly-structured ~47Dy proton spectrum originates mainly from the isorope's s~/2 ground state. These protons decayed with a 95 s half-life rather than with the 59 s half-life o f the h, ~/2 isomer, the p r i m a r y species p r o d u c e d in the 142Nd( 12C, 7n) reaction. This high-spin isomer in ~47Dy has a 41% isomeric decay branch to the ground state. We therefore p r o p o s e d [ 1 ] that the longer halflife observed for the protons was the result o f a second-order decay process. The d r a m a t i c increase [ 1 ] in the n u m b e r o f statistical protons in the J49Er spectrum can now be u n d e r s t o o d in terms o f an enlarged fl-decay Q windown and a decreased isomeric decay branch ( e s t i m a t e d [7] to be 2.7%) rather than as a departure from the Z = 64 closure. The persistence of the intense peaks in ~S~Yb decay, even though ~S'Tm has 5 protons b e y o n d Z = 64, clearly shows that the low level density o f states at ~ 4.5 MeV is a consequence o f the N = 82 shell. The effect o f the Z = 64 closure, in contrast to earlier speculation, [ 1 ] 153

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is m i n i m a l . T h i s c o n c l u s i o n is b u t t r e s s e d by the fact that the ~45Dy a n d 147Er p r o t o n spectra, w h i c h sample levels in the N = 80 isotones, t45Tb ( Z = 65) and 147H0 ( Z = 67) are essentially structureless [ 2,5 ]. T h e a u t h o r s wish to express t h e i r g r a t i t u d e to P. M611er for s t i m u l a t i n g discussions and for m a k i n g available his R P A c o m p u t e r code to calculate fl-decay strength functions. V a l u a b l e discussions w e r e also held with G. A. Leander. O a k R i d g e N a t i o n a l Laboratory is o p e r a t e d by M a r t i n M a r i e t t a Energy Systems, Inc. for the U S D e p a r t m e n t o f Energy u n d e r C o n t r a c t No. D E - A C O 5 - 8 4 0 R 2 1 4 0 0 . W o r k at the L a w r e n c e Berkeley L a b o r a t o r y is s u p p o r t e d by the Director, Office o f Energy R e s e a r c h , D i v i s i o n o f N u c l e a r Physics o f the Office o f H i g h Energy a n d N u c l e a r Physics o f the U S D e p a r t m e n t o f Energy under Contract DE-AC03-76SF00098. Additional s u p p o r t was p r o v i d e d by the U S D e p a r t m e n t o f Energy u n d e r C o n t r a c t No. D E - A S 0 9 - 7 9 E R 1 0 4 3 4 w i t h the U n i v e r s i t y o f S o u t h C a r o l i n a .

References [ 1] K.S. Toth, et al., Phys. Rev. C 30 (1984) 712.

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[2] D. Schardt et al., Proc. Seventh Intern. Conf. on Atomic masses and fundamental constants(Darmstadt, West Germany, September 1984) p. 229 [31 P.A. Wilmarth, J.M. Nitschke, P.K. Lemmertz and R.B. Firestone, Z.Phys. A 321 (1985) 179. [41 J.M. Nitschke, Nucl. Instrum. Methods 206 (1986) 341. [5] K.S Tothet al., Proc. Seventh Intern. Conf. on Atomic masses and fundamental constants(Darmstadt, West Germany, September 1984) p. 237. [6] Y.H. Chung et al., Phys. Rev. C 29 (1984) 2153. [7] K.S. Toth et al., Phys. Rev. C 32 (1985) 342. [81J.A. Macdonald et al., Nucl. Phys. A 288 (1977) 1. [9] S. Liran and N. Zeldes, At. Data Nucl. Data Tables 17 (1976) 431. [ 10 ] J. Krumlinde and P. M611er, Nucl. Phys. A 417 (1984) 419. [ 11 ] P. M611er, private communication (1985); see also P. M611erand J.R. Nix, At. Data Nucl. Data Tables 26 (1981) 165; R. Bengtsson, P. M611er, J. R. Nix and J. Zhang, Phys. Scr. 29 (1984) 402 [ 12] K.Takahashi, M. Yamada and T. Kondoh, At. Data Nucl. DataTables 12 (1973) 101. [ 13] J.C. Hardy, B. Jonson and P.G. Hansen, Nucl. Phys. A 305 (1978) 15. [ 14] C.E. Porter and R.G. Thomas, Phys. Rev. 104 (1956) 483. [ 15 ] B. Jonson et al., Proc. Third Intern. Conf. on Nuclei far from stability(Carg~se, 1976) CERN Report CERN 76-13(CERN, Geneva, 1976) p. 277. [ 16] T. Elmroth et al., Nucl. Phys. A 304 (1978) 493.