Levels in 102Pd populated by the 102Pd (P,P′: γ,γ) reaction

J. inorg, nucl. Chem. Vol. 40, pp. 1853-1857 © Pergamon Press Ltd.. 1978. Printed in Great Britain

0022-1902/7811101-1853]$02.0010

LEVELS IN 1°2pd POPULATED BY THE t°zpd (P,P': y,y) REACTION A. T. KANDIL't lnstitut fiir Experimental Physik (III), Ruhr Universit~t Bochum, 463 Bochum, West Germany

(First received 29 April 1977: in revisedform 6 October 1977: receivedfor publication 26 January 1978) A~tract--Low spin levels in ~°~Pd have been studied using the ~°2pd(P,P'; y,y). In-beam measurements of y-y coincidences are used in the construction of a level scheme of the ~°2pdnucleus levels upto an excitation energy of 3 MeV have been observed and their character is compared with the predictions of the nuclear vibrational model.

INTRODUCTION Scharff-Gotdhaber and Weneser[l] suggested more than twenty years ago that the low-lying states of even-mass medium-weight nuclei might result from harmonic vibrations of the nucleus around an average spherical shape. The large E2 transition probabilities of the y-rays from the first 2+ states and the two phonon vibration states (0÷, 2+ and 4 ÷) at about twice the energy of the first 2 + state are correctly predicted by this model. A significant test for this model remains still in the observation of the two phonon vibration states, their energies and their decay characteristics. Levels in t°Epd have been studied by several authors[2-5]. Hnatowich et a/.[6] [HMK], used a mass separated ~°2Ag isotope in their work. Gamma singles and (Y-Y) coincidences were used to construct a level scheme for '°2Pd. The low lying states observed by HMK consisted of the first 2+ state, the 4 + and 2+ members of the two phonon states. The 0 ÷ member of the two phonon vibration was not observed in the HMK work. Simms et al.[7] have made use of the reaction 99Ru (a,n; %y) to populate levels in t°2pd. Because of the large angular momentum brought in by the a-particles, only high spin states were expected to be populated in their work. A strong 1 0 - ~ 8 - > 6 ~ 4 - > 2 ~ 0 transition chain was observed and neither a strictly rotational nor a vibrational model could explain the states observed in the Simms et al. work. In this work, levels in ~°2pd have been investigated by using the reaction ~°2pd (P,P'; y,y). The purpose of this work is to construct a level scheme from this work and to compare the levels populated with those of the n°2Ag decay of HMK, and to compare the neclear structure of this nucleus with the other Pd isotopes whose structure is well known from Coulomb excitation and other investigations[8-10]. This comparision could determine if the 2d 5/2 shell is closed at J°zPd. EXPERIMENTAL

A 74.54% enriched 1°2Pd self-supported foil of approx. 1 mg/cm2 thickness was bombarded by 7.5 MeV protons at the Tandem accelerator of the Ruhr University of Bochum. The isotopic purity of the target material as provided by the Oak Ridge National Laboratory is J°2Pd (75.45%), ~°4Pd (12.13%), 1°sPd (6.46%), ~°rpd (3.81%), I°sPd (1.62%). Coincident gamma +Correspondence to: Department of Nuclear Chemistry, Atomic Energy Establishment, Cairo, Egypt.

rays were detected by two Ge(Li) detectors with volumes of 40 and 60 cm 3. The large detector has a resolution of 2.2 KeV, was located 10 cm from the target at 90°. The smaller detector with 2.5 KeV resolution at 1.28MeV was also placed equidistant at 53°. The chamber was not fitted with an exit beam line. The proton beam was collimated on the center of the target by a 1.5 mm dia. hole fixed at the entrance of a 14cm dia. thick brass chamber. The beam was stopped behind the target using a thick tantalum foil. The coincidence requirement of two gamma rays was defined by a fast slow coincidence circuit using two zero-strobe-generators. The three signals containing the two gamma energies, and the time difference of two coincident signals (the output of the pulse-height-converter) were fed into an Intertechnique tripple ADC system. The digitized energy and time information of the coincident gamma signals were transferred to a PDP-10 computer and stored on a magnetic tape in list mode. Total coincidence spectra from the prompt coincidence events of the two coincidence branches was constructed by the computer during the measurement. Such a spectra was used to identify such photopeaks which has coincident partners. After the measurement, energy windows (width 3--4KeV) were chosen, which enclose each photopeak, and time windows (width -20 ns) which enclose the prompt peak. Figure I shows a total coincidence spectra of the large detector obtained by opening the gate on the gammas of the smaller detector and setting a window on the half maximum of the time peak. RESULTS AND DISCUSSION The evaluation of the coincidence experiments involved the setting of narrow gates on the coincidence spectra of the smaller detector, with gates just off the peak to the high energy side and sometimes to the low energy side, in order to distinguish and subtract the coincidences with the compton events. The prompt coincident peaks in the other spectrum recorded by the large detector were then found and recorded. Figures 2(a-c) show examples of such treatment, where the coincidences with the 556.4KeV, 719.2KeV and 835.4 KeV y lines are drawn. The impurity contribution of t°4pd is evident in the 767.6 KeV y-ray representing the 4 + ~ 2 + transition in '°4Pd[8]. Impurity lines such as the 767.6 KeV gamma were eliminated based on intensity considerations, per cent impurity in the target, and by referring to the known decay schemes of the rest of the Pd isotopes. In some cases the small intensity of some of the photo-peaks in the coincidence spectra has hindered any definite conclusion about their coincidence relationship. t Table 1 presents the coincidence relationship of the

1853

1854

A.T. KANDIL

104

o

103

i ~o

102

200

400

600

800

1200

1000

1400

1600

1800

2000

Fig. 1, A total coincidencespectra of the 60 cm3 Ge(Li) detector at 90°. experiment. Figure 1 and Table 1 show the 556KeV .y-ray to be by far the most intense and has a very strong coincidence probability with all y-transitions except with the 1534 KeV y-ray. This result clearly identifies it as the ground state transition, in agreement, with HMK[6], Simms et al.[7], and the ~°3Rh (P, 2n) data of Sakai et a/.[5]. The 719 KeV transition follows the 557 KeV transition in intensity and is also in coincidence with most other transitions with the exception of the 1534, 1387, 1581, 1691 and 1835 KeV y lines. The 719KeV y has been identified to correspond to the transition of the 4 + member of the two vibration states to the first low lying 556 2+ state of the one phonon vibration[7]. The

978 KeV transition is in coincidence among other lines with the 556 transition and the 1534 KeV represents the 556 and 978 energy sum. The 1534 state stands for the 2+ member of the two phonon vibration states[7]. The 1387 KeV is only in coincidence with the 556 KeV giving a level at 1944. This level has not been observed in previous published experiments but has been observed in the '°2Pd (P, PLy) experiment[l l] where a y-transition to the ground state was also observed. This, and the fact that its energy is much higher than any of the two phonon vibration states clearly eliminates it to be the missing 0+ state which would complete the known triplet of the two phonon vibration states. The 0+ of the two

3 0 0 --

2 0 0 --

I00--

,:_g

500

I000

1500

Fig. 2(a).

2000

Levels in t°2Pd populated by the ~°2Pd(P,P", y,y) reaction

1855

:30O

20o

it30

IOOO Fig. 2(b).

15OO

IOOO

15o0

2000

6o

40

I

0 5oo

2000

Fig. 2(c). Fig. 2. (a) Coincidences with the 556.4KeV y. (b) Coincidences with the 719.2KeV y. (c) Coincidences with the 835.4 KeV y. phonon vibration has been also observed in the ]°2Pd ( P ; P ' , y ) experiment to lye at 1658KeV[II]. A very weak 1102 KeV y-line is seen in this work and is placed in Table 1 as a probable coincident member with the 556KeV y-line. The fact that the 0 + state was not populated in the ~°2Agdecay experiment of HMK[6] and in this work is an interesting observation. If the triplet of the two phonon vibration states has a similar vibrational character as the phonon model predicts, one would then expect a similar log ft values for these states and consequently the 0 ÷ state should be populated in this work and the ~°2Ag decay experiment. The fact that the

0 ÷ was only populated in the ( P ; P ' , y ) experiment clearly indicate that the 0 + state has different vibrational character than the other 4 + , 2+ members of the two phonon vibration states. This observation would seem to agree with Sakai[12], who has made a survey of the properties of even-even nuclei and observed that in the decay of most 1+ nuclei the//-branch to the 0 + and 2÷ members of the two phonon vibration to be largely different. In contradiction with our observation and that of Sakai, the log ft values of the two phonon vibration states of l°4pd are almost identical[8]. The possibility .that the 2d 5t2 shell might be closed at t°2Pd could

1856

A.T. KANDIL

Table I. Coincidence relationships in the J~Pd (P,P':y,y) experiment

3,0~

T

i

i E o•

y-ray (KeV)

Coincidence y-rays (KeV) "Definite. . . .

Probable" E2 •

556 604 719 835 864 892 1025 1257 1331 1800

719,835,864,893,178,1025, 1057, 1330, 1387, 1581, 1691, 1744, 1835, 2017, 2055 978 556,835,864, 892, 965, 1025, 1057, 1257, 1330. 1800 556,719.864, 892 556,719, 835 556,719,835 556,719 556,719 556,719 556,719

1102 ~ l ~~ 2,5 ~°1~~

-o-

102

I

I

I

104

106

108

Pd

explain the different vibrational character of the 0+ state and the very large difference in the energies of these two phonon states. The phonon model predicts 3 degenerate two phonon states at twice the energy of the first low lying 2' one phone level. Figure 4 shows a plot of the leo+ (2 phonon)/E2 + (I phonon)], [E2 + (2 phonon)/E2 + (I phonon)] as a function of the mass number of the

S

110

isotope

Fig. 4. The [Eo+(2phonon)lE2+(I phonon) and [E2+(2 phonon)/E2+(l phonon)] as a function of the mass number of the Even-Even Pd isotopes.

Even-Even Pd isotopes. These ratios are seen to be maximum at 1°2Pd. The decay scheme of 1°2pd constructed from this measurement is presented in Fig. 3. The energies of the levels observed in this work are within the experimental error of thos published by HMK. The levels at 1944.1 KeV and 2606.2 KeV are newly seen in this work. CONCLUSION The nuclear structure of 1°2pd is quite similar to the rest of the doubly even Pd isotopes and is characterized by: (l) A low lying one phonon 2+ state at 556 KeV; (2) A two phonon triplet at approximately twice the energy of the 2+ state. The fact that the 2d 5/2 shell might be closed at 1°2Pd affects the energy split of these states and has an obvious effect on the vibrational character of the 0 ÷ member of the triplet. (3) The state at 1944 KeV could not be a simple three phonon state, because if it were so, it would mainly populate the two phonon and not the one phonon state. (4) The crossover transition of the 1534 state is strictly forbidden according to the model, but experimentally it is always seen in vibrational nuclei. (5) These results clearly demonstrate that the harmonic model serves only as a first approximation to the description of spherical nuclei and that most nuclei vibrates quite an-harmonically and that the phonon number is no longer a good quantum number.

Acknowledgements--The author would like to acknowledge the help of Drs. J. Lange and K. Farsine in data accumulation and Prof. Dr. Harp yon Buttlov for his kind help in the discussion of the experimental data. The financial support of the Alexander yon Humboldt Stiftung in Germany is greatly acknowledged.

0 o

lO~pd Fig. 3. Level scheme of the ~°:Pdnucleus.

REFERENCES 1. G. Scharff-Goldhaber and J. Weneser, Phys. Rev. 98, 212 (1955). 2. P. Charoenkwan and J. R. Richardson, Nucl. Phys. A 94, 417 (1967). 3. H. Bakhru, R. !. Morse and I. L. Preiss, Nucl. Phys. A !00, 145 0967). 4. B. Greenebaun, A. D. Jackson, Jr. and R. A. Naumann, NucL Phys. A 196, 193 (1967).

Levels in ~°2Pdpopulated by the ~°2Pd (P,P': "r,~') reaction 5. M. Sakak T. Yamazaki and H. Ejiri, Nucl. Phys, 74, 81 (1%5). 6. D. J. HnatowicK F. Munnich and A. Kjelberg. Nucl. Phys. A 178, III (1971). 7. P. C. Simms, R. Anderson. F. A. Rickey, G. Smith, R. M. Steffen and J. R. Tesmer, Phys. Rev. C7, 1631 (1973). 8. N. C. Singal, N. R. Johnson, E. Eichler and J. H. Hamilton, Phys. Rer. C5. 948 (1972).

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9. K. Okano~ Y. Kawase and S. Uehara. Nucl. Phys. A 182. t31 (1972). 10. R. L. Robinson, F. K. McGowan, P, H. Stelson, W. T. Milner and R. O. Sayer, NuM. Phys. A 124, 553 (1969). II. A. T. Kankil, J. Lange, F. Farzine and H. V. Buttlar, Verb. Deut. Phys. Ges. (VI) 11,877 (1976), Nucl. Phys. A292, 301 (1977). 12. M. Sakai, Nucl. Phys. 33, 96 (1962),