Study of the side band in 188Pt

Nuclear Physics A289 (1977) 365-372 ; © North-Eollaed PuNishlnp Co ., Amsterdam Not to be reproduced by photoprint or micoillm without written permission from the publisher

STUDY OF THE SIDE BAND IN tssPt T .NUMAO Department of Physics, University of Tokyo, Tokyo, Japan and N . YOSHIKAWA, Y . SHIDA and M . SAKAI Institute for Nuclear Study, University

of Tokyo,

Tanashi-shl, Tokyo, Japan

Received 20 May 1977 Abstract : Low-lying levels in . . .Pt were studied using the "'Ir(p, 4ny)'s'Pt reaction and higher spin members of the quasi-y band were proposed. The results resemble a slightly perturbed phonon scheme, from comparisons with macroscopic calculations which would remove the degeneracy of the phonon multiplets. The importance of mass parameter renormaliration is pointed out .

E

NUCLEAR REACTION "'Ir(p, 4nT), E = 38 MeV ; measured E;, I;, Y)'-coin, 7(0) . "'I't deduced levels, J, x .

1. Introd~ The Pt nuclei have been considered to be soft against y-deformation, because the second excited 2 + state, which is described as the band head of the y-vibration in a rotational model, appears at low excitation energy. According to a theoretical prediction by Kumar and Baranger t~ Pt nuclei change their shape from oblate to prolate at mass number A = 188. The energy level systematics of the 4i and 22 states in the even Pt isotopes also suggest this phase transition. The study of ""Pt nuclei should help us understand the transitional nuclei and the phase transition mechanism. Decay properties of ""Au have been studied by several authors Z - s). Finger et al. measured y-rays and conversion electrons from the decay of Au isotopes which were in turn fed by the decay of on-line mass-separated Hg. A number of low-lying states were found in I "Pt as well as in other even Pt isotopes. Since the in-beam spectroscopy method was established by Mormaga and Gugelat e) in 1963, the technique has been applied to the study of the reaPt nucleus. Sakai et al. ') measured internal conversion electrons following the tsó0s(a, 2n)t e"Pt reaction. Lark and Morinaga s) . observed y-rays in (a, xny) reactions and deduced the members of the ground-state band up to the spin of 8+. Recent in-beam experiments 9,'o) have been 365

366

T. NUMAO et a/.

focussed on the backbending behavior and semi-decoupled band (the negative-parity sequence of 5 - , 7 - , . . .). The aim ofthe present work is to establish the low-lying states, and especially the higher spin states ofthe quasi-y band. The present experimental results are compared with available macroscopic calculations. 2. Experimentd pcooedures (t93Ir) Natural and enriched iridium targets were bombarded with protons from the INS FM cyclotron. The proton energy was reduced to be suitable for the IIt1r(p, 4n)ts"Pt reaction by means of several carbon plates . The targets were made ofmetallic powder deposited on thin mylar films and were 100 mg/cm 2 thick. A strip of 5 mm width was used as the target in the angular distribution measurements . Two 40 cm3 Ge(Li) detectors were used in the experiments, both having energy resolutions of 2.4 keV FWHM for 1332 keV y-rays. Singles y-ray spectra were taken at proton energies of 34, 38, 40, 44 and 48 MeV with the natural iridium target, and the excitation functions of the y-rays were deduced. These excitation functions and the y-ray spectrum at EP = 38 MeV with the enriched target were used to identify the y-rays from the t9 'Ir(p, 4ny) ta ePt reaction. The peak of the (p, 4n) cross section was determined to be at 38 MeV from the excitation functions. Most ofthe measurements were performed at this energy . Fig. 1 shows an example of the singles spectra

0

1000 CHANNEL Nt1dt3ER

MeV. The 7-rays from the Fig . 1 . Singles y-ray spectrum obtained with a natural at EP - 38"c". 19a lr(p. 4ny)'9°pt reaction are indicated by

Gamma-gammacoincidence measurements were performed using a small chamber. Both detectors were placed 3 cm from the target, and were perpendicular both to the incident beam and 'to each other. Channel numbers corresponding to the y-ray energies and to the time difference between them were stored in a buffer memory and written onto magnetic tape . The tapes were sorted later with a TOSBAC-3400 computer . The overall time resolution was 12 ns FWHM . The chance coincidences

Ieep t

367

were estimated from the counts of the prompt and the next beam burst in the time difference spectrum . The observed ratio ofchance to true events was negligibly small. The coincidence relations were determined quantitatively by using the relative efficiencies ofthe gating and gated systems, which were obtained from the comparison of the singles and coincidence intensities of well-known cascades. The angular distribution of the y-rays was measured at angles of 90, 110, 125, 135, and 147.5 degrees to the incident beam . Two detectors were placed 30 cm from the target ; one ofthem was fixed and was used to provide normalization for several runs . The spectra from the two detectors were recorded simultaneously and with similar dead times to each other. The angular distributions were fitted to the expression, W(0) = C(1 +A 2 p2(cos 0)+A4P4(cos 0)). 3. Results and anlysis Most ofthe y-rays shown in fig. l are from t "Pt and 19°Pt. Discrimination between them was made using the singles spectrum obtained with the enriched target, the Q-value dependence of the excitation functions and the results of the coincidence measurement. The energies, intensities and A 2 values of the 7-rays in "'Pt are given TABLE 1

The -1-transitions assigned to "'Pt E., (keV)

il

202.5 216.0 265.6 329.9 340.1 405.0 410.3 413.8 451.4 478.8 480.0 507.3 512.9 544.4 551 .3 583.5 597.6 605.2 654.6 670.3 894.3 1083 .7

10.5±0 .3 5.0±1 .4 ') 100.0±0 .7 5.9±0 .4 17 .7±0 .4 73.3±0 .8 7 .8±2 .3 ') 3.3±2 .1 ') 3.8±1 .3') 11 .6±0 .7 ') 2.5±0 .6') 10 .2±0 .5 49 .0±2 .4 6.7±0.4 5.5 ± 1 .3 ') 17 .7±0.7 14.6±2 .1 ') 12 .7±0.7 6.2±0.3 9.5±0.3 20.9±0.8 2.2±0.7

') From the y-y coincidence experiment .

A2 0.533±0.041 0.162±0.015 0.053±0.066 -0.102±0.037 0.206±0.021 0.181±0.047 0.161±0 .046 0.109±0 .074 -0 .109±0 .048 0.150±0 .063 -0.097±0.078 -0.193±0.060

368

T . NUMAO et al.

in table 1. The overall accuracies in the energy determination were about 0.3 keV for strong transitions and 0.8 keV for weak ones. Errors in the intensities and A2 values include only statistical deviations. Complex spectra may introduce additional uncertainties. The y1Y coincidence relationships are shown in table 2. The parentheses TABLE 2 Results of the coincidence experiment Gates (keV)

Coincident 7-rays (keV)

203 216 266

226, 265, (340), 405, 410, (414), 479, (544), (605) 266 203, (216~ 330,340,(357),405,(410),414,(45l),479,507,513,544,551,584, 598, (655), 670,894,(1084) 266,(340),(507),(605) (203),266,330,479,(507~ (551) 203, 266, 414, (451), 513, 544, 584, 598, (655), 894 203, 266, 405, 513, (584), (894) 266,405 (266),(405),(513) 203, 266, 340, (405), 414, 479, (544), (551), (605) 266, (330),340, (670) 266, 405, (410), (451), (544), 584, 598 203, 266, 405, 513, (584) 266, (405), (414), (479) 266, 405, (410), 513, (544) 266,405,513 (266),(405) 266,(507) 203, 266, 405, (410), (544)

330 340 405 410 414 451 479 507 513 544 551 584 598 655 670 894

indicate a probable coincidence. For the strong transitions, the intensities obtained from the coincidence measurements agree with the singles measurements to within 10 %. The relative intensities of the y-rays at 479 keV are obtained from the coincidence results, because this peak is found to be a doublet. When cascade relations are established, the construction of a level scheme is an easy problem. In the following we will discuss the assignments of some of the levels and their spins and parities. All the transitions observed are assumed to be E1, M1, E2 or M1 + E2. The level at 1084 keV. This level is established from the coincidences of the 479 and 340 keV (2+-2 +) transitions, and of the 414 and 405 keV (4+-2 + ) transitions. Since this level is fed by the 1565 keV state (5 -) through the 480 keV transition and decays to the 671 keV state (4+) and the 605 keV state (2+), possible spin assignments are 3- or 4+. The positive Az value of the 479 keV transition only allows the assignment of4+ to this level; the main part of the peak at 479 keV is from the transition to the 605 keV state. According to Finger et al. s~ the 414 and 479 keV lines were found in the y-rays following the decay of teeAu. The intensity ratio between them

Isspt

369

is consisten6with our result. The absence ofthe 894 keV y-ray (5- -4 +) in their result suggests that the 479 keV line comes purely from the decay of the 1084 keV state. Their multipole assignments to the 414 and 479 keV transitions were M1+E2 and E2, respectively. Therefore, their results support our assignment. The level at 1443 keV. The coincidence of the 507 keV transition with 266 and 340 keV transitions, and probably with the 670 and 330 keV transitions, suggests the existance of a new level. The absence of a transition to the 2 + states and the energy level systematics of neighboring even Pt isotopes are in favor of the spin and parity assignment of 5 +. The levels at 1363, 1767, 2178 and 2312 keV. The existance of these levels t°) is confirmed in the present work. The AZ values of the transitions to the states of the ground band are consistent with the assignment of 5- for the 1565 keV level and 7for the 1767 keV level.

A= ó m

(50 6f 3F

0,0_

2435 2312 2178 1767 1565 1443 1183 936

188

0

Fig. 2 . Level scheme of "'Pt. Closed and open circles indicate certain and probable coincidence relationships, respectively .

A summary of the present results is shown in the level scheme of fig. 2, where certain and probable coincidence relationships are indicated by closed and open circles, respectively. The lower (upper) circle at the end of a vertical line means that the transition is assured by the coincidence with the lower (upper) cascades . 4. Dieiamnion

The prominent features of the level scheme are that the 22 and 41 states are close to each other, and similarly also the 3i, 42 and 6; states. The decay properties of

370

T. NUMAO et al.

the side band are also very similar to those of the phonon model. The 605 keV (22-0+) and 670 keV (31-2i) transitions are hindered compared to the 340 keV (22 -2i) and 330 keV (3,-22 ) transitions, respectively. The B(E2) ratio of the 42-4i transition to that of the 42 -22' transition is 0.6, while that to the 42-2i transition is less than 0.01. Therefore, the groups of 22, 4, and 31, 61, 42 states can be considered as two- and three-phonon states, respectively . The first excited 0 + state at 798 keV, which is not observed in the present work, also has the character of a threephonon state °). In the y-unstable limit, the 0+ state of the two-phonon members goes to infinity"). It must be a good approach to adopt a y-unstable potential as a first approximation and to look for the most important additional terms in the Hamiltonian that will resolve the degenerate levels in the right order. In order to find such a term, two sorts of macroscopic calculations were carried out. One of them introduces stability against the y-deformation, while the other brings in the renormalization of the mass parameters for the collective motion . A calculation of the former type was carried out using the program HSPEC 12), which calculates numerically the Hamiltonian 2 2 H= ~, nra-w+V~+V1f -2)+V2e-e=leo~ßJ3p 1(cos3y) 2B N=-2 l o o +V3e- a= le°~ ß  16p2(cos 3y~ oo

(1)

where n,, is the momentum of the collective motion, and ß and y are the commonly used deformation parameters. The second term is the potential introduced by Myers and Swiatecki 13), which almost does not depend on the parameter y. The last two terms in Hamiltonian are important for resolving the degenerate states of y-unstable nuclei . When V2 is not zero, the 2s , .3i and 42 states become higher and the energy spectra approach those of axially symmetric nuclei. Hence, the effect of the term may be small in the case of '"Pt. However, light even Pt nuclei have such energy spectra and the magnitude of the term seems to become larger as the mass number decreases. The last term with positive V3 separates the 22 and the 41 states, and also separates the three-phonon states into the order 31, 61 and 42 from the lower energy state. Although it was possible to separately fit the ground band or the quasi-Y band well, we could not find any combination of V2 and V3 that would reproduce the two bands as a whole. It seems reasonable to think that the failure comes from the difference between the mass parameters for rotational and vibrational motion. As a calculation of the latter type, we applied the model ofref. ") . The Hamiltonian of the model is H = btb+btbtbb,

(2)

where bt(b) is a boson creation (annihilation) operator. Energy levels are obtained

1ssp t

371

with phonon number n, boson seniority v, and angular momentum L from the equation E(n, v, L) = en + fn(n-1)+g(n-vxn+v+3)+h(L(L+ 1)-6n).

In the present calculation, the parameters e, f, g, and h were obtained by least-square fits . Fig. 3 shows the result of the calculation. The selection rule of this model for 8+ 5+ 3+ 0+ + 2+ 0+

Exp .

Theory

Fig. 3. Comparison of positive-parity states with the model of ref. ").

transitions is the same as for the phonon model because the Hamiltonian does not contain the term which mixes different phonon states. Fairly good agreement between theory and experiment was obtained for the energy spectra as well as for transition strengths. To understand the effect of the last term of eq. (2), the term is expanded in a power series of the momentum 7c and collective coordinate a using a canonical transformation . Then the term becomes a linear combination of a4, n'a' and X 4. The a4 term may be compared to the y-unstable potential. The other terms have the effect of renormalizing the mass parameters and resolve the degenerate states. This is consistent with the conclusion deduced above. Therefore, we can conclude that ""Pt is unstable for the y-deformation and that the degenerate states are split by the potential which depends on the momentum of the collective motion but not so much on the y-stable potential. We wish to thank Drs. H. Kusakari and H. Kawakami for their collaboration and the INS cyclotron crew for their machine operation. We are grateful to Prof. N. Ohnishi for the use of the computer code. One of the authors (T.N.) wishes to thank Prof. R. Chiba for his encouragement and many suggestions. References 1) 2) 3) 4)

K. Kumar and M . Baranger, Nucl. Phys . A110 (1968) 529 N. Poffe, G. Albouy, R. Bernas, M . Gusakow, M . Rion and J . Teillac, J . Phys. Radium 21 (1960) 343 R . A . Naumann, R . F. Petry and J . S . Evans, Nucl . Phys. A137 (1969) 689 A . Johansson, B . Svahn, B. Nyman and S . Antman, Z. Phys . 230 (1970) 291

372

T. NUMAO er al.

5) M. Finger, R. Foucher, J. P. Husson, J. Jastrzebski, A. Johnson, G. Astner, B. R. Erdal, A. Kjelberg, P. Patzelt, À. Hoglund, S. G. Malmskog and R. Henck, Nucl. Phys . A188 (1972) 369 6) H. Morinaga and P. C. Gugelat, Nucl . Phys. 46 (1963) 210 7) M. Sakai, T. Ysmazaki and H. Ejiri, Phys. Lett. 12 (1964) 29 8) N. L. Lark and H. Morinaga, Nucl. Phys . 63 (1965) 466 9) J. Burde, R. M. Diamond and F. S. Stephens, Nucl . Phys . A92 (1969) 306 10) M. Piiparinen, J. C. Cunnane, P. J. Daly, C. L. Dors, F. M. Bernthal and T. L. Khoo, Phys. Rev. Lett . 34(1975) 1110 11) L. Wilets and M. Jean, Phys . Rev. 102 (1956) 788 12) N. Ohnishi, private communication 13) W. D. Myers and W. J. Swiatecki, Nucl. Phys. 81 (1966) 1 14) A. Arima and F. lachello, Phys . Rev. Lett. 35 (1975) 1069