J. C. Batchelder', K. S. Toth2, D. M. Moltz3, E. F. Zganjarl, T. J. Ognibene3, M. W. Rowe3, C. R. Binghan12.~,J. Powell3, and B. E.

Transcription

1 r Alpha Decay Studies of 189Bim, 190Po and 18oPb Using a Rapidly Rotating Recoil Catcher Wheel System J. C. Batchelder', K. S. Toth2, D. M. Moltz3, E. F. Zganjarl, T. J. Ognibene3, M. W. Rowe3, C. R. Binghan12.~,J. Powell3, and B. E. Zimmerman4 ' Oak Louisiana State University, Baton Rouge LA 70803, USA Ridge National Laboratory, Oak Ridge TN 37831, USA Lawrence Berkeley Laboratory and University of California, Berkeley CA 94720, USA University of Tennessee, Knoxville, TN 37996, USA The a decays of very neutrondeficient nuclei near the Z = 82 closed proton shell are of interest because they provide us with structure information that is relevant with regard to the shell model. n particular, the a decays of very protonrich eveneven Po and Pb nuclei have led to reduced widths (62) that help to delineate the extent of stability of the Z=82 magic shell [1,2]. Also, in the spherical shell model, the proton lh9/2 orbital lies above the 2582 closed shell while the 3~112orbital lies below. Any configurations of lh912 in Z<82 nuclei and 3~112in Z>82 nuclei are referred to as proton "intruder" states. Recent work has shown that the excitation energies of the lh912 intruder configurations in odda T1 and Au isotopes exhibit a parabolic dependence on its neutron number with a minimum midway between the major shell closures of N = 82 and 126 [3]. The parabolic energy dependence of the 1/2+ isomer in the odda Bi isotopes came into question, however, with the value 92( 10) kev reported by Coenen et al [4] for the 189Bim energy that indicated a continued drop of the x s 1 intruder ~ state energy for N < 108. We used a rapidly rotating recoil catcher wheel system, described in detail in Ref. [ 5 ], to study the a decays of ggbim, 19oP0, and 18oPb. Briefly, the system works as follows. Recoils from the back of the target, after passing through an A1 degrader placed behind the target, are stopped in 300pg/cm2 A catcher foils fixed at the edge of the wheel. These are inclined at an angle of 20 degrees with respect to the beam to maximize the catcher efficiency while keeping the thickness that a particles must travel in order to emerge out of the A1 foil to a minimum. This arrangement results in an effective thickness of 900 pg/cm2 for recoils, but only 150 pg/cm2 for the emitted a particles. Stopped recoils are then rotated between an array of 6 Si detectors in series; the six detectors have an overall solid angle of 8% of 4 7 ~Halflife information can be obtained by determining the difference in counts between the detectors. The beam is

2 DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect thosc of the United States Government or any agency thereof.

3 DSCLAMER Portions of this document may be illegible in electronic image products. mages are produced from the best available original document..

4 pulsed to avoid irradiating the spokes of the wheel and to allow the Si detectors to count while the beam is off. Wheql speeds between 30 to 2500 rpm can be used to maximize the efficiency for halflives of interest and to minimize it for longerlived species. n the present experiments, the wheel was rotated at speeds ranging from 240 (optimizedfor halflives of 8 ms) to 1500 (halflives of 2 ms) rpm. t should be noted that in our investigation, for a given mass number, product nuclei with the largest Qavalues are those i f resulting from the (xn) exit channels. Figure 1 shows an a spectrum observed in the first detector following the bombardment of a 144Smtarget with a 225MeV beam of 48Ti. The wheel speed was 240 rpm. One sees in Fig. 1 the 7.30(4) MeV a peak arising from the transition from the ( n s l ~isomer ) of 189Bi to the (nsil2) 185Tl ground state, and the 6.67MeV a peak arising from the (~hg/2)ground state of 189Bi to the (~hg12)isomer of 185Tl, as well as a particles from 186Pb (formed via the a2n exit channel) and lessdefined lower energy events that are attributed to heavier lead isotopes. Our energy of the ls9bim(xs1/2) + l85tl ( B s 1 ~ transition ) is 90 kev larger than the Ea previously reported [6], and establishes the excitation energy of 89Bim as 190(40) kev rather than the 92( 10)keV value adopted by Coenen et al. [4]. With our data, a halflife of 7.0(5) ms for 189Bim was determined, compared to 5 ms [6] and 4(2) ms [71. n a related investigation,the isotope lgop0 was produced in the 144Sm(48Ti,2n) reaction at a bombarding energy of 215 MeV. Figure 2 shows the total a spectrum observed in the first detector at wheel speeds of 1000 and 1500 rpm, (data from the two wheel speeds were added for better statistics). The inset is displayed as 40 kev per channel, while the main figure is displayed as 20 kev per channel. n this spectrum, one sees counts grouped above the 7.30(4) MeV 189Bim peak (see above). The energy of this group was determined to be 7.49 (4) MeV, in agreement with the 7482 (20) kev value previously assigned in Ref. [S to l9opo. However, our 190Po halflife of (2.0 ms is ):;YT much less than the 9.6ms halflife given i r Ref 18;. Experimentallydetermined reduced a widths (62)for the light eveneven Po isotopes have been of considerable interest as they seem to reach a saturation at 196Po (55k4 kev) through 192Po (53k6 kev). This saturation effect is believed [2] to be due to substantial mixing between the ground states x(2p) and lowlying O+ n(4p2h) intruder states in the Po parents. Alpha decay from a purely x(2p) Po ground state will be unhindered to the ground state of..

5 Pb, while a transition from a a(4p2h) state would be strongly hindered. Thus a Po ground state that is a mixture of a(2p) and a(4p2h) configurations will have its tola1 CY. decay strength hindered resulting in a lower 6* value. Our 62 value of 88 '81 kev for 190Po does not reinforce this ( 34) saturation effect, but its uncertainties are large enough to preclude us from reaching a definitive conclusion with regard to this effect. Finally, a search for the a decay of 18oPb was recently completed via the 144Sm(40Ca,4n) reaction. Figure 3 shows a spectrum recorded at a wheel speed of 500 rpm in the first three detectors (combined). A peak is seen at 7.23 (4) MeV, with a halflife of (4 ms. Spectra taken f24) with a wheel speed of 180 rpm revealed little of this peak, i.e., only scattered counts just above the 17% and 18%g 01 groups in the first detector; these data are consistent with a halflife of 5 ms for the 7.23MeV peak. We assign this new 01 emitter to 180Pb because its E a agrees with a value that one would anticipate from energy systematics, namely, kev greater than the 7.05MeV E a of 181Pb [9,101. n conclusion, the alpha decays of 189Bim, 19oP0, and 18OPb were studied by using a rapidly rotating recoil catcher wheel. This instrument has now been proven to be an effective tool for the study of nuclei far from stability with halflives in the range of 150 ms. References K.S. Toth, et al., Phys Rev. Lett (1984). +. J. Wauters, et al,f'hys. Rev C 47, 1447 (1993). K. Heyde, et al., Phys. Rep. 102,291 (1983). E. Coenen, etal., Phys. Rev. Lett. 54, 1783 (1985). J. E. Reiff, et al., Nucl. nstr. Meth. A276, 228 (1989). J. Schneider, Thesis GS Report A. N. Andreyev, et al., Nuc. nstr. Meth. A330,125 (1993). A. B. Quint, et ai.,gs Report 881, p. 16. K. S. Toth, et ai., Phys. Rev. C 39, 1150 (1989). K. S.Toth, et al., Phys. Rev. C 53, 2513 (1996). This work was supported in part by the U.S.D.O.E. under contracts # DEFGO584ER40159(Louisiana State University), DEACO596OR22464(OakRidge National Laboratory), DEACO376SFO0098 (Lawrence Berkeley National Laboratory), and DEFG0587ER40361 (University of Tennessee).

6 186 Pb Energy (MeV) 5.0 3)... PH% v) c. C 0" Energy (MeV) Figure captions 1). Alpha spectrum accumulated in the 1aSm(48Ti,p2n) reaction with a beam energy of 225 MeV showing 1g9Birn. 2). Alpha spectrum accumulated in the 1'%m(48Ti,2n) reaction with a beam energy of 215 showing 19*0. 3). Alpha spectra accumulated in the l%m(40ca,4n) rreaction with a beam energy of 230 MeV showing 18oPb..._ ~.