Investigation of excited levels in 193Ir from the beta decay of 193Os

Abstract

The excited states in 193Ir populated by the beta- decay of 193Os were investigated via gamma-gamma coincidence analysis. Three levels at 517, 563 and 621 keV, previsouly seen only in nuclear reaction studies, were observed in the present measurements; the 487 keV transition from the 849 keV level was identified, providing good evidence that the levels at 848.93 and 849.083 keV are the same. The level at 807 keV was positively identified through both the 449 and 668 keV transitions; also, there is evidence for four new levels at 880, 882, 890 and 986 keV, and several newly-found gamma transitions were identified and placed in the decay scheme.

NUCLEAR STRUCTURE

Investigation of excited levels in ¹⁹³Ir from the beta decay of ¹⁹³Os

G.S. Zahn^I; C.B. Zamboni^I; F.A. Genezini^I; M.P. Raele^I; J.Y. Zevallos-Chávez^I; M.T.F da Cruz^II

^IInstituto de Pesquisas Energéticas e Nucleares, Caixa Postal 11049, 05422-970, São Paulo, SP, Brazil

^IIInstituto de Física da Universidade de São Paulo, Caixa Postal 66318, 05315-970, São Paulo, SP, Brazil

ABSTRACT

The excited states in ¹⁹³Ir populated by the b^- decay of ¹⁹³Os were investigated via g-g coincidence analysis. Three levels at 517, 563 and 621 keV, previsouly seen only in nuclear reaction studies, were observed in the present measurements; the 487 keV transition from the 849 keV level was identified, providing good evidence that the levels at 848.93 and 849.083 keV are the same. The level at 807 keV was positively identified through both the 449 and 668 keV transitions; also, there is evidence for four new levels at 880, 882, 890 and 986 keV, and several newly-found g transitions were identified and placed in the decay scheme.

I. INTRODUCTION

The nucleus ¹⁹³Ir occupies a very interesting place in the nuclide table; whereas the ¹⁹²Os nucleus, which has one proton less than ¹⁹³Ir, has been successfully described as a prolate-deformed nucleus, the ¹⁹⁴Pt nucleus, with just one proton more than ¹⁹³Ir, shows oblate deformation [2]. The usual approach is to describe the ¹⁹³Ir nucleus as a triaxial rotor [2-5], but a recent study [6] has shown that it can be equally well described as a prolate rotor.

Moreover, while the ¹⁹³Ir nucleus has been recently thorougly studied through many different reactions, the last full examination of the b^- decay from ¹⁹³Os was made in 1972 [7], using Ge(Li) detectors with much lower detection efficiency than the presently-available HPGe detectors.

II. EXPERIMENTAL PROCEDURE

The present experiment was performed using the planar multidetector array system assembled at the Laboratório do Acelerador Linear (LAL), in the Physics Institute of the São Paulo University [8]. For this experiment, four HPGe detectors, with volumes ranging from 50 to 120 cm³, were used in coincidence mode; the detectors were placed around the sample as shown in Figure 1.

The coincidence electronics, schematically shown in Figure 2, check for a coincidence between two or more detectors within 200ns and, for each valid event, stores both time and energy information for each detector involved.

The samples were produced by irradiating 5mg of 99% enriched ¹⁹²Os for 5 minutes in the IEA-R1 reactor, under a neutron flux of about 10¹²cm^-2 · s^-1. Two sources were produced every week, and this procedure was repeated for two months, resulting in something around 1200h of counting.

The data obtained was analyzed using the BIDIM software [9], which allows the subtraction of accidental coincidences, through the use of a time gate, and bidimensional fitting of the peaks with compensation for the Compton remains of other peaks. It should be noticed that for the present analysis the events from all 6 detector pairs (AB, AC... CD, in the naming scheme used in Figure 1) were relocated, in order to equalize their energy calibration curves, and summed up to increase the overall detection efficiency - and, thus, the counting statistics.

Most of the g-ray intensities were taken from a previous singles spectroscopy experiment [10, 11]; in the cases where the spectroscopy results didn't agree (within 2s) with the reference values from [1], when the transition hadn't been seen in the spectroscopy experiment, and also for every new transition, I(g) was calculated from g-g coincidence analysis by assuming that å_AB,AC...CDW(q) @ k, where k is constant for all transitions, regardless of their multipolarity and mixing ratio; this would be true if the detectors covered 4p - in the present case, the assumption was tested on several well-established transitions, and the values obtained were in excellent agreement with the expected ones. This way, if two gamma transitions g₁ and g₂ are in direct coincidence with a third one g₃ (i.e., there are no intermediate transitions between g₁ and g₃ or between g₂ and g₃); then:

where A(ij) is the area of the coincidence photopeak for the pair g_i × g_j and e(ij) is the sum of the detection efficiencies of all six detector pairs for the cascade.

The internal conversion coefficients were calculated for the transitions with known multipolarity using the HSICC computer code; the levels' energies were calculated from a least-square fit of the energies of the related transitions using the GTOL package, and the b^- feedings were obtained by the same package from the intensity imbalance; the log(ft) values were obtained using the LOGFT package and the absolute intensities were then obtained through the product of the absolute level feeding by the branching ratio of the transitions that depopulate that level; the documentation of all used packages can be found in reference [12].

III. RESULTS AND DISCUSSION

In the present experiment, 90 g transitions were successfully identified; 2 of these transitions were known, but had no placement in the level scheme; 12 had never been seen in b^- decay experiments before, but were known from other reactions; and other 15 were completely new to the level scheme of ¹⁹³Ir. These results led to the verification of 4 levels only known from other reactions, as well as 4 completely new levels; Figure 3 shows simplified decay schemes for these new levels. also, the suspicion that the two levels at 848.9 and 849.1 keV were the same, reported in the ENSDF Adopted Levels for ¹⁹³Ir [1], was positively confirmed. On the other hand, four known transitions were not seen in this experiment; the 80 keV transition that depopulates the 80 keV isomer (T_1/2» 10d) wasn't seen because of its very long half-life; the 41 keV transition that depopulates the 180 keV level was below the energy threshold for this experiment (» 55 keV); the 486 keV transition that depopulates the level at 559 keV would only be coincident to the 73 keV transition, which is strongly contaminated by the 73.6 keV Ir K_b1 X-Ray or to very weak transitions (I < 0.05%), so it couldn't be positively identified; finally, the 338 keV transition that depopulates the level at 1078 keV couldn't be identified either, but in fact this very weak transition has never been seen in a b^- decay experiment before, and is just assumed to be there because it was seen depopulating a level known to be populated by the b decay in the ¹⁹¹Ir(nn,g) experiment [1, 13].

The 27 radiations seen for the first time in this work, together with the 2 tentatively-positioned ones, are presented in Table I, together with their placement, as deduced from the present g-g coincidence analysis. The placement of the transitions at 62 and 65 keV are considered tentative because of strong X-Ray contamination that prevent the definite assessment of the true coincidences.

A. The Previously Unplaced Transitions

The unplaced transition at 181 keV was successfully placed depopulating the 740 keV level; on the other hand, the unplaced transition at 65 keV was tentatively placed depopulating the 139 keV level, but this is a tricky placement because of the interference of the Ir K_a1 line at 64.9 keV.

B. The 849 keV Level

In the ENSDF Adopted Levels for ¹⁹³Ir [1], there was the suspicion that the two levels at 848.93 keV and 849.083 keV could be the same, but as the latter was seen only in the ¹⁹¹Ir(nn,g) experiment [13], which only studied E_g < 700 keV, and the former was only known to be depopulated by transitions above that energy (710, 776 and 849 keV), if the two transitions found to depopulate the 848.93 keV level in the (nn,g) experiment were too weak in comparison to the other ones, they could have remained unseen in the other experiments and thus the levels couldn't be positively identified to be the same.

In this experiment, though, the 487 keV transition was identified together with the 710, 777 and 849 keV transitions, and their energies confirm that they come from the same level at 849 keV. On the other hand, though, the transition with 389 keV, expected to be stronger than the 487 keV one, was not seen; one possible reason for this is that it could have been masked by the very strong (I_g = 31.5%) 387 keV transition that depopulates the 461 keV level.

C. Levels Previously Seen Only in Other Reactions

The levels at 517, 563 and 621 keV, known only from other reactions [1], were positively identified in the present experiment.

The level at 807 keV, reported as uncertain in the last compilation [1], was successfully seen, not only through the 668 keV transition (also cited as uncertain the the compilation), but also through another transition at 449 keV.

D. New Levels

A new triplet was found with excitation energies of 879, 882 and 890 keV; Figure 4 shows an excerpt of the bidimensional spectrum zoomed in the three coincidences that defined these levels, where the 282, 284 and 292 keV transitions can be seen in coincidence with the 299 keV transition which is the strongest transition to depopulate the 598 keV level, thus suggesting the placement of the three transitions populating that level, leading to the three new levels; the level at 879 keV was also confirmed by the placement of a new 418 keV transition, which was found in coincidence with the 461 keV transition that depopulates the level at 461 keV, which is very well established in the b^- decay scheme of ¹⁹³Os.

In addition to that, a new transition with 362 keV was also found in coincidence with the 299 keV transition, as shown in Figure 5, which led to the proposition of a new level at 956 keV.

IV. CONCLUSIONS

The present experiment extended the knowledge about the ¹⁹³Ir nucleus by adding 8 new levels and 27 new transitions to the scheme of the b^- decay of ¹⁹³Os. Two transitions left unplaced in the last compilation were placed in the decay scheme, and two levels were shown to be the same.

Acknowledgments

The authors would like to thank the Laboratorio do Acelerador Linear (LAL/IFUSP) for the experimental facilities.

Received on 8 July, 2005

• [1] A. Artna-Cohen, Nuclear Data Sheets 83, 921 (1998).
• [2] E. Bezakova, A. E. Stuchbery, H. H. Bolotin, W. A. Seale, S. Kuyucak, and P. Van Isacker, Nucl Phys A 669, 241 (2000).
• [3] F. K. McGowan et. al., Phys Rev C 35, 968 (1987).
• [4] A. E. Stuchbery, Nuclear Phys A 700, 83 (2002).
• [5] A. E. Stuchbery, J. Phys. G 25, 611 (1999).
• [6] B. Roussičre et. al., Phys of the Atomic Nucleus, 64, 1048 (2001).
• [7] R. H. Price, and M. W. Johns, Nucl. Phys. A 187, 641 (1972).
• [8] D. Bőrg Filho, R. C. Neves, and V. R. Vanin, Proceedings of the XX Brazilian Workshop on Nuclear Physics. Ed. by S. R. Souza et al. (World Scientific Publishing Co., Singapore, 1998).
• [9] Z. O. Guimarăes Filho, Medidas precisas de energias de transiçőes gama em coincidęncia: espectroscopia de séries do ²³²U e ²³³U. MSc. Thesis (Instituto de FÍsica da Universidade de Săo Paulo, 1998).
• [10] G. S. Zahn, C. B. Zamboni, F. A. Genezini, M. T. F. da Cruz, J. Y. Zevallos-Chávez, and H. Dias, Spectroscopy Investigation of Excited Levels in ¹⁹³Ir. XXV Reuniăo de Trabalho em FÍsica Nuclear no Brasil (Săo Pedro, 2002).
• [11] G. S. Zahn, C. B. Zamboni, F. A. Genezini, M. P. Raele, M. T. F. da Cruz, and J. Y. Zevallos-Chávez, Braz. J. Phys. 34, 719 (2004).
• [12] B. J. Barton, and J. K. Tuli, Physics analysis programs for nuclear structure evaluation. Brookhaven National Laboratory Informal Report BNL-NCS-23375/R. Available at web address http://www.nndc.bnl.gov/nndc/physco/ (1977).
• [13] S. E. Drissi, Nucl. Phys. A 621, 655 (1997).

Publication Dates

• Publication in this collection
  07 Nov 2005
• Date of issue
  Sept 2005

History

• Received
  08 July 2005