The 5.3 sec isomer of W183

Nuclear Physics 24 (1961) 422-430 ; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

THE 5.3 SEC ISOMER OF

W1sa

C. J. GALLAGHER, Jr ., and H. L. NIELSEN Institute for Theoretical Physics, University of Copenhagen, Denmark Received 17 January 1961 Abstract : A 5.3 sec isomer of W168 has been chemicaä :y separated from its parent 5.2 d Ta18'. The radiations from the isomer have been observed witi, a xenon proportional counter and a NaI scintillation crystal and have energies of 46, 52, ~:w 105 and ;s 160 keV. Gamma-gamma coincidence measurements involving the photons have been made . On the basis of these results and previously reported high resolution measurements of the decays of Ta 188 and Re18a, the energy of the isomeric level is assigned as 309.49 kel ', and its spin and parity as #-}- .

l . Introduction

The energy levels of beta stable W183 have been investigated through studies of the beta decay of Ta183 (ref. 1)), the electron capture decay of Re183 (ref. 3 )), fast and slow neutron reactions on wolfram 3-5) and Coulomb excitation of the W183 1evels directly d-8) . The very careful work of Murray et al. 1), using both high resolution gamma-ray and beta-ray spectroscopy, established the decay scheme of Ta183 shown in fig. 1. In their analysis of the experimental data, Murray et al. Tcoez

00-1068

45308

'h*)

nmm~n»r(Y2~) Qrg>453

L EC

--~772

cc

4l2.08-~--~~2

308.94

9~ 291.71

207.00-3j 20&e1

EC

IS

EC

9907 46.48 0

7AA!lIIIIIII!~/2 K .1/2 ,W183

Fig. 1. Level scheme of W188 and levels populated by the decays of Re188 and Ta18 3. The level scheme is as originally proposed by Murray et al . The analysis of the level scheme into intrinsic and rotational states was first proposed by Kerman . 422

THE b.8 SEC ISOMER OF W10

423

were able to fit all but two of the observed transitions into the decay scheme shown. One of these transitions unassigned in the decay scheme, at 102.49 keV, they assigned as an M2 transition on the basis of the K and L1 conversion coefficients (35 and 6, respectively) and the observed conversion in only the LI shell. Kerman 9) analyzed the level spectrum and showed that it could, be described in terms of intrinsic and rotational states as shown in fig. 1, assuming, however, that some of the states were highly K-admixed by the Coriolis interaction. Thulin et al., investigating the decay of Rels3 by the use of photographicrecording permanent-magnet spectrographs, found that all of the transitions they observed fit into the level scheme proposed by Murray et al. with no evidence for the conversion lines of the 102.49 keV transition 2) . Gallagher et al., using longer exposures than Thulin et al. on the same equipment, were also unable to find any evidence for the 102.49 keV transition 1°) . Der Mateosian and Goldhaber produced a 5.5 s isomer in wolfram by slow neutron reactions on wolfram, and reported that there were ;:w 80 keV electrons in the sample 3) . From the halflife and the electron energy they calculated that the isomeric transition was probably E3 or M3. Campbell and Goodrich reported gamma rays of 0.12 and 0.17 MeV for this isomer 4). Po6 has produced this isomer by fast neutrons on wolfram and has reported that the gamma-ray spectrum of the isomer included K X-rays (60 keV) and two gamma rays at 105 and 155 keV with an approximate ratio of intensities of 10 : 2.5 : 1, respectively . In (Ioulomb excitation experiments, the 99.07 and 46.48 keV levels have definitely been excited s. 7). Transitions of 162 and 292 keV have also been observed and assigned as arising from the excitation of the 208.81 keV 2-level and the 292 keV 2level 8 ) . We have investigated the beta decay of Ta183 again in an attempt to establish the position of the level in W183, which de-excites by the M2 transition reported by Murray et al. In so doing we have isolated and studied the decay of a 5.3 s isomer in W183 which is populated by Ta183 decay. 2. Experimental The 5.2 d Tala3 was produced by a 2 d irradiation of natural tantalum in a thermal neutron flux of 1014 n/cm2 . s. The tantalum activity was adsorbed from a weak HF-solution on top of a small column (3mm x 1 mm diameter) containing Dowex 1 X 10 anion-exchange resin of a grain size corresponding to a settling rate in water of 2-4 mm/min . Elution of wolfram activity was performed with a 0.3 M HF-1M HCl mixture 11) . An activity with a 5 .3 s halffife was found in the eluate . Determination of the halflife was carried out using a well-type scintillation crystal in connection with a single-channel analyzer set to accept the W K Xrays and a scaler . A few drops of the eluate were transferred to the detector

424

C . J . GALLAGHI:R, Jr., AND H . L . NIELSEN

immediately upon elution and photographs of the scaler were taken approximately every second for some minutes. A scaler counting the line current frequency of 50 Hz was photographed in the same picture . In this way, simultaneous values of accumulated counts and time were obtained . Analysis of the data

102 Fig. 2. Decay curve for the 5.3 s Wlasm eluted from a Dowex 1 x 10 cm ion exchange resin column loaded with radioactive Tares. The decay curve was measured directly by photographing the regis ter of the sealer in the counting circuit at 1 or 2 s intervals.

2000 H r 0

v a

1500

1000

500

CHANNEL NUMBER

Fig. 3. An Xe proportional counter spectrum of the low energy radiations from 5 .3 s W188m ob.

tai=fed by repeated rapid extractions of Wiesm from the parent Tales. Room and source background

were subtracted after each extraction .

yields a halflife for the activity of 5.34-0.2 s (fig. 2) . Applying the same chemical technique, we measured the gamma-ray spec-

THE 5 .3 SEC ISOMER OF W188

42 5

trum of the activity, using a 7.6 cm x 7.6 cm diameter NaI (TI) crystal as well as a xenon-filled proportional counter (gas pressure : 2 atm) and gamma-gamma coincidences. A256-channel pulse-height analyzer was used in all of these measurements t. Statistically significant numbers of counts were obtained in all three measurements by repeated rapid extractions, subsequently counting the decays of the isomer for 30 intervals. In order to account properly for the source background and analyzer dead time, which varied from extraction to extraction, we used the complement and lifetime operating modes of the analyzer to subtract the background after each extraction. The proportional counter spectrum (fig. 3) was energy calibrated, using sources of Cs137 and T1204 (emitting Ba and Hg X-rays, respectively) . Besides the strong W Ka and W K,8 lines, and their corresponding Xe-escape peaks, photopeaks and escape peaks of transition of 46 keV and 52 keV are present . The intensity ratio of the 46 keV line to the W K. is -110

8000

h FZ 34000 Q

H ~ 2000

Fig. 4. A NaI scintillation crystal pulse-height spectrum of the radiations from 5.3 s Wiaam . The spectrum was obtained in a manner identical to that described in the caption of fig . 3.

The NaI spectrum is shown in fig. 4. The energy spectrum was calibrated with a W178-Tal78 equilibrium mixture, Na22 and Cd' 09 . From fig. 4 it can be seen immediately that the spectrum contains peaks at 55, 105 and 160 keV. The experimental ratio of the peak intensities is 100 : 19 : 3.5, respectively . Pronounced coincidence sum peaks were observed at - 105 and ,. 160 keV when the solid angle was increased . In order to obtain an estimate of the energy of the isomeric level using sum coincidences, we measured the spectrum of the isomer with a source placed against the crystal. The sum background showed a definite cutoff at sw 260 keV.

t Manufactured by Radiation Counter Laboratories, Skokie, Ill ., USA .

426

C . J . GALLAGHER, Jr., AND H . L. NIELSEN

Gamma-gamma coincidence measurements using a conventional fast-slow of coincidence circuit were made, gating on all three photon peaks, but because the necessity for large initial counting rates and large geometries, we probably also observed coincidences with sum peaks . Quite unambiguously, however, the 55 keV photon is coincident with 55, 105 and 160 keV photons. The 160 keV transition was only strongly coincident with K X-rays, and some 160-105 keV coincidences were observed. The ratio of 55 to 105 keV photons was 2 .5 times less than the same ratio in the single spectrum, however. The mass assignment of the activity was made by determining the elution yield of the activity as a function of time. The experimental technique was a modified version of Campbell and Nelson's 12). A Dowex 1 x 10 column, as described above, but in which the Ta activity was evenly distributed on the resin, was continuously eluted with a 0.3 M HF-1M HCl mixture, which was recirculated in a steady flow to the top of the column. The system was shielded so that only a suitable length of the tubing immediately below the column was exposed to the detector. With the proportional counter as a detector, the spectrum was recorded for a given time, then the flow was stopped, and the "background" spectrum was subtracted . The area of the WXj-Xe Ka escape line was used as a measure of the yield of the activity. Aathough the method is subject to some uncertainty, a halflife of 6±4 d could be ascribed to the yield, Teas, thus indicating that the 5.3 s activity is W1s3m , the daughter of 5.2 d 3. Interpretation

The halflife of the NV1a3 isomer isolated from a Ta1e3 source is, within experimental error, equal to the halflife of the W183 isomer produced by neutron reactions on wolfram. The energies and the ratio of the intensities of the gamma transitions we observe are equal, within experimental error, to those reported by Poë. We, therefore, believe that this isomer is identical to that reported earlier. Our experimental data alone would not be enough to permit us to assign the energy, spin, or parity of the isomeric level, but taken together with the careful work of Murray et al . and Thulin et al. we believe we can assign all three . The energy of the isomer can be deduced in the following way. We have established that it is populated relatively strongly in the decay of Taxa3, hence probably has been observed in the work of Murray et al. Furthermore, because Murray et al. report only two unassigned transitions (the difference between which, or the sum of which, does not correspond to any energy sum or difference in the W183 level scheme) it is probable that (1) one of the unassigned transitions is the isomeric transition ; (2) this transition populates one of the levels reported by Murray et al . The first argument is reasonable because the 102.49 keV transition has been assigned an M2 multipolarity . Secondly, on the basis that the intensity of the other unassigned transition has an intensity only about

4

THE 5 .3 SEC ISOMER OF Wi83

42 7

the strength of the 102.49 transition (and is an M1-}-E2), we do not believe it can reasonably be assigned as a transition alone depopulating a level populated by the isomeric transition . Also because the isomer decays by at least two transitions, we conclude that the isomeric decay populates at least one of the levels reported by Murray et al. An examination of tie W183 level scheme shows that only one of all the levels previously assigned in WIN satisfies the requirements imposed by the fact that the highest energy transition in the spectrum of the isomer is 160 keV. That is the 207 .00 keV level previously assigned as the 1--rotational state of the K - 2 rotational band. Using the relative gamma intensities and conversion coefficients reported by Murray et al. for the transitions depopulating this level and the subsequently populated levels, and for the 102.49 keV isomeric transition, we conclude that, if this is the level populated by the isomer, the NaI spectrum of the isomer should (1) have peaks at ,. 55 keV (K X-rays, 46.48 and 52.59 keV photons), -,-s 105 keV (99 .07, 102.49, and 107.93 keV photons) and - 160 keV (160.53 keV photons) ; and (2) these peaks should have the intensity ratio 100 : 19.8 : 3.5. Both of these points are checked by the experimental results. In addition, the higher resolution Xe proportional-counter study of the lowenergy photon spectrum shows just the 46 and 52 keV photon and W K X-ray peaks expected with this interpretation . Furthermore, the principal sum peaks should be 2 K X-rays (at 110 keV), and K X-rays and 107.93 keV photons (at s:~:s 160 keV) with (because of the almost total conversion of the isomeric transition) a maximum energy for the coincidence sum peak at ,. 260 keV, again as observed in the experimental spectrum. The strong coincidence-sum peaks predicted by this level scheme can also account for the apparent 160-105 keV coincidences . The energy of the isomeric level is thus 309.49 keV. The spin and parity of the isomeric level can also be deduced, assuming the M2 assignment of the isomeric transition is correct. (In the following discussion we assume that there are no transitions with energies ;5 30 keV) . The K conversion coefficient reported by Murray et al. is 35, which is considerably larger than the theoretical value of Sliv and Band 13 ) of 27 .6. However, Murray et al. normalized their experimental conversion coefficient to the theoretical value for an M1 which had not been corrected for finite nuclear size effects, which results in an error of 20 % in this case t. When this correction is applied, the experimental conversion coefficient is 28, in excellent agreement with theory. However, the L-subshell ratio reported by Murray et al., aLI : xLII ' 7LIII = = 4.8 : -- : --, differs from the theoretical ratio for an M2, a LI : xLII ' MLIII = = 6.94 : 0.83 :1 .58 13 ) . We have not experimentally checked this apparent discrepancy between theory and experiment . The assignment of the transition as M2 is therefore at present only partially consistent with theory. t The conversion coefficients used in obtaining the ratio of transition intensities (i.e. Iy55 :IyIIO : Iyieo) reported in the paragraph above were also reduced by 20 %.

428

C . J . GALLAGHER, Jr., AND H . L . NIELSEN

We shall, however, accept Murray et al.'s assignment of the transftion as M2 on the basis of the K-conversion coefficient, and assign it to populate the 207.00 keV level to which Murray et al. assigned negative parity. The parity of the isomer is therefore positive . Because the isomer is populated by a beta branch with log ft = 8.5 (calculated from Moszkowski's nomograph 14), using a beta-decay energy calculated from the beta-endpoint energy of 615 keV for the transition to the 453.08 keV level, the decay fraction carried by the 102.49 keV transition reported by Murray et al. and the energy we have assigned to the isomeric level) from the Ta1a3 ground state, which is probably $ -}- (although in the range 5 to 2 . Because 2 - is possible), the level is expected to have a spin 83 , the level is not populated by the decay of Re' which has been postulated to have a 2 -}- ground state 2), the assignment of 2 -}- or 2 -}- seems unlikely, leaving 9-{- as the best possibility. Such an assignment is consistent with the systematics of energy levels in this region, as a 2 -}- state is generally assigned 15) as the ground state of nuclei with N = 107, and hence should be a low lying excited state in a nucleus with N = 109. If Ta1s3 has a 2-ground state, an additional possibility is ~+, but we consider this latter assignment less probable. [f the isomeric level is 2 --, the 210.4 keV cross-over M2 to the 5- state at 99.07 keV is possible . We can set an upper limit on the intensity of this crossover transition as < .1 the intensity of the 160 keV transition . This is -.v 80 times less than energy considerations alone would predict. However, the very large hindrance of the 102.49 keV transition (see below) indicates that it probably occurs only through small admixtures in the wave functions; hence, the weakness of the branch to the 99 .07 keV level may not be especially significant . The decay scheme we propose involving the isomer is shown in fig. 5.

9/2+) tK - 5.3

w183

Set

Fig. 5. The Ta 188 decay scheme involving the 5 .3 s W183 isomer.

THE 5 .3 SEC ISOMER OF Wi83

42 9

4. Discussion The log ft = 8.5 observed for the beta decay to the isomeric levels large for an allowed transition . Furthermore, althoughin the asymptotic-limit classification of the Nilsson states t the levels would be 404 (2 -{-) and 624 (2 -f-) and the transition would be hindered, the log ft for the same transition observed in Lu 177 decay is 6.3. Such variations in the log ft for a given transition have been observed before 15) but as yet have not been satisfactorily explained in a quantitative way. The argument we have used to assign a spin and parity to the isomer is largely dependent on the M2 assignment of the 102.49 keV transition, which we assign as the isomeric transition . In addition to the discrepancy in the Lsubshell conversion of the transition, the isomer is extremely retarded for an M2. The hindrance factor for the radiative transition relative to single-particle rates 16) Fr 2 °

tj(exp) .6x107 1 ti(theor)

is, we believe, a larger hindrance than has hitherto been observed for an M2 transition . Such a large hindrance might, of course, be associated with the v - 2(v = dK-L) K forbiddenness of the transition 17) . In view of the long halflife of the isomer it is surprising that an E3 transition does not compete. The hindrance for a 102.5 keV E3 would be .. 102. However, the L-subshell ratio reported by 1durray et al. effectively rules this out, as theoretically aLI : aLII : aLIII = 0.84 : 32 : 25 for an E3 transition . Most surprising to us, however, is that the El transition which might be expected to occur between 2 -{- and 2 - states is very weak, i f present at all, at least on the basis of the conversion coefficient . Even if we assume that the isomeric transition is pure El (ignoring the question of the large conversion coefficient), the hindrance factor for the radiative transition is FE1 = 1 .95 x 10 15 , which certainly classifies it as the most highly retarded K-forbidden E 1 (considering v = 3) yet observed. We thank Professor Niels Bohr for the excellent working conditions in his laboratory . The participation of one of us (C. J. G.) in this research was made possible by a U. S. National Science Foundation Postdoctoral Fellowship. t See ref. is) for a discussion about the use of the asymptotic limit selection rules.

References 1) Murray, Boehm, Marmier and Du Mond, Phys. Rev. 97 (1955) 1007 2) Thulin, Rasmussen, Gallagher, Smith and Hollander, Phys . Rev. 104 (1956) 471

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C . J . GALLAGHER, Jr., AND H . L . NIELSEN

3) E. Der Mateosian and M. Goldhaber, Phys. Rev . 76 (1949) 187 4) E. C. Campbell and M. Goodrich, private communication quoted in Strominger, Hollander and Seaborg, Revs. Mod. Phys . 30 (1958) 585 5) A. J. Poe, Phil. Mag. 46 (1955) 611 6) Huus, Bjerreg$rd and Elbek, Mat. Fys. Medd. Dan. Vid. Selsk. 30, No. 17 (1956) 7) Chupp, Clark, Du Mond, Gordon and Mark, Phys. Rev. 107 (1957) 745 8) P. H. Stelson and F. K. McGowan, Phys. Rev. 99 (1955) 112 ; F. K. McGowan and P. H. Stelson, Phys. Rev. 109 (1958) 901 9) A. K® Kerman, Mat. Fys. Medd . Dan. Vid. Selsk. 30, No. 15 (1956) 10) Gallagher, Strominger and Unik, Phys . Rev. 110 (1958) 725 11) Gallagher, Nielsen and Nielsen, Phys . Rev. to be published 12) E. C. Campbell and F. Nelson, J. Inorg. Nuclear Chem . 3 (1956) 233 13) L. A. Sliv and I. M. Band, Publication of the Leningrad Physico-Technical Institute (reproduced in USA as Reports 57 ICC K i and 58 ICC-L 1 by the Physics Department, University of Ill., Urbana, Illinois, USA) 14) S. A. Moszkowski, Phys. Rev. 82 (1951) 35 15) B. R. Mottelson and S. G. Nilsson, Mat. Fys. Skr. Dan. Vid. Selsk . 1, No. 8 (1959) 16) S. A. Moszkowski, in Beta- and Gamma-Ray Spectroscopy, edited by K. Siegbahn (North Holland Publishing Company, Amsterdam, 1955) Ch. XIII p. 391 17) Alaga, Alder, Bohr and Mottelson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 9 (1955)