The decay of 196Au

Nuclear Physics 31 (1962) 5 7 5 - - 5 8 3 ; (~) North-Holland Publishing Co., Amsterdam Not

t o be reproduced by photoprint or microfilm without written pernfission front the publisher

THE DECAY OF t96Au A. I-I. XYAPSTRA, J. F. W. J A N S E N , P. F. A. G O U D S M I T a n d J. O B E R S K I

Instituut voor Kernphysisch Onderzoek, Amsterdam, Holland Received 23 O c t o b e r 1961 A b s t r a c t : T h e electron c a p t u r e d e c a y e n e r g y of ~86Au, as d e t e r m i n e d f r o m t h e p e r c e n t a g e L - c a p t u r e in t h e b r a n c h to a 1447 k e V s t a t e in 196Pt, is 1485_+18skeV. In a d d i t i o n to t h e 1447 k e V s t a t e , levels a t 877 k e V a n d p o s s i b l y 1320 k e V are p o p u l a t e d in this decay. T h e halflife is f o u n d to be 6.15=t=0.15 d.

1. Introduction According to older data, 196Au decays with a halflife of 5.60~ 0.05 d (refs. 1,~) ), for 5.8 % to the 426.13:k0.15 keV level a) in 196Hg by a 270:k20 keV 4) fltransition, and for 68.0 % and 26.2 % 5) to the 355.73±0.05 and 688.78±0.07 keV 3) levels in 196Pt. In addition, a 1091.3±0.3 keV ~) gamma ray is present in the capture decay; according to Gupta 6) who discovered it, this gamma ray originates in a 1091 keV level in 19~pt, but Morinaga 7) found that the stop over gamma ray has an energy of 755 keV instead of the 735 keV required by Gupta's interpretation and therefore assumes both to come from a level at 1447.0 keV. This explanation fits slightly better with Cohen and Price's 195Pt (d, p) reaction measurements 8) that yielded levels in 196Pt at 870, 1130, 1270, and 1420 keV; it appears worthwhile to strengthen it by coincidence measurements. Combination of the recently computed 9) atomic mass of 196Au with Demirkhanov et al.'s C1sN2H12--196pt mass doublet lo) yields a capture decay energy of 870+110 keV, whereas Bhanot et al.'s C14H2s--196Pt doublet Jl) would yield 1600+100 keV. Evidently, a good direct determination of the electron capture decay energy would be desirable. Gupta's measurement 8) of the percentage K capture to the 688 keV level gave only a rather inaccurate value ll~n+aso . . . . . 10o keV; we propose to investigate whether the capture ratios in the transition to the level at 1091 or 1447 keV can give a nmch improved value. The intensity ratio < W 0 g of the 688.8 keV cross over transition to the stop over is curiously low a, r,); further information on the properties of the level scheme of a nucleus with such a deviating property is desirable.

2. Sources and Equipment Sources for gamma ray spectroscopy were made by Au (n, 2n) reactions on Au packed in Au and Cd foils in order to minimize the intensity of neutron OdO

576

A . H . WAPSTRA B• al.

capture. Beta spectroscopy samples and those used for investigation of the L X-rays were made b y (d, 2n) reactions on mass separated 19ePt obtained from Oak Ridge; they were purified b y extraction and electroplated as described before 13). The beta ray spectrometer used here is the iron free double focusing spectrometer of De Vries and Wapstra 14). The gamma ray spectra were measured in a 6 × 6 cm well type NaI (T1) crystal with a 6 mm diam. hole on a DuMont 6393 photomultiplier and a 4 × 25 mm NaI(T1) X-ray crystal on a DuMont 6292; the spectra were displayed on a R I D L 100- channel pulse-height analyzer. In coincidences between the 333-356 keV peak and the higher energy gamma rays, the former were measured in a 4 × 5 cm NaI (T1) crystal on a 6292 and selected with a Franklin model 348 amplifier-channel; in coincidences with the X-rays, the 6 cm crystal was used for detection of the 333-356 and 1090 keV peaks. In both applications, a combined A1-Cu-Pb-Cu absorber was placed between the source and the 6 cm crystal, as shown in fig. 1.

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3. Halflffe and S u m m i n g Spectrum An Au (n, 2n) source was left to decay for 20 d in order to make the contribution of 198Au completely negligible and then followed over 40 d. The value 6.154-0.15 d found iu this w a y for the halflife of both the 333-356 and 428 keV peaks is considerably longer than the value given in the introduction. It is not clear whether the deviation m a y be due to some remaining 19SAu activity in the measurements of earlier authors. We repeated Gupta's 0) summing spectrum measurements, using the 6 cm well type crystal mounted inside a 2½ cm Hg shield in order to reduce the background (see fig. 2; the background has been subtracted). The 1450 keV peak is certainly due to real 1091-355 keV summing: it is at least 20 times stronger than could be explained b y accidental summing, as is clear b y comparison with the 1020 keV peak due to accidental summing of the 688 keV real summing peak with 340 keV gamma rays. In addition, comparison of the 420 and 760 keV peaks due to summing with K X-rays, with the small intensity

TI-IE DECAY OF 196Au

577

at 1530 keV shows that the percentage K capture in the transition to the 1447 keV level is considerably less than those to the lower levels.

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4. Beta Ray Spectrometer Measurements Taking for the energy of the lowest gamma ray in 19sPt the value 355.6540.06 determined in this Institute la), we obtain values 332.94i0.07 and 426.094-0.08 keV for the two other main gamma rays, in agreement with the earlier values 8). We have been unable to find the conversion line of the 1091 keV transition due to the rather low intensity of our source; we found, however a weak line at 443.0 keV (fig. 3) that we should like to interpret as the K counts per minute 12C

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578

A . H . WAPSTRA et ~ .

conversion electrons of a 521.3~0.7 keV gamma ray in 19sPt. The intensity ratio of the electron line to the 356 K line is (3.0±1.0) • 10-4, both in sources made with natural Pt and with mass separated 196Pt.

5. G a m m a S p e c t r u m and Coincidences Fig. 4 shows the gamma spectrum as measured in the 6 cm crystal in the arrangement of fig. 1, fig. 5 shows the same spectrum as measured in coincidence with the 333-356 keV peak. The intensity of the continuous gamma distributions due to the electron capture process in the above two spectra has been calculated from the formulae of Glauber and Martin 15), using a total electron capture decay energy of 1480 keV. Also, the intensity of the summing spectrum of the 333 and 356 keV gamma rays was computed in the single spectrum. After subtraction of these two distributions, the spectra were analysed in the normal way. The results agree reasonably with those of earlier authors 3, s). Th ey are given in table 1, normalized to intensities of 100 and TABLE i Analysis of the single 7-spectrum of 19*Au and the spectrum in coincidence with a 333-355 keV channel G a m m a energy (keV) 333-356 426 435 520 570 640 689 755 860 980 1090

I n t e n s i t y singles 127 5.7

I n t e n s i t y coincidences 40

+1

0.0734-0.015 0.022±0.015 0.008±0.004 0.003±0.002 0.050 ± 0.007 0.003 ±0.001 0.002 ±0.001 0.20 ±0.01

0.035 ±0.009 0.052±0.009 0.016 ±0.009 ~ 0.016 ~ 0.015 0.078 ±0.008 ~ 0.002 ~ 0.003 0.20 ±0.01

27 for the 356 and 333 keV gamma rays; evidently, the intensities in the last column should be equal to those in the first column when the gamma ray in question leads to the 356 keV level, and almost doubly as large when it leads to the 688 keV level (to be exact, the ratio is only about 1.6 due to absorption: the efficiency for the 333 keV gamma ray is now slightly less than t hat for the 356 keV one). Thus, very clearly the 1090 and 755 keV gamma rays originate in a level at 1447 keV. The 520, 575 and 875 keV gamma rays should be explained as gamma rays to and from Cohen and Price's 8) level at 870 keV; the first of these radiations m a y be the same gamma ray as that found in the beta ray spectrometer measurements. The other three gamma rays (435, 640 and 970 keV) may be explained by another level at 1320 keV.

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Fig. 4. G a m m a r a y s p e c t r u m of X96Au. Thin d r a w n lines represent the c o m p u t e d intensities of the c o n t i n u o u s s p e c t r u m due to electron c a p t u r e and the s u m m i n g s p e c t r u m of the 333 and 356 keV g a m m a ra ;s; dotted lines represent the analysis of the remaining s p e c t r u m into single g a m m a rays.

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et al.

By following the decay, it was found that at the time of the above measurement half of the intensity of the 688 keV gamma rays was still due to XgSAu. The remaining intensity would give a ratio (1~0.6) • 104 for the cross over to stop over transitions from the 688 keV state. This value is compatible with the intensity limits found by Alburger 12) and Bergman 3). 6. D e c a y

Energy

We first measured, the percentage K capture to the 1447 keV level, essentially b y the same method as used below for L capture. Only 1.6=J=1.8 ~/o of the 1091 keV gamma rays was found to be preceded by K-capture. Tha L X-rays were measured, in the arrangement of fig. 1, in coincidence with gamma ray channels at 1 0 9 0 i 9 0 and 340=[=40 keV. The ratio of the numbers of coincidences per singles in the channel was x-

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- - 0.86:[:0.07.

The error in this result is mainly due to the uncertainty in the number of single counts in the 1090 keV channel, which was slightly less than twice the background. The percentage L-capture in the transition to the 1447 keV level was now derived in the following way. The number of primary holes in the Y shell ( Y ~ K , L I , L n , L m ) per gamma ray counted in the 340 and 1090 keV channels is N y34o/N a,o -~ ] l y + r ( R / 338y + / ahey ) / ( R + r ) , Nyxogo/Nlogo = / ~ v + ~ 3 6 e , .

Due to transfer of holes from the Z shell to the Y shell, the number of counted Y X-rays per counted gamma ray becomes NvJN E = ~ /zYNz~/Ndo.ev

•

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In the above formula, fly is the fraction Y-capture in the transitions to the 356 and 689 keV levels (equal within our precision). We used ]XK ~-- 0.815, /:L, = 0.137, [:L,, = 0.010, [:L,, = 0 (see ref. le), p. 59). The quantity [2Y is the same as ]lY but for the transition under consideration and [~y is the fraction Y-conversion for the transition with energy E; we used/35eK = 0.038, [a~L 0.006, ]a33K = 0.049, [333L ~ 0.013. Furthermore, ey is the efficiency for detection of X-rays due to radiative transition to the Y shell (equal within 4 ~o for the three L subshells, in our case), R ~ 1.6 the ratio of efficiencies for detection of 356 and 333 keV gamma rays in the 6 cm crystal, r = 0.27 the ratio of the intensities of the 333 and 356 keV transitions, ~y the fluorescent yield in the

T H E DECAY OF l°6Au

581

Y shell, and ]zY .the average number of holes in the Y shell produced by one hole in the Z shell (/zz = 1). The quantities ~oK and /KI., are accurately known 16), but the other yields are not, partly due to the fact that the assignment of a certain strong L Auger line to the L subshells is uncertain. Dependent upon this assignment, the fraction L capture to the 1447 level becomes either PL = 0.75X--0.03,

or

PL = 0.82x--0.04.

Thus, the uncertainty here causes an additional error of about 5 % in PL if the last quantity is computed with the average of the above numerical constants. We so obtain PL = 0.644-0.06. The decay energy was derived from this result with the help of the formula P(M+N+...)/PL = 0.264{ (Q-- 3.2)[(Q-- 13.9)} 2. The constant in this formula has been computed as discussed elsewhere 16), using Slater screening constants. This method yields a pretty accurate result for the ratio of L and K capture (0.143 as compared with the value 0.148 computed by more exact methods by Brysk and Rose lT)); the error in the present case m a y be somewhat larger but will yet probably be less than the experimental error. We then derive a decay energy 38+175keV for the transition to the 1447 keV state, so that the total electron capture decay energy becomes Q2 = 1485+1~ keV. 7. D i s c u s s i o n The decay scheme of 196Au following from the above data is shown in fig. 6. The spin assignment 2+ of the 689 keV level is certain from angular correlation measurements 18, 19); the very small intensity ratio 10-4 of the cross over to stop over gamma rays remains a puzzle. The same ratio is about 0.3 in 192Pt and 194Pt (see ref. 23)). One expects a positive parity for the 883 keV level; then, since a 564 keV M2 transition could scarcely compete with the E1 transitions from the 1447 keV level, the spin of the former level should be 2,3 or 4. Our above data yield a value 0.013±0.04 for the K conversion coefficient of the 521 keV gamma ray (if for the 356 keV gamma ray 2o) XK = 0.040±0.002), as compared to theoretical ~1) values 0.007, 0.017 and 0.060 for E l , E2 and M1 transitions. Thus, probably this gamma ray is rather purely E2. This, together with the high log /t value for the electron capture transition to the 877 keV level would point to a spin 4, as in the case of the analogous 784 keV level in 192 Pt (see insert in fig. 6 and refs. 22. 28)); the other two high levels shown there decay preferentially

582

A.H.

WAPSTRA 8t t~l.

to the second 2+ states). Yet, there is some evidence for a gamma ray at about 880 keV not occurring in the coincidence spectrum, that would be more easily explained if the spin of the 883 keV level were 2. 196 A u key

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The active sources were prepared and purified by Miss H. Kapteyn. We thank Prof. Dr. P. C. Gugelot, Dr. R. van Lieshout and Dr. G. J. Nijgh, for their interest in this work, which is part of the research programme of the "Stichting voor Fundamenteel Onderzoek der Materie (F.O.M.)" which is financially supported by the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (Z.W.O.)"

Note added in proo/: The recent results of Ikegami et al. 2,) agree with the above data; their interpretation involving levels at 1000, 1140 and 1280 keV in stead of the 1320 keV one proposed above requires more intensity in the coincidence spectrum at 435 and 570 keV than seems to be present in our result (table 1).

THE DECAY OF 196Au

583

References 1) 2) 3) 4) 5) 6) 7) 8) 9) I0) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)

22) 23) 24)

G. Wilkinson, Phys. Rev. 75 (1949) 1019 R. M. Steffen, O. Huber and F. Humbel, Helv. Phys. Acta 22 (1949) 167 O. Bergman, Arkiv. f. Fysik 18 (1960) 569 P. Staehelin, Phys. Rev. 87 (1952) 374 M. T. Thieme and E. Bleuler, Phys. Rev. 101 (1956) 1031 R. K. Gupta, Physica 26 (1960) 69 H. Morinaga, private communication 13. L. Cohen and R. E. Price, Phys. Rev. 118 (1960) 1582 F. Everling, L. A. K6nig, J. H. E. Mattauch and A. H. Wapstra, Nuclear Physics 18 (1960) 529 R. A. Demirkhanov, T. I. Gutkin and V. V. Dorokhov, J E T P 37 (1959) 1217 V. B. Bhanot, W. H. Johnson and A. O. Nier, Phys. Rev. 120 (1960) 235 D. E. Alburger, Phys. Rev. 108 (1957) 812 C. de Vries and J. H. Dijkstra, Nuclear Physics 18 (1960) 446 C. de Vries and A. H. \Vapstra, Nucl. Instr. and Meth. 8 (1960) 121 R. J. Glauber and P. C. Martin, Phys. Rev. 104 (1956) 158 A. H. Wapstra, G. J Nijgh and R. van Lieshout, Nuclear spectroscopy tables (North-Holland Publishing Co., Amsterdam, 1959) H. Brysk and M. E. Rose, USAEC report ORNL-1830 (1955) R. M. Steffen, Phys. Rev. 89 (1953) 665 V. R. Potnis, Indian J. Phys. 30 (1956) 375 J. F. W. Jansen, S. Hultberg, P. F. A. Goudsmit and A. H. Wapstra, unpublished M. E. Rose, Internal conversion coefficients (North-Holland Publ. Co., Amsterdam, 1958); L. A. Sliv and I. iV[. Band, Coefficients of internal conversion of gamma rays (USSR Academy of Science, Moscow-Leningrad, 1956) O. Bergman, Arkiv f. Fysik 18 (1960) 569 K. W a y et al., Nuclear Data Sheets (National Academy of Sciences, National Research Council, Washington, 1958-1960) H. Ikegarni, T. Yamazaki ank M. Sakai, J. Phys. Soc. Japan 16 (1961) 2350 and private communications