The isomeric transitions in Ti197 and Tl195

Nuclear Physics 3 (1957) 4 9 3 - - 5 0 5 ; North-Holland Publisking Co., Amsterdam

THE ISOMERIC T R A N S I T I O N S IN T1197 AND T119s G. A N D E R S S O N ,

P. A. TOVE, B. J U N G

AND

I. B. S V E N S S O N

The Gusta/ Werner Institute /or Nuclear Chemistry, University o/Uppsala, Uppsala R e c e i v e d 22 F e b r u a r y 1957 I n t e r n a l c o n v e r s i o n e l e c t r o n s f r o m t h e isomeric t r a n s i t i o n s in 2"119~ a n d TI I'5 h a v e b e e n s t u d i e d in h i g h r e s o l u t i o n / ~ - s p e c t r o m e t e r s . T h e i n t e n s i t y ratios of t h e lines, w h e n c o m p a r e d w i t h r e c e n t t h e o r e t i c a l values, were f o u n d to c o n f i r m t h e m u l t i p o l e a s s i g n m e n t E3 for b o t h t r a n s i t i o n s . T h e z - e n e r g i e s were e s t a b l i s h e d as 222.454-0.05 k e V (T1*~Tm) a n d 99.1::h0.i k e V (Tl1'Sm). T h e half-lives of T119vma n d T12eSmh a v e b e e n m e a s u r e d as 0.554- 0. I0 sec a n d 3.5::h 0.4 sec, respectively, u s i n g r a p i d c h e m i c a l t e c h n i q u e s a n d a m u l t i c h a n n e l m e t h o d of t i m e analysis. P u l s e h e i g h t s p e c t r a of t h e s h o r t - l i v e d a c t i v i t i e s were f o u n d to s u p p o r t t h e m a s s assignments.

Abstract:

I. Introduction E3 transitions between the proton levels h11/2 and ds/~ m a y be expected to cause isomerism in odd-Z, even-N nuclei near the 82 proton shell closure. Isomers of this type are known to exist in Au 197, Au 195, Au 193 i), and Ir 191 ~). In a recent communication from this institute a) the probable discovery of isomeric transitions also in T119~ and T1195 was announced. The K/EL conversion ratio of a 222 keV 7-ray, appearing in the decay of 42 min Pb ag~m and converted in TI, clearly indicated the multipole order E3 when compared with the value obtained from Rose's tables 4). The experimentally found ( L x + L n ) / L m ratio, however, was considerably smaller than expected. For the corresponding transition in T119s, where the low y-ray energy -- 99 keV -- prevented the K line from being detected, the assignment E3 was mainly based on a similar reduction of (LI+Lu)/Lm relative to the calculated value. These anomalous conversion intensities called for improved measurements of the electron spectra. Further a determination of the half-lives was needed in order to establish the isomers more firmly. In fact, a 0.54 sec period had earlier been assigned to a 384 keV z-ray in T119vm5). This was not quite contradictory to the findings reported in reference 3), as 385 keV and 387 keV transitions of lower multipolarities had been shown to follow in cascade after the 222 keV z-ray (cf. fig. 7). It remained to be proved, however, t h a t the 0.54 sec halffile was characteristic also of the last-mentioned transition.

2. fl-spectrometer Measurements 2.1. T H E

NUCLEUS

Tlle~m

The K and L conversion lines of the 222 keY y-ray in T1197were investigated m the ironfree double-focusing/3-spectrometer 6) of the Physics Department of this University. 493

June 1957

~94

G. A N D E R S S O N . P. A. T O V E . B . J U N G AND I . B . SVRNSSON

--% ,j,~~

"~I~~.~

V)

g ~N

~e

THE ISOMERIC TRANSITIONS IN T1197 AND TIl°6

~95

The source was obtained in the following way (el. 3)). Natural T1 was irradiated in the synchro-cyclotron with protons of 95 MeV average energy, corresponding to maximal yield of 42 rain Pb 197m. After chemical separation, using a few/zg Pb carrier, the activity in the form of PbC13 was vacuum evaporated onto 0.18 mg/cm 2 A1 foil. (The chemical procedure is described in sections 3.3 and 3.4). With the source dimensions 0.5 by 8 mm and an estimated spectrometer transmission of about 1 °/0o the relative half-width at the L-lines was found to be 0.9 °/0o. This was sufficient to permit the LI line to be resolved with a fair accuracy from the much stronger LIt as indicated in fig. 1. At the K-line the relative half-width was slightly more than 1 0/o0 -- even after the subtraction of an interfering line of longer half-life -- evidently depending on a non-negligible thickness of the sample. A careful analysis of the spectrum gave the results collected in table 1. TABLE

1

Ee-

Er

(keV)

(keV) (cony. TI)

K

136.931

222.451

0.556+0.028

Lx

207.108

222.452

0.223-4- 0.022

LI,

207.758

222.453

1 . 8 4 3 + 0.074

Line

i

J

E l e c t r o n i n t e n s i tT (arbitrary units)

L, + L n Lm

2 . 0 6 6 + 0.041 209.795

222.448

O. 856-}- 0.026

As no suitable calibration line was available, the absolute electron energies were determined by adjusting the spectrometer constant to give a satisfactory agreement between y-energies, cMculated from the different lines using T1 electron binding energies. A consideration of possible sources of error finally led to the transition energy E v = 222.45+0.05 keV. The electron line intensities in column 4 have been corrected for decay of the parent activity during the measurement, but the errors do not include the uncertainty in the half-life (42=52 min 3)). This was taken into account, however, in calculating the conversion ratios as K / Z L = 0.242+0.018

(0.272+0.011),

( L z + L n ) / L i n = 2.43+0.15

(3.984-0.14).

The figures given in parenthesis are the ratios for E3 originally obtained from Rose's tables 4), with their estimated interpolation errors. Thus the

496

G. A N D E R S S O N , P. A. T O V E , B. J U N G

A N D I, B. S V E N S S O N

results seemed to confirm the discrepancy mentioned in section 1. Several other E3 transitions in the same Z region were found to show a similar behaviour. A full discussion of this effect, leading to the conclusion that it could be caused neither b y M4-admixture nor b y the finite nuclear size, has been given elsewhere 7). Recently, Rose announced a recalculation of his Lm conversion coefficients for E3 transitions 8), which changed the theoretical ratios to K / Z L = 0 . 2 4 6 ! 0 . 0 1 0 and (LI+Lx~)/Lm = 2.59-1-0.10, in good agreement with the experimental values. counts/50s

/

I00~ 200G

t-- Idrnin L:E

t=O

\

7

150C I

z'24

1000

t =28rain Lj~

t = lOmin L!

half-~fe/

• unassigned line of longer 500

t=O

d

I d

2 22"

! 223

I 2.24

225

Amp

Fig. 2. Electron lines of t h e 99.1 keV F-ray in T1x", m e a s u r e d in an iron yoke double-focusing spectrometer. Counting t i m e per point = 50 sec. T h e inset illustrates t h e decay of the Lx+Lzl complex.

497

THE ISOMERIC TRANSITIONS IN T1197 AND T1195

2.2. THE NUCLEUS T1185m

The L lines of the 99 keV transitionin T119a,fed from the electron capture decay of 17 min Pb laSm 8), were measured in the 18.5 c m double-focusing fl-spectrometer of this institute 9). The same method of sample preparation as in the case of ]'i1~m was used, with the bombarding energy raised to about 120 MeV. A source width of 0.5 m m gave 2.2 0/0o relativehalf-width at a calculated solid angle of about 2 °/0o. Figure 2 shows the conversion electron spectrum. L m could relatively easily be corrected for a parasite on the low energy side and was used as a line shape standard in an attempt to decompose the LIq-L11 complex. Unfortunately LI turned out to be superimposed by a more long-lived (probably about 40 rnin) line of unknown origin (see the inset of figure 2). A detailed study of the decay of the complex was made impossible by the short parent half-lifeof the lines of interest (17+1 min). Thus only LH and LIH could be accurately determined as appears in table 2. TABLE 2

Line

E~ (keY)

E e-

(keV)

L~t/Lm

(conv. T1)

LH

84.354

99.049

LII!

86.424

99.077

1.52::[:0.12

The calculation of electron energies was performed using a calibration curve according to Arbman lo). Taking the calibration uncertainty into account we give the energy as Ev = 99.1±0.1 keV. As to the comparison between the experimental and the theoretical conversion ratios a similar discussion applies as in the case of "I"119~m. The corrected theoretical L m coefficient leads to LII/Lm = 1.42q-0.04. 3. H a l f - l i f e D e t e r m i n a t i o n s 3.1. G E N E R A L CONSIDERATIONS

Assuming the assignment of a 0.54 sec activity to T1lg~m (cf. section 1) to be correct and taking into account the dependence of y-decay lifetimes on energy and internal conversion 11), a half-life of about 5 sec could be predicted for the 99.1 keV transition in T1195. This was considered measurable using rapid chemical techniques to separate T1 from its Pb parent and following the y-decay with a scintillation counter connected to a multichannel time analyser. As it turned out, a refinement of the method even made possible a determination of the shorter-lived T1197m.

498

G. ANDERSSONp P. A. TOVE~ B. J U N G & n d I. B . SVENSSON

?

i

0

!

THE

ISOMERIC TRANSITIONS IN T1197 AND T1196

499

3.2. ELECTRONIC EQUIPMENT To record the decays and in complementary experiments to study the energy spectra of the short-lived activities a combined time analyser -pulse height analyser system 12) was used. In time analysis (see fig. 3) the pulses from the 7-counter are first passed through a single-channel kicksorter, set to select the photopeak pulses from the 7-ray under investigation. These are fed to the time analyser, consisting of 20 gated count channels, which are successively opened and shut by flip-flop circuits, each channel being open for a present time interval tk, variable between 2 ms and 13 sec. The process is controlled automatically by an independent pulse generator, giving pulses spaced tk in time. Thus the first channel is opened by the first control pulse appearing after pressing a start button (in the actual experiments when the T1 source had been put in position) and is shut after the time tk, simultaneously opening the second channel and so on. In this way the "channel sweep" proceeds through all the gates. A scale-of-eight circuit, acting on the control pulses, can be used to increase the open time to 2 × tk for channels 6--10, 4 X tk for channels 11--15, and 8Xtk for channels 16--20. Thereby improved background statistics can be achieved. The determination of tk is made by feeding pulses of known frequency to the channel inputs, instead of pulses from the 7-counter. In order to study the pulse height distribution of the short-lived activities, the equipment was rearranged to a 20-channel pulse-height analyser. In this operation the channel sweep, now with tk very short (about 10 /~s), is started by the pulse to be analysed. This pulse is also transformed into another pulse, having its length proportional to the height of the primary pulse. The trailing edge of the secondary pulse is fed to the inputs of the count channels and accepted in that channel which is open at the proper time. Thus the pulses accumulate as counts in the scalers of the various channels. The rate of filling of the scalers was normally recorded by rapidly repeated photographing. Alternatively and more suitably for the shorter half-life, a gating system could be used which opened the pulse height analyser for a short time interval at a predetermined time Ta after putting the T1 source in position. The scaler readings were photographed and with the same source the procedure was repeated after a time, which should be long compared to the half-life concerned, in order to determine the background. By varying Td it is possible to find the energy distributions and the approximate halflives of the activities present. 3.3. BASIC

CHEMICAL

PROCEDURE

After irradiation, the T1 target was dissolved in concentrated HNO3,

500

G. AND]~RSSON~ P. A. TOVI~ B. J U N G A N D I. B. SV~NSSON

containing about 5#g Pb ¢+ and 250/~g Fe 8+ as carriers. In order to reduce T1 to the monovalent state the solution was boiled with H~O= until excessive peroxide had disappeared. Fe and Pb were precipitated with NH3, the r61e of the Fe being to carry down the more soluble lead hydroxide as coprecipitate. After centrifuging, the precipitate was dissolved in HNO~, reprecipitated, and washed in water 3 times. Finally the hydroxides were dissolved in the smallest possible amount of HCI (about l0 drops of 0.1N acid). 3.4. THE

NUCLEUS

T11'6m

A. Milking o/ T1 /rom the P b solution Use was made of the possibility of extracting T1C18 from an aqueous solution into ether. The Pb activity was kept in 6 N HCh Three ether extractions were first made in order to get rid of the Fe carrier. (In preparing /~-spectrometer sources, the remaining Pb solution was then evaporated to dryness on a tantalum strip fitting the vacuum evaporation device.) The T1 formed in the decay of Pb most probably appears in the monovaient state. Therefore a drop o f KMnOa solution was added immediately before each (about 3 ml) ether portion. After stirring with a glass rod for some seconds in order to bring the ether into intimate contact with the aqueous phase, that part of the ether phase which is formed first (nearest to the surface) was pipetted off and transferred to a test tube, which as fast as possible was put into counting position over the y-detector and the analyser was started. The whole separation process was completed in some five seconds. In order to improve the ratio between short-lived and longer-lived activities in the ether fractions the T1 formed during the counting period was washed off with ether immediately before taking the next sample.

B. Measurements and results The 99.1 keV isomeric transition is strongly converted (~tot~l-~ 155 according to Rose l) with XM+~+... = 0.3 ~L), and for that reason the single channel energy selector was instead adjusted to 4204-50 keV in order to cover the two cascade transitions of 393 and 448 keV 8). About 20 T1 milkings were performed within the time limits set b y the 17 rain parent activity. The numbers of counts in the 20 channels were read after each run and those series showing the most favourable initial total count-to-background ratio were selected and combined to form the decay curve shown in fig. 4. From this curve the half-life T½ = 3.54-0.4 sec was obtained. (The error limits are meant also to include systematic errors.) In the pulse height analysis the same method of sampling was used. The counting rate in the 20 channels was recorded b y photographing the

T H E ISOMERIC T R A N S I T I O N S I N T1197 A N D T1196

~01

scalers after 3, 6, 9, 12, and 24 seconds. The combined result of 14 runs in the energy interval 200--870 keV is illustrated in fig. 5.

counts / tk

¢

10~f

L. ~ . w w " v

103

lO~.

Time in units of tk Fig. 4. D e c a y curve (including 13 r u n s ) of t h e 393 keV and 448 keV ~,-rays in T1x°s, a s s u m e d to be in cascade w i t h t h e 99.1 .keV E3 transition, t k = 1.51+0.01 sec.

The presence of a short-lived photopeak with its center of gravity slightly above 400 keV is clearly indicated. From the measurements it was even possible to estimate the half-life as about 4 sec. This proves t h a t the 3.5 sec period found in the time analysis was really due to a photopeak of the expected energy and not, for instance, to the Compton distribution of a ~-ray of higher energy.

502

G.

ANDERSSONj

P. A. T O V E ,

B. J U N G

AND

I. B. S V E N S S O N

coun~S/chom~e/

i

200(

.... ~=

= Total counts Background Short~lveff component

L- l

/00[

Channel out of function

,

,

.

1 2 3 4 5 6 7

.

91011121314151617181920

8bo

~anne~

~o k;v Energy scale

Fig. 5. Pulse height spectrum of the 3.5 sec activity, obtained b y subtracting the background (calculated from the counting interval 12 sec--24 sec) from the total n u m b e r of counts during the first three seconds. 3.5. T H E NUCLEUS T1t~Tm

A. Milking o/ T1 /rom the Pb solution Attempts to use ether extraction also for T1197m failed because the separation time could not be decreased sufficiently in relation to the halflife of the isomeric state. Instead a new technique, involving ion exchange, was worked out. The ion exchange resin (about 30 mg Dowex II, 2o0--400 mesh) was loaded with PO4s- ions. When the active Pb solution, containing also Fe 3+ and C1- ions, was put on the top of the exchange column, the inactive ions were found to be trapped in the upper part of the resin (visible b y a faint yellow-green discolouring), while the Pb accumulated immediately below this zone. The amount of resin was so chosen that the active layer was situated close to the b o t t o m of the column. The T1 formed in the decay of the radioactive P b was eluted with water, continuously pressed through the column at a rate of about three drops a second. One drop at a time was collected on a paper pulp disc and rapidly moved to the ?-counter b y means of a special sample changer, acting on a microswitch, which started the analyser as soon as the sample had reached counting position. It was estimated that 1--2 seconds escaped between the passage of the drop through the active layer and the start of the counting. B. Measurements and results As the total conversion coefficient of the 222.5 keV isomeric transition

THE ISOMERIC TRANSITIONS IN T~ID7 AND T119s

~0~

(~to~l -----2.26) indicated a good possibility of detecting its ),-radiation, the single channel analyser was set at 2 2 2 + 13 keV. The course of the experiment was mainly the one described for T1tg~m. Fig. 6 shows a resulting disintegration curve, where the slope of the background is ascribed to a small admixture of 3.5 sec T119~m.

c°unts/ tk 5.tO~

tO

20

3O

~0

5O 6O 7O Time in units of Pk

Fig. 6. Deca 7 curve (including 41 ~ n s ) of the 222.5 keV 7-ray in TlZgE ~k = 0.I~0~0.001 se¢.

As a mean value of several determinations T½-----0.554-0.10 sec was obtained. In additional runs the energy channel was adjusted to accept the cascade transitions (385 and 387 keV). Within the error limits the same half-life was found. Attempts to make a pulse height analysis of the 0.55 sec activity met with experimental difficulties, resulting in poor counting statistics. It can only be said that positive indications for a short-lived photopeak at 220 keV were found, b u t it was not possible to determine its half-life.

504

G. A N D E R S S O N j P . A. T o r E ,

B. J U N G AND I . B. SVENSSON

4. D i s c u s s i o n

The present investigation unambiguously confirms the E3 character of the 222.5 keV and 99.1 keV r-rays. Their mass assignment and association with the measured half-lives, however, m a y need an amplifying discussion. The 222.5 keV y-ray was shown b y Andersson et al. 3) to be in coincidence with transitions of 385 and 387 keV energy. All three y-rays were definitely converted in T1 and their strongest electron lines were found in r-spectra of mass 197 samples, electromagnetically separated. From the coincidence relation and the E3 multipolarity of the 222.5 keV v-ray it can be concluded that this y-ray is the cascade initiator. The present evidence for a 0.55 sec half-life associated with the y-energies 222 and 385 keV taken together with the similar decay period of a 384 keV y-ray found and ascribed to T1x~Tmb y Henrikson et al. form a strong support for the decay scheme of Pb x~7 shown in fig. 7.

TIt97

Pb197

811'116

h'?2

T/t95

82 ~5 o 55~

E3 0.222

M~lo2~,

f5/2 . . . . fl

MI*E, ~.385

^I,/2

.-/,2mtn d 5/2

E3,~0.099

82

e ~ , J.~sec --~

MI+Et L387

_

....

,,J

(E2

2.7h

/

,/E

, .~ s"

~

pb195

~ ~4

sT/2e~

L393 1.2h

Fig. 7. Proposed decay schemes of P b xgv and P b t°5.

The assignment of 99 keV, 393 keV, and 448 keV y-rays to the electron capture branch from Pb 195mwas originally based partly on their 17 rain halflife, which was also found in the decay curve of a mass 195 Pb sample, and partly on level systematics a). Recently, Jung et al. x3) have demonstrated the presence of a broad photopeak with the proper half-life at about 400 keV in the F-spectrum of a mass-separated Pb zgs sample. For the mass assignment of the 99 keV y-ray there is still only indirect proof. Its conversion in T1, however, is confirmed b y the present measurements (section 2.2). The 3.5 sec activity was shown b y pulse height analysis to have a photopeak at about 400 keV, which is in agreement with the assumption that this half-life is due to the 99.1 keV E3 transition and thus characteristic also of the 448 keV and 393 keV F-rays following it in cascade (fig. 7). In principle it would have been possible to determine the parent half-

T H E ISOMERIC T R A N S I T I O N S IN T1197 A N D T1195

505

lives from the T1 milkings described in sections 3.4 and 3.5. The sampling methods, however, were not sufficiently reproducible to allow any accuracy in this respect. It can only be stated, that the parent half-lives were short enough to exclude the possiblity of the 0.55 sec and 3.5 sec periods belonging to isotopes of lower elements, which could have been present in view of the bombarding energies used. Knowing the energies as well as the half-lives of the E3 transitions one can calculate the comparative lifetimes 14) as vr × A * × Er~ , where vr is the v-lifetime in sec and Er is the energy in MeV. This gives 1.9 for T119~ and 2.0 for T1195. The theoretical value 11) for a single proton transition of the hll/,--ds/2 type (statistical factor = 50/33) is 0.006. Thus the transition probabilities are reduced b y factors of 320 and 330, respectively. Such large "hindrance factors" seem to be a common feature of E3 systematics in this Z region ~). The similarity in comparative lifetimes of the 222.5 keV transition in T119~ and the 99.1 keV transition in T1195 forms a further argument for the proposed decay schemes (fig. 7), as the nuclear matrix elements for proton transitions of the same type in nuclei differing b y two neutrons should be substantially equal. We are greatly indebted to Prof. T. Svedberg for the working facilities given to us and for his continuous encouragement. Our thanks are also due to Prof. K. Siegbahn for putting the ironfree double-focusing spectrometer at our disposal and to Drs. E. Sokolowski and C. Nordling for aid in handling the instrument. Further we want to thank Dr. E. Arbman for the use of his r-spectrometer, Dr. G. Rudstam for valuable contributions in solving chemical problems, and Mr. S. Sundell for help with the electronic equipment. The investigation was financially supported b y the Swedish Atomic Energy Commission. References i) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)

J. W. Mihelich and A. de-Shalit, Phys. Rev. 93 (1954) 135 J. W. Mihelich, M. McKeown and M. Goldhaber, Phys. Rev. 96 (1954) 1450 G. Andersson, E. A r b m a n and B. Jung, Arkiv Fysik I1 (1957) 297 M. E. Rose et al., privately distributed tables A. Henrikson, S. W. Breckon and J. S. Foster, Nuclear Science Abstracts 8 (1954) 24 B 76 K. Siegbahn and K. Edvarson, Nuclear Physics I (1956) 137 G. Andersson and I. Bergstr0m, Nuclear Physics 3 (1957) 541 M. E. Rose, private communication E. A r b m a n and N. Svartholm, Arkiv Fysik I 0 (195{}) 1 E. Arbman, private communication S. A. Moszkowski in Beta- and G a m m a - r a y Spectroscopy, Ed. Kai Siegbahn (NorthHolland Publishing Co., 1955) Ch. X I I I 12) P. A. Tore, to be published; of. P. A. Tove, Nuclear I n s t r u m e n t s I (1957) 95 13) B. Jung, J. O. B u r g m a n and G. Andersson, unpublished work 14) M. Goldhaber and A. W. Sunyar in Beta- and G a m m a - r a y Spectroscopy, Ed. Kai Siegb a h n (North-Holland Publishing Co., 1955) Ch. X V I