194Au: Half-life and emission probabilities

Appl Rodrat Isot Vol 43, No 5, pp 6477649, 1992 Radrat Appl Instrum Part A Prmted I” Great Bntam All rights reserved

0883-2889/92

Int J

194A~: Half-life

and Emission

B. SINGH’ ’ Tandem

$5 00 + 0 00

Copynght 0 1992 Pergamon Press plc

Probabilities

and H. W. TAYLOR*

Accelerator Laboratory, McMaster Umverstty, Hamilton, Ontario, Canada 2Phystcs Department, Umverstty of Toronto, Toronto, Ontarto, Canada

and

(Recewed 16 Aprrl 1991, m rewed form 23 August 1991) The half-life of lq4Au decay has been measured using y-ray spectroscopy. The electron-capture (6) and fi + branchmg rattos to the ground state of ‘94Pt and the photon emtsston probabrhty of the 328 keV y-ray

have been deduced from a measurement of the total intensities of annihilation radiation and Pt K-x-rays

Introduction In a recent revtew of A = 194 nuclides (Singh, 1989) it was noted that the half-hfe of ‘94A~ had been measured with hmited precision some 40 years ago using /I+ spectra (Wdkmson, 1949; Steffen et al., 1949). It was felt that a more precise result could be obtained through y-ray spectroscopic procedures In addition it was also noted that measurements of the ratio I!+ (total)/1)(328 y) reported in the literature (HornshoJ et al., 1981; Thieme and Bleuler, 1956) differed by about 30%. Since a knowledge of this ratio is crucial to the determination of the electroncapture (c) and /I + branches to the g s of ‘94Pt and, therefore, the photon emtsston probabihties (Intensity per 100 decays of ‘94A~), a remeasurement of this ratio was considered necessary The determmation of the decay branch to the g s. of ‘94Pt also utihzes the L//I + ratios for the two p + transittons m the decay of ‘94A~ (g s J” = l-), one to the g.s. and the other to the first 2’ state of 19’Pt. It IS a general practice to use the theoretical values of the c/p+ ratio (Gove and Martin, 1971) which are calculable for allowed transittons only Through a measurement of the total Pt K-x-ray mtensity we have checked the validity of using the theoretical c/p+ ratios m the present case, where both transttions are first-forbidden Prehmtnary results of this work were reported m the survey of A = 194 nuchdes (Smgh, 1989)

Experimental The 194A~ source was produced through the ‘95Pt (p, 2n) reaction using enriched (-95%) “‘Pt metalhc foil as a target. The 2 h irradiation with 16 MeV protons was performed using the McMaster Tandem van de Graaff accelerator. Countmg was started 3 days after the irradiation to permit all short-lived activities to substantially decay from the source material. Gamma-ray spectra revealed the mam im-

trace impurity to be 186days ‘95A~; additional purities identified were 17.6 h ‘93A~, 4.3 days 193Pt, 6 2 days ‘96A~ and 2.7 days 19’Au formed through competing reacttons. The presence of these impurities, however, m no way hindered the spectral analysis of the y-ray peaks of interest. The measurement of y-rays, annihilation radiation and Pt K-x-rays were carried out using an mtnnsic Ge detector with a resolution of 2.3 keV at 1332 keV and an efficiency of 10% The source-to-detector distance was about 12cm; no summing effects were observed. The relative efficiency curve for the detector over the energy range 60-600 keV was obtained in the conventional manner using several standard radionuchdes including 13’Ba and s6Co For the measurement of the intensity of anmhilation radiation, the source was sandwiched between two pieces of lucite, each 1 cm thick to allow for complete annihilation of the positrons emitted by ‘94A~. For half-life and K-x-ray measurements, no absorbers were used The half-life measurement was made by momtormg the intensity of the 328 keV y-ray as a function of elapsed time over a total countmg period of about 170 h. These data were also normalized to the intensity of the 662 keV y-ray emitted by a 13’Cs source placed in the vicmtty to serve as a standard. The total mtenstty of the annihilation radiation from the source was measured relative to the intensities of the netghbourmg 482 keV and 528 + 529 keV y-rays Instead of the mtensity of the 328 keV y-ray so that the efficiency correcttons for the geometry used m this experiment were small (- 5%) and the intensity ratios could be determined more accurately.

Results and Discussion Figure 1 shows a semi-log plot of 328 keV y-ray from 19’Au. A the data points yields a half-hfe compartson with previous values

of the decay curve least-squares fit to of 38.02(10) h. In given in Table 1,

B SINGH and

648

2 m

R&o “\

0.50.

‘\

I, (to 2; )/I, (328 : ) I,_ (to g s ):J,(328 :,)

‘\

o.zo-

8 2 Y 2 ,$

Value

I/,_ (to 2,’ )//(32X ,‘)

z E 3

TAYLOR Table 2 Reldtwe 6 and 8’ branches deduced from experimental data

I .oo ,-

Y

H W

O.IOy

/,(to gs ):I (328 >)

*\ 0.05-

9

0.02 0

I50

100

50 TIME

(h)

Fig I Decay curve for 328 keV 7 -ray from ““AU The sohd hne represents the least-squares fitttng of the data pomts to the exponential function The uncertamtles on the data pomts he wlthm the drawn circles

our value IS more precise and probably more rehable because of the absence of any contammatlon m the 328 keV y-ray peak. The measured ratios of the mtensltles of anmhllatlon radiation and nelghbourmg y-rays are given m Table 1 The observed intensity of the anmhllatlon radiation was reduced by 5% to account for the presence of a long-hved component (background) suggested by the half-hfe curve for the 511 keV lme The intensity was corrected also by 2 0 (6)% for the contrlbutlon to anmhllatlon radiation from pau production through high energy ;J-rays This correction was estimated from y-ray spectra obtained m the same geometry with a 226Ra source which emlts y-rays of comparable (to ‘94A~) energies and mtenslties and has no positron emlsslon No contrlbutlon to the 511 keV line 1s expected from any of the lmpurlties present m the source material From the two ratios, the average I,, (total)/l, (328 7) IS deduced and given m Table 1 together with previously measured values Our value agrees with that from Thleme and Bleuler (1956) who used positron and conversion electron spectra, but differs by about 30% from the value given by HornshoJ ef al (1981) who used ;‘-ray spectroscopy and determined I(? ‘),‘I, (328 y) The reason for the 30% discrepancy 1s not obvious except

“From I, + p_ (to 2+):1,(328~)=0 48(2) [from the mten\ity balance tn the decay scheme (Smgh, 1989) and <;/j+ (theory)=47 I] hEstlmated by us from y * - 328 i’ comcldence spectrum shown by Benson e! al (1970) ‘From p+ and conversion electron d&d (Thleme and Bleuler 1956) “From [I, +,$+(to Z,i),‘I,(328y)]~[/,_(to 2; ):/,(328~)] ‘From [I,_(total):I,(328;)]~[Ip_(to 2:):1,(328 ;)I ‘From4_ (to g s ):I, (328 7) and ( :fl+ (theory) = 20 4

that there may be efficiency correction problems between 511 keV and 328 keV lines m the geometry used by HornshoJ et al (1981) Table 2 contams the /I+ and c branches to the g.s. and first 2+ state m ‘94Pt as deduced from the anmhllatlon radiation data, all values are relative to the photon intensity of the 328 keV y-ray In Table 3, the emlsslon probablhtles are given together with values available from the literature The emlsslon probablhty of 62 (3)% for the 328 keV y-ray as given m column 1 of Table 3 IS derived from I,, 8+ (to g s ),;1,(328~)=041 (4) (refer to Table 2) and I, _ ,,_ (to excited states)/1,(328 y)= I 23 (6) from the level scheme shown by Smgh (1989) The photon emlsslon probablhtles of all the other p-rays from ly4Au listed by Smgh (1989) can be obtained by use of a multlphcatlve factor of 0 61 (3) Our total Ill+ agrees with that from Thleme and Bleuler (1956) but the dlvlslon of intensity to the g s and the 2,+ state IS about 20% different which produces very different values for the t branchmgs to these states The measured intensity of the Pt K-x-rays, relative to the photon intensity of the 328 keV y-ray 1s 1 45 (15) as given m the last row m Table 1 About 20% of the observed Pt K-x-ray intensity belonging to

Table I Summdrv of exper~mentdl data for “‘AU Measurement

Present work

T, 2

3802(lO)h

I(y ‘)/I (482 , ) I(y ‘)/I:(528 ? + 529 Ip+ (totdl):f,(328 ;) I(Pt K-x-rdv):l

,)

(32X ;)

294(10)” I 51 (5)” 0 029 (2)h

0 OlOO(5) 00103(10)~ 0013 0 47(2)d 0 019 (2)’ 0016‘ 0 39 (4)’

Prrwous work 79 5 (5) h (Wllklnwn 1949) 3’) (2) h (St~llrn e,
0022(l) (Hornshoj t’f ul. 1981) (I 029 (Thxmr and Bleuler 1956)

I45(15)’

> *. Anmhllatlon rddlatmn (51 I keV) “Intensity corrected for 590 Lontrlbutmn from room background dnd 2% from dnmhllatlon radlatlon due to pd,r production of htyh energy ;‘-rdya (see text) bAVUX&. value deduced from I(;,-) given m two lows above and I(482 ,‘)/I (328 ~)=00185(10), I(528 jl + 529;);/ (328 ;)=O 037(2) (Benson el al, 1970, Heath, 1974) Correctmn for anmhlldtmn of positrons ,n flight IS estimated as 5% (Azeluor and Kltchmg. 1976) ‘Contrlbutlon from ~mpurlty Isotopes hsted m the text 15allmated as 20% and ha? been subtracted

lg4Au half-hfe Table 3 Emlssmn I. (328 i’) Present work Hornshoj er a/ (1981) Thxme and Bleuler (1956)

61 (3) 67” 64a

and emlsslon

probablhtles I,+ (to .a s )

I

16(12) 0 82 I 026

probablhtles

(per 100 decays of ‘%Au) I.(to .as) 24 (3) 17 21”

I,+(to

2:)

0 61 (6) 0 68 0 83’

L@o 2:) 29 (3) 32 39”

‘We have deduced these values from data given m the quoted references Uncertamtles are estrmated as 5% m Hornsho] et al (1981) and 10% m Thleme and Bleuler (1956)

Impurity radtolsotopes was subtracted The calculated Pt K-x-ray mtenstty relative to the 328 keV y-ray, based on the t and /?+ branches from the present work and the level scheme shown by Smgh (1989) 1s 1.31 (7) The predicted value mcludes a contrtbutton from internal conversion and assumes e(K)/t = 0 78 and fluorescence yield (wk ) = 0 96 External converston and self-absorption effects were constdered neghgrble. The agreement between the two values indicates the correctness of the deduced c and p+ branchmgs as well as the vahdtty of using the theoretical values of e//I + for the two first-forbidden p+ transtttons We have also estimated from the measured Pt K-x-ray intensity ratio, the experimental c/P+ ratto for the transttron to the g s as 30 (10) which IS consistent with the theorettcal value of 20 4 for an allowed transttton The experimental t//I+ ratio for the transrtton to the first 2; state IS 47 (5) as estimated from the y + - 328 ;J comcldence spectrum (Benson et al, 1970) and the intensity balance at the 2: state m the decay scheme (refer to Table 2 for details). Thts result IS m good agreement with the theoretrcal value of 47 1 for an allowed transition

Conclusion The half-life of ‘94Au has been determined with an improved prectsron using 7 -ray spectroscopic tech-

mques The absolute 6 and /I+ branches (per 100 decays of ‘94A~) to the g s. of ‘94Pt have been deduced from a measurement of the anmhrlatton radiation and the photon emrsston probabilities have been determmed From the present experimental data and level scheme constderatlons, theoretical c/p + values for allowed transttrons are found to be valid for the two first forbidden transttrons of ‘94A~ Acknowledgemenf-We wish to thank Mr Jim Stark for his help m the operation of the accelerator and the lrradlatlon procedures

References Azeluos G and Kltchmg J E (1976) At Nucl. Data Tables 17, 103 Benson G D , Ramayya A V , Albridge R G and O’Kelly G D (1970) Nucl Phys A150, 311 Gove N B and Martm M J (1971) Nucl Dara TablesAIO, 205 Heath R L (1974) ANCR-1000-2 Gamma-Ray Spectrum Catalogue HornshoJ P , Nielsen H L , Rud N and Ravn H L (1981) Nucl Insrrum Methods 186, 257 Smgh B (1989) Nucl Data Sheets 56, 75 Steffen R M , Huber 0 and Humbel F (1949) Helu Pbys Acra 22, 167 Thleme M T and Bleuler E (1956) Phys Rea 102, 195 Wllkmson G (1949) Phys Rev 75, 1019