Electron capture decay of 238Am and electric monopole transitions in 238Pu

1.E.I: I Nuclear Physics A186 (1972) 620--640; @) North-HollandPublishing Co., Amsterdam

1.E.4 [ Not to be reproduced by photoprint or microfilm without written permission from the publisher

E L E C T R O N C A P T U R E DECAY O F 2aaAnl

AND E L E C T R I C M O N O P O L E T R A N S I T I O N S IN 238pu

I. AHMAD, R. K. SJOBLOM, R. F. BARNES, F. WAGNER, Jr., and P. R. FIELDS

Chemistry Division, Argonne National Laboratory t, Argonne, lllinois 60439

Received 7 February 1972

Abstract: Mass-separated samples of 238Am (T_~_ = 98 rain) have been used to investigate the EC decay scheme of 23SAm. The y-ray and conversion electron spectra were measured with high-resolution Ge(Li) and Si(Li) spectrometers. Several electric monopole transitions were identified. The K/L ratios of the E0 transitions are in agreement with the theoretical values of Church and Weneser. A y-ray spectrum was measured in coincidence with the electron lines of the 44.1 keV transition in order to distinguish transitions going to the ground state from those terminating at the 2 + or 4 + member of the groLmd state band. From these data a decay scheme has been constructed. Assignments of levels already known fi'om the/3- decay of 23SNp and c~-decay of 242Cm have been confirmed. Several other low-spin states have been postulated to account for the observed y-rays and conversion electrons. From logf t value considerations the ground state of ~ ~8Am has been given an assignment of {n [743 ]½-; p [523 ]{-}1 + and the 962.8 keV level of 238pu has been assigned to {n [743 ]~-; n [622]~ -+ }1 -. The half-life of 2a SAm has been measured to be 98~3 rain. A 5.94 MeV c~-group has been assigned to 23SAm decay. The ~-branching has been measured to be (1.0=k0.4) × 10-* ~.

RADIOACTIVITY 238Am, [from 237Np(~¢, 3n), (SHe, 2n)]; measured T_~, E~, E~, Is, E~,, I~,, X~V- , cey-coin; deduced leg ft . 2a apt t deduced levels, y-multipolarity.

J, :z. Mass-separated 23 SAm .

1. Introduction

The v-rays associated with the electron capture decay of 238Am (T~ = 98 rain) were

first investigated by Glass eta[ . 1). Severs! high-energy v-rays were observed and ener-

gy levels at 980, 1350 and ~ t930 k e y were postulated to account for these transit ions.

However, spin-parity assignment was no t possibIe because of lack of detailed infor-

mat /on. The levels in 23SPu are also populated by the f l - decay of 23SNp and a-decay

of a42Cm. F rom conversion electron spectroscopy 2, 3) of 23aNp a level at 1030 keV

was identified as the V-vibrational state (K, U = 2, 2+). F r o m e-V and c~-ce- coin-

cidence studies levels at 605 and 941 keV were observed and they were interpreted as

the octupole-vibrat ional (K, U = 0, 1 - ) and the fi-vibrational state (K, I ~ = 0, 0+),

respectively 4). Recently, the "/-ray spectrum of 2aSNp has been measured s) with

high-resolution Ge(Li) spectrometers and new /evels at 962.7, 985.5, 1082.6 and

1202.4 keV have been deduced. Also, the energies of the octupole- and y-vibrational

Work performed under the auspices of the US Atomic Energy Commission.

620

23SAm DECAY 621

states have been more precisely measured to be 605.1 and 1028.6 keV, respectively. The 962.7 keV state has been given an assignment of K, U = 1, 1- and the 985.5 keV level has been interpreted as the 2- member of this band. The 1082.6 and 1204.4 keV states were given spin-parity assignments of 4 - and 3- , respectively and were interpreted as two-quasiparticle states formed by the singlet and triplet coupling of 7 - [743] and ½ + [631 ! neutron orbitals. However, the multipolarities of the transitions originating from the 605.1 and 962.7 keV levels have not been determined. The present paper describes our measurements of 7-rays and conversion electrons following the EC decay of 238Am.

2. Source preparation

Targets of approximately 60 nag 237Np were prepared by evaporating neptunium nitrate solution to dryness in a crucible-shaped water-cooled platinum target. During irradiation the Np was kept confined by covering the target with a 12 #m Pt foil. A stream of helium gas was passed over the surface of the foil to keep it cool. The target was irradiated with 40 MeV 4He ions in the Argonne 152 cm cyciotron at an average beam current density of 30/~A/cm 2. The irradiated Np was dissolved by heating with HNO3 and HF in a platinum crucible. After boiling offthe HF, the material was trans- ferred to a glass tube and evaporated to dryness in aqua regia. The residue was dissolved in 1 ml of 9 M HC1 and the Np was extracted four successive times with equal volumes of 0.4 M Aliquat-336 in xylene. To the aqueous phase, which contained Am, 1 mg of lanthanum carrier was added and it was precipitated as hydroxide. The pre- cipitate was dissolved in a minimum amount of H C I and evaporated to dryness. The residue was then dissolved in 3 M HN4SCN-0.01 M HzSO4 and loaded onto a 2 mm x 6 cm column containing Aliquat-336 chloride adsorbed on hydrophobic diatomaceous earth 6, 7). After loading, the elutriant was changed to 1 M NH4SCN- 0.01 M HzSO 4 which removed most of the fission products. The Am was then eluted with a 0.02 M H2SO~ solution. This solution was loaded onto a 2.8 m m x 10 cm column containing di-(2-ethylhexyl)orthophosphoric acid (HDEHP) adsorbed on hydrophobic diatomaceous earth s) and operated at 60°C. A solution of 0.1 M HC1 was passed through the column which removed N H , C N S and remaining fission products. The A m was finally eluted with 0.5 M HC1 solution. Isotopically enriched samples of 238Am were prepared in the Argonne Isotope Separator.

3. Experimental results

3.I. GAMMA-RAY SPECTRA

The 7-ray spectra of mass-separated 238Am samples measured with a 25 cm 3 Ge(Li) spectrometer are shown in figs. 1 and 2. For spectrum 1 a 220 mg/cm 2 A1 absorber was used and the sample was placed 3 em away from the detector end cap. The spectrum displayed in fig. 2 was measured by interposing a set of absorbers (0.7 g/em 2 Pb, 1.0 g/cm z Cd, 0.7 g/cm 2 Cu and 0.4 g/cm 2 A1) between the source and the

622 I. A H M A D et al.

detector. Several spectra were measured and the v-ray energies and intensities obtained from these spectra are given in table 1.

The spectrum of y-rays below 200 keV was also measured with a 2 cm 2 x 0.5 cm planar Ge(Li) spectrometer. This spectrometer had a 120/art thick beryllium window and had a resolution (FWHM) of 600 eV at 100 keV y-ray energy. A y-ray spectrum measured with the planar detector is shown in fig. 3. The intensity of the 44.1 keV y-ray was measured with respect to that of the Pu K,~ X-ray and it is included in table 1. The Au K X-rays present in the spectrum are produced by the fluorescence of the 40 #g/cm z gold electrode on the detector.

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238 ,~ 3 Fig . 1. The (50-580) keV r eg ion o f A m v - r a y s p e c t r u m m e a s u r e d w i t h a 2 5 c m G e ( L i ) spec t rome te r . The m a s s - s e p a r a t e d s a m p l e was p l a c e d 3 cm a w a y f r o m the de tec to r e n d cap. A 220 m g / c m 2 AI was

used as abso rbe r .

The y-rays were assigned to 23SAm for the following reasons: (i) the Z38Am sample was chemically purified and isotopically separated; (ii) they decayed with the charac- teristic half-life of 23SAm (T÷ = 98 min); (iii) the v-rays associated with the decay of neighboring Am isotopes are well known 9, lo). Hence, the 23~Am and 239Am v-rays could not be mistaken for 238Am y-rays; (iv) in one experiment, only Np was removed from the irradiated target leaving the fission products behind with Am. This Am was run through the Argonne Isotope Separator. The y-ray spectrum of this mass-separated 238Am source was identically the same as the spectra of other samples; and (v) the y-ray spectrum of a 238Am sample produced by 237Np(3He, 2n)23SAm

reaction did not show any variation in the y-ray intensities from those given in table 1.

2SSAm DECAY 623

3.2. E L E C T R O N SPECTROSCOPY

The internal conversion electron spectra were measured with a 80 mmZx 3 mm lithium-drifted silicon detector. The detector had a 40/~g/cm 2 gold window and could detect electrons as low as 10 keV. The detector and the input field-effect transitor

were cooled to liquid nitrogen temperature to minimize noise. The detector-preampli- fier system had a resolution (FWHM) of 1.0 keV for low-energy 7-rays, 1.2 keV

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Fig. 2. High-energy port ion of a 2a SAmy.ray spectrum measured with a 25 cm a Ge(Li) spectrometer. A set of Pb, Cd, Ca and A1 absorbers was used to reduce Pu K X-rays which would otherwise inter-

fere due to summing effect. A mass-separated aSSAm sample was used.

for low-energy electrons and 2.0 keV at 1 MeV electron energy. The e- energy was found lowered with respect to ~,-ray energy by 0.7 keV at 100 keV and 1.0 keV at 20 keV. The different response of the detector to electrons and y-rays could be accounted

624 I. A H M A D e t al.

TABLE 1

23 SAm .v-rays

Energy (keV) Relative intensity Transition

44 .1 ,0 .1 0.23=0.03 44.1 -+ 0 99 .5 ,0 .1 82 __45 Pu K~2

i03.8_4-0.1 124 , 8 Pu Kc~ , 117.1:{:0.2 46 , 3 Pu K/j., 120.6i0.2 16 , 1 Pu K/~, 2 301.4±0.2 1.8 ~0.15 962.8 -+ 661.3 324.2_--_'0.3 0 .22 ,0 .03 9 8 5 . 5 -+ 661.3 357.6,0.1 7.5 --0.5 962.8 -+ 605.1 515.4 ,0 .2 1.4 =:0.13 661.3 --> t46.0 56 i .0 ,0 .1 39.0 , 2 . 3 605.1 --> 44.1 565.8:t:-0.3 0.55±0.07 574.0±0.3 0.4 ±0.1 1560.0 -+ 985.5 597.0 ,0 .3 0 .52 ,0 .06 1560.0 -+ 962.8 605.1,0.1 27.0 =t:1.6 605.1 - + 0 6 i 7 . 4 , 0 . 2 2.6 , 0 . 2 661.3 --> 44.1 633 .0 ,0 .5 ~ 0.2 653 .3 ,0 .5 ~, 0.2 658.5 ,0 .3 0 .64 ,0 .07 1621.4 --> 962.8 679 .5 i0 .4 0 .91 ,0 .09 1621.4 -+ 941.5 821 .5 ,0 .4 1.1 , 0 . 1 1426.6 -+ 605.1 884.3 ,0 .3 0 .47 ,0 .06 897 .3 ,0 .2 2.0 ~0 .2 9%1.5 -+ 44.1 908 .8 ,0 .2 0.78--0.07 918.7~:0.1 82 , 5 962.8 --~ 44.1 935 .2 ,0 .3 ~ 0.3 941.4i0.1 8.0 , 0 . 5 985.5 --> 44.1 954.7±0.3 ~ 0.3 1560.0 -~ 605.1 962.8,0.1 100 (norm) 962.8 -+ 0 984 .0 ,0 .5 0 .41 ,0 .05 1028.5 -+ 44.1

1016.54-0.3 1.0 , 0 . 1 1621.4 --÷ 605.1 1028.5,0.4 0.4t 4_0.06 1028.5 -+ 0 1097.3:50.3 1.1 , 0 . 1 1118.2:50.3 0 .63 ,0 .07 1264.2 -+ 146.0 1130.2,0.5 0.18:50.03 1174.5 --> 44.1 1174.5,0.5 0 .15 ,0 .03 1.174.5 --> 0 1184.5,0.3 1.9 --0.16 1228.6 -+ 44.1 1220.1 ±0.3 0 .50 ,0 .06 1264.2 -+ 44.1 1226.4,0.3 0 .16 ,0 .03 1231.3=-0.3 0 .15,0 .03 1237.0,0.3 0.89:50.08 i266 .2 ,0 .3 6.0 , 0 . 4 1310.3 -+ 44.1 1293.2~0.3 1.1 , 0 . 1 1368.8±0.5 ~ 0.2 1403.2,0.3 2.4 , 0 . 2 1447.3 --> 44.1 1414.0:50.5 ~ 0.1 I458.5 --> 44.1 1447.3:50.3 1.5 , 0 . 1 14.47.3 -+ 0 1450.4±0.5 ~ 0.2 1458.5,0.3 0 .44 ,0 .05 1458.5 -> 0 1515.9±0.3 0.41 =t=0.05 1560.0 -+ 44.1 1552.2±0.3 0.26±0.04 1596.5 -+ 44.1

2SSAm D E C A Y

TABLE 1 (continued)

625

Energy (kcV) Relat ive intensi ty Transi t ion

1560.0-+0.3 0 .34±0 .05 1560.0 -+ 0 1577.3~0.3 10.3 :L0.8 1621.4 --> 44.1 1592.5±0.3 1.7 -4-0.17 1636.6 -* 44.1 1596.5-t-0.5 ~ 0.08 1596.5 ~4- 0 1607.0=0.4 0.34--0.05 1651.4 --> 44.1 1621.4=0.4 ~ 0.06 1621.4 -+ 0 1636.6~0.3 4.5 ± 0 . 4 1636.6 -+ 0 1651,4-F0.5 0 .06±0 .02 1651.4 ---> 0 1682.2~0.3 1.7 _--'0.17 1726.4 --> 44.1 1726.4±0.3 1.0 --0.1 1726.4 --> 0 1739.4±0.4 0 .10±0.02 1783.6 -~ 44.1 1761.5±0.4 0.32--0.04 1783.6--+0.4 0.21 :t_0.05 1783.6 --> 0 1789.0-+0.5 ~ 0.04 1835.1 -~0.5 0.12-+0.03

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Fig. 3, Low-energy por t ion of a 2s 8Am y-ray spect rum measured with a 2 cm 2 x 0.5 cm Ge(Li) detector. The mass-separa ted sample was placed 2 cm away f rom the detector end cap and a 350 rag/ cm 2 A1 absorber was used to reduce L X-rays reaching the detector. The A u K X-rays in the spect rum

are caused by the fluorescence of the gold electrode on the detector.

626 [. A H M A D et al.

for by an electron energy loss in the detector window. Isotopically enriched samples of Z3SAm prepared by the Argonne Isotope Separator were used for the present measurements. Several spectra were taken in order to determine the energies and intensities of electron lines accurately.

The low energy portion of an electron spectrum measured ~ 5 h after the end of irradiation is shown in fig. 4. The isotope ~a9Ana is produced in higher abundance in the irradiation and a small fraction of it appears in the mass-separated ZSSAm sample. The energies of ~39Am e- lines are known very accurately 9, ~ ) and hence, they were

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Fig, 4. Low-energy por t ion o f a a~SAm electron spectrum measure w i th a cooled Si (L i ) detector. A mass-separated sample was used and the geometry-efficiency product of the detector for the electrons in the peak was 1.0 %. The energy scale is g 0.21 keV per channel. The peaks not marked belong to

2a9Am EC decay.

used as standards for measuring the energies of low-energy transitions of Z3SAm. The higher-energy portion of the e- spectrum was measured with another 238Am sample and this is displayed in fig. 5. The e- energies were determined by calibrating the spectrometer with a reference pulser and the 661.6 K line of 13VCs. The transition energies thus measured agreed within 0.5 keV with the y-ray energies measured with the Ge(Li) spectrometer. Since the 7-ray energies are known more accurately (see table 1), the e- lines of strong transitions (561.0, 605.1,918.7, 962.8 and 1266.2 keV) were used as internal standards. The atomic electron binding energies were taken from ref. 9). The spectrometer was found to be linear over the entire energy range studied. For example, the K and L t lines of each transition did not vary more than one chan-

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s p e c t r o m e t e r . T h e geomet ry -e f f i c i ency p r o d u c t o f the d e t e c t o r fo r the e l ec t ron coun t s in the p e a k w a s

1.0 7/00. T h e e n e r g y scale is 0.422 k eV p e r channe l .

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whose corresponding 7-rays have not been observed in 7-singles spectra. The energies

of such lines are given in table 2. The transition energies were derived from K lines o n l y .

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2aSAm D E C A Y 631

The peak detection efficiency of the spectrometer was measured with calibrated sources of 2°3Hg and 137Cs and was found to be 1.0 ~o. The e - intensities normalized to the intensity of the 962.8 keV 7-ray as 100 are given in table 2. The same sample was also counted for 7-rays with a Ge(Li) spectrometer of known efficiency. A decay correction was applied to the ?-ray spectrum because the 7 and e - spectra were measured at different times. The conversion coefficients thus determined are included in table 2. Also included in the table are the theoretical conversion coefficients 12) for possible multipolarities.

TABLE 3

K / L ratios o f E0 t ransi t ions in 23Spu

Transi t ion (K/L)e~p (K/L)t~eor ") energy (keV)

658.4 4.6- '-0.2 4.4 673.4 4.64-0.5 4.4 939.0 5.1 4-0.2 4.8 941.4 4.9:k0.2 4.8

1016.2 4 . 9 ± 0 . 2 4.9 1031.3 4.84-0.5 4.9 1220.0 5.74-0.5 5.0 1228.7 5.54-0.5 5.0 1414.0 ~ 5.5 5.0 1426.6 5.64-0.7 5.0

a) The theoretical values have been taken f rom the calculations o f Church and Weneser .

In table 2 there are several transitions whose measured conversion coefficients are larger than the theoretical values 12) for M2 multipolarity. The large values of conversion coefficients establish predominantly E0 multipolarity for these transitions because a transition with multipolarity greater than 2 is highly retarded and cannot compete with allowed El, M1 or E2 transitions. Also, there are a few transitions which appear only in the electron spectra and are absent in the 7-ray spectra. Such transitions are also interpreted as E0 transitions. The experimental K/L ratios of these transitions are compared with the theoretical values of Church and Weneser 13) in taNe 3. The good agreement between the two quantities further supports the E0 na- ture of these transitions.

The energies of 23SNp 7-rays, recently measured by Bengston et al. 5) are ~ 1.5 keV lower than the transition energies derived from the conversion electron lines 3). Since we observe K, L and M e - lines of two prominent transitions with energies 939.0 and 941.5 ke¥ it was necessary to know accurate energies of e - lines in 238Np decay. For this reason the electron spectrum of a mass-separated 23SNp sample was measured with the cooled Si(Li) detector and the pertinent region of the spectrum is displayed in fig. 6. The energy of the previously observed 940.6 keV transition 3) was measured with respect to the 924.0 and 984.5 keY transitions and was found to be 938.9___0.2 keV. The 7-ray spectrum of the same sample was measured with the 25

632 I. A H M A D et al.

cm a Ge(Li) spectrometer. The y-ray energies were measured and were found to lie within 0.1 keV of the values reported by Bengtson et al. s).

104

I I ~ i { i I i i i L I i i 1 { i i i ( i I i i 1 i I i i

> ~ ~ , . 0 - _ - 2 ~ ~ /

.. ~ co -'Q5 co o~ O e 4

o . , ~ I - J ~ 5 ° I

d

l '~ I I / , t O

• . ; " . . . . . . . . . . . ." , ! / [ / t I

a

o_ I03 e~

o

i 0 z ~ J T I ? 1 F f I I I I t I I I r I r [ ~ 1 p [ I I I l 1900 1950 2000 2050 2I oo 2150

Channel number (eleclr0n energy)

Fig. 6. Conversion-electron spec t rum of 23 SNp showing the region o f interest. The spec t rum was measured with a cooled Si(Li) detector and the sample was prepared by the Isotope Separator. The geometry-efficiency product o f the detector for the electron counts in the peak was 1.0 %. The energy scale

is 0.424 keV per channel .

(3.

8

3 0 I I

2O-- E

('4

i 1D 1~

I i o - - ?~

f13 v

CO r,D . ~

_ u,-;

f " " • • • •

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E

+ C4 v

o

+1

0

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520 560 600

Channel number (a-par t ic le energy)

Fig. 7. The c~-particle spec t rum of a mass-separa ted 2SSAm sample measured 5 h after the end o f the irradiation with a 3 cmz Au-SI surface barrier detector. The sample was counted for 200 rain at a source-to-detector geometry o f 10 %. The 5.68 MeV :~-group did not decayin 12 h period and hence it

was at tr ibuted to background.

2aSAm DECAY 633

3.3. HALF-LIFE

The half-life of 238Am was determined by following the decay of the 962.8 keV y-ray. The y-ray spectra were measured with a 25 cm 3 Ge(Li) spectrometer by placing the sample at a fixed geometry. The counts in the 962.8 keV photopeak were used to determine the half-life. A least-squares fit analysis gave a half-life of 98 4- 3 min for 238Am decay.

3.4. ALPHA-PARTICLE SPECTRUM AND s-BRANCHING RATIO

The e-particle spectrum of a mass-separated 238Am sample was measured with a 3 cm 2 Au-Si surface-barrier detector. A spectrum measured at a source-to-detector geometry of 10 ~o is shown in fig. 7. The decay of the peaks was fo!iowed for a period of 12 h. The 5.68 and 6.04 MeV c~-groups d idnot die out after 12 h and hence they were attributed to background. The 5.94 MeV c~-group was assigned to 238Am for the following reasons: (i) it decayed with an approximate half-life of 23SAm; (ii) the cz- particle-spectra of z37Am and z 39Am are known i o, t 4-) and this peak does not belong

to either of them. The y-ray spectrum of the sample was counted with a Ge(Li) spectrometer. The

c~-branching ratio was calculated using an intensity of 29 photons per 100 EC decays for the 962.8 keV y-ray and was found to be (1.04-0.4) x 10 -4 %. This is the first time the c~-decay of 238Am has been observed. The hindrance factor, calculated from the spin independent theory of Preston 15), was found to be 110. This clearly shows that it is not a favored s-transition. We believe that this transition feeds either the ground state or some level very close to it.

3.5. COINCIDENCE MEASUREMENTS

A Pu K X-ray v e r s v s ?-ray coincidence experiment was performed to ascertain that the y-rays given in table 1 are associated with the EC decay of 23SAm. The 7-ray spec- t rum was measured with a 25 cm 3 Ge(Li) spectrometer and the gate X-rays were de- tected with a 4 cm 3 Ge(L]) diode. The single-channel analyser was set to include the Pu K,, peak only and the coincidence resolving time (2v) was 2#s. Absorbers were used to reduce the K X-rays incident on the 25 em 3 Ge(Li) detector. The coincidence y-ray spectrum was found to be identical to the 7-singles spectrum measured under the same conditions. This clearly indicates that all the prominent 7-rays given in table 1 follow the EC decay of 2aSAm.

An e - -? ray coincidence experiment was carried out to distinguish transitions going to the ground state from those feeding the 2 + or 4 + member of the ground state band. The electron detector was the same as used for the measurement of e - singles spectra and the y-ray detector was the 25 cm 3 Ge(Li) spectrometer. The coincidence resolving time (2v) was 2/ is and the gate included the L3, M and N electron lines of the 44. I keV transition. Since the gate also included Compton scattered photons of the Pu K X-rays, the y-rays going to the ground state were not completely eliminated from the

634 I. AHMAD et al.

y-ray spectrum measured in coincidence with the electrons. Instead, the v-rays going to the ground state (like 605.1 and 962.8 keV v-ray) were a factor of g 8 lower compared to the v-rays feeding the 2 + or 4 + member of the ground state band (like 561.0 and 918.7 keV v-ray). By comparing the v-singles spectrum with the coincidence spectrum it was found that the 515.4, 561.0, 617.4, 897.3, 918.7, 941.4, 1097.3, 1118.2, 1184.5, 1220.1, 1237.0, 1266.2, 1293.2, 1403.2, 1577.3, 1592.5, 1682.2 and 1761.5 keY transitions de-excite to the 2 + or 4 + member of the ground state band whereas the 605.1,962.8, 1636.6 and 1726.4 keV transitions go to the ground state. Other y-rays were too weak to be observed in the coincidence spectrum.

4. Discussion

4.1. DECAY SCHEME

The energy level diagram of 23Spu constructed on the basis of the present investi- gation is shown in fig. 8. The levels at 605.1,661.3, 962.8, 985.5, 941.5 and i028.6 keV were also observed in the/3- decay 2, 3) of 2aSNp and e-decay 4) of 242Cm. Our

assignments of spin-parity are identical with the previous assignments but because of additional information obtained in the present work these are discussed again. The spin-parity of a state is determined by the multipolarity of the transitions de-exciting that state to the ground state band. The two-quasiparticle character of the state is derived from a comparison of the logf t values in 238Am decay with the logf t values in the neighboring odd-A nuclei. In general, the /3- or EC transitions involving the same parent and daughter Nilsson states 16) in even-A and neighboring odd-A nuclei

have the same logf t values. The energies of the 2 + and 4 + members of the ground state band as determined

from the electron spectroscopy are 44.1 +_0.1 and 146.0 +__0.2 keV, respectively. These values are in excellent agreement with the previously measured values 5, 17) of 44.11

and 146.0 keV. The difference of 44.1 keV between two v-rays was used as an indi- cation of their originating from a common level and terminating at the 0 + and 2 + members of the ground state band. The fact that only one member of the pair was in coincidence with the 44.1 keV transition was used as a confirmation for the existence

of that level. The measured E1 multipolarity of the 962.8 and 918.7 keV transitions establishes

the spin-parity of the 962.8 keV level as 1 - . The logf t value of the EC transition to the 962.8 keV level is measured to be 6.1. In the 13- decay of 239Np [ground state 5+ (642)] the lowest value 18) of logf t observed is 6.6, whereas 239Am [ground state s - (523)]

s + [622] and 5 + [624] Nilsson states with logf t values of EC decay 9) populates the 6.0 and 5.8, respectively. Since the log J? values of Z3SAm and Z39Am EC transitions are the same, the ground states of both nuclei should involve the same proton state. The neutron configuration of the 23SAm ground state should be the same as that of 237•U with the unpaired neutron occupying the 5-[743] orbital. Hence, we assign the ground state of 23SAm to {n[743]~-; p[523]~-} 1 +. The 962.8 keV state of a38pu

23SAm DECAY 635

is given an assignment o f K, I S = 1, 1 - with the configurat ion {n [743]½- ; n [622]~+} 1 - .

This state is expected a 9) to be the lowest 1 - two-quas ipar t ic le state in 238Pu.

The level at 605.1 keV decays to the 0 + and 2 + members of the g round state band

by 605.1 and 561.0 keV E1 transi t ions. Hence, the only spin-par i ty possible for the

605.1 keV level is 1 - . The 661.3 keV level de-excites to the 2 + and 4 + members of the

{n[74317-; p[523] 5 - } i,i + 238~'m (gBrnin) A .C . > 99%

o [7431 ~/: n [622] 5/:

3 357,7

3:

( (

561.0 605.1 515.4 617.4

3.0 7,8

Yo, 8.7 V., 7.9

238pu

o

Fig. 8. Electron capture decay scheme of 238Am constructed on the basis of the present work. The numbers on the left hand side of the levels denote K, pr. The numbers on the right hand side of the

levels represent le'~el energies (keY), EC intensity ( ~ per decay) and logft values.

g r o u n d state b a n d and the mul t ipolar i t ies of these t rans i t ions are also E l . This

uniquely determines the spin-par i ty o f this state as 3 - . This is fur ther confirmed by

the pu re E2 charac ter of the 301.5 keV (962.8 ~ 661.3) t ransi t ion. The fact tha t we

observe only 1 - and 3 - members o f a band (and miss the 2 - member ) is character-

istic o f an oc tupole-v ibra t iona l band (K ~ = 0 - ) . The same assignment has also been

m a d e on the basis of c~-decay studies 4).

636 I. AHMAD et al.

A 941.4 keV v-ray was observed in the 238Np decay by Bengtson et al. 5) and it was interpreted as a 985.5 --* 44.1 transition. The 985.5 keV level was assigned to the 2 - member of the 962.8 keV band. We have also observed a 941.4_+0.2 keV transition in the v-ray spectrum of 23SAm. In addition, we have observed the K-, L-, M-electron lines of a 941.5 + 0.2 keV transition. The eK/y ratios of the (941.4 + 941.5) keV transitions in 23SNp and 23SAm decays have been measured to be 0.018 and 0.11, respec-

tively. This clearly shows that there are at least two 941.4 keV transitions involved in

(I, 2) (I, 2) (I, 21 (1,2) ( I, 2)

(0, 2 +) (0, 0 + )

(2%

( I , 2 + )

1t2- I ,I-

--821.5 El

. . . . 574.0 E 2 - M I --597.0

1783.6 0.1%, 7.9 1726.4 0.8%, 7.3 1651.4 0.1%, 8.2 1596.5 0.1%, 8.2 --1552.2

.LI515.9 1560.0 0.6%, 7.5 V 1458.5 0.2*/. 8. i

I 1426.6 0.4%; 7.9 ---1414.0 EO

I..310.3 1.8%, 7.3 -126' MI

1174.5 0.1%, 8.6

-1130.2

985.5 962.8

O, 3 - I 661.3 O, I - 605.1

0'4+ i i i 146.o

O, 2 + " ' " 44. I 0 , 0 + ~' 0

258pu

Fig. 9. A partial energy level diagram of 23sPu proposed to explain the weak y-rays and conversion- electron lines following the electron capture decay of 23SAm.

238Am decay. A 324.0 keV v-ray was observed in the decay of 238Np and it was interpreted as a 985.5 --* 661.3 transition. The ratio of the intensities of the 941.4 and 324.0 keV v-rays was found 5) to be 35. The 324.0 keV y-ray has also been observed in the 238Am decay and the ratio of the 941.4 and 324.2 keV v-ray intensities has been determined to be 364-4. This very clearly shows that all the photon intensity of the 941.4 keV transition in 238Am decay belongs to the 985.5 ~ 44.1 keV transition. This interpretation is further substantiated by the fact that the 941.4 keV y-ray is in coincidence with the 44.1 keV transition.

23SAm DECAY 637

Since all the photon intensity of the 941.4 keV transition belongs to the 985.5 44.1 keV transition, almost all the electron intensity must be associated with an E0 transition. From the present analysis it is not possible to determine whether the multipolarity of the 941.5 k e y transition is a pure E0 or predominantly E0 with some M1 and/or E2 admixture. The E0 character of this transition is further supported by the excellent agreement between the experimental and theoretical K/L ratios. We have observed a 897.3 keV 7-ray in the 7-singles spectrum, which is found to be in coincidence with the 44.1 keV transition. The multipolafity of the 897.3 keV transition is most likely F2 (from the measured conversion coefficient, E1 multipolarity cannot be ruled out).

The 941.5 and 897.3 keV transitions are interpreted as the 941.5 --+ 0 and 941.5 -~ 44.1 transitions and the 941.5 keV state is given an assignment of K, U = 0, 0 +. The 941.5 keV leveI has also been observed in the a-decay of 2¢aCm and has been interpreted as the fi-vibrational state (K, U = 0, 0+). The 941.5 e-/897.3 7-ratio, as measured in the present work, is 0.50_+0.05 which is in good agreement with the value of 0.7_+0.2 obtained from ~.-decay studies 4). The 939.0 keV E0 transition observed in the present work is interpreted as a 983.1 ~ 44.1 transition. The 983.1 keV level is assigned to the 2 + member of the fi-vibrational band. Additional support for this assignment comes from the fact that the 939.0 keV transition is ~ 8 times stronger 3) than the 941.5 k e y transition in the decay of 238Np. The ground state spin of 2aSNp is known 20) to be 2 and hence the 2 + member of the fi-vibrational band should be more populated than its 0 + member.

The multipolarities of transitions originating from the 1228.6 and 1264.2 keV levels determine the spin-parity of these states as 0 + and 2 +, respectively. From the upper limits on the K-conversion coefficients of the 1447.3 and 1403.2 keV transitions their multipolarities are deduced to be El. Hence, the only spin-parity possible for the 1447.3 keV state is 1 - . The level at 1636.6 keV decays to the 0 + and 2 + members of the ground state band by E1 transitions. Hence, it is given an assignment of 1 - . The spin-parity of the 1621.4 keV level is deduced from the E0 character of the 658.4 (1621.4 + 962.8) and 1016.2(1621.4-, 605.1) k e y transitions. However, the spin- parity assignment of 1 - is not consistent with the branching ratio of 7-rays de-exciting this level to the ground state band. The large intensity of the t577.3 keV (1621.4 -~ 44.1) 7-ray suggests that the spin-parity of the 1621.4 keV state should be 2 - . The present inconsistency can be removed by assuming that there are two 1621 keV levels with spin-parity 1- and 2 - .

The leveis in fig. 8 are proposed mainly on the basis of energy balance. Multipolari- ties of transitions originating from these levels have not been measured.

A 968 keV level was postulated by Bengtsol~ et aL s) to explain the presence of a 924.0 keV y-ray in a spectrum measured in delayed coincidence with 50-300 keV electrons. We did not observe any 924.0 keV 7-ray in 7-singles spectrum of Z3SAm. The upper limit of 0.06 ~ for the 924.0 keV 7-ray intensity gives a logf i vatue of > 9 for the EC transition to the 968 keV level. A logf t value of 9.3 was obtained for the fi-

638 I. AHMAD et al.

decay to this state. The high values of l og f t are inconsistent with the spin-parity assignment of 2 - to this state (the l o g f t values for allowed and first forbidden non- unique transitions usually range between 6 and 8).

The branching ratios o f v-rays are compared with the theoretical values calculated f rom the Alaga rule 21) in table 4. F r o m this comparison the value o f the K quantum

number of 1- states can be deduced. However, such analysis is not conclusive since Alaga rules are often violated 22).

TABLE 4

Branching ratios of reduced E1 transition probabilities

Transition Exp. ratio Theor. ratio

K ~r = 0- K 7r = 1-

605.1 -+ 0 0.55±0.05 0.5 2.0

605.1 -+ 44.1 661.3 --~ 44.1

1.08±0.14 0.75 1.33 661.3 --~ 146.0 962.8 ~ 0

1.06j=0.08 0.5 2.0 962.8 ~ 44.1

1447.3 -+ 0 0,57±0,05 0.5 2.0

1447.3 -~ 44.1 1636.6 --~ 0

2.4 ±0.25 0.5 2.0 1636.6 -~ 44.1 1726.4 -> 0

0.544-0.06 0.5 2.0 1726.4 ~ 44.1 1560.0 --~ 0

0.75~0.12 0.5 2.0 1560.0 -> 44.1

4.2. ELECTRON CAPTURE TRANSITION PROBABILITIES

The EC intensities (in ~o per decay) to various levels were determined in the following way. The EC intensity (in relative units) to each level was obtained by subtracting the intensities of v-rays and conversion electrons populating that level f rom the intensities o f the 7 and e - transitions de-exciting that state. The EC intensity was converted to K X-rays intensity with the theoretical value 23) o f K/(total) capture rat io and a correction for the fluorescent yield. The sum of K X-rays intensities populat ing all the excited states o f 23Spu was subtracted f rom the K X-rays intensity experi-

mentally measured (the K X-rays originating flora i n t e r n a l conversion were already removed from the experimental intensity). The excess K X-rays intensity gave the EC intensity to the ground state. The EC intensity (in % per EC decay) was then calculated by normalizing the total EC intensity to 100 ~ . The intensity o f the 962.8 keV v-ray was found to be 29__2 photons per 100 EC decays. The log J? values were calculated using a value of 2.3 MeV for the EC decay energy 18).

238Am DECAY 639

4.3. ELECTRIC MONOPOLE TRANSITIONS

In the present work several E0 transit ions have been identified. As shown in table 3

the K / L ratios are in excellent agreement with the theoretical values calculated by

Church and Weneser ,3). The levels at 941.5 and 1228.6 keV which are given K, U =

0, 0 + assignments de-excite to the 0 + and 2 + members of the ground state band by

E0 and E2 transit ions, respectively. The ratio of the observed e - and 7-ray intensities

was converted to the dimensionless quant i ty X = B(E0, 0 + ~ 0+)/B(E2, 0 + --* 2 +)

with the formula 24)

X = 2.54x 10 9 x (A)~xE~ x [~:*-(0+ --* 0+)

z,(0 + 2 +)

In the above equat ion A is the mass number of the nuclide, E~ is the energy of the E2

t ransi t ion in MeV, and IKo and I v are the K-convers ion electron and 7-ray intensities.

The term f2 is the electronic factor and was obtained by interpolat ion from the graph

given in ref. 13). The experimental values of X for the 941.5 and 1228.6 keV states were

found to be 0.24 and 0.13, respectively. These values are in good agreement with the

theoretical value of 0.23 calculated by Rasmussen 25) on the basis of a uniformly

charged spheroidal nucleus.

The authors wish to express their thanks to J. Lerner for the Isotope Separator

prepara t ion of the 23 SAm samples and the cyclotron crew for many irradiations. They

also acknowledge helpful discussions with E. P. Horwitz regarding chemical separa-

t ion procedures.

References

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Electron capture decay of 238Am and electric monopole transitions in 238Pu

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