Nuclear Physics All9 (1968)27--39; (~) North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

E N E R G Y L E V E L S O F 2aTu

IRSHAD A H M A D , A. M. FRIEDMAN and J. P. U N I K

Chemistry Division, Argonne National Laboratory *, Argonne, Illinois 60439

Received 12 July 1968

Abstract: The alpha decay of 2~lpu has been investigated using high-resolution semiconductor detec- tors in conjunction with coincidence techniques. The observed c~-particle and ?-ray transition energies and intensities were found to be consistent with the previously known energy levels of 23~U. The half-life of the ~+(622~) state (at 160.0 keV) in 237U has been measured by an ~-? delayed coincidence technique and found to be 3.1 zk0.1 nsec. The half-life of the 274-keV level, which de-excites by emission of a 114 zk 1 keV 7-ray has also been measured by an ~-7 delayed coincidence method and has a value of 155 ±6 nsec. Three alpha groups populating the 274 keV state and its rotational members were identified in an ~-spectrum measured in coincidence with ?-rays emitted 50 to 200 nsec after the emission of s-particles. By analogy with the energy levels of 2agPu, the 274 keV level has been given a Nilsson-state assignment of ~-(743~').

E I RADIOACTIVlTY241pu;measuredE~'I~'E~'l"77"7~'~7"c°in' [ ~7-delay, ~/fl ratio. 2aTU deduced levels, J, 3r, T].

1. In troduc t ion

The a lpha decay of 241pu has recent ly been invest igated by Baranov et al. 1) with a

h igh-resolut ion magnet ic spect rograph. In their work, mos t of the e-par t ic le decay

was found to popu la te the ro ta t iona l members of the ~2+(622t) and ½+(631+) bands in 237U. Recently, Bra id et al. 2) have s tudied the energy levels of 2aTU by means o f

the 236U(d, p)237U react ion. Due to lack of sufficient in format ion , these au thors

were no t able to make conclusive Ni lsson-s ta te 3) ass ignments to the newly observed

states. A decay scheme represent ing a compos i te of the results of Baranov et al. and

Braid et al. is shown in fig. 1. The purpose of the present invest igat ion was to identify

a n d elucidate the proper t ies of the excited states of 2 37 U weakly popu la t ed in the

~-decay o f 241pu by use o f coincidence studies.

2 . E x p e r i m e n t a l m e t h o d s 2.1. SOURCE

A p l u t o n i u m sample ( ~ 1 rag) conta in ing 99.9 % 241pu by a lpha act ivi ty was ob-

ta ined f rom the O a k Ridge Na t iona l Labora to ry . This sample was chemical ly pur i -

fied before each exper iment and was used for all spectral measurements . The Pu in the IV ox ida t ion state was abso rbed on an anion-exchange column, and the 241Am

daughte r was eluted with 10 M H C I . The Pu was then eluted with a 10M HC1 solut ion

t Work performed under the auspices of the U.S. Atomic Energy Commission.

27

28 I. AHMAD et al.

containing 1 ~o HI (HI was used to reduce Pu to III oxidation state), while the 237U daughter remained absorbed on the column. Thin sources on glass or platinum plates were made by dissolving the Pu in 1 MHNO3 (containing ~ 5 ~ tetraethylene glycol) and igniting the plate after drying.

[NnzAY~ ] KITr Energy (keV)

698 657

552

482

l~~ 321

-~4 258

-~4 202

5+~4 82 57

2 ~4 1~.7 [63, , ] ' '+ 2

237 U

Fig. 1. Energy level diagram of =87U proposed in refs. a, =).

2.2. COINCIDENCE APPARATUS

For delayed coincidence experiments and lifetime measurements, a two-parameter analyser and associated circuitry were used. Alpha particles were detected with a silicon surface-barrier detector and the photons with a NaI(T1) detector. Fast timing infor- mation was obtained from the leading edge of the detector pulses using transformer coupled time-pick-off units. The outputs of the alpha and NaI detector units were used as start and stop pulses, respectively, for a time-to-amplitude converter. The pulses from each detector were also branched to slow linear systems consisting of pre- amplifiers, bipolar amplifiers and zero cross-over timing single-channel analysers. The resultant pulse-heights from the two linear systems were digitized using analog- to-digital converters and stored in a 4096-channel two-parameter analyser. The out- put of a threefold coincidence circuit was used to gate the two-parameter analyser.

3. Experimental results 3.1. GAMMA-RAY SPECTRA

The ?-ray spectra of 241pu samples were studied with various lithium-drifted ger- manium detectors. About 0.2 mg of purified 241pu was spread on a glass plate and

laVU ENERGY LEVELS 29

was used for y-singles spectra. The spectrum measured with a 2 cm a Ge(Li) detector for 30 min, immediately after chemical purification is shown in fig. 2. The best values

I I I + I ~ q I I I I I I

UK,~ ~-UKol A J~ 103.5

t o ' f i l l / rUK#~ 148.5

KB e

:,-' ,6io 164.5 ( 23"tU )

%. ' . o

IOC 0 - : - - , , _ , , , , 4o 80 120 160 200 24o 2so 320 360 4oo

Chonnel number ()'-roy energy)

Fig. 2. The ~lPuT-ray spectrum taken with a 2 cm ~ x 1 cm Ge(Li) detector immediately after chem- ical purification. The sample was counted for 30 rain.

1000 I I I I I I I

=~ I00

o

=o 10 o

59.6 (241.&m) (114+-I) keV U L x-roys ,~

:'. 44 ¢o ,i : . UKctajKatO v

•. ." . .~. -~.",. io3.5 ." "

" " ' " -" . , , _ 4 , : ' .

".', •; ". • .. ,°;

148.5

,,m

• . . . -

• . . . . . . ~ . ° o • . . aw

I I I I I "1 " • I

40 80 120 160 200 240 280 Channel number ( ) ' - roy energy)

Fig. 3. The ~41Pu },-ray spectrum measured in coincidence with (80-130)" keY photons . The resolving t ime 2T of the coincidence circuit was 50 nsec.

of y-ray energies and intensities, obtained from this work are given in table 1. The absolute intensities of y-rays were obtained by measuring the y-ray spectrum of a thin 2*lpu source ( ~ 10 #g) of known disintegration rate.

30 I. AHMAD et al.

A y-y coincidence experiment was performed with a view to identifying any state which receives sufficient alpha population and de-excites through the 160 keV level (fig. 9). A 7 cm 3 Ge(Li) detector was used to measure the y-ray spectrum and a 5.1 cm x 1 cm NaI(TI) crystal was used to detect the gate signals. The y-ray spectrum taken in coincidence with (80-130) keV photons (mainly U K X-rays associated with the highly converted 148.5 keV transition depopulating the 160 keV state) using a coincidence circuit with resolving time 2z = 50 nsec is shown in fig. 3. An energy 114+ 1 keV y-ray was observed in the coincidence spectrum. The U K X-rays present in fig. 3 are partly due to the fact that the gate included the 114 keV y-ray and partly due to chance coincidences.

TABLE 1

m P u y-rays

Energy Intensity (keV) (photons/100 241Pu ~-decays) Transition

44.710.3 0.174-0.03 56.5 --~ 11.5 +

204 --~ 160.0 56.64-0.2 0.11:~0.03 56.5 ~ 0 76.94-0.2 0.734-0.07 160.0 --~ 83.2 94.64-0.2 12.7 4-1.2 UK~z 98.4-+-0.2 20.8 4-2.0 UK~I

103.54-0.2 4.5 4-0.5 160.0 ~ 56.5 111.04-0.2 7.7 4-0.7 UK~a 114.04-1.0 0.254-0.05 274 ~ 160.0 114.54-0.2 3.3 4-0.3 UK~2 120.74-0.5 0.054-0.02 204 ~ 83.2 148.54-0.2 9.0 ~0 .9 160.0 ~ 11.5 160.04-0.2 0.324-0.03 160.0 ~ 0

Attempts were also made to observe the gamma transitions de-exciting the high- lying states of 2 aTU. About 0.8 g of purified Pu containing 94 ~o 241pu by alpha activity was used for this purpose. The high-energy y-rays of 241Am, which grew in the Pu sample, obscured the region of interest, and hence the weak spectral lines of 241pu could not be identified. Only two new y-rays of energy 641_+2 and 687_+2 keV were observed in the y-singles spectrum, and most likely these transitions are associated with the 2 4 1 p u e-decay.

3.2. LIFETIME MEASUREMENTS

The half-life of the 160 keV state was measured with the coincidence apparatus. The coincident gamma rays were recorded as one parameter, and the output of the TAC (time difference between the detection of an alpha particle and a gamma ray) was recorded as the second parameter in a 32 x 128 channel matrix, respectively. The delayed coincidence time spectra of the 148.5 keV y-ray depopulating the 160 keV excited state and the K X-rays (essentially all due to internal conversion of the 148.5-

105

~S~'U ENERGY LEVELS 31

1 0 4

m

g

I03

IO z

i01 - I0 0 I0 20 30 40 50 60

Relative delay (nsec)

Fig. 4. Delayed coincidence time spectra of the U K X-rays and 160.0 keV y-ray measured by the ~-F coincidence method.

IO s

lO 5

I0 4

I0 3

I0 z

101

r i I i I ' I i I ' I '

Hal f - l i fe measurement of the 274-keV excited state of 257U

Decoy curve of 114-keV gamma ray

TI/2 =(0.155 -+ O.O06)Fsec

I i I i I ~ r r l , = = l 0 0.2 0.4 0.6 0.8 1.0

Relative delay (/~sec)

Fig. 5. Delayed coincidence time spectrum of the 114 keV y-ray measured in coincidence with 4.60-4.85 MeV ~-particles.

3 2 I . A H M A D et al.

keV transition) are shown in fig. 4. Both of these time spectra have been corrected for chance coincidences and a very small contribution due to a coincident longer-lived gamma ray. A weighted least-squares fit to the exponential decay of these spectra yielded a half-life for the 160 keV excited state of 3.1 _+0.1 nsec. A 23°Th ct to 68 keV gamma ray coincident time spectrum is also shown in fig. 4 as an example of the prompt coincidence spectrum (a measure of the electronic time resolution). The half- life of the 68 keV transition in the decay of 23OTh has previously been measured 4) as (6.3__+0.2) x 10 -1° sec.

114

6 0 0

5OO "E

ID

~ 40o C G)

" ~ 3 O 0

200

700

!

J oo

0 I I I I t I 1 I f I I - - T " I I 40 60 80 I00 120 140 160 180 200

T-roy energy (keV)

Fig. 6. The y-ray spectrum measured from 0.2 to 0.4 /~sec after the ~-particle emission.

For the measurement of the half-life of the 274 keV excited state, the timing pulses were obtained from the zero cross-over point of the bipolar amplifier signals using the timing single-channel analysers. The single-channel analyser on the a-particle spectrum was set to accept alpha pulses only between 4.60 and 4.85 MeV. The de- layed coincidence spectrum of the 114 keV gamma ray, which depopulates the 274 keV excited state, is shown in fig. 5. A weighted least-squares fit to the long-lived component yielded a half-life of 155 +__ 6 nsec for the 274 keV excited state. The spec- trum of v-rays emitted from 0.2 to 0.4 #sec after the emission of alpha particles is shown in fig. 6. The fact that the 148.5 keV ~-ray and U K X-rays arising from its

2S'7U ENERGY LEVELS 33

internal conversion are also found in the delayed ?-ray spectrunfdemonstrates that the 114 keV transition populates the 160 keV state.

3.3. A L P H A - P A R T I C L E SPECTRA

Various =-particle spectra of chemically purified 24tpu samples were measured at low geometries to determine the energies and intensities of the alpha groups. A typical c~-singles spectrum taken with a cooled 6 mm diam. Au-Si surface-barrier detector at a geometry of 0.2 % is shown in fig. 7. The c~-particle energies were mea-

10 4

10 3

,.$

o o $ Q.

102

I01

I I I

4.85

47f97

.

• ,

I I I -4.896 MeV

E 4.972 5.042

~14 I?.5-05~ ° ~ 99 ' 241pu

240p~ a

i00 I I I I I i00 140 180 220 260 500

• • o

I . - "--Zero events-

340 380

Ch0nnel number (e-particle energy)

Fig. 7. The 2~xPu co-particle spectrum measured with a cooled surface-barrier detector at a detector geometry of 0.2 %.

sured with respect to that of 24°pu c~0 group which was taken 5) as 5.1675 MeV. The final values of the energies and abundances obtained in the present work are given in table 2. The hindrance factors listed in the table were calculated from the simple bar- rier-penetration theory of Preston 6). For comparison, the ~-particle energies and intensities measured by Baranov et aL 1) are also included in the table.

In order to observe the alpha groups populating the 274 keV state (see fig. 9) and its rotational members, c~-particle spectra were measured in coincidence with delayed 7-rays. The co-particle spectrum measured in coincidence with (80-130) keV photons, emitted 50 to 200 nsec after the ~-particle emission is shown in fig. 8. The cq60 and C~2o 4 groups present in the spectrum are due to chance coincidences. In the spectrum,

34

I000

I . AHMAD e t aL

l l I I J

Random peaks

4.8~6a,eo ~

g 4z[2

g

.g

o I0

: ""

I00 120 140 160 180 200 Channel number (a-particle energy)

Fig. 8. The m P u u-particle spectrum measured in coincidence with 80-130 keV y-rays emitted 50 to 200 nsec after u-particle emission.

TAeI~ 2 u:Pu ~-groups

u-particle energy (MeV) Intensity

present Baranov excited-state present Baranov hindrance work s) et aL 1) energy b) work et aL 1) factor d)

(keY)

5.055-4-0.005 5.051 0 I 0.35 3200 5.042±0.003 5.041 12 j 1.5 -4-0.1 1.02 900

4.9994-0.004 4.995 56 0.414-0.05 0.36 1270 4.9724-0.003 4.971 83 1.3 4-4-0.1 1.12 260 4.8964-0.003 4.896 160 83.2 4-4-0.5 83.5 1.26 4.8534-0.003 4.853 204 12.1 ±0.2 12.3 4.3 4.7974-0.003 4.798 261 1.4 4-0.1 e) 1.18 15 4.7834-0.005 275 0.2 -4-0.1 85 4.7424-0.005 316 ~ 0.07 ~ 120

~4.732 -~325 ~ 0.03 ~300 4.6924-0.006 367 ~ 0.03 ~ 120

s) The u-particle energies are measured with respect to u°Pu ~e group (5.1675 MeV). b) The excited state energies are calculated with respect to the 160 keV level which is populated by the 4.896 MeV u-group. e) The intensity of the 4.797 MeV u-group represents the sum of the uzex and %74 intensities. d) The hindrance factors are calculated from Preston's equations using 5.73 × 10 s y for the half-life of 2~IPu u-decay. The abundances of u 0 and %9 groups were obtained by dividing their sum intensity (1.5 %) in proportion to the intensities measured by Baranov et al.

2~7U ENERGY LEVELS 35

the peaks at 4.783, 4.742 and 4.692 MeV are clearly identified. The energies, intensi-

ties and hindrance factors of these alpha groups are included in table 2.

3.4. ALPHA-TO-BETA-BRANCHING RATIO

The ~-spectrum of a 241pu sample was measured immediately after chemical puri-

fication with a high-resolution semiconductor detector. The sample was counted again after a week at the same geometry (0.2 ~) . From the growth of 24tAm in the sample and using a half-life 7) of 432.7___0.7 y for 241Am, the alpha-to-beta-branching ratio was found to be (2.45-t-0.08)x 10 -5. This value is in agreement with the previously reported value of (2.31 _ 0 . 1 ) x 10 -5 by Smith 8). In ref. a), the branching ratio was calculated with a half-life of 358 y for 241Am.

4. Discussion

4.1. SPIN AND NILSSON-STATE ASSIGNMENTS

The nuclear spin of the 24~pu ground state has been measured 9) by paramagnetic

resonance and found to be [. The most reasonable Nilsson a) state assignment for this state is [+(6221"), which is also the ground state ~0) of 24aCm. The favoured alpha transition of 241pu has been found to populate the 160 keV state of 237U. The

160 keV level should, therefore, have the same Nilsson quantum numbers as the ground state of 241pu, namely ~+ (6221"). The levels at 204 and 261 keV appear to be the rota- tional members of this band. The excited state energy E I of a rotational member can be calculated from the equation 11)

h 2 E1 = Eo + ~ [I (I-t- 1)-t-tSK, ~ a(-)(I+*)(1-1-½)1, (1)

where E o is constant and J the nuclear moment of inertia. The decoupling parameter a has a non-zero value only for K = ½ bands. The calculated energies of the 1 --- and ~ members using eq. (1) given in table 3 are in good agreement with the observed quantities.

TABLE 3 Calculated ct intensity to the favoured band

Spin I Excited state energy Observed (keV) Calculated ~-intensities b) ~ intensity

calc. a) observed L = 0 L = 2 L = 4

160.0 160.0 68.0 15.2 0.02 83.2 83.2 (norm) (norm) 204 (norm) 204 10.1 0.06 10.2 12.1 261 261 1.4 0.06 1.5 1.2

a) The rotational constant for this band is found to be 6.29 keV. b) The abundances are calculated from eq. (2) with values of (I-IF/.)e_ e as 1, 1.6 and 96 for L = 0, 2 and 4 alpha waves, respectively.

36 I. Ame~AD et aL

The energy spacings between the lowest four levels of 2 37 U indicate that these levels belong to a K = ½ band. The K = ½ assignment is consistent with the l~eta-decay 12) properties o f 237U. The Nilsson quan tum numbers for this state are expected to be the same as for the 239pu g round state ½+(6315). The rotat ional energies o f these levels calculated f rom eq. (1) for a K = ½ band given in table 4 are in excellent agree- ment with the observed energies. The value of the decoupling parameter is found to be - 0 . 4 0 .

TABLE 4 Calculated ~-intensities to the members of the K n = ½+ band of 287U

Spin I Excited state energy Observed (keV) Calculated intensity intensity

calc. a) observed L = 2 L = 4 32

½ 0 (norm) 11.5 (norm) 56.5 (norm)

½ 83.3 164.4

-~ 251.4

0 0.38 (norm) 0.38 0.38 11.5 0.36 0.76 (norm) 1.12 1.12 56.5 0.10 0.31 (norm) 0.41 0.41 83.2 0.02 0.57 0.59 b) 1.3

a) The values of the rotational constant h2/2J and decoupling parameter a are found to be 6.42 and --0.403, respectively. b) The calculated intensity does not include the contribution from the L = 6 alpha wave.

In the present work, the half-life of the 160 keV state has been measured to be 3.1 nsec. After making corrections for branching ratios and internal conversion, the partial half-lives o f the 160 and 148.5 keV ;,-rays are found to be 9.1 x 10 -7 and 3.3 x 10 -8 sec, respectively. The single-particle half-lives o f 160 keV E2 and 148.5 keV M1 ;,-rays as calculated f rom Weisskopf ' s formula 13) are 6.1 x 10 .8 sec and 6.9 psec, respectively. The large retardation factor for the 148.5 keV M1 transition (4.8 x 10 a) can be attr ibuted to K-forbiddenness (AK = 2) [ref. 14)]. Such retardation is also observed for the transition between the same excited states in 2 agpu where the retar- dat ion factor 15, 16) for the 277.6 keV ;,-ray is found to be 6.7 x 103. It should be

noticed that the hindrance factors in the two nuclei are very close. The similarity between the levels o f 2 a7 U and 2 a9pu is also shown by compar ing the reduced transi- t ion probabilities o f the corresponding transitions (see table 5). The quanti ty BQ.) for any transition o f multipole order 24 is obtained from its partial half-life after the correction for energy dependence 17).

The 44.7 keV transition observed in the ;,-singles spectrum probably consists of two separate transitions (56.5 ~ 11.5 and 204 --* 160.0). In 239pu ' the transitions involv- ing the same states are studied in detail 16), and the 49.4 keV transition (57.3 ~ 7.9) is found to be 84 70 M1 and 16 ~ E2. Assuming the same M1-E2 admixture for the corresponding 237U transition, the intensity of the 44.7 keV (56.5 ~ 11.5) ;,-ray is

237u ENERGY LEVELS 37

calculated with respect to that of the 56.5 keV E2 transition and found to be 0.07 ~o. The remaining intensity of 0.10 ~ comes f rom the 204 --, 160.0 transition.

TABLE 5

Comparison of the reduced transition probabilities between identical states in 237[.j and 2SSPu

Initial state Final state Multipolarity B(2, 2aTU) (KI n) (KI n) ~. B(Z, 2SgPu)

~+ ½ ½+ E2 3.0

½ ~+ M1 1.4

½ ~+ M1 1.2

½ ½+ M1 1.2

+

~ - 367

[NnzA~-] K ITr Energy (keY) 316

-~2 [743'] ]'2 -~'2 -(155 -+ 6)/nsec 274 4 - - 2 6 1

I" ~ 15_,5_ ¢3.1-+ iO~)ns t~.'~nO

237 U

Fig. 9. Alpha decay scheme of =4*Pu.

241pu

The ?-? coincidence experiment reveals the presence of an excited state at 274 keV. This level is found to decay by a delayed transition (114 keV), and the half-life of the state is found to be 1.55 x 10 -7 sec. From analogy with the energy levels of 239pu

38 i. AHMAD et al

where the 391.8 keV level has a half-life is) of 1.9 x 10 - 7 sec, the 274 keV level of 237 U is given a ~-(743T) assignment. It should be remarked that the E1 transitions in the actinide region are found, in general, to be highly hindered ~ 9, 2 o). The existence of the 274 keV level is also established by the identification of the 4.783 MeV alpha group. The complete alpha decay scheme of z4Zpu as deduced from this work is shown in fig. 9.

4.2. THE or-TRANSITION PROBABILITIES

The relative alpha intensities to the various members of a rotational band are func- tions of the appropriate vector-addition coefficients and can be calculated semi- empirically. The reduced co-transition probability P to a certain member of the fa- voured band is given by the equation 21)

P = PE/N ~ I(I~LKi(Kr-KOII~LIrKf)I2 (2) r = o, 2, 4 (HF/:)e_~

In eq. (2) Pe is the ~-transition probability calculated from the theory of Preston 6), (HFL)e-e the hindrance factor for an ~-wave of angular momentum L obtained from the adjacent doubly even nuclei, the term in the angular bracket the Clebsch-Gordan coefficient, N an empirical constant and the subscripts i and f the initial and final states. The hindrance factors for the L = 0, 2 and 4 ~-waves from 2 4 ° p u and 242pu decay are known 22) to be 1, 1.6 and 96, respectively. The calculated intensities are given in table 3.

It should be noticed that the ratio of the calculated intensities to the I = ~ and members is lower than that of the observed intensities. This trend has also been ob- served 2 a) in the ~-decay of other odd-mass nuclei, and in the case of 2 a a U it has been explained 24) in terms of the interaction of the nuclear quadrupole moment with the outgoing alpha wave.

The relative oc population to the members of an unfavoured band can be calculated by the equation 21)

P = PE ~ CLI(IiLKi(Kf- Ki)lliLlfKf) + bz(--)xr+Kr r

( IiLK~(- Kf - KOIt~ LI~- K,)I ~ (3)

where CL and br~ are two adjustable parameters which are determined empirically from known 0c-intensities. The intensities shown in table 4 are calculated with a bL value of +3.0 for the L = 4 wave. This value agrees well with the b4 value of +2.0 for 243Cm ~-decay.

The authors wish to express their thanks to Dr. D. W. Engelkemeir for the use of his electronic equipment.

|87U ENERGY LEVELS 39

References

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P. O. FrSman, Mat. Fys. Medd. Dan. Vid. Selsk. 1, No. 3 (1957) 22) C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes (John Wiley and Sons, New

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