Il.E.l:S I Nuclear Physics AZ39 (1975) 1-14; © North-Holland Publishing Co., Amsterdam

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

HiGH-RESOLUTION at-SPECTROSCOPY OF 253Es and 255Fm

IRSHAD AHMAD and JOHN MILSTED Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 t

Received 5 November 1974

Abstract: Alpha-particle spectra of 253Es and 25 5Fm have been measured with a high-resolution magnetic ex-spectrometer. Isotopically enriched samples prepared in the Argonne electromagnetic isotope separator were used for these measurements. Precise energies and intensities of 253Es and 255Fm ex-groups have been determined. Two new ex-groups have been identified in the decay of 255Fm. The relative intensities of ex-groups populating members of unfavored bands are found to be in good agreement with the theoretical values. For the favored bands the theoretical calculations cannot reproduce the experimental intensities. The enhancement in the intensities of ex-groups feeding the t + [642] band in 249Bk has been quantitatively explained by a Coriolis interaction calculation.

E

RADIOACTIVITY 253Es [from 253Cf e.;.; 253Cf produced by 252Cf(n, y)]. 255Fm [from 255Ese.;.; 255Es produced by 253Es(n, y)]; measured Ea.. Ia.; deduced hindrance factors. 249Bk and 25 I Cf deduced levels. Mass-separated 253Es and 255Fm.

Calculated ex-transition probabilities.

1. Introduction

The a-particle spectrum of 253E8 (20:47 d) was first measured by Asaro et al. 1) with a magnetic spectrograph. Recently Baranov et al. 2,3) have measured the 253£S spectrum with a magnetic spectrograph using much larger amounts of activity than was used by Asaro et al. They have identified new a-groups and their results are pub­lished in two separate articles 2, 3). However, the intensities reported in these two publications differ considerably from each other. The a-particle spectrum of 255Fm (20.1 h) was first investigated by Asaro et al. 4) with a magnetic spectrograph. More recently Ahmad et al. 5) have reported new a-groups which were identified in a-y coincidence experiments. However, it is difficult to obtain precise intensities by a-y coincidence method because of uncertainties in y-ray detection efficiencies and also because several y-rays are in coincidence with each a-group.

A precise knowledge of a-intensities is essential for the development of a-decay theories. Recently a-transition probabilities to the members of favored bands in 249Bk and 2S1Cf have been calculated by Soinski et al. 6). Intensities of a-groups populating five or more members of the favored band are needed to test the results of these coupled-channel calculations. Since there is scarcity of such data in literature we undertook the present measurements.

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

February 1975

2 I. AHMAD AND J. MILSTED

2. Source preparation

Approximately 100 Jlg of einsteinium containing 99.4 % 253Es (20.47 d), ~ 0.5 % 254Es (276 d) and ~ 0.04 % 255Es (38.3 d) was obtained from the USAEC as a part of the heavy element production program. The nuclide 255Fm, which is produced in the (r decay of 25 5Es, was chemically isolated several times and used for the pres­ent investigation. The Fm was first separated from Es and other actinides by a cation-exchange resin column using ammonium IX-hydroxy isobutyrate as the elu­triant 7). The Fm was then purified from fission products by an extraction chromato­graphic procedure 8). The sample was once again purified by a cation-exchange resin column to remove any Es left over from the initial separation. The purified Fm was used to prepare thin sources for the IX-spectrometer.

Sources of 253Es for the magnetic spectrometer were made from an Es sample containing> 99.9 % 2 53Es. This sample was supplied to us by the Oak Ridge National Laboratory and was obtained by isolating the 253Es produced in the P- decay of a purified 253Cf (17.6 d) sample. The einsteinium was chemically purified by the pro­cedure described earlier and was used for the preparation of thin sources for the lX"spectrometer.

All sources used for the present measurements were prepared 9) in the Argonne double electromagnetic isotope separator by the deposition of 200 e V ions on thin Al foils. A mask with a 1 mm x 6 mm slit was used to define the source deposit but the focussing was good enough to concentrate the mass in a spot of ~ 1 mm diam­eter. This step, in addition to providing a thin mass-free source, also enhanced the isotopic and chemical purity of the nuclide under investigation.

3. Experimental results

The IX-particle spectra of 253Es and 255Fm were measured with the Argonne focussing magnetic spectrometer. The spectrometer had a resolution (FWHM) of 5 keV at a transmission of 0.1 % of 4n for 6.0 MeV IX-particles. The arrival of IX­particles in the focal plane of the spectrometer is detected with an array of surface­barrier detectors instead of the conventional photographic plate. The detector as­sembly consists of 14 rectangular Au-Si surface barrier detector modules in a closely packed horizontal array 10). Each module has a collector electrode (anode) and 16 sensitive surface elements formed by evaporating the gold film through a grid of fine parallel wires 11). Each sensitive element is about 1.0 mm wide and 2.4 cm high. The arrival of an IX-particle in any of the 224 detector elements is stored as an event in the appropriate channel of a small computer memory and accumulation of the spectrum can be followed on a live oscilloscope display like that of a conventional multichannel pulse-height analyzer.

The decoder circuit which generates the binary address information is triggered by two coincident pulses: a "module" pulse derived from the collector electrode of the module in which the event occurred, and an "element" pulse derived from the

2S3Es, 2s5Fm cx-SPECTRA 3

ground return circuit of the individual element. The collector electrode of each module is provided with an individually adjustable positive bias voltage and is connected to a charge-sensitive preamplifier. The output of this preamplifier is connected to a linear amplifier/single-channel analyzer unit which provides the "module" logic pulse for the decoder. The upper- and lower-level discriminators for each module can be set to reject pulses corresponding to a-particle energies significantly above or below the energy range covered by this particular module at the magnetic field used in a given measurement. This feature provides effective rejection of a-particles which arrive in non-valid trajectories because of scattering in the vacuum chamber and beam defin­ing apertures. The corresponding surface elements of the 14 modules are connected in parallel to form 16 independent ground return circuits each of which contains a resistive load across which the "element" pulse is developed. The pulses are amplified by 16 high-gain voltage-sensitive amplifiers with threshold discriminators for noise rejection. The discriminator logic pulses provide the "element" inputs to the decoder circuit.

The energy calibration of the spectrometer was made with the following set of standards 12-14): 233U (4.824 MeV), 238pU (5.499 MeV), 244Cm (5.805 MeV), 242Cm (6.113 MeV), 211Bi (6.279 and 6.623 MeV) and 214pO (7.687 MeV). For in­tensity measurements the counts accumulated in each detector element (which cor­responds to one channel in the spectrum) were corrected for efficiency. This was ne­cessitated because of variations in the sensitive areas of the detector elements. The

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6 I. AHMAD AND J. MILSTED

relative efficiency of each detector element was determined by using a very thick 244Cm source. This source was prepared by diffusing the 244Cm activity into a glass substrate supported by a platinum plate at a temperature (~ 900°C) above the softening point of glass. Its IX-spectrum below 5.7 MeV was essentially a slowly de­creasing exponential, represented by a nearly horizontal straight line on a semi­logarithmic plot of counts versus channel number. The slope and intercept of this ideal line were determined by a least-squares fit to the spectrum of the thick 244Cm source, ignoring the end elements of each module which had consistently smaller sensitive areas. For this reference spectrum, more than 10000 counts were accumu­lated in each channel, giving statistical deviation of less than 1 %. A correction factor was calculated for each element on the assumption that the counts for all elements would have fallen on this ideal line (within statistical limits) if their sensitive areas

TABLE 1 253Es IX-groups

Energy Excited state Intensity Hindrance (MeV) energy (keV) (%) factor ")

6.633 ±0.002 0 89.8±0.2 1.7 6.624 b) 9 0.8 1.8 x 102

6,594 b) 40 0.7 1.5 x 102

6.592±0.002 42 6.6±0.1 15 6.552±0.OO2 82 0.71±0.02 92 6.540±0.OO2 94 0.85±0.02 68 6.498±0.OO2 137 0.26±0.01 1.4 x 102

6.480±0.002 155 (8.5±0.3) x 10- 2 3.6 X 102

6.432±0.OO2 204 (6.1 ±0.3) x 10- 2 2.9x 102

6.408 ±0.OO2 229 (1.3±0.1) x 10- 2 1.1 X 103

6.354±0.002 283 (8.2±0.4) x 10- 3 9.2 X 102

6.325±0.OO3 313 (4.0±1.0) x 10- 4 1.4 X 104

6.266±0.OO2 373 (8.0±0.8) x 10- 4 3.5 X 103

6.250±0.OO2 389 (4.5±0.2) x 10- 2 52 6.230±0.003 409 (1.2±0.4) x 10- 4 1.5 X 104

6.217 ±0.003 423 ~ 1.5 x 10- 3 ~ I X 103

6.211 ±0.002 429 (3.9±0.2) x 10- 2 38 6.166±0.OO2 475 (1.5±0.1) x 10- 2 59 6.122±0.002 519 (7.8±0.8) x 10- 4 6.7 X 102

6.100±0.OO2 542 (3.4±0.2) x 10- 3 1.2 X 102

6.084±0.OO3 558 (2.5±0.5) x 10- 4 1.3xl03

~ 6.071? ~ 571 ~ 5xlO- 5 ~ 6x 103

6.046±0.OO3 596 (4.0±0.9) x 10- 4 5.3 X 102

6.037 ±0.003 606 (2.9±0.7) x 10- 4 6.6 X 102

6.019 ±0.003 624 (1.8±0.5) x 10- 4 8.5 X 102

~ 5.974? 669 ~ 6x10- 5 ~ 1.5xl03

5.944±0.OO3 700 (1.5±0.5) x 10- 4 4.1 X 102

5.935 ±0.004 709 ~ 4x10- 5 ~ 1.4 X 103

a) The hindrance factors are calculated from Preston's equations 15) using a radius parameter of 9.400 fm.

b) The energies and intensities of these IX-groups are taken from ref. 1).

253Es, 255Fm a.-SPECTRA 7

TABLE 2 B5Fm a.-groups

Energy Excited state Intensity Hindrance (MeV) energy (keV) (%) factor a)

7.127 ±0.OO2 0 (7.0±0.7) X 10-2 4.4 X 103

7.103±0.002 24 (9.0±0.9) X 10- 2 2.7 X 103

7.080±0.002 48 0.40±0.03 4.9 X 102

7.022±0.OO2 107 93.4±0.3 1.2 6.983 ±0.OO2 146 0.13±0.01 5.9 X 102

6.963 ±0.OO2 167 5.04±0.06 12.6 6.953 ±0.003 177 (2.2±0.4) X 10- 2 2.6 X 103

6.918±0.OO2 212 (1.7±0.2) X 10- 2 2.4x 103

6.892±0.002 239 0.62±0.01 51 6.873 ±0.002 258 (8±1) X 10- 3 3.3 X 103

6.836±0.OO2 296 (8±1) X 10- 3 2.2 X 103

6.814±0.OO3 318 (2.0±0.5) X 10- 3 7.2 X 103

6.807 ±0.002 325 0.11 ±0.006 1.22 X 102

6.763 ±0.OO2 370 (1.6±0.2) X 10- 2 5.3 X 102

6.741 ±0.003 392 (1.2±0.4) X 10-3 5.7 X 103

6.710±0.002 424 (1.3±0.l) X 10- 2 3.8 X 102

6.699±0.OO2 435 (3.6±0.2) X 10- 2 1.23 X 102

6.692±0.OO3 442 (5±2) X 10- 3 8.2 X 102

6.621 ±0.OO3 514 (2.2±0.5) X 10- 3 8.9 X 102

6.592±0.002 544 (1.7 ±0.2) X 10- 2 84 6.546±0.002 590 (1.4±0.2) X 10- 2 63 6.488±0.003 649 (3.0±0.5) X 10- 3 1.6 X 102

a) The hindrance factors are calculated from Preston's equations 15) using a radius parameter of9.400fm.

had been equal. The variation in the average counts due to the slope of the reference line was about 10 % over the 224 channel range. A computer program was developed to make these corrections. The efficiency calibration of the detector elements was further checked by comparing the a-intensities of some doubly even nuclides mea­sured with the magnetic spectrometer with those determined with a Au-Si surface­barrier detector.

The a-spectra of 2 53Es and 25 5Pm samples measured with the magnetic spectrom­eter are shown in figs. 1-3. Several spectra of each nuclide were measured and the decay of each spectrum was followed in order to ascertain the identity of the assigned a-groups. The energies and intensities of a-groups determined from these measure­ments are given in tables 1 and 2. The hindrance factors were calculated from the spin-independent theory of Preston 15); the radius parameter used was 9.400 fm. The errors in energies and intensities are one standard deviation, (T. The errors in energies include contributions from spectrometer calibration, peak location and the standard. The errors in intensities include contributions from peak area, background and efficiency calibration.

8 I. AHMAD AND J. MILSTED

4. Discussion

4.1. THE 253Es Ct.-DECAY

The IX-decay scheme of 2 53Es is shown in fig. 4. The level energies and IX-intensities are from the present measurements and the spin-parity assignments are taken from previous studies 1.16). Levels identified in the present investigation and not shown in the figure have also been observed in the 249Cm Ir decay and have been given ten­tative assignments by Hoff et al. 17). Our energies of IX-groups are in good agreement with the values reported by Asaro et al. 1) and also with the more recent values of Baranov et al. 3). The intensities of 253Es IX-groups obtained in the present measure­ments also agree well with the intensities reported in refs. 1.3). However, our sample was chemically and isotopically pure (we did not observe any impurity peak in our

253Es [633]1212 + -""""l"",""--- 0 la

Energy (keV) a Intensity ("to)

13+ ______ 596.(f 4.0 x 10-4 '2

II + _____ 542.(f 3.4 x 10-3 '2

9+ -----475 # 1.5 x 10- 2 '2

7 + 429 # 3.9 x 10-2 '2

[642] -25 22

+ 389 # 4.5 x 10-2

373 # 8.0 x 10-4 12-2

17 + _____ 313.(f 4.0 x 10-4 "2 13-____ _

"2 283 # 8.2 x 10-3

15 + _____ 229# 1.3x 10-2 "2 ~-_____ 204 # 6.1 x 10-2

!f+ -----155#8.5XI0-29

_ '2 ----- 137 # 0.26

II + _____ 94 # 0.85 7- # '2 '2 82 0.71

9+ 42 # 6.6 5-______ 40 # 0.7 '2 '2

[633] t ~+ 0 # 89.8 -i- 9 # 0.8

[521] i-249Sk

Fig. 4. Alpha-decay scheme of 253Es. Level energies and Ct.-intensities are from the present measure­ments. Spin-parity assignments are taken from ref. 16). Levels at 409, 423, 519, 558, 571, 606, 624, 669, 700 and 709 keY, which have been identified in the present investigation, are not shown in the

figure because their spins and parities are not known.

253Es, 255Fm IX-SPECfRA 9

spectrum) and hence the sensitivity of our measurement was higher than that of Baranov et al. 3). We particularly looked for peaks at 6.460, 6.415, 6.367 and 6.304 MeV in our spectrum. We did not observe any peak at these energies and obtained an upper limit of 1.0 x 10- 4 % for the intensity of each of these ct-groups. Since the intensities of these ct-groups reported in ref. 3) are larger than our upper limit we conclude that these peaks are not due to 253Es. Hence, the existence of levels at 176,221,270 and 334 keV in 24gBk is doubtful.

In our spectrum we could not resolve ct g from the cto group and ct40 from the ct42

group. We measured the sum of the ct40 and ct42 intensities as (7.3 ±0.1) %. The in­tensities of ct g and ct40 groups given in table 1 are taken from ref. 1). The 254Es !X86

peak (6.429 MeV) falls at the same channel as the 2 53Es ct204 group. By follo-wingthe decay of the spectrum it was found that only < 0.1 % of ct204 intensity belonged to the 254Es ct86 group.

Level energy (keV)

9 40 82

137 204 283 373

TABLE 3

Alpha intensities to the members of the! - [521] band in 249Bk

]71 Observed intensity

(%)

0.8 b) 0.7 b) 0.71 0.26 6.1 x 10- 2

8.2 X 10- 3

8.0xl0- 4

") These intensities were calculated in ref. 19). b) These intensities were obtained from ref. 1).

TABLE 4 Alpha intensities to the members of the favored band in 249Bk

Theoretical a) intensity

(%)

0.76 1.10 0.71 (norm) 0.23 5.6 x 10- 2

8.2 X 10- 3

8.2x 10- 4

Level energy (ke V) 111 Observed Theoretical intensity (%)

obs. calc. a) intensity (%) Poggenburg b) Soinski et af. C)

0 0 !+ 89.8 89.8 (norm) 89.8 (norm) 42 42 !+ 6.6 5.3 6.4 94 94 .If+ 0.85 0.91 0.93

155 156 11-+ 8.5 X 10- 2 5.3 xlO- 2 0.11 229 228 .lj+ 1.3 X 10- 2 6.6xl0- 3 7.0xI0- 3

313 310 -¥-+ 4.0xIO- 4 3.6xl0- 4

403 ¥+ 2.7xl0- 5

a) These energies were calculated by taking the Coriolis interaction into account. b) These intensities were obtained from ref. 19). C) These intensities were calculated in ref. 6). These calculations include a rotational attenuation

factor with y = 0.018.

10 I. AHMAD AND J. MILSTED

TABLE 5 Alpha intensities to the members of the! + [642] band in 249Bk

Level energy (keY) [" Observed Theoretical intensity (%)

obs. calc. a) intensity (%) Poggenburg b) Coriolis B)

389 389 !+ 4.5 x 10- 2 4.5 X 10- 2 (norm) 4.5 X 10- 2 (norm) 429 428 i+ 3.9 X 10- 2 3.0x 10- 2 4.6x 10- 2

475 477 !+ 1.5 x 10- 2 9.7xl0- 3 1.5 X 10- 2

542 539 .y.+ 3.4xI0- 3 1.7x10- 3 5.3 X 10- 3

596 607 .lj+ 4.0xI0- 4 2.7 X 10- 4 4.0x 10- 4

a) Calculated by taking the Coriolis interaction into account. b) These intensities were obtained from ref. 19).

The IX-transition probabilities in odd-mass actinide nuclei have recently been cal­culated from the single-particle wave functions 18, 19). In tables 3-5 we compare the experimental intensities with the theoretical values given in ref. 19). We are essentially comparing the relative intensities because we normalize at one intensity. For the favored band the IX-intensities are calculated only for states up to spin 125. We calculated the intensities of IX-groups populating the 1} and 1£- members using the L = 6 IX­wave only. The reduced transition probability was calculated on the assumption that it is proportional to the square of the appropriate Clebsch-Gordan coefficient. The penetrability factor, PI(e), was determined from a plot of PI(e), given in ref. 19), against the excitation energy on a semilogarithmic graph paper. The product of the reduced transition probability and PI(e) gave the IX-intensity for each I-wave.

The experimental intensities to the members of the r [521] band are in excellent agreement with the theoretical values. However, in the case of the favored band, the agreement is poor. Even the coupled-channel calculations of Soinsky et al. 6) fail to reproduce the experimental intensities. These calculations take into account the interaction of the outgoing IX-wave with the nuclear quadrupole moment. They also include a term exp{ -y[Ir(Ir+ 1 )-IMi + I)]}, called rotational attenuation factor, to improve the agreement between the calculated and observed intensities. The IX­intensities to the members of the favored band, calculated with 0.018 as the value of y, are given in table 4.

The observed intensities of IX-groups feeding the members of the ~ + [642] band are consistently larger than the calculated values. This enhancement in the IX-intensity is caused by the Coriolis admixture 20) of the i + [633] state in the ~ + [642] band in 249Bk and the admixture of the ~+ [642] state in the 253Es ground state.

4.2. THE BSFm IX-DECAY

The IX-decay scheme of 2 5 5Fm is shown in fig. 5. The level energies and IX-intensities are our values and the spin-parity assignments are taken from ref. 5). The energies and intensities of 255 Fm IX-groups measured in the present work are in good agreement

253Es, 255Pm IX-SPECTRA 11

255 Fm [613] ~ ~+ _...,,... ___ 0

lIa

Energy (keV) a Intensity ("!o)

9+ 649# 3.0 x 10-3 '2

7+ 590# 1.4 x 10-2 '2

[622]i~+ 544# 1.7 x 10- 2

11- 514 {f 2.2 x 10-3 '2

5.0 x 10-3 9- [734]~-13- (f 442 435~ 3.6 x 10-2

2' '2 15+ 424 1.3 x 10- 2

11+ 392# 1.2 x IQ-3 '2 2'

[725]~ ~- 370# 1.6 x 10-2

~+ 318 # 2.0 x 10-3 # 8 x 10-3 13+ 325# 0.11 2 13+ 2'

2' 296

7+ 258# 8.0 x 10-3 '2 ll+ 240 11+ 239# 0.62

2 '2 5+ 212# 1.7 x 10- 2 '2

{622]i ~+ 177# 2.2 x 10-2 (10.13 9+ 167 {f 5.04

9+ '2 '2 146

7+ 106 1+ 107# 93.4 2' 2

[613]i+ 5+ 48# 0.40

~+ '2 24 # 9.0 x 10-2

[620]t ~+ o # 7.0 x 10-2

251Cf

Pig. 5. Alpha-decay scheme of 25 5Pm. Level energies and IX-intensities are from the present investi­gation. Spin-parity assignments are taken from previous studies 4. 5). Intensities of IXI06 and 1X240

groups are not shown because these were masked by more intense IX-groups.

with the values obtained in OC-')' coincidence experiments 5). Two new oc-groups with energies 6.692 (OC442) and 6.621 (OC5 14) MeV have been identified. The 442 keV level is assigned to the \3 member of the 121 - [725] band and the 514 ke V level has been interpreted as the 1/ member of the -! - [734] band. The observed level spacings of the two bands can be reproduced by a Coriolis interaction calculation using a Coriolis interaction matrix element of 2.23 [which is 0.36 ofthe value calculated with Nilsson 21)

wave functions] between the r [734] and ll-- [725] states. The oc-intensities obtained in the present investigation are compared with the

theoretical 19) values in tables 6-8. The intensities to the members of the t + [622] rotational band were recalculated using p/(r.) values corresponding to the observed

12 I. AHMAD AND J. MILSTED

TABLE 6 Alpha intensities to the mem~rs of the t+ [620] band in B1Cf

Level energy (keY)

o 24 48

106 146 240 296

I'T

a) These intensities were obtained from ref. 19).

Observed intensity

(%)

7.0X 10- 2

9.0xl0- 2

0.40 masked b) 0.13 masked b) 8.0xl0- 3

b) These IX-groups were masked by more intense IX-groups.

TABLE 7

Theoretical 8) intensity

(%)

9.6xl0- 2

0.12 0.40 (norm) 0.14 9.3 xl0- 2

1.1 X 10- 2

3.6 X 10- 3

Alpha intensities to the members of the favored band in 2 S lCf

Level energy (keY)

107 167 239 325 424

/1t Observed intensity

(%)

93.4 5.04 0.62 0.11 1.3 x 10- 2

a) These intensities were obtained from ref. 19).

Theoretical intensity (%)

Poggenburg a) Soinski et at. b)

93.4 (norm) 5.46 0.79 4.0xl0- 2

4.5xl0- 3

93.4 (norm) 4.3 0.83 8.9x 10- 2

7.4 X 10- 3

b) These intensities were calculated in ref. 6). The rotational attenuation factor was not included in this calculation.

TABLE 8

Alpha intensities to the members of the! + [622] band in B 1Cf

Level energy (keY)

177 212 258 318 392

[1t

a) These intensities were taken from ref. 19).

Observed intensity

(%)

2.2x 10-2

1.7 X 10- 2

8.0 X 10- 3

2.0x 10- 3

1.2xl0- 3

Theoretical 8) intensity

(%)

2.4x 10- 2

1.7xl0- 2

9.8 X 10- 3

3.4 X 10- 3

1.2 X 10- 3

(norm)

253Es, 255Fm a-SPECTRA 13

excitation energies. The experimental (X-abundances to the members of the 1- + [622] and 1-+ [620] bands are in good agreement with the calculated values. Again the observed and theoretical intensities to the members of the favored band do not agree with each other.

4.3. CORIO LIS INTERACTION CALCULATION OF 2s3Es a-INTENSITIES

The (X-intensities to the members of the t + [642] band were calculated by including the admixtures of other K-states in the parent and daughter wave functions caused by Coriolis interaction. These calculations were performed with a computer code BANDMIX 22). All seven bands arising from the i.y. shell state were included in the calculation. The energies of these states were obtained from Nilsson eigenvalues 21) and the Coriolis interaction matrix elements were calculated with Nilsson wave functions 21). The value of the rotational constant used was 6.4 keY. The energies of the t + [642] and 1- + [633] states and the matrix elements Aft and Au were varied until the best fit between the experimental and calculated energies was obtained. The values of Aft and Au for the best fit were 3.50 and 4.05, respectively, which are 0.57 and 0.72 of the respective theoretical values calculated from Nilsson wave functions. The level energies for the 1- + [633] and t + [642] bands obtained from the Coriolis calculations are given in tables 4 and 5. It should be pointed out that the calculated energies for the·V [642] band do not agree with the observed energies, especially for the \3.. + state. This disagreement could be caused by the presence of other positive parity states in this vicinity which we did not include in our calculations. The per­turbed wave functions of the members of the t + [642] band are

t/t129 = 0.972 t/tt- 0.137 t/tt ,

tfrt75 = 0.945 t/tt-0.202t/tt ,

t/t142 = 0.918 t/tt-0.254t/tt ,

t/t;96 = 0.880 Wt -0.299t/tt · (1)

In 253 Es the energy of the t + [642] single-particle state is not known. Since the level structures of 253Es and 249Bk are expected to be very similar, we use the 249Bk ground state configuration for the 253Es ground state. Thus the 253Es ground state is

t/tt = 0.990t/tt +O.137t/tt . (2)

The (X-intensities to the members of the t + [642] band were calculated with the equation 23)

(3)

In the above expression ak and t4 denote the admixture coefficients in the parent and daughter wave functions, Pl(e) is the penetrability factor for the (X-wave of angu­lar momentum I and HtKe is the product of the ex-decay matrix element and the appro­priate Clebsch-Gordan coefficient. The values of HilKe were obtained from ref. 19). The phases of all matrix elements except H~t4 were as given by Poggenburg 19).

14 I. AHMAD AND .J. MILSTED

The signs of the H~t4 matrix elements were changed because this improved the fit between the experimental and calculated intensities. Since the values of H~t are not calculated for 253Es decay, we obtained these quantities from 237Np decay 19). The (X-intensities for the members of the 1- + [642] band, calculated with eq. (3), are given in table 5. The agreement between the calculated and measured intensities is quite good.

As pointed out earlier the (X-intensities to the members of the favored band cannot be reproduced by any of the theoretical calculations. It has been shown by Soinski et al. 6) that the effect of Coriolis mixing on the intensities of (X-groups populating the favored band is insignificant and it cannot explain the disagreement between the experimental and calculated intensities. Thus further theoretical work is needed to explain this disagreement. Also precise measurement of (X-intensities to favored bands of other actinide nuclei will be of help here.

The authors wish to thank R. K. Sjoblom for the chemical purification of 253Es and 255Fm samples and J. Lerner for the preparation of thin isotopically enriched sources for the (X-spectrometer. Helpful discussions with J. K. Poggenburg, A. J. Soinski and R. W. Hoff are also acknowledged.

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