Alpha decay of 247Cm

F-I

Nuclear Physics A160 (1971) 460-470; Not to be reproduced

by photoprint

ALPHA P. R. FIELDS, Chemistry

I. AHMAD, Division,

@ North-Holland

Co., Amsterdam

or microfilm without written permission from the publisher

DECAY

OF “‘Cm

A. M. FRIEDMAN,

Argonne

Publishing

National

J. LERNER

Laboratory,

Argonne,

and

D. N. METTA

Illinois

60439 7

Received 6 August 1970 Abstract: An enriched sample of 247Cm (99.4 ‘A) has been used to investigate the cc-decay scheme of 247Cm. Seven cr-groups withenergies and intensities 5.265 (13.8 %), 5.210 (5.7 %), 5.145 (1.2 %), 4.983 (2.0 %), 4.941 (1.6 %), 4.868 (71.0 %) and 4.818 (4.7 %) were observed. Gamma-singles and uy coincidence spectra showed the presence of 278.0, 287.5, 346.0 and 402.4 keV y-rays. A two-parameter coincidence experiment established that the sources of the 402.4 and the 287.5 keV y-rays were 4.868 and 4.983 MeV cc-groups, respectively. The multipolarity of the 287.5 keV transition was found to be Ml. The K conversion coefficient of the 402.4 keV y-ray indicates an El multipolarity for this transition. The levels at 287.5 and 402.4 keV have been assigned to the #+(622) and Q-(734) Nilsson states, respectively. The half-life of 247Cm was found to be (1.56kO.05) x 10’ y from the c( pulse analysis and mass spectrometric analysis of the sample.

E

RADIOACTIVITY 247Cm [from 246Cm(n,y)]; measured T+, Eat Z,, Er, Zr, ay-coin; deduced hindrance factors. 243Pu deduced levels, cc, y-multipolarity. J, z. Mass-separated 247Cm.

1. Introduction The u-decay half-life of 247Cm has been measured previously ‘) to be (1.64kO.24) x 10’ y by determining the amount of 243Pu that was in radioactive equilibrium with a sample of curium which contained 0.684 mole percent 247Cm. The presence of other Cm isotopes, which are very radioactive, prevented direct observation of the 247Cm radiations. The presence of a 4.6 MeV a-emitter in natural sources, also found to contain enriched 235U and 23gPu, has been reported by Cherdyntsev and coworkers “*“). In one publication “) they reported that this a-emitter might be 247Cm, especially since 23gPu and 235U are the decay products of 247Cm. Several years ago a sample of Cm having the composition shown as A in table 1 became available as a result of the heavy element production program in the High Flux Isotope Reactor at Oak Ridge National Laboratory. It was apparent that successive isotopic enrichments could produce a sample sufficiently enriched in 247Cm to observe the radiations associated with its x-decay. The results of these a-decay measurements and a proposed decay scheme for 247Cm are reported in this paper. t Based on work performed

under the auspices of the US Atomic Energy Commission. 460

461

247Cm z-DECAY

2. Source preparation Approximately 20 mg of Cm having the isotopic composition given as A in table 1 was received from Oak Ridge National Laboratory, and after being separated in the Argonne Isotope Separator once, had the isotopic composition B. The 247Cm fraction frcm the first enrichment was dissolved, purified from aluminum and enriched once again in the separator to yield a sample of approximately 1 lug having the composition C. The final 247Cm sample was collected on an Al plate in the mass separator. TABLE

Isotopic composition Cm isotope

244 245 246 247 248 250

Initial sample (A) 65 1 30 1 3 ~0.25

1

of Cm sample (in atom percent) After first enrichment 0) 2.79 10.03 0.178 +0.004 81.28 +O.ll 14.10 *0.12 1.66 ~0.008

After second enrichment (C) 0.0018*0.0001

“)

0.564 10.004 99.401 30.004 0.033 io.003

“)

“) These numbers are obtained from pulse-height analysis and the known half-lives of these nucleides [refs. ‘3 s)].

The 247Cm and part of the Al plate which collected this mass fraction were dissolved in dilute HCl. The solution was evaporated to dryness and then evaporated several times with HNO, to destroy all Cl-. The residue was dissolved in 6 M LiNOs -0.01 M HNO, and extracted three times with 0.4 F aliquat-336 in xylene “). The three organic phases were scrubbed with 6 M LiNO, -0.01 M HNOs to remove all the Al. The Cm was then removed from the organic phase by re-extraction with 0.1 M HNO,. The dilute HNO, layer was extracted three times with 1 F HDEHP(di-2-ethylhexyl orthophosphoric acid) in heptane to separate the Cm from LiNO, [ref. “)I. The organic layer was washed once with 0.1 M HNO, and the Cm was re-extracted with 6.4 M HCl. The HCI solution was extracted twice with 0.4 F aliquat-336 to remove any Fe that was present and then washed with xylene to remove any residual extractant. The Cm solution was evaporated to dryness, dissolved in 0.05 M HCl and adsorbed on a 2 mm x 5 cm Dowex 50 cation-exchange resin column and eluted with ammonium a-hydroxy isobutyrate “) to separate the Cm from any other possible actinides. Finally, the Cm was adsorbed on another Dowex 50 cationexchange resin column and eluted with concentrated HCl to separate it from inert impurities and give a mass-free sample. The pure Cm was evaporated to dryness with the spreading agent, tetraethylene glycol, and ignited. This gave a thin source for alpha spectroscopy.

P. R. FIELDS et al.

462

3. Experimental results 3.1. ALPHA-PARTICLE

SPECTRA

Various a-particle spectra of the 247Cm sample were measured with Au-Si surfacebarrier detectors to determine the energies and abundances of the a-groups. Because of low activity, the spectra were counted for long intervals of time and the gain of the mounting system was stabilized with a digital-gain stabilizer. A spectrum taken with a 6 mm diameter detector is displayed in fig. 1. This spectrum, which was counted for

I04

v-

247Cm 0404

4.868

IO’ 400

I

I

500

600 Channel

number

I 700 (u-particle

I 800

energy)

Fig. 1. The a-singles spectrum of the Cm sample, showing its isotopic composition. The spectrum was measured with a 6 mm diameter Au-Si surface-barrier detector. The sample contained 98 a dis/min of 247Cm and was counted for a period of 8 d.

a period of 8 d, shows the relative abundances of 244Cm, 246Cm, 247Cm and 248Cm. Alpha spectra of mass-separated 244Cm, 246Cm and 248Cm were also measured in order to determine which peaks were associated with the m-decay of 247Cm. An enlarged spectrum of 247Cm is shown in fig. 2. The energies of the cx-groups were measured with respect to that of 242Pu CQ,group which was taken as 4.900 MeV [ref. ‘)I. The energies, intensities and hindrance factors of the 247Cm Mgroups are given in table 2. The hindrance factors in table 2 were calculated from the simple barrierpenetration theory of Preston ’ “)_ 3.2. GAMMA-RAY

SPECTRA

The y-singles spectra of 247Cm were measured with a 4 cm3 co-axial Ge(Li) detector. A spectrum obtained by counting the sample for a period of one week is shown in

463

247Cm x-DECAY

I

440

I 400

I

I

Channel

Fig.

2. An enlargement

I

I

520

1

560 number

of a portion

I

I 640

I

600

(a-particle

of fig. 1, showing

energy)

the 247Cm peaks

more clearly.

TABLE 2 247Cm

alpha

a-particle energy (MeV)

Excited state energy (keV)

5.265&0.004 5.210&0.004 5.145f0.004 4.98310.004 4.941 kO.004 4.868I-tO.004 4.818t_0.004

0 56 122 287 329 404 454

“) The hindrance factors are calculated the half-life of 247Cm cc-decay.

from

groups Hindrance factor *)

Intensity (%)

3.1 3.4 6.1 3.0 2.0 1.4 9.0

13X&0.7 5.7&0.5 1.2kO.2 2.OkO.2 1.6kO.2 71.OI_tl.O 4.7*0.3

Preston’s

equations

[ref. lo)]

X 103 v 103 x lo3 X 102 v 102

using

1.56 x 10’ y for

464

P. R. FIELDS et al.

I 180

I 220

I 260 IA I 280 v400

.*

..a.

.*

C.

..

”

1 560

”

402.4

/

v-.0-

energy)

1. 600

.

.nIII-..

1. 640

,

,..Il-. 680

w

m

..*.-..

720

Me...“_

-no-.

_

The counting

760

e.

?

time was

I 800

/i.Q._ .._..Y.:.i~* ....LJ

I

287.5

D

Channel numbet (y-ray

I 520

l

I)....**.

v.

“..

. .

27

Fig. 4. 247Cm y-ray spectrum measured with a 4 cm3 Ge(Li) detector in coincidence (2~ = 300 nsec) with a-particles. 10 d. Zero events are plotted at 0. I.

os’t40

t

‘F

100

466

P. R. FIELDS el al.

fig. 3. The 84.0 keV y-ray present in the spectrum is associated with the /3- decay of 243Pu. As 243Pu is in radioactive equilibrium with the parent 247Cm, the number of p- dis/min will be the same as the number of 247Cm c1dis/min. This gives a direct method

of measuring

the absolute

intensity

of the 84.0 keV y-ray. From

the present

experiment we obtain a value of 0.23 +0.02 photons/243Pu /3- decay, which is somewhat lower than the value of 0.276 photons/Bdecay obtained ‘I) by /3- counting of a 243Pu sample. TABLE 3 247Cm y-rays Energy (keV)

Intensity (photons/100 247Cm u-decays)

84.OkO.2 99.6kO.3 103.810.3 117.010.3 120.5zkO.4 278.010.8 287.5kO.7 346.0*0.8 402.410.5

23 f2 1.30+0.15 2.1 kO.20 0.8010.13 z 0.3 3.4 k-o.7 2.0 kO.3 E 1.3 72 16

Transition

243Pu y-ray Pu K,, Pu K,, Pu Kg’, Pu Kg’2 402.4 + 287.5 + 402.4 + 402.4 +

124 0 56 0

The peaks marked B in the spectrum are due to the background. This was checked by taking a background spectrum and also measuring a y-ray spectrum in coincidence with a-particles. A y-ray spectrum measured in coincidence (22 = 300 ns) with all 247Cm a-particles is shown in fig. 4. The spectrum was counted over a period of 10 d. The y-ray energies and intensities obtained from the present measurements are given in table 3. 3.3. COINCIDENCE

MEASUREMENTS

A two-parameter analyser was used for uy coincidence measurements. A 2 cm2 Au-Si surface-barrier detector was used for the detection of a-particles, and a 7.6 cm x 7.6 cm NaI(T1) crystal was used to detect the photons. The resolving time of the coincidence unit was 300 ns. The coincidence events were recorded on a magnetic tape and were later read back into the memory through a digital gate system. In this way CI- and y-spectra were measured in coincidence with selected gates. A y-ray spectrum measured in coincidence with 4.90 to 5.10 MeV u-particles is shown in fig. 5. Only the 288 keV y-ray and the Pu K X-rays were observed in this spectrum. The ratio of the Pu K X-rays intensity to that of the 288 keV y-ray was found to be l.lkO.3, which is in good agreement with the expected value I*) of 1.2 for a 288 keV Ml transition. In another two-parameter ay coincidence experiment a 4 cm3 coaxial Ge(Li) detector was used to detect the photons. The coincidence events were recorded for a period.

467

247Cm a-DECAY

of a week. The information marized in table 4. 3.4. HALF-LIFE

OF

obtained

from both two-parameter

experiments

is sum-

247Cm

The ratio of 247Cm a-activity to that of 246Cm was obtained from the a-particle spectrum shown in fig. 1. The isotopic composition of this sample was measured by a mass spectrometric method and is given in column C of table 1. From these ratios and the known half-life “) of 246Cm (4711+22 y), a half-life of (1.5650.05) x lo7 y was obtained for the x-decay of 247Cm. The present value is in good agreement with the previously reported value of (1.64kO.24) x lo7 y [ref. ‘)I.

L

I

I

I

I

I

I

268

keV I ”

0.1 -

0

60

120

Channel number (r-my

Fig.

I

J

I

I60

200

240

I 40

energy)

5. *47Cm y-ray spectrum measured with a 7.6 cm>:7.6 cm NaI(T1) crystal in coincidence (2~ = 300 nsec) with 4.90 to 5.10 MeV u-particles. Zero events are plotted at 0.1. TABLE 4

Results

of the two-parameter

ccy coincidence

a329

Pu K X-rays 278.0 287.5 402.4

“) This y-ray

yes no

yes no

yes no

yes no

is too weak to be seen in coincidence

with x454.

experiments

Go4

G4s

yes yes no

yes no “) no

yes

yes

P. R. FIELDS et al.

468

4. Discussion The decay scheme of 247Cm constructed from the results of the present investigation is shown in fig. 6. The ground state of 243Pu has been assigned to the $+(624) Nilsson state 13) from the study of its p- decay 11*14). This assignment is in agreement

with the results

of 242Pu(d, P)~~~Pu

reaction

studies 15). An assignment

of

q-(734) has been made to the ground state of 247Cm on the basis of the a-decay properties of 2 5‘Cf [ref. I”)]. Again, the assignment is consistent with the results of 248Cm(d, t)247Cm reaction studies I7 ). In the present investigation the favored a(734j t ;

Energy (keV)

(Nn,A)KIr

u2 (734)

9$-

(622)

sl+ 22

$+

;+ ;+ (624)

$$+

453 402.4

“7

(I.56 x ~O’Y)

a Intensity WA

4.7 71.0

330

I .6

267.5

2.0

124

1.2

56

5.7

0

13.6

Fig. 6. Alpha decay scheme of 247Cm.

transition of 247Cm is found to populate a level 402.4 keV above the ground state of 243Pu and hence the 402.4 keV state must have an assignment of s-(734). The K conversion coefficient of the 287.5 keV transition has been found to be 1.1 f0.3, which is in good agreement with the theoretical 12) value of 1.2 for a 287.5 keV Ml transition. Hence, the 287.5 keV state must be a K” = ++, 3’ or e+ state. The rotational constant (see table 5) and the u abundances to the 287.5 and 330 keV levels favor a K = 4 assignment for this band and hence, it is assigned to a j+(622) single-particle state. The 287.5 keV level has also been observed in 242Pu(d, P)‘~~Pu reaction studies r5) and h as b een assigned as the I = 3 member of the $+(622) band. The a-decay properties of 247Cm are very similar to those of 249Cf [ref. ‘“)I and the data of these two nucleides are compared in table 5. The Pu K X-ray intensity due to conversion of the 402.4 keV transition was obtained by subtracting the contribution due to the 285.7 keV y-ray from the total ob-

*“Cm

469

U-DECAY TABLE 5

cc-decay properties Nilsson state

Rotational

constant

Spin I

W/24)

-3Pu 6.2

6.1

++(622)

6.1

6.16

Q- (734)

4.6

5.0

WV)

5 :F K t H 9 $

“) The data

on 249Cf a-decay

and 249Cf “)

Level energy

Z45Cm

; + (624)

of 247Cm

have been taken

243Pu

245Cm

0 56 124 287.5 330 402.4 453

0 55.0 121.7 252.7 295.8 388.2 443

from

a-decay hindrance factor 243Pu 3.1 s 3.4x 6.1 x 3.0x 2.0 x 1.4 9.0

lo3 103 lo3 lo2 102

Z45Cm 6.7 x 6.6 x 1.0 x 2.5 x 1.6x 2.0 17

lo3 103 10’ 10’ 10’

ref. I”).

served K X-ray intensity. This gave a value of (3.010.5) x 10m2 for the K conversion coefficient of the 402.4 keV transition. This number is consistent with an El multipolarity for the 402.4 keV y-ray (theoretical value 12) of K conversion coefficient = 1.8 x 10d2 for El; 4.6 x 10e2 for E2). It should be remarked that higher values of K conversion coefficients than the theoretical values have also been obtained for the 388.2 and 333.2 keV El transitions in 245Cm [ref. ‘“)I. The relative intensities of the y-rays from the 402.4 keV level to the $+, 3’ and y’ members of the ground state band in 243Pu are found to be 72, x 1.3 and 3.4, respectively. These values disagree considerably with the relative intensities of 70, 16 and 0.7 for the analogous transitions in 245Cm (which are of nearly the same energy as those in 243Pu), and they also disagree with the theoretical values of 72, 10 and 0.5 given by the Alaga rules “). Such a strong deviation from simple theory is not unusual for El transitions in oddmass deformed nuclei 20). Among the effects that could cause this deviation are Coriolis coupling 21922) and the interaction of the expected low-lying octupole vibrational bands with the single-particle states ‘“). The authors wish to express their thanks to Dr. L. E. Glendenin for the use of the counting equipment and to R. K. Sjoblom and R. F. Barnes for their assistance in some of the chemical purifications.

References 1) P. R. Fields, A. M. Friedman, 2) V. 3) V. 4) E. 28

J. Lerner, D. Metta and R. K. Sjoblom, Phys. Rev. 131 (1963) 1249 V. Cherdyntsev and V. I. Mikhailov, Geokhimiya, 1 (1963) 3 V. Cherdyntsev, V. L. Zverev, V. M. Kuptsov and G. I. Kislitina, Geochem. Int. 5 (1968) 355 P. Horwitz, C. A. A. Bloomquist, L. J. Sauro and D. J. Henderson, J. Inorg. Nucl. Chem. (1966) 2313

470

P. R. FIELDS

et al.

5) D. F. Peppard, G. W. Mason, W. J. Driscoll and S. McCarthy, J. Inorg. Nucl. Chem. 12 (1959) 141; D. F. Peppard, G. W. Mason and I. Hucher, J. Inorg. Nucl. Chem. 18 (1961) 245 6) G. R. Choppin, B. G. Harvey and S. G. Thompson, J. Inorg. Nucl. Chem. 2 (1956) 66 7) W. C. Bentley, J. Inorg. Nucl. Chem. 30 (1968) 2009 8) D. N. Metta, H. Diamond and F. R. Kelly, J. Inorg. Nucl. Chem. 31 (1969) 1245 9) S. A. Baranov, M. K. Gadshiev, V. M. Kulakov and V. M. Matinskii, Yad. Fiz. 1 (1965) 557; Sov. J. Nucl. Phys. 1 (1965) 397 10) M. A. Preston, Phys. Rev. 71 (1947) 865 11) D. C. Hoffman, F. 0. Lawrence and W. R. Daniels, Nucl. Phys. A131 (1969) 551 12) R. S. Hager and E. C. Seltzer, Nucl. Data sect. A, vol. 4, No. 1 and 2, Febr. 1968 13) S. G. Nilsson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 16 (1955) 14) A. M. Friedman, I. Ahmad, J. Milsted and D. W. Engelkemeir, Nucl. Phys. Al27 (1969) 33 15) T. H. Braid, R. R. Chasman, J. R. Erskine and A. M. Friedman, Phys. Lett. 18 (1965) 149 16) A. Chetham-Strode, Jr., R. J. Silva, J. R.Tarrant and I. R. Williams, Nucl. Phys. A107 (1968) 645; E. Browne and F. Asaro, private communication (1970) 17) T. H. Braid, R. R. Chasman, J. R. Erskine and A. M. Friedman, private communication (1970) 18) I. Ahmad, Ph.D. thesis, Lawrence Radiation Laboratory Report, UCRL-16888 (1966) 19) G. Alaga et al., Mat. Fys. Medd. Dan. Vid. Selsk. 29, no. 9 (1955) 20) K. E. G. Lobner and S. G. Malmskog, Nucl. Phys. 80 (1966) 505 21) A. K. Kerman, Mat. Fys. Medd. Dan. Vid. Selsk. 30, no. 15 (1956) 22) M. N. Vergnes and J. 0. Rasmussen, Nucl. Phys. 62 (1965) 233 23) F. M. Bernthal and J. 0. Rasmussen, Nucl. Phys. Al01 (1967) 513