The alpha decay of 253Cf and 254Cf

Nuclear Physics A121 (1968) 433----439;(~) Nortlf,Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

T H E A L P H A DECAY O F ~S3Cf AND ~S4Cf C. E. B E M I S , Jr. a n d J. H A L P E R I N

Chemistry Division, Oak Ridge National Laboratory *, Oak Ridge, Tennessee 37830

Received 6 September 1968 Abstract: We have observed the alpha-branching decay of 17.6 d ~sscf and 60.5 d ~54Cfusing sources

of these activities produced by the intense neutron irradiation of ss~cf followed by electromagnetic isotope separation. Alpha-particle energies and relative intensities are 5.979+0.005 MeV (94.7:t=0.9) %, 5.9214-0.005 MeV (5.34-1.9) % for *sscf and 5.8344-0.005 MeV (834-1) ~o, 5.792 4-0.005 MeV (17 5=2) % for 2~4Cf.The alpha groups in s6aCfdecay populate the rotational band in s4acm arising from the Nilsson single-particle state, ~[613~]. The ]+ base state from the ½1613~] configuration has been located at an excitation energy of 50 keV in 24°Cm. The partial alpha half-life for ~4Cf is 53.44-2.7 y based on a total half-life of 60.24-0.2 d. E

RADIOACTIVITY ~aCf, ~6'Cf [from 2ss,s6scf(n,),)]; measured E=, I~, sf/~ ratio; deduced Q~, T½ (,t), Tj. (st'); 24°Cm, 2~°Cm deduced levels, J, ~. Isotope-separated source.

]

t

1. Introduction

The nucleides, 253Cf and 254Cf, were first observed in the debris of thermonuclear test explosions in 1952 as the result of multiple neutron capture from uranium (ref. l)). Negaton decay to 20.7 d 253Es is the principal decay mode of 253Cf while 254Cf decays primarily by spontaneous fission. Several workers have indirectly observed the fl- decay of 253Cf by observing the growth of the daughter nucleide 25aEs and have reported half-life values 2); the value a) with the smallest reported error is 17.6+0.2 d. Alpha-decay branching has recently been observed for 253Cf using samples produced in thermonuclear test explosions 4). These workers report an alpha-particle energy of 5.978+0.005 MeV with an abundance of (0.31+0.04)~o of the decays of 2 s 3Cf" A 55 d spontaneous fission activity was assigned 5) to 254Cf by noting that an activity of this approximate half-life grew from the electron-capture decay parent 2S4mEs. Later workers 2) with larger samples of 254Cf were able to make more accurate half-life determinations; the value 6) with smallest statistical uncertainty is 60.5___0.2 d. Asaro 7) has reported weak alpha branching, ~ 0.2 %, of the decays of 254Cf, and an alpha-particle energy of 5.84 MeV. Hulet s) has placed limits on the partial alpha half-life of 25 to 50 yrs, corresponding to an alpha-branching ratio in the range, 0.33 to 0.66 Yo. t Research sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation. 433

434

c . E . BEMIS, JR. AND J. HALPERIN

In the present work, 17.6 d 253Cf and 60,5 d 254Cf have been produced by reactor irradiations of 2.65 y 2s2Cf. The weak alpha decay of these activities is reported together with energy-level assignments in 249Cm and 25°Cm as derived from the alpha-decay data. 2. Experimental techniques and results

A sample containing approximately 1 mg of 2.65 y 252Cf (5.6 Vo 249Cf, 16.2 ~o 25°Cf, 4.4 ~ 25xCf and 73.8 ~o 252Cf) was irradiated in the High Flux Isotope Reactor ( H F I R ) in a thermal neutron flux of ~ 2.7x 10 is n • cm -2 • sec -1 for a period of 69 days. The time-integrated neutron flux, as determined from flux monitors was 1.37 x 1022 n • cm -2. This californium irradiation, as part of the transuranium element production program at Oak Ridge National Laboratory, was performed mainly for the production of 20.7 d 253Es and higher mass einsteinium and fermium isotopes. After removal from the reactor, the target was dissolved and chemically processed using conventional ion-exchange techniques to recover the californium fraction. The isotopic composition of the calitornium fraction at the end of the irradiation is given in table 1. A small quantity of this sample was mass separated using the 160 cm elecTABLE 1 Isotopic composition of irradiated californium sample at reactor discharge Nucleide 24,cf ~socf ~lCf ~bzcf ~cf 254Cf

Atom percent o. 124 0.812 0.217 97.411 1.347 0.088

tromagnetic separator at Lawrence Radiation Laboratory, Livermore. Singly-charged californium ions were collected at an incident ion energy of 55 keV on aluminum foil. The mass 253 position of the collector foil had an activity level of ,~ 6000 alpha disintegrations/min and the mass 254 position g 30 alpha disintegrations/min and 170 fissions/rain. The composition of the mass 253 and 254 samples, determined f r o m alpha-energy spectra and from the spontaneous fission activities, indicated an enhancement of ~ 100 in the 253Cf/252Cf mass ratio for the mass 253 source and an enhancement of g 4000 in the 254Cf/252Cf ratio for the mass 254 source. Alpha-particle energy spectra were measured using a silicon-gold surface barrier detector with an active area of 200 m m 2 and a depletion depth of 100 #m. The signal amplification system consisted of a low-noise charge sensitive preamplifier, linear amplifier and biased amplifier. The alpha pulse-height information was digitized and stored using 1024 channels of a 4096 channel pulse-height analyser equipped with a digital stabilizer which wa~ used to minimize instrumental drift.

=-DECAYOF Cf

435

The measured system resolution for the 6.119 MeV alpha particles in the decay of 2 ~2Cf was ~ 15 keV, full width at half-maximum height. Spontaneous-fission events, occurring in the system as overload pulses greater than I0 MeV, were recorded simultaneously with the alpha spectra using an external discriminator-scaler system. Precautions were taken during these experiments to minimize alpha-particle and fission-fragment scattering from the detector edges by using collimators between the source and detector. Scattering phenomena would have led to erroneous alpha/ fission ratios for 25aCf, The absence of instrumental abberations was verified by measuring the alpha/fission ratio of 2 S2Cf with the mass 252 portion of the collector plate from the isotope separation. We observed a spontaneous fission branching for 2s2cf of 3.1%, in agreement with previous determinations 3, 7).

t0,000

'

d t000 uJ Z Z

254Cf

o

n,- 100 L~ a_ "~zlt'f) t0~ S "00

100

I

5,,°

I 252Cf 6,119 /~ ~1 #I 6.076~1 A}I

I

1 255Es] 6,653 254Es 6,437 i~I 6.597j ll

/v

2530f fvt ef~ II J I 6.567 /HI (compl &~J 5'792/Ii ~ 5.9791.t I".~.~~I... ~. I(c°mpl ' . ex)][ II, ~'~¢'~"I t50

200

250 300 350 CHANNELNUMBER

400

Fig. 1. Alpha-particle spectrum of isotope-separated mass 254 californium source as observed with a silicon-gold surface barrier detector. A representative alpha spectrum for the mass 254 source is shown in figure 1. Alpha-particle groups from 2S2Cf (E~o = 6.119MeV, E~a = 6.076MeV) and 2S3Es (E~o = 6.633 MeV) are the predominant alpha activities. The presence of 280 d 2s4gEs in the sample (E~86 = 6.437 MeV) is attributed to the incomplete chemical separation of einsteinium prior to the isotope separation. The 25 aEs in this sample is the result of the fl- decay of 2 s a Cf which also contributes to the alpha spectrum of the mass 254 sample (E~ = 5.979 MeV). The new alpha-particle groups at energies of 5.834_+0.005 MeV and 5.792___0.005 MeV are assigned to the decay of 254Cf because their half-lives, observed over a three month period, are consistent with the 60.5 d half-life measured for 2 s4Cf spontaneous fission decay and because the alpha-particle energies are consistent with predicted values 9). Abundances are (83__+1)% and (17___2)% of the alpha decays of 254Cf for the 5.834 MeV and 5.792 MeV groups, respectively. After application of the recoilenergy and electron-screening corrections to the measured alpha-particle energies, a value of 5.967-t-0.005 MeV was obtained for the Q, value for 254Cf.

C. E. BEMIS~JR. AND J. HALPERIN

436

The alpha/fission decay ratio for 2 S4Cf was obtained from the integrated 254Cf alpha peak areas and from the number of fission events which were counted with twice the efficiency of the alpha particles. A correction factor of 6.8 ~o, determined using the isotope-separated 252Cf source, was applied to the alpha activity of 254Cf and accounts for the number of alpha events in the low-energy tail of the alpha peaks which were not included in the integration. The contribution of 252Cf to the fission activity in the mass 254 sample is extremely small ( < 0.1 ~o). The spontaneous-fission half-lives of 253Es and 254gEs, a r e 3) 6.4 x 105 y and lo) > 2.5 x 107 y, respectively, and thus no fission events would have been observed from their decay in these experiments. The corrected alpha/fission ratio for 2 5 4 C f is (3.10+0.16)X 10 -3 which corresponds to a partial alpha-decay half-life of 53.4 + 2.7 y, if we use 60.5_+0.2 d for the total half-life. This value is almost within the limits placed by Hulet s), but is somewhat larger than the approximate value reported by Asaro 7). Analyses of the alpha spectra of the mass 253 sample revealed the presence of a weak alpha group at an energy of 5.921 +0.005 MeV in addition to the main group at 5.979+0.005 MeV previously reported 4) and as mentioned above. The decay of these two alpha groups, observed over a period of two months, is in agreement with the 17.6 d half-life reported for 2 s aCf" Abundances are (94.7 +_0.9) ~ and (5.3-1- 1.9)~ of the alpha decays of 253Cf for the 5.979 MeV and 5.921 MeV groups, respectively. We were unable to measure the alpha-branching ratio for 253Cf in these experiments because 253Es, the fl- decay daughter of 253Cf, had been separated with an efficiency much larger than that of californium in the mass separation procedure. However, using the alpha branching of (0.31+0.04)~'0 previously reported for 253Cf4), we obtain 16.4-1-2.2 y and 293+__62y for the experimental partial alpha half-lives for the 5.979 MeV and 5.921 MeV groups, respective!y. TABLE 2 Summary of alpha-spectral data for m C f and ts*Cf Nucleide

mCf luCf

Alpha-particle energy (MoV)

Abundance per alpha decay ~)

Hindrance factor

5.~9~0.~5 5.~1i0.~5 5.834~0.~5 5.792±0.~5

~.7±0.9 5.3±1.9 83 ~1 17 ± 2

1.5~0.2 13.2~2.8 1 b) 3.0~1.2

%

=) The alpha decay branching is (0.31-b0.04)~ for m C f [ref. *)] and (0.31+0.16)~ for • C f

(this work). b) The ground-state to ground-state alpha transition was taken to be unhindered (H F = 1) in accord with other doubly even alpha decay data.

Alpha decay hindrance factors (HF), the ratios of experimental partial alpha half-lives to the theoretical half-life, were determined for the alpha-particle groups

a-DECXYOr C.f

437

in the decay of 253Cf and 254Cf using the spin-independent (l = 0) equations given by Preston 11) to calculate the theoretical half-lives. The resultant hindrance factors and a summary of the alpha decay data are given in table 2. 3. Discussion

3.1. THE s6~CfDECAY Alpha-decay hindrance factors less than ,~ 4 in odd-A nuclei indicate favored alpha decay where the initial and final states have the same asymptotic quantum numbers 12). For 25acf with 155 neutrons and K = 5, the configuration ~+[613T] in the Nilsson scheme is expected. This configuration is in accord with the alphadecay data 13) for 257Fm ' and with the log f t value of 6.8 for the p - decay of 253Cf to the ~+ [633T] ground state of 253Es" Thus the 5.979 MeV favoured alpha transition (HF = 1.5) from 253Cf populates a ~ ÷ state arising from the ~ ÷ [613~ ] configuration in 249Cm. Energy levels in 249Cm have recently been studied by Friedman and coworkers via the 2gSCm(d, p) reaction 14). These workers place the spin ~ member of the rotational band arising from the ~[613T] Nilsson state at an excitation energy of 110_+ 1 keV in 249Cm. The K = ~ base state was not located in these (d, p) studies because of the presence of a spin ~ rotational state, from the ground state ½16201'] band, with similar excitation energy and a much higher (d, p) cross section. We calculate that the ~+ ~[613T] state should lie 60 keV below the known 3 ÷ rotational state at an excitation energy of 50-t- 1 keV in 249Cm using the I ( I + 1) rule and a rotational energy constant of 6.69 keV as observed 12) for the ~[613T] band in 251Cf. Since alpha decay to the entire K = ~ rotational band in 249Cm is favoured, the 5.921 MeV alpha group populates the ~+ first rotational state. The energy difference between the two alpha transitions, 58 ___2 keV, is in agreement with the expected energy separation of 60 keV for the ~ and ~ members of this K = ~ band which supports the above conclusion. The relative alpha intensities to the various rotational states in the favoured K --band in 249Cm have been calculated using the methods outlined by Bohr et al. 16). Favoured alpha decay in odd-A nuclei is very similar to the alpha decay of doubly even nuclei to the ground state rotational band (K = 0) in the daughter. Assuming that the hindrance factors for the various alpha partial waves (L = 0, 2, 4) in favoured odd-A alpha decay are the same as for adjacent doubly even alpha decay, the relative alpha populations to the rotational states in the favoured band are given by the following relationship: p _ PE ~_, C(IiLXiOlIfKf) 2 N L=o,2,4 HFL(,-e~

In the above relationship, PE is the alpha-transition probability calculated from simple barrier penetration theory 11), N is a constant, which we have taken to be unity, C is a Clebsch-Gordan coefficient and HFLt..,) is the hindrance factor inter-

438

C.

E. BEMISJR,. AND J. HALPERIN

polated from adjacent doubly even nuclei. The indices i and f refer to the initial and final states respectively. We have used relative hindrance factors of 1 : 3.4 • 57 for the L = 0, 2 and 4 partial waves respectively as observed for the alpha groups in =S2cf. The L = 6 hindrance factor is ~ 1200 and was neglected in these calculations. The calculated alpha intensities to the K = ~ favoured band in 249Cm are compared with the experimental values in table 3. TABLE

3

Calculated alpha intensities to the favoured K = ~ band in ugCm from the decay of '53Cf Transition

L = 0

Predicted intensities

( I -+ I )

-+ -+ ~, ~ ~

~ ~ -~ ~

~

(~)

83.18

L = 2

L = 4

11.41 4.69 0.31

0.11 0.18 0.06 0.06

Experimental intensity (~o)

94.7 i) 4.87 0.37 0.06

94.7 5.3+1.9

a) The calculated relative intensities for the ½ -+ { transition were normalized to the experimental value.

The calculated intensity for the { -* { alpha transition is within the error limits on the experimental value. We were unable to observe the much weaker alpha transitions to the spin ~ and higher spin members of the K = { band due to the large lowenergy tails from the 252Cf and 253Es alpha activities in the mass 253 sample. The availability of larger, more highly enriched samples of 253Cf would aid in the identification of these groups. The Q value for 2 s aCf alpha decay was determined from the experimental alphaparticle energies, corrected for recoil-energy loss and electron screening together t7.6-d 253Cf

712 + 712 [6131']

a+aB. =(0.31 ~ 0.04)%

£N£RGY

(keY) 9/2 +

146

7/2 +

110 110

5/2" 3/24 [620'1'] I/2 1/2 +

~/2

HF =13.2 +

Ea=5.979(93.4°lo)J~HF = 1.5 48 50 f~/2÷7/2 [615'I'] 25 0 64-min

[N nz AT.] n l v

Ea=5"921(5"5%)

249Cm l~r,Q, [N nz A.~.]

Fig. 2. Level scheme for 1eCru showing ~-decay pattern of 2uCf.

~t-DECAYOF Cf

439

with the excitation energies in 249Cm. The resulting Q~ value is 6.165_+0.005 MeV and is in agreement with predicted values 9). The energy level scheme for 249Cm shown in fig. 2 is a combination o f some o f the 248Cm(d, p) work o f F r i e d m a n 14) together with the results of the present work. 3.2. THE 25~CfDECAY The alpha groups in the 254Cf alpha decay populate the g r o u n d state K = 0 b a n d in 2 s OCm" The excitation energy o f the first 2 ÷ state in 250Cm ' derived f r o m the energy difference between the two alpha groups, is 42 keV in accord with the energies o f other 2 + states in d o u b l y even nuclei in this region. The measured partial alpha-decay halflife, 53.4_+ 2.7 y, is approximately a factor of 2 less than values predicted using semiempirical correlations 9). This effect is presumably due to the decrease in stability towards alpha decay for nucleides beyond the 152-neutron subsheU. 4. S u m m a r y The alpha decay o f 17.6 d 253Cf and o f 60.5 d 254Cf has been measured. T w o alpha groups have been observed in the 2 5 3Cf decay, 5.979 +_0.005 MeV (94.7_+ 0.9) and 5.921___0.005 MeV (5.3_+ 1.9 ~o), which populate the ½1613T] favoured b a n d in 249Cm. The band head, 3 + ~[613T], is placed at an excitation energy o f 50 keV in 249Cm. The Q value for 2 53Cf alpha decay is 6.165 +_0.005 MeV. The alpha/fission ratio for 60.5 d 2 54Cf measured in these experiments is (3.10_+ 0.16) x 10 - 3 whicb corresponds to a partial alpha half-life o f 53.4-+2.7y based on a 60.5 d total half-life. Alpha-particle energies (relative intensities) in the decay o f 254Cf are 5.834-+0.005 MeV (83_+1 ~o) and 5.792_+0.000 MeV (17-t-2 ~ ) . The Q value for 254 C f alpha decay is 5.967_+0.005 MeV. We are grateful to Dr. E. K. Hulet o f Lawrence Radiation L a b o r a t o r y , Livermore, for arranging the isotope separation.

References 1) P. R. Fields et aL, Phys. Rev. 102 (1956) 180; H. Diamond et al., Phys. Rev. 119 (1960) 2000 2) E. K. Hyde, I. Perlman and G. T. Seaborg, The nuclear properties of the heavy elements, Vol. II (Prentice Hall, New Jersey, 1964) p. 942 3) D. Metta et aL, J. Inorg. Nucl. Chem. 27 (1965) 33 4) Combined Radiochemistry Group, Phys. Rev. 148 (1966) 1192 5) B. G. Harvey, S. G. Thompson, G. R. Choppin and A. Ghiorso, Phys. Rev. 99 (1955) 337 6) L. Phillips, R. Gatti, R. Brandt and S. G. Thompson, J. Inorg. Nucl. Chem. 25 (1963) 1085 7) F. Asaro, quoted by C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes (John Wiley, New York, 1967) 6th ed. 8) E. K. Hulet, private communication, 1968 9) V. E. Viola and G. T. Seaborg, J. Inorg. Nucl. Chem. 28 (1966) 697 10) P. R. Fields et al., Nucl. Phys. A96 (1967) 440 11) M. A. Preston, Phys. Rev. 71 (1947) 865 12) P. O. Fr6man, Mat. Fys. Medd. Dan. Vid. Selsk. 1, No. 3 (1957) 13) F. Asaro and I. Perlman, Phys. Rev. 158 (1967) 1073 14) A. M. Friedman, private communication, July 1968 15) F. Asaro, S. Bjornholm and I. Perlman, Phys. Rev. 133 (1964) B291 16) A. Bohr, P. O. Fr6man and B. R. Mottelson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 10 (1955)