Nuclear Physics A413 (1984) 423431

0 North-Holland Publishing Company

ALPHA-DECAY PROPERTIES OF 247Cf, 248Cf, *‘*Fm AND *“Fm

IRSHAD AHMAD and J. L. LERNER’

Chemistry Division, Argonne National Laboratory, 9700 South Cass Atienue, Argonne, Illinois 60439,

USA

Received 2 August 1983

Abstract: Alphadecay properties of 247Cf, ‘?Zf, ls2Frn and 254Fm were measured using thin mass- separated sources. Alpha spectra were measured with Au-Si surface barrier detectors and a magnetic spectrometer, and these were used to determine energies and intensities of a-groups. The energies and intensities are: 6.296 (95 %), 6.238 (5 %) in *47Cf decay; 6.258 (80.0 %), 6.217 (19.6 %), 6.118 (0.4 %) in *‘%f decay; 7.039 (84.0 %), 6.998 (15.0 %), 6.904 (0.97 %), 6.759 (0.023 %) in “‘Frn decay; 7.192 (85.0 %), 7.150 (14.2 %), 7.050 (0.82 %), 6.898 (0.0066 %) in 2s4Fm decay. Gamma-ray spectra of mass-separated z52Fm and 254Fm samples were measured with a high-resolution Ge(Li) spectrometer, and the 2+ -+ O+ and 4+ + 2+ transitions were observed in the decays of both nuclei. The half-lives of 247Cf and 252Fm were measured to be 3.11 kO.03 h and 25.39 kO.04 h, respectively. The a branching ratio a/(a+EC) for 247Cf was found to be (3.5k0.5) x 10e2 % and the fission/a ratio was measured to be (2.3 kO.2) x 10-j % which corresponds to a fission half-life of 125 +8 y.

RADIOACTIVITY 24’Cf, 248Cf [from 246Cm(a, xn)], 252Fm [from “?Zf(a,n)], 254Fm

E [from 254mEs (p-decay)]; measured E,, I,, E,, I,, T,,,, a branching ratio, SF branching ratio. Mass-separated 247Cf, *?Zf, 25ZFm and 254Fm.

1. Introduction

The cc-decay properties of 248Cf and 252Fm have not been measured carefully

and no u-group in the decay of 247Cf was observed before the present work was

undertaken. The u-decay branching of 247Cf is very small and hence it would have

been difficult to identify the 247Cf m-group in the presence of other interfering

peaks in the spectrum. The measurements of 247Cf, 248Cf, 2s2Fm and 254Fm c(-

spectra were undertaken because of the availability of mass-separated sources.

Some of the results have already been quoted in the Table of Isotopes ‘). In the

present article we describe these measurements in detail.

+ Deceased.

423

424 I. Ahmad, f. L. Lerner / Alpha decay

2. Source preparation

Sources of 247Cf (3.1 I h) and 248Cf (333 d) were prepared by the irradiation of

approximately one milligram of 246Cm (99.8 % by mass) with 40 MeV T-particles

in the Argonne 152 cm cyclotron. The irradiation and chemical purification

procedures have been described elsewhere 2)_ The chemically purified Cf samples

were run through the Argonne electromagnetic isotope separator 3, to prepare thin

isotopically enriched sources of 14’Cf and 248Cf. The zs2Fm (25.4 h) activity was

produced 4, by the 249Cf(a, n) reaction and the 2s4Fm (3.24 h) sample was

obtained “) as the daughter product of the 39.4 h 254mEs. Thin sources of both

isotopes were prepared in the isotope separator.

3. Experiment~I results

3.1. ALPHA-PARTICLE SPECTRA

Alpha-particle spectra of mass-separated 247Cf and 24”Cf sources were measured

with a 1 cm2 Au-Si surface barrier detector at a source-to-detector geometry

of ti 5 % and these are shown in figs. I and 2. The gain of the counting system

was held constant with a digital-gain stabilizer and the spectrum resolution [full

width at half-maximum (FWHM)] was _ 20 keV. Energy calibration was made

with a 244Cm (E,, = 5.805 MeV) source and the 246Cf peak (Eao = 6.750 MeV)

[ref. “)I present in the spectrum. The 6.296 MeV a-group decayed with a half-life of

3.3+0.3 h. This half-life plus the fact that the source was mass separated

unambiguously establish that the 6.294 MeV cc-group belongs to the 247Cf decay.

Energies, intensities and hindrance factors of cc-groups obtained from the present

investjgation are given in table 1. The errors denote one standard deviation 0. The

energies of the “‘Cf and 248Cf a-groups would increase by 5 keV if we had used

6.758 MeV [ref. ‘)I for the energy of the 246Cf q, group. The hindrance factors were

calculated from the spin-independent theory of Preston ‘) using radius parameters

of 9.285 fm and 9.273 fm for 247Cf and 248Cf, respectively. The a-decay half-life

for 247Cf was 370 d and for 248Cf it was 333 d [ref. “)I.

Alpha-particle spectra of mass-separated ” 2Fm and 2 54Fm sources were measured

with a 6 mm diameter Au-Si surface barrier detector and also with the Argonne

double-focussing magnetic spectrometer ‘). The semiconductor detector had a

resolution of 13 keV apd the magnetic spectrometer had a resolution of 5 keV at a

transmission of 0.1 % of 471. The 252Fm and 254Fm spectra recorded with the

magnetic spectrometer are displayed in figs. 3 and 4. The energy calibration of the

cr-spectrometer was made with the following set of standards lo): 233U (4.824

MeV), 238Pu (5.499 MeV), 244Cm (5.805 MeV), 242Cm (6.113 MeV), 211Bi (6.279

MeV and 6.623 MeV) and 214Po (7.687 MeVf. The intensities of ol-groups were

determined from spectra taken with the silicon detector. Intensities obtained from

I. Ahmad, .I. L. Lemer / Alpha decay 425

-

13NNVH3 kl3d SINflOg

426 1. Ahmad, J. L. Lerner 1 Alpha decay

TABLE 1

Energies and intensities of a-groups

Parent nucleus

Energy Excited state WeV) energy (keV)

Intensity (“/)

Hindrance factor

247Cf 6.296 f 0.005 not known 95 +3 1.5 6.238 ~0.006 not known 5 +1 1s

2‘Wf 6.258 +O.OOS 0 80.0 k 1.0 1.0 6.217~0.005 42 19.6 fl.O 2.6 6.118+0.007 142 0.4 +0.2 40

zszFm 7.039 _+ 0.002 0 84.0 +O.S 1.0 6.998 & 0.002 42 15.0 +0.2 3.8 6.904 + 0.002 137 0.97 ,0.04 23 6.759 & 0.003 285 0.023 F0.005 226

ZsJFm 7.192 * 0.002 0 85.0 kO.5 1.0 7.150 + 0.002 43 14.2 ,0.3 4.0 7.050 f 0.002 144 0.82 50.06 27 6.898 f 0.003 299 (6.6*0.8)x 1o-3 766

105, t I I I I -T- ’ / I I / ->

I I !I IO4

t i 1 1; I I: II

/ .I.- rLB9o._L_.-mNLb--L-- Lb_,: 1 I I I

l - 120 140 160 160 200

CHANNEL NUMBER

Fig. 3. Alpha-particle spectrum of a mass-separated ‘s2Fm source measured with the Argonne double focussing magnetic spectrometer. Energy scale is _ 3.3 keV per channel.

I. Ahmad, J. L. Lerner / Alpha decay 427

z

z 0

102+

IO'-

P l . . Il I t ’

I .

i . I . I ;

1 f

& B

I

I d !”

II.IdI. I I I I I I

120 140 160 180 200 220 CHANNEL NUMBER

Fig. 4. Alpha-particle spectrum of a mass-separated 254Fm source measured with the Argonne double

focussing magnetic spectrometer. Energy scale is _ 3.3 keV per channel.

spectra measured with the magnetic spectrometer had large uncertainties because

of the large contribution from the efficiency calibration of the elements of the

detector array used to detect the a-particles in the focal plane of the spectrometer.

Energies, intensities and hindrance factors of a-groups obtained in this study are

included in table 1. The hindrance factors were calculated from the theory of

Preston ‘) using radius parameters of 9.220 fm (252Fm) and 9.383 fm (z54Fm).

428 I. Ahmad, J. L. Lerner 1 Alpha decay

3.2. GAMMA-RAY SPECTRA

Gamma-ray spectra of mass separated “‘Frn and 2s4Fm samples were

measured with a 2 cm2 x 5 mm planar Ge(Li) detector. The spectrometer had a

resolution (FWHM) of 600 eV at 122 keV y-ray energy. Energy calibration was

made with 241Am, 243Am-239Np and ‘“9Cd y-rays and the detector efficiency was

determined with calibrated 241Am, 243Am, lo9Cd, and 57Co sources. Energies of

the 254Fm y-rays were found to be 42.76 +0.05 and 99.16 kO.05 keV. Absolute

intensities of these :)-rays were obtained by determining the cc-disintegration rate of

the 254Fm sample before measuring its y-ray spectrum. These measurements gave

the intensities of the 42.76 and 99.16 keV y-rays as (0.013 fO.OO1) y/i and

(0.031 f0.003) % per 254Fm a-decay, respectively. These intensities can also be

deduced from the measured M-intensities using measured conversion coefficients ‘I).

Using the total conversion coefficient of 1110 and 25.2 for the 42.76 and 99.16 keV

transitions and the a-intensities in table 1 we obtain the intensities of the two y-

rays as 0.014 y/, and 0.031 “/, per a-decay. These values are in excellent agreement

with the measured intensities.

The energies of the 252Fm y-rays were determined to be 41.53+0.06 and

96.28+0.06 keV with relative intensities of 1.0 and 3.3 f0.3, respectively. Using

the a-intensities in table 1 and the theoretical conversion coefficients 12) of 1450

and 27.5 for the 41.5 and 96.3 keV transitions, respectively, we determine the

intensities of these y-rays as (0.011 +O.OOl) and (0.035+0.003) 7” per 252Fm a-

decay, respectively.

3.3. HALF-LIFE AND BRANCHING RATIOS

The half-life of 247Cf was obtained by following the decay of the 294.1 keV y-

ray measured with a 25 cm3 coaxial Ge(Li) spectrometer. A least-squares fit to the

photopeak areas gave a half-life of 3.11 kO.03 h. The a-branching ratio for 247Cf

decay was obtained by measuring the a- and y-spectra of the same source with a

calibrated cc-detector and a Ge(Li) spectrometer, respectively. These measurements

gave a/K X-rays ratio of (4.7kO.5) x 10m4. Using the K X-ray intensity of 72.3 %

per 247Cf EC decay, determined in our earlier measurement 2), we obtain

or/(cr+ EC) = (0.035 +O.OOS) %.

The half-life of 252Fm was determined from the decay of its c(~ and c(42 peaks.

The a-spectrum of a mass-separated 252Fm source was measured with a 1 cm2 Au-

Si surface barrier detector at several intervals. The counts in the a, and c142 peaks

were obtained for each spectrum by adding the counts spanning the two peaks.

Each spectrum was counted for 10 h and a total of 15 spectra were measured. The

gain of the counting system was held constant with a digital-gain stabilizer. A least-

squares fit to these counts gave a half-life to 25.39 +0.04 h for “‘Frn decay.

The fission to cc-decay ratio was obtained by counting a mass-separated 252Fm

I. Ahmad, J. L. Lerner / Aipha decay 429

source in a 2n geometry proportional counter. The fission events and g-particles were counted simultaneously in two separate scalers. The measurement gave a fission/a ratio of (2.3 +0.2) x 10e3 % which corresponds to a fission’ half-life of 125$8 y for 252Fm decay. This half-life is in agreement with the previously measured value of 115 IL 60 y [ref. ’ )].

3.4. ALPHA-GAMMA COINCIDENCE MEASUREMENT

The small hindrance factor (WF = 1.5) of the main 247Cf z-group suggests that it represents the favored N-transition and hence the state populated in z43Cm has the same configuration as the 247Cf ground state, namely the $‘[624] state. The ground state of 243Cm is known to be the 2+[622] single-particle state. To measure the energy difference between the :+[624] and $+[622] states in 243Cm an xy-coincidence experiment was performed. The a-particles emitted by a mass- separated 247Cf source were detected with a 1 cm2 Au-S surface barrier detector (geometry = 15 “/,) and a 25 cm3 coaxial Ge(Li) detector was used for the detection of p-rays and X-rays. The resolving time (22) of the counting system was 2 ~LS. No y-ray or Cm K X-ray was observed in the y-ray spectrum gated by 147Cf a-particles. Only Bk K X-rays, which were in random coincidence with g-particles, were present in the spectrum. The source strength was strong enough so that Cm K X-rays resulting from the internal conversion of an Ml transition between the 4’[624] and $+[622] states would have been observed. The absence of Cm K X- rays in the gated y-ray spectrum indicates that the energy difference between the above two states is less than the K-electron binding energy in Cm, which is 128.2 keV [ref. ’ “)I.

4. Discussion

We have made the first ~enti~cation of the 247Cf a-groups. The low hindrance factor of the main z47Cf @-group suggests that the excited state populated in 243Cm by this a-group has the same configuration as the ‘07Cf ground state, the $+[624] state. The absence of Cm K X-rays in the y-ray spectrum gated by 247Cf a-particles indicates that the energy difference between the $+[624] state and the 243Cm ground state is less than 128.2 keV. Using this information and a closed cycle we obtain an upper limit of 6.54 MeV for Q,. This is the only experimental estimate of 247Cf Q, available. This is in agreement with the value of 6.55 MeV deduced from systematics ‘>.

The a-decay schemes of 2s2Fm and 254Fm constructed on the basis of the results of this investigation are displayed in fig. 5. The energies and intensities of 2s4Fm cc-groups measured in this work are in excellent agreement with the respective values of Asaro et af. 1*‘o*i4). In the case of 252Fm, we have made the

430 I. Ahmad, J. L. Lemer / Alpha decay

*s2Fm 25.4 h

Ogo+ 7

KJ” Energy I,, (keV) H.F.

6+ 265 /o&23%.

I

j K,I” Energy I,, (keV) H.F.

I I 6+

6.6 x 299 /766

lo-‘%,

I

Fig. 5. Alpha-decay schemes of fz;Frn and f$Frn

first identification of its y-rays. Also the present work provides the only careful

measurement of energies and intensities of 252Fm cc-groups.

The energies of the rotational members of the ground-state band can be

calculated from the equation

E, = AZ(Z+ l)+BI’(Z+ 1)2, (1)

where I is the spin of the state, A (= k2/2.Y) is the rotational constant and B is a

constant which is a measure of the rotation-vibration coupling. From the measured

transition energies (fig. 5) the values of A and B are calculated to be 6.935 k 0.013

keV and - (2.2 ? 0.5) eV for 248Cf and 7.140 f 0.011 keV and - (2.2 f 0.5) eV for

250Cf ground-state band. These values are quite similar to the values deduced for

the ground-state bands of other even-even actinide nuclei.

The authors wish to thank R. K. Sjoblom for the preparation and purification

of sources and J. Milsted for the measurement of a-spectra with the magnetic

spectrometer. This work was performed under the auspices of the Office of High

Energy and Nuclear Physics, Division of Nuclear Physics, US Department of

Energy under contract number W-31-109-ENG-38.

I. Ahmad, .I. 15. Werner / Alpha decay

References

431

I) C. M. Lederer and V. S. Shirley, ed., Table of isotopes, 7th ed. (Wiley, New York. 1978) 2) 1. Ahmad, S. W. Yates, R. K. Sjoblom and A. M. Friedman, Phys. Rev. C20 (1979) 290 3) J. Lerner, Nucl. Instr. 102 (1972) 373

4) 1. Ahmad, R. K. Sjoblom, A. M. Friedman and S. W. Yates, Phys. Rev. Cl7 (1978) 2163 5) 1. Ahmad, H. Diamond, J. Milsted, J. Lerner and R. K. Sjoblom, Nucl. Phys. A208 (1973) 287 6) S. A. Baranov and V. M. ShBtinskii, Yad. Fiz. (Sov. J. Nucl. Phys.) 26 (1977) 461 7) M. A. Preston, Phys. Rev. 7t (1947) 865

8) E. K. Hulet and J. F. Wild, Radiochem. Anal. Lett. 13 (1973) 217 9) I. Ahmad and J. M&ted, Nucl. Phys. A239 (1975) 1

10) A. Rytz, At. Data and Nucl. Data Tables 23 (1979) 507

II) M. S. Freedman, I. Ahmad, F. T. Porter. R. K. Sjoblom, R. F. Barnes, J. Lerner and P. R. Fields, Phys. Rev. Cl5 (1977) 760

12) F. RGsel, H. M. Fries, K. Alder and H. C. Pauli, At. Data and Nucl. Data Tables 21 (1978) 291 13) F. T. Porter and M. S. Freedman, J. Phys. Chem. Ref. Data 7 (1978) 1267 14) F. Asaro, S. Bjernholm and I. Perlman, Phys. Rev. 133 (1964) B291