Nuclear Physics A373 (1982) 43444Q North-Holland Publishing Company

Abstracts The a-decay scheme of ~°~Fm has been investigated by measuring the a-particle, electronand y-ray specVa of several sources containing -1000 ~'Fm a-disintegrationa per minute. Fivea-groups were identified : 6.756 MeV (0.60%), 6.695 MeV (3.5%), 6.520 MeV (93 .696),6.441 MeV (2.0%) and 6.346 MeV (0.3%) . y-ray spectra were measured with Ge(Li) spec-trometers and the intensity of the 179.4 keV y-ray was found to be 8.7f 0.7 photons per 100~~Fm a-decay:. A conversion-electron spectrum was measured with a cooled Si(Li) detector andtransition multipolarities were derived. On the basis of the present investigation the ~ s 'Cf groundstate has been assi~rsed to the ~+[613] single-particle state and the ~~Fm ground :fate and the241.0 keV state in s'Cf are assigned to the ~+[615] orbital . The a-intensities and y-ray branchingration have been explained in terms of Coriolis mixing between the ~`[613] and ~ +[615] states .Our observations are in agreement with the results of a previous study by Asaro and Penman .

E

ALPHA DECAY OF ~aoFmt

LRSHAD AHMAD and E. PHILLLP HORWITZ

Gisemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

Received 22 July 1981

RADIOAC'ITVTTY ss~Fm [from prolonged neutron irradiation of ~Cm in HFIR reactor] ;measured E� I� E~� I~� E� I,, ; deduced hindrance factors . ~ s3Cf deduced levels,

y-multipolanty, !, a.

1. Introdaction

The 100 d Zs'Fm is the heaviest nuclide produced in observable quantities in thehigh-flux neutron irradiation of transplutonitun targets. Because of its relativelylong half-life this nuclide finds application as target material for the synthesis ofother very heavy nuclides andfor the investigation of chemical properties of element100. u'Fm decays predominantly (>99%) by a-particle emission and its decayscheme was investigated by Asaro and Penman t) using samples which containedbetween 2 and 28 2s'Fma-disintegrations per minute. On the basis of this investiga-tion the ZS'Cf ground state was given the i+[613] single-particle state assignmentand the Bound state of Zs'Fm and the 242 keV state in ~S3Cf were assigned to thei+[615] orbital . The present investigation was initiated several years ago with theaim of determining energies and intensities of ZS'Fm radiations and identifyingadditional states in the daughter nucleus zssCf. These measurements were madewith several sources, each containing ~1000 2s'Fm a dis/min. In the present articlewe report the results of these measuuements .

' Work performed under the auspices of the Office of Basic Energy Sciences, Division of NuclearSciences, US Department of Energy under contract number W-31-109-ENG-38 .

434

!. AhmadEP. Horwirz/ Alpha decay

435

2. Sooroe prepantlon

Samples of 2s'Fm, which were produced by long irradiation of 244Cm in the highflux reactor HFIR were provided to us by the Oak Ridge National Laboratory.The Fm fraction contained -~-3 x 103 a dis/min of Zs'Fm, ~10' a dis/min of Zs3Es,--10' a dis/min of Z'ZCf and --~10' ß- dis/min of 91Y. Fermium was separatedfrom the above elements and other actinides and fission products using an ultra-highefficiency liquid-liquid chromatographic column s.3). The column which was 3 mmdiameter x2 cm in length was packed with 5 N.m diameter totally porous silicaspheres which contained 25 weight % of stationary phase consisting of 1 F di-(2-ethylhexyl) orthophosphoric acid (HDEHP) in dodecane 3).The fermium solution was loaded on the HDEHP column in 0.1 M HNO3 and

eluted with 0.5 M HNO3 solution . Es-Cf and 91Y were déoontaminated by factorsof >104 and >10', respectively, in a single column run. Two such columns wereused in succession to obtain essentially pure Fm samples. These. samples werespread on 0.5 mm thick quartz disks to produce thin sources for a, e- and y-rayspectroscopy. The disintegration rates of these sources were determined by a andfission counting in a 2~r geometry proportional counter and alpha pulse-heightanalysis . Four sources containing 500 to 2000 23'Fm a dis/min were used in thepresent investigation .

3.1 . ALPHA-PARTICLE SPECTRA

3. Experlmeetal r~aalts

Alpha-particle spectra of Zs'Fm samples were measured with a 6 mm diameterAu-Si surface barrier detector at a source-to-detector geometry of 0.6% . A spec-trum measured immediately after the chemical separation is displayed in fig. 1 .The spectrum contains a weak peak at 6.632 MeV which could be either due toZS'Fm, or due to the daughter Z'3Es or due to the combination of both . The areaof this peak gives an upper limit on the intensity of the 6.632 MeV a-group whichis expected to populate the 136keV level in ZS3Cf . A spectrum measured severaldays after the chemical purification and counted for a longer period is shown infig. 2. Energies of a-groups were determined with respect to that of the Z°3Es asgroup 4) (6.632 MeV). Hindrance factors were calculated from the spin-indepen-dent theory of Preston s) using a half-life of 100.5 d [ref . ~] and a radius parameterof 9.400 fm. Energies, intensities and hindrance factors of a-groups identified inthe present study are given in table 1 .

3.2 . GAMMA-RAY SPECTRA

Gamma-ray spectra of Zs'Fm samples were measured with several Ge(Li) spec-trometers . A spectrum measured with a 2cmZx 5 mm planar Ge(Li) detector is

436

I. ARmad, E.P. Horwin / Alpha decay

Fig . 1 . ~~Fm a-particle spectrum measured with a 6 mm diem Au-Si surface barrier detector immedi-ately after chemical purification . Source-to-detector geometry was 0.6% ; oountiag time was 25 h .

shown in fig. 3. This spectrometer had a resolution (FWHM) of 600 eV at 122 keVy-ray energy. The 46.4 keV peak in the spectrum showed growth indicating thatit does not belong to Zs'Fm decay. This y-ray was observed in spectra of differentZs'Fm samples with approximately the same intensity relative to the intensities of2"Fm y-rays . These two facts strongly suggest that the 46.4 keV y-ray is associated

i

v

v

v m ~ v340 380 ~ 420 460 500CHANNEL NUMBER (a-PARTICLE ENERGY)

CHANNEL NUMBER (a-PARTICLE ENERGY)

)ag . 2 . ~s~Fm a-particle spectrum measured several days after chemical purification . Counting timewas S d .

Ahmad, EP. Horwiu l Alpha decay

437

Twai.s 1~Fma-gmups

100 300 500 700 900 1100 1300 1500CHANNEL NUMBER (y-joy energy)

Fig. 3. ~'Fm y-ray spectrum measured with a 2 cm= x Smm planar C3e(li) detector several day: afterchemical purification. Counting time was 6.5 d.

with the decay of 2s3Cf, the a-decay daughter of ss'Fm. This y-ray is tentativelyassigned to the ?+-~z+ transition in the ground state band of sssEs.The zs'Fm y-ray spectrum measwed with a 25 ctn3 coaxial Ge(Li) spectrometer

is displayed in fig . 4. This spectrum was measwed immediately after the chemicalpurification, andhence it has zssFm (20.1 h) y-rays . The zssFm

is the decay productof ZssEs (38.3 d), which was present along with 2s3Es (20.3 d) in the Fm fraction.Decay of spectrawas followed for several months in order to ascertain the identityof ss'Fm ~ -rays. Energies and intensities were determined from hand-plotted graphsand these are summarized in table 2.

Energy(Mew

6.756 t0.0036.695 f0.003

6.622 (expaxed)

Excited Mateenergy (tee

062136

Intensity(96)

0.60t0.063.3 t0.3<1.2

Hindrancefactor

2.2 x 10'203

>2736.320 t0.002 240 93 .6t 1.0 1.186.441 t 0.003 320 2.0t0.2 236.346f0.005 416 0.3 t 0.1 54

100 200 300 400 500 600Chonnel Number (y - ray energy)

Fig . 4 . =°'Fm y-ray spectrum measured immediately after chemical purification with a 25 cm3 coaxialGe(Li) spectrometer . Counting time 10 h .

TnHLt? 2Zs'Fm y-rays and K X-rays

') The absolute intensity of the 179.4 keV y-ray has been measured to be (8.710.7)%per ~s'Fm a-decay .

Energy(keV)

Intensity(% per Zs'Fm a-decay)

Transition(initial-" final level)

61.6t0.1 1 .45t0.08 61.6y075.0t0.1 0.20t0.017 136.E-.61.680.2 t0.2 0.085 f 0.009 321 .2y 241.0104.4t0 .1 0.62t0.05 241.0y136.6109.8f0 .1 17.Ot1 .0 Cf K,2113.Of0 .1 27.Of 1 .6 Cf K, 1128.6f0.1~ 10.4t0.7 ~ total= CfKa3129.St0 .1 58.2t3.5 Cf Kal

133.6t0.11 3.St0.27~ Cf KaZ+Ka ,134.7t0.11 Cf Ko~, 3136.7t0.2 0.06t0.02 136.E-.0179.4t0 .1 8.70 (norm)') 241 .0-.61 .6241.Ot0 .1 11 .Ot0.06 241 .0-.0

3.3. CONVERSION-ELECTRON SPECTRA

The electron spectrum of a thin 2s'Fm source was measured with a cooled Si(Li)spectrometer') . The spectrometer consists of a 80 mm~x3 mm lithium-driftedsilicon detector coupled to a low-noise pre-amplifier, with the detector and theinput stage field-effect transistor (FET) cooled to liquid nitrogen temperature. Theinstrument had a resolution (FWHM) of 1 .0 keV at 100keV electron energy . Wefound that the electron lines appeared lower with respect to the photon energiesmeasured with the same Si(Li) detector by 0.7 keV at 100 keV and 1.0 keV at20 keV. The different response of the detector to electrons and y-rays could beaccounted for by an electron energy loss in the detector "window" .The electron spectrum of a s"Fm sample measured with the cooled Si(Li)

spectrometer at an efficiency-geometry product of 1 .1 % is shown in fig. 5. Allpeaks eaoept the 39.3 keV line in the spectrum have been assigned to the ZS'Fmdecay. The 39.3 keV peak is assigned to the 46.4 M conversion line in Zs3Es .

104

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w 10zayHZ

I. Ahnead, E.P. Horwitz / Alpha daay

439

200 400 600 800 1000 1200CHANNEL NUMBER (electron energy)

Fg . S . Conversion electron spectrum of a thin z°~Fm source measured with a cooled Si(Li) spectrometerat a source-to-detector geometry of 1.196 . Counting time was 51 h.

The efficiency-geometry product of the detector was measured with a calibratedz°dig source . The energy calibration was made with the 243Am?'9Np electronlines . Transition energies were obtained by adding appropriate electron bindingenergies e) to the measured electron energies . Transition multipolarities werederived from a comparison of the measured conversion coefficients with respectivetheoretical values 9), and these along with electron energies, intensities and conver-sion coefficients are given in table 3.

440

I. Ahmad E.P. Horwitz / Alpha decay

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4 .1 . LEVEL SCHEME

For a rotational nucleus, the value of the K quantum number and the rotationalconstant, flz/2.f, of aband can be derived from the spacings between the successivemembers of the band. Using the measured level spacings of 61 .6 and 75 .0 keV(see fig . 6) we calculate the K-value of 3.60t0.10 and the rotational constant of6.84t0.05 keV for the ground state band . The ground state of zs 3Cf (N =155) isexpected' ° ) to be the i+[613] neutron orbital . For the above reasons we assign thezss~ ground state to the i+[613] single-particle state. The measured gK value ofthis band provides additional support for this assignment .The level at 241 .0 keV decays to the i, z and ~ members of the ground state

band by Ml or Ml-E2 transitions which restrict the spin-parity of this state to i+ .Since this state is populated by the favored a-transition ~=1.2) it should havethe same configuration as the zs'Fm ground state. The only K' = z+ state whichcould be the ground state of zs'Fm (N =157) is the z+[615] single-particle state Io).

~IIn Z I~ K,I'

I. Ahniad, E.P. Horwltz / Alpha decay

441

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25TFmK]0.6 as ~991L

F~uçl a illy

H.F.It,v1 It1

2~CfFig . 6. Alpha-decay scheme of Z'~Fm constructed on the basis of results of the present investigation .

442

I. Akmad, EP. Horwitz / Alpha decay

We, therefore, give the i+[615] assignment to the zs'Fm ground state and the241 .0 keV state in zs'Cf. The rotational constant for the 241.0 keV band is foundto be 7.4t0.1 keV. The same assignments to the zs3Cf ground state and the241 .0 keV state were made by Asaro and Penman') .

4.2 . ALPHA-TRANSITION PROBABILITIES

The a-decay rates to the Zs3Cf levels have been discussed in detail by Asaro andPenman 1). They estimate the intensity of .the a-group to the ~ member of thefavored band as 3 .6% . Poggenburg' 1) has calculated an intensity of 3.9% for thisstate. Both values are appreciably larger than the intensity of 2.0% measured inthe present work . This discrepancy maybe due to the fact that the hindrance factorsfor the L = 2 and 4 a-waves in the decay of the adjacent even-even nuclei z56F'mand zsBFm are quite different from the respective values in z~Fm decay used inthe above calculations .The observed intensity of 3.5% for the aez group is much larger than the value

of 0.23% estimated by Poggenburg . This increase in a-intensity was explained interms of Coriolis mixing between the ground state band and the K =i favoredband . Using the Coriolis matrix element' of 0.67 between the z+[613] and i+[615]states we have calculated the wave functions of the observed states as follows:

~ez =0.997~y~~z +0.079~r9iz ,

~zai =0.997i/r9~z-0 .079i/r,~z ,

W 136 =0.993~,iz+0.116t/r9,z .

Assuming that the major fraction of the aez intensity occurs due to the Coriolismixed component, we can calculate the intensity by the formula

4.3 . GAMMA-TRANSITION PROBABILTTIES

(H~ez (HFir.~In the above expression a is the admixture coefficient in the wave function and(HF)ez and (HF)c.~ represent the hindrance factors for the aez and azai groups.Using a from eq . (1) and (HF)r�,=1 .2 we calculate (HF)ez as 192 which is inexcellent agreement with the experimental value of 203.

Relative reduced M1 transition probabilities, B(M1), for y-rays de-exciting the241.0 keV state to the members of the ground state band in zssCf were derivedfrom the measured y-ray intensities and M1-E2 mixing ratios and these are givenin table 4. These values are substantially different from the values calculated byAlaga rules' z). This discrepancy was attributed by Asaro and Penman') to Coriolis

Transition1fC~l~y1C,h

l. Ahmad, EP. Horwitz l Alpha decay

443

TAHt.E 4Reduced Ml transition probabilities of y-rays in~3Cf

Transition

Relative B(Ml) valuesenergy(keV) exp. theor.') theor.

241.0 33 440 33179.4

100 (norm)

100(norm)

100(norm)104.4 48 10 66

') Using the Alaga rule [ref. ~z)] .b) Using Coriolis-induced wave functions and G~~z~i~ _-2.08, Gq~4q~2 ~-0.95 and

Gq~s ~~ z = +0.037 .

mixing between the ~+[613] and i+[615] bands. Ml transition matrix elementsbetween single-particle states have been computed by Chasman et al. t~. At adeformation ß2 =0.26 the calculated matrix elements are : G7iZ,i2 = -2.08,Gg~zg~2 =-0.95 andG9~2Biz = -0.37. The last matrix isverysensitive to the deforma-tion used in the calculation ; its value is +0.30 at ßi =0.23. For this reason we haveused GgiZ ~i2 as a free parameter in ow calculation . Using the wave functions givenby eq . (1) and G,~s7is = -2.08, Gg~29~2= -0.95 and G9~2 7~2 = +0.037 we havecalculated the relative B(M1) values and these are given in the last column oftable 4. As can be seen the calculations reproduce the experimental branchingratios quite well .

4.4 . gK VALUE OF THE GROUND STATE BAND

The value of gK for a rotational band can be derived from the M1-E2 mixingratio, ô2, using the expression t3)

x

7

EY

I

Qo

Iz.S (I-~I-1)=8.7x10-

(3)(I-1)(I+1) gK -gR

In the above equation I is the level spin, E,, is the transition energy in keV andQ° is the intrinsic nuclear quadrupole moment. Using the measwed mixing ratiofor the 61 .6 keV (i ~ i) transition and a value t`) of 12.9 b for Q° we deduce avalue of ~gK-gR ~ as 0.62f0.05. This is in excellent agreement with the value of-0.64 calculated t°) for the z+[613] single-particle state. The other z+ orbital inthis energy region is the z+[624] state which has gK -gR = -0.24.

References

1) F. Asaro and I . Penman, Phys. Rev. 158 (1967) 10732) E.P. Horaitz, C.A.A . Blomquist and W.H . Delphin, J. Chromatog. Sci . 15 (1977) 41

444

I. Ahmad E.P. Horwltz / Alpha dccay

3) E.P. Horwitz, W.H . Delphin, C.A.A . Bloomquist and G.F. Vandergrift, J. Chromatog. Sci . 12S(1976) 203

4) A. Rytz, Atomic Data and Nucl. Data Tables 12 (1973) 4945) M.A. Preston, Phya. Rev. 71(1947) 8656) J.F . Wild, E.K. Hulet andR.W . Lougheed, J. Inorg. Nucl. Chem . 3S (1973) 10637) I . Ahmad and F. Wagner, Jr ., Nucl . Insu . 116 (1974) 4658) F.T. Porter and M.S. Freedman, J. Phya . Chem. Ref. Data 7 (1978) 12679) R.S . Hager and E.C. Seltzer, Nucl. Data A4 (1968) 1, for K, LandM shells ;O. Dragoun, Z. Plajner andF. Schmutzler, ibid. A9 (1971) 119, for (N+O+~ ~ ~) shells

10) R.R. Chasman, I. Ahmad, A.M. Freedman and J.R . Eakine, Rev. Mad. Phya . 49 (1977) 83311) J.K . Poggenburg, Jr ., Lawrence Radiation Lab. Report UCRL-16187, Ph.D . thesis (1965)12) G. Alaga, K. Alder, A. Bohr and B.R. Mottelaon, Mat. Fys. Medd. Dan. Vid. Selak. 29 (1955)

no . 513) I. Ahmad, R.K. Sjoblom andP.R . Fields, Phya . Rev. C14 (1976) 21814) J.L.C . Ford, Jr., P.H . Stetson, C.E . Bemis, Jr., F.K . McGowan, R.L . Robinaon and W.T . Milner,

Phys . Rev. Lett. 27 (1971) 1232