The mass distributions of fission fragments in the reaction 208Pb+48Ca

Volume 69B, number 3

PHYSICS LETTERS

15 August 1977

THE MASS DISTRIBUTIONS OF FISSION FRAGMENTS I N T H E R E A C T I O N 2 ° 8 p b + 48Ca R. KALPAKCHIEVA, Yu.Ts. OGANESSIAN, Yu.E. PENIONZHKEVICH, H. SODAN and B.A. GVOZDEV Joint Institute for Nuclear Research, Dubna, USSR

Received 25 May 1977

The mass distributions of fragments resulting from the fission of the compound nucleus produced in the reaction 208Pb + 48Ca have been measured. The asymmetric mass distribution obtained indicates the presence of shell effects in the nucleus 256102 at an excitation energy of 25 MeV.

The problem of the artificial production of heavy nuclei is essentially that of producing weakly excited compound nuclei, which deexcite by emitting a small number of neutrons. In refs. [1, 2], it was shown that the fusion of "magic" nuclei such as lead isotopes witk ions heavier than argon leads to the formation of compound nuclei having a very low excitation energy, sometimes ~ 2 0 - 3 0 MeV. Due to this circumstance, the compound nucleus emits as few as two or three neutrons, thus increasing substantially the yield of the final product in the ground state. The 48Ca ions seem to be unique in this respect [3]. For instance, it has been established experimentally that the cross section for the reaction 208pb(48Ca, 2n)254102 is equal to about 4/2b, i.e. a factor of nearly 100 larger than the cross section for 238U(22Ne, 4n) 256102 [4]. The high yield of the element 102 nuclei in experiments using 48Ca ions is conditioned by the fact that the compound nucleus 256102 has an excitation energy of 18 MeV in the vicinity of the reaction Coulomb barrier, and, therefore, competition between fission and neutron evaporation exists only at the first two stages of the neutron cascade. At the same time, we belive that at such a low compound nucleus excitation energy the fission process itself may show structural effects similar to those involved in the fission of uranium or plutonium at low energies [5]. Therefore, the purpose of the present work was to measure the mass distribution of fission products

formed in the reaction 208pb + 48Ca. Since 208pb and adjacent nuclei which can be produced in multinucleon transfer reactions have a high fission barrier, a radiochemical method has been used to separate the reaction products and to determine the mass spectrum of the fragments produced as a result of fission of the compound nucleus 256102. A target was prepared of a 208pb layer (enriched to 98.3%), 0.8 mg/cm 2 thick, deposited onto a thick A1 catcher foil. The admixture of heavy metals and rare earth elements in the catcher foil was not greater than 10 ppm. The target placed at 30 ° with respect to the beam direction was water-cooled. The particle intensity of the 48Ca ion beam was 1.5 X 1012 s -1, the ion energy was chosen as 220 MeV, i.e. 8 MeV above the reaction Coulomb barrier. For a "thick" 2°8pb target, this projectile energy determines the 256102 maximum excitation'energy as 25 MeV. The procedure of radiochemical separation of the isotopes of I, Te, Ag, Ba and rare earth elements took two hours. The accuracy of the determination of the chemical yield was better than 10%. The identification of the isotopes and the determination of their yields were performed by measuring the "r-radiation of the samples using a Ge(Li) detector with an energy resolution of about 2 keV (at E.t = 660 keV). The cross sections for the formation of different reaction products are presented in table 1. On the basis of the data obtained we have plotted the isotopic and mass distributions of fission fragments under the following assumptions used in such cases, i.e. 287

Volume 69B, number 3

PHYSICS LETTERS

15 August 1977

Table l Isotope production cross sections in the reaction 2°spb + 4SCa. Iso tope

T1/2

1621 1301 1311 1321 1331 1351 131roTe 132Te ll2Ag ll3Ag 92y 93y 133Bam 135Barn 140Ba 140La 137Ce 139Ce 141Ce

13d 12.4h 8.04d 2.28h 21h 6.7h 30h 78h 3.13h 5.3h 3.5h 9.6h 38.9h 28.7h 12.8d 40.2h 9.0h 137d 32.5d

E3,(K3,) (keY)

o (mb)

Isotope

T1/2

E3,(KT) (keV)

o (rob)

389(0.35) 536(1.00) 364(0.82) 773(0.75) 530(0.87) 1260(0.29) 150(0.36) 228(0.88) 617.4(0.42) 259(0.014) 943(0.14) 267(0.072) 176(0.183) 268(0.16) 537.2(0.34) 487(0.46) 446(0,023) 165.8(0.8) 145.4(0.48)

0.14 0.56 0.72 0.52 0.19 0.045 0.16 0.06 0.65 0.85 0.34 0.65 0.14 0.35 0.22 0.81 0.10 0.46 1.05

143Ce 144Ce 147Nd 148pm 150pm 151pm 153Sm 156Sm 149Eu 157Eu 159Gd 156Tb 160Tb 161Tb 157Dy 171Er 172Er 173Tm

35h 284d lld 5.4d 2.68h 27.8h 46.4h 9.4h 106d 15.1h 18.0h 5.4d 72d 6.9d 8.1d 7.5h 49.5h 8.2h

293(0.46) 133(0.11) 91(0,15) 550(0.26) 334(0.74) 340(0.21) 103(0.28) 88(0.3) 328(1,0) 413(0.27) 363(0.10) 534.3(0.7) 879(0.30) 75(0.10) 326(0.95) 308(0.64) 407(0.44) 399(0.89)

0.78 0.80 1.10 0.44 0.22 0.34 0.09 0.22 0.04 0.10 0.18 0.05 0.16 0.15 0.02 0.025 0.016 0.05

(i) the isobaric distribution of fission fragments is described by the Gaussian

=__!_1_ 1

W(Z-Zp)

-_Zp)21J

X/,--~z2 exp [ - ( Z Oz2

and is related to the isotopic distribution in the following way

°n

2[ Zp 7- 2

;

(ii) the relation Zp(Af) satisfies the equal charge displacement postulate, and (iii) the average number of neutrons emitted by fission fragments is proportional to their masses. The values of 02 and ~- are the parameters calculated in such a way as to obtain the best fit to the points of the isotopic distribution, which is, by definition, a Gaussian of the form

W(A°-

exp|A°" L

288

Fig. 1 shows the isotopic distributions of I, Te (symmetric fission) and rare earth elements (asymmetric fission), which are characterized by practically the same dispersion, but differ in the number of fissinn neutrons, V (these values are 6.5 and 8.5 for symmetric and asymmetric fragments, respectively). These ~- vales lie within reasonable limits as the experimentally obtained value of V for spontaneous fission of 252102 is 4.2 -+ 0.3 [6]. The mass distribution of fission fragments is shown in fig. 2. Despite the spread of the experimental points asymmetric fission involving the formation of a heavy fragment with mass lying between 142 and 145 proves to be predominant over symmetric fission to fragments with masses A h = A 1 ~ 128. As the projectile energy increases, the fission cross section increases rapidly, and, as shown in fig. 2, at Ela b = 250 MeV (E* = 53 MeV) the mass distribution becomes nearly symmetric. Therefore we think that the asymmetry fission fragment distribution observed in the fission of the nucleus 256102 with a maximum excitation energy of 25 MeV is due to shell effects that manifest themselves in the low-energy fission of practically all actinide elements up to 256Fm.

Volume 69B, number 3

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Fig. 1. Isotopic distribution of symmetric (upper curve) and asymmetric (lower curve) fission fragments in the reaction 2°sPb + 48Ca. The compound nucleus excitation energy is 25 MeV. The distribution dispersion a2n is equal to 12 in both cases, while v is 6.5 and 8.5 for symmetric and asymmetric fragments, respectively.

100 120 140 160 180 Moss Number. A

Fig. 2. Experimental mass distributions of fission fragments from the compound nucleus 256102 at two excitation energies: 25 MeV (circles) and 53 MeV (triangles). The dash-dotted lines are drawn through the experimental points to guide the eye. The histogram and dashed line show the mass distributions of spontaneous fission fragments from 252102 [ 8 ] and 256Fm [7 ]

The position of the maxima of the mass distribution of the fission fragments from 256102 is in agreement with analogous data obtained on spontaneous fission of 246Fm and 252102 [7, 8]. However, the mass distribbtion dispersion with respect to the avarage masses of the light and heavy fission product groups for the excited nucleus 256102 is somewhat wider than in the case of spontaneous fission. This fact seems to be the main reason for the substantial decrease in the ratio Yasymm/Y symm compared with the data obtained in experiments in spontaneous fission of 256 Fm and 252102.

The authors are grateful to S. Zaikin and Z.D. Pokrovskaya for their assistance in the data handling, the U-300 staff for providing intense 48Ca ion beams, and L. Pashkevich for preparing an English version of the paper.

It is noteworthy that the integral yield of fission fragments at Elab = 220 MeV is about 50 nab. This valUe constitutes a considerable portion of the total cross section for the reaction 208pb + 4 8Ca and is in good agreement with the cross section for the reaction 208pb(48Ca, 2n) 254102, equal to 4/ab. The data obtained indicate that similar reactions may be used to study nuclear fission from weakly excited states. It is of certain interest to investigate fission

[1] Yu.Ts. Oganessian et al., Nucl. Phys. A239 (1975) 157. [2] Yu.Ts. Oganessian, A.S. Iljinov, A.G. Demin and S.P. Tretyakova, Nucl. Phys. A239 (1975) 353. [3] G.N. Flerov et al., Nucl. Phys. A267 (1976) 359. [4] Yu.Ts. Oganessian, Nukleonika 22 (1977) 89. [5] J.L. Colby and J.W. Cobble, Phys. Rev. 121(1961) 1410 and 1415. [6] Yu.A. Lazarev, O.K. Nefediev, Yu.Ts. Oganessian and M. Dakowski, Phys. Lett. 52B (1974) 321. [7] K.F. Flynn et al., Phys. Rev. C5 (1972) 1725. [8] C.E. Bemis et al., Phys. Rev. C15 (1977) 705.

of superheavy nuclei, the mechanism of which may differ qualitatively from that of transuranic elements.

References

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