Yu. P. Kharitonov

Experimental Studies of the Formation and Radioactive Decay of Isotopes with Z = 104—109

B y Y U . T s . O G A N E S S I A N , M. H U S S O N N O I S * , A . G . D E M I N , Y U . P . K H A R I T O N O V , H . B R U C H E R T S E I F E R * * , O . C O N S T A N T I N E S C U * * * , Y U . S . K O R O T K I N , S . P. T R E T Y A K O V A , V . K . U T Y O N K O V , I. V . S H I R O K O V S K Y , and J. ESTEVEZ, Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, USSR

On the stability of the nuclei of element 108 withA=263–265

In bombarding209Bi and207, 208Pb targets by55Mn and58Fe ions the yields of “cold fusion” reactions have been determined using a sensitive technique for detectingT1/2≧1 ms spontaneous fission and theα-decay of heavy actinide elements. It has been shown that theA = 263–265 isotopes of element 108, including the even-even isotope264108, undergo mainlyα-decay. The obtained results, together with the known data on the properties of the isotopes of elements 104 and 106, provide evidence for the enhanced stability of theZ=108 nuclei against spontaneous fission.

Study of the stability of the ground states and K-isomeric states of 250Fm and 254102 against spontaneous fission

By employing the 249Cf(4He, 3n) and 208Pb(48Ca, 2n) reactions, experiments to study the stability against spontaneous fission of the nuclides 250Fm and 254102 as well as of the two-quasi-particle (2q-p) K isomers 250mFm (T1/2 = 1.8 ± 0.1 s) and 254m102 (T1/2 = 0.28 ± 0.04s) have been performed. The ground-state spontaneous fission of the two nuclides has been discovered and the corresponding branching ratios bsf and partial half-lives Tsf, respectively, have been determined to be: (6.9 ± 1.0) × 10−5, 0.83 ± 0.15 yr for 250Fm; (1.7 ± 0.5) × 10−3, (3.2 ± 0.9) × 104s for 254102. As a by-product of these studies, new data about cross sections of the 206,208Pb(48Ca, xn) reactions have been obtained. Experiments designed to search for the spontaneous fission decay of the 2q-p K-isomeric states in 250Fm and 254102 have not revealed the effect in question. The lower limits of the ratios of the partial spontaneous fission half-lives for the 2q-p K-isomeric states to those for the respective ground states, Tsf*/Tsf, have been established to be ≥ 10−1 for 250mFm/250Fm and ≥ 5 × 10−3 for 254m102/254102. This means that the stability of the 2q-p K-isomeric states in 250Fm and 254102 against spontaneous fission is practically not inferior to that of the ground states of these nuclei. In accord with the experimental findings, the theoretical estimates of Tsf*/Tsf made in the present paper show that, due to the influence of the specialization and blocking effects on the potential energy and the effective mass associated with fission, spontaneous fission from 2q-p K-isomeric states cannot be facilitated but, on the contrary, should be essentially hindered compared with ground-state spontaneous fission.

Ultra-pure 236Pu and 237Pu for environmental and biomedical research

Abstract Isotopically pure 236Pu (>99.999% by activity) has been produced by irradiating U (95% 235U) with 4He ions and refining the product by electromagnetic mass separation. Further developments in the production of 237Pu are also reported, giving material containing, by activity, only 3 × 10−6% 236Pu and probably only ∼ 10−5% 238Pu.

Cross sections of the (HI, αn) channel in the cold-fusion-type reactions 209Bi+40Ar and 208Pb+37Cl

Abstract By applying an off-line radiochemistry technique, cross sections for the production of the isotope 240 Cm (T 1 2 =27 d ) in the cold-fusion-type reactions 209 Bi+ 40 Ar and 208 Pb+ 37 Cl at the bombarding energy E lab ⩽230 MeV were determined to be 0.5±0.2 nb and 0.6±0.3 nb, respectively. The production of 240 Cm was attributed to the 1n-deexcitation channel of the composite systems 249 Md and 245 Es. At the same time, the measured 240 Cm production cross sections represent upper cross-section limits for the (HI, αn) channel of the reactions under study. These upper limits are about 100 times lower than the cross-section values reported recently by Nomura et al. for the ( 40 Ar , αx n ) channels with x =1,2 of the 209 Bi+ 40 Ar reaction at E lab =208 MeV. In the context of the above measurements, presented and discussed is the up-to-date summary of the available evidence on cross sections of the (HI, αx n) channels in the cold-fusion-type reactions induced by projectiles ranging from 37 Cl to 50 Ti on targets of 203,205 Tl, 208 Pb, and 209 Bi nuclei. Appreciable EC(β + )-delayed or/and spontaneous fission effects were detected in the 209 Bi+ 40 Ar, 208 Pb+ 37 Cl, and 206 Pb+ 37 Cl reactions. The obtained data point, in particular, to the EC(β + )-delayed fission occuring in the decay chains and .

Measurements of the maximum α-particle energy at forward angles in the reaction22Ne+197Au

The energy spectrum of α-particles has been measured for the system22Ne+197Au atE1ab=178 MeV. The reaction products emitted at 0° relative to the beam direction are analysed using a magnetic spectrometer and detected by means of a semiconductorΔE−E telescope. The measured α-particle spectrum strongly differs from the one expected from evaporation. The experimentally measured maximum α-particle energy almost amounts to the maximum possible value calculated from the reaction energy balance. The obtained results are discussed from the point of view of the mechanism of formation of high-energy α-particles.

Synthesis of260Ku in the irradiation of249Bk by15N ions

Conclusions1.In the249Bk(15N, 4n)260Ku reaction an isotope of kurchatovium (element 104) is obtained which decays by spontaneous fission with a half-life of ∼0.1 sec. This result is in complete agreement with conclusions of earlier works [1, 3, 5] in which260Ku was synthesized in other combinations of target and particle:242Pu+22Ne,246Cm+18O.2.The failure to observe260Ku with T1/2≈0.1 sec at Berkeley [10, 11] resulted from improper experimental arrangements and data processing.3.The detection of the spontaneous fission of nuclei of an unknown isotope when background is properly considered and suppressed permits the identification of the Z and A of the nuclei under study.

Analysis of the composition of a mixture of249Bk+249Cf on the basis of x rays

ConclusionThe results of the study enable us to conclude the following: the β decay of249Bk is accompanied by an L series of x rays from the daughter product Cf. The yield of this radiation is low: ω(Lα+Lβ+ Lγ) ≈ 10−4 (β disintegration)−1, but because of the high specific β activity of249Bk, 1 μg of this isotope emits ∼3.7·105 x-ray quanta per minute. This enables us to detect a fairly small amount of Bk (∼0.01 μg) on the basis of x rays in a period of 5–10 min. The rapid accumulation of the daughter product249Cf does not interfere with the determination of the249Bk on the basis of x rays. Even when the mixture contains equal numbers of Bk and Cf nuclei, the ratio of the Lα lines corresponding to them is 0.46. Therefore the composition of a mixture of249Bk+249Cf can be determined on the basis of x rays over broad ranges of variation of the relative concentrations of Bk+Cf, roughly from 0.03 to 80. It is important to note that for relative measurements it is not necessary to prepare special specimens, since x rays with energies of 15–20 keV are not strongly absorbed in the solution and in the walls of the chemical vessel.

Detectors for Spectrometry of the x-ray emission from germanium obtained by the hydride method

This paper investigates certain characteristics of p-n structures, prepared on a base of germanium obtained by the hydride method. The purpose of the study is to estimate the degree of purity of the germanium and its applicability for the preparation of the detectors with a high energy resolution. Monocrysstals of the p type were used. On completion of diffusion of the lithium in the samples by means of the procedure described, a protective groove was cut out. The apparatus spectra of Fe-55 and Am-241 are shown. The characteristics of the detectors obtained confirm the excellent quality and high purity of the germanium hydride.

Collecting nuclear reaction products by adsorption from a gas flow in a slit channel

Abstract Experimental results of collecting recoil atoms produced in reactions with heavy ions are presented. The recoil atoms thermalized in gas are transported by the gas flow into a 1 mm wide slit channel formed by the sensitive surfaces of radioactive decay detectors. Due to the adsorption from the gas flow it is possible to collect ≥50% of the initial recoil atoms on the detector surfaces within a time interval of ≤0.1 s.