A. N. Andreyev

The Vassilissa facility for electrostatic separation and study of complete fusion reaction products

Abstract The Vassilissa facility is described. The main component is an electrostatic separator of complete-fusion reaction products from the ion beam. The facility is capable of transmitting recoil nuclei emitted by the target in the beam direction within a solid angle of 10 −2 sr and has a ± 10% spread in energy and ionic charge. The time required to transport the nuclei to the detectors is 3 × 10 −6 s The separation of scattered ions is performed using a system of three electrostatic condensers. Separation factors obtained for Ar, P and Ne are equal to 10 10 , 10 11 and 10 12 , respectively.

Decay widths of highly excited Ra compound nuclei

Abstract Excitation functions were obtained for the x n, p x n and αx n decay channels of 216,218,220 Ra compound nuclei produced in the reactions 22 Ne+ 194,196,198 Pt in the excitation energy range of 40–160 MeV. Due to the employment of three platinum isotope targets, the cross section ratios were measured with a high degree of accuracy for evaporation reaction channels resulting in the formation of fixed cold products after the termination of evaporation cascades of a different numbers of stages. Experimental values for the reduced neutron widths of highly excited compound nuclei were obtained from the measured cross section ratios. Evaporation reaction cross sections were well described in the framework of the statistical model taking into account shell effects according to Ignatyuk. Our present, and also earlier, investigations showed that calculations making use of one single set of the model parameters (the scaling factor to the liquid-drop fission barrier of Cohen, Plasil and Swiatecki, C = 0.6–0.7 and the ratio of the level density parameters, a f a v = 1.0 ) reproduces correctly the cross sections of the evaporation reactions in a wide range of compound nuclei extending from Bi to U. Moreover, the measured cross section ratios are exceptionally sensitive to the value of the parameter a f a v , i.e. are very suitable for a high accuracy estimation of this value. By fitting two sets of the experimental data — the absolute cross section values and cross section ratios — we were able to independently estimate the values of two principal parameters of the model, C and a f a y . Our calculations showed that a large part of the pre-fission neutrons is evaporated by hot pre-actinide compound nuclei before they reach the saddle point (50 to 100% of the total number of pre-fission neutrons are pre-saddle).

Cross sections of 102 element isotopes formation in the reactions of22Ne+236U and26Mg+232Th

The production cross sections of the isotopes252102,253102, and254102 were measured for the heavy ion fusion reactions of22Ne+236U and26Mg+232Th by using the kinematic separator VASSILISSA. The obtained excitation functions and the maximum production cross sections are compared with the ones for more asymmetric reactions leading to the same compound nucleus258102. The experimental cross sections and the results of statistical model calculations are compared and discussed.

The new isotopes223, 224U

In the heavy-ion complete fusion reaction208Pb+20Ne the new isotopes223,224U were produced. These nuclei were identified after in-flight separation with the kinematic separator VASSILISSA, followed by their implantation into a passivated ion implanted silicon detector and the observation of the genetic relationships of subsequent α-decays.223U was found to decay withEα=(8780±40) keV and T1/2=18−5+10μs. For224U the α-line atEα=(8470±15) keV and T1/2=0.7−0.2+0.5 ms was observed.

Large area high-efficiency time-of-flight system for detection of low energy heavy evaporation residues at the electrostatic separator VASSILISSA

Abstract A large-area thin timing detector for slow heavy nuclei was developed. The apparatus is based on the transport of secondary electrons by crossed electric and magnetic fields with subsequentamplification by micro-channel plate multipliers. To increase the registration efficiency secondary electrons are collected from the both sides of the emitter foil. An intrinsic time resolution of (380±30) ps and registration efficiency of (95±5)% were obtained for the single TOF detector using a 226 Ra α-source. A time resolution of (400±30) ps and registration efficiency close to 100% were measured for heavy ions for the single TOF detector. A time-of-flight system consisting of two such detectors is extensively used in the experiments with heavy nuclei at the kinematic separator VASSILISSA [1,2].

The new isotope219U

In continuation of our experimental program on investigation of evaporation residues (ERs) in the region close to N=126 [1, 2], the new neutron-deficient isotope219U has been produced in the irradiations of an197Au target with a beam of27A1 ions. The identification of219U was made establishing genetic position and time correlations between the implanted ERs with their subsequentα-decays. Theα-decay energy and half-life were measured to be (9680±40) keV and (42−13+34)μs, respectively. To separate the evaporation residues in-flight the kinematic separator VASSILISSA [3] was used.

New nuclides228,229Pu

In continuation of our work on investigation of the neutron-deficient plutonium isotopes [1] we carried out the experiments aimed to produce new isotopes228,229Pu. The isotope228Pu has been produced in the bombardment of the208Pb target with a beam of24Mg and the isotope229Pu — in the reactions of207,208Pb targets with26Mg projectiles. The isotopes were identified after in-flight separation from the beam and target-like particles by the VASSILISSA electrostatic separator [2] and implantation into silicon strip detectors. The assignment of unknown α-decays was made by establishing their genetic position and time correlations with those belonging to known α-decay chains. The α-decay energies of228,229Pu were measured to be (7810±20) keV and (7460±30) keV, respectively.

The new isotope218U

The new neutron-deficient isotope218U was produced in the bombardment of197Au target with27Al ions and identified using theα-α correlation method. The α-decay energy and the half-life of218U were determined to be 8625±25 keV and 1.5−0.7+7.3 ms, respectively. Evaporation residues recoiling from the target were separated in-flight from the projectiles and from the products of other nuclear reactions by the electrostatic separator VASSILISSA [1].

4π-ionization chamber — a detector for a kinematic separator of heavy ion reaction products

Abstract Construction and properties of a double ionization chamber (DIC) are described. Heavy ion fusion-evaporation reaction products obtained at the focal plane of the kinematic separator VASSILISSA enter the DIC volume through a thin plastic window. After their thermalization in the gas, they drift in the electric field to the DIC cathode. Their α-decay and/or spontaneous fission are detected in 4π-geometry in two ionization chambers, divided by the common cathode. Using DIC the α-decay of Bi and Pb isotopes produced in the fusion reactions 40 Ar + 159 Tb and 40 Ca + 153 Eu were detected. A new isomer, 189m Bi was observed.

Investigation of the fusion reaction 27 Al+ 236 U→ 263 105 at excitation energies of 57 MeV and 65 MeV

The neutron-deficient isotopes 257,258105 were produced in the reaction 27Al+236U in 6n and 5n evaporation channels, respectively. The evaporation residues emerging from the target were separated in-flight from the projectiles and from products of different nuclear reactions by the electrostatic separator VASSILISSA [1]. The isotopes were then implanted into position-sensitive silicon detectors and identified using the α-α-correlation method. The measured production cross-section is σ(5n)=(0.45±0.20)nb at EP=154 MeV and σ(6n)=(0.075±0.055) nb at EP=163 MeV. These cross-sections are compared with data measured for the same isotopes in the more symmetrical reaction 50Ti+209Bi.