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

Nuclear Instruments and Methods in Physics Research A330 (1993) 125-131 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

47-ionization chamber - a detector for a kinematic separator of heavy ion reaction products A.N. Andreyev a, D.D. Bogdanov ", V.I. Chepigin a, V.A . Gorshkov a, K.V . Mikhailov A.P . Kabachenko a, G.S. Popeko a, S . Saro ", G.M. Ter-Akopian a, AN. Yeremin a and Sh.S. Zeinalov a

a,

Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, Russian Federation n Faculty of Mathematics and Physics, Comenius University, Bratislava, Slovak Republic

Received 9 December 1992

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 a-decay and/or spontaneous fission are detected in 4,Tr-geometry in two ionization chambers, divided by the common cathode. Using DIC the a-decay of Bi and Pb isotopes produced m the fusion reactions 40Ar+i 59 Tb and 4° Ca+ i53Eu were detected . A new isomer, lsy"'Bi was observed .

1 . Introduction The lifetimes of many heavy nuclei are too short to investigate their properties using off-line methods, and their production cross sections are very low, therefore there is a need to use on-line detecting systems with a high counting efficiency and a low background . The advanced detecting systems are the combinations of different kinematic separators with multidetector systems [1-4]. In general, a detector system is an array of position sensitive Si-strip detectors. The end-detector registers the recoil nuclei (evaporation residues) passing through the kinematic separator and a-decay and/or spontaneous fission of these nuclei . Observed recoil-alpha and alpha-alpha correlations have been successfully used for unambiguous identification of variety of heavy-ion fusion-evaporation residues including the discovery of the heaviest elements [1-3]. About 50% of the a-decay detection probability leaves room for attempts to increase it as this would greatly enhance the revealing efficiency of genetic links in long (x-decay chains originating from initial reaction products . Hofmann [5] developed a new detecting system in which the stop detector array consists of 16 position sensitive Si-strip detectors and an additional array of side Si-strip detectors is added in order to increase the a-decay detection efficiency . In this paper the results of our attempt to approach the problem in another way are given. The basic idea

of our detecting system is as follows: the evaporation residues (ERs), after passing through a kinematic separator, enter a double Frish-type ionization chamber having a single common cathode for both chambers . The residues are stopped in the filling gas not far from the entrance window and drift as positive ions to the cathode. Such a system of a pair of combined ionization chambers creates a 47r-spectrometric detector of a particles and fission fragments. 2. The end-window double ionization chamber 2.1 . Design of the chamber

The diagram of the detector module designed for the kinematic separator VASSILISSA [4] is shown in fig. 1 . DIC has a system of parallel plate electrodes placed into a common vacuum tank . The setup was tested on ERs obtained with beams from the U-400 cyclotron of Flerov Laboratory of Nuclear Reactions, JINR, Dubna. The separated ERs entered DIC through a vacuum window which was made of a thin (from 70 to 400 Wg/em z) metallized Mylar foil supported by a stainless steel grid . This window was located at a distance of 1 .5 mm from a wire grid forming the central part of the anode (Al) of the first chamber (ICI). The transparency of the grid supporting the vacuum window, together with the anode and the Frish

0168-9002/93/$06 00 © 1993 - Elsevier Science Publishers B.V All rights reserved

126

AN Andreyec et al. / 47r-ionization chamber

grid, amounted to 90%. The possibility was provided to apply the high voltage to the whole anode Al . The mixture of 90% Ar + 10% CH, was used as a gas filling of DIC at the pressure ranging from 125 to 500 Torr . After thermalization, the ERs drifted as positive ions to the cathode and stuck to a thin 70 wg/cm2 metallized Mylar foil covering an opening made in the center of the cathode plate. The diameter of this opening was equal to 100 mm . The same was the diameter of the entrance vacuum window . All the electrodes (i .e ., anode Al, grid G1, cathode-collector C, grid G2 and anode A2) had the same diameter of 400 mm . The distances between the electrodes in the second ionization chamber IC2 were fixed (see table l), whereas the distances between the electrodes of the first chamber ICI were variable . The field correction rings shown in fig. 1 provided a uniform field distribution in the working space of both chambers . Four preamplifiers followed by amplifiers provided the registration of signals from the anodes and the Frish grids of both ionization chambers . Each amplifier had two spectrometric outputs differing in their gains by a factor of 10 . The four higher gate outputs connected to ADCs were adjusted for amplitude measurements of pulses of a particles and recoil nuclei . The lower gate signals were fed to other four ADCs and used for fission fragment spectroscopy . The presence of a coinciding pair of signals from the anode and the grid of any ionization chamber was used for the event selection. For each event, the pulse amplitudes from the anodes and the Frish grids of both chambers were stored in the computer memory together with the arrival time of the event counted from the beginning of the experiment with an accuracy of 10 [,s. One-dimensional spectra of a particles and fission fragments as well as two-dimensional fission fragment spectra were obtained from the data . For each event, the anode-togrid amplitude ratio was used to obtain the angle between the a particle trajectory and the symmetry axis of the DIC . An event was excluded from the

TARGEMIÜII .

Table 1 Specifications of the double ionization chamber (DIC) Entrance window : Effective diameter Thickness Transparency Anode A1 : Diameter Anode A2: Diameter Frish grids Gl, G2 : Effective diameter Grid wire diameter Grid step Common cathode: Diameter Mylar window diameter Window thickness Guard rings: Diameter Wire diameter Distance between rings Distances: Anode A1-grid G1 Grid GI-cathode C Anode A2-grid G2 Grid G2-cathode C Gas filling: Gas pressure for fission for E,< 8 MeV for E < 11 MeV Field intensities: Anode Al-grid G1 Grid G1-cathode C Anode A2-grid G2 Grid G2-cathode C

80 mm 70-400 Wg/cm2 90% 400 mm 400 mm 400 mm 125 p m 2 mm 400 mm 100 mm 70 Wg/cm2 420 mm 3 mm 10 mm 0 or 5 mm 50-200 mm 30 mm 200 mm 90% Ar+ 10% CH 4 125-150 Torr 350 Torr 500 Torr 100-250 V/cm 40-100 V/cm 150 V/cm 60 V/cm

spectrum, if this angle surpassed a limiting value (typically, 85°) . The amplitudes of the anode pulses of the second chamber (IC2) were corrected for the energy

F I ECTR0',7 AITC ~EPARATOR

ANODEI

hNODU

(A MODE

DFC.RADFR

Au-FOIL

GRIDI

RUTHERFORD S( ATTERING

CON( RETF WAI.I . 2 m

6R11)

11

Fig. 1 . The diagram of the kinematic separator VASSILISSA together with the DIC in the separator focal plane.

127

AN Andreyeu et al. / 47r-ionization chamber

loss in the cathode window . Such a correction for the case of fission fragments could be accomplished according to the prescriptions of ref. [6]. The a particles and/or fission fragments emitted by ERs adsorbed on the cathode foil were detected by ionization chambers ICl and IC2. Recoil nuclei originating from heavy targets as a result of fusion reactions occurring with heavy ions ranging from 2° Ne to 40Ar have rather short ranges in matter. Large fluctuations of the energy losses are typical for ERs. Taking into account energy losses in the target and in the entrance window we came to the conclusion that the distribution of the ranges of the ERs within the DIC volume covered the distances from a few to about 40 mm from the entrance window even at the lowest gas pressure (125 Torr) which could be used only for the case of fission fragment detection. To avoid the screening of a large fraction of ERs we reduced to 5 mm the distance between the anode and the Frish grid of the first chamber using a wire grid with 0.5 mm steps between the wires. It was also possible to remove the grid from the first chamber. The ability of this setup to detect the ERs depends mainly on two parameters : the efficiency and the collection time of the ERs. 2.2. Collection efficiency

After their thermalization in the mixture of Ar and CH 4 , the heavy ERs remain as positive ions mainly in the charge state + 1. The efficiency of their collection on the cathode depends on the neutralization rate in atomic collisions during the drift. One could expect a low neutralization rate, if the first ionization potential of atoms and molecules of the gas gas is higher than the first ionization potential of the drifting positive ions . The concentration of impurities in the gas should be low enough to avoid recombination and charge exchange in atomic collisions . For our gas filling these conditions were satisfied: VÂr = 15 .75 eV, Vj+Ha = 12 .99 eV, and VZ < 10 eV for all the atoms with Z > 80 [7]. We measured the collection efficiency of polonium ions created as a result of the reaction n"Dy (4°Ar, 4-5n)199Po . The 199Po half-life is 4.2 min and its a-decay energy is E a = 6.059 MeV. At the gas pressure of 200 Torr, Po nuclei had the mean range of about 3 cm in ICl. We varied the field strength between the grid and cathode of ICl from 40 to 85 V/cm and measured, by aid of the second chamber IC2, the intensity of the a-decay line of 199Po nuclei collected on the cathode. At a statistical error level of 5% we did not observe any dependence of the 199Po a-decay rate on the applied field intensity. The absolute value of the collection efficiency of (90 ± 10)% was determined in another experiment in which we compared the 199Po

alpha activity of the cathode foil measured by IC2 with the activity measured by a Si-detector fixed at the position of the cathode foil. In the last case the gas pressure in the volume of the DIC was reduced to < 10 -3 Torr. To determine the absolute collection efficiency a similar experiment was made with 214Ac nuclei from the reaction 197Au(22 Ne, 5n) 214Ac. We obtained the same efficiency value : > 90%. 2 .3 . Ion drift time

The drift time t d of the thermalized ERs is a critical parameter because it limits the possible applications of DIC only to ERs with half-lives T112 > t d . To determine the drift time we compared the number of ERs emitting a particles during their drift with the number of a particles emitted by the nuclei that were deposited on the cathode. This method demands the use of nuclei having a half-life comparable with their drift time . The measurements were carried out with the a activity of "'Ac obtained from the reaction 197Au(22 Ne, 4n)215Ac . The a-decay branch of 215Ac is practically 100%, T112 = 170 ms, and E. = 7.60 MeV. After the passage of the entrance window, kinetic energy values of the 215Ac nuclei occurred in the range of 6 ± 3 MeV and, at the gas pressure of 250 Torr, 215Ac nuclei were thermalized at the distance of 19-20 cm from the cathode surface . The range of the alpha particles with E a = 7.60 MeV was 22.5 cm . Therefore, during all the time of their drift from the thermalization region to the cathode, the actinium nuclei emitted a particles which could be registered by both ionization chambers ICI and IC2. The number of actinium nuclei satisfying this condition is

= JI0 "N~P exp(-At) dit,

Nd

where N, is the number of "-Ac nuclei entering ICI, A is the decay constant, P is the probability that the a decay is registered by both chambers . This probability depends on the ion drift velocity (w) and the nucleus decay time (t) which is the integration variable . The drift time (td ) is defined through the known drift path and the searched drift velocity .

Table 2 215Ac ion drift velocity as a function of the field intensity Field intensity E [V/cm]

Drift time t d [ms]

Drift velocity w [cm/s]

50 75 100 125

200+20 20 120+12 64+_ 6 38+ 4

100+10 10 170+10 320+30 530+50

128

A.N. Andreyec et al / 4a-ionization chamber

We obtained this velocity from the measured ratio NdIN, where Nc =No exp(-At d ) is the number of 215Ac nuclei decaying on the cathode surface It . appears to us that one can neglect the influence on this ratio of the ion diffusion and recombination processes. The obtained drift times and drift velocities for different field intensities are given in table 2. All measurements were made at the same gas pressure of 250 Torr . The given errors are determined by the statistical error of the counting rates and by the uncertainty of the drift path length . The data in table 2 show a rapid increase of the drift velocity with the field strength . 3. Experimental results

To determine the properties of the DIC for fission fragment registration at the absence of the Frish grid in ICI, we employed a 248Cm calibration source, deposited on the cathode window (70 Wg/cm), on the IC1 side of the window . The gas pressure was 150 Torr . In fig. 2 the energy spectra are shown obtained in this experiment . In the left side the two-dimensional fission spectrum is shown measured with ICI and IC2 in coincidence . The right side shows the corresponding linear energy spectra. From the measured data we obtained the total kinetic energy (TKE) distribution having a width of 29 MeV (FWHM) in comparison with the 25 MeV width reported in ref. [8]. From this we deduced the energy resolution of 14 MeV which is limited due to the absence of the Frish grid in ICI and the effect of the cathode foil . We tested DIC in an experiment in which the 24'Fm spontaneous fission was detected . This nuclide was

100

El

50

50

100

375

250 aa 125

5000

3. I. Registration of fission fragments

20

500 r-

150

El (MCV)

Fig. 2. Calibration spectra of

248

Cm fission fragments.

tn 5500

6000

6500

7000

E, keV

7500

Fig. 3. The a-spectrum of the ERs produced in the reaction 40 Ca + 153Eu at the 40 Ca bombarding energy 200 MeV. produced in the reaction 201 Pb(4°Ar, 2n) and separated by the VASSILISSA setup. The energy of the Ar beam was 193 MeV and the thickness of the Pb target was 0.5 mg/cm. The cross section of this reaction is 16 ± 10 nb [9], the 246 Fm half-life is 1.1 s and it has fission branch of 8% . In agreement with our calculations, we registered five fission events at an integral flux of 10 17 argon ions on the target . The mean TKE value for these five fission events was 195 ± 12 MeV in agreement with an earlier work [10] . 3.2. Registration of the evaporation residues produced to the reactions 40Ar + 759n and 4° Ca + 153Ett The beams of 217, 250 and 293 MeV 4°Ar and 215 and 270 MeV 40 Ca ions were produced by the JINR U-400 cyclotron. After passing through Al or Ti degraders and a thin (200 wg/cm2) Au foil used for the measurement of the energy the scattered ions (see fig. 1) the beam bombarded targets made of terbium (600 wg/cm2) or europium (500 [-0g/cm') oxide deposited on thin (1 .35 mg/cm2 ) Ti backings . Use was made of an isotopically enriched Eu target containing 99 .1% of 153Eu . The recoil nuclei emerging from the target as a result of fusion reactions were separated from the beam and products of other reactions and focused onto the DIC entrance window located at the distance of 12 m from the target . In these experiments, the gas pressure in the DIC volume was 400 Torr, the distance between the anode and Frish grid in the first chamber (ICI) was equal to 5 mm . The distance between the grid GI and cathode (see fig. 1) was varied between 95 and 42 mm . The ERs obtained as a result of particle evaporation from the

AN. Andreyev et al. / 4,ir-ionization chamber Table 3 The characteristics of neutron deficient isotopes observed in the experiments Isotope

E a [MeV]

T1/2 [s]

a,,, [%]

Bi 19oBi 191' Bi 191 Bi 192' Bi 192 Bi 193' B1 193Bi 194' Bi 194 Bi 195' Bi 195 Bi 188 Pb 189 Pb 190 Pb

7 .010 6 .820 7 .206 6 .675 7 .116 6 .455 6 .429 6 .876 6 .310 6.052 6 .058 6.475 5 .899 5 .598 5 .645 6 .106 5 .420 5 .980 5 .720 5 .580

0 .044 0.210 0 .005 0 .680 5 .9 5 .7 0 .15 13 41 34 1 .9 67 115 95 87 182 24 .5 51 72

75 +25 75 +25 70 90 75 +25 60 +20 10 12 75 +25 3 .5 ± 1 .5 0.2 0.46+ 0 .25 4 0.03+ 0.02 22 +_ 7 0.42 0 .9 ± 0 .2

188' Bi 188Bi 189' Bi 189Bi 190'

and 193Bi compound nuclei were stopped, on average, at a distance of 15 mm from the entrance window . Therefore, the 5 ± 2 ms drift time and near 100% efficiency of their collection on the cathode window were attained at the grid-cathode distance of 42 mm and the field strength of 150 V/cm in the space between grid and cathode. 19913i

12 9

The a-decay spectra of the nuclei collected on the cathode were detected by the second chamber (IC2). After correction for the energy loss of a particles in the cathode window, the energy resolution of 40-50 keV (FWHM) was obtained in the a-particle energy range of 5-6 MeV in long-term (two to three days) experiments . An example of the registered spectra is shown in fig. 3 . The first chamber ICl was used in these experiments only for thermalization and drift of the evaporation residues . In fact, all the ions passing through the separator together with the ERs were stopped within this chamber, thus leaving the second chamber completely free of this disturbing background . The total efficiency of the setup was measured directly in each experiment by making use of the longlived (T i ., > 1 min) evaporation residues produced in the 4°Ar +,atDy reaction . We accomplished the measurements of the efficiency by comparing the counting rates of the IC2 with the counting rate obtained from a thick catcher foil which could be inserted periodically at a position close to the target . The efficiency values around 15% were obtained typically. This result agrees well with the known value of the separation efficiency of about 20% [4], 100% cathode collection efficiency of the ERs stopped in the ICl and 1 .51r sr solid angle of the a-particle detection of the IC2 . Such measurements, performed repeatedly, convinced us that the intrinsic parameters of DIC determining ERs detection efficiency did not show visible changes during the experiments. The self-consistency of many obtained results about the cross sections of the reactions leading to different ERs led us to the conclusion that the

Table 4 Excitation functions for the xn and pxn evaporation channels measured in the 40 Ca+ 159 Th reaction E* [MeV]

Cross section [wb] 4n

5n

6n

62 65 72 74 78 84 90 92 96 98 102 108 111 120 126 133 139

16000 13900 2000 520

4300 6000 4900 2300 1050

300 400 2190 2690 4250 3520 1275 1030 360 330 14

7n

2 340 575 680 680 510 300 120 57 7

8n

9n

2 42 78 156 230 320 250 200 60 16 5 1 .3

6 26 40 40 33 18 7 .5

IOn

p8n

p9n

p10n

2.5 6 .5 9 .5 6.0

240 700 960 970 550 480

40 310 860 1680 2480 2180

4 16 20 55 90

13 0

A .N. Andreyec et al. / 4-rr-tonization chamber

relative error of these measurements did not exceed 20% . This error value was also obtained in other experiments carried out at VASSILISSA with the use of silicon detector arrays in the separator focal plane. Alpha activities obtained in these experiments originated basically from Bi and Pb isotopes formed after evaporation of neutrons and protons from ' 99 13i and 193Bi compound nuclei . We identified these activities according to their a-decay energy, decay time and excitation functions. The list of the observed nuclides is presented in table 3. The nuclear characteristics listed in table 3 are taken from refs . [I1-15]. Tables 4 and 5 give the reaction cross sections measured at different compound nucleus excitation energies . The cross sections for the 159Tb(40Àr, xn) reactions presented in table 4 are consistent with the results of ref. [16]. We evaluated 70% total error for data given in tables 4 and 5. This includes the errors of the efficiency and beam current measurements, the target thickness fluctuations and possible inhomogeneity of the target, as well as the errors of the a-decay branches given in table 3. Most of the Bi isotopes were formed both, in their ground and isomeric states . We obtained the mean isomeric ratio of 15 ± 7 for the isotopes '91Bi, 193Bi and 195 Bi produced in the argon induced reactions. The given error originated practically only from the uncertainties of the values of the corresponding a-decay branches (see table 3) . The value of 1 .4 t 0.3 was obtained for the ratio of the a-decay yields from the ground and isomeric states of 194 Bi produced in the reaction 159Th(40Ar, 5n). This value was 0.9 ± 0.09 for 18'Bi produced in the reaction 153Eu(40 Ca, 4n). The isotope 's9Bi was synthesized in two reactions: 159Tb(40Ar 10n) and 153Eu(40 Ca, 5n). We assigned to 189Bi three a-decay lines: 6.67, 7.12 and 7.34 MeV. The first two lines are known for the 189Bi ground state, and their assignment to 189Bi was justified additionally by the excitation curves (see tables 4 and 5) . The third line had essentially the same excitation curve (see fig. 4) . Therefore we ascribed it to the a-decay branch of Table 5 Excitation functions for the xn and pxn evaporation channels measured in the 40 Ca+ 153 Eu reaction E* [MeV]

Cross section [wb]

41 47 53 55 62 65 67

3 8 22 23 6 5 3

3n

4n

5n

p3n

p4n

p5n

2.5 14 30 20 17 8

0.5 1 .5 6.5 8.0 7.5

620 990 1200 1800 900

4 5 17 220 310 350

2.0 2.5 16 .0

103

100 35

45 55 65 Excitation Energy, MeV

75

Fig. 4. The yields of the a-activities obtained in the reaction 40 Ca+ 153Eu in function of the excitation energy of the compound nucleus 193Bi . an isomer 189'Bi . In total, more than 600 a particles with the energy of 7.34 f 0.03 MeV were detected in our experiments . The ratio of a-particle yields from the 189Bi ground and isomeric states (the 6.67 and 7.34 MeV lines) was 12 .4 ± 0.6 for the reaction 153Eu(40Ca, 4n). In another experiment [17], in which we used a silicon detector array in the focal plane of the separator, we also obtained the same a lines: `89 Bi(Ea = 6.67 and 7.12 MeV) and 189m Bi(Ea = 7.34), produced in the reaction 153Eu(40Ca, 4n). The value of 6.7 ± 0.3 was measured for the yield ratio of the 6.67 and 7.34 MeV a lines. From the values of this ratio measured with the DIC and a Si-detector array we extracted the half-life T t~z - 4 ± 2 ms for the obtained isomer 189mBi (E~ = 7.34 MeV). Other authors reported [18], on the basis of the observation of two a-decay events, the a-decay energy of a 189Bi isomer to be 7.20 ± 0.02 MeV. This a line was not present in our spectra at the level of < 10% of the intensity of the 7.34 MeV line . The obtained data on the reaction excitation functions presented in tables 4 and 5 are a subject of discussions in terms of fissility of heavy neutron-deficient compound nuclei [17,19]. 4. Side-window double ionization chamber The possible applications of the end-window DIC are limited by the drift time t d . A DIC with a sidewindow in its central part, near the cathode foil can give, in principle, shorter drift times. The schematic view of this variant of DIC is shown in fig. 5. The entrance window is inside the first ionization chamber. The average distance between the region of the recoil nuclei thermalization and cathode foil is reduced to about 4 cm . This reduces the drift time for which the

A .N. Andreyer et al. / 4a-ionization chamber side entrance window located at the half-distance between the Frish grid and cathode. At properly chosen parameters of the chamber (dimensions, gas pressure, field strength) the detection efficiency close to 100% should be obtained for the reaction products having lifetimes of > 0.2 s due to their drift far away from the entrance window . The short-lived nuclides will be detected with a lower efficiency due to the screening effect of the entrance window . For nuclides with halflives shorter than a fraction of the second the total kinetic energy of spontaneous fission fragments could be measured during the drift time to the cathode.

r Rf COILS

VP

References

s d 2 G2 ~ Fig. 5. The diagram of the side-window DIC tested in our work, A1,A2 - anodes, G1,G2 - Frish grids, C - common cathode, VR - recoil nucleus velocity, Vd - recoil nucleus drift velocity . A

value of 4-8 ms should be attainable in real experimental conditions . In experiments with ERs entering the DIC through the side window we obtained cathode collection efficiency varying between 30 and 70% depending on those conditions which we tried to change in our attempts to increase this value. We tested different possibilities : (i) the whole tube carrying the entrance window and its supporting grid were manufactured from insulators, (ü) the wire grids were installed in front of the window and around the tube, and this allowed us to control, to some degree, the distribution of the potential in the ion drift region . The reason of the low efficiency was the attraction of a considerable fraction of the stopped recoil nuclei to the entrance window . A successful solution of the problem could give the possibility to detect, in nearly 4,ir-geometry, a particles and fission fragments emitted by recoil nuclei with half-lives of the order of a few ms or less. One could take in such a case the first ionization chamber with a

[1] G. Muenzenberg, Rep. Prog . Phys . 51 (1988) 57 . [2] P. Armbruster, Ann. Rev. Nucl . Sci. 35 (1985) 135. [3] S. Hofmann, in : International School-Seminar on Heavy Ion Physics, Dubna, 1989 (JINR, D7-90-12, Dubna, 1990) p. 338. [41 A.V . Yeremin et al ., Nucl . Instr. and Meth . 274 (1989) 528. [5] S. Hofmann et al ., Int. Conf. NFFS6+AMC09, Bernkastel-Kues, Germany (July 1992). [6] V.I . Chepigin et al ., Nucl . Instr. and Meth . 220 (1984) 419. [7] K. Kikoin, Tablicy fizicheskich velichin (Atomizdat Moskva, 1976) (In Russian) [8] Yu .P . Gangrskil, B. Dalchsuren, B.N . Markov, Oskolki delenia jader (Energoatomizdat Moskva, 1986) (In Russian) [9] H. Gaeggeler et al ., Z. Phys . 316 (1984) 291. [10] D.D . Bogdanov et al ., Preprint JINR, P15-81-706, Dubna (1990). [111 W. Westmeier and A. Merklin, Catalog of Alpha Particles from Radioactive Decay. No . 29-1, Karlsruhe, 1985 . [121 M. Huyse, Phys . Lett . B201 (1988) 293. [13] P . Van Dupper, Nucl . Phys . A529 (1991) 268. [14] E. Coenen et al ., Phys Rev. Lett . 54 (1985) 1783 . [15J A.N. Andreyev et al ., Preprint JINR, P15-89-684, Dubna (1989) . [16] Y. Le Beyec et al ., Phys . Rev. C9 (1974) 1091 . [17] A.N. Andreyev et al ., JINR Rapid Communications 2 1531 (1992) p. 35 . [18] J. Schneider, GSI, Darmstadt, Report GSI-84-3 (1984) . [19] A.N. Andreyev et al ., Int. Nucl . Phys . Conf., Wiesbaden, 1992, Book of Abstracts, r. 3.2 .11.