The Dubna double-arm time-of-flight spectrometer for heavy-ion reaction products

Nuclear Instruments and Methods in Physics Research A257 (1987) 197-208 North-Holland, Amsterdam

197

THE DÜBNA DOUBLE-ARM TIME-OF-FLIGHT SPECTROMETER FOR HEAVY-ION REACTION PRODUCTS K.D . SCHILLING, P. GIPPNER, W. SEIDEL, F. STARY and E. WILL Zentralinstitut für Kernforschung Rossendorf 8051 Dresden, GDR

K. HEIDEL, S.M. LUKYANOV, Yu .E. PENIONZHKEVICH, V.S . SALAMATIN and H. SODAN Joint Institute for Nuclear Research, Dubna, USSR

G.G . CHUBARIAN Yereuan Physics Institute, Yereuan, USSR

Received 15 August 1986 The double-arm time-of-flight spectrometer DEMAS designed for the detection and identification of heavy-ion reaction products at incident energies below 10 MeV/amu is presented. Based on the kinematic coincidence method, the relevant physical information is obtained from the measurement of the two correlated velocity vectors of the binary fragments. Construction and performance of the different detector systems applied to measure the time-of-flight values, the position coordinates and the kinetic energies of both fragments are described in detail. The description of the data acquisition and analysing procedures is followed by a discussion of some experimental examples to demonstrate the spectrometer performance . A mass resolution of typically 4-5 amu (fwhm) is routinely achieved . 1. Introduction In heavy-ion reactions at bombarding energies below 10 MeV/amu, the following mechanisms contribute to the total reaction cross section: elastic and quasielastic scattering, deep-inelastic or damped processes, fusionlike processes (fast-fission and quasi-fission) and compound-nucleus formation followed by fission and/or light-particle emission . These types of reactions are characterized by a growing degree of interpenetration of the projectile and target nuclei resulting preferentially in the emission of two heavy fragments. By studying the velocity, energy and angular distributions of the two correlated reaction products, information about the contributions of the different types of reactions to the total reaction cross section can be obtained. Based on the correlation method introduced in ref. [1], the spectrometer to be presented here is designed for investigating such binary fragmentation processes. Instead of determinin g the fragment-mass distributions by a combined velocity-energy measurement [2] of the reaction products (which requires high individual resolving powers of the time and energy detectors, as reported, e.g., in ref. [3J) we preferred the measurement of the two correlated velocity vectors. The fragmentation processes have to be isolated in the presence of a large 0168-9002/87/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

background of undesired events, such as gamma quanta, electrons, light charged particles and neutrons. Therefore, the experimental arrangement should be based on large-area position-sensitive gas detector systems. Combined with time-of-flight (TOF) techniques, such systems are effective multiparameter detection devices for the identification of the heavy reaction products . This caused us to build the double-arm time-of-flight spectrometer DEMAS *, which is installed at the heavy-ion beam of the cyclotron U-300 of the JINR Laboratory of Nuclear Reactions in Dubna. Parts of the spectrometer have been briefly described previously in refs . [4-6]. A general description of the spectrometer and its principle of operation is given in sect . 2. The start detectors for the TOF measurements as well as the gas detector systems are described in detail in sect . 3. Sect. 4 is devoted to the data acquisition and analysis . Examples of experimental results demonstrating the spectrometer performance, together with some conclusions, are contained in sect . 5.

* DEMAS is the abbreviation for double-arm energy and mass spectrometer.

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2. Principle of operation The spectrometer DEMAS - designed for operation as a kinematic coincidence setup - has been developed for the identification of reaction products in heavy-ion collisions with incident energies below about 10 MeV/ amu, where binary processes play the dominant role. The application of the kinematic coincidence method, based on the measurement of the two correlated velocity vectors, enables one to study the primary reaction product distributions before particle evaporation, such as, e.g., primary fragment masses and total kinetic energies [7-9,41] . The principle of operation of the spectrometer is illustrated in fig. 1. The binary reaction products are detected by recording the following parameters of both coincident fragments event by event: time-of-flight (TOF) values, energies (E) as well as position coordinates in (X) and perpendicular (Y) to the horizontal plane. The fragment identification - i.e . determination of their mass and nuclear charge numbers, kinetic energies as well as in-plane and out-of-plane scattering angles (in both the laboratory and the centre-of-mass systems) - is then performed off-line on the basis of two-body kinematics . Furthermore, the detection and identification of simultaneously emitted light charged particles is taken into consideration by means of d E-E semiconductor telescopes as indicated in fig. 1. All essential components of the spectrometer DE-

Beam

MAS presented here are included in fig. 1. The tightly collimated beam of the heavy-ion cyclotron hits the target at the centre of the scattering chamber. The binary reaction products are detected in two TOF arms positioned in accordance with the reaction kinematics. The start signals for the TOF measurements are supplied by microchannel-plate (MCP) detectors. The stop pulses are generated by large-area parallel-plate avalanche counters (PPACs) located in front of the active volumes of the position-sensitive ionization chambers (ICs). The ICs at the end of the flight paths are used to determine the kinetic energies E of both fragments by stopping the latter in the active gas volumes . The specific energy loss values d E can also be measured if necessary. The out-of-plane coordinate Y is deduced from the drift time of the electron tracks in the homogeneous electric field of the IC . In order to measure the in-plane coordinate X a special position-sensitive detector is installed behind the slit between the d Eand E-anode plates of each IC (cf. fig. 2) . Since the spectrometer is designed to be used for the investigation of heavy-ion reactions, where the projectile mass is usually less than the target mass, the reaction products are generally scattered into a wide angular range of the reaction plane. The positions of the two TOF arms can, therefore, be adjusted independently of each other within the medium (horizontal) reaction plane. The construction of the scattering chamber enables one to fix the two arms at definite positions in steps of 15 ° . With an angular acceptance of each TOF arm of about 15' in-plane, an angular range from about 20 ° up to 160 ° in either half-plane can thus be covered in successive measurements . The out-of-plane acceptance angle of each arm is about 4° . The solid angle covered by each spectrometer arm is equal to 18 msr. 3. The detector systems

IC 2

Faraday cup

Fig. 1 . Schematic arrangement of the kinematic coincidence spectrometer DEMAS. MCP - microchannel-plate detector as start detector ; PPAC = parallel-plate avalanche counter as stop detector for the time-of-flight (TOF) measurement ; Ll, L2 = flight paths of roughly 60 cm length for determination of the time-of-flight values TOFl and TOF2, respectively ; IC 1, IC 2 = position-sensitive ionization chambers; Mon. = monitor de tector ; AE, E-Tel. = semiconductor-detector telescopes for identification of light charged particles.

The mass identification of the heavy-ion reaction products with the spectrometer DEMAS is essentially based on the application of the TOF technique [10,11]. The start detectors, which provide the time-zero signals for the TOF measurements (fig . 1), are secondary electron emission detectors with microchannel plates (MCP) as electron multipliers . These detectors best meet the requirements of a fast timing for the measurement of the fragment velocities. Moreover, they impose a minimum possible energy loss on the fragments passing through and are also insensitive to neutron and gammaray background radiation. The secondary electrons ejected from the thin transmission foils are accelerated and subsequently deflected by magnetic or electrostatic fields ((1) and (2) in table 1) towards the chevron arrangement consisting of two

K.D. Schilling et al. / The Dubna double-arm TOF spectrometer

ENTRANCE WINDOW supporting wires 70 pm transmission 91%

JII

D

CORRECTION ~GR wires 180 tum

P PAC

CUT THROUGH A

19 9

SCREEN INSULATOR DELAY-LINE PROPORTIONAL WIRE ANODE

CUT THROUGH B

Fig. 2 . Schematic view of the gas detector system. The geometrical dimensions are indicated in millimeters. MCPs acting in series . The deflection angle is 180' when applying a homogeneous magnetic field (1) and 90' when using an electrostatic mirror (2). We have used both types of MCP detectors, but finally preferred the type (2) to cover a larger effective solid angle . Some features of the MCP detectors are listed in table 1 . Details have extensively been discussed elsewhere [12] or were partly presented earlier (for references see ref. [12]) . MCP detectors with a magnetic electron transport system were first introduced by Zebelman et al. [13] . The present detectors are of the same construction . The time resolution achieved with our MCP detectors of magnetic type for a-particles of a radioactive source is competitive with that in refs. [13,14]. For heavier particles like fission fragments an even better resolution can be expected . The present MCP detectors with an electrostatic mirror are similar to those developed by Busch et al. [15]. These detectors are of simple construction and relatively small size, they are reliable and easy to handle. The time resolution obtained is - in spite of the enlarged solid angle - also at the 100 ps level (table 1) . The time-zero detectors are placed within the scattering chamber at distances of 10 cm from the target . They are shielded each by a combined Pb-Cd-Cu absorber

(8 mm total thickness) from X- and gamma-ray background. Undesired events are further reduced by requiring a coincidence between the two TOF arms as well as by suppressing the target 8-electrons by applying a high voltage of + 18 kV to the target . At the end of either flight path (fig. 1), a large-area gas detector system is installed . It consists - as displayed schematically in fig . 2 - of a parallel-plate avalanche counter (PPAC) and a two-dimensionally position-sensitive ionization chamber (IC) . The PPAC delivers a fast timing signal, which is used as stop signal for the TOF measurement and, simultaneously, as start signal for the drift-time measurement defining the outof-plane coordinate . The IC measures the kinetic energy and - in combination with the PPAC and a special coordinate detector - the two spatial coordinates of the particle to be detected . The installation of a large-area PPAC in front of an IC has proved successful by other authors [9,16,17] . In the present spectrometer, the PPAC was placed behind the entrance window of the IC inside the same gas volume for the first time [18-20] . Somewhat later than in ref . [18], Prete et al . [21] operated a parallel-grid avalanche counter in a common gas volume with an IC . The present solution is based on the following arguments : The main purpose of use of the spectrometer in

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its actual configuration is to detect comparatively heavy products of nuclear fragmentation processes with low specific energies of 1-2 MeV/amu. The IC can, thereTable 1 Specification of the features of the detector systems included in the spectrometer DEMAS Parameter

Value

Microchannelplate (MCP) detectors (1) With magnetic deflection

transmission foil a> diameter thickness range acceleration voltage variable magnetic field strength MCP diameter voltage across each MCP anode impedance apparent time resolution of a single MCP detector for 5.3 MeV a-particles (2) With electrostatic deflection transmission foil diameter 8> reverse mirror voltage apparent time resolution of a single MCP detector for 5.3 MeV a-particles [the remaining parameters are the same as in case (1)]

50-80 G 34 mm =1000 V 50 ù

proportional-wire diameter delay-line impedance time delay

Value 50 pm Cu-Be 1.1 kdl 22 ns/cm

Position resolutions (fwhm)

d X =1 mm b) dY=1 mm b)

Typical mass resolution `)

4M = 4-5 amu (fwhm)

b)

30 mm 2800 V d t =120 ps (fwhm)

b)

180 x 50 mm2 18 msr t7 .5° ±2.1* 25 Torr pentane 50-130 i~g/cm2 30-4011g/cm2 1 .6 mm d t = 340 ps (fwhm)

b)

Position-sensitive ionization chambers (IC)

entrance window active area thickness range thickness variations gas-pressure range overall energy resolution for 5.5 MeV a-particles instrumental energy resolution for 5.5 MeV a-particles

Coordinate detector (6 read-out)

Self-supporting carbon or collodium (nitrocellulose) foils, the latter evaporated with thin conductive gold layers ( - 30 ilg/cm2). b) Cf. ref. [12] . `) Arranged in a common gas volume with the IC. d> The contributions of the PPAC, the coordinate detector and the radioactive source are subtracted from the overall energy resolution. `~ Cf. sect . 5 of this work .

Parallel-plate avalanche counters (PPAC) `)

active area solid angle (PPAC and IC) angular acceptances (PPAC and IC) in plane out of plane operating gas pressure (typically) thickness range of the individual electrodes formvar conductive gold layer gap apparent time resolution of a single PPAC for 5.5 MeV a-particles

Parameter

e>

16 mm 20-30 1£ g/cm2 =1000 V

d t = 80 ps (fwhm)

Table 1 (continued)

polypropylene 180 x 50 mm2 50-100 pg/cm2 < ±10% 5-50 Torr pentane d E = 70 keV (fwhm)

b)

Q E = 55 keV (fwhm)

b.d)

fore, be operated at low gas pressures (e.g . _< 50 Ton pentane) in order to stop the fragments. Theoperating gas pressure for a PPAC is usually reported to be in the range 5-30 Ton [22] . The installation of the PPAC and the IC in a common gas volume thus simplified the mechanical construction and the gas handling system . An important point is that one or two additional gastight foils for the PPAC housing could be avoided. As the mass resolving power for relatively slow ions is mainly limited by the energy straggling in the detector foils [16], any additional foil would further deteriorate the resolution of the TOF and the energy measurements. In the present case, comparatively thin foils could be used for the entrance windows because of the low operating gas pressure . On the other hand, two problems encountered with the present construction should also be mentioned: (1) since the PPAC is placed immediately in front of the sensitive volume of the IC, thorough field corrections are required in the region behind the PPAC in order to keep the electric field of the IC as homogeneous as possible; (2) since both counters are operated at the same gas pressure, the PPAC does not always work in the optimal pressure range of 8-15 Torr [23] . Some features characterizing the PPACs and ICs used in the present spectrometer are also summarized in table 1. For further details we refer to ref. [12] . The entrance windows of the gas detector systems (fig . 2) and thus the PPACs - are located at roughly 70 cm distance from the target. The resulting solid and acceptance angles are compiled in table 1. A field correction grid (fig . 2) compensates for the influence of the PPAC on the electric field of the IC. The electric field and potential distributions were optimized with a computer

M

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K. D. Schilling et al. / The Dubna double-arm TOF spectrometer

program [24] . In accord with the calculations, the dependence of the E and A E signals on the Y-coordinate was found experimentally [19,20] to be negligible . The signals are taken off on the narrow side of the PPAC electrodes . Corrections concerning the internal time delay of the signals are performed within the analysing procedure (sect. 4) by taking into account the propagation velocity of the signals from the impact point to the output connector. The instrumental time resolution of either TOF system (spectrometer arm) determined for 5.5 MeV a-particles is A t = 360 ps (fwhm) . For heavily ionizing particles it can be expected to improve to about 250 ps (fwhm) [12] . The ICs are of transversal type (fig . 2), i.e . the uniform electric field is applied perpendicularly to the particle trajectories . The electrodes are oriented parallel to each other. The trapezoidally shaped, active anode area (divided into a A E and an E' part) corresponds to an opening angle of 20" in-plane . This area is electrically disconnected from the outer region to decrease the effective capacitance . The active depth of the IC is 180 mm . The IC volume is trilaterally surrounded by equipotential stripes to shield the inner region and to compensate for electric field inhomogeneities in the edge region. The thin polypropylene films for the 90 cm2 entrance windows are manufactured in a special stretching procedure [12,25]. As counting gases we chose high-molecular hydrocarbons, e.g . pentane and hexane, which best meet the present requirements [12] . The vapour pressure can easily be regulated by changing the temperature, which is adjusted using a Peltier element. The gas handling system can be operated either in the stationary regime or in the flow mode. The energy resolution of a Frisch-gridded IC is restricted by various processes contributing to the observed line width. They are discussed in detail in refs . [12,16,26]. In the present case, moreover, the energy resolution deteriorates by the influence of the PPAC and the coordinate detector (CD) . The PPAC causes an additional energy straggling . The influence of the CD turns out to be twofold: (1) it reduces the number of primary electrons generating the energy signal and (2) the necessary screening of the CD (fig . 2) increases the anode capacitance and, thus, the electronic noise contribution . The overall as well as the instrumental energy resolutions obtained with the present ICs are demonstrated in table 1. These values, which are extensively discussed in ref. [12], are competitive to those reported in ref. [26] . Although only moderate, they are in most cases sufficient for heavy-ion reaction studies. The impact point of an ionizing particle is determined by the position coordinates X and Y defined in fig. 2. The determination of the in-plane coordinate X (and, respectively, the corresponding angle 0) is performed with the coordinate detector, which is located

MCP

PPAC

CD

IC

N

PA

CFD

PA

CFD

a

TAC

TFA

CFD

TAC

STOP

SA

PA

PA

TFA

TFA

CLD

CLD

START

TAC

STOP (DELAYED)

CO

ADC

ADC

á ADC Y J E CAMAC

ADC

SM-3

Fig. 3. Block diagram of the electronics used for operaton of one TOF arm of the spectrometer . PA = preamplifier, SA = spectroscopy amplifier, TFA =timing filter amplifier, CLD = constant-level discriminator, CFD = constant-fraction discriminator, TAC = time-to-amplitude converter, ADC = analog-to-digital converter, CO = coincidence unit, T = TOF, E = energy . behind the slit between the d E- and E'-anode segments (fig . 2) . The CD consists essentially of a proportional wire and a delay line carefully screened against the anode. The propagation-time difference of the signals derived from both ends of the delay line is proportional to the X-coordinate (fig. 3) . The Y-coordinate is determined by measuring the drift time of the electrons of the ionization track from the point of their origin to the Frisch grid. For this purpose, the time difference between the PPAC and the anode signals is measured (fig. 3) . The position resolutions achieved are also given in table 1 . They correspond to angular resolutions of better than 0.1' . 4. Data acquisition and analysis For every event to be detected, the counter signals are recorded utilizing the electronics shown schematically in fig. 3 (for the detection of a single fragment) . Since one event consists of two coincident fragments, eight physical parameters are measured simultaneously in the present configuration : TOF, E, X and Y for either fragment . The arrangement shown in fig. 3 comprises fast and slow branches of conventional nuclear electronics (for further details see ref. [12]). The signals representing the physical parameters are digitized in CAMAC ADCs and, after having fulfilled the coincidence requirement in the corresponding unit conveyed as data words in list mode to a CAMAC register . The

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K. D. Schilling et al. / The Dubna double-arm TOF spectrometer

latter is placed in a second CAMAC crate, which is coupled with the UNIBUS of a small computer of the type SM-3 (28K words memory) with standard periphery. Supported by a data acquisition program (EVMESS) [27,28] running on the SM-3 computer, the data words - eight of them representing a physical event - are written blockwise onto magnetic disk or tape. In order to ensure a permanent on-line control of the experiment, the accumulated data are visualized simultaneously on a colour display having 256 x 256 points .

Four two-dimensional correlations (" scatter plots") as well as alpha-numerical information can be displayed in eight colours at the same time and printed as hard copy by request . The possibility of setting conditions for the data is also included in the program EVMESS . The data analysis is performed off-line on computers of the type EC-1055/EC-1060 with a special program system [29] . Its main features are summarized in fig. 4 and will be described in the following. In a binary reaction, a projectile of mass number Ml

Sorting of of

one- and

two-dimensional spectra

the measured uncalibrated data .

Evaluation of TOF- and energy values for calibration of

the

spectrometer .

Program KINTAR calculates :

TOF S , X3 . 0 3 . 03 . Y 3' E 3' M 3' z3' TOP X 04' 04' 4' Y4 ' E 4 ' M 4 ' z4' 4' M3+M4 . T3 + ~4' 0 3 +04 , É3 +É4 . Some one- and two-dimensional printed for the

3' 4'

03 ' E4'

spectra are

on paper and written on magnetic sake of preliminary evalu ation .

tape

Program EVAL determines 20 physical parameters per event : TOF S , X3 . Y3' E 3 ' M 3 ' z 3' 03' B 3' 1 3' E3' TOF 4' X 4 ' Y4 ' E 4 ' M 4 ' z 4' 04' U4' 4' É4, Results are written event by event on magnetic tape . Program SUSI The

sorts 24 one-dimensional spectra .

spectra are printed

on

paper and written

on magnetic tape (i .e . for 20 physical parameters M 3 +M 4' 3+ +É 4 ) . plus f4' 0 3 +0 4 and É 3

f

Program TWOO sorts two-dimensional spectra of any combination of the 20 physical parameters . The spectra are printed on paper and written on magnetic tape .

------~

Three-dimensional

plots of

the

results

of

program TWOD .

-1Isoline-plots of

the results of program TWOD .

Fig. 4. Summary of the computer programs used for the off-line data analysis .

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KD. Schilling et al. / The Dubna double-arm TOFspectrometer

and kinetic energy El interacts with a target nucleus M2 resulting in two fragments M3 and M4, which are emitted at the laboratory angles 03 and 04, respectively. This type of reaction can be completely analysed by measuring three kinematic quantities [30]. If more observables are measured, the reaction kinematics are determined redundantly, what is in general advantageous. In the present case, the TOF values, energies and spatial coordinates of both correlated fragments are experimentally observed for every event. For the subsequent off-line analysis, i.e. the identification of the reaction products, however, only the TOF values and position coordinates (in other words: the velocity vectors) were chosen as the most appropriate kinematic observables. The reason is that they are unchanged on the average by particle evaporation and allow one, therefore, to reconstruct the primary kinematics from measurements of secondary products [30]. Moreover, the velocity can, in principle, be measured with better resolution than the energy for two reasons : (1) the velocity dispersion of the reaction products is only half as large as their energy dispersion, (2) with decreasing kinetic energy of the fragments, the relative time resolution improves, whereas the energy resolution deteriorates. The decisive role of the conditions of collinearity and coplanarity in the analysis of binary reactions will be discussed below . In order to calculate the relevant physical quantities such as mass and nuclear charge numbers, kinetic energies and emission angles for each fragment from the raw data, the spectrometer must first be calibrated. The calibration and calculation procedures take advantage of the linear dependence of the physical parameters to be determined on the response functions of the corresponding detector systems . To give an example, the TOF values are determined according to the equation TOF = a,K, + b, (P=3,4), where K, are the recorded TOF data words, which can be sorted by the aid of the program DSORT (fig. 4). The coefficients a the conversion factors of the corresponding TACs, are simply determined using a time calibrator. The terms b, in eq. (1) are deduced from the peaks of elastically scattered projectiles and their respective recoil partners. For this purpose, the TOF, values in eq. (1) are evaluated according to the reaction kinematics for elastic scattering taking into account the energy losses the collision partners suffer while passing through thin layers of material. The latter include the target before and after the elastic scattering process, the MCP detector foils, the IC entrance windows, the first PPAC foils and the gas dead layers between the entrance windows and the second PPAC foils. The stopping power values needed result from a scaling of the corresponding values for a-particles [31] according to the model of Ward et al. [32], which is valid for the

whole energy region regarded here. The projectile-target interaction itself is assumed to take place in the midplane of the target layer. This calibration procedure is supported by the code EICAL (fig. 4). The TOF, values deduced with eq. (1) from the raw data after calibration are used to calculate the velocities v,, of the reaction products. In connection with the position determination, the TOF,, values are corrected beforehand with regard to the internal time delay of the PPAC signals, which can be represented by a linear relation between time and position [12]. The position coordinates are calibrated by putting diaphragms in front of the IC entrance windows or by utilizing the shadows of the mechanical entrance window supports. The mathematical dependence of the X- and Y-coordinates on the measured data words is completely analogous to that expressed in eq . (1) . As already stated in ref. [7], the precision in determining the emission angles is usually not limited by the resolution of the positionsensitive detectors, but by the angular straggling of the reaction products in the target and the detector foils as well as by the emittance of the incident beam. The experimentally observed velocities v, and in-plane laboratory angles B, (P = 3, 4) of the two coincident fragments allow inference of the fragment masses M3 an M4 by applying the following relations derived from the mass and the linear-momentum balances in a twobody reaction : Ml +M2 =A=M3 +M4 , M3v3 sin 03 = M4v4 sin 04,

(2)

where A is the total mass . The values obtained for M, and v, deliver the laboratory fragment energies E,. As mentioned above, the direct measurement of the E, is, however, less advantageous, since it leads to a larger mass dispersion. The knowledge of the velocities v, and the angles 0, allows the transformation of the motion into the centre-of-mass (cm) system, where the physical quantities are labelled with a bar, for instance B É, (fig . 4). The nuclear charge numbers Z, of the two fragments have not yet been measured, although the IC is, in principle, designed for operation as a AE-E gas detector . For fragment energies in the region of 1 MeV/amu, the specific energy loss is almost independent of the nuclear charge, which, consequently, cannot be deduced experimentally in such a case. The AE and E' electrodes of the ICs are therefore electrically connected in the present configuration of the measurement of the total kinetic (residual) energy . The Z, values are determined using the following expression Z =0 .5M (1-0 .006M,2/3 )

(P=3,4) .

(3)

Since eq. (3) is valid for nuclides situated close to the valley of beta-stability in the N-Z diagram, the Z,

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values derived from eq . (3) are expected to be systematically somewhat too large for the mainly neutron-rich fragments. The values of the physical quantities My , ZV and E calculated on the basis of eqs. (1)-(3) are a first estimate, because they are still affected by the energy losses the reaction products suffer in the target and detector foils. Starting from these uncorrected values, the corrected mass, charge and energy distributions of the fragments are obtained by an iteration procedure, which compensates for the energy losses . The average values of the final distributions for M,, Zv , E, and related quantities correspond to the pre-evaporative values, provided that the two-body assumption is verified . The half-widths of the distributions, however, turn out to be increased - compared to the real primary ones - due to the velocity spread introduced by neutron and chargedparticle evaporation [3,33] . The validity of the conditions of collinearity and coplanarity (see below) is tested for each event in order to select only events of interest. The program KINTAR is intended for the preliminary evaluation of the physical parameters indicated in fig. 4 with the aim to optimize the calibration coefficients . The program EVAL finally calculates the relevant physical quantities displayed in fig. 4 taking into account the results obtained from the preceding optimization procedure. The sorting programs SUSI and TWOD allow the sorting of the physical parameters calculated with EVAL and their storage as one-dimensional or any combination of two-dimensional spectra on paper or magnetic tape. Two-dimensional (isoline) and three-dimensional graphic representations are calculated from the spectra of the program TWOD and plotted with the aid of the programs ISO and RELIEF, respectively . Two-dimensional scatter plots can also be produced in advanced representations on a CALCOMP-565 plotter or on a DIGIGRAPH plotter . In a two-body reaction, the emission of the two fragments is collinear if considered in the cm system. Thus, the sum of the cm angles B3 + B4 must be equal to 180' in the reconstructed velocity diagram. This implies, of course, that the fragment velocity vectors are also coplanar with respect to the beam axis in the laboratory (lab) system . In the present analysing procedure, the velocity vectors of the two reaction products are determined independently of each other by a combined TOF and position measurement in either spectrometer arm. This double TOF technique offers besides the insensitivity to light-particle evaporation the possibility of unambiguously selecting true binary events and, thus, of discriminating efficiently against nonbinary processes as well as random coincidences. Already deviations from the two-body kinematics concerning only one single velocity vector of the correlated fragments become evident as a violation of the 180 ° rule .

120

Fig. 5. Yield-surface plot of relative yields (the square-root scale was chosen for convenience) as a function of the cm angular sum 63 + B4 and of the fragment mass M3 (recorded in IC 1) for the reaction 4OAr(E, /M1 =5 .5 MeV/amu) + 232 Th. The intensities were measured at a fixed position of IC 2 (04 = 60 °) by integration over the IC 1 positions centered at B° = 60 ° and 75 °. As a result of an event-by-event calculation, the relative yields as a function of the angular sum B3 + B4 and of the fragment mass M3 , which has been recorded in IC 1, are displayed in fig. 5 for the reaction 40Ar(El/Ml = 5.5 MeV/amu) +232 Th, The following conclusions can be drawn from this picture : the two dominating peaks centered around the projectile and target masses (resulting from elastic and quasielastic processes) as well as the broad distribution between them (resulting from compound-nucleus and fusion-like reactions) obey the condition of collinearity . All these binary events fall around the line 03 + 94 =180 ° independently of their fragmentation. The half-width of the in-plane distribution, if projected onto the (e3 + B4) axis, is typically about 5°-10' dependent on the type of reaction . This dispersion contains information about the light particles evaporated from the fragments, supposing the emission takes place only after the fragmentation. Outside of the ridge along 180 °, two regions of random coincidences at symmetric mass division (03 + B4 = 150 ° and = 230 ° ) can be observed, which result from the intense flux of elastically scattered projectiles and recoil nuclei, respectively . These "islands" are connected via "diagonals" of the same origin with the quasielastic peaks. Perpendicular to the main ridge in fig. 5, there exists a smaller one centered around the projectile mass and extending from B3 + B4 =180 ° to = 210 °. It is probably caused by ternary processes, like the emission of fast light particles at an early stage of interaction . These light particles carry away a noticeable amount of linear momentum preferentially in the forward direction, thus enlarging the folding angle of the two remaining fragments. Sequential fission events, which - a priori - do not obey the condition of collin-

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earity, can also easily be excluded by the double TOF technique. It should be emphasized in this context that - compared to the double TOF technique - the measurement of the TOF difference between the reaction products [7-9] has the advantage not to rely on start detectors, but - on the other hand - includes a substantial disadvantage: the absolute values of the velocity vectors of the two correlated fragments are mutually dependent. Thus, the collinearity condition (03 + B4 = 180 ° ) can be simulated in cases of nonbinary processes. Only the less selective coplanarity condition can then be applied to discriminate against nonbinary events. This is, however, merely possible with an efficiency of not more than 90% [8]. The remaining events lying within the reaction plane can no longer be selected unambiguously. 5. Experimental test and conclusions In order to illustrate the operation of the spectrometer DEMAS in its present configuration, some examples obtained in the course of recent experiments will be considered . First, some remarks will be made on the mass resolution and related problems. The values presented in sect . 3 concerning the time and energy resolutions of the detector systems were achieved under laboratory conditions with a-particle sources. Under beam conditions at the heavy-ion cyclotron, these values deteriorate because of beam and target effects as well as by the presence of a strong rf field. The overall time resolution of a TOF arm derived under experimental conditions for elastically scattered projectiles from 22 Ne to 56 Fe turns out to be about 1 ns (fwhm), sufficient for the unambiguous analysis of binary events. It should be mentioned that the AE resolution of the ICs, which has also been measured with elastically scattered projectiles at gas pressure values optimized for the higher ionizing heavy fragments, has been observed to lie in the range of 3-5% . The overall E resolution of about 2% and worse is insufficient . This parameter has, therefore, hitherto not been included in the analysing procedure. The fragment mass resolution achieved by using the double TOF technique is demonstrated in fig. 6. The values of AM= 4-5 amu (fwhm) were obtained in the reactions 4°Ar (El = 220 MeV) +232 Th and 56 Fe (El = 354 MeV) +208 Pb. The results presented in fig. 6 are comparable with those reported in ref. [34] . It should be stressed that there is an unavoidable limitation to the mass resolution due to the contributions of light-particle evaporation (3,33] . As has been shown experimentally [33], the mass resolution achievable in the double TOF technique deteriorates continuously the higher the fragments are excited. Therefore, substantially better values of the mass resolution can be achieved only in

10  N8 0

N6 z û4

0

Fig. 6, Kinematically reconstructed peaks of elastically scattered projectiles and recoil nuclei demonstrating the mass resolving power of the spectrometer, which operates on the basis of the double TOF technique. The outer peaks were obtained in the reaction 4°Ar (El = 220 MeV)+ 232 Th, the inner (hatched) peaks result from the reaction 56 Fe (E l = 354 MeV)+ 2°sPb. fragmentation processes, where relatively cold fragments are produced, e.g. in correlation studies of spontaneous or neutron induced fission. Now we will discuss an example of the kinematic selectivity of the experimental setup. Fig. 7 shows the measured angular correlation 03 vs 04 in the lab system 90

80

60 50 50

60

70

80 0136 [de91

90

100

Fig. 7 . Contour plot of the 63 -04 correlation in the laboratory system measured in the reaction 4°Ar (El = 220 MeV)+ 232 Th. The intensities shown were recorded at three successive chamber positions of IC 1 : B° = 60 °, 75 ° and 90 °, whereas IC 2 was fixed at 04 = 60 °. EL +QE denotes elastically and quasielastically scattered events, respectively. CN + FL stands for fission events from compound-nucleus or fusion-like reactions. The correlation curves for elastic scattering (dashed lines) as well as for fission events (dashed-dotted lines) are indicated, the latter for different mass ratios R = M31M4 of the two fragments (cf. also fig. 8).

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for binary reaction products in the 'Ar (El = 220 MeV) +232 Th interaction. The intensity distributions were obtained by integrating three angular settings of IC 1, while IC 2 remained at a fixed position (cf. figure caption) . Two groups of binary events can clearly be discerned : the left-hand group arises from elastic and quasielastic events, whereas the right-hand group results from fission events, which originate from compoundnucleus and fusion-like reactions [35] . Since both groups of events arise from different physical processes, they can be unambiguously distinguished from each other by means of the reaction kinematics in the correlation diagram . It should be remembered that the events of both groups in fig. 7 satisfy, of course, the collinearity condition, as has been shown in fig . 5 . Thus, it can be concluded that the influence of the primary processes is to a high degree eliminated by demanding fragment collinearity [36]. The interpretation of the experimental distributions in fig . 7 is supported by the calculated correlation curves displayed in fig . 8, which were evaluated assuming the conservation of energy, momentum and mass . For the estimate of the total kinetic energy (TKE) values of the fission fragments, we made use of the Viola systematics [37] for symmetric as well as asymmetric fragmentation . The relevant part of the diagram exhibited in fig . 8 is also inserted in the experimental distributions of fig. 7 (cf. also figure cap-

Fig. 8. Calculated 03-04 correlation between the emission angles of binary reaction products in the 4°Ar (El = 220 MeV) +232 Th collision . The curves were calculated in the laboratory frame for elastic scattering (dashed lines) and fission events (solid lines) . The solid lines are arranged according to different mass ratios R = M3/M4 of the two correlated fission fragments. For elastic scattering events, the curve with R = 0 .172 (5 .8) is valid in case the projectile is detected in IC 1 (IC 2) . For further details see text.

tions). More details on this subject have been described elsewhere [38] . As already stated in ref. [36], the main information from the measured 03 -84 correlation is about the primary fragmentation but also about the evaporation process . The latter is to a certain degree responsible for the dispersion observed experimentally around the correlation curves for elastic scattering in fig . 7. Some geometrical limitations of the contour lines of the distributions are also visible. They will be discussed below. As already mentioned in sect . 2, the opening angle of either TOF arm is about 15 ° in the reaction plane. Therefore, two or three successive angular settings of one IC are usually necessary - at a fixed position of the other IC - to cover the relevant mass and angular range of the reaction products. To completely reconstruct the mass, energy and angular distributions of the correlated fragments, these separate measurements have then to be integrated in angle and normalized to each other. The limited angular range of each TOF arm limits the mass range of the reaction products to be detected at fixed chamber positions . For definite reaction kinematics, relevant information about these limitations can be obtained from mass-angle acceptances, which are calculated for the spectrometer DEMAS with a special procedure [38] . To get a first estimate, e .g . for planning of experiments, calculated correlation curves like those presented in fig . 8 are useful . An example of a measured mass-angle acceptance is given in fig. 9. The limited angular acceptance of the spectrometer for each individual fragment mass is demonstrated. Such rather complicated distributions can be well reproduced by model calculations as discussed in ref. [38] . Moreover, the distributions shown in fig. 9 allow one to draw conclusions concerning the mass drift and reaction times . These aspects are dealt with in more detail elsewhere (e .g. in ref. [8]) . Finally, we present an example to TKE vs mass distributions that have been obtained using the analysing procedure reported here. These distributions result from a series of recent experiments performed with the aim to form composite systems with identical nuclear charge number Z = 108 . The experimental results and their physical interpretation are described in detail in ref. [39] . Preliminary results have been published previously [40] . Fig . 10 shows two-dimensional TKE vs M representations of the yields of the two correlated fragments measured in the 'Ar + 232 Th collision for three different incidence energies. The peaks of elastically scattered projectiles and their respective recoil partners are clearly separated from the broad distributions of the fission fragments . The symmetry with regard to A/2 originates from the assumption of mass conservation in the analysing procedure . In conclusion, it can be stated that the double-arm time-of-flight spectrometer DEMAS turns out to be a

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K D. Schilling et at / The Dubna double-arm TOF spectrometer 500

O gr (Ar)

40Ar (220 MeV) + 232Th

ci)

400 300 200

75°

100 d

w Y f-

20F

300

u7

w ww 200 U

w

Z Y Q

FO

100

300

200

100 MASS Camul

Fig. 9. Contour plots of relative yields as a function of the fragment mass and of the lab angles (a) and cm angles (b) for binary events observed in the reaction °°Ar (E, = 220 MeV) +232Th. The distributions were measured at a fixed position of IC 2 (04 = 60 ° ) by integration over the IC 1 positions centered at B° = 60 ° and 75 °, as indicated in the upper part of the figure. The symbols B., and 0., refer to the grazing angles in the lab and the cm frame, respectively .

versatile instrument for studying nuclear fragmentation processes near the Coulomb barrier of the interacting partners. The kinematic coincidence method based on the determination of the velocity vectors of the two correlated fragments offers the possibility of deducing the primary fragment masses and energies. It thereby proves advantageous to select the events of interest by kinematic conditions . In order to be able to recognize the underlying physics, one should endeavour to detect the reaction product distributions as completely as possible. Improvements should be made regarding the active depths of the ICs to extend the dynamic range of the gas detector systems. If the energy resolution might be improved, correlated velocity-energy measurements

0

40

80

120 160 200 MASS [amul

240

280

Fig. 10. Contour plots of relative yields as a function of the fragment mass and total kinetic energy for binary events observed in the projectile-target combination 40Ar+ 232 Th at three different incidence energies. The yields have been obtained by integration over successive angular regions in the reaction plane. The dashed lines represent the predicted TKE release in fission according to ref. [371. would become possible, which are not influenced by light-particle evaporation and allow inference of preequilibrium processes. Moreover, the detection of the light associated particles must be included in such an extended correlation experiment . At higher bombarding energies, the PPACs and ICs should be arranged in separate gas volumes for optimal conditions of operation. Acknowledgements The authors are indebted to Prof. K.H. Kaun and Prof . Yu .Ts. Oganessian for their permanent interest and active support of the project. We would like to express our thankful appreciation to Drs. P. Manfrass

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and V.D. Dmitriev for their participation in the detector development and to Mr. R. Kirchbach for essential contributions to the mechanical construction . We are grateful to Drs. W. Enghardt and W.D . Fromm for providing the first variant of the data acquisition program and to Dr . H.G. Ortlepp and Mr. R. Kotte for valuable support in solving electronic and timing detector problems. The comprehensive and steady technical assistance of Mrs. J. Fiedler, Mrs. R. Förster, Mr . E. Schuster, Mr . D. Walzog, Mrs. I. Heidel, Mr . V.I . Nosokin and Mrs. Z.D . Pokrovskaya is gratefully acknowledged . Thanks are due to the "Akademie der Wissenschaften der DDR" and to the "Ministerium für Wissenschaft and Technik der DDR", Berlin, as well as to the Joint Institute for Nuclear Research, Dubna, for financial support.

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