|
|
|
|
K.-H. Schmidt, GSI Darmstadt, November 1998 Parameters of ALADIN (see also GSI 88-08 Report ISSN 017-4546 and GSI-Nachr. 02-89)Length: 1760 mm with additional field clamps: 2160 mm Gap height: 500 mm Horizontal acceptance: 1500 mm Bending power (field ´ length) 2.3 Tm Deflection angle: » 18 Tm / Brho ´ 7.3 degrees = 18 Tm / Brho ´ 127.4 mrad Properties of the detectors: Two Ionization chambers around the target Length: 90 mm each Active area > 50 ´ 50 mm2 Filling P10, atmospheric pressure Field in beam direction (secondary lead target as common cathode, equiped with a field cage, no Frisch grid)) Cerenkov detector Active area of radiator 50 ´ 50 mm2 Radiator MgF2 5 mm Mirror 0.4 mm ´ Ö 2 float glass (in beam direction) Velocity resolution D b /b £ 1.3´ 10-3 Homogeneous multi-hit position-sensitive detector (Parallel-plate detector) Active area 120 ´ 120 mm2 Resolution about 0.2 mm (horizontal) Fibre detector Two layers of 0.5 mm fibres with partial overlap, 100% detection efficiency, readout by PSPM; two separate detectors to cover the lower and the upper range, corresponding to the division of the Twin MUSIC, Thickness £ 1 mm Area 2 times. 400 ´ 200 mm2 Position resolution about 0.2 mm (horizontal) Twin MUSIC Total length 915 mm Total height 765 mm Total width 1000 mm Lenght of active volume 600 mm Height of active volume 2 ´ 200 mm Width of active volume 600 mm Vertical position resolution 0.5 mm at 4 positions (4 anodes, each 150 mm long) Horizontal position resolution 5 mm at 4 positions (4 anodes, each 150 mm long) (The information on the vertical position and angle from the Twin MUSIC is usefull for higher-order corrections of the magnetic deflection.) LAND (see also Th. Blaich et al., Nucl. Instr. Meth. A 314 (1992) 136-154) Active area: 2m ´ 2m Time resolution for neutrons 0.5 to 1 ns Position resolution for neutrons 5 to 10 cm
Parameters of the ions along the beam line Primary beam 238U, 1 A GeV Primary target Be, 1 g/cm2 Secondary beam (e. g.) 226Th, 930.6 A MeV Degrader Al, 4 g/cm2 Secondary beam 226Th, 639.2 A MeV at entrance of secondary target assumed: 226Th, 600 A MeV Secondary target Pb, 1 g/cm2 Typical fission assumed with 1 A MeV: Average fission fragment (e.g.) 119Pd, 493 A MeV minimum, 632 A MeV maximum, maximum angle 37.7 mrad Additional angles of neutrons of 1 MeV: 37.7 mrad; of 2 MeV: 50 mrad, of 3 MeV: 61.3 mrad. -> Emission of neutrons with 3 MeV from fission fragments is covered if the magnet accepts 100 mrad in all directions around the beam. This limits the distance from target to exit ALADIN to 2500 mm, the distance from target to entrance ALADIN to 740 mm. The distance from target to the radiator of the Cerenkov detector is 425 mm. For all fission events the velocities of the two fission fragments can be determined separately. The distance from target to exit of Twin-MUSIC is about 3500 mm. Fission fragments are accepted up to a vertical angle of 200/3500 rad = 57 mrad; this is definitely sufficient. In horizontal direction, the range of angles due to the different magnetic rigidities is 35 mrad. The total horizontal range of fragments at the exit of the Twin MUSIC is estimated to 2 ´ 37.7 mrad ´ 3500 mm (from the angles in fission) plus 35 mrad ´ 1880 mm (from range of magnetic rigidities). In total, this makes 330 mm which is fully covered by the active width of the Twin MUSIC of 600 mm. One might even think to put the Twin MUSIC at a somewhat larger distance from the ALADIN magnet in order to reduce the fraction of insensitivity near the cathode of the Twin MUSIC and to reduce the influence of angular straggling on the magnetic resolution. Tables: Kinematic parameters of fission fragments behind target:
Table of interactions of 119Pd, 600 A MeV in different layers:
Further considerations: Secondary beam: It is assumed that only one isotope is transmitted to Cave B. The emittance of the beam has been determined to be about 20 to 30 p mm mrad. The intensity of the most frequently produced isotopes is about 10-4 of the primary beam times the transmission factor from FRS to CAVE B. The isotopic identification of the secondary beam can be performed just like it was done in the experiments directly behind the FRS. Two scintillation detectors at F2 and at F8 are necessary. In addition, a detector in front of the secondary target is helpful as a TOF and a position detector. Active target: By mounting the reaction target inside a double ionization chamber, the location of the reaction can be determined and reactions leading to fission from other layers, exept in the active volume of the detector itself, can be suppressed. The reaction rate in the counting gas amounts to 8 % of the nuclear reaction rate in the lead target. In case of silicon detectors used for this purpose the reaction rates in two 0.3 mm thick detectors would amount to 55 % of the nuclear reaction rate in the lead target which is certainly too much. A second measurement with another target layer (probably aluminum) with mostly nuclear reactions has to be used as a reference to subtract the nuclear reactions inside the lead target. Mass resolution: Mass 160 and 161 (extremely heavy fission fragments) at about 600 MeV have a relative difference of D b /b = 2.3 10-3 = 1/435 (for constant Br ) and D Br /Br = 6.25 10-3 = 1/160 (for constant velocity). That means that the velocity resolution should be better than 1/1000 and the magnetic resolution should be better than 1/300. The necessary velocity resolution may only be obtained by the Cerenkov detectors. A short estmation of the possibilites of a time-of flight measurement is given below. The magnetic resolution will be discussed in more detail in the following. Magnetic resolution: We assume 3 position detectors to be installed: 1. detector 200 mm in front of ALADIN, l1 =1280 mm before centre of ALADIN 2. detector 200 mm behind ALADIN, l2 = 1280 mm behind centre of ALADIN 3. detector behind Twin MUSIC, l3 = 1200 mm behind the second position detector (The beam position at the target cannot be used for the magnetic resolution, because the straggling in the radiator is too important.) The deflection angle
In the simplified case when l1 = l2 = l3 = l (which is almost realised here) this corresponds to
This shows that the position x2 enters most sensitively into the definition of the magnetic rigidity. We assume a position resolution of 0.2 mm FWHM for all position detectors. From this we obtain a resolution of the deflection angle of
The value of ALADIN is assumed to be filled with a helium bag, because the angular straggling in air would be too large. For a deflection angle of about 210 mrad (» 120) this means a resolution of D Br /Br = 3.2 ´ 10-3 or Br /D Br = 313. Together with a velocity resolution of D b /b = 10-3, one obtains A/D A = 250, sufficient to separate the fission fragments up to mass 160. In order to obtain this resolution, the higher-order corrections from the field mapping of ALADIN have to be applied. For completeness: What about the perspectives to replace the Cerenkov detector by a time-of-flight? The velocity resolution of a time-of-flight wall in 10 m distance to the target for 600 A MeV ions would be D v/v = D t/t = 0.1 ns/42 ns = 2.4 ´ 10-3 , if we assume an excellent time resolution of 100 ps. After considering the relativistics, this gives a mass resolution of D A/A = 1/91 which is by far not sufficient to resolve fission fragments up to mass 160. Nuclear-charge resolution: From preceding experiments we know that the Twin MUSIC in combination with a velocity measurement (precision D b /b £ 5 ´ 10-3) yields a nuclear-charge resolution of better than D Z = 0.4 charge units for all fission fragments. Thus, the requirements on the velocity resolution are much less severe than those for the mass resolution. The inhomogeneity of the fibre detectors (± 0.5 mm) introduces a variation of the velocity of the fission fragments. This changes the D E signal of the Twin MUSIC by about 3.6 ´ 10-3. The D E signals of Z = 60 and Z = 61 differ by about 3.3 ´ 10-2. Therefore, the inhomogeneity of the fibre detector in front of the Twin MUSIC does not deteriorate the Z resolution noticeably. Resolution in total kinetic energy for single events The resolution in total kinetic energy (TKE) released in fission is mainly limited by the magnetic resolution of ALADIN and by the angular straggling in the radiator of the Cerenkov counter. Angular straggling in radiator of a typical fission fragment: 1.2 mrad (s ) or 2.8 mrad (FWHM) Estimated magnetic resolution of ALADIN Br /D Br = 313
|
in the magnet can
be expressed by the measured horizontal positions xi in the three
position detectors by the following relation:
includes the influence of the straggling of 0.19
mrad in the second detector and half the straggling of 0.383 mrad in the Twin MUSIC: 