Scientific report, GSI Darmstadt , 1993, p 280

An Improved Detector for Direct Mass Measurements of Exotic Nuclei at the ESR
Ch.Klein 1, J.Trotscher 1, H.C.Jung 1, H.Wollnik 1, B.Hartmann 1, E.Schneider 1, D.Scholten 1, T.Radon 1, M. Hausmann 1, K. E. G. Lobner 2, T. Beha 2, S. Issmer 2, Th. Kerscher 2, K. Rudolph 2, A.Magel 1, H. Geissel 3, G.Munzenberg 3, B.Franczak 3, B.Franzke 3, Y.Fujita 4, K.Mezilev 5, Yu.Novikov 5 and D.J.Viera 6
  1. II Physikalisches Institut der Universitat Giessen, 
2. Sektion Physik der Universitat Munchen, Garching, 
3. GSI Darmstadt, 
4. College of General Education, Osaka University, Japan, 
5. St.Petersburg Nuclear Physics Institute, Russia, 
6. Los Alamos National Laboratory, USA


For the proposed time-of-flight mass measurements of exotic nuclei in the ESR [1] the existing time pick-off detector [2] has been upgraded to allow the detection of secondary electrons on both sides of the detector foil [3]. This allows to operate a position sensitive detector in addition to a fast timing detector. Furthermore the ion detection in coincidence allows to reduce background noise.

The overall timing uncertainties, which limit the mass resolving power, are influenced by higher order aberrations of the ESR. These become minimal after each fourth turn because of ion-optical symmetry. The leftover higher order terms can be corrected and should improve the mass resolution, if the time aberrations due to different start positions and different initial angles of inclination can be determined [4]. The aberration coefficients can be calculated while the positions at which individual ions pass the timing detector can be taken from a position sensitive readout of this detector. Therefore a fast timing position sensitive channelplate detector has been developed [5], with ±1 mm position accuracy and readout times <5 ns.

The conical anode of the timing detector was replaced by a newly developed flat design, which gives a better signal quality. 

  A test of the detector system has been done with U72+ ions of various energies in the range of 400 - 900 MeV/u in a parasitic run at the final focus (S4) of the FRS. The timing properties were measured with both detectors in coincidence. A time resolution of < 85 ps has been achieved (see fig.2), with a detection efficiency of about 70%.

Since the existing prototype of the Time recording system (TRS) was not running reliably, an approved hardware version of the TRS is under construction.


Through a modification of the ADC control unit a reduction of dead time by one half will be possible. The maximum recording time is increased to some milliseconds. With this equipment, a count rate of < 50MHz will be recordable event by event, with a pulse pair resolution better than 5 ns.

The original transputer system has been replaced by a UNIX workstation, since the expected further developments of the transputers did not occur. To handle the high data rate of 1 ESR filling per second, the operating system has been changed from MSDOS to LINUX. Now the time measuring system can be controlled by any Xwindow terminal. Software adaption and improvements are done at the moment.


[1] H. Wollnik, Th. Schwab, and M. Berz, GSI-Report, GSI-86-1, 372.

[2] J. Trotscher et al., Nucl. Instr. and Meth. B70 (1992) 455.

[3] J. Trotscher 1993; PhD Thesis, (Univ. Giessen)

[4] B. Hartmann 1993; PhD Thesis, (Univ. Giessen)

[5] Ch. Klein, J. Trotscher and H.Wollnik, Nucl. Instr. and Meth. A335, 1993, p 146.