The modified mini-FOBOS setup Last update 30.11.2004

Nowadays, the modified mini-FOBOS setup (MMF) is a combination of the double arm time-of-flight-energy spectrometer assembled employing the standard detector modules of the 4π spectrometer FOBOS1, and a neutron bell consisting of 3He-filled counters. The modular structure being the heart of the design of the mini-FOBOS allows to adapt it easily to satisfy the demands of a particular nuclear experiment. This special feature has been already used successfully in a number of experiments devoted to the search and study of Collinear Cluster Tripartition (CCT) of heavy nuclei, a rare phenomenon poorly investigated so far. One of the main advantages of the FOBOS detector modules consists in independent measurement of the velocity vector, mass and charge for each fragment without any kinematical assumptions about the reaction mechanism. This makes possible precise study of reactions in the most general case - non-binary processes with some missing mass, which come out to be decisive for investigation of characteristics of CCT.

The general idea of the MMF consists in using a small reaction chamber which can be then unique for each experiment and the basic universal system maintaining the detectors. The special mounting cones are used to join the rather large detectors with the reaction chamber (Figs.1, 2) Such a design benefits, in particular, in a multi-detector facility requiring a high efficiency fragment trigger. By means of such mounting cones one can easily build the high efficiency system joining six modules in the form of 3D-cross. The solid angle of an individual detector module amounts to 0.26 sr meaning flight-path of 50 cm.

Fig. 1.
Layout of the MMF spectrometer. Dimensions are given in cm

Fig. 2. Major components of the MMF spectrometer

Our spectrometer is currently equipped by the universal reaction chamber of 44 cm in diameter with the available arm-angles of 65˚, 90˚, 135˚ in the reaction plane for two detector modules (Figs.1). The direction of the beam can be reversed increasing thus the number of possible opening angles between the detectors. Such a configuration of the spectrometer is well suited for study of heavy-ion induced reactions and for spontaneous fission as well. Although the flight path of 50 cm is an optimum due to a number of reasons, the geometry of the spectrometer can be changed if needed. The lowest flight path of approx. 25 cm is defined by the construction of the mounting cones (Figs.1, 2).

Depending on demands of the particular experiments additional detectors can easily be installed into the MMF setup (like gamma-detectors, neutron-detectors, forward-angle arrays, etc.). In particular, the currently operating universal reaction chamber has two free flanges for additional applications independently from the angles of the arms.

Coupling the TOFE spectrometer with the high-efficiency neutron detector was neaded for experiments aimed at seach for CCT. The possible distortion of the neutron field due to thick metallic constructions was carefully analyzed. The degree of such a distortion was calculated with the help of Monte-Carlo simulations2. It was shown that the major distortion is introduced by the concrete floor of the experimental cave; however, the systematical error seems to be correctable.

Fig. 3. The view of the setup duing the experiment in Februrary 2004.
Reaction chamber is surrounded by 3He-filled counters

The detector module of the mini-FOBOS spectrometer is the standard FOBOS module which is described carefully here.

The gas-supply system has been designed by analogy with the gas supply system of the FOBOS spectrometer, which reliability has been tested in a series of experiments. The gas-filled detectors are operated in a flow-through regime to guarantee stable long-time working conditions. The flow-through regime makes the detectors insensitive to radiation dose, what is of great advantage in comparison with solid-state detectors. The status of the whole evacuation and gas-supply system is controlled at every time with a PC operating via interface RS485. The P-10 gas for the BIC is mixed on-line by controlling the respective mass-flows of the components. A gas analyzer permanently checks the gas composition that came out as necessary test for fixing the electron drift-time and, therefore, the energy resolution of the BIC during long-time experiments. The gas exchange rate of the BIC can be adjusted at the inlet collectors. A full gas exchange takes about 6 hours. All pressures are stabilized to an accuracy of 1 %.

Data acquisition system

Detection system is equipped now by the standard electronics of FOBOS system including the following specialized CAMAC blocks (per one detector): 3CFT 5386, 4TDC КА251, BCD 5387, BDP 5385, LBIN (manufactured in FZR and JINR). Original data acquisition system optimized for open architecture of the MMF is based on code package supporting format type HOOPSY/OLYMP under NT-Windows or under real time systems in VME standard. Connection between CAMAC crates are performed using VDB-line with original PCI-VSB controller on the PC side and VSB crate-controllers CVB 1000. Other equipment is standard. Additional logical blocks are used for processing of events, TDCs with time interval up to 8 μs for the drift time measurements. Electronics for the mosaic of scintillators comprise 7 TDC channels and 2х7 QDС channels (better 3х7) and gate generator.

The first-level trigger of the mini-FOBOS is usually generated by the gas detector part, and the scintillators are read-out in the slave mode. In special cases a trigger can also be generated by the scintillator mosaic. The entire TTL/ECL-based hardware of the trigger logics fills one CAMAC crate. It delivers either the LAM demand for data storage or a general RESET after a certain event inspection leading to a rejection of the event. Provided the digital processor (BDP) is not busy, a timing signal of the corresponding PSAC passes a special blocking and pile-up inspection unit (LBIN) connected with the control logics of the BDP, opens an event gate of duration Dt = 200 ns, sets a bit in the coincidence pattern register and starts a TDC
which will be stopped by the next arriving RF-signal from the cyclotron or common STOP signal from START detector.
One CAMAC crate housing the digitizing electronics of the gas-detector part and the control logics, and one FASTBUS mini-crate used for the photomultiplier read-out of the scintillators, are connected with a main processor by means of the parallel VSB Differential Bus Extension (VDB bus). The VDB bus is well suited for multi-crate systems where different bus standards have to be controlled. A single-board computer EUROCOM-6 with a 68030 CPU in the VME crate builds the event data blocks3.

The CAMAC-to-VSB interface is a single-width CAMAC crate controller STR 610 / CBV4 driven from the VME Subsystem Bus (VSB) via the VSB Differential Cable. The specification of the CBV is similar to the CAMAC crate controller of type A1. It maps a portion of the VSB address space to the CAMAC - “C,N,A,F” and generates single CAMAC cycles from each proper VSB cycle.
The FASTBUS mini-crate contains a 68030-processor board (CERN Host Interface, CHI), an I/O-Port, a LAN Ethernet module 4 and 96-channel FASTBUS QDC’s. The VSB I/O-Port provides an efficient interface between the CHI and the VME workstation where the CHI is operating in the VDB slave-mode. The CHI data memory is directly mapped into the local VSB address space, and the EUROCOM-6 processor module is treated in the same manner as any local memory.
The VME workstation sends the data blocks via Ethernet (LAN) and a fiberoptical link to a PC-station which writes them “event by event” to the mass storage memory.
The main above described properties of the data-acquisition system are generally the same in its version oriented to PC under NT-Windows.

Further upgrade of the MMF

Besides of the possible additional detectors which can be easily attached to the MMF spectrometer the following improvements of the detector modules are under consideration:
  • Extension of fragment charge interval up to Z = 30 at fragment energies about 1 AМeV when Bragg’s peak disappears. A method of fragment charge determination using measurements of track drift time in a uniform electric field5 is supposed to be adapted in the modern form of obtaining of digital image current pulse and its off-line analysis.
  • Improvements of the energy resolution of BIC up to 1% for 1 AМeV fission fragments
Development of method of discrimination of events result from scattering of fragments on wires and supporting grids. This discrimination is absolutely necessary in the case of measurements of rare events (for example, superasymmetric fission).
Providing a simultaneous registration in PSAC of some particles connected with one physical event. It can let as well to construct a low-level trigger with a decision- time of less than microsecond. Modern multihitting TDC will be the base of this development. A reconstructed PSAC will be able to detect two fragments intersected by 2 ns interval or 16 fragments with a guarantied interval of 10 ns between every pair of detected fragments.

[1] H.-G. Ortlepp et al. NIM A 403 (1998) 65-97.
[2] D.V. Kamanin, E.A. Kuznetsova, B.V. Florko,”Modelling of the neutron field of the mini-FOBOS setup”, Heavy Ion Physics, FLNR JINR Scientific Report 2001-2002, Dubna 2003, p. 217-218.
[3] P. Ziem, T. Kiehne, L. Dietterle, and V.V. Trofimov, FZR-92-11, Rossendorf, Germany, 1992, p. 19.
[4] STRUCK - Product Summary, Hamburg, Germany, 1990.
[5] A. Oed et al., Nucl. Instr. Meth. 205(1983) 455.

The first experiments with the Modified Mini-FOBOS Setup and their preparations
The Mini-FOBOS Setup at the beam line 6b of the IBR-2 reactor