Development of a time of flight spectrometer based on position sensitive multi-wire proportional counters for fission fragment mass distribution studies

Abstract

Characteristics and performance of a time of flight (TOF) spectrometer developed for performing fission mass distribution studies are presented. The spectrometer contains two TOF arms based on multi-wire proportional counters (MWPCs). Each arm has two MWPCs to form a start–stop detection system for TOF measurements. The start detector has an active area of 4 × 4 cm2. The stop detector is a two-dimensional position sensitive MWPC with an active area of 16 × 11 cm2. Salient features of the MWPCs are the use of reduced sub-millimeter wire pitches of 0.635 and 0.317 mm in the electrodes along with the use of gold plated tungsten wires of diameters 10 and 20 µm. A delay line for position electrodes is prepared using chip inductors and capacitors. Ten different configurations of MWPC were investigated for the start detector, which involved the use of three and four electrode geometries, use of different wire pitches, and use of aluminized mylar for timing electrodes. Performance results close to micro-channel plate detectors have been observed with some designs of MWPC, displaying rise times better than 2 ns with an estimated inherent time resolution of ∼100 ps FWHM. A position resolution of ∼1 mm (FWHM) has been observed. Design features of the MWPCs and their test performance results are described in this article.

Measurement of mass-angle and mass-total kinetic energy distributions from the fission of 190Pt compound nucleus

“…Fission fragments were detected using two large area (16 cm × 11 cm) position-sensitive multiwire proportional counters (MWPCs) [40], mounted on each arm of the scattering chamber. Forward and backward detectors were placed at the distances of 38 cm and 33 cm from the target, at angles 45°and −115°with respect to the beam axis, respectively.…”

“…The unexpected asymmetric fission behaviour of 180 Hg has generated immense interest in the fission of neutron deficient nuclei in this mass region, and experimental studies have been performed in the pre-actinide region to look for the signatures/presence of competing asymmetric FF and QF in relatively neutron deficient nuclei [16-20, 23, 25, 28-30]. Evidence of asymmetric fission has been found in the measured mass distributions of the fission of 180,190 Hg [20], 182 Hg [16] and 179 Au [18] compound nuclei, formed in the fusion reactions induced via 36 Ar, 40 Ca and 35 Cl projectiles, respectively. However, in a recent study of fission fragment mass distribution measurements for the fission of 191 Au, populated via 16 O + 175 Lu and 37 Cl + 154 Sm [17] reaction systems, no substantial contribution of mass asymmetric fission was observed.…”

Hg178 and asymmetric fission of neutron-deficient pre-actinides

“…The PS-MWPC provided the stop of a second ToF (with respect to the VAMOS++ start) and the (X , Y ) position of the coincident fission partner, while the SSD measured its energy. As compared to previous designs [37,38], a salient feature of the current SED system is the stacking of a transmission-type low-pressure PS-MWPC followed by a SSD in the same detector housing, i.e., a single aluminium chamber filled with isobutane at a pressure of 4 mbar. The PS-MWPC has a three-electrode geometry with the central timing cathode (for ToF) sandwiched between two position-sensitive anodes.…”

“…Position information is extracted using the delay-line technique. The reduced wire pitch of 0.317 mm for the timing electrode and 0.635 mm for the position electrodes significantly improves the avalanche gains and timing resolutions [37]. Another salient feature is the integration of the timing preamplifiers with the detector body, eliminating cables between them.…”

“…Another salient feature is the integration of the timing preamplifiers with the detector body, eliminating cables between them. The position resolution of the PS-MWPC was found to be 1.2 mm (FWHM), while its intrinsic timing resolution has been estimated to be 200 ps (FWHM) [37]. The second layer consists of a 300 μm thick SSD (model TTT12 from Micron Semiconductors) with 20 strips on the front side (each 4.8 mm wide and 97 mm long) and interstrip separation of 50 μm.…”