P.A.A. Perera

A solenoidal spectrometer for positron-electron pairs produced in heavy-ion collisions

Abstract A new solenoidal spectrometer, designed to study the production mechanism of electrons and positrons in heavy-ion collisions, has been constructed at Argonne National Laboratory. The spectrometer uses a 300 G magnetic field to transport the leptons to two highly segmented silicon arrays that are centered on the solenoid axis, 1.2 m from the target. Positrons are identified by detecting their annihilation radiation with two arrays of position sensitive NaI(Tl) crystals that surround the silicon arrays. A novel design feature of the spectrometer is the ability to measure the angles of emission of the leptons relative to the solenoid axis. The measured response of the apparatus to electrons and positrons is in very good agreement with the calculated response obtained from Monte Carlo simulations.

FERA readout system for APEX

Abstract A data acquisition system is described for processing energy and timing signals from large arrays of solid-state detectors and photomultiplier tubes. Signals are digitized using Fast Encoding and Readout ADCs (FERAs), and then downloaded to Data Stacks for subsequent readout via CAMAC. The acquisition electronics uses additional ECL modules to make logical decisions about data compression and synchronization. Three modes of readout are available, providing varying degrees of event compression. The system is characterized by a very short readout period, as well as the ease with which readout mode can be reconfigured. This FERA readout system is currently being used in the ATLAS Positron EXperiment (APEX) at Argonne National Laboratory.

The sixteen-channel APEX constant fraction discriminator

Abstract A sixteen-channel constant fraction discriminator (CFD) has been constructed for the ATLAS Positron EXperiment (APEX). An integrated circuit, recently introduced commercially, contains all the electronic building blocks required to construct a CFD and makes it possible to fit sixteen CFD channels into a single-width CAMAC module. An important new feature is the time-to-charge converter that is part of each CFD channel. Its calibration constant is controlled via CAMAC. The time-to-charge converter allows the use of low-cost charge-sensitive analog-to-digital converters for timing measurements.

Segmented focal plane detector for light and heavy ions

Abstract A segmented focal plane detector for an Enge split-pole spectrograph has been developed for the study of breakup reactions at very low relative energies. It consists of a 61 cm long segmented position-sensitive parallel plate avalanche counter backed by a large Bragg curve detector. A segmented plastic scintillator is mounted behind the anode of the Bragg curve detector and is used for particle identification of low-ionizing particles. The dead space between the two sections of the focal plane detector is 2.5 mm. The intrinsic position resolution of the detector is 1 mm. The intrinsic energy resolution depends on the energy of the incident ion and can be as good as 0.55%. The nuclear charge and mass resolutions are 0.3 e and 0.3 u, respectively.

New focal plane detector system for the rochester recoil mass spectrometer

Abstract A focal plane detector system has been developed for the Rochester Recoil Mass Spectrometer. It consists of a parallel grid avalanche counter (PGAC) backed by an ionization counter (IC). The PGAC is used to obtain the horizontal and vertical position of the incident particles and also to provide a fast timing signal. The PGAC has an active area of 5.1 cm x 5.1 cm. The IC consists of three charge collecting anodes (2.0 cm, 6.2 cm, 21.6 cm long) and provides nuclear charge identification and a total energy measurement. Both the PGAC and the IC are filled with isobutane as the counter gas. The intrinsic position resolution of the PGAC was determined to be 0.9 mm (FWHM). The IC is able to resolve neighboring nuclear charges for light ions with energies as low as 0.7 MeV/u. The nuclear charge resolution of the IC for heavy ions with IC energies below 0.7 MeV/u was found to be 1.3 e (FWHM).

The sixteen channel CAMAC constant fraction discriminator for APEX

The authors report on the construction and the performance of a sixteen-channel constant fraction discriminator (CFD) for the Atlas Positron Experiment (APEX). An integrated circuit which contains all the electronic building blocks needed to construct a CFD has been used. Sixteen channels of CFD have been placed into a CAMAC module. An important feature is the time-to-charge converter (TQC) included for every CFD channel. Its calibration constant is controlled via CAMAC. The TQC allows the use of charge-sensitive analog-to-digital converters for timing measurements. Results for CFD walk, resolution, and crosstalk as well as for TQC linearity are presented.<<ETX>>

Apex and the e+/e− Puzzle: Recent Results

Narrow structures first reported in positron singles energy spectra [1, 2, 3, 4, 5, 6, 7] and later in electron-positron energy distributions [5, 8, 9, 10, 11, 12] associated with collisions of very heavy ions near the Coulomb barrier have been an outstanding puzzle in nuclear physics for nearly fifteen years. In a series of positron and positron/electron experiments by three different groups (EPOS, ORANGE and TORI) ([5] and citations therein) at the GSI UNILAC at Darmstadt, Germany narrow structures (‘lines’) were observed by EPOS and ORANGE for several heavy-ion collision systems, with some features such as the line energies and widths having apparently similar values for the different collision systems.