Upgrade to the focal-plane detector system of the S800 spectrograph at FRIB

,


Introduction
The S800 is a large-acceptance, high-resolution spectrograph [1] used for experiments with radioactive beams (RIBs) produced by projectile fragmentation at the Facility for Rare Isotope Beams (FRIB) [2].The S800 started its operation at the end of the '90s at the National Superconducting Cyclotron Laboratory (NSCL), and it was a key instrument to capitalize on the high-luminosity and energy (up to ~100 MeV/u) RIBs experiments through the use of thick targets.It has been an important experimental device for the international low-energy nuclear experimental user community.The S800 will also be a crucial tool also at FRIB, with about 40% of the experiments expected to be performed on the S800 until the completion of the High Rigidity Spectrometer [3].
The typical experimental scheme is based on a fast, exotic beam (v/c >0.4) delivered by the FRIB fragment separator, impinging on a reaction target located at the S800 pivot point.The emerging reaction products are identified and characterized by the spectrograph, using its focal-plane detection system (FPD) [4], via the simultaneous measurement of magnetic rigidity (Bρ), time-of-flight (ToF) and energy loss (dE).
The basic configuration of the S800 FPD comprises [5]: two drift chambers (DC) that provide tracking capability; an energy-loss measurement device used to identify the atomic number (Z); a large timing plastic scintillator, which combined with a small scintillator placed in the beamline prior to the target, or combined with the accelerator radiofrequency phase information, yields ToF information; and a CsI(Na) hodoscope readout by a 2D array of photomultiplier tubes (PMTs) [6], used to measure either the total kinetic energy of implanted nuclei (allowing the identification of different charge states) or gamma rays emitted from isomers (isomer tagging).All of the particle detectors described above have an effective area of 30x60 cm 2 and are capable of event-by-event particle detection at maximum rate up to 5 kHz, with 100% transmission efficiency, with the exception of the hodoscope, which absorbs all particle energy.A focal-plane detector system upgrade, with innovative readout technologies and improved detector designs, has been initiated for the S800 to cope with beams of the high-intensity heavy-ion beams planned for the FRIB science program.In this work, the plan for the upgrade of the S800 FDP will be presented and described in detail.This includes the implementation of a novel position-sensitive gas-avalanche readout for the tracking system [7], an innovative high-rate energy-loss detector concept for Z-identification [8], and a revised high-resolution optical-readout for the timing plastic scintillator.The improved FPD is being designed to improve high-rate tracking capabilities, identification performance, and overall reliability.Furthermore, a new, high-performance front-end electronics and data acquisition system is being upgraded and implemented in order to take advantage of the new capabilities of the novel detector systems.
Through the development of a high-performance tracking and particle identification detector system for the S800 spectrometer as well as the optimization of the data acquisition system hardware, FRIB will be able to continue its rich program of nuclear physics with exotic beams.In addition, it would also serve as a benchmark for the development of similar detector systems to be used at FRIB.This includes, for instance, the Sweeper Magnet [9] and the future High Rigidity Spectrometer [3], [10].

The tracking system
The tracking system comprises two drift chambers (DCs), 1 meter apart, for the measurement of the transversal position and the angle of the charged particles that reach the focal-plane.The design of the DC is based on the old cathode readout drift chamber (CRDC) [5], with the exception of the single wire avalanche architecture, being replaced by a more advanced micro-pattern gaseous avalanche structure.The latter consists of a two-layer M-THGEM [11], which serves as a pre-amplification stage, and is suspended over a position-sensitive micromegas detector [12] -see Fig. 2.This novel hybrid readout configuration has the advantages of stable, high-gain operation at low pressure (typically 40 Torr).For reducing heavy-ion energy/angle straggling, low pressure operation is necessary to reduce the material budget.In addition, it provides excellent position resolution (0.25 mm RMS in both directions), and a better reduction of ion backflow.The latter is crucial for an overall improvement of the operational stability of the detector at high rates.
Furthermore, in the conventional CRDC, operated in the slow CF4/20%iC4H10 gas to delay aging problems, the typical drift times of the electrons to the anode wires are from a few hundred ns to around 20 μs, depending on the position of the track in the non-dispersive direction.The relatively long drift times limit the maximum rate that the detector can process properly to around 5 kHz.With the newly developed detector configuration, the operation in a faster counting gas is possible (i.e., pure CF4), resulting in an increase in electron drift velocity, and thus a better rate capability (>20 KHz).
The localization of the charged particle along the dispersive direction is achieved by computing the center of gravity of the avalanche charge induced on the pads (1.25 mm wide pitch) of the segmented micromegas board -a total of 480 pads along the 60 cm long direction.The localization along the non-dispersive coordinate is given by measuring the drift time of the primary ionization electrons to reach the micromegas mesh from the point of their origin in the drift region, measured with respect to an external trigger (e.g. the trigger signal from the ToF plastic scintillator).Fig. 3. Details of the mechanical design of the drift chamber and of the gas avalanche readout scheme.
The 3 cm wide effective volume (drift region) is sandwiched between two printed circuit boards (PCB) and two 12-µm PPTA windows mounted on frames (Fig. 3).Each PCB is made of un-masked G-10, and includes a field shaping foil (70 µg/cm 2 polypropylene) to ensure a uniform field in the active region of the detector.The shaping foils are made of 1.9 mm pitch of evaporated (0.1 µm thick) gold strips perpendicularly oriented to the electric field.
The signals from the micromegas pads are read out and processed by a Generic Electronics for TPCs (GET) electronics based DAQ system [13], characterized by low noise, high data transfer capabilities and multi-hit capability for efficient digital rejection of pile-up events.The updated tracking system for the S800 will be commissioned in early spring 2023.
An ionization chamber with optical readout, denoted as an energy-loss optical scintillator system (ELOSS) [8], will be installed downstream of the DCs and it will be used to identify the atomic number (Z) of the transmitted isotopes from their energy loss.ELOSS will replace the conventional "charge" ionization chamber (IC) [5] used for the identification of the atomic number (Z) in the last three decades at the S800.It has an active area that is the same as that of the DC and an effective depth of about 100 mm.
The ELOSS vessel is filled with high scintillationyield gas, such as Xenon (Xe).The scintillation light emitted along the track of a beam particle that cross ELOSS is intense (approximately 75 photons/keV [14], [15]) with prompt photons are emitted within a few nanoseconds.The scintillation light is read out by an array of vacuum photomultiplier tubes (PMTs) that are placed around the detector volume (Fig. 4).The optical readout comprises 120 Hamamatsu PMTs (Model R8520-406 [16]) arranged in four sectors (30 PMTs uniformly spaced per sector).The sectors are sandwiched between aluminized thin foils (1.5 um thick), which allow scintillation photons to be directed towards the optical readout for increased photon collection efficiency.Alternatively, the aluminized foil can be biased to create a uniform electric field with a strength comparable to the end of the ionization region of Xe (proportional scintillation mode).Stimulated electroluminescence produces a higher light yield and thus better photon statistics.The PMT signals are processed by a Mesytec QDC-MDPP-32 digitizer [17], which provides high resolution timing and amplitude on an event-by-event basis.
The ELOSS detector is a cutting-edge experimental device that offers several noteworthy technical advantages compared to traditional charge readout systems.Unlike conventional ICs, the detector readout of ELOSS is completely decoupled from the drift region, hence the small detector capacitance from an optical readout, together with the low limit of the photosensor threshold (down to single-photon detection efficiency), allow ELOSS to reach larger signal-to-noise ratios, and consequently a better resolving power.ELOSS is expected to achieve a 0.4% (σ) energy resolution compared to the 1.2% (σ) resolution [5] achieved by conventional IC.The better intrinsic energy resolution of ELOSS will be able to extend Zidentification to elements with an atomic number Z>50.
ELOSS is based on recording electroluminescence emission, and it is therefore a much faster detector than a conventional IC.In the case of Xe gas, more than 90% of the scintillation photons are emitted within 100 ns providing an extremely short signal rise time.This results in a rate capability (up to 50 kHz) that is several times higher compared to the traditional IC (limited to 5 kHz).
Finally, ELOSS may also provide good timing information (<100 ps) and moderate localization capability (a few mm) compared to virtually no usable timing/position information provided by the traditional IC.
The ELOSS detector and interfaces (gas handling system and DAQ) will be commissioned at the S800 in early fall 2023.

The timing plastic scintillator
A large-area scintillation detector is placed behind ELOSS to provide timing information, as well as energy loss (at a moderate resolution).The upgraded version of the timing detector features a large (30 cm × 60 cm) plastic scintillator sheet (Eljen EJ-230), with thickness between 1-3 mm, depending on the nuclear charge of the fragments to be detected.The EJ-230 is designed to handle very fast timing (rise time of 0.5 ns) over large areas, with light output of about 11,000 ph/MeV and a long light-attenuation length.The light is readout by a series of very fast Hamamatsu PMTs (model H6533), positioned around the scintillator area.The PMTs are directly coupled to the scintillator through an optical pad, with no need for a light guide.A single PMT is mounted on each of the horizontal sides of the plastic scintillator -i.e.along the nondispersed beam direction, and five PMTs along each of the two vertical sides -i.e.along the dispersed beam direction (Fig. 5).However, the design of the detector frame allows one to include up to 20 PMTs.The number and arrangement of the PMTs around the plastic scintillator was defined through a systematic Monte Carlo simulation and measurements performed with a prototype unit, with the aim of attaining the highest photon-collection efficiency and a uniform fast timing response across the full detector area.Details of the procedure and the results of the readout optimization will be reported in a future publication [18].
The PMT signals of the scintillator detector are processed by a Mesytec MCFD16 (selectable constant fraction discriminator (CFD) or leading edge (LE) [19], followed by a multi-hit Mesytec MTDC32 [20] for timing.A second branch of the detector signals is processed by a Mesytec digitizer MDPP-32 [17] for extracting the integrated charge.
Different time-of-flight paths can be selected by the timing signals from the S800 scintillator and other upstream timing detectors along the beam line, the fragment separator, and the RF signal.The timing resolution for a point-like beam spot in the focal-plane is expected to be <35 ps (σ).However, the resolution worsens significantly (up to 1 ns) when the whole focalplane is illuminated, because of the different path lengths of the fragments along the beam line and the spectrometer.Good time resolution can be recovered by including in the calculation the trajectories of the fragments using the S800 tracking system.The plastic scintillators can withstand maximum rates of up to 10 6 particles per second.
The new upgraded timing plastic scintillator, which replaced the old configuration based on only two PMTs, will be commissioned in early spring 2023.

The FRIB data acquisition system
The focal-plane detector systems of the S800 spectrometer is read out via a set of different Data Acquisition Systems (DAQs), specific of the various detector technologies.FRIB takes advantage of the experience accumulated over many years of experiments running at the NSCL by adopting a software suite that provides a flexible and extensible framework known as FRIBDAQ (previously NSCLDAQ).FRIBDAQ features a generalized data flow architecture that supports data transfer from an arbitrary set of source processes to an arbitrary set of destination processes (consumers) reachable over the network.To construct the data stream, also known as the data pipeline, consumers can, in turn, become sources for successive stages to build more complex architectures.
The backbone of the data flow in FRIBDAQ is the ring buffer, a memory location that acts as simple proxy object used to perform traffic aggregation and accessible to multiple processes.Access is however controlled in such a way that only one process can contribute data to it, while many processes can read from it.FRIBDAQ features a timestamped generic event builder.It can accept event fragments from segments of the dataflow system, order these data by timestamp and emit events that consist of fragments that are within a user specified coincidence window.This data is then submitted back to the FRIBDAQ data flow system where they can be used in later stages of the event builder.This design allows for the combination of detectors with independent data acquisition systems without modification to each individual component, a routine procedure at FRIB.A collection of standalone executables has been provided to readout data from what have become standard data source types.These include support for VMUSB and CCUSB modules from Wiener/JTEC as well as a generic framework that is used both for legacy SBS/Bit3 PCI/VME bus bridges, XIA Digital systems based on the PIXIE-16 module, and General Electronics for TPCs (GET), which will be used to readout the new S800 tracking detectors.FRIBDAQ is continuously improved and adapted for new hardware by the FRIB Laboratory staff, in coordination and collaboration with the users of the facility.
A very important aspect of FRIBDAQ is to be designed to be open both in terms of ability to extend its functionality through plugins and to attach and analyze to the data pipeline with a multiplicity of data analysis framework, including CERN ROOT system [21] and Python scientific libraries for analysis.FRIBSpecTcl is for data analysis what FRIBDAQ is for data flow.It is a C++ framework that allows experimenters to write custom analysis software that seamlessly integrates with a histogramming engine and viewer.The Graphical User Interface (GUI) allows users to quickly create histograms from the data and to draw and apply on the fly one-and two-dimensional gates for more restrictive event selection.FRIBSpecTcl has integrated support for both ROOT and Python libraries for more advance data analysis tools and it is maintained and continuously improved by the Laboratory staff.

Summary
The upgraded configuration chosen for the S800 focalplane detector system has been presented and discussed.Through the newly developed readout scheme based on micro-pattern gaseous detectors for the tracking system and high-resolution particle identification, it is possible to detect isotopes with a wide range of energies and stopping powers.In addition, the rate capability is increased by about 2-3 fold when compared to the old detector configuration.The upgraded detector systems will also serve as models for other applications, including experiments with the Sweeper Magnet and the future High Rigidity Spectrometer.

Fig. 1 .
Fig. 1.Schematic drawing of the focal-plane detector system used for particle identification at the FRIB S800 spectrograph.

Fig. 2 .
Fig. 2. Updated readout structure of the drift chamber for the S800 tracking system, comprising a segmented micromegas board preceded by an M-THGEM as a pre-amplification stage.

Fig. 5 .
Fig. 5. Mechanical design of the upgraded scintillator detector for timing measurements.