P. Thuiner

The mu TPC method: improving the position resolution of neutron detectors based on MPGDs

Due to the He-3 crisis, alternatives to the standard neutron detection techniques are becoming urgent. In addition, the instruments of the European Spallation Source (ESS) require advances in the state of the art of neutron detection. The instruments need detectors with excellent neutron detection efficiency, high rate capabilities and unprecedented spatial resolution. The Macromolecular Crystallography instrument (NMX) requires a position resolution in the order of 200 mu m over a wide angular range of incoming neutrons. Solid converters in combination with Micro Pattern Gaseous Detectors (MPGDs) are proposed to meet the new requirements. Charged particles rising from the neutron capture have usually ranges larger than several millimetres in gas. This is apparently in contrast with the requirements for the position resolution. In this paper, we present an analysis technique, new in the field of neutron detection, based on the Time Projection Chamber (TPC) concept. Using a standard Single-GEM with the cathode coated with (B4C)-B-10, we extract the neutron interaction point with a resolution of better than sigma = 200 mu m.

First measurements with new high-resolution gadolinium-GEM neutron detectors

European Spallation Source instruments like the macromolecular diffractometer (NMX) require an excellent neutron detection efficiency, high-rate capabilities, time resolution, and an unprecedented spatial resolution in the order of a few hundred micrometers over a wide angular range of the incoming neutrons. For these instruments solid converters in combination with Micro Pattern Gaseous Detectors (MPGDs) are a promising option. A GEM detector with gadolinium converter was tested on a cold neutron beam at the IFE research reactor in Norway. The μTPC analysis, proven to improve the spatial resolution in the case of 10B converters, is extended to gadolinium based detectors. For the first time, a Gd-GEM was successfully operated to detect neutrons with a measured efficiency of 11.8% at a wavelength of 2 Aand a position resolution better than 250 μm.

Effects of high charge densities in multi-GEM detectors

A comprehensive study, supported by systematic measurements and numerical computations, of the intrinsic limits of multi-GEM detectors when exposed to very high particle fluxes or operated at very large gains is presented. The observed variations of the gain, of the ion back-flow, and of the pulse height spectra are explained in terms of the effects of the spatial distribution of positive ions and their movement throughout the amplification structure. The intrinsic dynamic character of the processes involved imposes the use of a non-standard simulation tool for the interpretation of the measurements. Computations done with a Finite Element Analysis software reproduce the observed behaviour of the detector. The impact of this detailed description of the detector in extreme conditions is multiple: it clarifies some detector behaviours already observed, it helps in defining intrinsic limits of the GEM technology, and it suggests ways to extend them.

Charge transfer properties through graphene for applications in gaseous detectors

Graphene is a single layer of carbon atoms arranged in a honeycomb lattice with remarkable mechanical and electrical properties. Regarded as the thinnest and narrowest conductive mesh, it has drastically different transmission behaviours when bombarded with electrons and ions in vacuum. This property, if confirmed in gas, may be a definitive solution for the ion back-flow problem in gaseous detectors. In order to ascertain this aspect, graphene layers of dimensions of about 2×2 cm2, grown on a copper substrate, are transferred onto a flat metal surface with holes, so that the graphene layer is freely suspended. The graphene and the support are installed into a gaseous detector equipped with a triple Gaseous Electron Multiplier (GEM), and the transparency properties to electrons and ions are studied in gas as a function of the electric fields. The techniques to produce the graphene samples are described, and we report on preliminary tests of graphene-coated GEMs.

Charge transfer properties through graphene layers in gas detectors

Graphene is a single layer of carbon atoms arranged in a honeycomb lattice with remarkable mechanical, electrical and optical properties. For the first time graphene layers suspended on copper meshes were installed into a gas detector equipped with a gaseous electron multiplier. Measurements of low energy electron and ion transfer through graphene were conducted. In this paper we describe the sample preparation for suspended graphene layers, the testing procedures and we discuss the preliminary results followed by a prospect of further applications.

submitter : Implementation of the VMM ASIC in the Scalable Readout System

The Scalable Readout System (SRS) developed by the RD51 collaboration is a versatile and multi-purpose approach, which is used with different front-end chips to transfer data from detectors to computers. Targeting mainly micropattern gaseous detectors, the system is also applicable for silicon strip or pad detectors. The most frequently used front-end chip today is the APV25, originally developed for the CMS pixel detector. In the scope of the ATLAS New Small Wheel upgrade, a new front-end chip, the VMM, is developed, which has significantly improved specifications compared to the APV25. We report on the implementation of the VMM in the Scalable Readout System carried out by the RD51 collaboration in the framework of a detector project related to the European Spallation Source ERIC. Due to the hierarchical design of the Scalable Readout System, only specific parts of the readout chain need to be adapted or designed, which is the carrier board for the front-end chip, an adapter card that connects to the common hardware of the system and the firmware for a field programmable gate array. In addition, we have developed dedicated software for slow control, data acquisition and online monitoring. The ∗Corresponding author Email address: michael.lupberger@cern.ch (M. Lupberger) Preprint submitted to Nuclear Instruments and Methods in Physics Research Section A, June 11, 2018 readout system has been tested in the laboratory and in particle beams and we present results which proof the functioning of the system, even though it is still in a prototype state.

Combined Optical and Electronic Readout for Event Reconstruction in a GEM-Based TPC

Optically read out time projection chambers (TPCs) based on gaseous electron multipliers (GEMs) combine 3-D event reconstruction capabilities with high spatial resolution and charge amplification factors. The approach of reconstructing particle tracks from 2-D projections obtained with imaging sensors and depth information from photomultiplier tubes is limited to specific cases such as straight particle trajectories. A combination of optical and electronic readout realized by a semitransparent anode placed between a triple-GEM stack and a camera in an optically read out TPC has been realized and used to reconstruct more complex particle tracks. High spatial resolution 2-D projections combined with a low number of charge readout channels enable accurate 3-D event topology reconstruction. Straight alpha tracks as well as more complex cosmic events have been reconstructed with the presented readout concept. Relative depth information from electronically read out charge signals has been combined with drift time information between primary and secondary scintillation pulses to absolute alpha track reconstructions.