SiD Calorimeter Report Outline



VI. Hadron Calorimetry

1. Introduction

The Hadron Calorimeter (HCal) is a sandwich of absorber plates with gaps instrumented with active detector elements. To satisfy the stringent imaging requirements of the PFA algorithm transverse segmentation is required to be small of order a few cm2 and each layer is read out separately. This in principle allows an analog or digital treatment of the signals.

The current baseline uses steel for the absorber and resistive plate chambers as the detector. Alternative detector possibilities, consisting of Gas Electron Multipliers (GEMs), Micromegas or scintillator are also under study. One of the design criteria for the HCal is to minimize the size of the active gap, because an increase in the gap size has a large impact on the overall detector cost. The current gap size is 12mm.

The absorber consists of steel plates with a thickness of 20 mm or approximately 1.1 X0.

The cell structure, which is identical for the barrel and the endcaps, is repeated 34 times, leading to an overall depth of the HCal corresponding to four interaction lengths. Tungsten has been and will also be considered as a absorber, but is currently not the

baseline.

The inner radius of the HCal is 138 cm, the outer radius of the barrel is 233.7 cm and its length is 554.0 cm. The endcaps start at a distance of 179.65 cm from the interaction point. The SiD baseline design uses steel as absorber material in the HCAL. Steel with a radiation length Xo = 1.8 cm and an interaction λI = 16.8 cm offers the smallest X0/λI of all commonly used absorber materials. A small ratio permits a one radiation sampling, while keeping the number of active layers manageable for a hadron calorimeter with a depth equivalent to four interaction lengths at 90o to the beam pipe.

The first attempts at a mechanical structure foresee a barrel and two endcaps, which are

inserted into the barrel structure. The barrel is subdivided along the beam direction into

three sections (Fig. XX), and there are 12 modules to make a complete ring in azimuth.

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Fig. XX Schematic of SiD Hadron Calorimeter (left), and single module (right).

The steel plates are held in place by a set of picture frames located at each end (Fig. XX). Readout cables, high voltage cables and the gas lines are routed to the outer radius of the barrel structure through the openings in the picture frames. In azimuth the barrel structure is subdivided into twelve modules. Each module weighs approximately ten tons and will

be held in place through supports in the cryostat of the solenoid. Deflections have been calculated and do not exceed 0.5 mm at any point of the structure.

Prototyping and testing of active gap technology options has been ongoing for several years. The basic characteristics and initial viability of each option have been established through a series of cosmic ray and beam tests. For the gas-based technologies the next stage will involve a “slice test” of a number of RPC and GEM chambers with absorber layers. The scintillator based medium has been validated with test beam studies and focus has moved to production issues. Beyond this we plan to build one or more ILC prototypes of approximately 1m3 size to contain hadron showers, and read out using these technologies. This scale of testing, involving up to 400,000 channels is presently limited by funding in the US.

We have defined a set of criteria and requirements for the selection of the final active medium. We have established a time schedule for the convergence of the hardware tests and the PFA results. We anticipate making a selection on the timescale of the SiD CDR – around middle to late 2008.

2. Technologies under Consideration

Digital Hadron Calorimeter with Resistive Plate Chambers

Resistive Plate Chambers (RPCs) with small readout pads are an ideal candidate for the active medium of a hadron calorimeter optimized for the application of PFAs. RPCs can provide the segmentation of the readout pads, of the order of 1 to 4 cm2, which is necessary to keep the ‘confusion term’ small. They can be built to fit small active gaps (less then 10 mm) to maintain a small lateral shower size and to keep the longitudinal extend of the hadron calorimeter as short as possible. Glass RPCs have been found to be stable in operation over long periods of time [1], especially when run in avalanche mode, and their rate capabilities are adequate for the ILC and for test beam studies of hadronic showers. RPCs are inexpensive to build since most parts are available commercially. Signals in avalanche mode are large enough (in the range of 100 fC to 2 pC) to simplify the design of the front-end electronics.

Figure H1 shows a schematic diagram of a single-gap RPC. The chamber consists of two plates with high electrical resistance. Readily available window glass of thickness 0.8 to 1.1 mm is used. High voltage is applied to a resistive coating on the outside of the glass plates. The resistance of this coating must be low enough to allow for a fast local re-charge of the glass plate after a hit, and high enough to avoid screening of the electron avalanche in the gas from the readout pads located on the outside of the chamber. The glass plates enclose a gas volume in which ionization and electron multiplication takes place. Particles traversing the gas gap ionize the gas, creating an avalanche of electrons drifting towards the glass plate at positive high voltage. The signal is picked up inductively with pads located on the outside of the glass.

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Detailed simulation studies have shown that the energy response is preserved with a simple one-bit (or digital) readout of the pads, see Fig. H2. The single particle energy resolution with one-bit readout is found to be significantly better at lower energies, due to its insensitivity to Landau fluctuations. At higher energies the resolution with analog readout is superior, due to the high density of particles in the core of the shower. However, for the application of PFAs, only the response in the energy range below say 20 GeV is important. Using a simple digital readout, pads with an area of 1 x 1 cm2 appear to provide the best possible resolution without unnecessarily increasing the channel number.

Gas Electron Multipliers

The essential requirements for DHCAL include a robust design with stable, reliable operation; a thin sensitive layer for compact calorimeter design; on-board amplification/discrimination/digitization for digital readout; high efficiency for minimum ionizing particle (MIP) tracking in a hadron calorimeter; flexible design for implementation of varying cell sizes; in addition to the basic requirements of minimal supports/intrusions for hermeticity, ease of construction, and cost containment. As a member of the SiD concept group, the University of Texas at Arlington (UTA) group have been exploring an implementation based on the Gas Electron Multiplier (GEM) [H5] technology developed at CERN by Fabio Sauli and the GDD Group [H6], shown in Fig. H3. As shown in Fig. H3a – c, thanks to its structure of having 70μm holes separated by 140μm, GEM detectors can provide wide range of granularity, starting from microstrips to macro-pads. Figure H3d shows a conceptual design of a GEM based hadronic calorimeter in which double GEM active layers are alternated with layers of absorber to form a sampling calorimeter.

Figure H3. (a) A field line diagram of a GEM foil. (b) A simulated electron avalanche process through a hole on a GEM foil. (c) A microscopic photograph of a GEM foil. (d) A schematic diagram of a conceptual DHCAL using double GEM foil sensitive layers.

In order to study the viability of using GEMs as a sensitive medium in a DHCAL, the GEM layer geometry has been implemented into Mokka [H7], a GEANT 4 [H8] based simulation package, replacing the scintillation counter sensitive layers in the TESLA TDR hadronic geometry (stainless steel/ scintillation counter) with the double GEM layer structure. All other detector structure as in TESLA TDR detector design [H9] was retained. Using this simulation package, nearly linear behavior was observed for a wide variety of single particle incident energies and for the number of hits above the threshold. Much more detailed simulation studies of GEM DHCAL have been documented in two master’s students’ theses [H10, H11].

Figure H4. (a) GEM DHCAL single pion energy resolution. (b) GEM DHCAL jet energy resolution using a “perfect” PFA.

Simulated performance studies show that GEM response for analog and digital calorimetry very close to one another. Figure H4a shows single pion energy resolution for GEM DHCAL (green) comparable to the TESLA TDR detector (red) except at low energies. On the other hand, the GEM analog mode resolution is significantly worse than other detectors. Figure H4b compares jet energy resolution for a GEM DHCAL using a “perfect” PFA (blue), which has a sampling term of 30%/(E, to other detector technologies, using the single pion energy resolution obtained for GEM DHCAL. While the geometry is not that of SiD, the above studies give us confidence that GEM will perform well in a digital hadron calorimeter, providing the high jet energy resolution required for ILC physics.

Micromegas

A micromegas detector was proposed few years ago by Y. Giomataris, P. Rebourgeard, J.P. Robert, and G. Charpak. As in the case of Gas RPC it is a very good candidate for the active medium of a fine granularity hadronic calorimeter. The Micromegas detector is a gaseous detector also, based on the micro-pattern detector technology and widely used by many experiments: COMPASS, CAST, NA48, n-TOF, ILC TPC project, and the T2K TPC project.

A schematic view of the detector is shown in Figure H5: a commercially available fine mesh supported by pillars of 400 μm diameter separates the drift gap (~ 3mm ) from the amplification gap (~ 100 μm). A thin copper plate glued to the calorimeter absorber medium, defines the cathode, without adding extra space. Assuming a thin PCB for the anode pixel read-out (~0.8mm), this compact structure provides an economic active medium with a total depth of about 3.1mm, reducing the total radius of the HCAL. The cathode and the mesh are connected to high voltage (~800 and ~400 V respectively). The anode PCB is segmented into pads of 10x10mm2.

This simple structure allows full efficiency for MIPs and thanks to the thin pillars provides a good uniformity over the whole surface. The rate obtained with Micromegas chamber is not constrained. Another advantage of such a chamber is the tiny size of the amplification avalanche, resulting in fast signals without physical cross talk.

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Fig. H5 : Schematic view of a Micromegas chamber Fig H6 : T2K Micromegas TPC Prototype

A new promising technology ‘Bulk Micromegas’ has been recently developed in collaboration with the CERN PCB workshop. The basic idea is to build the whole detector in one process: the anode PCB plane with the copper pads, two photo-resistive films summing up to the correct pillar thickness, and the cloth mesh are all laminated together at high temperature, forming a single object. By a photolithographic method the photo resistive material is then etched which produces the pillars supporting the mesh. The new industrial process allows easy implementation and provides uniform, light, low cost and robust detectors. Employing the bulk technology, large detectors have been built for the TPC prototype of T2K experiment as shown in Fig. H6.

Scintillator/SiPM

The scintillator-based hadron calorimeter option capitalizes on the marriage of proven detection techniques with novel solid-state photo-detector devices called silicon photomultipliers (SiPMs). SiPMs are multi-pixel photo-diodes operating in the limited Geiger mode. Their high gain (106), small size and low operating voltages ( ................
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