The COMPASS experiment was designed as a “Common Muon and Proton Apparatus for Hadron Structure and Spectroscopy”. This setup consists of high-precision spectrometer and the world’s largest polarized target. The nucleus of deuterium or hydrogen, which are a part of working substance of a target ⁶LiD or NH³, can be polarized longitudinally or transversely and can save the direction of the “frozen” polarization during a long time. The COMPASS is placed at the unique SPS M2 channel at CERN, which generates beams of pions or polarized muons at energies between 50 GeV and 280 GeV. The COMPASS setup has already proved its viability, giving a unique opportunity to find solutions to a number of fundamental problems both in structure of a nucleon and in hadron spectroscopy

The COMPASS-II experiment, approved in 2010, has laid the foundation for the studies of the 3-dimensional nucleon structure in the next decade. One of the main objectives of the COMPASS-II is the study of functions of generalized parton distributions (GPD) in nucleons. Recent theoretical GPD developments, based on the experimental data, provide information on transverse localization of partons in the nucleon, as a function of the nucleon transverse momentum fraction carried by the parton. Obtaining such 3-dimensional picture is called “nucleon tomography”. In addition, the GPD concept has attracted great attention of scientists after it was shown that the total angular momentum of a certain type of partons, J ^ƒ for quarks (ƒ = u; d or s) or J ^g for gluons, which depends on the second moment of the sum of two GPDs H and E. Collecting data for these GPDs via measurement of exclusive Deep Virtual Compton Scattering (DVCS), μp → μγp, or Virtual Deep Scattering with meson production (DVMP), μp → μMp, is the only known method to determine the components of the “contribution” to the nucleon spin 1/2 = ∑ _{ƒ= u;d;s} J ^ƒ + J ^g and consequently to finish the so-called “spin crisis”. The DVCS and DVMP reactions are exclusive. In order to measure their cross-sections, the existing setup COMPASS should be upgraded with 2 new detectors – Proton Recoil Detector (the RPD), which measures characteristics of protons, and the electromagnetic calorimeter ECAL0 placed in front of the first spectrometer magnet (SM1) (Figure 1). Inside of the RPD a new liquid hydrogen target with 2.5 m length was installed. The new ECAL0 calorimeter will provide registration of events in a significantly wider kinematic region of response in comparison with the existing ECAL1 and ECAL2 calorimeters. ECAL0 was proposed and developed at JINR in collaboration with groups of physicists from Munich, Freiburg, Warsaw, Saclay (France), Prague, CERN and colleagues from Kharkov (Ukraine).

The new electromagnetic calorimeter is a unique device of “shashlyk”-type (scintillator, lead), in which the most advanced photodetectors – Micro-pixel Avalanche Photo Diodes (MAPD) with ultra-high pixel density (up to 15 thousand pixels / mm²) were applied, instead of the traditional photomultiplier tubes. Thus, it was the first time when the MAPD were applied for the electromagnetic calorimeter at a large physical setup. It should be noted that this type of photo detector has been developed and tested at many institutions for more than 20 years, and JINR is one of the leading centers in this field. The MAPD with the pixel density of 15 thousand pixels/mm² were developed at our institute by the Z. Sadygov’s team and were used in the calorimeter as a part of the experimental setup COMPASS in 2012 during the pilot data collection. To serve this purpose, more than a quarter of the calorimeter modules were produced. After successful tests the final stage of the calorimeter production began and 250 modules were produced and tested at the Institute for Scintillation Materials (ISMA, Kharkiv, Ukraine).The module design was developed by I.E. Chirikov-Zorin (JINR, DLNP). In December 2013 the modules were delivered to CERN. Further tests of the MAPD calorimeter, associated with some new physical tasks – the use of this detector in intense hadron beams, – led to the need to use the new fast MAPD of MPPC S12572-10P-type of Hamamatsu company (10 thousand pixels / mm²). The development, production and testing of the registration blocks on the basis of these MAPD took almost one year and a half and were successfully completed in cooperation with the Russian company Rusalox (Vladimir). This Russian company is a manufacturer of motherboards with high thermal conductivity based on the aluminium oxide technology that provides high cooling efficiency for any heat-generating electronic components. Printed circuit boards manufactured using this technology are made of a conducting layer of aluminum and a dielectric material with nanoporous structure. We needed a new technological process – the drilling of holes, their oxidation and metallization, with leakage currents on the level of 1 pA at the voltage of 100 V. JINR additionally performed the soldering of the pins and the gluing of the MAPD to the alumina board, the gluing of Winston cones to the MAPD and other assembly operations regarding the registering units. The first recording blocks were ready at the beginning of June 2015 and at the end of June 2015 20 recording blocks were delivered to CERN and tested on the beam. As a result, after many years of hard work on the development and optimization of the detector systems, multiple tests of unit prototypes and of calorimeter reading system on beams at CERN, at DESY (Hamburg) and at the ELSA complex (Bonn) the final version of the module calorimeter was developed, which is shown in Figure 2. The registration units were developed and produced by I.E. Chirikov-Zorin (JINR, DLNP).

It is noteworthy that groups from other scientific centers and production companies took part in the development of the calorimeter systems. Apart from the colleagues from ISMA and Rusalox, who have already been mentioned above, power systems, monitoring and thermostabilizaton systems were developed and produced by the HVSYS company, the reading system (ADC) and the amplifiers were created by the groups from Munich and Warsaw, and the group from Prague produced the optical splitters for the ECAL0 slow control and monitoring system.

In March-April 2016 the ECAL0 was fully assembled, tested and included in the COMPASS setup, and currently is successfully used for data collection. The main features of the new calorimeter can be formulated as follows: ECAL0 effectively registers direct photons from the DVSC and DVMP reactions in a wide energy range (0.2 – 40 GeV); and together with ECAL1 (Figure 1) effectively registers π⁰, which can significantly reduce the background of photons produced by π⁰; furthermore, it provides the opportunity for additional measuring reactions with the release of other mesons. These properties significantly expand our measuring range with minimal systematic uncertainties.

Stable operation of our calorimeter within the first months of data collection was appreciated by the leadership of the COMPASS collaboration. We expect to obtain new physical results with the data from the ECAL0 already in 2017. Moreover, the acquired experience will be used by development of similar calorimeters for MPD and SPD setups in the NICA complex.

Krumstein Z.V., Nagaytsev A.P., Olchevskyi A.G., Savin I.A.

Figure 1: Experimental COMPASS setup (left) and the front of the spectrometer, upgrated by the detectors for studying of generalized parton distributions

Figure 2: ECAL0 calorimeter module

Figure 3: Assembled and ready to work calorimeter as a part of the COMPASS setup

Read the article “COMPASS and COMPASS-II. Twenty Years of Successful Measurements” as of 22 July 2020