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The dilution refrigerator used to cool the large COMPASS polarized target is monitored through a PC running LabVIEWTM 6.1 under Windows 2000TM. About 60 parameters of the target (temperatures, pressures, flow rates) are continuously plotted and checked. They are periodically recorded in an OracleTM database and in a data file. An alarm for every parameter can be individually activated and optionally connected to a GSM (Global System for Mobile Communication) delivery message system. A web server receives and publishes the online status of the target with online tables and graphics on a dedicated COMPASS polarized target information web site. A Siemens programmable logic controller (PLC) powered by an uninterruptable source keeps the cryogenic system safe and stable during the long beam periods by controlling valves and interlocks. This safety feature protects the dilution refrigerator against potential damages in case of power failure. [Copyright &y& Elsevier]

Abstract: The COMPASS polarized target at CERN operates with irradiated ammonia (NH3) as a material having a reasonable content of polarizable nucleons and the highest resistance against radiation damages. We study the magnetic structure of ammonia polarized by the Dynamic Nuclear Polarization (DNP) method at 0.2K and 2.5T. In this material, electron spins, induced by ionizing radiation, couple proton and nitrogen nuclear spins by indirect J-interactions. This coupling and the dipole–dipole interactions between nuclear spins produce an asymmetry in the proton NMR line shape depending on the value of nitrogen polarization. We consider the asymmetry as an indirect imaging of the actual nitrogen spectra, useful for research developments and, in practice, for monitoring of nitrogen polarization in the long target, instead of a complicated analysis of NMR nitrogen spectra. [Copyright &y& Elsevier]

The transversely polarized target (PT) of the COMPASS (NA58) collaboration at CERN has been used for Drell–Yan measurements in 2015 and 2018. The transverse spin structure of the proton has been studied using a negative pion beam and a solid ammonia target. Employing the dynamic nuclear polarization (DNP) method, proton polarization values of more than 80% have been routinely achieved after one day, at a homogeneous magnetic field of 2.5 T and using a 3 He/ 4 He dilution refrigerator. During the data-taking the target operates in a transversely oriented magnetic dipole field at 0.6 T. This so-called frozen spin operation mode without the DNP pumping process leads to a slow depolarization of the target material, which is further accelerated by the heat input of the pion beam, produced secondary particles and radiation damage effects to the target material. Ammonia has the highest resistance against radiation-induced depolarization among known solid target materials. The proton polarization has been measured by the nuclear magnetic resonance (NMR). Relaxation times of about 1100 h have been observed for the proton polarization resulting in an average polarization between 68% and 76% during about two weeks long data-taking periods. To achieve a systematic uncertainty of the polarization Δ P / P as low as 3.2% and a statistical one of less than 1.8% two large target cells with appropriate positioning of the NMR-coils have been built. [ABSTRACT FROM AUTHOR]

The spin structure of the nucleons is investigated by studying spin dependent process of deep inelastic scattering of a polarized muon beam and a polarized nucleon target in the COMPASS experiment since 2001. In order to achieve high luminosities it is necessary to use a large solid polarized target. The target consists of two independent and oppositely longitudinal polarized cells. The cells are filled with irradiated6LiD granulated material. One of the characteristics of the polarized target is the spin reversal by magnetic field rotation in order to cancel the systematic error. The polarized target apparatus consists of a large refrigerator providing a low temperature of below 100 mK and a high cooling power with 200–300 mK, a superconducting magnet producing 2.5 T with good field homogeneity, two high power of a few watts microwave systems producing 70 GHz for Dynamic Nuclear Polarization (DNP) and 10 NMR signals detection system for the polarization measurement. [ABSTRACT FROM AUTHOR]

The dynamic nuclear polarization (DNP) of 6LiD requires 1 mW/g or more microwave power in the beginning of the process. With the material of 350 g, this gives more than 350 mW in total for the COMPASS polarized target. A temperature around 300 mK or below is needed for an efficient polarization. These low temperatures can only be achieved with a dilution refrigerator designed to operate with a 3He flow of 100 mmol/s. In order to keep the polarization in the frozen mode, temperatures of about 65 mK are used with typical magnetic relaxation times of more than 1400 h at 0.42 T and of more than 15 000 h at 2.5 T. Low lattice temperature is important in achieving high nuclear polarization. The base temperature is limited by the heat brought to the mixing chamber by the inlet 3He and by radiation and conduction of heat. [Copyright &y& Elsevier]

Continuous wave nuclear magnetic resonance (NMR) is used to determine the target polarization in the COMPASS experiment. The system is made of the so-called Liverpool Q-meters, Yale-cards, and VME modules for data taking and system controlling. In 2001 the NMR coils were embedded in the target material, while in 2002 and 2003 the coils were mounted on the outer surface of the target cells to increase the packing factor of the material. Though the error of the measurement became larger with the outer coils than with the inner coils, we have performed stable measurements throughout the COMPASS run time for 3 years. The maximum polarization was +57% and -53% as the average in the target cells. [Copyright &y& Elsevier]

We have experimentally studied the first and the second moments of D, 6Li and 7Li (I>1/2) NMR lines in a granulated LiD-target material as a function of nuclear polarizations and the data has been compared with a theory elaborated by Abragam, Roinel and Bouffard for monocrystalline samples. The experiments were carried out in the large COMPASS twin-target at CERN. The static local magnetic field of the polarized nuclei was measured by frequency shift between the NMR-signals in the two oppositely polarized cells and lead to the first moment, whereas the investigation of the second moment was done through Gaussian approximation. The average field magnitude in granulated material was estimated 20% larger than the value given by the calculations for monocrystalline samples of cylindrical shape. The second moment shows a qualitative agreement with the theory but it is slightly larger at the negative than at the positive polarization. In a polarized mode, the moments depend on the saturated microwave field. [Copyright &y& Elsevier]

Due to the vanishing quadrupole interaction only a single narrow 2.8 kHz full-width half-maximum nuclear magnetic resonance (NMR) line is observed in the 6LiD target material for deuterium and 6Li with spin 1. The second and fourth moments are obtained from a Memory function fit to the measured NMR signal. The values are compared to the calculations from dipole interaction between nuclei. The NMR line shape during polarization up to +55% and -52% is discussed. [Copyright &y& Elsevier]

The COMPASS (NA58) experiment at CERN operates with a large solid polarized target (PT) to study the spin structure of the nucleon. The COMPASS PT system started its operation with the target material 6LiD in 2001. Deuteron polarizations of +54.2% and −47.1% were achieved in a 3He/4He dilution refrigerator at a magnetic field of 2.5 T. The equal spin temperature (EST) concept was found to hold among the deuteron, the 6Li and the 7Li nuclei during the dynamic nuclear polarization (DNP) process. The agreement with the EST concept allows the permanent monitoring of only one nuclear species by the nuclear magnetic resonance (NMR) method. [Copyright &y& Elsevier]

The 6LiD was weighed after unloading of the target. With the help of the measured cold volumes of the target cells the packing factor of the target material in each cell could be determined. From the known isotopic content in the target material a table of elements was produced for each cell. [Copyright &y& Elsevier]