Indirect detection of nitrogen spins in ammonia target at superlow temperatures

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 0.2 K and 2.5 T. 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.