GOOSE, a guard of the cold neutron source, uses the Larmor precision (read more) to check the performance of the cold neutron source. The (cold) neutron source produces slowly moving neutrons that enter the GOOSE apparatus in which the wavelength spectrum is measured. The ratio of the spectrum intensity after and before the installation represents the gain factor, hence the name of the instrument: ‘Gain Of Oyster Spectrometer Equipment’
The Larmor precession, named after Joseph Larmor, is a precession of a magnetic moment of an object in an external magnetic field. An example of such an object is a neutron, a particle that is comparable to a spinning top (read: ‘A new project’) that turns its magnetic moment around a magnetic field. GOOSE actually is built from several components. The neutron beam is defined by two diaphragms (D1 and D2 in the figure). This beam first passes the monitor (M in the figure). In the monitor, the initial intensity of the neutron beam is determined.
Then, it passes the polariser (P) where the neutrons magnetic moment is selected. After the polariser all neutrons that enter the magnetic field (B) have their magnetic moments into the same direction. Since not all neutrons are moving equally fast, the time spent in the magnetic field differs per neutron. After the magnetic field region the neutrons pass the analyser, where they are transmitted or absorbed depending on their magnetic moment. Finally the neutrons that were transmitted are detected (D in the figure). The count rate depends on the direction of their magnetic moment before entering the analyser. All neutrons that are deviating from the ‘set’ direction, will be removed from the beam. The number of turns the neutrons magnetic moment make depend on the magnitude of the magnetic field and the velocity of the neutron, hence by scanning the magnetic field a spectrum of the Larmor frequencies available in the beam will be made.