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The ISIS pulsed neutron source operates 25 instruments for neutron scattering applications. Many of these instruments use one or more neutron beam monitors to measure neutron beam flux. These monitors are used for data normalisation and in a diagnostic role to determine correct settings of beam line equipment including choppers, jaw settings and beam line scrappers. To date more than eighty ISIS Neutron Beam Monitors of this type are in use on ISIS.
The majority of the ISIS neutron beam monitors are based on scintillation technology. A typical scintillation monitor is shown in the figure and characteristics are given below. The scintillator material used is GS20 glass scintillator which is an alumino-silicate doped with enriched lithium-6 oxide. To reduce detector efficiency to a level suitable for monitor applications the scintillator is in the form of 0.25 mm cubes of glass which are attached to glass fibres. Typically the cubes are distributed in an array with a 7 mm pitch in both the horizontal and vertical directions. Thus these cubes sample the neutron beam flux depending on the density of the cube array. Typical beam areas covered are 40 x 30 mm² whist efficiencies are in the region of 10 -4 at 1 Angstrom. The density of the array of glass cubes and the area of the array can be varied to suit beam areas from 10 x 10 mm² up to 30 x 80 mm².
The scintillator array is viewed with a 10 stage photomultiplier tube which is screened with a mu metal tube. The photomultiplier tube is connected to an active voltage divider network and a pre amplifier. The monitor is housed in a steel tube to provide further magnetic shielding, The housing is equipped with an aluminium end cap. The end cap is machined to 100 μm in the active area of the beam to minimise neutron scattering.
The ISIS Neutron Beam Monitor is available with an optional discriminator card in NIM format. This unit collects the output from the beam monitor and outputs a NIM Pulse out on LEMO. The discriminator card has been developed alongside the ISIS Neutron Beam Monitor, specifically to preserve signal quality and reduce noise. This complete solution to the monitor requirement enables the simplest integration option.
Scintillator MaterialGS20 Glass Scintillator
Sensitive AreaCustomisable from 10 x 10 mm² up to 30 x 80 mm²
ISIS Neutron Beam Monitor Efficiencyapprox 10-4 at 1 Angstrom
ISIS Neutron Beam Monitor Installed Duration5 years in harsh environments. Longer elsewhere.