Isis Neutron Beam Monitor

Highly efficient reliable neutron beam monitor. Used at the world leading Isis Neutron and Muon source at the Rutherford Appleton Laboratory at the Harwell Science and Innovation Campus in Oxfordshire, United Kingdom.

What is it? How does it work?

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 Material – GS20 Glass Scintillator
  • Sensitive Area – Customisable from 10 x 10 mm² up to 30 x 80 mm²
  • ISIS Neutron Beam Monitor Efficiency – approx 10-4 at 1 Angstrom
  • Vacuum MountableYes
  • ISIS Neutron Beam Monitor Installed Duration – 5 years in harsh environments. Longer elsewhere.