Correction factors

Monte Carlo calculations allow us to quantify effects that would be difficult or impossible to measure experimentally.

The NPL manganese bath as modelled in MCNP (displayed using the visualisation code SABRINA). Left: overall view. Right: close-up of the central cavity containing the source being measured.

We measure the neutron emission rate from radionuclide sources by placing them at the centre of a large (1 m diameter) tank of manganese sulphate solution, and measuring the activity induced in the manganese. Some of the neutrons are captured by nuclei other than manganese, or escape completely, and a Monte Carlo calculation allows us to calculate the necessary factors.

Detector characterisation

When characterising a detector in terms of its response or efficiency, one needs results for a large number of different configurations (incident energies, source / detector distances, etc.). It is usually impractical to make measurements at all of these, so instead we model the detector in a Monte Carlo calculation, and verify a few selected results experimentally. We have characterised in this way:

• Organic liquid scintillators
• Hydrogen-filled proportional counters
• Bonner sphere spectrometers
• Long counters (neutron counters with an almost energy-independent response over a wide range of energies).

Neutron tracks from a Monte Carlo calculation of neutron interactions in a Long Counter. Neutrons are incident from the right, and only tracks that contribute to the detector response are shown. For 144 keV neutrons (left), the contributing tracks originate from a narrower range of locations than for 5 MeV neutrons (right).

For more information, please contact Nigel Hawkes or Graeme Taylor