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Thermal neutron production

Thermal neutron fluence and dose standards

Well characterised thermal neutron fields are available at NPL for the calibration of neutron measuring devices, for instance in area survey instruments or personal dosemeters, and for irradiation purposes.

These standard thermal neutron fields are produced by bombarding two beryllium targets, set into a large graphite moderator, with a beam of deuterons from the NPL 3.5 MV Van de Graaff accelerator. The thermal neutron fluence rate at the centre of the ‘pile’ is controlled by signals from three boron coated ionisation chambers placed within the graphite, below the beam line. These signals are used to set the voltages on pairs of horizontal and vertical deflection plates which in turn determine the number of deuterons striking each beryllium target in order to give a stable, uniform neutron field. A small cavity at the centre of the pile provides for the irradiation of artefacts of modest size in a standard thermal neutron field.

Larger samples, suchas area survey instruments, can be irradiated in a beam of thermal neutrons extracted through an evacuated tube or ‘thermal column’. Corrections for epi-cadmium neutrons are derived from the results of irradiations under cadmium cover.

The value of the fluence rate is determined in terms of known thermal neutron capture cross sections by the activation of gold or manganese foils. The fluence rates near the central region and below the thermal column are continuously monitored with small fission chambers. Dose quantities, both ambient or personal dose equivalent, can be derived from the fluence values using standard conversion coefficients.

Standard thermal neutron facility

Volume of cavity 150 cm3
Diameter of access hole to cavity 8.5 cm
Range of fluence rate 104 to 3 x 107 cm-2 s-1
Uncertainty of value (95% confidence limit) ± 2%
Long term stability Better than ± 0.25%
Spatial uniformity ± 0.2%
Epithermal fluence component 1%
Epithermal spectrum E-1.05
Cadmium ratio for a thin 1 / v detector 270
Gamma absorbed dose (in tissue) in cavity .06 Gy per 1012 cm-2

Thermal Column

Cross sectional area 1000 cm2
Length 1m to 1.5m
Maximum dose equivalent rate at 1.5m 0.6 mSv h-1
Neutron fluence component > 0,5 eV ~ 19% of total fluence
Photon doesrate
Air Kerma/ thermal personal dose equivalent
4.0 Gy/Sv
Maximum steady fluence rate 4 x 104 cm-2 s-1
Minimum steady fluence rate 2 x 102 cm-2 s-1

Our reported uncertainties are based on a standard uncertainty and multiplied by a coverage actor k = 2, providing a level of confidence of approximately 95%.

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