National Physical Laboratory

Manganese Bath

The manganese bath is the primary standard for neutron emission rate at NPL. It measures the number of neutrons per second emitted by sealed radionuclide neutron sources such as 241Am-Be and 252Cf. The sources can then be used to calibrate neutron sensitive devices such as area survey instruments and personal dosemeters.

Manganese BathThe manganese bath, used to measure the neutron emission from radionuclide sources 

The neutron source is placed inside a large spherical bath containing almost 500 litres of manganese sulphate solution. The manganese nuclei capture neutrons to form an unstable isotope of manganese (56Mn) which decays with the emission of a gamma ray. By measuring the gamma radiation it is possible to determine accurately the neutron emission rate of the source. Corrections are made for neutrons escaping from the bath and for those captured by other nuclei in the solution and in the source mounting assembly.

In 2007 a new facility to house the manganese bath was completed. Features include a large concrete source handling cell with manipulators, a new linear transfer system to place sources in the bath, and greatly enhanced security for storing sources. A program of upgrading the hardware is also underway. The facility is a major improvement on the previous one and ensures that the manganese bath will remain in operation at NPL for many years to come.

The NPL manganese bath is one of about 10 such facilities worldwide and is arguably the most advanced and most extensively used. A neutron source calibration service is offered to customers from around the world many of which are themselves national standards laboratories. International comparisons of neutron source measurements are held regularly and NPL has always demonstrated excellent agreement with other laboratories.

Mangenese bath cut-away Cut-away representation of the manganese bath modelOne of the most important developments of recent years has been the work to improve the corrections for neutron escape and for capture by nuclei and materials other than manganese. This has been done using a very widely used Monte Carlo radiation transport code called MCNP. It enables the complete geometry of the source, source mounting assembly, manganese sulphate solution, the bath itself and even the room it is located in to be modelled. Computational simulations are then run to determine where, in which material, by which nuclei and by which interaction mechanism each neutron from the source is captured. As the sources have different energy spectra and come in different capsule sizes it is necessary to perform the calculations for each different source type. Monte Carlo modelling can also be used to solve other problems related to the manganese bath such as how much additional irradiation does the solution receive whilst the source is travelling to and from the bath at the start and end of each calibration.

New software is also being developed that will analyse the data as it is being acquired. This will streamline the analysis part of the calibration and enable provisional values for the emission rates of sources to be obtained as soon as the measurement has been completed. It will also allow the progress of the calibration to be monitored more closely.

Anisotropy measurements

When using a radionuclide neutron source to calibrate an instrument or a dosemeter, in addition to knowing the neutron emission rate of the source, the degree to which the source emission is not isotropic (i.e. the same in all directions) also has to be known. As sources are nearly always in cylindrical rather than spherical capsules there is usually significant deviation from isotropic emission. This is called the anisotropy of the source and NPL has the capability to measure it. This is offered as a measurement service to outside customers.

With the cylindrical axis of the source horizontal, measurements are made in a low scatter area at 10° intervals around the source. This enables the relative emission from the waist of the source to be determined which can be several percent higher than if the source was isotropic.

Source capsules have also been modelled using the Monte Carlo codes MCNP and MCBEND to determine the anisotropy. Agreement with measured values is generally very good except for a small number of sources where there is uncertainty as to the exact location of the active material within the encapsulation.

Further measurements are planned in order to build up a library of anisotropy factors for all common source capsules. These will be aided by the planned improvements to the measurement procedure which will reduce the time and effort required to set up the equipment.

Last Updated: 15 Aug 2012
Created: 18 Oct 2007

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