National Physical Laboratory

HSE performance testing of person dosimetry services for fast neutron radiation

Personal Dosemeters
Personal dosemeters free-in-air

NPL is one of the few laboratories in the UK that can offer performance testing of dosimetry services for fast neutron radiation. As a result of changes in the Health and Safety Executive’s ‘Requirements for the Approval of Dosimetry Services under the Ionising Radiations Regulations 1999’, all dosimetry services are now required to undertake performance tests, if there is a relevant HSE performance test for the type of dosimetry for which they seek approval. The tests are carried out under well defined irradiation conditions enabling the basic performance of a service to be assessed for approval.

Sources used for Performance Tests

  • 241Am-Be or 252Cf.
  • NPL can provide all the dose equivalent ranges required by the HSE, the highest range being between 30 to 50 mSv for both 241Am-Be and 252Cf.
  • Irradiations are usually performed at 75 cm from the neutron source, but are also possible at 50 cm if required.

Neutron Fluence

Dosemeters on Phantom
Personal dosemeters
mounted on a phantom

  • The neutron fluence rate at the reference point of the dosemeter is calculated from the absolute neutron emission rate of the source, as measured in the NPL manganese bath. Dose equivalent rates are derived from the fluence rate using internationally agreed conversion coefficients.
  • When specifying the fluence, allowance is also made for the source emission anisotropy.

ISO Recommendations

The 241Am-Be and 252Cf sources are both ISO recommended calibration standards.
(See ISO 8529 Reference neutron radiations – Part 1: Characteristics and methods of production (2001))

Calibration Facilities for Performance Testing

  • Irradiations of personal dosemeters are performed in a low scatter facility (18m x 18m x 25m). The source is mounted 6 m above the floor keeping the scattered neutron component to a minimum.
  • Personal dosemeters are usually mounted on an ISO recommended water phantom but can also be mounted free-in-air if necessary.

Typical fluence and dose equivalent rate:

Fluence & DER

Certification

The results obtained and the HSE band achieved are given. Although not included in the test report, a separate uncertainty budget for the conventional true value of the personal dose equivalent is available upon request.

Contact

Customer Services tel: +44 20 8943 8637
E-mail: neutron_enquiries@npl.co.uk

Last Updated: 25 Mar 2010
Created: 17 Apr 2007

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