Intercomparison Transfer Standards for Neutron Source Calibrations

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Objectives for MPD C.3.4

  • Develop and promulgate protocols for the use of thermoluninescent dosimeters as intercomparison standards that will be effective on a national and international level.
  • Appraise and report on the reliability of other intercomparison transfer standards and instruments for neutron source calibrations.

Background: The calibration of personnel dosimeters and area survey meters used for radiation pro¬tection purposes in neutron fields is difficult. The devices used for measurements in neutron fields have dose equivalent responses that are dependent on the neutron energy spectrum and on the scattering environment. In addition, the reference calibration neutron sources maintained by NIST are not available for routine calibration or intercomparison measurements. These measurement services are supplied by secondary calibration laboratories.

In order to ensure the consistency of calibrations performed by secondary calibration laboratories with NIST standards, ongoing measurement quality assurance (MQA) interactions between the laboratories and NIST must take place.

The MQA program for photon (X-ray and gamma-ray) radiations has been in place for many years, and NIST and the secondary laboratories produce consistent results. The situation for neutrons is more complex. NIST maintains neutron reference radiations recommended by ISO 8529-1. The physical characteristics of these sources have been publicly documented. However, the critical elements of a neutron calibration include more than the source spectrum and intensity. The calibration depends upon having knowledge of the interaction of the neutron source with its surrounding material, the phantom (for dosimeters), and the detector itself. The methods required for neutron calibrations are discussed in ISO 8529-1, 2 and 3.

An MQA program for neutron dosimetry needs to incorporate methods that will either incorporate or evaluate the effects of the items mentioned previously. Each neutron calibration facility is virtually unique, and all variables affecting the calibration need to be considered in the design of a method for MQA measurements. Therefore, a method needs to be developed that will permit evaluation of all of the variables or that has a response to the variables that is close to that of the devices calibrated.

The original effort on transfer standards was completed and was not successful at the precision needed. The MPD has been revised to include intercomparisons with both instruments and passive dosimeters. Currently efforts are underway for direct intercomparisons using personnel dosimeters. Typical personnel dosimeters have been irradiated under nearly identical conditions at NIST and PNNL. The results of this study are presently being evaluated. Follow-on experiments will determine optimal reader parameters and appropriate irradiation and readout protocols for use of the Thermoluminescent Dosimeter (TLD) system as transfer standards in intercomparison measurements and for proficiency testing of calibration laboratories seeking accreditation by National Voluntary Laboratory Accreditation Program (NVLAP) for dosimetry. When the irradiation conditions have been established the study needs to be extended to additional US and foreign laboratories to fully evaluate the technique. Results will be published and presented at a CIRMS meeting or CIRMS workshop.

Additional efforts will be undertaken to evaluate the use of instruments including a survey meter as a transfer standard for general calibrations of neutron survey meters. Another approach that will be further evaluated is the use of the tissue equivalent ion chamber. Current research efforts on electronic dosimeters (ED) will result in detector based methods of neutron dosimetry. The devices under consideration (combinations of diodes, ion chambers, and multi-cell tissue equivalent proportional counters) will have energy responses that are different from conventional dosimeters and different from instruments. Ensuring that the transfer standards are suitable for these devices will require additional investigations in the next 1-3 year time period.

Through participation in ISO standards efforts NIST personnel and personnel from other US secondary laboratories (PNNL) will seek optimization of intercomparison methods and seek international standardization to ensure worldwide consistency of neutron dose measurements for radiation workers throughout the world. NIST and PNNL will be primary participants in these efforts. Other laboratories and vendors that wish to be involved will need to perform experimental irradiations, establish a pool of transfer dosimeters/ins¬truments and develop capability to analyze and tabulate the results.

Action Items:

1 – Evaluate and establish protocols for the use of thermoluminescent dosimeters (TLDs) to be used in intercomparison studies and as transfer standards.

2 – Extend the results of the TLD program to involve non-US laboratories. 3 – Evaluate neutron survey meters, ion chambers and electronic dosimeters for their reliability as transfer standards for general measurement of neutron dosimetry.

4 – Optimize the intercomparison and standardization protocols for neutron dosimetry through participation in international standardization efforts (ISO) so that they become applicable on a world-wide basis.

Resource Requirements:

1 – The neutron calibration program will require one person-year per year over the next three-year time frame and approximately $150,000 for equipment and supplies. 2 – Funding must provided for personnel to track and participate in international standards efforts. It is estimated that this will require 5-10% of an individual’s effort per year plus travel costs $30,000 per year.