Dosimetry and Medical Radiation Physics
The IAEA/WHO SSDL Network
Criteria for Establishing an SSDL
1. SCOPE
This document sets forth the criteria to be met when a member state wishes a national SSDL to be accepted for membership in the IAEA/WHO network. It may also serve as a recommendation to governments in the process of designating a laboratory to become an SSDL. These criteria are minimum requirements only.
2. THE IAEA/WHO NETWORK OF SECONDARY STANDARD DOSIMETRY LABORATORIES
2.1. In a working arrangement (1976) between the IAEA and WHO it was agreed to set up an "IAEA/WHO Network of Secondary Standard Dosimetry Laboratories (SSDLs)" in order to improve accuracy in applied radiation dosimetry throughout the world. The working arrangement specifies details of the structure of the network and lays down procedures for membership.
2.2. A laboratory fulfiling the general criteria set forth here may become, at the request of its government, a member of this network. In cases where these criteria are not initially met, provisional membership may be appropriate during the period needed to satisfy these requirements. Application for membership in the IAEA/WHO SSDL Network must be based on a nomination from the Government of the Member State concerned. The nomination specifies a laboratory which is suitable for the purpose and names the responsible person who will be in charge of the work. Membership in the network facilitates international co-operation, enables the member to obtain assistance in solving specific problems and provides access to the international primary measurement system. There is no membership fee.
2.3. Only one SSDL organization should be nominated for each Member State. If national radiation standards do not already exist, the Member State should recognize as national measurement standards the radiation measurement standards maintained by the SSDL.
2.4. Membership of a laboratory in the network does not impose any liability on the IAEA, WHO or other collaborating institutions in connection with the performance of work within the laboratory.
3. DEFINITIONS
3.1 Secondary Standard Dosimetry Laboratory (SSDL).
An SSDL - in this context - may be either national or regional. A national SSDL is a laboratory which has been designated by the competent national authorities to undertake the duties of a calibrating laboratory within that country. A regional SSDL is an SSDL which is designated, by intergovernmental agreement or by an international organization, not only to carry out national functions but also to provide calibration services and advice to other countries within the geographical area concerned.
An SSDL is equipped with secondary standards which are calibrated against the primary standards of laboratories participating in the international measurement system.
3.2. Primary Standard Dosimetry Laboratories (PSDLs)
A PSDL is a national laboratory designated by the government for the purpose of developing, maintaining and improving primary standards in radiation dosimetry. A PSDL participates in the international measurement system by making comparisons through the medium of the Bureau International des Poids et Mesures (International Bureau of Weights and Measures), and provides calibration services for secondary standard instruments.
3.3. The IAEA/WHO network of SSDLs
The SSDL network is an association of SSDLs which agree to co-operate in promoting the objectives of that network under international auspices. Its objectives are:
- to improve dosimetric accuracy, particularly in radiation therapy and radiation protection, by supporting centres and laboratories for the creation and distribution of knowledge in applied dosimetry;
- to further the exchange of experience between members and affiliated members and to provide support to each other where necessary;
- to establish and facilitate links between members and the international system of radiation measurements through PSDLs; and
- to promote the compatibility of methods applied for calibration and performance of dosimetry in order to achieve uniformity of measurements throughout the world.
3.4. Classification of instruments
For the purpose of this publication the various classes of instruments referred to are defined as follows:
3.4.1. Primary standard. An instrument of the highest metrological qualities, which allows determination of the quantity to be measured from measurements of basic physical quantities, and the accuracy of which has been verified by comparison with equivalent standards of other institutions participating in the international measurement system.
3.4.2. Secondary standard. An instrument of long-term precision and stability which has been calibrated against a primary standard.
3.4.3. Field instrument. An instrument used for routine measurements.
4. LABORATORY ORGANIZATION AND STAFF
4.1. Organizational structure
The work of the laboratory should be independent in character and free from any external influence which could adversely affect the quality or impartiality of the service it offers.
4.2. Head of laboratory
4.2.1. The head of the laboratory should be a physicist with several years of experience in radiation measurement and calibration.
4.2.2. The head of the laboratory is responsible for the work performed at the laboratory and should hold a full-time appointment.
4.2.3. The head of the laboratory is responsible for adequately documented calibration procedures , certification and the implementation of QA programmes.
4.3. Laboratory staff
As the prime obligation for the correctness of a measurement lies with the person making it, laboratory staff should possess adequate qualifications and experience in measurement procedures and practice appropriate to their responsibilities.
4.4. SSDL calibration certificates
When a laboratory has been designated, arrangements must be made by the competent national authority to authorize the head of the laboratory to sign SSDL calibration certificates.
5. LABORATORY ACCOMMODATION
5.1. Location
For an appropriate location of the laboratory the following criteria need to be considered:
5.1.1. It should be located in a geographically central position in relation to the demand for its services and the practical performance thereof.
5.1.2. It should be free from external environmental disturbances which are likely to affect the measurements (consideration must also be given to the laboratory’s own impact on the environment).
5.1.3. Allowance should be made for the possibility of extending the facilities.
5.1.4. If accommodation is found in existing buildings, compromises on some of the above criteria may be unavoidable; the basic principles should, however, be observed.
5.2. Premises
The following minimum requirements for premises should be fulfiled:
5.2.1. It is desirable that the laboratory should not share space with other activities.
5.2.2. At least one large room (e.g. 6 m X 3 m X 3 m) is required for X-ray calibrations. A second room is required for calibrations with cobalt-60 radiation, but, if this cannot be provided initially, access to a cobalt-60 source elsewhere must be made available as an interim measure.
5.2.3. Shielded control space next to the calibration rooms is necessary.
5.2.4. Structural shielding, particularly for cobalt-60 radiation, may be required in order to avoid unacceptable radiation exposure of staff and the public and to keep background radiation at levels consistent with protection-level calibrations.
5.2.5. One laboratory room should be available for electronic measurements and other physical experiments, e.g. checking and preparing dosimeters for calibration, operation of TLD services, etc.
5.2.6. A mechanical and electronic workshop should be available.
5.2.7. Office space for the head of the laboratory, scientific-technical staff and secretarial staff should be available.
5.3. Services
The following are essential:
5.3.1. Appropriate stability of the mains voltage supply (for which voltage stabilizers may be necessary).
5.3.2. Appropriate water supply and water removal for cooling systems (X-ray machines, etc.).
5.3.3. Control of environmental conditions, particularly air conditioning, with automatic temperature and humidity control.
6. CALIBRATION FACILITIES AND EQUIPMENT
6.1. An SSDL must be able to provide calibration services for either therapy or protection levels, or both.
6.2. Duplicate secondary standard instrumentation should be provided to cover each of the energy and dose-rate ranges for which services are offered. One of the secondary standards must be retained in the laboratory solely as a basic reference instrument. Methods of checking stability are essential, and rely on the use of radioactive check sources.
6.3. The secondary standards must initially be calibrated for the ranges of interest against a primary standard of a recognized national standards laboratory, and then re calibrated at least every 5 years.
6.4. Radiation sources giving appropriate dose rates must be provided to cover the energy ranges of interest. In the case of X-ray generators they should be of the constant potential type, and highly stabilized power supplies are required.
6.5. Essential ancillary equipment includes diaphragms, a set of filters, a shutter mechanism, a transmission monitor chamber with measuring assembly, and calibration benches with positioning devices.
6.6. Other equipment required includes precision instruments for measurement of time, temperature, pressure and humidity. Additional laboratory equipment is also desirable, either as back-up in case of malfunction of instrumentation, or to provide alternative techniques to confirm the measurements made. For example, current/charge measurements and precision voltage and current sources are needed for this purpose.
6.7 The design of the calibration facilities must be in accordance with the relevant national and international safety regulations and should take into account the International Basic Safety Standards( Safety Series No 115).
Depending on the services to be made available, the requirements may include equipment or instrumentation for thermoluminescence dosimetry, film dosimetry and chemical dosimetry.
7. RESPONSIBILITIES OF THE SSDL
The responsibilities for the SSDL include, but are not limited to, the following activities:
7.1. Maintaining secondary standard instruments in agreement with the international measurement system, and performing re-calibrations at least every 5 years.
7.2. Performing calibrations of radiation measurement equipment and issuing calibration certificates with all necessary information, including the estimated uncertainties.
7.3. Organizing dose comparisons for radiation therapy for institutions within the country or region, and participating in measurement comparisons within the IAEA/WHO SSDL network, and with other standardizing laboratories.
7.4. Co-operating with the IAEA/WHO network and with other metrological laboratories in the exchange of information and improvement of measurement instruments and techniques.
7.5. Documenting and preserving records of all procedures and the results of calibrations.
7.6. Keeping up to date on progress in radiation measurement, so as to improve calibration techniques as required, and thereby provide a better service to the users of radiation.
7.7. Providing training in radiation measurement and calibration techniques and in the use and maintenance of the relevant instrumentation, appropriate to the users of radiation served by the SSDL.
7.8. Reporting to the Secretariat, at least annually, on the status of its secondary standards, radiation sources, calibrations performed and related activities.