Radiation Metrology Unit

SECONDARY STANDARD DOSIMETER LABORATORY (SSDL)

The SSDL was established in 1983, and is a member of the IAEA/WHO Network of SSDLs. The laboratory has acquired the status of national standard laboratory with the basic aim of improving accuracy in radiation dosimetry in the country. It is also the national focal point for the calibration of radiation measuring instruments used in radiation protection and radiotherapy. Radiation measuring instruments need to be calibrated to ensure that they give accurate and correct reading with a certain uncertainties and to comply with the regulations imposed by the relevant authority. This is achieved by calibrating the laboratory’s protection and therapy levels dosimeters against those in the Primary Standard Dosimetry Laboratories (PSDLs), the International Atomic Energy Agency (IAEA), or by participating in the international comparison on dosimetry measurements programmes.

External radiation quantity can be measured in terms of exposure, air kerma, absorbed dose, dose equivalent, ambient dose equivalent by using radiation measuring instruments. Various kinds of radiation measuring instruments such as survey meters, area monitors, personal dosimeters, contamination-monitoring instruments are used in irradiation facilities for radiation protection purposes. Area dosimeters or dose rate meters are calibrated in terms of the ambient dose equivalent, H*(10), or the directional dose equivalent, H’(0.07).

In radiotherapy centers, the radiation dosimeters are normally used for determinations of the output of Cobalt-60 teletherapy units and medical linear accelerators (linacs) and should be calibrated in terms of exposure, air kerma or absorbed dose to water.

Gamma Radiation

Radionuclide sources are used to perform calibration at photon energies above 300 keV. There are several gamma sources i.e. Caesium-137 and Cobalt-60 with different activities available to provide radiation standards for protection and therapy level dose rates. The reference gamma radiation fields are determined in terms of air kerma rate. A teletherapy 60Co unit model Picker C 9 is used to provide national standards for air kerma and absorbed dose to water at therapy level and calibration service for therapy level dosimeters.

              

              Figure 1. Gamma Radiation Laboratory

X-Radiation

Calibration at photon energies below 300 keV is usually carried out using an x-ray system. SSDL has created a series of x-ray beam qualities which were selected to match the beam qualities offered by the International Bureau of Weight and Measures (BIPM) and based on ISO Narrow Spectrum Series. The Yxlon constant potential x-ray systems type MG 160 with a 160 kV tube and Siemens Stabilipan with a 300 kV tube are used to generate ISO narrow spectrum series x-ray reference radiation for the calibration of radiation protection instruments. The beam characteristics are shown in table 1 and table 2, where the values of the mean energies have been adopted from the ISO document. X-rays generated between 100 kV and 250 kV in accordance with the BIPM therapy qualities are used for calibration of therapy level dosimeters.

           

             Figure 2. X-Radiation Laboratory

ACCREDITATION OF LABORATORY

The laboratory is established as the national focal point for the calibration of radiation measuring instruments used in radiation protection and radiotherapy. To maintain the radiation dosimetry standard in accordance with an international standard, a comprehensive quality assurance programme based on the ISO/IEC 17025 is adopted and implemented. SSDL will be accredited as a calibration laboratory for radiation survey meters and therapy dosimeters by the Turkish Accreditation body (TURKAK).

Table 1. SSDL X-radiation ISO narrow spectrum series 

Radiation quality

Mean energy

Tube potential

Additional filtration [mm]

HVL [mm]

 

[keV]

[kV]

Al

Cu

Sn

Pb

Cu

N40

33

40

4.0

0.21

-

-

0.079

N60

47

60

4.0

0.66

-

-

0.235

N80

65

80

4.0

2.0

-

-

0.60

N100

83

100

4.0

5.0

-

-

1.18

N150

117

150

4.0

-

2.53

-

2.5

N200

164

200

4.0

2.0

3.0

1.0

4.0

N250

207

250

4.0

-

2.0

3.0

5.0

Table 2. SSDL X-radiation qualities 

Tube potential [ kV ]

Additional filtration [mm]

HVL [mm]

Al

Cu

Al

Cu

100

3.5

-

3.9

-

135

1.0

0.23

-

0.50

180

1.0

0.51

-

1.01

250

1.0

1.65

-

2.50