ARPANSA - Australian Radiation Protection and Nuclear Safety Agency

Get a PDF version (19kb) of this page.

Background

ARPANSA maintains two standards for air kerma for X-rays. A medium-energy free air chamber (MEFAC) is used for the measurement of beams generated by tube potentials in the range 50 – 300 kV, and a low-energy free-air chamber (LEFAC) is used in the range 20 – 70 kV. For many years these chambers have been the Australian primary standards of air kerma. The majority of calibrations performed by ARL/ARPANSA are traceable to the MEFAC, however some low energy calibrations are traceable to the LEFAC.

These chambers have been known to differ by approximately 2% when they are compared using beams in the overlap region of their energy ranges. The magnitude of the difference is significant but is comparable to the combined uncertainties of the two free-air chambers (typical standard uncertainties are 0.5% for the MEFAC and 1.5% for the LEFAC).

In 2002 these standards were compared with the Bureau International des Poids et Mesures (BIPM) in Paris. The results showed that the MEFAC was in good agreement with the international primary standard, and confirmed the difference between the MEFAC and LEFAC.

Using the results of the intercomparison, and the results of previous comparisons between the two ARPANSA chambers, calibration factors were determined for the LEFAC. In April 2003 the status of the LEFAC was downgraded to that of a secondary standard. Correction factors were then used to make measurements with the LEFAC consistent with ARPANSA’s other standards and with the 2002 intercomparison. The changes are presented in Table 1.

Changes to the air kerma rate for ARPANSA RT100 beams

The air kerma and exposure rate has been adjusted upwards as shown in Table 1. As a result of this change, chamber calibration coefficients also increase by this amount.

Table 1: Adjustment of low energy air kerma and exposure rates for ARPANSA RT100 beams for measurements traceable to the LEFAC.

Beam ID	kV/mA	Filter (mm Cu)	Filter (mm Al)	Measured HVL (mm Al)	Effective energy (keV)	Air kerma and exposure rate increase	Increase in chamber calibration coefficient (%)
RT1	20/10		0.15	0.125	11	1.0349	3.5
RT2	30/10		0.30	0.23	13	1.0340	3.4
RT3	37/10		0.40	0.38	16	1.0328	3.3
RT4	45/10		0.55	0.58	18	1.0312	3.1
RT5	55/10		0.78	0.87	21	1.0291	2.9
RT6	70/10		1.25	1.40	25	1.0255	2.6
RT7	100/8		1.70	2.34	30	1.0203	2.0
RT8	100/8	0.25	1.02	6.67	49	1.0156	1.6

Change in chamber calibration coefficient

Since April 2003, ARPANSA calibrations for RT100 X-rays have included the corrections given in Table 1. For a chamber calibrated before this time, the calibration coefficient may need to be revised upwards to agree with the changed standard. For example, a chamber’s calibration coefficient measured with RT1 in 2002 (expressed in mGy/nC or C/kg/C), should be increased by 3.5% to agree with the new interim standard. ARPANSA has sent letters to hospitals with revised calibration coefficients for reports issued between 2001 and 2003.

Please note that not all RT100 calibrations before 2001 are affected. Some of the older calibrations are traceable to the MEFAC, and in this case no revision of coefficients is necessary. If your dosimetry is based on a calibration prior to 2001, please contact ARPANSA to determine if changes need to be made.

All measurements on ARPANSA’s Seifert X-ray generator, whether of low or medium energy, are traceable to the MEFAC and do not require revision.

Traceability

The LEFAC response is now determined by an interpolation of results obtained against the BIPM and the ARPANSA MEFAC. The results can be traced to the ARPANSA standard through the known relationship between the MEFAC and LEFAC on the RT100 beams.

Future

The physical reasons for the LEFAC’s behaviour are not known. Possible causes are distortion in the electric field or an error in the collecting volume. This problem has been investigated and further study is required. For this reason the current changes are for the interim period until the primary standard can be re-established.

Prepared for ARPANSA by D Butler (IRS section, Yallambie) 2nd edition, February 2005

Top of Page