BS EN ISO 16526-2-2020 Non-destructive testing – Measurement and evaluation of the X-ray tube voltage Part 2: Constancy check by the thick filter method.
This part of ISO 16526 specifies a constancy check of a X-ray system where mainly the X-ray voltage is checked and also the tube current and the constitution of the target which can he changing due to ageing of the tube.
The thick filter method is based on a measurement of the dose rate behind a defined thick filter using defined distances between the X-ray tube, the filter and the measuring device.
This method is very sensitive to changes of the voltage, but it does not provide an absolute value for the X-ray tube voltage. Therefore, a reference value is needed and, it is recommended to find this reference, for example, within the acceptance test of the system.
The thick filter method is a rather simple technique and may be applied by the operator of an X-ray system to perform regularly a constancy check of the system.
The method can also be applied for consistency checks after changing components which may affect the X-ray tube voltage.
This method can he applied for all types of X-ray systems, i. e. for constant potential, half wave and impulse wave generators with a tube current larger than I mA.
2 Principle and equipment
The equipment to be used includes the following components, see figure 1:
— the X-ray system;
— a specified collimator;
— a specified filter;
— suitable dose meter or dose rate meter;
— a film for the prove of good collimation and dose meter or dose rate meter adjustment.
3 Measurement
In order to carry out the constancy check of an X-ray system or the consistency check of an X-ray system
to compare different components the dose rate is measured using a filter as given in figure 2 or thicker.
3.1 Leverage factor
For the first application of the thick filter method the leverage factor L, shall be determined corresponding to the selected setup. The dose value shall be measured at 100 % (D10o) and at 95 % (D95) of the test voltage.
NOTE The leverage factor should be adjusted between 6 and 8. This is a compromise which considers limitations in exposure time. A high leverage factor is recommended for high accuracy of the method.
The leverage factor is calculated by equation (1)
L=(1—Dgs/D1oo)/0,05 (1)
3.2 Constancy check
The constancy test of the tube shall be repeated regularly at the given voltage. The arrangement corresponding to figure 1 shall be used with the same collimator, thick filter (thickness and material) and the same calibrated dose meter for the constancy check. The constancy of the tube voltage is confirmed if the measured dose value or dose rate value has been reproduced within the required tolerance T (in %) which is calcualted by equation (2).
T = 100. I (DlOomcacured — Dioonominai) I / (Dioonomnai L) (2)
Within the reference measurement all essential parameters (distances Dl and D2, filter material and thickness, type of dose meter or dose rate meter) shall be documented. The arrangement shall be identical for all subsequent measurements.
An example is given in Annex A.BS EN ISO 16526-2 pdf download.