Published on March 1, 2014
By Vibha Chaswal, Ph.D.
The ion-‐chamber should be tested along with the electrometer and the cable it is going to get used with For reproducibility of results the ion-‐chamber should also be tested against diﬀerent electrometers and the results should have minimum variability It is important to be aware of various sources of noise in the system that contribute to charge-‐ collection at the ion-‐chamber electrodes in addition to charge collected due to ionizations by radiation
Acceptance testing is recommended when the chamber is back from ADCL before using it for reference or absolute dosimetry.
PTW 0.6cc SN1315 farmer type ion-‐chamber by PTW FREIBERG. Model # TN 30013-‐1315
IC set-‐up: 100 cm SSD; ion-‐chamber active volume at the center of a 20cmx20cm ﬁeld; solid-‐water phantom with 1.5 cm build-‐up and 5 cm back scatter Electrometer: -‐300 V (-‐100%) bias Energy: 6 MV photon beam Procedure: Record charge collection measurements for MU delivery ranging between 2 MU to 200 MU
The uncertainty in the charge collection due to stem eﬀect should be less than 0.5% This can be checked by taking exposures using a ﬁeld size that irradiates just the thimble and comparing it with charge collection reading taken when the whole stem is in the ﬁeld. Set up: 100 cm SSD; FS 5cmx30cm; tape the IC in two orientations – IC parallel to the 30 cm dim of ﬁeld, IC perpendicular to it with only thimble inside direct radiation beam
stem eﬀect slightly greater than 0.5% but 5cmx30cm has greater penumbra uncertainties repeat test using 7cmx30cm or 10cmx30cm FS stem eﬀect is 0.2%, within manufacture’s speciﬁcations
Collect charge readings using both bias polarities with all other set-‐up parameters constant
Which means check the cylindrical symmetry of the ion chamber’s active volume’s construction Ion chamber suspended in-‐air with the ion chamber’s build-‐up cap on This provides same build-‐up from all diﬀerent directions of irradiation
(left) Ion chamber major axis perpendicular to the CAX. (right) Ion chamber major axis parallel to the CAX.
ion chamber response for an orientation perpendicular to the central beam axis
Ion chamber response for an orientation parallel to the central beam axis (Set up: 10 x 10 ﬁeld, 100 SAD, 06 MV X-rays, 100 MU, CAX parallel)
Measurements involving ionizing current at 2 diﬀerent voltages are used to assess the collection eﬃciency of an ion chamber [Boag 2-‐volatge technique] This is ‘Pion’ or recombination correction factor [TG 51] Use electrometer 100 % and 50 % voltage settings using negative and positive bias (Vh and Vl settings)
Collection Eﬃciency (Pion):
A feel of your ion chamber Physicist’s extended hand and mind in clinical reference dosimetry JEB’s way of keeping things in head for an easy reference *Average nC/cc MU for a 0.6 cc farmers chamber = 0.2092 nC/cc/ MU *Average nC/cc MU for a 0.125 cc farmers chamber = 0.1833 nC/cc/ MU *Average nC/cc MU for a 0.015 cc farmers chamber = 0.1714 nC/cc /MU OR, simply put… ….the average charge collected per cubic centimeter of the chamber’s active volume for a monitor unit of radiation is roughly ~0.2 nC So, expected charge collection for 100 MU ~ 20 nC…..right, Happy Physicist! * Averages derived from detailed linearity measurements using 3 chamber types
The SNR is derived from the same irradiation data set over the three ion-‐chambers SNR = mean signal (your data)/ Std Deviation SNR test Results observations: SNR generally increases with the increase in Signal Normalized SNR values show that the SNR decreases with the decrease in ion chamber collection volume (reason I wouldn’t try to use a 0.015 cc ion-‐chamber to assess doses at very low signals) At 10 MU using 0.015 pin-‐point -‐ noise and the detected signal are almost equal
1. 2. 3. To quantify the leakage from various components the following were quantiﬁed: Ionizations in the ion-‐chamber cable in the ﬁeld Ionizations in the triax cable in the ﬁeld Leakage when no irradiation but power supply is on. This was quantiﬁed with and without the ion-‐ chamber being connected to the electrometer. Leakage only due to electrometer electronics can be characterized by letting the electrometers run on battery for a given duration of time without any connecting wires or ion-‐chamber.
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