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Output calibration using TRS 398
Sothing vashum
Table of contents
Beam calibration in Co-60 gamma ray beam - dosimetry equipment
- phantom and chamber sleeves
- reference conditions
- determination of absorbed dose under reference condition
- worksheet
Dosimetry equipment Recommended reference instrument for calibration - cylindrical and plane-parallel chamber
In cylindrical – chamber axis at the centre of the cavity volume
In plane-parallel – the inner surface of the entrance window at the centre of the window
THE REFERENCE POINT SHOULD BE POSITIONED AT THE REFERENCE DEPTH IN A WATER PHANTOM
Phantom and chamber sleeve
Extend 5cm – four sides –field size – at depth of measurement
Extend at least 5g/cm^2 beyond max depth of measurement.
Reference conditions
SSD/SCD – 80cm/100cm field size - 10cm * 10cm
Co-60
After the positioning of the chamber according to the reference conditions, the absorbed dose to water is given by
For a dosimeter in a reference beam of quality Qo, where Qo is Co-60, in this situation,
DW,Q = DW , KQ,QO = KQ ( which has a value of unity) and ND,W,QO = ND,W.
DW,Q = MQND,W,QOKQ,QO
then we have,
Here, MQ = corrected meter reading which includes KTP,
Kpol, Ksat where KTP, which is the correction factor for temperature and
pressure given by,
KTP = (273.2 + T) Po/ (273.2 + To)P
To = 20oC ; Po = 760 mmHg
DW = MQND,W.
Kpol, which is the factor to correct the response of an
ionization chamber for the effect of change in polarity of the polarizing voltage applied to the chamber given by
Kpol = |M+| + |M-|/2M
M+ = meter reading for +300V
M - = meter reading for -300V
Ksat, factor to correct the response of an ionization chamber for the lack of complete charge collection given by
Ksat = (V1/V2)^2 – 1/ (V1/V2)^2 – (M1/M2)
V1 = voltage (+300V) V2 = voltage (+150V)
M1 = meter reading for voltage V1(+300V)
M2 = meter reading for voltage V2(+150V)
The Ksat for a pulsed beam is given by,
Ksat = ao + a1(M1/M2) +a2(M1/M2)^2
Where ao, a1 and a2 are constants and the values are
2.303, -3.636 and 2.299 respectively.
Biblioligraphy
TRS 398