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