Cobalt-60 source for reference irradiations.
The cobalt irradiator is used as local reference. It has a well defined (almost monoenergetic) photon beam and a highly stable dose rate.
Model: Terabalt T100 from UJP Praha, Czech Republic.
Radionuclide: cobalt-60 (gamma energies: 1.17 MeV and 1.33 MeV)
Half-life of cobalt-60: 5.2710 y = 1925.21 d (u =0.015 % = 0.3 d) for 365.242 d/y. This reference value is from Table of radionuclides (comments on evaluations) vol. 1-3, Monographie BIPM-5 (2006). The effective or "apparent" half-life describing the exponential decrease in dose rate at the reference point for the standardized DTU set-up is 1918.0 d (u=2.5 d with k=1).
Dose rate: ~1 Gy/min @ 100 cm distance (2018).
Beam: Fixed horizontal beam.
Collimator: 5x5 cm2 to 40x40cm2.
Special features: Full computer control using the DTU scripting software (MELab).
A special rig has been designed for irradiations (see sketch below). The rig enables accurate positioning of ionization chamber free in air at the 1000.0 mm reference distance using the motorized xyz-stage. When the ionization chamber is in place, the water tank is lifted into position.
Calibration of a PTW31021 ionization chamber in the cobalt beam.
The reference distance (1000.0 mm from the source) is realized using a micrometer.
Roos chambers and other plane parallel ionization chambers can be positioned at the iso-center line using a low-force indicator dial that previously has been set to zero using the optical telescope. The chamber can thereafter we moved 1.12 mm or 1.00 mm with the motorized xyz-stage in order to have the chamber at the desired reference point.
The details of the geometry for absorbed-dose to water calibrations are given below. The reference point is at 5.00 g/cm2 of water. Distance C from the water tank front surface to the reference plane on the cobalt source head is established using a special tool involving an indicator dial with a well defined force.
