Introduction

In July 2004, a new irradiation facility for ⁶⁰Co gamma radiation was put into operation. This facility was developed and constructed at PTB especially for metrological requirements. Particular importance was placed on the stability (reproducibility) of the properties of the radiation field.

Tasks of the irradiation facility

The new irradiation facility generates a ⁶⁰Co radiation field which is used for the following tasks:

• realization of the unit of absorbed dose to water,
• dissemination of absorbed dose to water by the calibration of dosemeters,
• irradiations for scientific purposes and
• participation in international comparison measurements at the highest accuracy level.
  
  
  

Characteristics of the irradiation facility

The most important characteristics of the new facility are:

• At a distance of 1 m from the ⁶⁰Co radiation source, the absorbed dose rate amounts to approx. 1.3 Gy/min.
• The geometric form of the radiation field is determined by a collimator (acc. to ISO 4037) which can be manually exchanged to realize, if and when required, different field sizes.
• The time between switching on the radiation and reaching of the maximum dose rate amounts to maximally 50 ms. The same is valid for switching-off of the radiation.
• In the case of repeated switching on and off of the radiation, the (decay-corrected) absorbed dose rate at the reference point changes by maximally 0.01%.
• At the measuring location, the ratio of the dose rates amounts to approx. 10⁵ when the radiation is switched on and off.
• The ⁶⁰Co radiation emerges horizontally from the irradiation facility so that the object under irradiation is easily accessible also in the case of large measurement distances.
• When switched-off, the facility shields the radiation source to such an extent that the requirements of radiation protection are met.
• The facility is electrically operated; in case of failure of the supply voltage, the radiation is automatically switched-off.
• The radiation source in the facility can be easily exchanged.

Construction of the irradiation facility

General

When the radiation is repeatedly switched on and off, the dose rate generated by the facility may vary by maximally 0.01%. Investigations performed by PTB have shown that the ⁶⁰Co facilities available on the market, which have mostly been designed for medical applications, do not meet this requirement. Here, variations of the dose rate up to a few 0.1 % occur when the radiation is repeatedly switched on and off.

This variation is often caused by a movement of the radiation source when the radiation is switched on and off. When the radiation is switched on, it is, for example brought from a shielded resting position into the radiation position. This may cause slight inaccuracies in the source positioning which lead to variations of the dose rate (under reference conditions). In addition, the inside of many radiation sources consists of ⁶⁰Co granulate; when the source is moved, its internal structure may change and this also leads to a change of the dose rate.

Principal set-up of the facility

To prevent these effects, the 60Co radiation source is fixed in the facility and is not moved - unless the source must be exchanged. The radiation is therefore switched on and off with a moveable shutter made of heavy metal which unblocks or covers the beam exit. The shutter must be moved very quickly and precisely so that the maximum dose rate is very quickly reached (maximally 50 ms after switching-on) and the temporal variation of the dose rate is very well reproducible from zero to the maximum value when the radiation is switched on. As the shutter must also have a very large mass - which is required for sufficient shielding - this precise movement is difficult to realize. The shutter is therefore designed in two parts; the "inner" and the "outer" part can be moved independently of each other. Form and size of the radiation field are determined by a collimator which is integrated into the outer shutter and is automatically brought into the correct position for irradiations when the shutter is opened.

Principal set-up of the 60Co irradiation facility (schematically, without lead bricks for shielding of the radiation source).

The inner shutter

The internal shutter is arranged directly in front of the ⁶⁰Co radiation source. It is 15 cm in thickness (in the direction of radiation) and consists of tungsten-copper alloy. Its attenuation factor for ⁶⁰Co radiation amounts to approx. 10⁵. The internal shuter moves in vertical direction and is driven by an electric motor via an eccentric.

The internal shutter serves to switch the effective radiation on and off for irradiation. The time the internal shutter needs to completely unblock or cover the radiation source is shorter than 50 ms, and this rapid movement is performed with high reproducibility (cf. "dynamic" properties of the radiation field). This allows the fraction of the dose which is already generated when part of the radiation source is unblocked to be considered for irradiations.

A film showing the movement of the internal shutter in combination with the eccentric is shown here (approx. 8 MB).

The internal shutter is constructed so that it automatically closes when the line voltage fails. The internal shutter attenuates the radiation to such an extent that - with the internal shutter closed - the range around the irradiation facility is the control range.

The external shutter

The external shutter further attenuates (in addition to the internal shutter) the dose rate, when work is to be performed in the irradiation room (e.g. arrangement of experiments, adjustment of ionization chambers in the phantom etc.). It is 21 cm in thickness (in the direction of the beam radiation) and made of lead. Its attenuation factor for ⁶⁰Co radiation amounts to approx. 10⁵. The external shutter moves in horizontal direction and is driven by an air cylinder. Complete opening or closing of the external shutter takes approx. 3 s.

Before an irradiation process is started and when all persons have left the irradiation room, first the external shutter is opened; irradiation then begins and ends with opening or closing of the internal shutter.

The collimator is integrated into the external shutter. It is automatically brought into the correct position for irradiations when the shutter is opened.

The collimator

The collimator determines the form of the radiation field. Its type of construction is in compliance with that described in ISO 4037-1; it is composed of a total of 7 orifices made of tungsten-copper alloy and has a total length of approx. 21.3 cm. The standard collimator of the irradiation facility generates a square radiation field whose field size amounts to 10 cm×10 cm at a distance of 100 cm from the radiation source. To be able to realize - if and when required - also other geometries or field sizes of the radiation field, the collimator can be (manually) exchanged when the external shutter is closed.

Schematic view of a collimator acc. to ISO 4037-1.

Shielding of the radiation source

The irradiation facility at the same time serves as a storage and shielding container for the ⁶⁰Co radiation source. The housing of the facility is completely filled with lead bricks so that the radiation source is in all directions surrounded by at least 32 cm of lead (when both shutters are closed). It is guaranteed that all over the outer side the facilitiess dose rate is smaller than 3 µSv/h.

Properties of the radiation field

The geometric properties of the radiation field as, for example, the form or field size, are determined by the collimator. This collimator is integrated into the outer shutter and can be exchanged if and when required so that different geometries of the radiation field or different field sizes can be realized.

The "dynamic" properties of the radiation field as, for example, the temporal variation of the dose when the effective radiation is switched on or off, are determined by the internal shutter.

Geometric properties

The irradiation facility is frequently used to calibrate dosemeters as secondary standards for radiation therapy. According to international and national standards (e.g. DIN 6800-2), the reference field size for the calibration amounts to 10cm×10cm. The standard collimator therefore generates a radiation field whose field size amounts to 10cm×10cm at a distance of 100 cm from the radiation source. The field size is determined by the shape of the 50% isodose lines.

The following figures show the relative dose distribution generated with the standard collimator in a plane vertical to the beam axis at a distance of 100 cm from the radiation source in the water phantom (source-surface distance: 95cm, depth in the phantom: 5 cm).

Relative dose distribution in the water phantom at a depth of 5 cm with a source-surface distance of 95 cm.

Relative dose distribution in the water phantom at a depth of 5 cm with a source-surface distance of 95 cm. The Figure also shows some isodose lines.

The figures show that the requirement of the protocol with respect to the field size is met. Moreover, the radiation field has a good symmetry and a pronounced plateau.

In addition to this dose distribution in a plane vertical to the beam axis, knowledge of the depth dose distribution is also of interest for a detailed characterization of the radiation field. The following figures show the dose distribution in the horizontal plane in the phantom which contains the beam axis.

Relative depth dose distribution in the water phantom in the horizontal plane which contains the beam axis. The source-surface distance amounts to 95 cm.

Relative depth dose distribution in the water phantom in the horizontal plane which contains the beam axis. The source-surface distance amounts to 95 cm.

"Dynamic" properties

Here, the "dynamic" properties of the radiation field are understood to mean the temporal variation of the dose when the radiation is switched on and off or the variation of the (maximum) dose rate after repeated switching on and off.

hen the radiation is switched on (this is performed by opening of the iinternal shutter, the maximum dose rate is not immediately available. Instead, the dose rate increases to its maximum value with increasing unblocking of the radiation source by the internal shutter.

The following figure shows the change of the dose rate as a function of the rotational angle of the motor which moves the internal shutter. The measurements were performed quasi-statically, by adjusting a specified (fixed) rotational angle of the motor for each measurement point and measuring the associated dose rate.

Development of the dose rate as a function of the rotational angle of the motor when the internal shutter is opened. The rotational angle can be converted into an equivalent time (upper axis). The two measurements were performed at different times with different ionization chambers.

When the rotational speed of the motor is known, the rotational angle can be converted into an equivalent time (upper axis in the figure) which gives the change of the dose rate with time when the internal shutter is opened.

The time between the moment when the radiation is switched on and the moment when the maximum dose rate is reached amounts to 50 ms at the most. As the figure shows, increase in the dose rate during switch-on is very well reproducible. The dose rate generated during the switch-on process can therefore very exactly be taken into account for irradiations. The same applies to the switch-off process.

For characterization of the radiation field it is moreover of interest to know whether the movement of the internal shutter during switch-on and switch-off of the effective radiation causes a change of the maximum dose rate. The irradiation facility has been constructed with the aim of minimizing this effect as far as possible (see: General).

Measurements with an ionization chamber show that the maximum dose rate does not change when the internal seal is moved. As the following figure shows, the measured values of the dose rate scatter by maximally ±0,02% independent of whether the internal seal is closed and opened before each measurement or if it remains constantly open. As the same dispersion of the values was also observed when the shutter was not moved, it is to be assumed that mainly the noise of the dosemeter used is concerned. A possible change of the dose rate due to a movement of the shutter can therefore be only much smaller than the dispersion observed here (and is therefore not measurable).

Relative variation of the measured values at repeated measurement of the dose rate. Measurements were performed both with constantly opened internal shutter and with the internal sbhutter moving.