Investigation and Performance Tests of a Designed and Constructed Cylindrical Ionization Chamber (0.6cc) in Iran

There are a number of codes, reports and protocols by national and international organizations, including IAEA, which provide physicists with a systematic approach to dosimetry of photon beams. Most of these dosimetry recommendations have explicitly recognized the advantages of using cylindrical ionization chambers for dosimetry of therapeutic beams, especially for photon beams with energies from kilo voltage (kV) to megavoltage (MV) x-ray. A commercial cylindrical ionization chamber (W-30001) was used as the reference chamber for compare measurements. The homemade 0.6cc ionization chamber (CC1) have been designed, fabricated, tested and calibrated. Measurements were made using a Farmer type 2670 electrometer together with these chambers. Leakage current, short-term stability, cable effect, repeatability and angular dependence of the CC1 and W-30001 were all tested and found to be in consistence with the international standard IEC 60731. Ion collection efficiency and polarity effect were determined during calibration of the chambers in Co-60. According to the preliminary test results the CC1 homemade chamber is found to be in consistence with the international standard IEC 60731. An advantage of CC1 chamber is a very low leakage current i.e. its specific insulation design and material. J o u r n a l o f N uc lea r M edicine & Riatio n T h e r a p y ISSN: 2155-9619 Journal of Nuclear Medicine & Radiation Therapy Citation: Moradi MS, Ghafoori M (2012) Investigation and Performance Tests of a Designed and Constructed Cylindrical Ionization Chamber (0.6cc) in Iran. J Nucl Med Radiat Ther S3:003. doi:10.4172/2155-9619.S3-003


Introduction
There are a number of codes, reports and protocols by national and international organizations, including IAEA, which provide physicists with a systematic approach to dosimetry of photon beams [1,2]. Most of these dosimetry recommendations have explicitly recognized the advantages of using cylindrical ionization chambers for dosimetry of therapeutic beams, especially for photon beams with energies from kilo voltage (kV) to megavoltage (MV) x-ray [3]. Radiotherapy machines are widely used in developed countries and to a lesser extent in developing countries. Regarding the possibility of installing more radiotherapy machines in radiotherapy departments in Iran, attempts have been made to construct and make use of a 0.6cc cylindrical chamber for photon beam dosimetry according to the IAEA TRS No.277 [1], based on air kerma standards and the other IAEA code of practice IAEA TRS No.398 [2], based on absorbed dose to water standards ( , ).

Design and fabrication of chamber
The constructional details of our 0.6cc cylindrical ion chamber, the CC1, are based on the description given in IAEA TRS 277(based on International Standard IEC 60731:1997, A1:2002 [4]. The design characteristics, mainly the shape and height of the collecting volume, make cylindrical chamber theoretically ideal for ionization measurements in regions with sharp dose gradients in the beam direction or whenever the uncertainty in the position of the effective point of measurement of the ionization chamber is to be minimized. Figure 1 shows different pieces and connectors of the CC1. Figure  1A and B show TNC connector for 0.6cc ion chamber connection to electrometer, Figure 1C and D show Al and PMMA pieces and the connectors of the chamber, Figure 1E shows the completed configuration of the fabricated chamber and Figure 1F shows the completed configuration of the fabricated chamber plus build up cap for Co-60 measurement in air.
The diagram and basic design characteristics of the CC1 are given in Figure 2 and Table 1. For comparison, Table 1 gives the CC1 dimensions and also those for a commercial cylindrical ionization chamber, the PTW W-30001. Both chambers are not waterproof.

Preliminary tests
Leakage current, short-term stability, cable effect, repeatability and angular dependence of the CC1 and W-30001 were all tested and found to be in consistence with the international standard IEC 60731:1997, A1:2002 (ion collection efficiency and polarity effect were determined during calibration of the chambers in Co-60). An electrometer type Farmer 2670 was used for all measurements and tests.
A Picker V9 Co-60 therapy unit was used as a radiation source. For all tests, except for the test of angular dependence and short term stability, the chambers were placed at a depth of 5 cm in a water phantom. The solid-water phantom dimensions were 20 cm × 20 cm × 10 cm with 5 cm fixed depth ( Figure 3).

Leakage test
The pre and post-irradiation leakage currents for the CC1 were about

Abstract
There are a number of codes, reports and protocols by national and international organizations, including IAEA, which provide physicists with a systematic approach to dosimetry of photon beams. Most of these dosimetry recommendations have explicitly recognized the advantages of using cylindrical ionization chambers for dosimetry of therapeutic beams, especially for photon beams with energies from kilo voltage (kV) to megavoltage (MV) x-ray. A commercial cylindrical ionization chamber (W-30001) was used as the reference chamber for compare measurements. The homemade 0.6cc ionization chamber (CC1) have been designed, fabricated, tested and calibrated. Measurements were made using a Farmer type 2670 electrometer together with these chambers. Leakage current, short-term stability, cable effect, repeatability and angular dependence of the CC1 and W-30001 were all tested and found to be in consistence with the international standard IEC 60731. Ion collection efficiency and polarity effect were determined during calibration of the chambers in Co-60. According to the preliminary test results the CC1 homemade chamber is found to be in consistence with the international standard IEC 60731. An advantage of CC1 chamber is a very low leakage current i.e. its specific insulation design and material. respectively. Also, within 5 seconds after a 10-min irradiation, the transient leakage currents of the chamber decreased to less than 1% of the ionization currents during the irradiation.

Repeatability
The relative standard deviation calculated from ten successive measurements in different intervals was less than 0.5% in most cases [6].

Short-term stability
A reference Sr-90 check source was used for evaluation of short term stability of the chamber response ( Figure 4). The difference between check source measurements during 5 month relative to the CC1 response of a reference date was less than 1% for different intervals.

Cable effect
The CC1 and W-30001 were irradiated in a rectangular field, 6 cm × 24.5 cm (at phantom surface) in two configurations. First, the cable was positioned parallel to the larger side of the irradiation field. In the second irradiation, the cable was positioned perpendicular to the larger side of the field. For both configurations, the difference of the collected signal for both chambers was less than 1%.  Figure 1A and B show TNC connector for 0.6cc ion chamber connection to electrometer, Figure 1C and D show Al and PMMA pieces and the connectors of the chamber, Figure 1E shows the completed configuration of the fabricated chamber and Figure  1F shows the completed configuration of the fabricated chamber plus build up cap for Co-60 measurement in air.

Angular dependence
The CC1 and W-30001 were irradiated in air at a distance of 80 cm from the source. The responses of the chambers were then obtained for several incident angles (θ) from -90° to +90°. The results of the measurements showed that the response variations of the chambers with respect to the incident angle were not significant. This particular W-30001 chamber displayed a maximum response variation with angle of 0.9%, and this particular CC1 chamber displayed a maximum response variation with angle of 0.3% ( Figure 5).

Ion collection efficiency and polarity effect
Ion recombination corrections were performed during calibration of the CC1 and W-30001, according to the two-voltage method [2]. Two different voltages, 1 = 400 V V + and 2 = 100 + V V , were used to determine the recombination correction factors, k s and two polarizing voltages, 1 = 400 + V V and 2 = 400 − V V were used in determine the polarity effect ( pol k ). The results are shown in Table 2 and 3. Relative Response of CC1 with applied voltage is shown in Figure 6.
for Co-60 s k =     When the chamber has an k N factor, the air kerma ( Where Q M is the dosimeter reading corrected for influence quantities.
When the chamber has a calibration factor in terms of absorbed dose to water at the reference quality 0 Q , , the absorbed dose to water at the reference depth is given by [7]: is a chamber-specific factor which corrects for difference between the reference beam quality 0 Q and actual beam quality Q.
In order to determine the k N factor of CC1, the k N factor of the reference cylindrical chamber should be known.