Primary standard calibrations in terms of air kerma in Co-60 beam

Air kerma primary standard CC1 chamber serial number 104

As is the case in all national primary standard labs, LNK Ghent uses a standard graphite cavity chamber as primary standard for air kerma. This chamber is shaped in such a way that the active volume was determined with high precision, making it a primary standard. For more conventional ion chambers, used as secondary standard, this volume determination is not possible. Therefore these chambers need to be calibrated against a primary standard.

The chambers to be calibrated are positioned free in air at the reference distance (source to chamber distance SCD = 100 cm) in the beam and are calibrated with a build-up cap (to provide electronic equilibrium) in combination with/without the user’s electrometer. The calibration procedure always involves two measurements with the primary standard, one before and another after the chamber to be calibrated. The second measurement with the primary standard is meant to check the stability of the setup during the short time interval.

The combined expanded uncertainty for a calibration in terms of kerma in air is 1.4 % for k = 2 (confidence level 95.4 %).

The charge created by the photon beam is collected by a central electrode inside the chamber leading to a current which can be measured by an electrometer. The electrometer used in the LNK Ghent is a Keithley 6517A which is a very reliable electrometer even when the collected charge is of the order of 1 pC. The electrometer is remotely controlled by a PC in the control room via a GPIB-ENET interface. User friendly data acquisition software was created to control the electrometer. This acquisition software was optimized for an automatisation of the measurement sequence reducing the interference of the operator with the data taking process. All the data read with the electrometer and the corresponding settings during the measurements are stored in log files.

The measurement has to be corrected for ambient pressure, temperature and humidity as these parameters have an influence on the air density and thus on the mass of the air inside the chamber. These quantities are measured using digital probes that are calibrated regularly in an ISO-17025 accredited laboratory. The output of these probes is read remotely by a PC in the control room via a GPIB-ENET interface as well. The corresponding data acquisition software reads and saves the output of the probes with a frequency of few minutes.

Further, corrections are required for the presence of the chamber: a stem correction, a scatter correction and a wall correction. Especially the wall correction is very important.

For the measurements with the standard graphite cavity chamber (CC01), the air kerma (kinetic energy released per unit of mass) is obtained using the following equation:

• M = measured charge as indicated by the electrometer.
• k_Tp is the correction factor for the ambient atmospheric conditions. The reading, M, of the electrometer depends on the variations in ambient temperature and pressure. The air kerma is calculated for standard values p = 1013.25 mbar and T = 20°C.

where T is the air temperature expressed in °C.

• k_elec is the calibration factor of the electrometer (see Sec: 4.3.3 for details)
• V is the volume of the cavity ionization chamber; V = 1.0220.10^-6 m³
• ρ_air is the air density at normal ambient conditions ( 20°C and 1013.25 kPa) and ρ_air = 1.2048kg/m³
• W is average energy to produce an ion pair in dry air by secondary electrons and e is electron charge with W/e = 33,97 J/C from Ref [2,4].

• 	is the fraction of energy lost by brehmsstrahlung,
  	= 0.003 for 60Co and
  	= 0.002 for 137Cs from Ref [4].

• is the ratio of mean mass energy absorption coefficients in air and graphite;

= 0.9989

The values are taken from Ref.[1]. (μ_en/ρ)_air =2.666 10^-2 cm²/g and (μ_en/ρ)_{Carbon,graphite} =2.669 10^-2 cm²/g

• c,a is the graphite to air mass stopping power ratio; sc,a =1.0007 for 60Co and sc,a =1.010 for 137Cs

• k_s is the recombination correction; k_s =1.0025
• k_h is the humidity correction and k_h=0.9970 (from Ref. [2]). The measurements should be done in the range 20 %-70 % humidity
• k_stem is the stem scattering correction; k_stem = 1.0034 for ⁶⁰Co and k_stem = 1.000 for ¹³⁷Cs
• k_wall is the correction for wall scattering and attenuation; k_wall = 1.0159 for ⁶⁰Co and and k_wall = 1.019 for ¹³⁷Cs . Both the k_stem and k_wall were determined by Monte Carlo simulations. The absolute uncertainty of the k_wall was approximated as being equal to 0.001 (Ref. [7]).
• k_an is the correction for the axial non-uniformity of the photon beam; k_an = 1.
• k_rn is the correction for the radial non-uniformity of the photon beam; k_rn = 1.

Bibliography:

1. J. H. Hubbell and S. M. Seltzer , Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients from 1 keV to 20 MeV for Elements Z = 1 to 92 and 48 Additional Substances of Dosimetric Interest, Version 1.4, July 12, 2004 - http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html
2. Joakim Medin, Pedro Andreo and Stefaan Vynckier, Comparison of dosimetry recommendations for clinical proton beams, Phys. Med. Biol. 45 (2000) 3195–3211.
3. Allisy-Roberts P.J., Burns D.T., kesler C., Summary of the BIPM.RI(I)-K1 comparison for air kerma in ⁶⁰Co gamma radiation, Metrologia, 2007, 44, Tech. Suppl., 06006.
4. Comite Consultatif des Rayonnements Ionisants 1985 (CCEMRI-1985)
5. Theratron user manual – 1979
6. IAEA TRS-381 " The use of plane-parallel ionization chambers in high-energy electron and photon beams. An international code of practice for dosimetry ", IAEA, Vienna, 1997
7. D.T.Burns, "Calculation of k_wall for ⁶⁰Co air kerma standards using PENELOPE", CCRI (I)/03-40