Comparison of the NIST and NPL Air Kerma Standards Used for X-Ray Measurements Between 10 kV and 80 kV

Journal of Research of the National Institute of Standards and Technology, Sept, 2000 by M. O'Brien, P. Lamperti, T. Williams, T. Sander

A direct comparison was made between the air kerma primary standards used for the measurements of low-energy x rays at the National Institute of Standards and Technology (NIST) and the National Physical Laboratory (NPL). The comparison was conducted at the NPL using NPL reference radiation qualities between 10 kV and 80 kV. The results show the primary air-kerma standards to agree within 0.6 % of their values for beam qualities up to 80 kV.

Key words: air kerma; free-air ionization chamber; primary standard; reference radiation qualities.

Accepted: July 28, 2000

Available online: http://www.nist.gov/jres

1. Introduction

A direct comparison was made between the air kerma primary standards used for the measurements of low-energy x rays at the National Institute of Standards and Technology (NIST) and the National Physical Laboratory (NPL). The comparison was conducted in June 1998 at the NPL using NPL tungsten reference radiation qualities between 10 kV and 80 kV and the new mammography 28 kV entrance and exit beam offered at NPL. The two NIST primary standards shipped to the NPL for this comparison were the Lamperti (10 kV to 60 kV) and the Ritz (20 kV to 100 kV) free-air ionization chambers. Prior to this comparison these primary x-ray standards at these energies have only been compared indirectly through comparisons at the Bureau international des Poids et Mesures (BIPM).

2. NPL Irradiation Facilities

Two x-ray irradiation laboratories at NPL were used for this comparison. A constant-potential low-ripple generator connected to Machlett [1] OEG-50A x-ray tubes with either a tungsten or molybdenum anode, each having 1 mm of beryllium inherent filtration, is used to perform calibrations in the NFL low-energy x-ray laboratory. The x-ray tube voltage may be varied from 8 kV to 50 kV in 0.1 kV steps and the tube current is adjustable from 10 [micro]A to 17 mA in 10 mA steps. In the medium-energy x-ray laboratory a constant-potential low-ripple generator connected to either a Philips 160 kV x-ray tube with an inherent filtration of 1 mm beryllium, or a Muller 300 kV tube with an inherent filtration equivalent to 4 mm aluminum, is used to perform calibrations. The voltage may be varied from 50 kV to 300 kV and the current is adjustable from 10 [micro]A to 25 mA. The output of each x-ray system was measured through the use of a transmission monitor chamber. All charge measurements were normalized to the response of the monitor chamber. In the low-energy facility the comparison measurements were made at a distance from the x-ray focal spot of 0.5 m with a field size of 40 mm diameter at the point of measurement. In the medium-energy facility the comparison measurements were made at a distance from the x-ray focal spot of 0.75 m with a field size of 63 mm diameter at the point of measurement. The x-ray beams produced in both NPL calibration facilities are sufficiently uniform to perform primary standard comparisons and calibrations. The NPL reference radiation qualities used for the comparison are listed in Table 1.

3. Determination of the Air Kerma Rate

The air kerma rate is determined by the relationship

K = (1/m) (W/e) [(1 - g).sup.-1] [pi][k.sub.i], (1)

where

I/m is the mass ionization current as measured by the free-air ionization chamber,

W/e is the mean energy per unit charge expended by electrons in dry air with SI unit in joules per coulomb (J/C),

g is the fraction of the initial kinetic energy of secondary electrons dissipated in air through radiative processes, but is negligible for x rays with energy less than 300 keV, and

[pi][k.sub.i] is the product of the correction factors to be applied to the free-air ionization chamber.

The physical constants used in the calculation of air kerma follow in Table 2. The calculation of air kerma involves some physical measurements of the primary standards, which are listed in Table 3. The dimensions of the chambers are used to determine the mass of air in which the ionization occurs.

4. Characteristics of Air Kerma Standards

4.1 Description of Standards

The measurement of the mass ionization current for the determination of air kerma is obtained at both NPL and NIST through the use of primary standard free-air ionization chambers. All four of the free-air ionization chambers used in the comparison are of the conventional parallel plate design. The diameter of the chamber aperture and the length of the collecting region define the mass of air in which the ionization is collected for this type of free-air ionization chamber. The NPL 50 kV chamber is used to measure exposures and air kerma for x rays generated between 8 kV and 50 kV. Reference [1] presents a detailed design description of the NPL chamber and a complete explanation of the correction factors. The NPL 50 kV primary standard has a companion chamber that allows the direct measurement of the air attenuation correction. The air attenuation chamber is of similar construction as the standard, but is fitted with two collector plates separated by the distance equal to the distance between the defining pl ane of the aperture and the center of the collecting volume of the primary standard. The NFL 300 kV standard is a new chamber which has been designed for use with x rays generated between 40 kV and 300 kV. The design has been described [2], but additional work on the scattered photon correction is continuing and a full report is being prepared. The NIST Lamperti chamber is designed for x-ray exposure and air kerma measurements in the 10 kV to 60 kV region, but is generally used at NIST for the 10 kV and 15 kV qualities. The Lamperti chamber, described in detail in Ref. [3], utilizes a guard-ring system to maintain a uniform electric field. The NIST Ritz chamber, designed for x-ray exposure and air kerma measurements between 20 kV to 100 kV, uses a guard plate and guard strip system to diminish the distortion to the electric fields. The Ritz chamber is described in Ref. [4].