Comparison of the NIST and NPL Air Kerma Standards Used for X-Ray Measurements Between 10 kV and 80 kV.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 National Institute of Standards and Technology, governmental agency within the U.S. Dept. of Commerce with the mission of "working with industry to develop and apply technology, measurements, and standards" in the national interest. (NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. ) and the National Physical Laboratory (NPL 1. NPL - New Programming Language. IBM's original (temporary) name for PL/I, changed due to conflict with England's "National Physical Laboratory." MPL and MPPL were considered before settling on PL/I. Sammet 1969, p.542. 2. ). 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 ionization: see ion. ionization Process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules (ions) by the removal or addition of negatively charged electrons. chamber; primary standard; reference radiation qualities. Accepted: July July: see month. 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 June: see month. 1998 at the NPL using NPL tungsten tungsten (tŭng`stən) [Swed.,=heavy stone], metallic chemical element; symbol W; at. no. 74; at. wt. 183.85; m.p. about 3,410°C;; b.p. 5,660°C;; sp. gr. 19.3 at 20°C;; valence +2, +3, +4, +5, or +6. reference radiation qualities between 10 kV and 80 kV and the new mammography mammography, diagnostic procedure that uses low-dose X rays to detect abnormalities in the breasts. The early diagnosis of breast cancer made possible by the routine use of mammography for screening women increases a woman's treatment alternatives and improves her 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 Ritz elegant and luxurious hotel opened in Paris in 1898 by César Ritz; hence, ‘ritzy, putting on the ritz.’ [Fr. Hist.: Wentworth, 429] See : Luxury (20 kV to 100 kV) free-air ionization chambers. Prior to this comparison these primary x-ray X-ray Electromagnetic radiation of extremely short wavelength (100 nanometres to 0.001 nanometre) produced by the deceleration of charged particles or the transitions of electrons in atoms. standards at these energies have only been compared indirectly through comparisons at the Bureau international des Poids et Mesures (body, standard) Bureau International des Poids et Mesures - (BIPM) The standards body that ensures world-wide uniformity of measurements and their traceability to the International System of Units (SI). (BIPM BIPM - Bureau International des Poids et Mesures ). 2. NPL Irradiation irradiation /ir·ra·di·a·tion/ (i-ra?de-a´shun) 1. radiotherapy. 2. the dispersion of nervous impulse beyond the normal path of conduction. 3. Facilities Two x-ray irradiation laboratories at NPL were used for this comparison. A constant-potential low-ripple generator generator, in electricity, machine used to change mechanical energy into electrical energy. It operates on the principle of electromagnetic induction, discovered (1831) by Michael Faraday. connected to Machlett [1] OEG-50A x-ray tubes X-ray tube An electronic device used for the generation of x-rays. X-rays are produced in the x-ray tube by accelerating electrons to a high velocity by an electrostatic field and then suddenly stopping them by collision with a solid body, the so-called with either a tungsten or molybdenum molybdenum (məlĭb`dənəm) [Gr.,=leadlike], metallic chemical element; symbol Mo; at. no. 42; at. wt. 95.94; m.p. about 2,617°C;; b.p. about 4,612°C;; sp. gr. 10.22 at 20°C;; valence +2, +3, +4, +5, or +6. anode anode (ăn`ōd), electrode through which current enters an electric device. In electrolysis, it is the positive electrode in the electrolytic cell. anode Terminal or electrode from which electrons leave a system. , each having 1 mm of beryllium beryllium (bərĭl`ēəm) [from beryl ], metallic chemical element; symbol Be; at. no. 4; at. wt. 9.01218; m.p. about 1,278°C;; b.p. 2,970°C; (estimated); sp. gr. 1.85 at 20°C;; valence +2. inherent filtration filtration: see sewerage; water supply. Filtration The separation of solid particles from a fluidsolids suspension of which they are a part by passage of most of the fluid through a septum or membrane that retains most of the solids , is used to perform calibrations in the NFL NFL abbr. National Football League NFL (US) n abbr (= National Football League) → Fußball-Nationalliga 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 (company) Philips - A Dutch multinational electronics company. It produces washing machines, consumer electronics, integrated circuits and light bulbs. Together with Sony they set the Compact Disc standard, especially Green Book CD-ROM. 160 kV x-ray tube with an inherent filtration of 1 mm beryllium, or a Muller Mul·ler , Hermann Joseph 1890-1967. American geneticist. He won a 1946 Nobel Prize for the study of the hereditary effect of x-rays on genes. Mül·ler , Johannes Peter 1801-1858. 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 focal spot, n See spot, focal. focal spot the area on the target of the x-ray tube which the electron stream strikes and from which x-rays are emitted. Called also focus. 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 x-ray beam, n the spatial distribution of radiation emerging from a radiograph generator or source. The colloquial term for radiographic beam. See radiographic beam. produced in both NPL calibration calibration /cal·i·bra·tion/ (kal?i-bra´shun) determination of the accuracy of an instrument, usually by measurement of its variation from a standard, to ascertain necessary correction factors. 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 ex·pend tr.v. ex·pend·ed, ex·pend·ing, ex·pends 1. To lay out; spend: expending tax revenues on government operations. See Synonyms at spend. 2. by electrons in dry air with SI unit in joules per coulomb coulomb (k  `lŏm) [for C. A. de Coulomb], abbr. coul or C, unit of electric charge. The absolute coulomb, the current U.S. (J/C J/C Just CuriousJ/C Just Checking J/C Just Chilling ), g is the fraction of the initial kinetic energy kinetic energy: see energy. kinetic energy Form of energy that an object has by reason of its motion. The kind of motion may be translation (motion along a path from one place to another), rotation about an axis, vibration, or any combination of of secondary electrons Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation (the primary radiation). This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. dissipated dis·si·pat·ed adj. 1. Intemperate in the pursuit of pleasure; dissolute. 2. Wasted or squandered. 3. Irreversibly lost. Used of energy. in air through radiative processes In particle physics, a radiative process refers to one elementary particle emitting another and continuing to exist. This typically happens when a fermion emits a boson such as a gluon or photon. See also
 This article or section is written like a personal reflection or and may require .Please [ improve this article] by rewriting this article or section in an . 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 An orifice. It often refers to an opening in which light is allowed to pass in optical systems such as cameras and lasers. See f-stop and numerical 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 Loss of signal power in a transmission. Attenuation The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. 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 Scattered Used for listed equity securities. Unconcentrated buy or sell interest. 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 distortion, in electronics, undesired change in an electric signal waveform as it passes from the input to the output of some system or device. In an audio system, distortion results in poor reproduction of recorded or transmitted sound. to the electric fields. The Ritz chamber is described in Ref. [4]. 4.2 Standard Correction Factors Although free-air chambers are designed to keep all corrections to the mass ionization current to a minimum, some corrections must be applied. The air attenuation correction, the largest of all the corrections, is the correction for the attenuation of the x rays in the air between the defining plane of the chamber aperture and the center of the collecting volume. The air attenuation correction is expressed by [k.sub.a] = exp exp abbr. 1. exponent 2. exponential ([micro]L) (2) where [micro] is the air-attenuation coefficient coefficient /co·ef·fi·cient/ (ko?ah-fish´int) 1. an expression of the change or effect produced by variation in certain factors, or of the ratio between two different quantities. 2. and L is the air absorption length, the distance between the defining point of the chamber aperture and the center of the chamber volume. All of the principal corrections for each chamber are listed in Tables 4 through Table 9. The air attenuation corrections are adjusted to the conditions of 293.15 K and 101.325 kPa. The humidity humidity, moisture content of the atmosphere, a primary element of climate. Humidity measurements include absolute humidity, the mass of water vapor per unit volume of natural air; relative humidity (usually meant when the term humidity correction applied to all chambers, as well as its associated uncertainty, was taken from Ref. [5]. No investigation into the polarity (1) The direction of charged particles, which may determine the binary status of a bit. (2) In micrographics, the change in the light to dark relationship of an image when copies are made. effects was conducted for this comparison; previously determined polarity corrections were applied. The NPL 50 kV chamber was designed to achieve no measurable front face penetration. The correction factors for wall transmission and ion loss for the NPL 300 kV chamber are considered to be negligible at the energies used in this comparison. The Lamperti chamber corrections for wall transmission, [k.sub.p] and aperture transmission, [k.sub.1], are negligible with negligible uncertainties. The Ritz chamber is also considered to have negligible corrections for wall transmission and aperture transmission with relative standard uncertainties of 0.01 % and 0.04 %, respectively. The ion recom bination correction for the Ritz chamber was evaluated for an applied potential of 3 kV, less than the routinely used 5 kV. 5. Comparison Procedure The collection of the charge measurements and the positioning of the primary standards was performed by an automated au·to·mate v. au·to·mat·ed, au·to·mat·ing, au·to·mates v.tr. 1. To convert to automatic operation: automate a factory. 2. procedure for the majority of the reference radiation qualities used in this comparison. In the NPL low-energy range, the positioning of each standard is accomplished through the use of linear positioning motion controllers and verified ver·i·fy tr.v. ver·i·fied, ver·i·fy·ing, ver·i·fies 1. To prove the truth of by presentation of evidence or testimony; substantiate. 2. manually with micrometers before and after each measurement series. Adjustments in positioning are made to the chamber position since the x-ray tube is held in a fixed position. The typical measurement routine, used in the low-energy NPL calibration facility, involved measuring the air attenuation correction with the NPL air attenuation chamber, measuring the charge with the NPL 50 kV standard, followed by the NIST standard and completed with repeat measurements with the NPL standard and the NPL air attenuation chamber. The complete comparison measurement routine was conducted at least twice for each of the NIST standards. The positioning routine used for this comparison in the NPL medium-energy facility was not ideal; the process was laborious la·bo·ri·ous adj. 1. Marked by or requiring long, hard work: spent many laborious hours on the project. 2. Hard-working; industrious. and imprecise im·pre·cise adj. Not precise. im  pre·cise ly adv. . Due to the size and weight of the NIST standard, the NPL
normal calibration positioning procedures were unusable. Since the NIST
standard could not be positioned on the same alignment support structure
as the NPL chamber, the NIST standard was independently positioned
before each measurement. This resulted in a higher positioning
uncertainty. The air temperature and pressure for each measurement was
monitored using NPL instrumentation instrumentation, in music: see orchestra and orchestration. instrumentation In technology, the development and use of precise measuring, analysis, and control equipment. in both facilities. 6. Measurement Uncertainties The individual uncertainty components, considered applicable to the comparison, are shown in Table 10 for all of the primary standards. The NIST uncertainties were evaluated according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. Ref. [6]. In general, the uncertainties are representative of the uncertainties associated with routine air-kerma measurements at both institutions. The alignment and the charge measurement uncertainties for the NIST chambers result from the NPL alignment and charge collection methods and are different than those for air-kerma measurements conducted at NIST. 7. Results and Conclusions The comparison results are shown in Tables 11 through Table 13 as the ratio of the corrected mass ionization current of the NIST primary standard to that of the NPL primary standard for each reference radiation used in the comparison. The Lamperti chamber response agreed with that of the NPL 50 kV chamber response at the 0.5 % level for the reference radiation qualities produced at 10 kV and at the [+ or -] 0.1 % level for the reference radiation qualities produced between 11.5 kV and 20 kV. The Lamperti and the NPL chambers have good agreement, considering the uncertainty of this comparison. The Ritz chamber also compared favorably fa·vor·a·ble adj. 1. Advantageous; helpful: favorable winds. 2. Encouraging; propitious: a favorable diagnosis. 3. with the NPL 50 kV chamber; agreement was found to be between 0.2 % and 0.6 % for both the tungsten and the molybdenum reference radiation qualities up to 50 kV. The Ritz chamber and the NPL 300 kV chamber compared favorably, 0.1 % and 0.6 %, for 50 kV and 80 kV respectively, despite the alignment difficulties. The Ritz chamber electron loss corrections and the photon scatter scat·ter v. 1. To cause to separate and go in different directions. 2. To separate and go in different directions; disperse. 3. To deflect radiation or particles. n. corrections are currently being reevaluated for new reference radiation qualities being developed at NIST. Slight changes to the corrections used for 80 kV are expected; however the results for this comparison are based on the correction factors available at the time of the comparison. About the authors: Michelle O'Brien and Paul Lamperti are physicists Below is a list of famous physicists. Many of these from the 20th and 21st centuries are found on the list of recipients of the Nobel Prize in physics. A
n. High-energy radiation capable of producing ionization in substances through which it passes. Ionizing radiation Division, Dosimetry dosimetry /do·sim·e·try/ (do-sim´e-tre) scientific determination of amount, rate, and distribution of radiation emitted from a source of ionizing radiation, in biological d. and Interaction Group of the NIST Physics Laboratory. Tudor Williams and Thorsten Sander are physicists in the Centre for Ionising Radiation Metrology metrology Science of measurement. Measuring a quantity means establishing its ratio to another fixed quantity of the same kind, known as the unit of that kind of quantity. at the National Physical Laboratory, United Kingdom. The National Bureau of Standards National Bureau of Standards: see National Institute of Standards and Technology. National Bureau of Standards - National Institute of Standards and Technology and Technology is an agency of the Technology Administration, U.S. Department of Commerce. (1.) Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. 8. References (1.) A. R. S. Marsh and T. T. Williams, 50 kV Primary Standard of Exposure--1978 Design Of Free-Air Chamber, RS (EXT EXT Extension EXT Extended EXT External Ext Extraction EXT Exterior (screenwriting) EXT Extinguisher EXT Extruded EXT Extinguished EXT Exeter, England, United Kingdom - Exeter (Airport Code) )54, National Physical Laboratory Report, Teddington, Middlesex, UK, April 1982. (2.) J. A. Palmer, S. Duane, D. R. Shipley, and C. J. Moretti, The Design and Construction of a New Primary Standard Free Air Chamber for Medium Energy X-rays, Medical and Biological Engineering and Computing computing - computer , 35, 1086 (1997) (3.) P. J. Lamperti and H. O. Wyckoff, NBS (National Bureau of Standards) See NIST. NBS - National Bureau of Standards: part of the US Department of Commerce, now NIST. Free-Air Chamber for Measurement of 10 to 60 kV X Rays, J. Res. Natl. Bur. Stand. (U.S.) 69C, 39-46 (1965). (4.) V. H. Ritz, Design of Free-Air Ionization Chambers for the Soft X-Ray Region (20-100 kV), Radiology radiology, branch of medicine specializing in the use of X rays, gamma rays, radioactive isotopes, and other forms of radiation in the diagnosis and treatment of disease. , 73(6), 911-922 (1959). (5.) International Commission on Radiation Units and Measurements The International Commission on Radiation Units and Measurements (ICRU) is a standardization body set up in 1925 by the International Congress of Radiology. Its objective is to develop internationally acceptable recommendations for quantities and units of radiation and , Average Energy Required to Produce an Ion Pair, ICRU ICRU International Commission on Radiation Units and Measurements ICRU Iceland’s Crisis Response Unit 31, Washington, D.C. (1979). (6.) B. N. Taylor and C. E. Kuyatt, Guidelines guidelines, n.pl a set of standards, criteria, or specifications to be used or followed in the performance of certain tasks. for Evaluating and Expressing the Uncertainty of NIST Measurement Results, 1994 Edition, NIST Technical Note 1297, September 1994.
The NPL reference radiation qualities used
for the comparison
NPL reference Generating Half-value A1 filtration Air kerma
number potential layer rate
(kV) (mm A1) (mm) (mGy [s.sup.-1])
Tungsten anode
2.4.2 10 0.036 0.025 2.1
2.4.3 11.5 0.050 0.050 2.5
2.4.4 14 0.07 0.11 2.2
2.4.5 16 0.10 0.20 2.5
2.4.6 20 0.15 0.30 3.0
2.4.7 24 0.25 0.45 2.2
2.4.8 34 0.35 0.47 3.7
2.4.9 41 0.50 0.56 4.1
2.4.10 44 0.70 0.74 3.7
2.4.11 50 1.00 1.01 3.1
2.2.1 50 1.00 0.75 1.5
RQR6 80 2.9 2.7 1.3
Molybdenum Anode
Entrance 28 0.30 0.03 Mo 0.6
Exit 28 0.62 0.03 Mo 0.02
+ breast phantom
Physical constants used in the determination
of air kerma
Physical Value Relative standard
constant uncertainty
(%)
[[rho].sub.air] [a] 1.293 kg * [m.sup.-3] 0.01
[W.sub.air]/e 33.97 J [C.sup.-1] 0.15
1-[g.sub.air] 1.0000 0.01
(a.)Density of dry air at T = 273.15 K and p = 101 325 Pa.
Dimensions of primary standard
chambers used in the comparison
Aperture diameter (mm) NIST Lamperti NIST Ritz NPL 50kV NPL 300 kV
Air path length (mm) 39.18 127.39 89.2 493
Aperture diameter (mm) 4.9943 10.0017 8.007 10.014
Applied voltage (V) 1500 3000 [a] 1500 3000
Collector length (mm) 10.135 70.03 19.827 100.258
Volume ([mm.sup.3]) 198.55 5502.05 998.5 7896
Plate separation (mm) 40 90 62.5 264
(a.) An applied voltage of 5 kV is routinely used at
NIST for the Ritz chamber.
Correction factors used for the Lamperti
chamber for the NPL comparison
Correction factor Generating potential
(kV)
10 11.5 14 16
Air attenuation [k.sub.a] [a] 1.0721 1.0520 1.0342 1.0244
Electron loss [k.sub.e] 1.000 1.000 1.000 1.000
Field distortion [k.sub.d] 1.000 1.000 1.000 1.000
Humidity [k.sub.h] 0.998 0.998 0.998 0.998
Recombination [k.sub.s] 1.0001 1.0001 1.0001 1.0001
Photon scatter [k.sub.sc] 0.9960 0.9962 0.9963 0.9965
Correction factor Relative standard
uncertainty (%)
20 Type A Type B
Air attenuation [k.sub.a] [a] 1.0169 0.05 0.23
Electron loss [k.sub.e] 1.000 0.1
Field distortion [k.sub.d] 1.000 0.2
Humidity [k.sub.h] 0.998 0.06
Recombination [k.sub.s] 1.0001 0.04
Photon scatter [k.sub.sc] 0.9966 0.2
(a.)These are nominal values Nominal Value The stated value of an issued security that remains fixed, as opposed to its market value, which fluctuates. Notes: When referring to fixed-income securities, the nominal value is also the face value. for T = 293.13 K and p = 101 325 Pa; each measurement is corrected using the air temperature and pressure measured at the collection time.
Correction factors used for the Ritz chamber
fro tungsten reference radiation qualities
used in the NPL comparison
Correction factor Generating potential
(kV)
20 24 34 41
Air attenuation [k.sub.a] [a] 1.0557 1.0371 1.0334 1.0253
Aperture transmission [k.sub.l] 1.0000 1.0000 1.0000 1.0000
Electron loss [k.sub.e] 1.0000 1.0000 1.0000 1.0000
Field distortion [k.sub.d] 1.000 1.000 1.000 1.000
Humidity [k.sub.h] 0.998 0.998 0.998 0.998
Recombination [b] [k.sub.s] 1.0012 1.0010 1.0015 1.0016
Photon scatter [k.sub.sc] 0.9939 0.9944 0.9947 0.9950
Wall transmission [k.sub.p] 1.0000 1.0000 1.0000 1.0000
Correction factor Relative standard
uncertainty (%)
44 50 80 Type A
Air attenuation [k.sub.a] [a] 1.0169 1.0113 1.0054 0.05
Aperture transmission [k.sub.l] 1.0000 1.0000 1.0000
Electron loss [k.sub.e] 1.0000 1.0000 1.0012
Field distortion [k.sub.d] 1.000 1.000 1.000
Humidity [k.sub.h] 0.998 0.998 0.998
Recombination [b] [k.sub.s] 1.0015 1.0007 1.0007 0.04
1.0013 [c]
Photon scatter [k.sub.sc] 0.9953 0.9956 0.9965
Wall transmission [k.sub.p] 1.0000 1.0000 1.0000
Correction factor
Type B
Air attenuation [k.sub.a] [a] 0.23
Aperture transmission [k.sub.l] 0.04
Electron loss [k.sub.e] 0.10
Field distortion [k.sub.d] 0.20
Humidity [k.sub.h] 0.06
Recombination [b] [k.sub.s]
Photon scatter [k.sub.sc] 0.20
Wall transmission [k.sub.p] 0.01
(a.)These are nominal values for T = 293.13 K and p = 101 325 Pa; each measurement is corrected using the air temperature and pressure measured at the collection time. (b.)The recombination recombination, process of "shuffling" of genes by which new combinations can be generated. In recombination through sexual reproduction, the offspring's complete set of genes differs from that of either parent, being rather a combination of genes from both parents. correction was evaluated for an applied potential of 3 kV. (c.)Two values are shown for the recombination correction at 50 kV due to the use of different air kerma rates.
Correction factors used for the Ritz chamber
for molybdenum reference radiation qualities
used in the NPL comparison
Correction factor Generating potential Relative standard
(kV) uncertainty (%)
28 28 Type A
with phantom
Air attenuation [k.sub.a] [a] 1.0268 1.0219 0.05
Electron loss [k.sub.e] 1.0000 1.0000
Field distortion [k.sub.d] 1.000 1.000
Humidity [k.sub.h] 0.998 0.998
Recombination [b] [k.sub.s] 1.0004 1.0002 0.4
Photon scatter [k.sub.sc] 0.9945 0.9952
Correction factor
Type B
Air attenuation [k.sub.a] [a] 0.23
Electron loss [k.sub.e] 0.10
Field distortion [k.sub.d] 0.20
Humidity [k.sub.h] 0.06
Recombination [b] [k.sub.s]
Photon scatter [k.sub.sc] 0.20
(a.)These are nominal values for T = 293.13 K and p = 101 325 Pa; each measurement is corrected using the air temperature and pressure measured at the collection time. (b.)The recombination correction was evaluates for an applied potential of 3 kV.
Correction factors for the NPL 50 kV
chamber for reference radiation qualities
generated between 10 kV adn 20 kV
Correction factor Generating potential
(kV)
10 11.5 14 16
Air attenuation [k.sub.a] [a] 1.1749 1.1224 1.0804 1.0570
Electron loss [k.sub.e] 1.0000 1.0000 1.0000 1.0000
Field distortion [k.sub.d] 1.0002 1.0002 1.0002 1.0002
Humidity [k.sub.h] 0.998 0.998 0.998 0.998
Recombination [k.sub.s] 1.0004 1.0005 1.0005 1.0005
Photon scatter [k.sub.sc] 0.9949 0.9954 0.9959 0.9962
Polarity effect 1.0004 1.0004 1.0004 1.0004
Correction factor Relative standard uncertainty
(%)
20 Type A Type B
Air attenuation [k.sub.a] [a] 1.0398 0.05 0.23
Electron loss [k.sub.e] 1.0000 0.006
Field distortion [k.sub.d] 1.0002 0.012
Humidity [k.sub.h] 0.998 0.058
Recombination [k.sub.s] 1.0008 0.029
Photon scatter [k.sub.sc] 0.9967 0.115
Polarity effect 1.0004 0.023
(a.)These are nominal values for T = 293.13 K and p = 101 325 Pa; each measurement is corrected using the air temperature and pressure measured at the collection time.
Correction factors for the NPL 50 kV
chamber for reference radiation qualities
generated between 24 kV and 50 kV
Correction factor Generating potential
(kV)
24 34 41 44
Air attenuation [k.sub.a] [a] 1.0262 1.0233 1.0169 1.0121
Electron loss [k.sub.e] 1.0000 1.0000 1.0000 1.0000
Field distortion [k.sub.d] 1.0002 1.0002 1.0002 1.0002
Humidity [k.sub.h] 0.998 0.998 0.998 0.998
Recombination [k.sub.s] 1.0007 1.0012 1.0014 1.0013
Photon scatter [k.sub.sc] 0.9971 0.9973 0.9975 0.9977
Polarity effect 1.0004 1.0004 1.0004 1.0004
Correction factor
50 Mo 28 Mo 28
exit
Air attenuation [k.sub.a] [a] 1.0083 1.0231 1.0132
Electron loss [k.sub.e] 1.0000 1.0000 1.0000
Field distortion [k.sub.d] 1.0002 1.0002 1.0002
Humidity [k.sub.h] 0.998 0.998 0.998
Recombination [k.sub.s] 1.0011 1.0000 1.0000
Photon scatter [k.sub.sc] 0.9979 0.9972 0.9977
Polarity effect 1.0004 1.0004 1.0004
Correction factor Relative standard uncertainty
(%)
Type A Type B
Air attenuation [k.sub.a] [a] 0.05 0.23
Electron loss [k.sub.e] 0.006
Field distortion [k.sub.d] 0.012
Humidity [k.sub.h] 0.058
Recombination [k.sub.s] 0.029
Photon scatter [k.sub.sc] 0.115
Polarity effect 0.023
(a.)These are nominal values for T = 293.13 K and p = 101 325 Pa; each measurement is corrected using the air temperature and pressure measured at the collection time.
Correction factors for the NPL 300 kV
chamber for reference radiation qualities
generated at 50 kV and 80 kV
Correction factor Generating potential
(kV)
50 80
Air attenuation [k.sub.a] [a] 1.0401 1.0202
Electron loss [k.sub.e] 1.0000 1.0000
Field distortion [k.sub.d] 1.0003 1.0003
Humidity [k.sub.h] 0.998 0.998
Recombination [k.sub.s] 1.0004 1.0004
Photon scatter [k.sub.sc] 0.9908 0.9926
Polarity effect 1.0004 1.0004
Correction factor Relative standard uncertainty
(%)
Type A Type B
Air attenuation [k.sub.a] [a] 0.20 0.23
Electron loss [k.sub.e] 0.006
Field distortion [k.sub.d] 0.012
Humidity [k.sub.h] 0.058
Recombination [k.sub.s] 0.029
Photon scatter [k.sub.sc] 0.115
Polarity effect 0.023
(a.)These are nominal values for T = 293.13 K and p = 101 325 Pa; each measurement is corrected using the air temperature and pressure measured at the collection time.
Applicable relative standard uncertainties for the indicated
chamber for the 1998 NPL-NIST comparison, in percent
Source of uncertainty NIST Lamperti NIST Ritz NPL 50 kV
Combined relative standard Type A Type B Type A Type B Type A
Ionization current 0.100 0.017 0.100 0.017 0.100
Volume 0.04 0.01 0.04 0.01
Positioning 0.10 0.10
Correction factors 0.064 0.326 0.064 0.385 0.05
(excluding [k.sub.h])
Humidity [k.sub.h] 0.06 0.06
Physical constants 0.15 0.15
Quadratic sum 0.13 0.38 0.13 0.43 0.11
Combined relative standard 0.40 0.45 0.38
uncertainty for the
measurement of uncertainty
Source of uncertainty NPL 300 kV
Combined relative standard Type B Type A Type B
Ionization current 0.017 0.100 0.017
Volume 0.155 0.155
Positioning 0.10
Correction factors 0.267 0.2 0.238
(excluding [k.sub.h])
Humidity [k.sub.h] 0.06 0.06
Physical constants 0.15 0.15
Quadratic sum 0.36 0.22 0.33
Combined relative standard 0.40
uncertainty for the
measurement of uncertainty
Comparison of Lamperti chamber to the 50kV NPL standard
NPL reference Generatin42,0
g potential Half-value layer Ratio of the NIST
number (kV) (mm A1) to the NPL standard
chamber response
2.4.2 10 0.036 0.9951
2.4.3 11.5 0.05 1.0006
2.4.4 14 0.07 0.9996
2.4.5 16 0.1 0.9995
2.4.6 20 0.15 0.9992
Comparison of the Ritz chamberto the NPL 50 kV standard
NPL reference Generating Half-value layer Ratio of the NIST
number potential (mm Al) to the NPL standard
(kV) chamber response
2.4.6 20 0.15 0.9977
2.4.7 24 0.25 0.9978
2.4.8 34 0.35 0.9989
2.4.9 41 0.5 0.9973
2.4.10 44 0.7 0.9983
2.4.11 50 1.0 0.9983
Mo 28 28 0.30 0.9958
Mo 28 Exit 28 0.62 0.9937
Comparison of the Ritz chamber
to the NPL 50 kV Standard
NPL reference Generating Half-value layer Ratio of the NIST
number potential (mm Al) to the NPL standard
(kV) chamber response
2.2.1 50 1.0 0.9987
RQR6 80 2.9 0.9941
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