Radiological protection - X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy - Part 1: Radiation characteristics and production methods
|Publication Date:||1 January 2019|
|ICS Code (Radiation measurements):||17.240|
This document specifies the characteristics and production methods of X and gamma reference radiation for calibrating protection-level dosemeters and doserate meters with respect to the phantom related operational quantities of the International Commission on Radiation Units and Measurements (ICRU). The lowest air kerma rate for which this standard is applicable is 1 μGy h-1. Below this air kerma rate the (natural) background radiation needs special consideration and this is not included in this document.
For the radiation qualities specified in Clauses 4 to 6, sufficient published information is available to specify the requirements for all relevant parameters of the matched or characterized reference fields in order to achieve the targeted overall uncertainty (k = 2) of about 6 % to 10 % for the phantom related operational quantities. The X ray radiation fields described in the informative Annexes A to C are not designated as reference X-radiation fields.
NOTE The first edition of ISO 4037-1, issued in 1996, included some additional radiation qualities for which such published information is not available. These are fluorescent radiations, the gamma radiation of the radionuclide 241Am, S-Am, and the high energy photon radiations R-Ti and R-Ni, which have been removed from the main part of this document. The most widely used radiations, the fluorescent radiations and the gamma radiation of the radionuclide 241Am, S-Am, are included nearly unchanged in the informative Annexes A and B. The informative Annex C gives additional X radiation fields, which are specified by the quality index.
The methods for producing a group of reference radiations for a particular photon-energy range are described in Clauses 4 to 6, which define the characteristics of these radiations. The three groups of reference radiation are:
a) in the energy range from about 8 keV to 330 keV, continuous filtered X radiation;
b) in the energy range 600 keV to 1,3 MeV, gamma radiation emitted by radionuclides;
c) in the energy range 4 MeV to 9 MeV, photon radiation produced by accelerators.
The reference radiation field most suitable for the intended application can be selected from Table 1, which gives an overview of all reference radiation qualities specified in Clauses 4 to 6. It does not include the radiations specified in the Annexes A, B and C.
The requirements and methods given in Clauses 4 to 6 are targeted at an overall uncertainty (k = 2) of the dose(rate) value of about 6 % to 10 % for the phantom related operational quantities in the reference fields. To achieve this, two production methods are proposed:
The first one is to produce "matched reference fields", whose properties are sufficiently wellcharacterized so as to allow the use of the conversion coefficients recommended in ISO 4037-3. The existence of only a small difference in the spectral distribution of the "matched reference field" compared to the nominal reference field is validated by procedures, which are given and described in detail in ISO 4037-2. For matched reference radiation fields, recommended conversion coefficients are given in ISO 4037-3 only for specified distances between source and dosemeter, e.g., 1,0 m and 2,5 m.
For other distances, the user has to decide if these conversion coefficients can be used. If both values are very similar, e.g., differ only by 2 % or less, then a linear interpolation may be used.
The second method is to produce "characterized reference fields". Either this is done by determining the conversion coefficients using spectrometry, or the required value is measured directly using secondary standard dosimeters. This method applies to any radiation quality, for any measuring quantity and, if applicable, for any phantom and angle of radiation incidence. In addition, the requirements on the parameters specifying the reference radiations depend on the definition depth in the phantom, i.e., 0,07 mm, 3 mm and 10 mm, therefore, the requirements are different for the different depths. Thus, a given radiation field can be a "matched reference field" for the depth of 0,07 mm but not for the depth of 10 mm, for which it can then be a "characterized reference field". The conversion coefficients can be determined for any distance, provided the air kerma rate is not below 1 μGy/h.
Both methods need charged particle equilibrium for the reference field. However, this is not always established in the workplace field for which the dosemeter is calibrated. This is especially true at photon energies without inherent charged particle equilibrium at the reference depth d, which depends on the actual combination of energy and reference depth d. Electrons of energies above 65 keV, 0,75 MeV and 2,1 MeV can just penetrate 0,07 mm, 3 mm and 10 mm of ICRU tissue, respectively, and the radiation qualities with photon energies above these values are considered as radiation qualities without inherent charged particle equilibrium for the quantities defined at these depths.
To determine the dose(rate) value and the associated overall uncertainty of it, a calibration of all measuring instruments used for the determination of the quantity value is needed which is traceable to national standards.
This document does not specify pulsed reference radiation fields.