ISO 6980-2:2023 核エネルギー | ページ 6

3.1

extrapolation curve

curve given by a plot of the corrected ionization current versus the extrapolation chamber depth

3.2

ionization chamber

ionizing radiation detector consisting of a chamber filled with a suitable gas (almost always air), in which an electric field, insufficient to induce gas multiplication, is provided for the collection at the electrodes of charges associated with the ions and electrons produced in the measuring volume of the detector by ionizing radiation

Note 1 to entry: The ionization chamber includes the measuring volume, the collecting and polarizing electrodes, the guard electrode, if any, the chamber wall, the parts of the insulator adjacent to the sensitive volume and any additional material placed in front of the ionization chamber to simulate measurement at depth.

3.3

extrapolation (ionization) chamber

ionization chamber (3.2) capable of having an ionization volume which is continuously variable to a vanishingly small value by changing the separation of the electrodes and which allows the user to extrapolate the measured ionization density to zero collecting volume

3.4

ionization density

measured ionization per unit volume of air

3.5

leakage current

Ι_B

ionization chamber (3.2) current measured at the operating bias voltage in the absence of radiation

3.6

maximum beta energy

E_max

highest value of the energy of beta particles emitted by a particular radionuclide which can emit one or several continuous spectra of beta particles with different maximum energies

3.7

mean beta energy

E_mean

fluence averaged energy of the beta particle spectrum at the calibration distance free in air

3.8

parasitic current

Ι_p

negative current produced by beta particles stopped in the collecting portion of the collecting electrode and diffusing to this electrode and the wire connecting this electrode to the electrometer connector

3.9

phantom

artefact constructed to simulate the scattering properties of the human body or parts of the human body such as the extremities

Note 1 to entry: A phantom can be used for the definition of a quantity and made of artificial material, e.g. ICRU tissue, or for the calibration and then be made of physically existing material, see ISO 29661:2012, 6.6.2, for details.

Note 2 to entry: In principle, the ISO water slab phantom, the ISO rod phantom, the ISO water cylinder phantom, or the ISO pillar phantom should be used, see ISO 29661. For the purposes of this document, however, a polymethyl methacrylate (PMMA) slab, 20 cm × 20 cm in cross‑sectional area by at least 2 cm thickness, is sufficient to simulate the backscatter properties of the trunk of the human body, while tissue substitutes such as polyethylene terephthalate (PET) are sufficient to simulate the attenuation properties of human tissue (see 6.2).

[SOURCE:ISO 29661:2012, 3.1.22, modified — Note 2 to entry added.]

3.10

reference point of the extrapolation chamber

point to which the measurement of the distance from the radiation source to the chamber at a given orientation refers, i.e., the centre of the back surface of the high‑voltage electrode of the chamber

3.11

reference absorbed dose

D_R

absorbed dose to tissue, D_t(0,07), in a slab phantom (3.9) made of ICRU 4‑element tissue with an orientation of the phantom (3.9) in which the normal to the phantom (3.9) surface coincides with the (mean) direction of the incident radiation

Note 1 to entry: The absorbed dose to tissue, D_t(0,07), is defined in ICRU 51^[4] as personal absorbed dose, D_p(0,07). For the purposes of this document, this definition is extended to a slab phantom.

Note 2 to entry: It is considered that the rear part of the extrapolation chamber approximates a slab phantom with sufficient accuracy by the material surrounding the standard instrument (extrapolation chamber) used for the measurement of the beta radiation field^[7][8].

Note 3 to entry:H_p(0,07) is obtained by the multiplication of the absorbed dose to tissue at 0,07 mm depth, D_t(0,07) = D_R, with the conversion coefficient 1 Sv Gy^‑1, see ISO 6980-3:2023, 5.2.2.2, Formula (3).

3.12

reference beta‑particle absorbed dose

D_Rβ

reference absorbed dose, D_R , (3.11) at a depth of 0,07 mm only due to beta particles

Note 1 to entry: As a first approximation, the ratio D_Rβ/D_R is given by the correction factor for bremsstrahlung, k_br, and other photons (see C.3).

3.13

tissue equivalence

property of a material which approximates the radiation attenuation and scattering properties of ICRU tissue

Note 1 to entry: See ISO 6980-1:2023, Annex A; more tissue substitutes are given by ICRU 44.

Note 2 to entry: Further details are given in 6.2.

3.14

transmission function

T_m(ρ_m · d_m; α)

ratio of absorbed dose, D_m(ρ_m·d_m; α), in medium m at an area depth, ρ_m·d_m, and angle of radiation incidence, α, to absorbed dose, D_m(0; 0°), at the surface of a phantom (3.9)

3.15

tissue transmission function,

T_t(ρ_t · d_t; α)

ratio of absorbed dose, D_t(ρ_t·d_t;α), in ICRU tissue at an area depth, ρ_t·d_t, and angle of radiation incidence, α, to absorbed dose, D_t(0; 0°), at the surface of an ICRU tissue slab phantom (3.9)

3.16

zero point

reading of the extrapolation chamber depth indicator which corresponds to a chamber depth of zero, or no separation of the electrodes