Measurement of H*(10) and Hp(10) in Mixed High-Energy Electron and Photon Fields

The physical basis for the measurement of Hp(10) and H*(10) in mixed high-energy electron and photon fields was developed for the general case of non-existing secondary charged particle equilibrium at the reference depth of 10 mm. Several mixed fields, with different electron fluence distributions were produced and the corresponding depth ionisation curves were measured in a Makrolon phantom. By means of Monte Carlo simulations, conversion coefficients to directional or personal dose equivalent were calculated for three mixed fields. By variation of the electron fluence impinging on the phantom surface with respect to the photon fluence, the values of Hp(10) and H*(10) were varied by a factor of up to 1,9, in an otherwise unchanged photon field. Introduction The operational quantities personal dose equivalent, Hp(10), and directional dose equivalent, H*(10), are defined for purposes of routine individual and area monitoring (1) for strongly penetrating radiation. The personal dose equivalent, Hp(10), is the dose equivalent in soft tissue below a specified point on the body at a depth of 10 mm. For calibration purposes, Hp(10) is considered as the dose equivalent in a 30 cm × 30 cm × 15 cm phantom, made of ICRU tissue-equivalent material (2). ICRU tissue-equivalent material has a mass composition of 76,2 % oxygen, 11,1 % carbon, 10,1 % hydrogen and 2,6 % nitrogen and a density of 1 g/cm. The directional dose equivalent, H*(10), at a particular point in a radiation field, is the dose equivalent that would be produced in the corresponding “aligned and expanded field”, at a depth of 10 mm in the ICRU sphere. The "aligned field" has the same spectric fluence as the actual field while being mono-directional. The "expanded field" is a hypothetical field, homogeneous over the volume of interest, possessing the spectric fluence of the actual field at the point of measurement. The phantom defined as the ICRU sphere is a 30 cm diameter sphere of ICRU tissue-equivalent material. At present, the calibration of individual and area dosemeters in photon fields is performed by measuring the collision air kerma free in air under conditions of secondary electron equilibrium. Then the appropriate operational quantity is derived applying a conversion coefficient that relates the air kerma to the appropriate operational quantity. The conversion coefficients for photon radiation published by ICRU (3), ICRP (4) and ISO (5) pertain to conditions of secondary electron equilibrium. A problem arises in photon fields with photon energies above 2 MeV, for which the maximum range of secondary electrons in tissue-equivalent material is larger than the reference depth of 10 mm. Thus, at the reference depth, the condition of secondary electron equilibrium is not fulfilled. It has been shown (6,7) that in 6-7 MeV photon fields, as they occur in nuclear power reactor facilities, the secondary electron equilibrium is achieved only at a depth corresponding to about 3 g/cm or more in tissue-equivalent material. Moreover, depending on the properties of the radiation source and on the geometric arrangement, the depth dose curve can rise or fall with increasing depth. The reason for this behaviour is the electron contamination of the photon beam, which is an integral part of any high-energy photon source (6,8). A depth dose curve increasing with increasing depth means that the electron fluence is smaller than the one under conditions of equilibrium and vice versa. This implies that the values of Hp(10) and H*(10) depend not only on the properties of the photon, but also on those of the electron field impinging on the phantom. They can differ significantly from those that would be obtained under conditions of equilibrium. The methods and data currently available for the calibration of instruments in terms of these operational quantities cannot be applied to mixed fields. A procedure for determining the operational quantities was developed for the general case of fields in which there is no secondary electron equilibrium at the reference depth of 10 mm. For this purpose, several mixed highenergy electron and photon fields were produced, with different fluence distributions of electrons relative to photons. This provides the possibility of studying the response of radiation protection dosemeters with respect to the operational quantities depending on the different fluence distributions of electrons relative to photons. Description of fields The objective of this paper is to determine, as a first step, the operational quantities under conditions with no electron equilibrium at reference depth. For this purpose a number of reference fields have been established which are characterised by different magnitudes of the electron fluence with respect to the photon fluence. The experiments were carried out in fields produced at PTB by means of a cyclotron and of a Van-deGraaff accelerator. The fields are slightly different from those termed R-C and R-F according to ISO 4037-3 (5). These ISO-fields are at present routinely used for the calibration of radiation protection dosemeters under