11.1 Development of a Specially Shaped Detector with Human Response Characteristics, for Measuring the Effective Dose for External Photon Exposures


Fig. 11-1 Directional sensitivities of the NaI(Tl) crystals with various shapes

The directional response changes with the ratio of detector diameter and length. The optimum shape, showing similar directional characteristics with human body, is determined.


Fig. 11-2 The NaI(Tl) detector developed to have a similar angular sensitivity with the human body

The dimensions of the crystal are 75 mm in diameter and 210 mm in length. A scale of 30 cm is shown above the detector.


Fig. 11-3 Comparison of angular dependence with the human body and newly developed NaI(Tl) detector

At two photon energies of 124 keV and 10 MeV the relative angular dependences of the detector, and the doses HE and E are shown. Each response is normalized by the response at 90 degrees. As can be seen in the figure, the angular responses of the detector agree well with those of the effective dose equivalent HE. For photon energies ranging between 124 keV and 10 MeV good response characteristics were obtained as shown in the figure.





The absorbed dose of human organs and tissues changes according to the irradiation conditions. In addition, their radiation sensitivities are not same. So, it was very difficult to evaluate the radiation risk between different tissues and organs distributed in the body using a common scale. Under these circumstances, the concept of "effective dose, (E)" or "effective dose equivalent, (HE)" was introduced from the viewpoint of comparing radiation risk under a common criterion. These quantities are constructed to be proportional to the stochastic effects arising from radiation exposure to the human body and can be conveniently used for the practical evaluation of radiation risk, as a comparative index, in the field of radiation protection. It is difficult to directly evaluate these qualities from measurements because they are calculated from the specific dose absorbed in every tissue and organ and depend not only on the energy distribution but also on the angular distribution of the gamma ray field. So the quantities are usually calculated by using a conversion factor from the absorbed dose in air, which can be measured directly by a dose measuring instrument. Conversion factors, however, vary greatly according to the radiation energy and angular distribution incident to the human body. Therefore, it is generally very difficult to obtain accurate conversion factors for E or HE, because of the lack of precise information on the radiation field, such as the energy and directional distribution of the incident photon at the measuring point. JAERI has carried out the development of a radiation detector with the same energy and angular response characteristics as the human body, and a NaI(Tl) scintillation detector with a novel shape has been developed to realize direct measurement of E or HE for external photon exposures. This work solved the problem concerning the angular dependence, by considering a detector shape that satisfies the angular response similar to the human body. The optimum dimensions of the NaI(Tl) crystal for HE and E were determined respectively by a Monte Carlo simulation. The detector subject to HE was manufactured and tested. The angular characteristics of the developed detector indicate a fairly good coincidence with those of the theoretical values, HE. Together with an energy-dose conversion method, the so called "G(E) function method," it was possible to directly measure the effective dose (equivalent).



Reference
M. Tsutsumi et al., Development of a Detector for Measuring Effective Dose (Equivalent) for External Photon Exposures in Natural Environment, J. Nucl. Sci. Technol., 37(3), 300 (2000).

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