Dose and Photon-energy Measurements for Bone Marrow in a Human Phantom by Thermoluminescence

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Abstract
The shape of the curve of dose to bone marrow against photon energy for a human phantom exposed to rotational whole body X-radiation has been in some doubt. A monotonic curve has been measured by chemical dosimetry. A hump is seen at around 100 keV effective on the curve measured by the thermoluminescence method. This hump could possibly be artificial if there were sufficient energy degradation inside the phantom to shift the response of the dosimeter system. The author has remeasured this curve and confirmed its general shape. The amount of energy degradation was investigated by depth-energy curves, obtained with a combined thermoluminescence system (LiF + CaSO4: Sm). The depth-energy curve shows two mechanisms-a softening of the incident radiation in the soft tissue, followed by a hardening in bone. The mean energy for dose to bone marrow was shown to be 120 keV effective for 168-keV effective incident, and 77 keV for 100-keV incident. This degradation was insufficient to create an artificial hump. Energy effects inside water phantoms are discussed, together with mechanisms operating on the magnitude of the absorbed dose (particularly the effect of the size of irradiated area, and the effect of bandwidth). These produce a satisfactory qualitative explanation of the shapes of the depth-energy curves and of the dose to bone-marrow curve.