The calculated value of the effective atomic number ( Z T ) of 7.23 for lithium tetraborate (Li,B,O,) agrees closely with the value of 7.22 for soft tissue of composition C,H,,O,,N.For excised muscle specimens the experimental value (7.12) obtained by Spiers for 2, is very nearly equal to the corresponding calculated value (7.46) for the composition (ICRU) of striated muscle. d similar comparison shows that the composition for bone given by Woodard with a higher content of calcium and phosphorus gave a value of ZT = 13.3, which is in closer agreement with the measured value of 13.8 obtained by Spiers. The calculated values of Z, and kerma per roentgen for lithium tetraborate and lithium fluoride for photon energies below 1 MeV show that lithium borate (Li,B,O,) is B better substitute for soft tissue than lithium fluoride.
Calculations of response to mono-energetic photons show that lithium borates containing 0, 0.1%, 0.250/, and 0.340,6 of Mn are equivalent to soft tissue (C,H,,O,, N), water, ICRU muscle, and air respectively. Results obtained with monochromatic and broad-spectrum x-rays of effective energies between 12 and l15 kev are in agreement with this finding. The monochromatic and broad-spectrum x-ray beams were also used to explore the discrepancies in response of LiF TLD-100 to low energy photons. Only small discrepancies were found which were attributed either to breadth of x-ray spectra or to additional absorbed dose due to scattered x-rays amounting to about 504 at 40 kevetf.
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