. tee te wg pa eee ele dale foie dD ee, Ali htDad atin eiellbaa 36 NATURAL THORIUM IN HUMAN BONE H. F. Lucas, Jr., D. N. Edgington, and Frank Markun The *”Th concentration in rib bone from 38 humans ranged from <0.1 to 72 ng/g ash. Values increased with age and were fitted by the expression Y = (0.16 + 0.02)7, where Y is the ’Th concentration in ng/g bone ash and T is the age in years. Evaluation of retention of the “Ra produced by decay of *“Th is not feasible because direet intake of naturally-occurring **Rais too large. INTRODUCTION The measurement of thorium in normal, non-in- dustrially exposed humans is required for the evaluation of its radiological hazard. Studies with dogs have shown that the ***Ra daughter of *“8Th is es- sentially completely retained by bone. If one as- sumes that 228Ra produced by the decay of 232Th is similarly retained, then the radiation dose to the bone would be higher than that previously assumed. In addition, the metabolism of thorium under natural environmental conditions is needed for the evaluation of its toxicity. Edgington has reviewed present knowledge and finds many questions unanswered.’) The reactor for 24 hr at a flux of about 2 x 10* sec, and then allowed to cool for about 7 days. The samples and standards were washe the quartz irradiation tubes with about 9 ml HCl. Less than 1% of the *38Pa activation remained in the vial. Four sets of duplicate t standards were run, and the average deviatio the mean of each set ranged from 2.7 to 4.7% \ average of 3.6%. The reproducibility of the stz was acceptable and indicates that the neutr was quite uniform. The bone samples were dissolved in 10 ml HC! te which several drops of 30% hydrogen ide and 0.1g aluminum nitrate were added. reagents reduce chromium to its trivalent sta complex any fluoride ion present in the sampl: solution was heated for several minutes in a water bath to insure complete dissolution of th: ple. The sample was then transferred to a colt Dowex 1-X8 anion exchange resin (100 mesh), in diameter by 35 cm long. The2°*Pa activation uct of 73°Th is quantitatively retained on the c purpose of this study was to determine the concentration of thorium in samples of human bone taken under these conditions. EXPERIMENTAL METHOD were collected directly in counting tubes. Ap mately 90% of the ?33Pa was obtained in the s. at surgery or autopsy. The samples were obtained through the interest and courtesy of many physicians as part of a survey of the natural radium content of human bone. All samples selected for this study were rib bone and were obtained at the time of either surgery or autopsy. The residence, age, sex, and occupation of each patient was recorded in almost all cases. On receipt at Argonne National Laboratory, these samples had been scraped to remove soft tissue, dried at 110° C, and stored at —10° C until prepared for thorium assay. All samples were ashed for 16 hr at 600°C in quartz crucibles. Weighed aliquots were transferred to 10-mm OD xX 10-cm-long quartz tubes.* These Cations and weakly bound anions are elutec 90m] 9’4f HC!L.® The 733Pa was then elutec four 10-ml1 portions of 4M HCl 03M HF, and third fractions. The amount of *3%Pa in bot samples and the standards was determined by c ing with a 4” x 2” Nal (TI) crystal, an ND-120 tichannel analyzer, and an iterative least sq computational method. 5) This method has found to be very sensitive and is not affected b small amounts of other radionuclides present 1 sample.**) The amount and concentration determined from the ratio of decay-corrected cc ing rates of °°%Pa in the sample to that in the st ards. The standard deviation of the result is d mined by the standard method from the cow tubes were sealed with a crushed aluminum foil plug. Thorium standards (5.5 4g) were aliquoted into the quartz tubes, evaporated to dryness under a heat lamp, and sealed in the same manner as the bone samples. Four bone samples plus standards were com- statistics and the deviation between duplicate st: ards. * “Spectrasil,” Thermal American Fused Quartz Co., Montville, New Jersey. est value was observed in bone from an indivi: bined in a single irradiation can. All samples were irradiated in the isotope tray of the Argonne CP-5 DISCUSSION The 7°°Th concentration in bone from 38 indi uals is summarized in Table 22. The concentra ranges from <0.1 to 72 ng/g ash. However, the h who had a 40-year occupational exposure to th