Isbell Horace S, Frush Harriet L, Holt Nancy B
J Res Natl Bur Stand A Phys Chem. 1960 Jul-Aug;64A(4):363-367. doi: 10.6028/jres.064A.038. Epub 1960 Aug 1.
A convenient procedure is described for the radioassay of both carbon-14 and tritium in water-soluble, nonvolatile compounds by means of a windowless, gas-flow, proportional counter. The materials are counted in uniform films of sodium -(carboxymethyl) cellulose that are "infinitely thick" to the radiation of tritium but not to the radiation of carbon-14. Films of uniform thickness are obtained by new techniques which are described in detail. If only carbon-14 is present, its absolute activity can be calculated conveniently by means of an empirically established curve for the counting-efficiency. If both carbon-14 and tritium are present, the films are counted in the proportional counter and are then recounted in the presence of a screen that stops all radiation from tritium but only a portion of that from carbon-14. From a film with a thickness of 0.8 mg/cm, approximately 43 percent of the radiation of carbon-14 is counted. Of this emerging radiation, approximately 50 percent passes through a screen of ¼-mil double-aluminized "Mylar." By use of suitable calibration curves for counting-efficiency, carbon-14 and tritium in the same sample can be calculated from the counts with, and without, the screen. Satisfactory analyses can be made with samples containing less than 0.001 microcurie of carbon-14 and 0.005 microcurie of tritium. The method is suitable for the radioassay of a wide variety of labeled materials.
本文描述了一种简便的方法，用于通过无窗气流正比计数器对水溶性非挥发性化合物中的碳 - 14和氚进行放射性测定。将材料制成均匀的羧甲基纤维素钠薄膜进行计数，这种薄膜对氚的辐射是“无限厚”的，但对碳 - 14的辐射不是。通过详细描述的新技术可获得厚度均匀的薄膜。如果仅存在碳 - 14，其绝对活度可通过根据经验建立的计数效率曲线方便地计算得出。如果同时存在碳 - 14和氚，则先在正比计数器中对薄膜进行计数，然后在有能阻挡所有氚辐射但仅阻挡部分碳 - 14辐射的屏蔽物存在的情况下再次计数。对于厚度为0.8mg/cm的薄膜，大约43%的碳 - 14辐射被计数。在这种出射辐射中，大约50%能穿过1/4密耳双铝化“聚酯薄膜”屏蔽物。通过使用合适的计数效率校准曲线，可根据有无屏蔽物时的计数计算出同一样品中碳 - 14和氚的含量。对于含碳 - 14小于0.001微居里和含氚小于0.005微居里的样品，也能进行令人满意的分析。该方法适用于多种标记材料的放射性测定。
