论文部分内容阅读
现已开发了一个定量评估α粒子中止距离对(U-Th)/He 年龄影响的数学模型。在~(238)U、~(235)U 和~(232)Th 链中,α粒子中止距离在约10～30μm 之间变化,它取决于衰变能和中止介质的密度与组成。在富 U 和 Th 的副矿物情况中(例如磷灰石、锆石、金红石),长中止距离的主要影响是α粒子发射到邻近矿物。对小于几百微米的颗粒,发射效应将造成测定的 He 年龄大大低于真实年龄。例如,100μm 半径的球体仅保留其约82%的α粒子。若母体核素为均匀分布,则发射的α粒子的比例是晶体表面积与体积之比乘以平均α粒子约1/4的行程,而与几何尺寸无关。在定年之前,晶体外层20μm 的剥离消除了经历过α粒子丢失区域,但当晶体依 U 和 Th 被严重分带时,这可能会导致错误的年龄。通过从一个单样中分离出4种筛选的磷灰石,既使发射超过总衰变的35%,α粒子发射也可以准确地校正(U-Th)/He 年龄。其结果对确定最适合(U-Th)/He 定年的颗粒尺寸和外形非常有用,并为在发射效应明显时校正年龄提供了基础。有意义的(U-Th)/He 年龄需要大的晶体,或者必须对测定的年龄进行α粒子发射的校正。
A mathematical model has been developed to quantitatively assess the influence of the alpha particle stop distance (U-Th) / He age. In ~ (238) U, ~ (235) U and ~ (232) Th chains, the stopping distance of α particles varies from about 10 to 30 μm depending on the decay energy and the density and composition of the stopping medium. In the case of secondary minerals rich in U and Th (eg apatite, zircon, rutile), the main effect of the long stop distance is the emission of alpha particles to neighboring minerals. For particles smaller than a few hundred micrometers, the emission effect will cause the measured He age to be much lower than the true age. For example, a sphere of 100 [mu] m radius retains only about 82% of its alpha particles. If the parent nuclide is uniformly distributed, the proportion of alpha particles emitted is the ratio of the crystal surface area to volume times the average alpha particle by about a quarter of a stroke, regardless of geometry. Prior to dating, exfoliation of 20 μm in the outer layer of the crystal eliminates experiencing areas of alpha particle loss, but this can lead to erroneous ages when the crystals are heavily zoned by U and Th. By separating the four selected apatites from a single sample, the alpha particle emission can be accurately corrected (U-Th) / He age even if the emission exceeds 35% of the total decay. The results are very useful for determining the particle size and shape for the U-Th / He dating and provide the basis for age correction when the emission effect is significant. Significant crystals are required for U-Th / He age, or alpha particle emission correction must be performed for the measured age.