高通量试验堆(HFETR)辐照40Ar-39Ar定年样品条件初探

国内目前用于40Ar-39Ar定年样品辐照的反应堆较少且开堆频率低，样品辐照周期长，且不同类型样品辐照条件缺乏系统研究.首次对高通量试验堆（HFETR）用于40Ar-39Ar定年样品的辐照工作进行了研究.通过辐照一定质量的黑云母标准物质ZBH-25，确定了辐照孔道39Ar_K产率，为不同年龄和不同含钾量样品的辐照时间提供了参考依据.辐照孔道轴向中子通量梯度仅为3.3%/cm，且均呈二次曲线特征分布（R2>0.99），中子通量在径向上存在差异，最大差异达到7.1%/cm；通过辐照纯的钾盐和钙盐，得出辐照孔道内校正因子（36Ar/37Ar）_Ca的值为（3.52±0.11）×10-4，且在样品辐照罐内不同位置基本一致，而校正因子（40Ar/39Ar）_K和（39Ar/37Ar）_Ca的值在辐照样品罐内存在明显的差异；Cd皮对于降低校正因子（40Ar/39Ar）_K的影响在样品罐底部更加明显，而在样品罐顶部几乎没有影响，这可能是由辐照孔道内中子能谱的差异造成的；以FCs透长石为中子通量监测物质，对标准物质ZBH-25黑云母和BSP-1角闪石进行了年龄测定，ZBH-25黑云母获得了理想的坪年龄，证明此反应堆满足40Ar-39Ar定年样品辐照的要求；BSP-1角闪石的坪年龄准确度和精度较差，与样品本身异常老的年龄和较低的K/Ca比有关，对于此类样品，准确确定干扰校正因子并合理延长照射时间对获得高精度40Ar-39Ar定年结果非常重要. 

Primary Research of High Flux Engineering Test Reactor(HFETR) for Irradiation of 40Ar-39Ar Dating Samples

The domestic reactors which could be utilized for the irradiation of 40Ar-39Ar dating samples are not only rare but the operation frequency is very low, which makes the period of the irradiation for samples extremely long.Besides, the optimized parameters of irradiation for different kinds of samples are short of systematic research.To solve the above-mentioned problems, primary research was done on the HFETR in the application of the irradiation of 40Ar-39Ar dating samples for the first time.Through the irradiation of certain amount of biotite standard ZBH-25, the production efficiency of 39Ar_K is determined, which could provide the basis for the irradiation time for samples with different ages or potassium contents.The axial neutron flux gradient of the irradiation channel, which shows approximative conic function (R2>0.99), is only 3.3%/cm.However, the radial flux gradient is much more obvious, which is as high as 7.1%/cm.The interference factors are determined through the irradiation of pure potassium salt and calcium salt.It is found that the (36Ar/37Ar)_Ca factor is uniform with the value (3.52±0.11)×10-4, but the interference factors (40Ar/39Ar)_K and (36Ar/37Ar)_Ca are scattered at different positions of the irradiation channel.The shielding effect of cadmium for the decrease of the interference factor of (40Ar/39Ar)_K is prominent at the bottom of the sample vessel but negligible at the top of the sample vessel, which may result from the difference of the neutron spectrum along the axial direction of the irradiation channel.Taking the international standard sanidine FCs as neutron flux monitor, the domestic standards biotite ZBH-25 and hornblende BSP-1 are dated.Excellent plateau age is obtained for the ZBH-25 biotite standard, indicating that the HFETR could satisfy the requirement of the sample irradiation.However, the precision and accuracy of the plateau age of the BSP-1 hornblende is somewhat worse, which maybe has something to do with the relatively old age and low ratio of the K/Ca.Precise determination of the interference factors and proloning of the irradiation time are necessary for the improvement of the age quality for such kind of sample.