Establishment of a community-wide 252Cf source for enhancing production of confined track-lengths for apatite fission track thermochronology

磷灰石裂变径迹（AFT）方法记录了岩石在地质时间尺度上温度低于110℃的冷却过程。由于238U的放射性衰变，裂变径迹以恒定速率形成窄长度分布，随后缩短，最终可能因温度升高而消失。这个过程称为退火。因此，径迹长度分布是晶体热历史的敏感监视器，特别是在约60-110°C之间的温度区间，部分退火区；在明显更高的温度下，退火在地质上是瞬时的。测量大量密闭轨道的长度为热历史提供了一个代理，热历史可以通过热模拟来反转。磷灰石的裂变轨迹只有在用硝酸蚀刻后才能在显微镜下看到。封闭的轨迹完全位于磷灰石晶体内部，只有当酸从表面通过另一条轨迹时才会被蚀刻，因此才会被看到。通常，操作人员试图测量每个样品约100个受限轨道。对于含有大量U的磷灰石晶体，它们在低温下保存了很长地质时间，通常存在足够的轨迹，为逆建模提供可靠的数据集。然而，对于较年轻的样品和低U丰度的颗粒，很少有自发径迹穿过表面，因此有机会与相对较少的受限径迹相交。在这些情况下，使用传统的样品制备技术来测量大量的受限长度是极其困难或不可能的，因此要么排除了热建模，要么极大地限制了这种模型的分辨率。为了增加可测量的受限径迹的数量，可以用252Cf源发射的裂变粒子轰炸样品，这为蚀刻剂穿透磷灰石晶体创造了大量额外的途径，从而允许蚀刻许多相交的受限径迹。由于低温热建模是AFT方法的最大优势之一，因此显然需要提高测量受限轨道的能力。因此，我们寻求资金，通过购买252Cf源，在德国建立磷灰石裂变径迹辐照设施。在辐照过程中，还需要建立一个保存裂变径迹样品的装置。这必须得到适当的允许和屏蔽，以免使用者暴露于不必要的放射性剂量。该设施将供德国裂变径迹研究小组使用。

The apatite fission-track (AFT) method records the cooling of rocks below temperatures of ca. 110°C over geologic timescales. Fission-tracks form with a narrow length distribution at a constant rate due to radioactive decay of 238U and are subsequently shortened and may eventually disappear in response to elevated temperatures. This process is termed annealing. As a result, the track length distribution is a sensitive monitor of a crystal's thermal history, particularly in the temperature interval between ca. 60-110°C, the partial annealing zone; at significantly higher temperatures, annealing is geologically instantaneous. Measuring the lengths of a large number of confined tracks provides a proxy for the thermal history, which can be inverted by thermal modeling.Apatite fission tracks are only microscopically visible after etching with nitric acid. Confined tracks lie completely within the apatite crystal and can only be etched and hence seen when there is a pathway for the acid from the surface via another track. Typically, the operator seeks to measure ~100 confined tracks per sample. For apatite crystals with high amounts of U and which have remained at low temperatures for long geologic times, sufficient tracks are usually present to provide a robust dataset for inverse modeling. However, for younger samples and grains with low U abundance, there are very few spontaneous tracks which cross the surface and therefore have a chance to intersect the relatively small number of confined tracks. In these cases, it is extremely difficult or impossible to measure a large number of confined lengths using traditional sample preparation techniques, thus either precluding thermal modeling or drastically limiting the resolution of such models.In order to increase the number of measurable confined tracks, it is possible to bombard the sample with fission particles emitted from a 252Cf source, which creates a large number of additional pathways for the etchant to penetrate the apatite crystal, allowing numerous intersected confined tracks to be etched. As low-temperature thermal modeling is one of the greatest strengths of the AFT method, it is clearly desirable to improve the ability to measure confined tracks. Therefore, we seek funding to establish a German facility for apatite fission track irradiations by purchasing a 252Cf source. It is also necessary to build an apparatus to hold the fission track samples during irradiation. This must be properly permitted and shielded so that users are not exposed to unnecessary radioactive doses. This facility will be available to German fission track research groups.