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低于约110°C的温度。由于238U的放射性衰变，裂变径迹以恒定的速率形成了窄长度分布，随后被缩短，并可能最终因温度升高而消失。这一过程称为退火法。因此，磁道长度分布是晶体热历史的灵敏监测器，特别是在约60-110°C(部分退火区)之间的温度区间；在显著更高的温度下，从地质上讲，退火是瞬时的。测量大量受限径迹的长度提供了热历史的替代物，可以通过热模拟来反演热历史。受限的轨迹完全位于磷灰石晶体内，只有当表面的酸通过另一条轨迹存在路径时，才能被蚀刻。通常，操作员寻求测量每个样本约100个受限轨道。对于含铀量较高的磷灰石晶体，其在低温下保持较长的地质时间，通常存在足够的径迹来为反模拟提供可靠的数据集。然而，对于较年轻的样品和铀丰度较低的颗粒，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。在这种情况下，使用传统的样品制备技术很难或不可能测量大量的受限长度，从而排除了热模拟或极大地限制了此类模型的分辨率。为了增加可测量的受限径迹的数量，可以用从252Cf源发射的裂变粒子轰击样品，这为腐蚀剂穿透磷灰石晶体创造了大量额外的路径，使得许多相交的受限径迹得以蚀刻。由于低温热模拟是AFT方法的最大优点之一，因此提高测量受限轨道的能力显然是可取的。因此，我们寻求资金，通过购买252Cf源来建立一个用于磷灰石裂变径迹照射的德国设施。还有必要建立一种装置，在辐照过程中保持裂变径迹样品。这必须得到适当的允许和屏蔽，这样使用者才不会暴露在不必要的放射性剂量下。这一设施将提供给德国裂变径迹研究小组。

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.