低能核反冲在液氙暗物质探测器中的发光和电离效率

发布时间：2018-04-11 21:00

本文选题：液氙探测器 + 暗物质　；参考：《上海交通大学》2014年硕士论文

【摘要】：大质量弱相互作用粒子(weakly interacting massive particles WIMPs)是目前暗物质粒子的最具吸引力的候选之一。近二十年来,无数暗物质探测实验被开发用于WIMPs粒子的直接探测。其中,液氙探测器是所有直接探测试验中最有希望的一种。液氙探测器用来探测暗物质粒子(WIMPs)与探测器中原子发生弹性散射作用所引起的核反冲,从而达到对暗物质粒子的直接探测的目的。当液氙探测器中的某个氙原子与暗物质粒子发生弹性散射,该原子会获得大约几到几十千电子伏特(keV)的动能成为一个反冲原子。反冲原子在探测器介质中运动并减速,在此期间会激发或者电离探测器中其他原子,产生实验中容易探测到的瞬时发光信号(以下简称为发光信号)S1或者电离正比发光信号(以下简称为电离信号)S2。其中发光信号是由激发电子退激发,或者被电离电子与负离子重新结合并退激发而产生,而电离信号则是由于被电离的电子在外加电场作用下,漂移到气态探测器室中而产生。在液氙探测器中,有两个重要的参数,即相对发光效率(relative scintillation e?ciency Leff)和电离效率(ionization yield Qy)。这两个参数有效地的将实验易探测到发光信号S1或者电离信号S2与暗物质粒子(WIMPs)在探测器中原子反冲的初始能量联系起来。如果Leff或者Qy能够精确的计算出来,那么结合实验对原子反冲所产生的发光信号S1或者电离信号S2测量,实验学家可以重构原子反冲的能量,从而对暗物质粒子的相关性质进行进一步分析。原则上讲,Leff和Qy可以通过实验进行测量。然而这项任务在低能原子反冲区却很有挑战性。大多数实验通过中子源来产生核反冲,从而模拟暗物质粒子产生的原子反冲信号来测量Leff和Qy,但是实验的性质决定了核反冲的能量越低(keV能量区域附近),该实验的系统误差就会越大。同时,由于轻暗物质粒子(WIMPs)的质量多数在几个吉电子伏特(GeV)左右,其产生的原子反冲能量多数在数个千电子伏特(keV)。针对这种情况,本文对液氙探测器在低能区的发光和电离过程进行了详尽的研究和理论分析,并且对Leff和Qy在低能区的行为做出了相应的理论预测。基于Lindhard的基本积分方程以及二体碰撞模型,我们开发了一个计算机程序来模拟原子反冲在液氙探测器中的减速过程。利用该程序,我们可以计算出原子反冲在减速过程中的电子能损的具体数值。通常来说,这个数值与原始原子反冲能量的比值被称为Lindhard系数(Nuclear Quenching Factor qnc)。从而可以进一步来计算发光信号和电离信号的数量。为了得到qnc在低能区的精确值,我们对低能区的氙原子在液氙介质中的电子能损的过程,现存的理论模型,以及实验数据进行了分析,对传输截面法(Transport Cross Section)求电子能损Se的方法做了修正,从而重新计算了Se。我们的理论结果与中等能量区域(40到100千电子伏特)的实验结果符合得很好。为了进一步计算光信号和电离信号的数值,我们对电子与负离子的结合过程进行了分析,修正并推广了现有的Thomas-Imel模型。通过该修正,我们预测了电子与负离子在外场下的结合效率,从而能够精确计算出发光信号和电离信号的数值。将我们所做的研究结合起来,我们得到Leff和Qy在低能区的理论预测值。我们对Leff和Qy的理论预测与中子散射实验的测量结果符合的很好。在无实验数据区域(低于3千电子伏特的区域),我们所预测的Leff迅速降低。该现象与之前文献中所做的假设相矛盾。我们所预测的Qy随着原子反冲能量的降低而升高,在2到3千电子伏特的区域达到最大值。这个预测结果可以将探测器的探测极限进一步降低到大约1千电子伏特左右。由于电离信号相对易于探测,该预测有可能被实验进一步证实或者证伪。
[Abstract]:Wimps (weakly interacting massive particles WIMPs) is currently the dark matter particles. One of the most attractive candidates for the past twenty years, numerous dark matter detection experiment was developed for the direct detection of WIMPs particles. The liquid xenon detector is a direct test of the most promising. The liquid xenon detector is used to detect dark matter particles (WIMPs) and nuclear recoil detector in atomic elastic scattering effects, so as to achieve the direct detection of dark matter particles. The elastic scattering occurs when the particle liquid xenon detector in a xenon atom and dark matter, the atom will be about a few to tens of electron V (keV) kinetic energy become a recoil atom. The recoil atom movement in the detector medium and slow, can stimulate or in other atomic ionization detector during the test Instantaneous easily detected light signal (hereinafter referred to as the luminescence signal) or light signal is proportional to the ionization S1 (hereinafter referred to as the S2. ionization signal) the luminescence signal is excited by electron de excitation, or ionizing electron and negative ion recombination and de excitation generated, and ionization signal is due to the ionized electrons under electric field, drift to the room in which gaseous detector in liquid xenon detector, there are two important parameters, namely the relative luminous efficiency (relative scintillation e ciency? Leff) and ionization efficiency (ionization yield Qy). These two parameters effectively will be easy to detect the luminescence signal S1 experiment or ionization signal S2 and dark matter particles (WIMPs) in the probe atom recoil initial energy together. If Leff or Qy can be calculated accurately, then combined with the experiment of atomic recoil The luminescence signal S1 or S2 ionization signal measurement, experimental scientists can reconstruct atomic recoil energy, further analysis and related properties of dark matter particles. In principle, Leff and Qy can be measured by experiments. However, this task in the low-energy atomic recoil region is very challenging. Most of the experiments by neutron source to produce nuclear recoil, thereby simulating atomic recoil particles of dark matter signal generated by measurement of Leff and Qy, but the nature of the nuclear recoil energy is low (near the keV energy region), the system error of the experiment will be bigger and bigger. At the same time, because the light dark matter particles (WIMPs) in the majority of quality a few GeV (GeV), the atomic recoil energy most in a number of thousands of electron volts (keV). In this case, the liquid xenon detector in the low energy region of the light and electricity from the detailed process As the research and theoretical analysis, and the Leff and Qy in the low energy region acts made the corresponding theoretical predictions. The basic integral equations of Lindhard and two body collision model based on deceleration process we developed a computer program to simulate the recoil atoms in liquid xenon detector. By using this program, we can calculate the the specific value of atomic recoil electron energy loss during deceleration. Generally, the ratio of this value with the original atomic recoil energy is called Lindhard (Nuclear Quenching Factor coefficient QNC). The number of which can further calculate the light signal and the ionization signal. In order to get the exact value of QNC in the low energy region, we the low energy region of xenon atoms in liquid xenon electronic medium energy loss, the existing theoretical model, and the experimental data were analyzed on the transmission cross section method (Transport Cross Section) for Electronic energy loss Se method modification is made to re calculate the theoretical results of Se. and medium energy region (40 to 100 thousand EV). The experimental results are in good agreement. In order to further numerical calculation of optical signal and ionization signal, we on the electron and negative ion binding process was analyzed and the correction and extend the existing Thomas-Imel model. Through the correction, we predicted the binding efficiency of electrons and negative ions in the presence of magnetic field, which can accurately calculate the numerical luminescence signal and ionization signal. The research we have done together, we get Leff and Qy predictive value in the low energy region of the measurement result of our theory. The Leff and Qy theoretical prediction and neutron scattering experiments are in good agreement with experimental data. In the area (less than 3 thousand EV, area) we have predicted Leff decreased rapidly. The phenomenon of literature In the assumption made in contradiction. We predicted Qy increased with decreasing atomic recoil energy, 3 thousand EV in 2 areas reaches the maximum value. The prediction results can further reduce the detection limit of detector to about 1 thousand ev. The ionization signal is relatively easy to detect, the prediction of possible the experiment was further confirmed or falsified.

【学位授予单位】：上海交通大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：P145.9;O572.2

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