节点文献
相对论重离子实验中的MRPC/TOF研制
【作者】 孙勇杰；
【导师】 李澄；
【作者基本信息】 中国科学技术大学 ， 粒子物理与原子核物理， 2006， 博士
【摘要】 相对论重离子物理是当今人类对物质世界进行研究的最前沿。人类对宇宙的总体特征以及质量起源等一系列重大问题的认识被确信与物质相变有关。在极高的温度、密度状态下，核物质将发生夸克-胶子等离子体（QGP）的相变。这一过程与大爆炸理论预言的宇宙在最初的10-5s内发生的相变过程相同，但方向相反。对夸克-等离子体状态的研究需要具有相当高的能量的大型强子对撞机来实现。美国布鲁克海文国家实验室（BNL）的相对论重离子对撞机（RHIC）和CERN的大型强子对撞机（LHC）的建设，为相对论下重离子对撞物理的研究提供了强有力的工具，能够达到或超过理论模型预言的相变发生的临界温度(～170MeV，相应的能量密度为εc～1GeVfm-3)，使得对粒子物理模型的验证和发展更进一步的理论成为可能。 相对论重离子对撞将产生大量的末态粒子，这对探测器系统的粒子鉴别能力提出了新的挑战。在KHIC／STAR和LHC／ALICE探测装置上，都将采用飞行时间探测器（TOF），通过对粒子飞行时间的测量，结合其它探测器给出的粒子动量信息，来提高粒子鉴别的能力。作为飞行时间探测器，首先要具有很高的本征时间分辨和探测效率，同时必须能够在对撞产生的高粒子通量环境下工作而性能不会降低。传统的解决办法是使用快闪烁体+光电倍增管的方案。由于相对论重离子对撞实验的高粒子多重数（STAR实验为每单位快度～1000个末态粒子，ALICE实验预计每单位快度可达到8000个末态粒子），即对探测器的粒度和读出电子学通道提出了大量要求。能够在探测器所处的强磁场环境下工作的光电倍增管价格昂贵，大量的探测通道加大了建造成本的压力，使得这种闪烁体+光电倍增管方案几乎不可能。为了实现重离子对撞物理实验中对粒子鉴别的要求，研制新型的高分辨的TOF探测器，人们开展了大量的研究，提出了一些新思想和新方法，特别是将气体探测器引用到TOF中的新模式，为进一步拓宽气体探测器的应用领域，提高TOF的性能／价格比提供了一条新途径。近年来一种具有悬浮电场内部结构的多气隙电阻板室（Multi-gap Resistive Plate Chambers，简称MPRC）使研制这种新型的高时间分辨TOF成为可能。 对具有平行板结构的气体探测器而言，减小板间的气隙以减小雪崩发生的时间随原初电离位置的变化，可以提高探测器的时间分辩。在这方面人们对各种结构的气体探测器开展研究，如小间隙多丝室（MGC）、电阻板室（RPC）等。但是减小气隙使得原初电离离子数目和电子的漂移距离减小，从而探测器输出的感应电荷减小。以大面积的RPC为例，气隙减小到1mm，工作在正比模式下，对最小电离粒子，时间分辨达到几百ps，其输出电荷仅为几十fC，信噪比很低。MRPC采用多个子气隙的结构特点较好地解决了上述问题，探测器最终输出的信号是各个子气隙中感应信号之和，降低了对电子学灵敏度的要求，同时也充分提高了探测效率。美国布鲁克海文国家实验室（BNL）STAR实验组计划用4000多块类似结构的MRPC建造飞行时间谱仪（TOF），预计造价约是相同指标的常规闪烁体-光电倍增管TOF的1／5-1／10；在CERN LHC上的ALICE实验也将采用大面积的MRPC建造飞行时间探测器，在～2个快度单位的接受度上提供0.5GeV／c到2.5GeV／c动量范围内的粒子鉴别。
【Abstract】 Relativistic Heavy Ion Physics has always been one of the most forward nature study of the world. Global features of our Universe and the origin of mass are believed to be linked to the phase transitions of matter. At extremely high temperature or density, the nuclear matter will transit into Quark-Gluon Plasma （QGP） state. This transition took place some 10-5s after the Big Bang in the early universe in an inverse direction. To study the physics of Quark-Gluon Plasma needs large hadronic colliders of extremely high energy. The constructions of the Relativistic Heavy Ion collider （RHIC） at Brookhaven National Laboratory （BNL） and the Large Hadron Collider （LHC） at CERN provide powerful tools for the study of the ultra-relativistic heavy-ion physics. They are expected to reach or even exceed the critical energy density (εc1GeVfm-3 corresponding to a temperature of 170MeV) predicted by the theoretical models. Thus one can check the existing particle physics models and develop new physics theories.In these relativistic heavy ion collisions, mass final state particles will be produced which makes great challenge to the particle identification of the detectors. A Time-Of-Flight （TOF） detector will be used both on RHIC/STAR and LHC/ALICE to enhance the particle identification capability with the measured flight time and also the momentum information measured by other sub-detectors. As a detector for the TOF measurement, it should have a good intrinsic time resolution and a high detection efficiency of all; it should also endure the high flux of particles produced in the collisions without any loss in performance. A traditional solution is based on the fast scintillators and good photomultiplier tubes （PMT）. The high multiplicity of the ultra-relativistic heavy-ion collisions requires a high granularity of the detector and a lot of electronics channels. The PMTs can work under the high magnetic field of the detector region are extremely expensive. The total cost will be extremely high for the large number of detection channels. All these facts indicate that the scintillators + PMT solution is impossible. To fulfill the particle identification requirement in heavy ion collisions, a lot of study has been taken to make a new type of detector with high time resolution at acceptable price. Some new ideas and methods have been considered. The introduction of gaseous detectors into this area offers a new access to develop such a TOF detector and opens a new area for the use of the gaseous detectors. A new kind Multi-gap Resistive Plate Chamber （MRPC） with internal plates electrically floating makes it possible to construct such a high time resolution TOF system.For a gaseous detector with parallel plate structure, the reduction of the gas gap size can reduce the time jitter related to the position of the primary ionization, thus the time resolution is enhanced. Some kinds of gaseous detectors have been considered such as the Microgap Chamber （MGC）, the Resistive Plate Chamber （RPC）, et al. But with the smaller gap size, the number of primary ionization and the distance that the electrons drift through are reduced. This leads to a reduction of the induced charge. For the large area RPCs working under proportional mode,