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概述 当前SiPM读出测试系统进行到了一定程度，最初SiPM型号的选定是由项目团队敲定，个人对此经过一段时间的学习，对应为何要如此选型也有了一定的理解。 目前，项目计划在当前基础上衍生额外的项目分之，即SiPM需要选用其它型号。本文基于个人理解试图初步探讨如何对于即将采用的SiPM型号进行选择。 当前SiPM型号 当前使用的SiPM来自滨松的S14161-6050HS-04，这是一个4x4的SiPM阵列，总共16个通道，其电气指标如下图所示。 图1：S14161系列SiPM指标。 新项目SiPM选型指导 新衍生的项目设计根本目标是提升符合分辨率。当前项目有哪些是除了电子电路设计方面无法优化，而需要通过改变SiPM型号来进行优化的呢？这方面主要有两个因素可以考虑： 提高光子探测效率 降低暗电流 新的SiPM型号无法从根本上大幅度提高所谓的符合分辨率，只能在当前物理条件基础上进行优化。如图1所示，当前使用的SiPM像素尺寸是50um，单通道尺寸为6mm，暗电流典型值为2.5uA，最大值为7.5uA。 从上述两个指导建议，即提高PDE，降低暗电流出发，可以寻求增大像素尺寸，减小单通道尺寸，这样在不改变其它指标情况下可以初步得到一个型号S14161-4075HS-04。从滨松公开网站上无法查询到S14161系列有该型号。 SiPM相关指标的理解 这里对于指标的理解还有两个疑问，即首先是图1给出暗电流整个阵列的暗电流还是单个像素的暗电流？其次是图1给出的增益是整个阵列增益还是单个通道或单个像素的增益？ 从滨松官网有关MPPC的基础知识分享，可以了解到增益计算公式如下所示： Gain = 1/q * Cp * Vov 根据上述公式，增益似乎是基于像素计算，所以图1给出的增益是否可以理解为也是对于型号中单像素增益。 SiPM其它因素考虑 通过咨询，得知滨松应该是很愿意为特定用户进行定制化MPPC的生产。那么在考虑增大PDE方面考虑除了增大像素尺寸外，是否还有其它因素可以是该目标呢？同样在滨松官网给出了PDE计算公式，如下式所示。 PDE = FF * QE * AP 上述公式右侧，第一项是填充系数，第二项是量子效率，最后一项是雪崩概率。理论上来说，大的像素尺寸会带来大的雪崩概率，从而增大PDE，这是为何上述指导意见首先像素尺寸从50um到75um的原因。 那么这里探讨的其它因素是什么呢？和第二项的量子效率无关，和第一项的填充系数应该是有关系的。如图2所示，在SiPM器件手册里给出的封装尺寸可以看出，SiPM通道 之间存在比较大的“沟道”间隔。 图2：S14161-6050HS-04封装尺寸示意图 如上图所示，这里通道之间有个0.2mm的间隔，0.2mm相对50um或75um的像素尺寸是非常巨大间隔。这个间隔必然会降低整体阵列的PDE，这里提出一个想法，通过将这个沟道尺寸尽量降低来提升或优化整体器件的PDE。唯一担心的问题是，这有可能是滨松封装生产上的限制。因为遍查其它阵列器件，该尺寸都是固定的0.2mm。而且如图3所示，竞争对手的这个指标也是0.2mm。 图3：ON Semi（SENSL）家SiPM芯片封装尺寸示意图 如图3所示，SiPM阵列其实就是由多个单通道SiPM封装而成，那么如果用正片原料直接代替分割好的小颗粒封装成类似尺寸SiPM是否可行？这样就解决了通道间过大间隔问题。内部通过“电气分割”再将正片材料分成同样的通道。这是一种思路。

As I am a bear of simple brain like Winnie the Pooh, many things confuse and confound me. One such topic is that of dark matter. When Sir Isaac Newton presented his Theory of Universal Gravitation in 1687, everyone quickly came to the conclusion that "this was it" – a theory that truly described the way in which the universe worked. After a while, however, they came to realise that the planet Mercury wasn't orbiting the Sun in quite the way it should. Since the folks of the time absolutely believed in the concept of Newtonian gravity, they looked for an explanation that would fit into this "world view." The solution they came up with was that there must be an undiscovered planet (which they called Vulcan) in orbit between the Sun and Mercury. Based on this proposal, many folks devoted huge amounts of effort and ingenuity trying to find this planet... that we now know does not exist. It was more than two hundred years later, in 1916, that Albert Einstein published his theory of General Relativity, whose description of space-time curvature sorted out the problem of Mercury and appeared to provide all of the answers (gravity-wise). For close to 100 years, General Relativity has been accepted by the majority of folks as fully describing gravity. But once again there's a problem. Astronomers have discovered that the stars at the edges of rotating galaxies are travelling much faster than they should be... so fast that they should fly off into space... but they don't. In order to address this, astronomers and physicists came up with the concept of Dark Matter. The idea in a nutshell is that Dark Matter is something we can't "see" or "taste" or anything like that... except through its gravitational interactions. Am I the only one who finds this conclusion to be a tad dubious? Thus it was that I was somewhat irked by a recent column in Discover Magazine, which was titled Largest Map of Dark Matter Across the Cosmos . The associated image (see below) came from the NASA Website ( Click here to see the original posting of this image and associated text).   A snippet from the article in Discover is as follows: Astronomers believe dark matter makes up a quarter of the universe, yet it does not absorb or emit light, and nobody has detected a particle of it. Fortunately, dark matter does reveal itself in a subtle way: As light approaches a clump of the mysterious stuff, it bends around it in a phenomenon known as gravitational lensing. The more massive the clump, the more the light bends. Later on, talking about a catalogue of gravitational lensing that has been compiled over the last five years, the article goes on to say: Their map, which covers 100 times as much sky as previous surveys, reveals giant heaps of dark matter enveloping galaxies. "Whenever there was a dark matter peak, there was a massive cluster of galaxies." (The quote was from astrophysicist Catherine Heymans who is one of the principles on the project.) Now call me "Mr. Fuddy Duddy" if you wish, but I don't find this argument ( "Wherever there was a dark matter peak, there was a massive cluster of galaxies" ) to be particularly convincing. The thing is that gravitational lensing is also associated with the real mass constituting the galaxies. Proponents of dark matter will counter that the dark matter is "wrapped around" galaxies. For myself, having read a lot of "stuff" from a lot of sources, I'm much more inclined to think that the answer lies in another direction, which is that the Einsteinian theory of gravity is incomplete, not the least that – thus far – we haven't managed to tie gravity into the other fundamental forces that manifest themselves at the quantum level. I don't know why, but I have a "gut feeling" that we are on the verge of making some really big discoveries that will dramatically change the way in which we view the universe. I cannot wait to see whatever developments come our way in the next 10 or 20 years. Watch this space...