Radiation imaging by a micro photomultiplier using micromachining thchnique

我们研究了一种新的位置敏感阵列型光电倍增管，它可以用于辐射成像探测器在非常高的计数率，如在SR设施。这个想法是基于一个标准的光电倍增管（PMT）。利用微机械加工技术在倍增极上形成许多微小孔。这些倍增电极板用绝缘体分层，并且偏置电压施加在板之间。入射到第一倍增极板的电子在倍增极孔上产生二次电子，并且这些发射的电子被吸引到下一层倍增极板。连续的二次电子发射发生在每个像素中，就像一个独立的光电倍增管。针对该倍增极孔的结构，设计了两种不同类型的平板倍增极。一个具有通过各向异性Si蚀刻形成的多个金字塔形穿透孔（100 μ m * 100 μ m尺寸），另一个具有通过LIGA类工艺模制的多个倾斜孔（100 μ m * 80 μ m尺寸）。在12个阶段之后计算的最大增殖因子对于金字塔型为10^5，对于斜孔型为10^7。这两种类型的倍增电极板制造。使用每个孔配置的两个倍增极板测量每个倍增极的电子倍增因子。在偏置电压为150V时，锥孔型为1.62，在偏置电压为400V时，斜孔型为2.14。这些值与通过Monte Carlo模拟获得的1.7（金字塔）和2.19（倾斜）一致。为了研究多层器件的可行性，在一个器件中堆叠了四个金字塔倍增器电极。不幸的是，在硅多层器件中发现了一个充电效应，它极大地降低了倍增系数。虽然这种效应必须研究和克服，但我们确保了多层器件的可行性。

We have investigated a new position sensitive array type photomultiplier which can be used for a radiation imaging detector operated at very high counting rate such as in SR facilities. This idea is based on miniaturizing a standard photomultiplier tube (PMT). Many miniature holes are formed on the dynode using micromachining technique. These dynode plates are layred with insulators, and bias voltages are applied among plates. Incident electrons to the first dynode plate make secondary electrons on the dynode hole, and these emitted electrons are attracted to the next layred dynode plate. The successive secondary electron emissions occur in each pixel that works like an independent photomultiplier. As to the structure of this dynode hole, two different types of plate dynodes were designed. One had many pyramid shape penetrating holes (100mum*100mum size) which were formed by anisotropic Si etching, and the other had many slanting holes (100mum*80mum size) which were molded by a LIGA like process. Calculated maximum multiplication factors after 12 stages were 10^5 for the pyramid type and 10^7 for the slanting hole type. Both type of dynode plates are fabricated. The electron multiplication factor per one dynode was measured using two dynode plates of each hole configuration. It was 1.62 at the bias voltage of 150V in pyramid hole type, and 2.14 at 400V in slanting hole type. These values agreed with 1.7 (pyramid) and 2.19 (slanting) obtained by Monte Carlo simulation. In order to study the feasibility of a multi-layred device, four pyramid dynodes are stacked in one device. Unfortunately, a charge-up effect was found for the silicon multilayred device which extremely lowers the multiplication facotor. Although this effect must be investigated and overcome, we ensured the feasibility of multilayred devices.