Development of New type PET using liquid Xe scintillation light

液体铋是透明液体，原子序数为54，密度约为3.0 g/cm^3。对于γ射线，其闪烁效率与NaI（T_1）几乎相同，闪烁衰减时间常数为2nsec和22 nsec，比NaI（T_1）快得多。据估计，通过用多个PMT检测直射光，而不使用反射光，将获得高能量分辨率。此外，通过比较来自多个PMT的输出，可以估计伽马射线的反应位置。通过对0.5MeV γ射线的模拟计算，在较好的条件下，获得了几mm的位置分辨率。据此，我们认为新型PET采用液体石蜡作为探测介质的可能性，并制作了液体石蜡探测器，为PET的研制提供了所需的实验数据。腔室为2层圆柱形，外层抽真空隔热，光电倍增管的探测器部分包含在内层。内层充有液态氙，内部装有44个光电倍增管的探测器部分，窗口为500 μm厚的不锈钢蜂窝窗和100 μm厚的密耳窗。有效体积为12 X12 X12 cm^3，由44个1英寸和2英寸的PMT包围。这次开发的PMT是滨松光子公司制造的2英寸圆柱型（R6041- 06 MOD）和1英寸矩形型（R5900- 06 MOD），这些PMT保持石英窗对真空紫外光具有高传输效率，并且被设计为即使在低温（-110 ℃）和高压（3atm）下也没有任何问题。此外，减少了成为闪烁光减少的原因的脱气或杂质。位置分辨率为7.8mm，时间分辨率为0.69nsec。在有效体积的中心部分，对511 keV γ射线的能量分辨率为45%.

Liquid Xe is transparent liquid with atomic number 54 and density about 3.0 g/cm^3. For gamma rays its scintillation efficiency is almost same as NaI(T1) and the decay time constants of scintillation, 2nsec and 22nsec, are much faster than that of NaI(T1). It is estimated that the high energy resolution will be obtained, by detecting direct light with a plural PMTs, without using the reflected light. Also it is possible to estimate the reaction position of the gamma ray by comparing the output from plural PMTs. By the simulation for the 0.5 MeV gamma ray, the position resolution of several mm was obtained with the good condition. Thereupon, we think the possibility of new-type PET using liquid Xe as detector medium and produced the liquid Xe detector and made to obtain the experimental data needed for PET development. The chamber is the cylinder forms of 2 layers and the outside layer is evacuated vacuously for heat insulation and detector part of PMTs is included in the inside layer. The inside layer is filled with liquid xenon and the detector part of 44 PMTs is set in it. The windows are the honey cam window made of the stainless steel of 500 μm thick at inside and the miler window of 100 μm thick at outside. The effective volume is 12X12X12 cm^3 that is surrounded with 44 PMTs of 1 inch and 2 inch. PMTs that we developed this time are 2 inch cylinder type(R6041-06MOD) and 1 inch rectangular type (R5900-06MOD) made by Hamamatsu Photonics company, and these hold the quartz window to have high transmission efficiency for vacuum ultraviolet light and is designed to act without any problem even with the low temperature (-110 ℃) and the high pressure (3atm). Also, it is made to decrease the outgas or the impurities that becomes the cause of the decrease of scintillation light. The position resolution is 7.8 mm and the time resolution is 0.69nsec. In the central part of effective volume, the energy resolution is 45 % for 511 keV gamma ray.