Lutetium oxyorthosilicate (LSO) or lutetium yttrium oxyorthosilicate (LYSO) are the scintillator materials most widely used today in PET detectors due to their convenient physical properties for the detection of 511 keV annihilation photons. Natural lutetium contains 2.6% of ¹⁷⁶Lu which decays beta to excited states of ¹⁷⁶Hf producing a constant background signal. Although previous works have studied the background activity from LSO/LYSO, the shape of the spectrum, resulting from β-particle and γ radiation self-detection, has not been fully explained. The present work examines the contribution of the different β-particle and γ-ray interactions to provide a fuller comprehension of this background spectrum and to explain the differences observed when using crystals of different sizes. To this purpose we have shifted the continuous β-particle energy spectrum of ¹⁷⁶Lu from zero to the corresponding energy value for all combinations of the isomeric transitions of ¹⁷⁶Hf (γ-rays/internal conversion). The area of each shifted β-spectrum was normalized to reflect the probability of occurrence. To account for the probability of the γ-rays escaping from the crystal, Monte Carlo simulations using PENELOPE were performed in which point-like sources of monoenergetic photons were generated, inside LYSO square base prisms (all 1 cm thick) of different sizes: 1.0 cm to 5.74 cm. The analytic distributions were convolved using a varying Gaussian function to account for the measured energy resolution. The calculated spectra were compared to those obtained experimentally using monolithic crystals of the same dimensions coupled to SiPM arrays. Our results are in very good agreement with the experiment, and even explain the differences observed due to crystal size. This work may prove useful to calibrate and assess detector performance, and to measure energy resolution at different energy values.

中文翻译：

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了解LYSO / LSO闪烁晶体中176Lu的本征放射性能谱。

氧正硅酸（（LSO）或氧正硅酸钇（LYSO）是当今在PET检测器中最广泛使用的闪烁体材料，因为它们具有方便的物理性能，可检测511 keV an灭光子。天然含有¹⁷⁶ Lu的2.6％，可将β分解为^176的激发态HF产生恒定的背景信号。尽管先前的工作已经研究了LSO / LYSO的背景活性，但尚未完全解释由β粒子和γ辐射自我检测产生的光谱形状。本工作研究了不同的β粒子和γ射线相互作用的贡献，以提供对该背景光谱的更全面理解，并解释了使用不同尺寸的晶体时观察到的差异。为此，我们已将¹⁷⁶ Lu的连续β粒子能谱从零转移到¹⁷⁶异构转变的所有组合的对应能量值Hf（γ射线/内部转换）。将每个移动的β光谱的面积归一化以反映出现的可能性。考虑到γ射线从晶体逸出的可能性，使用PENELOPE进行了蒙特卡洛模拟，其中在不同尺寸的LYSO方形基础棱镜（均厚1 cm）内部生成了单能光子的点状光源：1.0厘米至5.74厘米。使用变化的高斯函数对解析分布进行卷积，以说明所测量的能量分辨率。将计算的光谱与使用相同尺寸的单晶晶体耦合到SiPM阵列实验获得的光谱进行比较。我们的结果与实验非常吻合，甚至可以解释由于晶体尺寸而观察到的差异。