Department of Radiology, Stanford University, Stanford, CA, United States of America.
Radiation Center for Ultrafast Science, Korea Atomic Energy Research Institute, Daejeon, Republic of Korea.
Phys Med Biol. 2024 Nov 29;69(23). doi: 10.1088/1361-6560/ad8832.
Achieving ultra-precise temporal resolution in ionizing radiation detection is essential, particularly in positron emission tomography, where precise timing enhances signal-to-noise ratios and may enable reconstruction-less imaging. A promising approach involves utilizing ultrafast modulation of the complex refractive index, where sending probe pulses to the detection crystals will result in changes in picoseconds (ps), and thus a sub-10 ps coincidence time resolution can be realized. Towards this goal, here, we aim to first measure the ps changes in probe pulses using an ionizing radiation source with high time resolutionWe used relativistic, ultrafast electrons to induce complex refractive index and use probe pulses in the near-infrared (800 nm) and terahertz (THz, 300m) regimes to test the hypothesized wavelength-squared increase in absorption coefficient in the Drude free-carrier absorption model. We measured BGO, ZnSe, BaF, ZnS, PBG, and PWO with 1 mm thickness to control the deposited energy of the 3 MeV electrons, simulating ionization energy of the 511 keV photons.Both with the 800 nm and THz probe pulses, transmission decreased across most samples, indicating the free carrier absorption, with an induced signal change of 11% in BaF, but without the predicted Drude modulation increase. To understand this discrepancy, we simulated ionization tracks and examined the geometry of the free carrier distribution, attributing the mismatch in THz modulations to the sub-wavelength diameter of trajectories, despite the lengths reaching 500m to 1 mm. Additionally, thin samples truncated the final segments of the ionization tracks, and the measured initial segments have larger inter-inelastic collision distances due to lower stopping power (d/d) for high-energy electrons, exacerbating diffraction-limited resolution.Our work offers insights into ultrafast radiation detection using complex refractive index modulation and highlights critical considerations in sample preparation, probe wavelength, and probe-charge carrier coupling scenarios.
实现离子化辐射探测的超高精确时间分辨率至关重要，特别是在正电子发射断层扫描中，精确的定时可提高信号与噪声比，并可能实现无需重建的成像。一种有前途的方法是利用复折射率的超快调制，通过向探测晶体发送探测脉冲，会导致皮秒（ps）级的变化，因此可以实现低于 10 ps 的符合时间分辨率。为了实现这一目标，我们旨在首先使用具有高时间分辨率的离子化辐射源测量探测脉冲的 ps 级变化。我们使用相对论性超快电子来诱导复折射率，并在近红外（800nm）和太赫兹（THz，300m）波段使用探测脉冲，以测试在德劳德自由载流子吸收模型中吸收系数随波长平方增加的假设。我们使用 1mm 厚的 BGO、ZnSe、BaF、ZnS、PBG 和 PWO 来控制 3MeV 电子的沉积能量，模拟 511keV 光子的离子化能量。对于 800nm 和 THz 探测脉冲，大多数样品的透射率都降低了，表明发生了自由载流子吸收，BaF 的感应信号变化为 11%，但没有出现预期的德劳德调制增加。为了理解这种差异，我们模拟了离子化轨迹并检查了自由载流子分布的几何形状，将 THz 调制的不匹配归因于尽管长度达到 500m 至 1mm，但轨迹的亚波长直径。此外，薄样品截断了离子化轨迹的最后部分，由于高能电子的低阻止能力（d/d），测量的初始部分具有更大的非弹性碰撞距离，从而使衍射极限分辨率恶化。我们的工作为使用复折射率调制进行超快辐射探测提供了深入了解，并强调了在样品制备、探测波长和探测电荷载流子耦合场景中需要考虑的关键因素。
