Experimental set-up and first measurements of the neutron lifetime using PENeLOPE

自由中子的寿命是一个基本的物理常数，在物理学的许多领域起着重要的作用。在PENeLOPE实验中，超冷、极慢的中子将被磁性储存在一个陷阱中，并测量tn；磁场是由超导磁体产生的。通过这种方式，避免了材料壁上不被理解的损失。PENeLOPE磁铁目前正在建设中（DFG项目PA 762/8-1）。在这个实验中，中子寿命将以两种方式确定；储存周期后在阱中幸存的中子将在不同的储存时间后被计数，最值得注意的是，中子衰变中产生的质子将被实时记录。质子探测结合了两个优点：它不受每次填充中子数变化的影响，并且与中子计数相比，只需要四分之一的测量时间就能达到期望的< 0.1 s的统计精度。在早期的DFG项目（PA 768/5-2）中，证明了衰变质子可以通过在光导上蒸发一层薄薄的碘化铯（CsI）来检测，并通过雪崩光电二极管（APD）从侧面读出。为此，有必要将质子加速到30 keV左右的能量。此外，还验证了可以区分电子和质子，并且探测器可以在低温和大磁场的PENeLOPE环境中工作。下一个合乎逻辑的步骤是使探测器适应PENeLOPE的几何形状：一个半径为180毫米的内环和360毫米的外环，总面积约为3000立方厘米。CsI薄蒸发层的柱状结构是一个很大的挑战；如果从侧面读出，会严重影响光的收集效率。因此，应该研究该探测器解决方案的其他替代方案：直接耦合到apd的大型CsI晶体，薄有机（塑料）闪烁体，微通道板和转换箔，以产生二次电子。

The lifetime of the free neutron tn is one of the fundamental physical constants and plays an important role in many areas of physics. In the experiment PENeLOPE, ultra-cold, very slow neutrons shall be stored magnetically in a trap and tn shall be measured; the magnetic fields are created by superconducting magnets. In this way, not understood losses at material walls are avoided. The magnet for PENeLOPE is currently under construction (DFG project PA 762/8-1). The neuton lifetime will be determined in two ways with this experiment; the neutrons surviving in the trap after the storage cycle will be counted after varying storage times and, most notably, the protons generated in neutron decay will be recorded in realtime. Proton detection combines two advantages: it is independent from a variation in the number of neutrons per filling and requires only a fourth of the measuring time compared with neutron counting to achieve the aspired statistical precision of < 0.1 s. During an earlier DFG project (PA 768/5-2) it was proven that decay protons may be detected with a thin layer of cesium iodide (CsI) evaporated on a light guide and read out from the side via an avalanche photodiode (APD). To this end, it is necessary to accelerate the protons to energies of around 30 keV. In addition, it was verified that electrons and protons may be distinguished and that the detector works in the PENeLOPE environment at cryogenic temperatures and in large magnetic fields. The next logical step is to adapt the detector to the geometry of PENeLOPE: a ring of 180 mm inner and 360 mm outer radius, resulting in a total area of around 3000 cm3. The columnar structure of a thin evaporated layer of CsI turned out to be a big challenge; it impedes the light collection efficiency significantly if read out from the side. Hence, other alternatives to this detector solution shall be investigated: large CsI crystals, directly coupled to the APDs, thin organic (plastic) scintillators, microchannel plates and conversion foils to generate secondary electrons.