Chapter II: UCN 𝜏 experimental design
2.7 UCN detectors
2.7 UCN detectors
Clear polyester sheets coated with 3.25±0.25 mg/cm2of ZnS:Ag were purchased from a commercial vendor [40]. The thickness of the ZnS:Ag layer is ∼ 10 𝜇m.
Vacuum evaporation is used to deposit∼20 nm of 10B on top of the ZnS:Ag. The thickness of the10B layer was chosen while considering that a thicker layer increases the UCN capture efficiency per interaction, but also decreases the scintillation light yield in the ZnS:Ag. The clear polyester sheets are backed with ∼ few mm of poly(methyl methacrylate) (PMMA, or sometimes referred to as acrylic) which provides a rigid structure to the UCN detector. The probabilities that a UCN incident on the detector is captured or reflected can be calculated by modelling the detector as a series of layers with different Fermi potentials and considering a quantum mechanical wave incident on the layers, as discussed in Section 2.1 of [41]. All UCN detectors in the UCN𝜏 experiment use this detection method. The general process by which the 10B and Zn:S were layered is common among all UCN detectors, but the way in which the scintillation light is collected and detected by PMTs differs among different types of detectors. Sections 2.7.2 and 2.7.3 will discuss how PMTs collect the scintillation light from different types of detectors.
2.7.2 Primary detector
The primary detector counts the number of UCN in the trap. The number of UCN in the trap at the beginning and the end of each storage time are direct inputs to extracting a lifetime, so it is critical that the primary detector accurately and precisely counts the number of UCN in the trap. Therefore, the primary detector was built to a higher standard than the upstream monitors, which will be described in Section 2.7.3.
The key feature of the primary detector is the coincident detection of scintillation light in two PMTs. This coincident detection significantly suppresses the back- grounds caused by the dark rates of the PMTs. A set of 1×1 mm2 grooves with 2 mm spacing are machined out of a PMMA plate that is 3 mm thick. Wavelength shifting fibers (WLSF) are glued into each groove using optical epoxy. The WLSF are coupled to two PMTs in an alternating fashion so that any two neighboring WLSF are coupled to different PMTs. A second 3 mm thick PMMA plate is glued onto the first to completely surround the WLSF with PMMA. 10B-coated ZnS:Ag screens described in Section 2.7.1 are glued to both sides of the 6 mm of PMMA such that the10B-coated surface faces outward [42]. Figure 2.8 shows a schematic of the layers of the primary detector. Figure 2.9 shows the primary detector in various stages of construction.
Figure 2.8: A schematic of the layers of the primary detector. The WLSF alternate which PMT they are connected to, as denoted by the numbering. The relative locations and size of the WLSF and the PMMA are as shown, but the layers of ZnS:Ag and10B are actually much thinner than shown.
The primary detector is connected to an actuator that raises and lowers it through the volume of the trap. The primary detector is lowered in multiple steps to sample the energy spectrum of UCN in the trap. The primary detector is positioned in the middle of the midplane of the trap and covers∼ 20% of the area of the midplane.
The curved bottom edge of the primary detector was chosen to match the curve of the bottom of the trap. The primary detector is raised above the entire usable volume of the trap while UCN are stored in the trap. When a UCN is captured by the primary detector scintillation light is produced. Then, some of that scintillation light enters the PMMA, is transferred to the WLSF, and is directed to the PMTs. The voltage signals from the two PMTs are amplified by a 10× gain and are discriminated at thresholds of 16 and 13 photoelectrons. The two thresholds allow systematic effects due to gain drifts and background to be studied. The higher threshold data are more sensitive to gain drifts, so only the lower threshold data are used in the analyses presented in Chapters 3 and 4.
Figure 2.9: The primary detector. Top: a fully-constructed version of the primary detector. Bottom: the internal workings of the primary detector, observed prior to completing the construction [42]. 10B-coated ZnS:Ag sheets are mounted on both sides of the acrylic (PMMA) plate. WLSF are interleaved through the acrylic plate which directs the scintillation light to the two PMTs. The WLSF are interleaved in an alternating manner so that any source of significant scintillation light is detected in both PMTs.
2.7.3 Upstream monitors
The primary detector described in Section 2.7.2 counts the number of UCN in the trap, but in doing so it removes the UCN from the trap. The number of UCN in the trap at the beginning and the end of each storage time are used to extract a lifetime from the data. The number of UCN in the trap are not directly measured at the beginning of each storage time, but instead the number is estimated using measurements of the UCN density at various positions in the guides between the UCN source and the trap via the upstream monitors.
The UCN density in the guides are measured using 10B-coated ZnS:Ag detectors.
It is much less critical to make high-precision measurements of the local UCN densities than it is to make a high-precision measurement of the number of UCN in the trap, so the upstream monitors have a simpler design than the primary detector.
The 10B-coated ZnS:Ag screens of each upstream monitor are optically observed by a PMT, but the PMT is not physically coupled to the scintillator with WLSF.
The voltage signal from each PMT is integrated by a shaping amplifier and then discriminated. The parameters of the shaping amplifier are tuned with the goal that one captured UCN should produce one discriminated count.
Some of upstream monitors sample, and therefore remove, UCN that can be stored in the trap. In order to limit the decrease in the number of UCN loaded into the trap, upstream monitors that sample UCN with energies that can be stored in the trap are coupled to the UCN guides with only very small openings in the guides.
One monitor of this type is the “gate valve” monitor, which is located near the gate valve and is shown in Figure 2.1.
The sD2source produces, and the guides contain, many UCN with too much energy to be stored in the trap. These high-energy UCN can not be used to directly measure the lifetime, but they can be used to measure the density of UCN in the guides. The
“standpipe” monitor is installed inside of a vertical offshoot of the guide system at a height just above the lowered height of the cleaners. The standpipe monitor is also shown in Figure 2.1. At this height no trappable UCN can be sampled by the standpipe monitor, so there is no need to limit the coupling of the standpipe monitor to the guide system. Therefore, the standpipe monitor is designed with a large active area and detects UCN at a rate∼ 7×the rate at which UCN are detected in the gate valve monitor.
Between the 2017 and 2018 run cycles, a large buffer volume (also shown in Figure 2.1) was installed between the UCN source and the trap. Upstream monitors are installed at the bottom of the buffer volume and near the top of the buffer volume.
Following the same logic in the above two paragraphs, the upstream monitor at the bottom of the buffer volume (the “buffer volume monitor”) is coupled to the interior of the buffer volume with a very small opening. In contrast, the upstream monitor at the top of the buffer volume (the “buffer volume pre-cleaner”) has∼1000 cm2of active surface.