Detectors
4.1 E158 Calorimeter
The E158 calorimeter is the primary detector of the E158 experiment [46]. It is divided into four separate annular regions, denoted as the In ring, Mid ring, Out ring, and eP detector. The spectrometer (Section 3.7) focuses electrons scattered from the target so that the In, Mid, and Out regions are dominated by Møller scattering, while the eP detector is dominated by electron-proton scatters.
The In and Mid region are collectively known as the Møller detector, and provide the primary measurement of AP V. The Out ring could also have been included, but Sec- tion 6.7.2.2 will demonstrate that it was found to be susceptible to large systematic effects.
The eP detector supplements the Møller detector by providing information on the back- ground electron-proton scattering events.
4.1.1 Calorimeter Design
The calorimeter is mounted on the movable detector cart located in End Station A (Fig- ure 4.2). It is 16 radiation lengths thick, chosen as a compromise between large signal size and minimizing sensitivity to the pion background. The calorimeter is composed of 100 copper plates interspersed with quartz fibers. The three inner regions are 10% quartz by vol- ume, while the eP detector is 2%. Electrons shower in the copper and produce Cherenkov light in the quartz, which is then directed through light guides to photomultiplier tubes (PMTs) for detection. The Cherenkov angle in quartz is close to 45◦, so the plates and fibers are positioned at this angle to maximize light collection efficiency. The resulting geometry is depicted in Figure 4.3. The In, Mid, and Out rings make up the dark inner region while the eP detector is the light outer region.
Figure 4.3: Partially constructed E158 calorimeter.
The quartz fibers are bundled to divide the calorimeter into the four concentric zones.
Each region is further subdivided with bundles of fibers servicing separate photomultiplier tubes. Figure 4.4 depicts the different regions of the detector. The In and eP rings are serviced by 10 tubes, while the Mid and Out rings have 20 tubes.
Figure 4.4: Calorimeter channel map.
To protect the PMTs from radiation damage, they are located roughly 70 cm from the beamline. The 60 light guide periscopes that direct the Cherenkov light from the calorimeter to the PMT locations are shown in Figure 4.5.
The PMTs are also encased by a large slab of lead for further protection. Figure 4.6 presents a diagram of the lead shielding. The PMTs are positioned in the cylinders drilled in the back shield. The two rows of PMTs are located at radial distances of 67.3 cm and 75 cm. Over the course of the experiment, the E158 calorimeter absorbed a radiation dose
Figure 4.5: Light guide configuration.
of approximately 500 MRad, while the PMT dose was only ∼1 Rad.
Figure 4.6: E158 calorimeter lead shielding.
4.1.2 Calorimeter Electronics
The electronics for the E158 calorimeter are depicted in Figure 4.7 [47]. The RLC circuit is employed to increase the length of the signal from the PMTs. The longer time constant allows the ADCs (analog-to-digital converters) to integrate for a longer period of time,
suppressing random noise. Employing this method, it was found that the ADCs had only one to two counts of noise, compared to the full range of 64,000 counts.
Figure 4.7: E158 calorimeter electronics diagram.
The power supplies and ADCs are located in the electronics hut, which is accessible at all times. The hut is connected to the End Station through 200 feet of cable laid in an underground tunnel. The amplifiers are located in the End Station near the detector to avoid amplifying pick-up noise from the cables.
4.1.3 Calorimeter Resolution
The asymmetry resolution of the Møller detector is the dominant contributor to the overall uncertainty on the measured value of AP V. Because of the large cross section for Møller scattering, the detector receives a signal of≈20 million scattered electrons for a beam cur- rent of 5×1011 electrons. The counting statistics contribute roughly 160 parts-per-million (ppm) to the resolution of the detector. Additionally, common-mode electronics noise con- tributes 110 ppm to the resolution, so that the overall resolution is near 200 ppm. Figure 4.8 depicts the Møller detector asymmetry distribution for a standard one-hour data run. The correlation of the detector with the beam monitors has already been removed through the regression process, covered in Section 6.3.
Figure 4.8: Møller detector asymmetry resolution.
The Out detector receives an additional four to seven million Møller electrons (depending on whether the insertable eP collimator is in or not). If the Out detector is included with the In and Mid regions, the detector resolution improves slightly, as seen in Figure 4.9.
Because of its large systematic susceptibility and only marginal resolution gain, the Out detector is not included in the measurement ofAP V.
Figure 4.9: Møller plus Out ring asymmetry resolution.
The eP ring is dominated by electron-proton scatters, and is used to measure the parity-
violating asymmetry in this background. Figure 4.10 depicts the eP detector asymmetry distribution. The resolution of the eP detector is adequate to perform its function, which will be covered in Section 6.7.1.
Figure 4.10: eP ring asymmetry resolution.
4.1.4 Calorimeter Linearity
A non-linear response can introduce an error between the asymmetry observed with the detector AM easured and the physics asymmetry AP V. Section 5.10.1 demonstrates that the shift introduced by the non-linearityof the detector is given by
AM easured= (1−)AP V −AT oroid, (4.1)
whereAT oroid refers to the charge asymmetry of the beam measured with a toroid.
The regression procedure is employed to reduce the detector’s sensitivity to beam pa- rameters, including the charge asymmetry, effectively removing theAT oroid term in Equa- tion 4.1. The remaining (1−)AP V term is a source of systematic uncertainty. Because AP V is≈-150 ppb, it is important to keepat the level of 1% to insure that the systematic
Run I Ring Linearity
In 0.996 ±0.013 Mid 0.986 ±0.010 Out 0.995 ±0.012
Run II Ring Linearity
In 0.994 ±0.012 Mid 0.989 ±0.009 Out 1.009 ±0.011
Table 4.1: Measured linearity of the In, Mid, and Out rings of the E158 calorimeter, for Run I and Run II.
uncertainty is only a few ppb.
The linearity of the calorimeter was constrained by comparing the observed value of a large asymmetry measured at multiple PMT input light levels [48]. The asymmetry was provided by the iron foil used for polarimetry, described in Section 4.2. The foil was inserted simultaneously with the liquid hydrogen target so that the signal flux was nearly the same as during normal production running. Filters were placed in front of some of the PMTs, reducing the light level to 1/3 or 1/2 of the signal seen during normal running. By comparing the asymmetry recorded by each class of PMT, it is possible to determine the linearity. Linearity of 100% would mean that all tubes would record the same asymmetry.
Table 4.1 presents the Møller detector linearity found by this method in Run I and Run II.
The results indicate that the detector non-linearity is at the 1% level, as required.