Development of the Recoil Detector for the HESR Day-One Experiment and Commissioning at COSY by Measuring pp Elastic Scattering
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Monday, 16 March, 2015
The PANDA experiment is one of the future experiments at FAIR, it will pursue a broad physics program with high quality antiproton beam in the momentum region from 1.5 to 15 GeV/c. The goal of the PANDA luminosity detector is to determine the absolute in- tegrated luminosity with precision of ∆L/L ≤ 3%. The antiproton-proton elastic scattering differential parameters, σ tot , ρ and b are needed for the PANDA luminosity calibration in order to achieve this high precision of absolute normalization. However, the existing data of σ tot , ρ and b in the PANDA beam momentum range are not sufficient by precise for the PANDA luminosity calibration. The HESR Day-One experiment is dedicated to determine the differential param- eters by measuring antiproton-proton elastic scattering in a large range of squared 4- momentum transfer, |t|. The HESR Day-One experiment will measure the elastically scattered antiprotons and recoil protons with the PANDA luminosity detector and a recoil detector, respectively. One recoil detector consists of two silicon and two germa- nium sensors by covering the polar angle range from θ = 71 ◦ to 91.5 ◦ . The silicon and germanium detectors are single-sided strip structure. Both silicon detectors have the di- mensions 76.8 mm × 50 mm × 1 mm. The two germanium detectors have the same ac- tive area of 80.4 mm × 50 mm, but with thickness of 5 mm and 11 mm, respectively. The silicon detectors will measure the recoil protons below 12 MeV, and the germanium de- tector will measure the recoil protons up to 60 MeV. A dedicated recoil detector for tests with proton beam at COSY has been built. After assembly, the silicon and germanium detectors have been tested in the laboratory with the radioactive source 244 Cm. The optimal operation temperature, 125 K, of the germa- nium has been determined. The energy resolutions of the silicon and germanium detec- tors at 125 K are better than 20 keV and 30 keV, respectively. Energy calibration for the silicon and germanium detectors has been studied. The uncertainty of ADC’s nonlinearity is about 0.31%. The silicon detectors have been calibrated with several standard sources and the uncertainty of the energy calibration of the silicon detectors has been estimated to be around 0.33%. The germanium detectors have been calibrated with the radioactive sources 60 Co, 137 Cs and 244 Cm. Based on the α particle’s energy deposited in the sensitive area of the germanium detectors, the dead- layer thickness of the Ge #1 and Ge #2 sensors have been determined to be 0.72 μm and 0.82 μm of equivalent silicon, respectively. The uncertainty of energy calibration of the germanium detector is about 0.31%. Simulation studies of proton-proton elastic scattering at 3.2 GeV/c has been per- formed. The detector’s acceptance has been obtained. The differential counts as a func- tion of |t| has been reconstructed with acceptance correction. The proton-proton elastic scattering differential parameters σ tot , ρ and b have been determined by analyzing the characteristic shape of the |t| distribution with optical theory and parameterized expres- sions. Due to the recoil particle and kinematics to be measured in pp→pp are the same as that in pp →pp at a certain beam momentum, it is reasonable to learn about pp →pp from pp→pp. Therefore, the recoil detector was installed at the ANKE cluster target sta- tion at COSY and commissioned by measuring proton-proton elastic scattering in 2013. Data were taken at beam momenta of 1.7, 2.5, 2.8 and 3.2 GeV/c. For 2.8 GeV/c and 3.2 GeV/c, the differential counts as a function of |t| distributions have been reconstructed based on the centroid energy and elastic events in the identical strips. After analysing the characteristic shape of the |t| distribution, the elastic scattering differential parame- ters and integrated luminosity have been determined. The uncertainty of σ tot and b were better than 1% and of ρ was about 2%. The absolute integrated luminosity within preci- sion of ∆L/L ≤ 3% was achieved. Comparing the measured σ tot and ρ with the predicted values based on existing data, the differences between our results and the predicted values are less than 1.5%, but the measured b values are about 20% greater than the predicted ones. The differential cross section obtained. The dσ/dt at 2.8 GeV/c and 3.2 GeV/c have been distribution of our results in the range of |t| ∈ [0.02, 0.095] (GeV/c) 2 are in good consistent with the existing data at 3.0 GeV/c. These results demonstrate that the HESR Day-One experiment is feasible and the data analysis method is reliable.