Detecting Protons in the aCORN Neutron Beta Decay Experiment

The aCORN experiment will measure the angular correlation coefficient ``little a'' between the electron and the anti- neutrino in neutron beta decay. The goal is to reduce the error on $a=-0.104\pm 0.004$ (PDG value) to less than 0.001 to check the completeness of the $V-A$ model of the weak interaction and the value of $V_{ud}$ in the CKM matrix. The aCORN apparatus selects electrons and protons that, after a +2~kV acceleration, have oppositely directed momenta by restricting the acceptance using a series of tungsten collimators and a co-axial 350-G magnetic field. The lack of transverse anti-neutrino momentum for the selected decays creates two event groups with the anti- neutrino momentum either parallel or anti-parallel to the electron momentum and whose rate asymmetry measures $a$. These groups are distinguished using proton time of flight to a silicon detector. Proton detector electronics was tested for installation in a confined tube where it will be cooled with liquid nitrogen and mounted on a $-30$-kV acceleration platform to separate the proton signal from noise. An optical link transfers the signal to the data acquisition system. Spectra of 60-keV gamma rays from $^{241}$Am were used as a test signal.