Understanding the Dead Layer: Examining experimental conditions affecting proton energy loss

The Nab experiment at the Spallation Neutron Source will precisely characterize neutron beta decay to further understand the weak interaction. It uses an asymmetric, magnetic spectrometer with an unpolarized cold neutron beam to measure the correlation, a, between the electron and the antineutrino. By means of the conservation of momentum, this correlation only requires the detection of proton momenta and electron energy. These measurements are performed by using silicon detectors to determine the decay proton's momentum and decay electron's energy. The "dead layer" at the entrance of the detector is unable to produce detectable energy signals, thus resulting in proton energy loss. In poor vacuum conditions, there could be a gradual increase in the build-up of frozen particles on the detector, which could expand the effective dead layer. If the dead layer increases with time, that will decrease the detected proton energy and therefore the accuracy of the Nab results. I will discuss measurements of the proton energy as a function of detector cooling time, the possible relationship to an increased dead layer, and the implications for the Nab experiment requirements.