Neutron Beam Line at SLAC

Direct searches for Dark Matter (DM) require full understanding of the detector's characteristics, and careful calibration of its energy response. The neutron is the one standard model particle whose energy deposition mechanisms at low energies in matter parallel those expected from WIMP-like Dark Matter particles, and thus, should allow a direct calibration to be attained. Low energy neutron generators (typically 2.5 MeV) have been used in earlier experiments to calibrate detectors for the small nuclear recoil energies expected for DM particles in the 10 GeV/c mass region and above. However, if the Dark Matter particle mass is very small, say ~1-10 GeV/c or even below, one will need to explore the phase space of very low energy nuclear recoils, using correspondingly low neutron energies. In this note, we explore the feasibility of developing an low energy neutron beam (10 keV to a few MeV) at SLAC, produced in one of the beam dumps with the primary electron beam from the SLAC Linac, where one would use neutron time-of-flight (TOF) to define the neutron energy. In this note, we explore two possible neutron sources at SLAC: the ESA and/or the LCLS beam dumps. Introduction As experimental limits on WIMPS in the mass region above 10 GeV/c have improved, attention has become more focused on the lower energy region. Any measurements made with DM detectors that probe this mass region require that the low energy response of the detectors be well understood. Fig. 1 illustrates an example of one measurement attempting to understand this low recoil energy regime with a small LXe detector, as shown in Fig. 1b [1]. One could imagine similar tests with other types of Dark Matter detectors. Fig. 1a shows a neutron beam from the H(d,n)He-neutron generator, providing ~2.5 MeV energy, entering the LXe volume, where one measures a scintillation response for a given nuclear recoil energy, determined using two neutron detectors EJ-301 at a given polar angle and calculated using equation (1). Fig. 1b shows the PMT arrangement in this LXe detector. Fig. 1c shows the scintillation response Leff as a function of the nuclear recoil. The lowest energy nuclear recoils measured are ~3 keV in this particular test [1]. Note that for a fixed scattering angle, the recoil energy scales by the neutron beam energy.