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SANE utilized frozen ammonia ($^{14}\textrm{NH}_3$) as a proton target, polarized via dynamic nuclear polarization (DNP)   in a 5 T magnetic field at around 1 K. DNP is a mature technique for target polarization in the study of nuclear   structure\cite{crabb97}, and this target has had extensive use at SLAC \cite{PhysRevD.58.112003,e155,e155x}   and Jefferson Lab \cite{PhysRevLett.87.081801}. Nuclear target polarizations of greater than 90\% are   achieved by leveraging electron--proton spin coupling in a high magnetic field, using microwaves to   excite the spin flip transitions. The open-geometry of the superconducting, split-pair magnet   allowed polarization both longitudinally and transverse to the electron beam incidence.  \subsubsection{Dynamic Nuclear Polarization}  Figure \ref{fig:DNP} shows a simplified diagram of the DNP mechanism. In a target   material with a suitable number of unpaired electron spins, hyper-fine splitting from   the spin-spin interaction of the proton and electron in the magnetic field gives four discrete energy levels. The spins of the electron and proton can be simultaneously flipped by applying microwaves of frequency lower or higher than the electron paramagnetic resonance by the proton magnetic resonance, which will result in the protons becoming aligned or anti-aligned, respectively, with the magnetic field. The electron will tend to relax into the lowest energy state, allowing it to be used to polarize another proton, making possible a continual driving of protons into positive polarization.   \begin{figure}[htb]   \begin{center}  %\includegraphics[width=2in,natwidth=4.07in,natheight=3.71in]{../experiment/target/DNP-diag.pdf}  \includegraphics[width=2in,natwidth=4.07in,natheight=3.71in]{../experiment/target/DNP-diag.eps}  \end{center}  \caption{\label{fig:DNP} Simplified diagram of the dynamic nuclear polarization mechanism.}  \end{figure}  At low temperature, the spin relaxation time of the protons is many times   greater than that of the electrons; while the protons maintain their spin   orientation, the electrons relax and can spin couple with other protons.   This creates a rate of polarization higher than the rate of depolarization   due to proton relaxation and allows polarization to be constantly built and maintained by microwaves.  Neighboring nuclear spins are coupled by dipole--dipole interactions   which allow spin flips which conserve energy. These flips are thus   frequently occurring, and allow for the transport of nuclear   polarization away from the unpaired electrons. This   process, called spin diffusion, tends to equalize the polarization throughout the material\cite{Borghini:1968gd}.  \subsubsection{NMR Polarization Measurements}  An accurate measure of the degree of the material's nuclear polarization is obtained   through nuclear magnetic resonance measurements. An RF field at the proton's Larmor   frequency will induce the spin system to either absorb or emit energy. The magnetic   susceptibility of the system describes this absorption/emission response, and the integral over frequency of the absorptive portionof the susceptibility is proportional to the absolute polarization of the material.  By embedding an NMR coil in the   target material, inductive coupling between the spins and the coil results   in an impedance which is a function of the magnetic susceptibility\cite{abragam1983}.   Integrating the voltage due to this impedance with a Q-meter\cite{Court1993433} as the RF   is swept through frequency gives proportional measure of the   absorptive part of the magnetic susceptibility, after the baseline behavior of the circuit is removed.   The polarization obtained by the NMR signal integration must be   calibrated using the calculable polarization of the material   at thermal equilibrium $P_{\textrm{TE}}$. When the material is allowed to   relax to thermal equilibrium at known temperature $T$ and   magnetic field $B$, the proton polarization is due only to the Zeeman   interaction between the field and the proton magnetic moment $\mu$, and is thus known via Boltzmann statistics:  $$P_{\textrm{TE}} = \tanh \left (\frac{\mu B}{kT} \right).$$  By integrating the Q-meter response curve at thermal equilibrium, the area   obtained $A_{\textrm{TE}}$ allows the calculation of the polarization during dynamic polarization with enhanced area $A_{\textrm{Enh}}$: $P_{\textrm{Enh}} = P_{\textrm{TE}} A_{\textrm{Enh}}/A_{\textrm{TE}}$.   \subsubsection{Material Preparation and Performance}  Irradiated ammonia ($^{14}$NH$_3$) is an attractive target material   due to its high polarizability and radiation hardiness, as well   as its favorable ratio of free, polarizable protons to   total nucleons---dilution factor. Ammonia freezes at 195.5 K, and   can be crushed through a metal mesh to produce beads of   convenient size, allowing cooling when the material is under a liquid helium bath\cite{Meyer200412}.   Before DNP is feasible, the ammonia must first be doped   with paramagnetic centers, which provide the crucial   free electrons used for spin coupling to the protons; for SANE, the target   material was irradiation doped at a small electron accelerator. Free   radicals were created by 19 MeV electrons at a beam current of   between 10 to 15 $\mu A$ which struck the frozen ammonia in a 87 K LAr$_2$ bath, until an approximate dose of 10$^{17}$ e$^-$/cm$^2$ was achieved.   After irradiation, proton polarizations in ammonia can   routinely surpass 90\% under dynamic nuclear polarization, however when   used experimental conditions, the electron beam will cause depolarization.   Beam heating will create a reduction in polarizing efficiency, and reduce the polarization by as much 5\% in a matter of seconds\cite{Liu19981}. Over hours of beam on target, excess free radicals are built up in the material which provide extra decay paths for the proton spin, and these radicals become the primary source of depolarization. This radiation damage of the material with electron dose from the beam appears as two or three exponential decays in the polarization vs.\ dose accumulated.  To recover the polarization lost to radiation damage the material can be ``annealed'' by heating it to between 70 to 100 K, allowing the recombination of certain free radicals.   This process will often allow the   polarization to achieve its previous maximal values, although the   build-up of different radicals with subsequent anneals will result in the increased decay   rate of the polarization, when the material must be replaced\cite{McKee200460}.  In the more than 300 hours of beam on target taken during SANE, 23 thermal equilibrium calibrations were taken, 26 anneals performed, and 7 material replacements were required for the 11 different samples of frozen ammonia target material used.