\chapter{Conclusions and Suggestions} A new High Repetition Rate Linac (HRRL) beamline located in the Beam Lab of Physics Department at Idaho State University has been successfully reconstructed to produce and transport positrons to the experimental cell. The electron beam energy profile and emittance of the HRRL were measured using a Faraday cup and an OTR based diagnostic system. The electron spatial profile measured using the OTR system was not described by a Gaussian distribution but by a super Gaussian or Lorentzian distribution. The unnormalized rms emittance of HRRL at 15$\pm$1.6~MeV and 11~pC micro pulse charge is $0.37\pm0.02$ for horizontal projection and $0.30\pm0.04$ for vertical projection. Positrons generated when the electron beam impinged on a tungsten target (T1) were transported to another tungsten target, T2. Positrons were collected by the quadrupole triplet downstream of T1 and bent 45$^\circ$ by the first dipole D1. The beam bent by D1 diverged and was refocused using the quadrupole in the 45 degree beamline, Q7. The beam was bent another 45$^\circ$ by the second dipole, D2. The quadrupole triplet in the 90 degree beamline transported positrons to T2. The momentum/energy spread of the beam can be selected using the energy slit located between D1 and Q7. Positrons transported to T2 annihilated inside T2 and created 511~keV photons. The 511~keV photons were detected using two NaI detectors placed horizontally to T2. T2 is a 1.016~mm thick retractable tungsten target placed in the 6-way cross. NaI detectors were operated in triple coincidence between electron gun pulse and two NaI detectors. Positrons were measured at energies between 1 and 5~MeV. The ratio of positrons to the electrons (e$^+$/e$^-$ ratios) are shown in Figure~\ref{fig:e+2e-exp-sim} (illustrated by the hollow diamond symbols). The e$^+$/e$^-$ ratios are on the order of $10^{-15}$. The ratio is the highest near 3~MeV. The positron beam creation, beam loss in the transportation, and detection process were studied using the simulation package G4beamline and compared to this experiment. The simulated e$^+$/e$^-$ ratios are shown in Figure~\ref{fig:e+2e-exp-sim} for the energies measured in this experiment. The simulation includes electron beam generation with the measured electron energy profile, beam losses during transportation, positron annihilation in T2, and detection of 511~keV photons. While the simulation result agrees with the experiment in that the peak energy distribution is near 3~MeV, it predicts a higher positron to electron ratio as shown in Figure~\ref{fig:e+2e-exp-sim}. The original goal of the experiment was to measure the positron rate at 3~MeV and the beamline was optimized accordingly. The decision to measure the rates at other energies was made few hours before the end of the beam time. The beamline was not well optimized, because of the short time limit, and resulted in lower rates than estimated in the simulation. Systematic error study using simulation demonstrated that a mis-alignment in the beamline can reduce the e$^+$/e$^-$ ratios by 20\% to 40\%. In the worst case scenario, the ratios dropped by 60\% to 80\%. This might explain why the e$^+$/e$^-$ ratios in experiment are much lower than predicted in simulation. The ratio of the positrons contained within the 90 degree beampipe to the 511~keV photons detected in coincidence mode when the dipoles were set to bend 3~MeV positrons is predicted to be 1655:1 by the simulation. The ratio of the positrons on T2 to the 511~keV photons is 620:1 under the same conditions as above. The 3~MeV positron rate measured in experiment was $0.25\pm0.2$~Hz when the HRRL was operated at a 300~Hz repetition rate, 100~mA peak current, and 300~ns (FWHM) RF macro pulse length. Based on this simulation, a measured $0.25 \pm 0.02$~Hz coincidence rate by the NaI detectors would correspond to a $155 \pm 12$~Hz positron rate incident on T2. In simulation, the number of positron collected was insensitive to the quadrupole triplet collection field setting (see section 4.5). The ratio of solid angles subtended by the quadrupole (Q4) and dipole (D1) entrance windows approximated the ratios of positron transported. Dipoles defocus in one plane and defocus in the other. Thus, one can only collect positrons in one plane while loses in the other. Solenoids, on other hand, focus in both planes. A solenoid may be a better option to improve the collection efficiency. Positioning the target T1 at the entrance the solenoid may be the optimal choice for capturing positrons. %7. Experimental results show quadrupole magnets are not efficient in collecting positrons, since positrons have large angular distribution. Solenoid might be able to improve the collection efficiency of positrons~\cite{kim-bindu-solenoid} and should be placed as close the production target as possible for better efficiency. \begin{figure}[htbp] \includegraphics[scale=0.79]{5-Conclusion/Figures/Overlay_Exp-Sim-Ratio/R.eps} \caption{Ratio of positrons detected to electrons measured in the experiment (hollow diamond) and simulation (full circle) in coincidence mode. The black solid error bars are statistical and dashed ones are systematic. The experimental systematic errors (red dashed lines) are discussed in section 3.4 of Chapter 3 and the systematic errors in simulation (red dashed lines) estimation is described section 4.6 of Chapter 4.} \label{fig:e+2e-exp-sim} \end{figure} %An OTR based diagnostic tool was designed, constructed, and used to measure the beam emittance of the HRRL. The electron spatial profile measured using the OTR system was not described by a Gaussian distribution but by a super Gaussian or Lorentzian distribution. The unnormalized projected emittances of the HRRL were measured to be less than 0.4~$\mu$m by the OTR based tool using the quadrupole scanning method when accelerating electrons to an energy of 15~MeV. %OTR used, Not Guassian, Changed magnet, measured emttiance