Sadiq Proposal Defense

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File:Sadiq proposal.pdf


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\author{Sadiq Setiniyaz (Shadike Saitiniyazi)\thanks{Email:}} %{address={Department of Physics, Idaho State University}} \title{PROPOSAL FOR POSITRON PRODUCTION EFFICIENCY STUDY USING HIGH REPETITION RATE LINAC AT IAC}

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\abstract{ \indent

I propose to measure the positron production efficiency for a positron source that uses a quadrupole triplet system to collect positrons from a tungsten target that are produced when the target is impinged by electrons from the High Repetition Rate Linac (HRRL) at Idaho State University's (ISU) Idaho Accelerator Center (IAC). Positrons were observed in May of 2008 at the IAC without the use of a quadrupole triplet collection system. When a 10~MeV electron beam is used on the tungsten target, positrons escaping from the downstream side of the tungsten have a wide momentum spread of 0 to 2~MeV and a large divergence of $\pi$ rad. A quad triplet collection system, after the tungsten target, is used to focus the positron beam and as a result increase our positron collection efficiency. I will install the collection system and associated beam line components and measure the positron production efficiency using the HRRL.}

\section{Introduction} \indent

I propose to measure the positron production efficiency for a positron source that uses a quadrupole triplet system to collect positrons from a tungsten target that are produced when the target is impinged by electrons from the HRRL. A polarized positron source, as a new probe to explore nuclear and particle physics at Jefferson Lab, is being studied at the Continuous Electron Beam Accelerator Facility (CEBAF). While their main mission is to optimize polarization, ISU's goal is to optimize positron production efficiency. Additionally, a positron beamline at ISU is also a potential tool for nuclear physics studies. I have measured the emittance of the HRRL electron beam and constructed PMT bases for four NaI detectors. I will install the collection system and associated beam line components to measure the positron production efficiency using the HRRL.

\section{Previous Measurements} \indent

Earlier positron production measurements were conducted at ISU's IAC in the May of 2008. The setup is shown in Fig.~\ref{fig:2008-pos-beamline} and the beamline elements are described in Table~\ref{tab:2008-pos-beamline-elements}. The accelerator was operated at a 300~Hz repetition rate and 10~MeV energy. Electrons were bent by the first dipole and sent to a 2~mm thick tungsten target. Any positrons produced were focused using two quadrupoles and bent 45 degrees by a second dipole which was set to transport 3~MeV positrons. Positrons were transported to the end of the linac where they annihilated in a Ta target. A HpGe and a NaI detector were used to detect the 511~keV positrons produced as a result of annihilation. Fig.~\ref{fig:2008-spectrum} shows the spectrum taken over a 600 second time interval.

\begin{figure}[htbp] \centering \includegraphics[width=80mm]{2008_positron_measurement_at_IAC.eps} \caption{The HRRL beamline configured for positron production at IAC in 2008. } \label{fig:2008-pos-beamline} \end{figure}

\begin{table}[htbp] \caption{Beamline elements for positron production at IAC in 2008.}

\begin{tabular}{ll} \hline

      \textbf{Item} & {$\textbf {Description}$} \\


          Tantalum foil   &  6 mm thick 20 mm x 20 mm area   \\
          Tungsten foil   &  2 mm thick 20 mm x 20 mm area    \\
          Phosphorus flag  & 1 mil aluminum backing             \\
          HpGe detector & 81.3mm Diameter, 55.5mm Length \\
          %NaI detector	&

\hline \end{tabular} \label{tab:2008-pos-beamline-elements} \end{table}

\begin{figure}[htbp] \centering


\caption{Spectrum from HpGe Detector and NaI detecotrs.} \label{fig:2008-spectrum} \end{figure}

%\begin{figure}[htbp] %\centering % \includegraphics[scale=0.45]{2008_PositronYield_SweeperMagnet_run60-61} %\caption{Spectrum.} %\label{fig:2008-spectrum-zoom} %\end{figure}

\section{Proposed Beamline} \indent

I propose a measurement of the positron production efficiency using the HRRL. The HRRL can provide electron beams with energies between 3~MeV and 16~MeV, and a maximum repetition rate of 300~Hz. The HRRL beamline has recently been reconfigured to generate and collect positrons, see Fig.~\ref{fig:HRRL-e+-line} and Table~\ref{tab:hrrl}.


  \caption{Operational Parameters of The HRRL Linac.}
      Parameter     & Unit   & Value \\
       maximum electron beam energy $E$   &  MeV     &  16   \\
      electron beam peak current $I_{\textnormal{peak}}$ &  mA      &  80     \\
       macro-pulse repetition rate                   &   Hz       &  300  \\
       macro-pulse pulse length (FWHM)          &   ns       &  250    \\
       rms energy spread                                &  \%      &   4.23   \\

\end{tabular} \label{tab:hrrl} \end{table}

The new beamline was first designed by Dr. G. Stancari to use a quadrupole triplet system to collect positrons~\cite{stancari}. The design was further optimized by Dr. Y Kim. The final design of the beamline is shown in Fig.~\ref{fig:HRRL-e+-line}. The HRRL accelerator room is divided into two parts by an L-shaped cement wall. The accelerator cell houses the cavity and other elements needed to transport electrons to an experimental cell. The experimental cell is located in a room adjacent to the accelerator cell. The HRRL beamline was reconfigured into an achromat by moving the accelerator cavity to accommodate two dipoles and a system of quadrupole magnets optimized for collecting positrons.

In the new beamline, shown in Fig.~\ref{fig:HRRL-e+-line}, the electron beam exits the cavity and passes through a quadruple triplet that will focus the electron beam onto the positron target. Positrons produced from the positron target will be collected by the second quadruple triplet that will be optimized to collect positrons. The first dipole magnet bends the positrons/electrons, depending on the magnet polarity, by 45 degrees towards the second dipole magnet. The second dipole will bend the beam another 45 degrees, thus completing a 90 degree bend. A third quadruple triplet will focus the e-/e+ beam, as users desire. All beam elements are described in Table~\ref{tab:new-hrrl-line-elements}.

%\begin{figure*}[htbp] \begin{sidewaysfigure*}[htbp]

\centering %\includegraphics[scale=0.28]{HRRL_Pos_and_Ele_Go} \includegraphics[scale=0.35]{HRRL_Pos_and_Ele_Go.eps} \caption{The new HRRL beamline cofiguration for positron generation.} \label{fig:HRRL-e+-line} \end{sidewaysfigure*}



  \caption{The new HRRL positron beamline elements.}
        Item   &  Description \\
        T1    & Positron target \\
        T2    &  Annihilation target \\
        EnS    & Energy Slit  \\
        FC1, FC2& Faraday Cups \\
        Q1,...Q10	     & Quadrupoles \\
         D1, D2	    & Dipoles \\
        NaI     &  NaI Detecotrs \\
        OTR     &  Optical Transition Radiaiton screen\\
        YAG    & Yttrium Aluminium Garnet screen\\

\end{tabular} \label{tab:new-hrrl-line-elements} \end{table}

%00000000000000000000000000000000000000000000000000000000000 \section{Preparation for the Positron \\ Production Experiment} \subsection{HRRL Emittance measurements} \indent

Emittance, a key parameter in accelerator physics, is used to quantify the quality of an electron beam produced by an accelerator. The beam size and divergence at any point in the beamline can be described using emittance and Twiss parameters.

An Optical Transition Radiation (OTR) based viewer was installed to allow measurements at the high electron currents available from the HRRL. The visible light from the OTR based viewer is produced when a relativistic electron beam crosses the boundary of two mediums with different dielectric constants. Visible radiation is emitted at an angle of 90${^\circ}$ with respect to the incident beam direction~\cite{OTR} when the electron beam intersects the OTR target at a 45${^\circ}$ angle. These emitted photons are observed using a digital camera and can be used to measure the shape and intensity of the electron beam based on the OTR distribution.

The emittance of the HRRL was measured to be less than 0.4~$\mu$m using the OTR based tool at an energy of 15~MeV. The details of this emittance measurement using the quadrupole scanning method were described in the IPAC12 proceedings~\cite{setiniyaz-q-scan}. The results are summarized in Table~\ref{results}.


  \caption{Emittance Measurement Results.}
       {Parameter}         & {Unit}     &    {Value}    \\
        projected emittance $\epsilon_x$        &   $\mu$m    &    $0.37 \pm 0.02$     \\
         projected emittance $\epsilon_y$            &   $\mu$m    &    $0.30 \pm 0.04$     \\

% normalized \footnote{normalization procedure assumes appropriate beam chromaticity.} emittance $\epsilon_{n,x}$ & $\mu$m & $10.10 \pm 0.51$ \\ %normalized emittance $\epsilon_{n,y}$ & $\mu$m & $8.06 \pm 1.1$ \\

        $\beta_x$-function                            &  m                           &   $1.40  \pm  0.06$          \\
        $\beta_y$-function                                &  m                           &   $1.17   \pm 0.13$         \\

$\alpha_x$-function & rad & $0.97 \pm 0.06$ \\ $\alpha_y$-function & rad & $0.24 \pm 0.07$ \\ micro-pulse charge & pC & 11 \\ micro-pulse length & ps & 35 \\ energy of the beam $E$ & MeV & 15 $\pm$ 1.6 \\ relative energy spread $\Delta E/E$ & \% & 10.4 \\



\subsection{Positron Detection using NaI crystals} \indent

A tungsten target will be placed at the end of the 90 degree beamline to annihilate positrons. I want to use two NaI detectors to detect the 511~keV photons created when positrons annihilate. I acquired some NaI crystals from Idaho Accelerator Center (IAC). Since their original bases used a slow post-amplifier, I built new PMT bases. I modified the design of model PA-14 from Saint-Gobain Crystals \& Detectors Ltd. These detectors are tested, calibrated, and ready to be used for the measurement. Fig.~\ref{fig:IAC-dets} shows the crystals and the bases I built. Fig.~\ref{fig:IAC-dets-Co60-Na22-spec} shows the spectrum taken by the detector using button sources. %I expect by doing coincidence, the resolution of 511~keV peak in the spectrum will be improved.

\begin{figure}[htbp] \centering \includegraphics[scale=0.08]{IAC_NaI_Detectors} \caption{The NaI detector and base built.} \label{fig:IAC-dets} \end{figure}

\begin{figure}[htbp] \centering \includegraphics[scale=0.18]{Na22_Co60Spectrum_by_IAC_Detectors} \caption{Detector 3 calibrated Spectrum.} \label{fig:IAC-dets-Co60-Na22-spec} \end{figure}

%\subsection{Positron Target Installation} %\indent % %A step motor is ready to be installed once the vacuum chamber is ready. The step motor, shown in the Fig.~\ref{fig:step-motor}, will hold 8 tungsten targets. % %\begin{figure}[htbp] %\centering %\includegraphics[scale=0.08]{setep_motor} %\caption{Step motor for holding W targets.} %\label{fig:step-motor} %\end{figure}

\section{Future Plan} \indent

We want to produce positrons using the HRRL beam line. We can improve positron collection efficiency by applying following methods:

1. By using a quadrupole triplet before tungsten a target, we will have control over the beam size and divergence at the target.

2. Cryogenically cooled converter will be installed, and these targets will be able to take on more beam power and increase positron yield.

3. Positrons will be collected by the quadrupole triplet system, which will improve collection efficiency.

4. Simulations will optimize beam elements for positron collection.

%\bibliographystyle{unsrt} % Order by citation %\bibliography{report}

\begin{thebibliography}{9} %{stancari} %@techreport{stancari, % title =Template:Stancari's proposal-------, % month ={Nov.}, % year = {2005}, % author ={J. Stancari}, % address ={Frascati, Italy}, % number ={}, % institution ={DAFNE Technical Note} \bibitem{stancari}

G. Stancari and T. Forest "Design of a new beamline for electrons, positrons and photons at the HRRL lab", Pocatello, ID, USA (2009).

%@techreport{OTR, % title =Template:Optical Transition Radiation, % month ={}, % year = {1992}, % author ={B. Gitter}, % address ={Los Angeles, CA 90024}, % institution ={Particle Beam Physics Lab, Center for Advanced Accelerators, UCLA Department of Physics} %}

\bibitem{OTR} B. Gitter, Tech. Rep., Los Angeles, USA (1992).

%\bibitem{setiniyaz-q-scan} %@InProceedings{setiniyaz-q-scan, % author = {S. Setiniyaz, K. Chouffani, T. Forest, and Y. Kim}, % title = {TRANSVERSE BEAM EMITTANCE MEASUREMENTS OF A 16 MeV LINAC AT THE IDAHO ACCELERATOR CENTER}, % booktitle = {IPAC2012},%pages = {151--158}, % year = 2012, % address = {New Orleans, USA} %} \bibitem{setiniyaz-q-scan} S. Setiniyaz, K. Chouffani, T. Forest, and Y. Kim, in $Proc$. $IPAC2012$, New Orleans, USA.

%\bibitem{emit-mat} %C.F. Eckman $et$ $al$., in $Proc$. $IPAC2012$, New Orleans, USA.