Session R7: Instrumentation I

Development of Spin-Exchange Polarized 3He Target Cells that Incorporate both Glass and Metal

Approved experiments at Jefferson Lab following the 12 GeV upgrade will include a measurement of the elastic electric form factor of the neutron using a polarized $^3$He target at a luminosity more than ten times higher than previous experiments. Historically, polarized $^{3}$He targets at JLab have been made out of glass. At higher beam currents, it will be desirable to incorporate metal windows through which the electron beam can enter and exit. There is only limited data on nuclear spin relaxation due to metal surfaces and the alkali metal we use in spin-exchange optical pumping (the technique we use to polarize the $^3$He) has adverse effects on certain metal surfaces causing it to be more relaxing over time. For this reason we have been studying spin relaxation on a variety of metals that may be incorporated in our targets. Measurements from Mainz show, under significantly different conditions, that gold is a promising candidate\ [A.$\,$Deninger et.$\,$al., Eur.$\,$Phys.$\,$J.$\,$D \textbf{38},$\,$439$\,$(2006)\@]. Among the results we will present are spin-relaxation measurements of cells made of glass and OFHC copper in which the copper has been plated with gold. Our measurements show considerable promise for a new generation of high-luminosity polarized $^3$He targets.   

Development of a Thin-Walled Tritium Gas Target System for $(\gamma,p)$ Measurements

The planned $^3$H$(\gamma,p)$nn experiment at the TUNL High Intensity Gamma-ray Source will provide data that test three-nucleon ab-initio calculations, and will enable a determination of the $^1 S_0$ neutron-neutron scattering length. This experiment will involve measurement of the emitted protons with energies down to about 1.5 MeV, and requires an overall uncertainty in energy measurements of less than 250 keV in the region of interest. The target will be approximately 230 Ci of gaseous tritium contained in a series of thin-walled cylindrical cells, constructed from 2.5-$\mu$m thick Havar foil. We report on methods for building, filling, handling, and leak testing the target cells.   

Cross calibration of the JLab, Hall C, Compton and M\o ller polarimeters and a study of systematic uncertainties of the Compton electron detector

A Compton polarimeter was commissioned at Jefferson Lab, Hall C, for continuous non-invasive measurement of the electron beam polarization. It uses $\sim 1.5~kW$ of green light for the $\vec{e}-\vec{\gamma}$ scattering. The polarimeter has several planes of diamond micro-strip detectors to detect the Compton scattered electrons and a $PbWO_{4}$ crystal for detecting back-scattered photons. It was successfully used to measure the electron beam polarization along with periodic polarization measurements by the standard Moller polarimeter. The diamond micro-strip electron detector provided a standalone measurement of the beam polarization with $< 1\%$ statistical uncertainty per hour, for a 1.16 GeV, $180~\mu$A electron beam. The systematic uncertainties are projected to be better than 1\%. We will discuss the various contributions to the systematic uncertainties for the electron detector. We also collected data at low current for a Moller-Compton cross calibration. The preliminary results from the analysis of these data will be presented.   

Improving the quantum efficiency of the CLAS12 LTCC PMTs with a p-Terphenyl wavelength shifter

An improved version of the CLAS Cherenkov detector will be used as the Low-Threshold Cherenkov Counter (LTCC) for the CLAS12 spectrometer at JLAB. The original detector used 216 UV-glass PMTs, which have a poor quantum-efficiency (QE) below 300nm due to the UV-glass transparency. The application of a p-Terphenyl wavelength shifter to the face of these PMTs dramatically improves the QE for short wavelengths, rivaling that of a much more expensive quartz PMT. This translates into an improved detector performance for the LTCC, especially in the higher-energetic regimes in the CLAS12 spectrometer. We will discuss the coating process, as well as the performance testing taking place at Temple University.   

Improved Detection of Cherenkov Radiation using Wavelength-Shifting Paints

Photomultiplier Tubes (PMTs) are often used to detect Cherenkov radiation in accelerator-based physics experiments. Since the Cherenkov spectrum is inversely proportional to the square of the photon's wavelength, PMTs with relatively good quantum efficiencies in the ultraviolet region can produce on average a higher number of photoelectrons. The application of certain paints, which absorb light at ultraviolet wavelengths and emit in the visible spectrum, to the surface of some PMTs allows for better sampling of the Cherenkov spectrum. The effects of various wavelength-shifting (WLS) paints designed by \textit{Eljen Technologies} were tested on \textit{ET Enterprises, Model: 9390KB} PMTs. Using a $^{\mathrm{106}}$Ru $\beta $-source, Cherenkov light was produced in disks of fused silica. The charge spectrums of the PMTs were measured before and after application of the paint. The average number of photoelectrons produced from the Cherenkov radiation could be determined by knowing the value of the single-photoelectron peak and the mean of the charge spectrum. Four paints were tested, and the gain in the number photoelectrons produced varied from 10-35{\%} for the different paints.   

Online SVT Commissioning and Monitoring using a Service-Oriented Architecture Framework

Silicon Vertex Tracker detectors are devices used in high energy experiments for precision measurement of charged tracks close to the collision point. Early detection of faulty hardware is essential and therefore code development of monitoring and commissioning software is essential. The computing framework for the CLAS12 experiment at Jefferson Lab is a service-oriented architecture that allows efficient data-flow from one service to another through loose coupling. I will present the strategy and development of services for the CLAS12 Silicon Tracker data monitoring and commissioning within this framework, as well as preliminary results using test data.   

Measuring the Rate of Muon Capture on the Deuteron

The goal of the MuSun experiment is to measure the rate of nuclear muon capture on the deuteron with a precision of 1.5\%. This rate will be used to fix the low-energy constant that describes the two-nucleon weak axial current in effective field theory models. It will therefore calibrate evaluations of proton-proton fusion and neutrino-deuteron scattering. The experiment uses many of the techniques and much of the apparatus that were developed for the successful MuCap measurement of the rate of muon capture on the proton. However, to optimize the molecular kinetics, the deuterium gas is cooled to 30 K in a cryogenic time projection chamber (TPC). Progress in the analysis of the data taken during the production run of 2011 will be presented, as well as a description of the hardware upgrades and performance during the 2013 run, in preparation for another high-statistics run in 2014.   

Near-Threshold Measurement of \boldmath $\gamma n \rightarrow p \pi^-$ at MAX-lab

One of the outstanding questions in nuclear science is to be able to describe the dynamical properties of the nucleon using the framework provided by QCD. Pion photoproduction near threshold is one process where both experimental measurements and theoretical calculations can produce valid and useful results. Consequently, high-quality measurements of this fundamental nuclear process can be used to test the predictions of various theoretical approaches. These measurements also provide data to improve the SAID and MAID partial-wave analyses used in the interpretation of other measurements. The Photon Tagging Facility at MAX-lab in Lund, Sweden is uniquely suited to perform measurements of pion photoproduction at energies between threshold and the $\Delta$-resonance. The PIONS@MAXLAB Collaboration is performing a measurement of the $\gamma n\rightarrow p\pi^-$ channel very close to threshold. Using a LD$_2$ target and the reaction $\gamma d\rightarrow pp\pi^-$, the $\pi^-$ is captured on another deuteron creating a high-energy photon which was detected using three large NaI spectrometers. These new near-threshold data will be used to better evaluate the threshold E$_0^+(\pi^-p)$ amplitude, which can be compared with the theoretical predictions. Initial results will be presented.   

A measurement of two-photon exchange in unpolarized elastic electron-proton scattering

Jefferson Lab experiment E05-017 was designed to study 2-photon exchange contributions to elastic electron-proton scattering over a wide kinematic range. By detecting the scattered proton instead of the electron these measurements will be very sensitive to the $\epsilon$ dependence of the cross section and consequently the ratio $\frac{G_E}{G_M}$. The goals of the experiment, the experimental technique and the kinematic range will be presented. The analysis sequence and results of the early steps will be outlined.