Session D11: Nuclear Instrumentation and Applications

Neutron Beam Characterization for the NPDGamma Experiment at the Spallation Neutron Source

The NPDGamma experiment is being carried out at the FnPB at the Spallation Neutron Source to study the effect of the weak nucleon-nucleon interaction in the $\vec{n} + p \rightarrow d + \gamma$ reaction. In the experiment, polarized low-energy neutrons capture on protons in a liquid hydrogen target. The parity-violating gamma spin-asymmetry, detected in a 3$\pi$ detector, is a clean signature of the weak interaction. To control systematic errors and achieve statistical precision, it is important to know the characteristics of the polarized neutron beam. We will discuss simulations and comparative data for the neutron beam flux, profile, and optimized neutron chopper settings.   

Neutron Polarization Measurements with a 3He Spin Filter for the NPDGamma Experiment

The Fundamental Neutron Physics Beamline (FNPB) at the Spallation Neutron Source (SNS) provides a pulsed beam of polarized cold neutrons for the NPDGamma experiment which intends to measure the parity violating asymmetry in the emitted gamma rays from the capture of polarized neutrons on protons. The neutrons are polarized by a multi-channel super mirror polarizer, and the polarization can be flipped with an RF spin rotator. The accuracy of the NPDGamma experiment and various commissioning experiments is dependent on the polarization of the neutron beam and the efficiency of the RF spin rotator. These parameters are measured with a polarized $^{3}$He spin filter at multiple points in the beam cross section and with multiple $^{3}$He polarizations. The analysis methods, background effects, and preliminary results will be discussed.   

A liquid parahydrogen target for the NPDGamma Experiment

The NPDGamma Experiment, which is running at the SNS, is in the process of measuring the parity-violating correlation $A_\gamma$ between neutron spin and gamma momentum in the reaction $\overrightarrow{n} + p\rightarrow d + \gamma$ with a projected statistics-limited precision of $10^{-8}$. To achieve this statistical goal, a polarized cold neutron beam is captured in a 16 liter liquid parahydrogen target which includes a cryogenic system, containment mechanisms designed for safe filling, venting and storage of the hydrogen, and a data acquisition system. We will describe the target system and its performance.   

Calculation of the Detector Solid Angles and Geometrical Factors for the NPDGamma Experiment

The goal of the NPDGamma experiment is to measure the parity-violating asymmetry in $\gamma$-ray emission in the capture of polarized neutrons on para-hydrogen. The detector array consists of 48 gain-matched CsI-crystals arranged in 4 rings around the hydrogen target. The detector array has approximately $3\pi$ acceptance angle. The sensitivity in the measurement of a physical asymmetry is dependent on detector solid angles and to the position of detectors. The position of detectors is given by geometrical factors. Initial calculations of the geometrical factors were performed with an analog Monte Carlo approach. The geometrical factors and solid angles are modelled in modified MCNPX and calculated to approximately 1\% uncertainty and compared to experimental results.   

Cryogenic Design for the nEDM Experiment

The neutron electric dipole moment (nEDM) measurement proposed for the Spallation Neutron Source (SNS) will provide a precision test of time reversal symmetry. Lowering the upper limit on the nEDM, currently $\sim$ 10$^{-26}$ $e\cdot$cm, by a factor of 100 would test proposed extensions to the Standard Model. Thermal neutrons from the SNS will be trapped as ultracold neutrons in two $\sim$ 2.2~liter cells filled with superfluid $^4$He at 0.45~K. Polarized $^3$He atoms dissolved into the $^4$He will serve as co-magnetometers. A large dilution refrigerator (DR) will cool the cells and a surrounding 1000~liter volume of liquid helium. Transporting the $^3$He to and from the cells via heat flush, and the large diameter tubes that connect volumes of superfluid liquid helium at $T \leq 0.5$~K to 4.2~K and above cause significant heat loads on the DR. We have developed thermal models to estimate these heat loads and guide the design of the DR heat exchangers. Previous theoretical and experimental results indicate that large heat flows occur due to superfluid film creep up the tubes and reflux of evaporated gas at higher temperatures. We have prepared an experiment to measure this effect at temperatures relevant to the nEDM experiment.   

Characterization of segmented Silicon detectors for neutron beta decay experiments

The ``Nab'', and ``UCNB'' collaborations will measure the correlation parameters ``a'', ``b'', and ``B'' that are found in the triple differential rate equation from neutron $\beta$-decay ($ n \rightarrow p + e + \bar{\nu}_{e} $). These parameters that offer an atractive platform for searches of signals of new physics beyond standard model will be measured using unpolarized cold neutrons (Nab) at SNS, ORNL,and polarized ultracold neutrons (UCNB) at LANL. Following a neutron $\beta$-decay the electron and proton, will be accelerated in a $4\pi$-field spectrometer, and detected by a novel detector design consisting of two opposite large area and thick silicon detectors segmented in 127 pixels per detector, and operated at $\sim$ 100 Kelvin. We have successfully completed the first phase of detector characterization, operating 0.5, 1.0, and 1.5 mm thick Silicon detectors of 11 cm in diameter for neutron $\beta$-decay experiments at Los Alamos National Laboratory, and detected $\sim$ 300 Hz protons from 15 to 35 keV at NCSU with a FWHM resolution of $\sim$ 3.2 keV with a potential of another factor of two improvement. Custom amplifiers based on FETs mounted directly on the detector reduced the noise and made possible the proton detection.   

The fused-silica Cherenkov detector system for the Qweak experiment

The Qweak experiment will measure the proton's weak charge via the parity-violating asymmetry in electron-proton scattering, measuring the asymmetry to a precision of 5~parts per billion. A continuous $\sim 180~\mu$A beam of polarized 1.1~GeV electrons interacts with a 35~cm liquid hydrogen target. Elastically scattered electrons with momentum transfer $Q^2 \approx 0.03$~GeV$^2$ are focused onto an array of eight fused silica detectors, each 2~m long. Cherenkov light emitted inside the silica is collected by photomultiplier tubes. The typical electron rate in each detector is $\sim$700~MHz; during production data-taking, the photocurrent is measured continuously by low-noise integrating electronics. The accelerator can also deliver the same beam at much lower currents ($\sim100$~pA); for these measurements, the phototubes can be connected to higher-gain fast electronics capable of resolving single-electron events. In this talk I will describe the construction, operation, and performance of these detectors.   

Metal-loaded Liquid Scintillator for Neutrino Experiments

Success of neutrino experiment requires a combined knowledge of chemistry and physics for its detector design and operation. Organic liquid scintillator (LS) has been selected as one of the main detection mediums for neutrino interactions since the early demonstrated experiment of Reines and Cowan. Metal-loaded LS (M-LS) can be used for different applications in neutrino researches, where diverse choices of metals serve different functions. Fundamental aspects of the M-LS for neutrino detection are extended chemical stability, long attenuation length, high photon production, and ultra-low radioactive impurity. BNL Neutrino {\&} Nuclear Chemistry group has been focusing on the development of new metallic-ion loading techniques for numerous neutrino experiments since early 2000. The challenges and applications of M-LS for neutrino experiments will be reported.   

Indium-loaded Liquid Scintillator for the Low Energy Neutrino Spectrometer (LENS)

The Chemistry Department at Brookhaven National Laboratory has a long history of neutrino research since Ray Davis's Homestake experiment. The Solar Neutrino and Nuclear Chemistry group has been successfully building large neutrino detectors over the past decade for various physics experiments, using tens to hundreds of tons of liquid scintillator. Among them, LENS aims to use 8{\%} indium-loaded LS (In-LS, first investigated by Raghavan in the 1970s) for a real-time measurement of over 95{\%} of sub-MeV solar neutrinos, mainly from pp-, CNO-, and $^{7}$Be-processes. A nearly background-free spectral image from neutrino interactions on $^{115}$In can be obtained via a triple coincidence tag in space and time. LENS detector R{\&}D has made major progress in the recent years. The development of In-LS, in collaboration with Virginia Tech, now meets the challenging requirements of light yield, optical clarity, and chemical stability; and the collaboration is in the process of building a 410-L prototype (\textit{mini}LENS). In this talk, the preparation and properties of In-LS for the \textit{mini}LENS detector will be presented.   

Scintillator-Layered Imaging Microscope for Environmental Research

We are developing a detection mechanism to observe fluorescent light from a fluorescently-tagged biological sample as well as the electron from the beta decay of $^{14}$C within the same sample while maintaining the position information of the interaction. This system will be used to study the carbon uptake of microorganisms from permafrost soil samples, which will help understand the output of CO and CH$_4$ from thawing permafrost. Our system is called SLIMER, the Scintillator-Layered Imaging Microscope for Environmental Research. The microscope component of SLIMER is a fluorescence microscope already capable of detecting the fluorescent signal. We chose to use CsI(Tl), which, when vapor-deposited on a fiber-optic plate, grows with a microcolumnar structure. These columns channel the light produced within them, thus maintaining the position information of the beta decay. We will present our progress using this method of detection.