Session Y6: Instrumentation for Nuclear Physics

Observation of far-ultraviolet signatures of the $^3$He($n,tp$) reaction

In previous work [1] on the production of Lyman $\alpha$ radiation in the the $^3$He($n,tp$) reaction in a cell of $^3 $He gas at atmospheric pressure, we found that $\approx 50$ Lyman $\alpha$ photons were produced per absorbed neutron. These photons are produced by excited states of T and H created by charge-exchange and excitation collisions of $t$ and $p$ with the background gas [2]. This provides a method of neutron detection with intrinsic discrimination against gamma rays. We are now conducting studies of far-ultraviolet emission in $^3$He mixtures with other gases, e.g. $^4$He, N$_2$, Ne, Ar, Kr, and Xe. The heavier noble gases amplify by factors of over 100 the signals seen in pure $^3$He. The results suggest the possibility of an efficient optical neutron detector with improved economy of use of $^3$He.\\[4pt] [1] A. K. Thompson, {\it et al.}, {\it J. Res. Nat. Inst. Standards Tech.} {\bf 113}, 69 (2008)\newline [2] J. W. Cooper, {\it et al.}, {\it J. Res. Nat. Inst. Standards Tech.} {\bf 114}, 185 (2009)   

Neutron energy response of a hybrid scintillator/$^3$He calorimeter

Fast neutron energy spectrum above 1 MeV provides a unique window for nuclear material detection and identification. We describe a neutron calorimeter consisting of an array of plastic scintillators and low-pressure (200 mbar) $^{3}$He drift tubes. Fast neutrons transmit their kinetic energies to protons in the plastic through elastic collisions, generating one or multiple scintillator pulses. Thermalized neutrons are then detected through the capture reaction $^{3}$He(n,p)$^{3}$H. The hybrid calorimeter implements a few coincidence schemes to measure fast neutron energy spectrum. By requiring a signal in the $^3$He tube, only fast neutrons that deposit their full energies in the scintillators are counted. By requiring multiple scintillator responses within a certain time window and certain directions, only the proton recoil events due to fast neutron collisions are preferentially selected. The performances of the calorimeter are calibrated using neutrons of known energies. Practical issues such as $\gamma$-ray background reduction, nonlinear energy response of the scintillators, edge loss of protons, and configuration of the detector cells will be discussed. The measurements are compared with MCNPX simulations.   

Efficiency Measurement of VANDLE Modules

The Versatile Array of Neutron Detectors at Low Energy (VANDLE) is a new array of plastic scintillator bars being developed at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL). The modular design enables optimization of different configurations for particular experiments, such as (d,n) and beta-delayed neutron-decay experiments, with rare ion beams. Two prototype modules were moved to the Edwards Accelerator Laboratory at Ohio University to measure their efficiency using a calibrated $^{27}$Al(d,n) reaction as a neutron source. Results show that one bar with a cross section of 3x3~cm$^{2}$ is over 25\% efficient to neutrons around 1~MeV with sensitivity down to 100~keV neutrons. Other design features such as wrapping and coupling will be presented, as well as results from resolution tests.   

Measurement of Neutron Detection Efficiency of Crystal Ball and TAPS Detector System

Accurate Cross section measurement of reactions such as $\pi^0$ and $\pi^0\eta$ production on the neutron requires accurate simulation of detector acceptances. The neutron detection efficiency of the combined Crystal Ball and Two Arm Photon Spectroscopy, TAPS, detector system currently employed by the A2 collaboration in the Tagged Photon hall at MAMI accelerator in Mainz, Germany, is measured to test the accuracy of such simulation. To this end, photo disintegration of the deuteron - $d(\gamma,p)n$ and $\pi^0$ production off the deuteron - $d(\gamma,p,\pi^0)n$ channels are investigated. Preliminary neutron detection efficiency results are produced using liquid deuterium target data with 885 beam energy which are to be compared using recent liquid deuterium target data with 1557 beam energy. Ultimately, the efficiencies will be compared to results obtained from a GEANT-4 simulation of the complete detector setup in order to validate the neutron response provided therein.   

A simple charged particle spectrometer for a pion production experiment

Measurement of a charged particle energy is not a new task, but inexpensive ways of such measurement are of interest, in particular as they open new opportunities for advanced charged particle radiography. We describe a magnetic spectrometer we have recently built for pion production measurement experiments at LANSCE (Los Alamos). The spectrometer consists of four modules of drift tubes and a bending magnet. A maximum magnetic field in the bending magnet was about 7500 Gauss. Drift tubes of 2 inches in diameter were made of thin carbon fiber to minimize multiple scattering in their walls. The spectrometer was used in the scattering experiment with primary beam of 800 MeV protons and C or Al target. We present measurements of secondary protons, pions and muons scattered in the reverse direction. Energy deposited in 1 cm thick plastic scintillator and 4" CsI was compared to the energy measured from the particle bending in the magnetic field. Experimental data are compared to GEANT4 modeling. We discuss, how this technology may be applied to the particle identification and to the energy loss measurements.   

Development and commissioning of the CARIBU project

The Californium Rare Ion Breeder Upgrade (CARIBU) will enhance the radioactive beam capability of the ATLAS accelerator by providing high quality neutron-rich beam from a $^{252}$Cf fission. The whole apparatus consists of four main components: 1)A Helium filled gas catcher and RFQ ion cooler that thermalizes fission products and forms a low-energy ion beam; 2)An isobar separator that magnetically purifies ion cocktails to a mass resolution of approximately 1/20000; 3)A charge breeder ECR ion source where ions of low charge states are further ionized by electron bombardment in the plasma; 4)A low energy experimental area where ions are trapped and bunched to suit high precision experiments. Ion optical simulations for CARIBU ion cooling, bunching and transmission will be presented. Experimental results from commissioning will be compared with the corresponding calculations. Other technical details of the facility and insight gained in its commissioning will also be presented. Current status of CARIBU will be given.   

Measuring absolute cross sections with HELIOS

The HELIOS device at Argonne National Laboratory provides a means of analyzing outgoing ions produced in direct reactions in inverse kinematics, with good energy resolution. To best exploit this device it is imperative that there is a means to measure absolute cross sections: ingredients essential, for instance, to the extraction of spectroscopic factors. A method has been developed to determine absolute cross sections and to monitor the luminosity in the course of a measurement, which relies on the magnetic properties of HELIOS and the elastic scattering cross section at small center-of-mass angles. This method was tested recently using a 680-MeV $^{136}$Xe beam impinging a CD$_2$ target of nominal thickness 110~$\mu$g/cm$^2$---here ($d,d$) elastic scattering was measured in the Rutherford regime. Using the same system the deuterium content of the target was monitored at higher (10~MeV/u) beam energies to obtain information on the extent to which the target degrades with beam dose.   

A Novel Holding Field Coil for the SNS nEDM Experiment

Magnetic shielding requirements demand rigid constraints on the holding field for neutrons entering the SNS nEDM cryostat. The field must be uniform in the neutron path but vanish in the measurement cell and mu-metal shielding. In addition, the coil can only be formed of nonmagnetic materials. A modified double cosine-theta coil has been designed to satisfy these conditions. This coil was designed using the desired B-fields as boundary conditions and solving Maxwell's equations directly to obtain the coil winding geometry. A prototype coil has been built and field-mapped. A comparison of calculated and measured fields of this prototype will be presented.   

A Study of Multi-Shower Discrimination in the New Muon $(g-2)$ Calorimeter

We have used a GEANT3 simulation of the New Muon ($g-2$) Calorimeter to study discrimination between single $e^+$ and double $e^+$ electromagnetic showers. The calorimeter was defined as a $20\times 16\times 16$RL cm$^3$ box with alternating layers of plastic scintillator and tungsten. Assumed energy resolution was matched to the measured resolution of a detector prototype. An artificial neural network algorithm (ANN) was trained to discriminate between a single positron shower and the background events. The five ANN input variables were: (1) maximum energy deposited in the single module $E_{\rm M}$, (2) module index with maximum energy deposition $i_{\rm M}$, (3) energy deposition $E_{\rm R1}$ in the ring of modules around $i_{\rm M}$, (4) energy deposition $E_{\rm R2}$ in the next-to-nearest-neighbors around $i_{\rm M}$, and (5) outer energy sum $E_{\rm OUT}$. The efficiency of the ANN algorithm in recognizing a single $e^+$ signal while suppressing false positives (two $e^+$'s misidentified as a single) was studied as a function of energy and the calorimeter segmentation. The results were used as an input design parameter for the 2D tracking calorimeter hodoscope.