Ronald G. Cooper

A detector for neutron imaging

A bright neutron source such as the Spallation Neutron Source (SNS) places extreme requirements on detectors including excellent 2-D spatial imaging and high dynamic range. Present imaging detectors have either shown position resolutions that are less than acceptable or they exhibit excessive paralyzing dead times due to the brightness of the source. High neutron detection efficiency with good neutron-gamma discrimination is critical for applications in neutron scattering research where the usefulness of the data is highly dependent on the statistical uncertainty associated with each detector pixel.. A detector concept known as MicroMegas (MicroMEsh GAseous Structure) has been developed at CERN in Geneva for high-energy physics charged-particle tracking applications and has shown great promise for handling high data rates with a rather low-cost structure. We are attempting to optimize the MicroMegas detector concept for thermal neutrons and have designed a 1-D neutron strip detector which we have tested In addition, we are performing research into the compatibility of various converter coatings. Our goal is to develop a manufacturable detector that could be scaled to a 1m/sup 2/, 2-D array for use at the SNS and other facilities.

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ~55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ~30 mA energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mA level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ~few hundreds μistrain for 300s of data acquisition time.

Wavelength-shifting-fiber scintillation detectors for thermal neutron imaging at SNS

We have developed a wavelength-Shifting-fiber Scintillator Detector (SSD) with a 0.3 m2 area per module. Each module has 154 × 7 pixels and a 5 mm × 50 mm pixel size. Our goal is to design a large area neutron detector offering higher detection efficiency and higher count-rate capability for Time-Of-Flight (TOF) neutron diffraction in the Spallation Neutron Source (SNS). A ZnS/6LiF scintillator combined with a novel fiber encoding scheme (v.3) was used to record the neutron events. A Cross-fiber Read-Out-Card (CROC) based digital-signal processing electronics and position-determination algorithm was applied for neutron imaging. Neutron-gamma discrimination was carried out using Pulse-Shape Discrimination (PSD). A sandwiched flat scintillator detector can have a detection efficiency close to He-3 tubes (about 10 atm). A single layer and sandwiched flat scintillator detectors have count rate capabilities of about 6,000 and 35,000 cps/cm2, respectively, which can satisfy the count rate requirement of powder diffractometers at SNS. Detectors with v.3 fiber encoding have better image quality and higher spatial resolution than those with previous v.2 fiber encoding.

First Measurements of the Inclined Boron Layer Thermal-Neutron Detector for Reflectometry

A prototype detector based on the inclined boron layer principle is introduced. For typical measurement conditions at the Liquids Reflectometer at the Spallation Neutron Source, its count rate capability is shown to be superior to that of the current detector by nearly two orders of magnitude.

High resolution neutron imaging at high counting rates with noiseless readout

We describe the design and report on the experimental results of a novel thermal and cold neutron imaging detector utilizing neutron sensitive (10B-doped) microchannel plates (MCPs). In this detector, the incoming neutron interaction products produce secondary electrons at the pores adjacent to the absorption point within the MCP glass. This electron signal is then multiplied by a stack of conventional MCPs within those adjacent pores limiting the spread of the signal to less than two pore diameters (currently 6-10 mum pores on 8-12 mum centers). The event position then can be encoded by a number of readout techniques already developed for photon/charged particle counting applications. This paper presents the results of experimental evaluation of neutron sensing MCP detector with Medipix2 readout allowing operation at high counting rate mode (>100 MHz level) at a spatial resolution limited by the 55 mum pixel size of the Medipix2 readout. Other attractive features of MCP neutron detectors are their high detection efficiency (approaching 50% levels) for thermal and cold neutrons and the absence of readout noise.

Optical readout for imaging neutron scintillation detectors

The Spallation Neutron Source (SNS) under construction at the Oak Ridge National Laboratory (ORNL) will be the most important new neutron scattering facility in the United States. Neutron scattering instruments for the SNS will require large area detectors with fast response (< 1 microsecond), high efficiency over a wide range of neutron energies (0.1 to 10 eV), and low gamma ray sensitivity. We are currently developing area neutron detectors based on a combination of a 6LiF/ZnS(Ag) scintillator screen coupled to a wavelength-shifting fiber optic readout array. A 25 x 25 cm prototype detector is currently under development. Initial tests at the Intense Pulsed Neutron Source at the Argonne National Laboratory have demonstrated good imaging properties coupled with very low gamma ray sensitivity. The response time of this detector is approximately 1 microsecond. Details of the design and test results of the detector will be presented.

First measurements of the inclined boron layer thermal-neutron detector for reflectometry

A prototype detector based on the inclined absorber layer principle is introduced. For the Liquids Reflectometer at the Spallation Neutron Source, it is shown to be a significant improvement over its current detector, which imposes an instantaneous count rate limitation of 50 kcps.

A detector for 2-D neutron imaging for the spallation neutron source

We have designed, built, and tested a 2-D pixellated thermal neutron detector. The detector is modeled after the MicroMegas-type structure previously published for collider-type experiments. The detector consists of a 4/spl times/4 square array of 1 cm/sup 2/ pixels each of which is connected to an individual preamplifier-shaper-data acquisition system. The neutron converter is a /sup 10/B film on an aluminum substrate. We describe the construction of the detector and the test results utilizing /sup 252/Cf sources in Lucite to thermalize the neutrons.

Development of a Position Sensitive Neutron Detector with High Efficiency and Energy Resolution for Use at High-Flux Beam Sources.

We are developing a high-efficiency neutron detector with 1 cm position resolution and coarse energy resolution for use at high-flux neutron source facilities currently proposed or under construction. The detector concept integrates a segmented 3He ionization chamber with the position sensitive, charged particle collection methods of a MicroMegas detector. Neutron absorption on the helium produces protons and tritons that ionize the fill gas. The charge is amplified in the field region around a wire mesh and subsequently detected in current mode by wire strips mounted on a substrate. One module consisting of a high-voltage plate, a field-shaping high-voltage plate, a grid and wire strips defines a detection region. For 100 % efficiency, detector modules are consecutively placed along the beam axis. Analysis over several regions with alternating wire strip orientation provides a two-dimensional beam profile. By using 3He, a 1/v absorption gas, each axial region captures neutrons of a different energy range, providing an energy-sensitive detection scheme especially useful at continuous beam sources.