Ulisse Bravar

Design and Testing of a Position-Sensitive Plastic Scintillator Detector for Fast Neutron Imaging

We describe the design and performance of a position sensitive scintillator detector developed for neutron measurements. Several of these detectors are to be used in the assembly of the Fast Neutron Imaging Telescope (FNIT), an instrument with imaging and energy measurement capabilities, sensitive to neutrons in the 2-20 MeV energy range. FNIT was initially conceived to study solar neutrons as a candidate instrument for the Inner Heliospheric Sentinels (IHS) program under formulation at NASA. It is now being adapted to locate Special Nuclear Material (SNM) for homeland security purposes by detecting fission neutrons and reconstructing the image of their source. The detection principle is based on multiple elastic neutron-proton scatterings in organic scintillator. The detector presented here utilizes wavelength-shifting (WLS) fibers, grooved into the plastic scintillator and read out by multianode photomultiplier tubes (MAPMTs) to determine scattering locations. By also measuring the recoil proton and scattered neutrons energies, the direction and energy spectrum of incident neutrons can be determined and discrete sources identified

PAMELA's Measurements of Magnetospheric Effects on High-energy Solar Particles

The nature of particle acceleration at the Sun, whether through flare reconnection processes or through shocks driven by coronal mass ejections, is still under scrutiny despite decades of research. ...

Calibration of the Fast Neutron Imaging Telescope (FNIT) Prototype Detector

The paper describes a novel detector for neutrons in the 1 to 20-MeV energy range with combined imaging and spectroscopic capabilities. The Fast Neutron Imaging Telescope (FNIT) was designed to detect solar neutrons from spacecraft deployed to the inner heliosphere. However, the potential application of this instrument to Special Nuclear Material (SNM) identification was also examined. In either case, neutron detection relies on double elastic neutron-proton (n-p) scattering in liquid scintillator. We optimized the design of FNIT through a combination of Monte Carlo simulations and lab measurements. We then assembled a scaled-down version of the full detector and assessed its performance by exposing it to a neutron beam and an SNM source. The results from these tests, which were used to characterize the response of the complete FNIT detector to fast neutrons, are presented herein.

Development of the fast neutron imaging telescope

We report on the development of a next generation neutron telescope, with imaging and energy measurement capabilities, sensitive to neutrons in the 2-20 MeV energy range. The fast neutron imaging telescope (FNIT) was initially conceived to study solar neutrons as a candidate instrument for the inner heliosphere sentinels (IHS) program under formulation at NASA. This detector is now being adapted to locate special nuclear material (SNM) for homeland security purposes by detecting fission neutrons and reconstructing the image of their source. In either case, the detection principle is based on multiple elastic neutron-proton scatterings in organic scintillator. By reconstructing the scattering coordinates and measuring the recoil proton energy, the direction and energy of each neutron can be determined and discrete neutron sources identified. We describe the performance of the FNIT prototype, report on the current status of R&D efforts and present the results of recent laboratory measurements

The Fast Neutron Imaging Telescope (FNIT) - hardware development and prototype testing

We describe the design and performance of a prototype instrument with capabilities for both imaging and spectroscopy of sources of 1-10 MeV neutrons. The laboratory prototype is composed of custom-designed liquid scintillator detectors and employs measurements of double neutron-proton scattering to determine the direction and energy of incident neutrons. The detectors provide measurements of the position, energy deposit and pulse shape for each interaction. Pulse shape discrimination and a time-of-flight measurement between coincident interactions in two detectors are used to identify the neutron events, reject background due to Compton scattered gamma radiation and provide a measure of the scattered neutron energy. Calculations employing double n-p scattering kinematics are used to accumulate images and spectra of the neutron source. The quality of these images and spectra is correlated with the energy, position and timing resolution of the detectors. The prototype instrument achieves ~5deg angular resolution and 20% energy resolution (1sigma). This paper presents the design and the most recent results.

Atmospheric neutron measurements with the SONTRAC science model

The Solar Neutron Tracking (SONTRAC) telescope was originally developed to measure the energy spectrum and incident direction of neutrons produced in solar flares, in the energy range 20-250 MeV. While developed primarily for solar physics, the SONTRAC detector may be employed in virtually any application requiring both energy measurement and imaging capabilities. The SONTRAC science model (SM) is presently being operated at the University of New Hampshire (UNH) as a ground-based instrument to investigate the energy spectrum, zenith and azimuth angle dependence of the cosmic-ray induced sea-level atmospheric neutron flux. SONTRAC measurements are based on the non-relativistic double scatter of neutrons off ambient protons within a block of scintillating fibers. Using the n-p elastic double-scatter technique, it is possible to uniquely determine the neutrons energy and direction on an event-by-event basis. The 3D SM consists of a cube of orthogonal plastic scintillating fiber layers with 5 cm sides, read out by two CCD cameras. Two orthogonal imaging chains allow full 3D reconstruction of scattered proton tracks

FNIT: the fast neutron imaging telescope for SNM detection

We report on recent progress in the development of the Fast Neutron Imaging Telescope (FNIT), a detector with both imaging and energy measurement capabilities, sensitive to neutrons in the 2-20 MeV range. FNIT was initially conceived to study solar neutrons as a candidate design for the Solar Sentinels program under formulation at NASA. This instrument is now being configured to locate fission neutron sources for homeland security purposes. By accurately identifying the position of the neutron source with imaging techniques and reconstructing the energy spectrum of fission neutrons, FNIT can locate problematic amounts of Special Nuclear Material (SNM), including heavily shielded and masked samples. The detection principle is based on multiple elastic neutron-proton (n-p) scatterings in organic scintillators. By reconstructing the n-p event locations and sequence and measuring the recoil proton energies, the direction and energy spectrum of the primary neutron flux can be determined and neutron point sources identified. The performance of FNIT is being evaluated through a series of Monte Carlo simulations and lab tests of detector prototypes. The Science Model One (SM1) of this instrument was recently assembled and is presently undergoing performance testing.

Nuclear material detection techniques

Illicit nuclear materials represent a threat for the safety of the American citizens, and the detection and interdiction of a nuclear weapon is a national problem that has not been yet solved. Alleviating this threat represents an enormous challenge to current detection methods that have to be substantially improved to identify and discriminate threatening from benign incidents. Rugged, low-power and less-expensive radiation detectors and imagers are needed for large-scale wireless deployment. Detecting the gamma rays emitted by nuclear and fissionable materials, particularly special nuclear materials (SNM), is the most convenient way to identify and locate them. While there are detectors that have the necessary sensitivity, none are suitable to meet the present need, primarily because of the high occurrence of false alarms. The exploitation of neutron signatures represents a promising solution to detecting illicit nuclear materials. This work presents the development of several detector configurations such as a mobile active interrogation system based on a compact RF-Plasma neutron generator developed at LBNL and a fast neutron telescope that uses plastic scintillating-fibers developed at the University of New Hampshire. A human-portable improved Solid-State Neutron Detector (SSND) intended to replace pressurized 3He-tubes will be also presented. The SSND uses an ultra-compact CMOS-SSPM (Solid-State Photomultiplier) detector, developed at Radiation Monitoring devices Inc., coupled to a neutron sensitive scintillator. The detector is very fast and can provide time and spectroscopy information over a wide energy range including fast neutrons.

Imaging solar neutrons below 10 MeV in the inner heliosphere

Inner heliosphere measurements of the Sun can be conducted with the proposed Solar Sentinel spacecraft and mission. One of the key measurements that can be made inside the orbit of the Earth is that of lower energy neutrons that arise in flares from nuclear reactions. Solar flare neutrons below 10 MeV suffer heavy weak-decay losses before reaching 1 AU. For heliocentric radii as close as 0.3 AU, the number of surviving neutrons from a solar event is dramatically greater. Neutrons from 1-10 MeV provide a new measure of heavy ion interactions at low energies, where the vast majority of energetic ions reside. Such measurements are difficult because of locally generated background neutrons. An instrument to make these measurements must be compact, lightweight and efficient. We describe our progress in developing a low-energy neutron telescope that can operate and measure neutrons in the inner heliosphere and take a brief look at other possible applications for this detector.

Advanced characterization and simulation of SONNE: a fast neutron spectrometer for Solar Probe Plus

SONNE, the SOlar NeutroN Experiment proposed for Solar Probe Plus, is designed to measure solar neutrons from 1-20 MeV and solar gammas from 0.5-10 MeV. SONNE is a double scatter instrument that employs imaging to maximize its signal-to-noise ratio by rejecting neutral particles from non-solar directions. Under the assumption of quiescent or episodic small-flare activity, one can constrain the energy content and power dissipation by fast ions in the low corona. Although the spectrum of protons and ions produced by nanoflaring activity is unknown, we estimate the signal in neutrons and γ−rays that would be present within thirty solar radii, constrained by earlier measurements at 1 AU. Laboratory results and simulations will be presented illustrating the instrument sensitivity and resolving power.