Wavelength-shifting Fibers Research Articles

An empirical formula for calculating the spatial resolution of a wavelength-shifting fibre detector coupled with a ZnS/6LiF scintillator for detecting thermal neutrons

An empirical formula that predicts the spatial resolution of a thermal neutron detector comprising of wavelength-shifting (WLS) fibre arrays coupled with a ZnS/6LiF scintillator screen was derived based on experimental results. The spatial resolutions of the test detectors were measured separately in order to evaluate the contributions due to light spreading within the scintillator and the WLS fibres layers. The empirical formula involved the scintillator thickness and the diameter of the WLS fibres. The results calculated using the derived formula faithfully reproduced the results obtained experimentally with the test detectors, demonstrating the feasibility of using the formula when designing the neutron detecting head for this type of detector.

Recording of relativistic particles in thin scintillators

Results of investigating an assembly of thin scintillators and silicon photomultipliers for registering relativistic particles with the minimum ionization are presented. A high efficiency of registering relativistic particles using an Ej-212 plastic scintillator, BSF-91A wavelength-shifting fiber (Saint-Gobain), and a silicon photomultiplier (Hamamtsu) is shown. The measurement results are used for creating a scintillation hodoscope of the magnetic spectrometer for registering γ quanta in the GlueX experiment.

Upgrade of the POLDI diffractometer with a ZnS(Ag)/6LiF scintillation detector read out with WLS fibers coupled to SiPMs

A thermal neutron detector based on ZnS(Ag)/6LiF scintillator, wavelength- shifting fibers (WLS) and silicon photomultipliers (SiPM) is under development at the Paul Scherrer Institute (PSI) for upgrading the POLDI instrument, a pulse-overlap diffractometer. The design of the detector is outlined, and the measurements performed on a single channel prototype are presented. An innovative signal processing system based on a photon counting approach is under development. Its principle of operation is described and its performances are evaluated on the basis of a Monte Carlo simulation.

A real-time, large area, high space resolution particle radiography system

In this paper we describe a new detection system for the high resolution measurement of the residual range of charged particles, designed and developed with the aim of achieving real-time data acquisition and large detection areas. A prototype of the residual range detector, with a sensitive area of about 4 × 4 cm2, consisting of a stack of sixty ribbons of scintillating fibers (Sci-Fi) has been designed and tested. Each layer is read-out by two wavelength shifter (WLS) fibers and a position sensitive photomultiplier (PSPM). The Bragg peak shape is calculated real-time by the time over a suitable threshold for each channel. The results of the measurements taken using the prototype and a 62 MeV proton beam and a comparison with the GEANT4 simulations of the detector are presented. The main concepts on which the prototype is based have been used to demonstrate the technique patented by the INFN. The next step will be to design and validate the final detector which will have 30 × 30 cm2 FOV and cover the 250 MeV proton range with about 150 micron range resolution. These performances are suitable for almost all medical imaging applications.

Test and simulation of plastic scintillator strips readout by silicon photomultipliers

We studied the light collection in plastic scintillator strips, optimized for the detection of Minimum Ionizing Particles (MIPs).The light is collected by Wave Length Shifter (WLS) fibers and detected by Silicon Photo Multipliers (SiPMs). The study is based on prototypes developed for the muon detector of SuperB experiment. In parallel to measurement made on various type of geometries, a complete simulation suite, based on FLUKA, was developed. The simulation parameters were tuned by comparison with real data. In this way, we were able to study the effects of geometries and assembling procedures on light collection and provide a useful simulation tool for the design of future prototypes.

Uniformity measurements and new positioning algorithms for wavelength-shifting fiber neutron detectors

Wavelength-shifting (WLS) fiber scintillator detectors were successfully installed at two neutron powder diffractometers at the Spallation Neutron Source (SNS). However, they have the following second-order disadvantages: (i) they cannot have both high efficiency and images free of ghosting (position misassignment) concurrently; (ii) the apparent detection efficiency and spatial resolution are not uniform. These issues are related to the diffusion of scintillation photons and the fluctuation in the number of photons (quantum noise) collected by photo-multiplier tubes (PMTs). To mitigate these two issues, we developed two statistics-based positioning algorithms, i.e., a centroid algorithm (CEA) and a correlation algorithm (CA). Compared with the generally used maximum-photon algorithm (MPA), the CEA eliminates the ghosting with only about a 10% loss in detection efficiency, and provides better uniformity in detection efficiency and intrinsic background and lower gamma-ray sensitivity. The CA can effectively eliminate ghosting too, but the loss of efficiency at the group boundaries of PMTs is large. The results indicate that both algorithms can reduce the influence of quantum noise on the neutron positioning.

Silicon photomultiplier characterization with a scintillating bar detector

This paper describes a setup dedicated to the test of silicon photomultipliers interfaced to a scintillating bar detector. The main item of the setup is a scintillator bar tracker made of extruded bars, readout with wavelength shifter fibers: multiple configurations in terms of number of fibers and readout bar sides are available to test different readout approaches. This setup has been used to test the prototype SiPMs produced by FBK-irst which are characterized by a larger fill factor and photodetection efficiency with respect to standard devices. The SiPMs have been characterized in terms of efficiency, signal to noise ratio and mean photoelectron number using cosmic rays.

Segmented scintillation detectors with silicon photomultiplier readout for measuring antiproton annihilations

The Atomic Spectroscopy and Collisions Using Slow Antiprotons experiment at the Antiproton Decelerator (AD) facility of CERN constructed segmented scintillators to detect and track the charged pions which emerge from antiproton annihilations in a future superconducting radiofrequency Paul trap for antiprotons. A system of 541 cast and extruded scintillator bars were arranged in 11 detector modules which provided a spatial resolution of 17 mm. Green wavelength-shifting fibers were embedded in the scintillators, and read out by silicon photomultipliers which had a sensitive area of 1 × 1 mm(2). The photoelectron yields of various scintillator configurations were measured using a negative pion beam of momentum p ≈ 1 GeV/c. Various fibers and silicon photomultipliers, fiber end terminations, and couplings between the fibers and scintillators were compared. The detectors were also tested using the antiproton beam of the AD. Nonlinear effects due to the saturation of the silicon photomultiplier were seen at high annihilation rates of the antiprotons.

A position-sensitive tubular scintillator-based detector as an alternative to a 3He-gas-based detector for neutron-scattering instruments

A position-sensitive tubular scintillator-based neutron detector is proposed as an alternative to a 3He-gas-based detector. The detector has a neutron-detecting element constructed from rolled ZnS/6LiF scintillator screens that sandwich wavelength-shifting (WLS) fibre coils (SFC element). Multiple SFC elements are enclosed in an aluminium tube in a row to form a one-dimensional position-sensitive neutron detector. The design of the WLS fibre coil, which was determined by performing basic experiments, comprised two 0.75-mm-diameter WLS fibres wound in parallel at a pitch of 1.5mm. A 64-element detector with a pixel size of 22mm×20mm (width×length) successfully demonstrated the detection principle. The tubular shape of the new detector is similar to the usual 25-mm-diameter 3He tube, making this an alternative detector with the potential to be installed in a vacuum tank for inelastic-neutron-scattering instruments.

A scintillator-based detector with sub-100-μm spatial resolution comprising a fibre-optic taper with wavelength-shifting fibre readout for time-of-flight neutron imaging

A scintillator-based neutron-counting imaging detector with a sub-100-μm spatial resolution was developed for energy-selective neutron imaging. The detector head of the detector comprised a thin ZnS/6LiF scintillator screen, a fibre-optic taper and crossed wavelength-shifting (WLS) fibre arrays. A high spatial resolution was achieved by constructing the scintillator with a thickness of 100μm and placing it in contact with the fibre-optic taper at a magnification ratio of 3.1:1. WLS fibres with a diameter of 100±5μm (mean±SD) were specially made, and their dye content was optimized for use in crossed WLS-fibre arrays. The developed detector had a pixel size of 34μm×34μm, and exhibited spatial FWHM resolutions of 80±7μm and 61±6μm in the x and y directions, respectively. A small prototype detector demonstrated the capability of neutron imaging using Bragg edges of a Cu/Fe sample when using the pulsed-neutron source in the Materials and Life Science Experimental Facility at the Japan Proton Accelerator Research Complex.

Development of a wavelength-shifting fiber-based photon detector for LBNE

The proposed LBNE experiment will employ liquid argon TPCs for the far detector. We are developing a photon detector prototype based on wavelength-shifting fibers and utilizing silicon photomultipliers for potential use in the LBNE far detector. This paper describes progress and plans of the prototype development. An update on the development of a cryogenic detector development test facility, which includes a 500 L cryostat designed for testing full-scale photon detector components for LBNE will also be covered.

Time dependent measurements of induced fission for SNM interrogation

Gammas from induced fissions were measured and separated into prompt and delayed particles. To this end, a dedicated detector was realized, based on a plastic scintillator, a wavelength shifter fiber and a silicon photomultiplier (SiPM). Results are presented from the interrogation of Special Nuclear Materials (SNM), employing a pulsed neutron generator in the PUNITA graphite moderator incorporating the above detector assembly. The detector response is presented, as well as the sensitivities for prompt and delayed processes within the same experimental setup.

COMPET: High resolution high sensitivity MRI compatible pre-clinical PET scanner

COMPET is a pre-clinical MRI compatible PET scanner which decouples sensitivity and resolution by the use of a novel detector design. The detector has been built using 8×8cm2 square layers consisting of 30 LYSO crystals (2×3×80mm2) interleaved with 24 Wavelength Shifting Fibers (WLS) (3×1×80mm3). By stacking several layers into a module, the point-of-interaction (POI) can be measured in 3D. Four layers form a PET ring where the sensitivity can be increased by stacking several layers. The layers can be stacked so that no inter-crystal or inter-module gap is formed. COMPET has used four assembled layers for module and scanner characterization. The modules are connected to the COMPET data-acquisition chain and the reconstructed images are produced with the novel geometry-independent COMPET image reconstruction algorithm. Time and energy resolution have been resolved and found to be around 4 ns and 14% respectively. Tests for MRI interference and count rate performance have been carried out. The reconstruction algorithm has been verified with data acquired by means of a COMPET full ring PET scanner.

Design of a muon tomography system with a plastic scintillator and wavelength-shifting fiber arrays

Recently, monitoring nuclear materials to avoid nuclear terrorism has become an important area of national security. It can be difficult to detect gamma rays from nuclear material because they are easily shielded by shielding material. Muon tomography using multiple -Coulomb scattering derived from muons can be utilized to detect special nuclear materials (SNMs) such as uranium-235 and plutonium-239. We designed a muon tomography system composed of four detector modules. The incident and scattered muon tracks can be calculated by two top and two bottom detectors, respectively. 3D tomographic images are obtained by extracting the crossing points of muon tracks with a point-of-closest-approach algorithm. The purpose of this study was to optimize the muon tomography system using Monte Carlo simulation code. The effects of the geometric parameters of the muon tomography system on material Z-discrimination capability were simulated and evaluated.

Characterization of a small CsI(Na)-WSF-SiPM gamma camera prototypeusing 99mTc

A small field of view gamma camera is being developed, aiming for applications in scintimammography, sentinel lymph node detection or small animal imaging and research. The proposed wavelength-shifting fibre (WSF) gamma camera consists of two perpendicular sets of WSFs covering both sides of a CsI(Na) crystal, such that the fibres positioned at the bottom of the crystal provide the x coordinate and the ones on top the y coordinate of the gamma photon interaction point. The 2D position is given by highly sensitive photodetectors reading out each WSF and the energy information is provided by PMTs that cover the full detector area. This concept has the advantage of using N+N instead of N × N photodetectors to cover an identical imaging area, and is being applied using for the first time SiPMs. Previous studies carried out with 57Co have proved the feasibility of this concept using SiPM readout. In this work, we present experimental results from true 2D image acquisitions with a 10+10 SiPMs prototype, i.e. 10 × 10 mm2, using a parallel-hole collimator and different samples filled with 99mTc solution. The performance of the small prototype in these conditions is evaluated through the characterization of different gamma camera parameters, such as energy and spatial resolution. Ongoing advances towards a larger prototype of 100+100 SiPMs (10 × 10 cm2) are also presented.

Elimination of ghosting artifacts from wavelength-shifting fiber neutron detectors

Misassignment of neutron position (ghosting) produces artifacts which have been observed in wavelength-shifting (WLS) fiber detectors developed for time-of-flight (TOF) neutron powder diffraction. In position-sensitive detectors (PSDs) with WLS fiber encoding, thermal and cold neutrons interact with a monolithic (6)LiF/ZnS:Ag scintillator screen, and scintillation photons are generated and transported through the crossed fibers to photomultipliers (PMTs). The neutron position is determined by photon counts in the PMTs within a preset time window. Ghosting occurs when neutrons hit the group boundaries of two neighboring PMTs for x-position multiplexing, which is modeled as resulting from a long travel length (about 3-5 mm) of a small number of scintillation photons. This model is supported by the change observed in aperture images when the threshold number for photon-pulses was adjusted for neutron event determination. When the threshold number of photon-pulses was set above 10 for each PMT, the ghost peaks in the aperture images and TOF spectra of powder diffraction were strongly suppressed or completely eliminated, and the intrinsic background levels of the WLS detectors were significantly reduced. Our result indicates that WLS fiber detector is a promising alternative for (3)He PSDs for neutron scattering.

Multilayer scintillation spectrometer for charged pionium detection

The design description and characteristics of a 14-layer scintillation spectrometer for meson recording are given. The results from testing the spectrometer, calibrating it with cosmic-ray particles, and using the particle beams at energies reaching 1 GeV are presented. The spectrometer design is based on flat scintillation plates glued with wavelength-shifting optic fibers.

Long term performance of the MINOS calibration procedure and stability of the MINOS detectors

The MINOS detectors are steel-scintillator sampling tracking calorimeters and are calibrated using an in-situ light-injection system and cosmic ray muons. The MINOS Near and Far Detectors have been operating almost continuously since 2003 and 2005, providing opportunity to quantify the behavior of the various detector components, many of which are used in the next generation neutrino oscillation experiments, under long-term experimental operation. We report on the calibration procedure and its stability, as well as the time and temperature dependencies of the scintillator, wavelength-shifting fibers and photo-multiplier tubes.

Detection of extensive cosmic air showers by small scintillation detectors with wavelength-shifting fibres

A set of three small scintillation detectors was employed to measure correlated events due to the passage of cosmic muons originating from extensive air showers. The coincidence rate between (any) two detectors was extracted as a function of their relative distance. The difference between the arrival times in three non-aligned detectors was used to reconstruct the orientation of the shower axis.

A large-area two-dimensional scintillator detector with a wavelength-shifting fibre readout for a time-of-flight single-crystal neutron diffractometer

A two-dimensional scintillator-based neutron detector that has a neutron-sensitive area of 256×256mm2 with a pixel size of 4mm was developed. The detector was designed to be compact and modular with the smallest dead area for the SENJU time-of-flight Laue single-crystal diffractometer to be constructed in the Materials and Life Science Experimental Facility at the Japanese Proton Accelerator Research Complex. The detector employed wavelength-shifting (WLS) fibres to collect scintillation light generated in a neutron-sensitive scintillator. The 64 WLS fibres with a diameter of 1mm were regularly spaced at a pitch of 4mm both in the x and y directions to produce a detector with a large neutron-sensitive area and a small number of electronics channels. Two ZnS/10B2O3 scintillator screens with an optimised scintillator thickness sandwiched the cross-arranged WLS fibre arrays to ensure a high detection efficiency for thermal neutrons. The prototype detector exhibited a detector efficiency of 40±1%(mean±SD) for 1.6Å neutrons and a 60Co gamma-ray sensitivity of 6.0±0.1×10−6, which fulfilled the required detector specifications for SENJU.