Wavelength-shifting Fibers Research Articles

Neutronics aspects of position-sensitive neutron scintillator detectors using wavelength-shifting readout fibers

Recent investigations in using wavelength-shifting fibers with scintillator screens for two-dimensional position sensitive neutron detection have shown this method to be effective and affordable. However, hydrogen and carbon atoms in the readout fibers scatter many neutrons traveling into the scintillator and affect the detector's efficiency and position resolution. In this work we seek to quantify these effects for various neutron energies using Monte Carlo simulations of different geometrical cases and suggest methods for mitigating negative effects from neutron scattering.

Neutron imaging device using position-sensitive photomultipliers

A historical survey of the technical development on neutron imaging devices using the position-sensitive photomultiplier technique is given on the basis of works conducted by several collaborations in Europe as well as in Japan since 1986. Two different methods are compared: the first one based on the direct coupling of scintillator to the photocathode, and the second by using wavelength-shifting fibres for transfer of light signals from scintillator to the photocathode. Some perspectives for future developments are also discussed taking into account these experimental results.

Shifting scintillator prototype large pixel wavelength-shifting fiber detector for the POWGEN3 powder diffractometer

A prototype neutron scintillation detector has been developed, specifically aimed at the needs of the powder diffractometer POWGEN3 at the Spallation Neutron Source at Oak Ridge National Laboratory. This instrument requires a detector array with large (6 mm×40 mm) pixels and an area of several square meters. The prototype uses a two-dimensional grid of wavelength shifting fibers, with the fiber axes parallel to the scintillator screen, to collect the scintillation photons. The fiber ends for each pixel go to a specific set of four photomultiplier tubes, so that the position of each event can be determined by a 4-tube 2C n × 2C n coded coincidence. The observed maximum light yield with a 6LiF/ZnS:Ag neutron scintillation screen, summed over four tubes, is greater than 200 photons/neutron. This is about 0.13% of the ∼150,000 photons/neutron produced in the scintillator. The light yield is sufficient to allow pulse discrimination between neutron signals and gamma-ray background. Further light collection gains should be achievable using double-clad fiber and green-enhanced photomultiplier tubes. Currently, the shape, structure, and specific composition of the scintillator are being investigated, on the assumption that 6LiF/ZnS:Ag will be the chosen scintillator material.

The aluminization of 600 k WLS fibers for the TileCal/ATLAS/LHC

The TILE CALorimeter, the hadronic sampling calorimeter of ATLAS/LHC/CERN, is made of iron and polystyrene scintillating tiles. The light produced in scintillating tiles is absorbed and guided to the photomultipliers (PMTs) through wave length shifter (WLS) optical fibers. To optimize the detection of jets and muons, the top of the fibers away of the PMTs is coated with an aluminum mirror. This aluminum mirror is produced by planar magnetron sputtering. This process adds to an excellent reproducibility a minimal thermal aggression, important for a proper film adhesion to the plastic surface. To satisfy schedule and optical critical requirements, a dedicated mass production machine named SIDELO II was projected and constructed. A reflectivity of R/spl sim/75% is achieved and the light output uniformity improved by /spl sime/10%. The aluminization of the fibers and their quality control started in August 1999 and went on continuously until May 2002. The quality control results showed a reproducibility over the optical characteristics of the fibers that fully accomplish the ATLAS requirements.

Recent development of position-sensitive neutron detectors employing wavelength-shifting cross-fiber

For evaluating neutron optical devices, we have designed and tested two-dimensional position-sensitive neutron detectors (PSND) which employ two wavelength-shifting fiber arrays optically coupled to a scintillator plate. Feasibility on applications of the PSND having a 50×50 mm 2 sensitive area and a pixel size of 0.4×0.4 mm 2 has been tested at pulsed neutron sources. While, for realizing the large sensitive area, another PSND having a 128×128 mm 2 sensitive area and a 0.5×0.5 mm 2 pixel size with a dynode coding technique were developed and tested.

Development of a new photomultiplier tube with high sensitivity for a wavelength-shifter fiber readout

We have developed a new photomultiplier tube (PMT) with high sensitivity for a wavelength-shifter fiber readout of plastic scintillators. Improvements of the photoelectron yield were made on a 2-in. PMT (Hamamatsu R329) by adopting (1) a prism-shaped photocathode, (2) an extended-green photocathode material, and (3) an optical mirror surface of the electrodes. As a result, the number of photoelectrons obtained by the new PMT has increased to be 1.8-times that by R329. Other PMT characteristics, such as uniformity of sensitivity, time response and darkcurrent are not so much different from those of R329 for practical applications.

On the linearity of the MINOS light-injection calibration system

The MINOS light-injection calibration system has been fully documented in a previous article (Nucl. Instr. and Meth. A 492 (2002) 353). Upon commissioning in the MINOS detectors, the system was found to give a non-linear response to variations in the intensity of injected light. The source of this non-linearity has been traced to a small change in the spectrum of the injected light as a function of the current applied to the original blue LEDs, in combination with a rapidly varying spectral response function of the wavelength-shifting fibre used in the detector. Both aspects of the problem have been addressed successfully by use of LEDs with different spectral characteristics, and the system now has a linear response.

MINOS: THE PHYSICS PROGRAM AND THE CONSTRUCTION STATUS

The MINOS experiment will study muon neutrino oscillations in the range of parameter space implied by the measurements with atmospheric neutrinos. The two magnetized toroidal tracking calorimeters, 1 kton at Fermilab and 5.4 kton in the Soudan Mine 735-km away, are constructed out of extruded scintillator strips and use wavelength-shifting and clear fibers read out by multi-anode photomultipliers. The far detector is now more than 3/4 completed and commenced collecting cosmic ray data. The neutrino beamline and the near detector will be finished by the end of 2004. In addition, a calibration detector was also constructed and exposed to the test beams. Over the last two years the entire project has progressed on schedule and budget and with most of the civil construction out of the way the road to this interesting physics program is finally cleared.

Development of a fast neutron radiography converter using wavelength-shifting fibers

A fluorescent converter for fast neutron radiography (FNR) comprising a scintillator and hydrogen-rich resin has been developed and applied to electronic imaging. The rate of the reaction between fast neutrons and the converter is increased by thickening the converter, but its opaqueness attenuates emitted light photons before they reach its surface. To improve the luminosity of a fluorescent converter for FNR, a novel type of converter was designed in which wavelength-shifting fibers were adopted to transport radiated light to the observation end face. The performance of the converter was compared with that of a polypropylene-based fluorescent converter in an experiment conducted at the fast-neutron-source reactor YAYOI in the University of Tokyo.

Photomultiplier pulse Read Out system for the preshower detector of the lhcb experiment

The second generation experiment for CP violation studies in B decays, LHCb, is a 20-m-long single-arm spectrometer to be installed on the future Large Hadron Collider at CERN. For its precision measurement purpose, it combines precise vertex location and particle identification, in addition to a performance trigger system able to cope with high flux. The first level of trigger is mainly based on the fast response of the calorimetric subsystem. Of major importance is the 6000 channels preshower detector that aims to validate the electromagnetic nature of calorimetric showers. It consists of two-radiation-length lead sheet in front of a scintillator plane. Scintillator signals are extracted from plastic cells using wavelength-shifting fibres coupled to multi-anode photomultiplier tubes. The preshower Read Out system has to cope with fluctuating photomultiplier pulses caused by small amounts of photoelectrons, in addition to strong constraints imposed by the 40MHz LHC bunch-crossing frequency.A special Read Out electronics including perfect 40MHz integrators able to shape fluctuating photomultiplier pulses has been designed, and successfully realized. The temporal shape of photomultiplier pulse and the upstream Read Out system for preshower are described in this document.

Study of multianode photomultipliers for the electromagnetic calorimeter preshower read out of the lhcb experiment

The Lhcb experiment will study the CP symmetry violation in the system of the beauty particles. The detector is a 20-m-long spectrometer, to be installed on the proton–proton collider Lhc. Of major importance for the level 0 trigger is the preshower of the electromagnetic calorimeter designed to discriminate between electrons, hadrons and photons. The preshower cells consist of a two-radiation-length lead sheet located between two plastic scintillator planes. The scintillation light is extracted with wavelength-shifting fibres. The best candidates to read out the light of the 6000 detector cells are multianode photomultipliers. The HAMAMATSU photomultiplier R5900-00-M64 has been studied and its responses in terms of gain, linearity, uniformity within and between the anodes and cross-talk between the channels are addressed in this document.

Recent developments for the readout of a matrix of scintillators by means of a multi-pixel HPD

We have been investigating the possibility to readout a large number of crystals (more than 400) by means of WaveLength Shifting (WLS) square section fibers and multi-pixel Hybrid PhotoDiodes (HPDs) for PET applications. However, due to the small light yield, the detection efficiency for 511keV photons could be too low. To overcome this limitation, we have recently proposed to shape the surface of each finger crystal like an “half-pipe” to achieve a better scintillator—WLS fiber coupling. In this paper, we present the results of the light yield experimental measurements on a test 5×5 “half-pipe” YAP:Ce crystal matrix and a comparison with square fibers—standard matrix configuration.In view of future availability of large-area HPD, we have also assembled a small detector head using a test 4×4 YAP:Ce crystal matrix directly coupled to a 61-pixel HPD. Experimental measurements on the imaging capabilities are also presented.

High counting rate two-dimensional neutron-imaging method using rectangular scintillators with WLS fibers

A high counting rate two-dimensional neutron-imaging method using rectangular scintillators with wavelength-shifting (WLS) fibers has been developed for neutron-scattering experiments at a next-generation high-intensity pulsed-neutron source. Rectangular scintillators are arranged in longitudinal and transverse directions and WLS fibers are attached along four sides of these scintillators. Luminescences generated in the scintillator are absorbed by the four fibers and wavelength-shifted luminescences are detected by multi-channel photomultipliers. The position of an incident neutron is decided by coincidence of the four signals. By experiments using 5 mm×5 mm×2 mm-thickness 6Li glass scintillators and 0.5-mm square WLS fibers, it was confirmed that the maximum counting rate for each pixel was 3 Mcps with the coincidence time of 100 ns.

Development of a two-dimensional imaging detector based on a neutron scintillator with wavelength-shifting fibers

For evaluating neutron optical devices, a two-dimensional (2D) detector based on a neutron scintillator with wavelength-shifting fibers has been developed at RIKEN. We have investigated a ZnS(Ag)+LiF and a Li glass plate as neutron scintillators with the coding technique for realizing the large sensitive area of 50×50 mm2. After fabricating the 2D detector, its performance was tested using cold neutrons at JAERI. As a result, a spatial resolution of ∼1.0 mm was obtained.

High-position-resolution scintillation neutron-imaging detector by crossed-fiber readout with novel centroid-finding method

Aiming at high-position-resolution and high-counting-rate neutron imaging, a novel centroid-finding method is proposed for a scintillation neutron-imaging detector with crossed-fiber readout. Crossed wavelength-shifting fibers are arranged on and under the scintillator. Luminescences generated in the scintillator are emitted and detected by a few fibers surrounding the incident point of a neutron. In the novel method, X and Y positions of the incident neutron are decided by coincidence of a central signal and neighboring signals, respectively. By fundamental experiments using a ZnS:Ag/6LiF scintillator of 0.5-mm thickness and crossed wavelength-shifting fibers with a size of 0.5×0.5 mm2, it was confirmed that the position resolution is about 0.5 mm and the limitation of the neutron-counting rate is 320 kcps.

The QCAL tile calorimeter of KLOE

The quadrupole tile calorimeters of KLOE (QCAL) are two compact detectors placed close to the interaction point and surrounding the focusing quadrupoles. Their purpose is to increase the hermeticity of KLOE calorimetry. Each QCAL consists of a sampling structure of lead plates and scintillator tiles with wavelength shifter (WLS) fibers and mesh photomultiplier readout arranged in 16 azimuthal sectors. The arrangement of WLS fibers allows the measurement of the longitudinal position of the showers from time of flight. In this paper we describe the QCAL design and assembly and present preliminary results obtained with both cosmic rays and photons from K L decays. The time and energy calibration procedures are also discussed in detail.

A gamma ray detector with lead-scintillator tiles and WLS fiber readout for the experiment at SPring-8 to study the GDH sum rule

A prototype gamma ray detector for the detection of π 0 in the proposed experiment at SPring-8 to study the Gerasimov–Drell–Hearn (GDH) sum rule (GDH experiment) has been built and tested. The detector is fabricated with lead-scintillator tiles and wave length shifter fiber readout. This detector will be placed inside the bore of the superconducting magnet which supplies the polarized target with a 2.5 T magnetic field. High-detection efficiency (above 85%) for the gamma rays is the most important requirement for this detector to be employed in the proposed experiment. It has been tested with minimum ionizing particles, electrons and gamma rays. The results of the tests in comparison with some Monte Carlo simulations are presented. The evaluated efficiency for gamma rays ranging in energy above 50 MeV is higher than 90%.

Large liquid-scintillator trackers for neutrino experiments

Results are given on tests of large particle trackers for the detection of neutrino interactions in long-baseline experiments. Module prototypes have been assembled using TiO 2-doped polycarbonate panels. These were subdivided into cells of ∼1 cm 2 cross-section and 6 m length, filled with liquid scintillator. A wavelength-shifting fibre inserted into each cell captured a part of the scintillation light emitted when a cell was traversed by an ionizing particle. Two different fibre-readout systems have been tested: an optoelectronic chain comprising an image intensifier and an electron-bombarded CCD (EBCCD); and a hybrid photodiode (HPD). New, low-cost liquid scintillators have been investigated for applications in large underground detectors. Testbeam studies have been performed using a commercially available liquid scintillator. The number of detected photoelectrons for minimum-ionizing particles crossing a module at different distances from the fibre readout end was 6–12 with the EBCCD chain and a mirror at the non-readout end; 4–10 with the HPD and no mirror. The light-attenuation lengths in the fibres were ∼9.4 m with the EBCCD and ∼6.4 m with the HPD. The detector response to electron showers has also been measured. After ten radiation lengths of lead, the transverse position of the incoming electron was determined with a precision of a few millimetres for electrons of 4 GeV , and with a precision of 1.5– 2.0 cm for electrons of 1 GeV .

Development of scintillation imaging device for cold neutrons

As an application of the wavelength-shifting (WLS) fiber technique recently developed in the field of high-energy physics, a novel type of imaging device for neutrons has been successfully investigated; a space resolution of 1 mm FWHM with detection efficiency of 55% for 10 A ̊ neutrons has been experimentally confirmed with a prototype made of a 0.5-mm-thick ZnS(Ag)+ 6LiF scintillator plate optically coupled to WLS fiber arrays. In addition to promising results obtained in this study, its simple structure and reliable operation allow us to foresee a new generation of imaging devices to meet the increasing demand for large-area and high space-resolution imaging devices for several new projects on spallation neutron sources in the world.

Two-dimensional neutron imaging method using scintillators with wavelength shifting fibers

We have developed a two-dimensional neutron imaging method using rectangular scintillators with wavelength shifting (WLS) fibers for neutron scattering experiments using a next-generation high-intense pulsed-neutron source. In the method, rectangular scintillators are arranged at longitudinal direction and transverse direction and WLS fibers are arranged on the four sides of these scintillators. Luminescences generated in the scintillator are absorbed by the four fibers and wavelength-shifted luminescences are detected by multi-anode photomultipliers. The position of incident neutron is decided by coincidence of the four signals. By preliminary experiments using 5 mm×5 mm×2 mm t 6Li glass scintillators and 0.5 mm∅ WLS fibers, it was concluded that detection efficiency for thermal neutron was 13%. By scanning experiment using a 4×4 6Li glass scintillator array neutron detection system, we obtained the clear spot of incident neutron beam that the cross talk was <10%.