John W. Epstein

Scintillating optical fiber trajectory detectors

Abstract Measurements of attenuation in several types of plastic scintillating optical fibers give attenuation lengths varying from 0.8 to 1.5 m. By comparing attenuation as a function of wavelength in fibers of different thicknesses we infer the contributions to the attenuation from reflection losses and bulk scintillation losses. We find good agreement between these values and calculated estimates of attenuation in scintillator. We have also calculated the effective scintillation efficiency of small fibers relative to that of bulk scintillator (for scintillator with dimethyl POPOP as the waveshifting dye) for the two cases of optically coupled and decoupled fibers. Scintillating fiber ribbons made of 200 μm square cross section fibers were exposed to relativistic iron nuclei at the LBL Bevalac, and positional resolution of 70 μm was obtained. Relativistic neon and carbon were also detected in these ribbons. In a similar exposure of 100 μm fibers to 50 MeV/n nitrogen nuclei at the NSCL cyclotron, Michigan State University, a positional resolution of about 50 μm was obtained.

Development of a second-generation fiber-optic on-line image verification system☆

PURPOSE We have previously reported the development of a fiber-optic fluoroscopic system for on-line imaging on radiation therapy machines with beam-stops because of space limitation. While the images were adequate for clinical purposes in most cases, an undesirable grid artifact existed and distracted visualization. The resolving power of the system, limited by the 1.6 mm x 1.6 mm dimension of the input fibers, appeared insufficient in some cases. This work identifies solutions to reduce grid artifact and to improve the resolution of the system. METHODS AND MATERIALS In the clinical system, it was found that the scanning mechanism of the newvicon camera was deflected differently at various gantry positions because of the different orientation of the earths magnetic field. The small image misregistration produced grid artifact during image normalization, particularly near boundaries of the fiber bundles. One approach taken to reduce magnetic field effects was to shield the camera with mu-metal. Alternatively, a charged-coupled-device camera was used instead of the newvicon camera. As for improving spatial resolution, fibers with smaller input dimension were used. A 20 cm x 20 cm high resolution fiber-optic prototype consisting of 250 x 250 fibers, each with an input dimension of 0.8 mm x 0.8 mm was constructed. Its performance was tested using several phantoms studies. RESULTS Both shielding the newvicon camera with mu-metal or replacing it with a charge-coupled-device camera reduced grid artifact. However, optimal shielding could not be made for our clinical system because of the space limitation of its housing. High contrast resolution was improved, the 30% value of the modulation transfer function occurred at 0.3 linepairs per mm for the clinical system and at 0.7 linepairs per mm for the high-resolution prototype. However, because of the larger degree of transmission non-uniformity of the prototype, it was less effective using the current setup in detecting low contrast objects. CONCLUSIONS The results are encouraging and demonstrate successful reduction of grid artifact and improvement of high contrast spatial resolution using the proposed methods. The less effective low contrast detection was related to reduced light collection efficiency due to use of prototype fibers whose productions were not closely monitored. The findings are being considered in our construction of a second generation clinical fiber-optic on-line image verification system.

Large area pulse ionization chamber for measurement of extremely heavy cosmic rays

Abstract Parallel-plate ionization chambers with nearly 1 m 2 active area have been constructed and successfully operated in a series of high-altitude balloon flights. The chambers are used for identification of relativistic cosmic-ray nuclei with charge greater than or equal to 20. For relativistic iron nuclei (charge 26) the charge resolution (fwhm) achieved is 5.5 percent (i.e., less than 1.5 charge units).

A scintillating optical fiber track imaging detector

Abstract In this paper we describe a prototype of the scintillating optical fiber isotope experiment (SOFIE) and give results of a Bevalac calibration using iron nuclei to study the measurement precision in range and trajectory which can be obtained. We have measured the range of iron nuclei with approximate energy 500 MeV/amu entering the SOFIE instrument to a precision of 200–300 μm, and their transverse position coordinate to better than 35 μm. These results indicate that scintillating optical fibers are very useful as a range detector and as a high precision hodoscope for heavy nuclei.

Towards two-dimensional brachytherapy dosimetry using plastic scintillator: localization of the scintillation process

Abstract Detecting the scintillation light coming from a thin sheet of plastic scintillator (PS) provides a promising fast and precise tissue equivalent method for radiation dose measurements in two dimensions. The successful implementation of such technique requires high efficiency, dosimetric tissue equivalence and high localization of the scintillation process. The last is needed to assure that the light photons originating from a pixel of the scintillator sheet correspond to energy deposited in the same pixel. Since no such information is available for PS material with standard or modified chemical composition we have developed two experimental methods for assessing the scintillation locality by measuring the optical spectra and the scintillation light profile (SLP) of PS samples with different thickness. The results of the two types of measurements are consistent with each other and with a simple theoretical model of the energy conversion process. We have demonstrated that comparing the relative intensities of the primary and secondary photon peaks in the optical spectra of the scintillator is a sensitive approach to determine the delocalization of the secondary photon emission. The ratio of the number of primary to secondary photons shows strong dependence on PS dye composition. Two types of plastic scintillator materials were tested and the advantages of one of them for radiation dosimetry are demonstrated.

Development and testing of a fiber/multianode photomultiplier system for use on FiberGLAST

A scintillating fiber detector is currently being studied for the NASA Gamma-Ray Large Area Space Telescope (GLAST) mission. This detector utilizes modules composed of a thin converter sheet followed by an x, y plane of scintillating fibers to examine the shower of particles created by high energy gamma-rays interacting in the converter material. The detector is composed of a tracker with 90 such modular planes and a calorimeter with 36 planes. The two major component of this detector are the scintillating fibers and their associated photodetectors. Here we present current status of development and test result of both of these. The Hamamatsu R5900-00-M64 multianode photomultiplier tube (MAPMT) is the baseline readout device. A characterization of this device has been performed including noise, cross- talk, gain variation, vibration, and thermal/vacuum test. A prototype fiber/MAPMT system has been tested at the Center for Advanced Microstructures and Devices at Louisiana State University with a photon beam and preliminary results are presented.

Construction and performance of a large-area multiwire ionization hodoscope for use in a cosmic-ray detector

Abstract The mechanical construction and electronics of a large-area multiwire ionization hodoscope are described. Each chamber is 0.83 m by 1.65 m with wire spacing of 2.5 cm. The chambers are assembled to compose a hodoscope for a 6.6 m 2 ·sr cosmic ray detector. Operating characteristics and in-flight performance from two 30 h high-altitude balloon flights are presented.

The scintillating optical fiber isotope experiment: Bevalac calibrations of test models

Abstract The Scintillating Optical Fiber Isotope Experiment (SOFIE) is a Cherenkov d E /d x -range experiment being developed to study the isotopic composition of cosmic rays in the iron region with sufficient resolution to resolve isotopes separated by one mass unit at iron. This instrument images stopping particles with a block of scintillating optical fibers coupled to an image intensified video camera. From the digitized video data the trajectory and range of particles stopping in the fiber bundle can be determined; this information, together with a Cherenkov measurement, is used to determine mass. To facilitate this determination, a new Cherenkov response equation was derived for heavy ions at energies near threshold in thick Cherenkov radiators. Test models of SOFIE were calibrated at the Lawrence Berkeley Laboratorys Bevalac heavy ion accelerator in 1985 and 1986 using beams of iron nuclei with energies of 465 to 515 MeV/ nucleon. This paper presents the results of these calibrations and discusses the design of the SOFIE Bevalac test models in the context of the scientific objectives of the eventual balloon experiment. The test models show a mass resolution of σ A ≈ 0.30 amua nd a range resolution of σ R ≈ 250 μ m. These results are sufficient for a successful for a successful cosmic ray isotope experiment, thus demonstrating the feasibility of the detector system. The SOFIE test models represent the first successful application in the field of cosmic ray astrophysics of the emerging technology of scintillating optical fibers.

High altitude balloon flights of position sensitive CdZnTe detectors for high energy X-ray astronomy

Cadmium Zinc Telluride (CZT) is a semiconductor detector well suited for high energy X-ray astronomy. The High-Energy X-ray Imaging Spectrometer (HEXIS) program is developing this technology for use in a hard X-ray all-sky survey and as a focal plane imager for missions such as FAR_XITE and Constellation X. We have designed a novel electrode geometry that improves interaction localization and depth of interaction determination. The HEXIS program has flown two high altitude balloon payloads from Ft. Summer, NM to investigate background properties and shielding effects on a position sensitive CZT detector in the energy range of 20–350 keV.