Scintillation Light Collection Research Articles

Methods to extract more light from minute scintillation crystals used in an ultra-high resolution positron emission tomography detector

Recently, there has been great interest in developing finely pixellated position-sensitive scintillation detectors for ultra-high-resolution Positron Emission Tomography (PET) systems designed for breast cancer detection, diagnosis, and staging and for imaging small laboratory animals. We are developing a different high-resolution PET detector design that promotes nearly complete scintillation light collection in ⩽1mm wide, >10mm thick lutetium oxyorthosilicate (LSO) crystals. The design requires the use of semiconductor photodetector arrays in novel configurations that significantly improve the light collection aspect ratio for minute crystals. To reduce design complexity and dead area we are investigating the use of 1mm thick sheets of LSO in addition to discrete crystal rods, and the use of position-sensitive avalanche photodiodes (PSAPDs) which require only four readout channels per device, in addition to pixellated APD arrays. Using a 1mm thick scintillation crystal sheet coupled to a finely pixellated APD array results in a pseudo-discrete response to flood irradiation: due to a very narrow light spread function in the thin sheet we observe sharp (<1mm wide) peaks in sensitivity centered at the APD pixel locations in a very linear fashion all the way out to the crystal edge. We measured an energy resolution of 13.7% FWHM at 511keV for a 1mm LSO crystal coupled to two APD pixels. Using a 1mm thick crystal sheet coupled to a PSAPD the response to flood and edge-on irradiation with a 22Na point source shows a compressed dynamic range compared to that observed with discrete crystals or direct X-ray irradiation. With a discrete LSO crystal array the flood response is peaked at the crystal location where light is focused onto one spot on the PSAPD. We observed strong pin-cushioning effects in all PSAPD measurements. All LSO–PSAPD configurations studied had high aspect ratio for light collection and achieved energy resolutions ⩽12% FWHM at 511keV.

Optimization of fiber-optic readout of LSO scintillation crystals with acid etching

Optimizing the collection of scintillation light is essential for good positron emission tomography scanner performance, and even more so when the crystals are read out through optical fibers. Acid etching has been proposed as a cost-effective alternative to mechanical polishing, but there are discrepancies between the results published so far. The aim of this paper is to gain a better understanding of acid etching and its effects on the light yield, and to assess its application to fiber-optic-based configurations. We have examined the surface state of LSO crystals etched for various times and measured their light output in different configurations, with both direct and fiber-optic readout. Our results indicate that crystal etching is a complex process, where different crystal faces may be etched in different ways. Acid etching always resulted in an improved light yield and energy resolution of the crystals, and was as efficient as, or in many cases superior to, mechanical polishing. While this improvement was somewhat limited for the configurations with direct readout of the crystals we tested, it was much larger for the fiber-optic-based configuration.

A Method for Calculating the Scintillation Light Collection Time in PWO Crystals

A method for estimating the scintillation light collection time in PWO crystals is proposed. This method has a low sensitivity to statistical noise due to the integration of experimental data over a wide time range. As a result, the data acquisition time can be reduced. The method is noniterative, so that the calculation time is significantly reduced, and the operation is simplified. This method is used for the certification of PWO scintillation crystals in their mass production.

PICsIT detector for γ-ray astronomy: pixels qualification campaign

Position sensitive detectors coupled to coded masks are used for imaging in high energy astronomy. PICsIT is the γ-ray position sensitive detector of the IBIS telescope to be flown onboard the INTEGRAL satellite in 2001. PICsIT detector functionalities and scientific performances as a whole are described in details elsewhere in this Conference (Labanti et al. (Nucl. Instr. and Meth. A, this conference)). In this work, we present the aim, methodology, and results of the tests and calibrations performed on the first qualification lot of 136 PICsIT pixels carried out at SCIONIX laboratories in The Netherlands, in April 1999. Before being accepted for PICsIT, each detection unit has in fact to be fully characterized in terms of energy resolution and lower energy threshold. The principal aim of PICsIT pixel qualification campaign described in this work has been to measure the key parameters of each CsI(Tl)+photodiode detection unit: CsI(Tl) crystals light output, gain variation with temperature, energy resolution degradation due to scintillation light production and collection, inhomogeneity in the crystal, and the variation of these quantities after a thermo-vacuum stress treatment.

Investigation of a new readout scheme for high resolution scintillation crystal arrays using photodiodes

We are exploring the possibility of using PIN photodiodes to readout the scintillation crystals used in positron emission tomography (PET) detector designs. Semiconductor photodetectors typically have a lower signal to noise ratio than photomultiplier tubes (PMTs). However, they have the advantage of compactness, and, thus, scintillation crystal readout schemes not available to PMTs because of their size and geometry limitations, are readily available to photodiodes. With current PET detector designs, only a small fraction of the available scintillation light, created from 511 keV gamma ray interactions within the crystal, is collected. Scintillation light collection studies were performed for several crystal geometries and surface treatments using both simulations and measurements. In this report, we present a feasible photodiode readout scheme that allows greater than 90% of the available scintillation light created in either BGO or LSO scintillation crystals to be collected by the photodetector. This improvement in light collection with the photodiode readout somewhat compensates for its lower inherent signal to noise ratios and makes it feasible for use in PET detectors. A coincident timing spectrum resolution of 9.4 ns FWHM was measured for 511 keV interactions with one LSO crystal coupled to a photodiode, the other to a PMT.

Detection of single photon ionization events using a contactless microwave technique

For the first time, to the best of our knowledge, the electron-hole pairs generated in a material by a single photon ionization event have been detected using a microwave technique. The detection mechanism is based on the attenuation of a resonant electromagnetic wave and, unlike conventional particle detectors, does not rely on charge or scintillation light collection. The current technique has an inherent fast response (microwave frequency of 9 GHz) and is not affected by the problems associated with charge sweeping and/or light measurements. Experimental results for single high energy x-ray photons absorbed in high purity p-germanium are presented. The results demonstrate the potential for a new class of gamma/particle detectors and a useful method to study the kinetics of particle interactions in materials.

Collection of scintillation light from small BGO crystals

We propose to develop a high resolution positron emission tomography (PET) detector designed for animal imaging. The detector consists of a 2-D array of small bismuth germanate (BGO) crystals coupled via optical fibers to a multi-channel photomultiplier tube (MC-PMT). Though this approach offers several advantages over the conventional BGO block design, it does require that a sufficient number of scintillation photons be transported from the crystal, down the fiber and into the PMT. In this study we use simulations and experimental data to determine how to maximize the signal reaching the PMT. This involves investigating factors such as crystal geometry, crystal surface treatment, the use of reflectors, choice of optical fiber, coupling of crystal to the optical fiber and optical fiber properties. Our results indicate that using 2/spl times/2/spl times/10 mm BGO crystals coupled to 30 cm of clad optical fiber, roughly 50 photoelectrons are produced at the PMT photocathode for a 511 keV interaction. This is sufficient to clearly visualize the photopeak and provide adequate timing resolution for PET. Based on these encouraging results, a prototype detector will now be constructed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simulation of light collection in scintillators with rough surfaces

A simple model to evaluate the light collection in scintillators with rough surfaces by Monte Carlo simulation is proposed. The model has a single parameter related to the degree of roughness. The results are compared with measurements for a long tapered BGO crystal.

Position estimation and error correction in a 2D position-sensitive NaI(Tl) detector

Removal of spatial distortion for a two-dimensional position-sensitive NaI(Tl) detector used in a positron-emission tomography (PET) scanner is investigated. The Anger-type detector has an active area of 45 × 9 cm 2 and is 2.5 cm thick and is optically coupled to a 3 × 10 array of 5 cm square photomultiplier tubes. Due to the nature of the scintillation light spread and collection there is a substantial difference between the true and calculated scintillation position as calculated from a biased centroid algorithm. To generate an error-correction lookup table the detector is illuminated with a tightly collimated gamma-ray beam at regular intervals and the measured positions at these locations are compared with the known true positions. Since it is impractical to collect this data for all the possible position channels, a table of 1024 × 200 distortion correction factors is interpolated from a sparse grid of measured points that covers the detector surface. The effects of different sampling distances for the measured points and two different interpolation schemes are compared. We conclude that the optimal combination is a sampling distance of 7.5 mm and a two-dimensional bilinear interpolation algorithm, which results in an average rms error in the corrected position of 0.2 mm. The bilinear interpolation algorithm is modified to work on the distorted rectangular grid that results from the measured locations of the data points.

Effect of geometrical modifications and crystal defects on light collection in ideal rectangular parallelepipedic BGO scintillators

The major loss of signal in ideal rectangular parallelepipedic scintillators results from the presence of permanent modes which cannot escape the crystal volume. When the photodetector is coupled with an index of n d=1.55, this value can be as high as 46% in BGO. The purpose of this study is to investigate some alternatives to this geometry with the aid of Monte Carlo simulations and direct measurements on a set of crystals. The study is focused on long narrow-shaped BGO scintillators designed to be used in conjunction with avalanche photodiodes in positron emission tomography. Results show that slightly unpolishing the crystal surfaces efficiently eliminates trapped modes but that very-high-reflectivity reflectors are needed to transport photons to the detection plane. Minor modifications to the geometry eliminating all symmetry from the crystal also efficiently eliminate trapped modes and generally lead to the highest light output.

Effects of scintillation light collection on the time resolution of a time-of-flight detector for annihilation quanta

Monte Carlo methods were used to simulate the production and collection of scintillation light in a BaF/sub 2/ crystal. The calculated results were compared to measured values for detectors with different heights. Based on this simulation, a TOF (time-of-flight) detector consisting of two conical BaF/sub 2/ crystals (base radius 18 mm, top radius 15 mm, height 25 mm) was assembled. The total time resolution of this system was (210+or-5) ps FWHM (full width at half maximum) for 511-keV annihilation quanta and was found to be independent of the source position between the detectors. Since the time resolution of the TOF detector system consisting of two conical BaF/sub 2/ crystals is sufficiently constant in the volume of interest and does not change during operation, this detector meets the requirements for application in a clinical routine. >