Showing papers on "Photomultiplier published in 1990"

A 10‐GHz frequency‐domain fluorometer

Abstract: We describe the design and performance of a 10‐GHz harmonic‐content frequency‐domain fluorometer. The modulated excitation is provided by the harmonic content of a train of ps pulses. High‐speed and/or high‐frequency detection was attained with a triode‐type microchannel plate photomultiplier tube (MCP PMT) from Hamamatsu, R‐2566‐6, with 6 μm channels. To minimize the cost of the electronic components, and to minimize the noise due to these components, the detection circuits consists of two frequency ranges, 10 MHz–2 GHz and 2–10 GHz. The upper frequency limit of 10 GHz is determined by the current MCP PMT, so the usual configuration includes a low‐noise 2–10‐GHz amplifier. This amplifier is easily replaced with a 2–18‐GHz amplifier to allow operation to 18 GHz and the use of faster PMTs, should they become available in the future. Measurement of known optical delays demonstrates the accuracy of the instrument. For instance, a 1.69 ps optical delay was measured as 1.7±0.4 ps from 0.5 to 10 GHz, and 1.7±0.2 ps from 2 to 8 GHz, where the uncertainty indicates the maximum deviation from the expected value. The data were shown to be free of systematic errors by measurements on fluorophores with single exponential decays, with decay times ranging from 61 ps to 1.24 ns. Measurement of anisotropy decays with correlation times of 24 ps are shown and it is predicted that correlation times as short as 1 ps could be measured with this instrument. And finally, the sensitivity of the instrumentation was demonstrated by measurements of the very weak intrinsic tryptophan emission of deoxyhemoglobin, which displays decay times ranging from 2 to 820 ps.

The MSU Miniball 4π fragment detection array

Abstract: A compact and highly granular charged particle detection array, covering 89% of 4π in solid angle, is described. In its present configuration, the array consists of 188 fast-plastic CsI(Tl) phoswich detectors arranged in 11 rings coaxial about the beam axis. Each phoswich detector is comprised of a 40 μm thick plastic scintillator foil and a 2 cm thick CsI(Tl) crystal selected for good scintillation uniformity. The detectors are read out by photomultiplier tubes. Elemental identification up to Z ≈ 18 and isotopic identification of H and He nuclei is achieved by exploiting pulse shape discrimination techniques.

A High Resolution Imaging Detector for Tev Gamma-Ray Astronomy

Abstract: Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques.

High resolution block detectors for PET

Abstract: New high-resolution blocks detectors in which BGO scintillators are coupled to a position-sensitive photomultiplier tube (PMT), were developed for positron emission tomography (PET) applications. The block detectors, which use two different BGO scintillator structures, were constructed and evaluated: one is a 1.7-mm-wide BGO array which provides a coincidence detector resolution of less than 2 mm FWHM (full width at half maximum) and the other is a BGO block having comb-shaped slits in it, which has a potential capability of improving the resolution uniformity of a PET system because of its ability to detect the depth-of-interaction of gamma -rays in the scintillator. The application of these detectors to a PET system for animal studies is discussed. >

Readout techniques and radiation damage of undoped cesium iodide

Abstract: Several readout techniques for undoped CsI have been studied which utilize the fast scintillation component for speed and the high photon yield for good energy resolution. Quantum yields have been measured for samples up to 30 cm in length using photomultiplier tubes, wavelength shifters, and silicon photodiodes. A study has also been made of the scintillation properties of undoped CsI. It is found that the light output and decay time of the 310-nm fast component increases and the emission spectrum shifts to longer wavelengths at lower temperatures. The effects on the optical transmission and scintillation light output due to radiation damage from /sup 60/Co gamma rays have been measured for doses up to approximately 10/sup 6/ rad. It is found that the radiation resistance of undoped CsI is substantially higher than has been reported for thallium-doped CsI. >

Detector, shielding and geometric design factors for a high-resolution PET system

Abstract: The authors evaluated the resolution, efficiency, and scatter rejection of a novel high-resolution PET (positron emission tomography) system designed for animal studies which is based on a 2-D modular detector system. A digital positioning system was evaluated by testing different encoding methods. Tungsten interplane septa of different thicknesses and geometries were evaluated by Monte Carlo simulations and experiments. The detector system consists of a 6*8 array of BGO crystals coupled to two dual photomultiplier tubes (PMTs). The crystals are 3.5 mm wide with 4-mm transaxial spacing and are 6.25 mm long with 6.75-mm axial spacing. PMT outputs are digitized, and Anger-camera-type logic is used to determine the X and Y location of the scintillator event. Data are then sorted into the appropriate detector using a column lookup table and the column dependent row lookup tables. >

A study of the scintillation light induced in liquid xenon by electrons and alpha particles

Abstract: The time dependence and the intensity of the primary scintillation light in liquid xenon excited by /sup 241/Am alpha particles and /sup 207/Bi internal conversion electrons were measured at different electric field strengths. High-purity liquid xenon was used to fill a parallel-plate ionization chamber equipped with a CaF/sub 2/ window coupled to a UV (ultraviolet) sensitive photomultiplier tube. The effect of the specific ionization density on the scintillation light as well as the time correlation between the light signal and the charge signal is reported. It is demonstrated that the fast scintillation signal produced in liquid xenon by an ionizing particle provides an ideal trigger in a detector aiming at a complete three-dimensional event reconstruction with an excellent background rejection capability. >

Gated photomultiplier response characterization for DIAL measurements

Abstract: The characteristics of various detector responses are studied to understand the cause of various systematic biases and to minimize these undesirable effects in measurements of transient signals with large dynamic range. We quantitatively evaluated signal induced bias, gain variation, and the linearity of commonly used gated photomultipliers in the current integrating mode. Analysis of the results indicates that impurity ions inside the photomultiplier tube are the source of the signal induced bias and gain variation. Two different photomultiplier tubes used in this study show significant differences in the magnitude and decay behavior of signal induced bias. We found it can be minimized by using an external amplifier to reduce PMT gain, and by applying a low potential between the cathode and first dynode. The linearity of a photomultiplier tube is also studied over a large dynamic range of input intensities employing a new technique which does not require an absolute calibration. The result of this study shows that the photomultiplier response is linear only for a limited input intensity range below a certain anode current.

High resolution gamma ray detectors for positron emission tomography (pet) and single photon emission computed tomography (spect)

Abstract: A device and method for detecting gamma rays in a diagnostic imaging system preferably utilizing fast step index scintillating fibers and photomultipliers is provided by the present invention. A plurality of scintillating fibers are formed into layers comprising substantially parallel fibers positioned lengthwise each other in each layer. Each layer is positioned parallel each adjacent layer with each layer having its respective fibers at an angle of substantially either 60 or 90° to each adjacent layer. Preferably, position sensitive photomultipliers are provided and each photomultiplier is coupled to a plurality of fibers and each fiber has at least one end coupled to a photomultiplier. Data output from each photomultiplier is collected and processed to determine spatial coordinates of the origination position of a detected gamma ray. The present invention provides a high resolution gamma ray detector having an improved signal-to-noise ratio. This improves significantly the diagnostic power of the presently used PET and SPECT systems, and greatly extends the field of their application both in medical diagnosis and research. In addition, the present invention enables use of longer-lived positron emitting radiactive isotopes than presently used in typical PET systems.

Plasma constituent analysis by interferometric techniques

Abstract: An interferometer (18 or 40) is used to identify trace constituents in a plasma during processing semiconductor devices such as transistors. Light emissions collected from the processing chamber (10) are collimated by lens (14) and transmitted to the interferometer (18 or 40) which selectively allows therethrough particular wavelengths of light which are characteristic of the excitation emissions of certain atoms such as sodium and copper. The light intensity at the selected wavelengths is sensed by a photomultiplier tube (30). In one embodiment, the interferometer (18) is a Fabry-Perot type interferometer and the separation of the plates (20 and 22) which form the Fabry-Perot etalon is controlled using a piezoelectric driver (26). A signal processor (34) correlates the sensed light emissions from the photomultiplier tube (30) with the selected wavelength that is determined by the piezoelectric driver (26). In another embodiment, the interferometer (40) is a narrow bandpass interferometric filter which is tiltable with respect to the collimated incident light from the processing chamber (10). Tilting a narrow bandpass interferometric filter (42) with respect to incident light changes the path length through the filter (42) and allows for the selective transmission of certain wavelengths of light. By rapidly tilting the narrow bandpass interferometric filter (42) at a rate between 5-300 Hz with respect to the incident light, a narrow range of wavelengths on the order of 3 nm can be scanned.

A fundamental limit on timing performance with scintillation detectors

Abstract: A new lower bound on the mean-squared error of postdetection gamma -ray time-of-flight estimators has been derived. Previously, the Cramer-Rao bound has been used, but for nearly exponentially decaying scintillation pulses it gives an extremely optimistic picture of the achievable performance, depending critically on the dark current and photomultiplier characteristics. The new bound has been derived under the assumption that excited states in the scintillator leading to the emission of scintillation photons have an exponential lifetime density. The bound is a function of the mean state lifetime, the spectrum of energy deposited, and the energy conversion efficiency of the scintillator, and it is exact for the observation of a mono-exponentially decaying photoelectron rate at the first dynode of the photomultiplier tube given the gamma -ray arrival time. >

Devices for detecting and/or imaging single photoelectron

Abstract: A wafer of neutron transmutation doped silicon having a p-n junction between extended opposite surfaces is formed with bevelled edges. A single or plurality of reverse biased signal contacts is disposed on one surface to provide a single or integrated array of avalanche photodiodes. In addition, an avalanche photodetector (APD) capable of detecting a single photoelectron or imaging multiple photoelectrons comprises a light sensitive photocathode, similar to that in a photomultiplier tube, acting as a converter to produce photoelectrons, which are then accelerated to an anode. The anode comprises a single avalanche photodiode (AP) for detecting or an array (APA) for imaging photoelectrons. The energetic photoelectrons striking the AP or the APA serve as the AP or APA's input signal, respectively.

A Monte Carlo study of the timing resolution of BaF/sub 2/ for TOF PET

Abstract: A Monte Carlo study of the timing resolution of small BaF/sub 2/ crystals for time-of-flight (TOF) positron emission tomography (PET) is presented. The interaction of 511-keV gammas, the emission and transport of ultraviolet light in the crystal, and the pulse formation in the photomultiplier are simulated. The individual contributions to the timing resolution and the overall effect are calculated and compared to experimental measurements. It is found that for small crystal lengths the choice of photomultiplier can have strong effect on the resolution. >

Current-mode detector for neutron time-of-flight studies

Abstract: A system for the detection of high-intensity neutron bursts with instantaneous rates as high as 10 11 Hz is presented. This system uses a transient digitizer to sample the output voltage of a high-current photomultiplier tube as a function of time. The output voltage is proportional to the number of neutrons striking the detector. This detector is used at the Los Alamos Neutron Scattering Center to study fundamental symmetries. Design considerations, construction details and performance benchmarks are presented.

Gamma camera device

Abstract: A radiation ray (gamma ray) radiated from a test subject previously dosed with radioisotope is incident on a scintillator and converted into scintillation light. A plurality of photomultipliers arranged in a hexagon receive and convert the scintillation light into electrical signals. Photomultipliers arranged in parallel with sides of the hexagon constitute a group. Output signals of the photomultipliers are supplied to a first position detector for detecting a group of photomultipliers which receives the scintillation light and deriving position information Xo and Yo representing the approximate gamma ray incident position. The position information Xo and Yo and the output signals of the photomultiplier are supplied to a second position detector for deriving position information x and y representing the precise gamma ray incident position. Outputs of the photomultiplier are supplied to a pulse height analyzer which derives incident radiation energy information z.

Monte Carlo simulation of BaF/sub 2/ detectors used in time-of-flight positron emission tomography

Abstract: To better understand the performance of the SP3000 time-of-flight PET (positron emission tomography), machine, the authors performed Monte Carlo calculations of photon interactions in a BaF/sub 2/ detector module and calculations of crystal optics for BaF/sub 2/ UV scintillation light. Evaluated were photon detection sensitivities, the amount of photon spillover, crystal crosstalk, interring miscodes, and true-to-random coincidence ratios as a function of the lower-level energy discriminator setting and crystal/photomultiplier tube energy resolution. UV light collection efficiencies for the timing tube and decode tube were calculated assuming different crystal geometries, reflective materials, and reflectances. For a typical SP3000 detector module the detection efficiency for single 511-keV photons varied between 20 and 60% depending on the angle of incidence. >

Wide dynamic range detection with a charge injection device (CID) for quantitative plasma emission spectroscopy

Abstract: The operation and performance of an analytical plasma emission spectrometer based on an echelle polychromator and a charge injection device (CID) two-dimensional multichannel array detector is described. Quantitative analysis methodology for measuring the intensity of emission lines over a wide dynamic range is presented. The procedure involves varying the integration time on the detector for each spectral line, based on the intensity of that line, in a process called random access integration (RAI). The factors that determine the maximum number of spectral lines that may be simultaneously observed are presented. With current CID technology these include the size of the array, the intensity of spectral lines at the focal plane, and the size of the subarrays used to record the line intensities. The sensitivity and dynamic range of the system equal those of emission spectrometers employing photomultiplier tubes, and the flexibility to use any spectral line, or group of lines, for a particular element provides unprecedented ability to perform simultaneous multielement trace analysis in complex mixtures. Background intensity in the vicinity of each spectral line is measured simultaneously, enabling precise spectral background corrections. The simultaneous availability of the information from the entire emission spectrum allows spectral features of diagnostic value such as argon, hydroxide, and carbon emission to be monitored, as well as spectral features due to the principal components of the sample matrix. Changes in analysis conditions or matrix composition can be detected because of the availability of this additional information, thereby increasing the reliability of the analytical results.

A γ-ray detector with good time resolution and high signal-to-noise ratio

Abstract: A coincidence detector for an energy range between 8 and 50 keV with a good time resolution and a high signal-to-noise ratio was developed for time-differential experiments using nuclear Bragg diffraction with synchrotron radiation as a source.

Photon migration in scattering media and tissue

Abstract: We used fiber-optic phase-modulation methods as well as time-correlated single-photon counting to characterize time-dependent migration of 580-740 nm photons in living tissue, in particular in human fingers, forearms, calfs and foreheads. The phase-modulation measurements were extended to 4.2 GHz using an internally cross-correlated gatable MCP photomultiplier. Owing to the very high modulation frequencies, accurate measurements could be accomplished even at short distances (6 mm) between the source fiber and detector fibers. In order to estimate the influence of the optical tissue properties, and of geometrical factors, we performed model experiments in scattering media. The time-dependence of the reemerged light appears to be characterized by three parameters: 1) a time delay, 2) a sample transit-time spread, and 3) a (complex) exponential decay. All three parameters depend on fiber distance, absorption coefficient, scattering coefficient, detector geometry, and photon wavelength. The phase-modulation data are unique in that the time delay of the reemerged photons results in phase angles as high as 300 degrees, and the transit-time spread results in a dependence of the modulation on frequency which is a faster-than-exponentially decreasing.

Timing performance of scintillators read out by silicon avalanche photodiodes

Abstract: Solid-state photodetectors have several distinctive features making them suitable for many applications in scintillation detection. However, unity-gain devices such as silicon PIN or mercuric iodide photodiodes show relatively poor timing performance as compared to photomultiplier tubes, especially for sub-MeV energy radiation. Avalanche photodiodes (APDs) have an internal gain which enables significantly better timing capabilities. The purpose of this work is to investigate the potential of avalanche photodiodes for timing discrimination with five types of scintillators, organic and inorganic, spanning a wide range of conditions in terms of decay time constant, wavelength of emission and signal amplitude. Silicon APDs of reach-through structure with a 5 × 5 mm2 active area have been used for the measurements. All scintillators were 4 × 4 × 4 mm3 in dimensions and optimized for light collection. Results show that for the 511 keV annihilation radiation, timing as fast as 1.4 ns is achieved with fast organic scintillators and well below 10 ns FWHM for the most common inorganic scintillators such as NaI(TI), CsI(TI) and BGO. All measurements were performed at room temperature.

Theoretical analysis of confined quantum state GaAs/AlGaAs solid-state photomultipliers

Abstract: A detailed theoretical analysis of the design considerations of a solid-state photomultiplier based on avalanche multiplication of carriers out of confined quantum states is presented. Since these devices are unipolar, much lower noise and higher speed of performance are anticipated as compared with interband avalanche photodiodes. As an example of the design criteria for confined-state photomultipliers, a GaAs/Al/sub 0.32/Ga/sub 0.68/As multiquantum well structure is analyzed as to impact ionization rate, gain, dark current, and multiplied dark current. It is found that the highest gain is achieved in an asymmetric quantum well structure in which the second barrier height is half as large as the initial barrier height. The gain is further evaluated for a symmetric quantum well device. The effects of the applied electric field, quantum well doping concentration, and layer widths on device performance are examined. >

Directional gamma ray spectrometer

Abstract: This disclosure is directed to a scintillation crystal surrounded by a sleeve and having a window formed therein. The scintillation crystal has end located first and second photomultiplier tubes which provide output signals. A scintillation event observed in the crystal is seen by both photomultiplier tubes and both form output signals. The output signals have a relationship determining the point of impingement along the length of crystal of the photon gamma radiation; in turn, that photon is located relative to a surrounding shield having a helical window formed therein so that the relative angle of impingement can be determined. The relative angle is then converted into the absolute angle relative to the well borehole by referencing a north seeking gyroscope supported by the sonde. The output signals are also used to determine the energy of the impinging gamma radiation using well known pulse height analysis spectroscopy techniques.

An ‘‘on’’‐gated photomultiplier circuit for the determination of phosphorescence lifetimes

Abstract: An ‘‘on’’‐gated photomultiplier circuit is described which possesses a cutoff ratio >103 by switching dynodes 1 and 3 of a nine‐stage side‐on phototube (Hamamatsu models 1P28, R955, etc.). ‘‘On’’‐gate delay and gate‐width times are continuously variable between 150 ns and 10 ms. The ‘‘ringing’’ usually observed after the gate opening in similar circuit designs, due to the propagation delay in the dynode chain and the capacitive coupling between the anode and the switched dynodes, has been greatly reduced using a ‘‘spike compensator’’ circuit. When −1000 V was applied to the anode of the phototube, the resulting output of the circuit was linear 80 ns after the unit was triggered. The circuit will also operate as an ungated photomultiplier.

A Li-glass scintillation detector for thermal-neutron TOF measurements

Abstract: A unique Li-glass scintillation detector has been developed for time-dependent neutron spectroscopy by the linac-chopper method. It is required that the detector can measure neutrons against the background of the electromagnetic radiation from a linac target, and that it has high detection efficiency for the neutrons from a spectrum source. The detector consists of a Li glass scintillator, two photomultiplier tubes, two hemispherical reflectors, tube base circuits for photomultipliers and a dark chamber. The pulse-height resolution of the detector with thermal-neutron events was about 56%. The detector also provides pulse-shape discrimination. A figure of merit for the discrimination between photons and neutrons was about 0.25.

Contribution of photocathode nonuniformity to energy resolution in NaI(Tl) scintillation detectors

Abstract: An automatic method for measuring the photocathode nonuniformity is described. Monte Carlo calculations have been performed to estimate the importance of the photocathode's contribution to the resolution, compared to photon and photoelectron statistics, secondary electron statistics, variations in the light collection throughout the crystal, and the “intrinsic” resolution of NaI(Tl). It is concluded that the photocathode contribution is only significant for thin crystals in direct contact with the photomultiplier. In most cases where good resolution is required, it is more important that the photomultiplier has a good single electron resolution.

A system for simulation of scintillator light signals

Abstract: A fast triggerable light source using a green light emitting diode (LED) has been developed to reproduce the response of fast plastic scintillators for e.g. photomultiplier testing and calibration purposes. Duration and amplitude of the light signals can be tuned by varying the length of an open and clip cable which is connected to the rear end of the LED. This solution is very inexpensive because only off-the-shelf components are used.

Signal to noise ratios for continuum source atomic absorption spectrometry using a linear photodiode array to monitor sub-nanometre wavelength intervals

Abstract: The development and initial evaluation of a pulsed continuum source atomic absorption spectrometer using a linear photodiode array and weak pulsing of the source is discussed. A 256-pixel array, mounted in a high-dispersion monochromator, was used to monitor a narrow interval (0.4–0.8 nm) around the wavelength of interest, resulting in rapid (<10 ms) readout times. Experimental results confirmed the readout noise limitation of the array. The minimisation of readout lag, quantisation noise and relative analogue-to-digital (A/D) converter noise resulted in a readout noise limit that was twice the manufacturer's specifications. Pixel averaging, in conjunction with a large entrance slit width, reduced the absorbance noise. With flame atomisation, use of a linear photodiode array resulted in a factor of 3.9–6.2 improvement in the signal to noise ratio of five elements (zinc, cadmium, nickel, cobalt and iron) in comparison with photomultiplier tube detection. Lamp pulsing resulted in a 50% increase in integrated intensity and was limited by the power rating of the power supply. The combined effect of a pulsed source and photodiode array detection improved the signal to noise ratios by factors ranging from 5.8 to 8.5.

4‐GHz internal MCP‐photomultiplier cross correlation

Abstract: We have performed sinusoidal and pulsed internal cross correlation within a Hamamatsu model R2024U gatable microchannel‐plate photomultiplier (MCP‐PMT) by biasing and modulating the gating mesh which is located 0.2 mm behind the photocathode. The light source used was a sync‐pumped and cavity‐dumped dye laser. Near the photocathode center, no effective high‐frequency photocurrent modulation was achieved. However, for a circular area near the photocathode edge, useful modulation up to 4.5 GHz, the frequency limit of our amplifier, was obtained. The modulation frequency response is characterized by resonances caused by the gating mesh which represents a strong disturbance with respect to the modulation‐cable termination. The shortest measured rise time for pulsed modulation was 178 ps at 700 nm. The measured pulse response is a ‘‘mirror image’’ of a response curve as observed normally because the gating pulse is sampled by the δ‐like photoelectron pulse. From this observation it can be reasoned that an even...

Highly Sensitive Raman Spectroscopy by a Position-Sensitive Photomultiplier and a Triple-Stage Spectrograph with Stigmatic Optical Correction

Abstract: A Raman spectrometer with extremely high sensitivity was constructed through the use of a position-sensitive photomultiplier and a triple-stage spectrograph with stigmatic optical correction Raman spectra of Si wafer were demonstrated with the low-level excitation intensity down to 10 nW The longitudinal optical phonon spectrum of an individual 3-layer-thick AlAs thin film buried in GaAs was obtained for the first time The system is useful for studying a small number of atoms and molecules of various surfaces, interfaces, and ultrathin layers