Photon Sensitivity Research Articles

A TL/OSL emission spectrometer extension of the Risø reader.

The integration of a low cost, compact sized spectrometer with the Risø reader is described. The luminescence light emitted by the sample is transmitted by an optical fibre onto a fixed entrance slit of a spectrograph. The light is measured with a high sensitivity 2048-element CCD-linear array detector (Avantes PC2000) sensitive in the UV-VIS region. The CCD array has a low readout noise and a photon sensitivity of 86 photons per count. Examples are given of both thermally and optically stimulated 3-D spectra showing the luminescence spectra as a function of temperature and time respectively. Spectra from CaF2:Tm (TLD-300) chips could be distinguished from the background (2 SD) at a 0.7 Gy dose level.

Photon quality correction factors for ionization chambers in an epithermal neutron beam

Photon quality correction factors (kQγ) for ionization chamber photon dosimetry in an epithermal neutron beam were determined according to a modified absorbed dose to water formalism which was extended to mixed radiation fields. We have studied two commercially available ionization chambers in the epithermal neutron beam optimized for BNCT at the facility at Studsvik, Sweden. One of the chambers is nominally neutron insensitive; a magnesium-walled detector flushed with pure argon gas (denoted by Mg/Ar). The second chamber has approximately the same sensitivity for neutrons and photons; it is considered a ‘tissue equivalent’ detector, with A-150 walls flushed with methane-based tissue-equivalent gas (denoted by TE/TE). The kQγ-factors in epithermal neutron beams have previously been assumed to be equal to unity or estimated from measurements in clinical accelerator produced photon beams. In this work the kQγ-factors have been determined from absorbed dose calculations using cavity theory together with Monte Carlo derived electron fluences obtained with the MCNP4c system for water and PMMA phantoms. The calculated quality correction factors differ substantially from unity, being in the order of 10% for the Mg/Ar detector at shallow phantom depths, and between 2 and 4% for other depths and for the TE/TE chamber.

A Visual Pigment Expressed in Both Rod and Cone Photoreceptors

Rods and cones contain closely related but distinct G protein-coupled receptors, opsins, which have diverged to meet the differing requirements of night and day vision. Here, we provide evidence for an exception to that rule. Results from immunohistochemistry, spectrophotometry, and single-cell RT-PCR demonstrate that, in the tiger salamander, the green rods and blue-sensitive cones contain the same opsin. In contrast, the two cells express distinct G protein transducin α subunits: rod α transducin in green rods and cone α transducin in blue-sensitive cones. The different transducins do not appear to markedly affect photon sensitivity or response kinetics in the green rod and blue-sensitive cone. This suggests that neither the cell topology or the transducin is sufficient to differentiate the rod and the cone response.

A new noise source in superconducting tunnel junction photon detectors

We report on the development of an all-in-one detector that provides spectroscopy, imaging, photon timing, and high quantum efficiency with single photon sensitivity: the optical/UV single-photon imaging spectrometer using superconducting tunnel junctions. Our devices utilize a lateral trapping geometry. Photons are absorbed in a Ta thin film, creating excess quasiparticles. Quasiparticles diffuse and are trapped by Al/AlO/sub x//Al tunnel junctions located on the sides of the absorber. Imaging devices have tunnel junctions on two opposite sides of the absorber. Position information is obtained from the fraction of the total charge collected by each junction. We have measured the single photon response of our devices. For photon energies between 2 eV and 5 eV we measure an energy resolution between 0.47 eV and 0.40 eV respectively on a selected region of the absorber. We see evidence that thermodynamic fluctuations of the number of thermal quasiparticles in the junction electrodes leads to current noise that far exceeds the expected shot noise of the dc bias current. We believe that this may limit the resolution of our present generation of detectors at the operating temperature of 0.22 K.

Spectroscopic identification of light emitted from defects in silicon devices

A comprehensive study of different fundamental aspects of light emission from defects in Si semiconductor devices is presented. Based on an experimental analysis, using a new highly sensitive spectroscopic photon emission microscope (SPEM) for continuous wavelength analysis (2.5 eV–1.2 eV), a unique assignment of the spectrum of the emitted light and the corresponding failure mechanism is established. Three distinguishable basic spectral categories were identified. They were attributed to gate oxide breakdown, metal shorts, and electro-static discharge caused junction spiking. The focused ion beam technique was used to look at the damage sites for confirmation of the SPEM results.

A Novel Delay-Line Anode Design for Position and Time Sensitive Read-Out of MCP-Based Detectors

We have developed a novel delay-line anode design based on L/2 shifted maeander-lines for the readout of open-faced and sealed MCP-based detectors. In combination with the sealed detector (image intensifier) we are able to provide position and time sensitive single photon detection from near UV to near IR.

Photon sensitivity of superheated drop at room temperature

It has been reported that superheated drops made of R-12 at room temperature are sensitive to neutrons yet insensitive to photons. This property makes its use attractive as one of the most useful neutron dosimeters. The photon sensitivity of R-12 at room temperature (25°C) when exposed to 59.54 keV photons obtained from radioactive 241Am has been noted for the first time in our laboratory. This discovery is of importance not only from the point of view of basic science but also to the users of R-12 in neutron dosimetry in assessing the neutron dose correctly. This discovery also indicates a new direction of investigation with R-12 detector.

A comparison of the potential therapeutic gain of p(66)/Be neutrons and d(14)/Be neutrons

Purpose: To determine the relationship between photon sensitivity and neutron sensitivity and between neutron RBE and photon resistance for two neutron modalities (with mean energies of 6 and 29 MeV) using human tumor cell lines spanning a wide range of radiosensitivities, the principal objective being whether or not a neutron advantage can be demonstrated. Methods and Materials: Eleven human tumor cell lines with mean photon inactivation doses of 1.65–4.35 Gy were irradiated with 0–5.0 Gy of p(66)/Be neutrons (mean energy of 29 MeV) at Faure, S.A. and the same plating was irradiated on the same day with 0–10.0 Gy of Cobalt-γ-rays . Twelve human tumor cell lines, many of which were identical with the above selection, and spanning mean photon inactivation doses of 1.75–4.08 Gy, were irradiated with 0–4 Gy of d(14)/Be neutrons (mean energy of 6 MeV) and with 0–10 Gy of 240 kVp X-rays at the Essen Klinikum. Cell survival was determined by the clonogenic assay, and data were fitted to the linear quadratic equation. Results: 1. Using the mean inactivation dose, a significant correlation was found to exist between neutron sensitivity and photon sensitivity. However, this correlation was more pronounced in the Faure beam ( r 2 = 0.89 , p ≤ 0.0001) than in the Essen beam (r 2 = 0.65, p = 0.0027). 2. No significant relationship could be established between neutron RBE and photon resistance for both modalities ( p = 0.69 and p = 0.07, respectively). 3. Using α-coefficients as a criterion, the neutron sensitivity for the Faure beam correlated with photon sensitivity ( p = 0.001), but this did not apply to the Essen beam ( p = 0.27). 4. The neutron RBE for the Essen beam derived from α-coefficients showed a steep increase with photon resistance ( p = 0.003). In the Faure beam there was no increase of RBE with photon resistance ( p = 0.494). Conclusion: Radiobiological differences between high-energy and low-energy neutrons are particularly apparent in the dependence of the neutron RBE on photon sensitivity. The increase of RBE with photon resistance is more pronounced in the low-energy Essen neutrons than in the high-energy Faure neutrons. An RBE advantage is indicated for photon-resistant cell lines and this is particularly apparent in the low-dose range using α-coefficients as compared to the mean inactivation dose. The clinical application of low-energy neutrons may be more restricted because of poor penetration and lack of skin sparing. However, these neutrons discriminate better between photon-sensitive and photon-resistant cells giving an RBE range of 2–6 and a mean RBE of 4.1, than high-energy neutrons where the RBE range is 1.6–3.5 and the mean RBE is 2.4. From the radiobiological point of view it, therefore, appears that the clinical potential of low-energy neutrons is considerably underrated.

A model for photon detection and dosimetry with superheated emulsions.

A model is presented for the analysis and prediction of the photon response of detectors based on superheated emulsions of light halocarbons in tissue equivalent gels. It is shown that on the basis of a nondimensional thermodynamic quantity, called reduced superheat, it is possible to identify the degree of superheat, or the operating temperature, corresponding to the photon sensitization of the emulsions. Moreover, on the basis of the mass energy absorption coefficients, it is possible to determine the energy dependence of the photon response. The vaporization energy necessary for bubble nucleation is estimated by means of the thermal spike theory developed for bubble chambers. The energy deposition requirements are consistent with the energy transferred by secondary electrons at the end of their range in the halocarbons. These findings provide design criteria for photon detectors based on superheated emulsions. It is shown that light halocarbons of low effective atomic numbers present the best dosimetric properties. In particular, by manufacturing superheated emulsions with octafluoropropane, or halocarbon R-218, photon sensitivity is achieved at room temperature along with a fairly constant air-kerma response.

443産業用高エネルギーX線CTとその応用

A high energy industrial X-ray computed tomography system using silicon semiconductor detectors (Si-SSD) and electron linear accelerator (LINAC) was developed to obtain high resolution 3D images of high density and large scale object. To increase photon sensitivity, detector elements were placed parallel to the X-ray beams. The obtained sensitivity of 20% for X-ray of 3MeV was 3×104 times higher than that of X-ray films. It was clarified that the photon sensitivity of X-ray detectors restricts the performance of high energy X-ray CT, and it is shown that a CT image of 200mm thick iron object can be obtained with 0.2mm space resolution.

Mesopic state: cellular mechanisms involved in pre- and post-synaptic mixing of rod and cone signals.

At least twice daily our retinas move between a light adapted, cone-dominated (photopic) state and a dark-adapted, color-blind and highly light-sensitive rod-dominated (scotopic) state. In between is a rather ill-defined transitional state called the mesopic state in which retinal circuits express both rod and cone signals. The mesopic state is characterized by its dynamic and fluid nature: the rod and cone signals flowing through retinal networks are continually changing. Consequently, in the mesopic state the retinal output to the brain contained in the firing patterns of the ganglion cells consists of information derived from both rod and cone signals. Morphology, physiology, and psychophysics all contributed to an understanding that the two systems are not independent but interact extensively via both pooling and mutual inhibition. This review lays down a rationale for such rod-cone interactions in the vertebrate retinas. It suggests that the important functional role of rod-cone interactions is that they shorten the duration of the mesopic state. As a result, the retina is maintained in either in the (rod-dominated) high sensitivity photon counting mode or in the second mode, which emphasizes temporal transients and spatial resolution (the cone-dominated photopic state). Experimental evidence for pre- and postsynaptic mixing of rod and cone signals in the retina of the clawed frog, Xenopus, is shown together with the preeminent neuromodulatory role of both light and dopamine in controlling interactions between rod and cone signals. Dopamine is shown to be both necessary and sufficient to mediate light adaptation in the amphibian retina.

Noninvasive Assessment of Tumor Cell Proliferation in Animal Models

Revealing the mechanisms of neoplastic disease and enhancing our ability to intervene in these processes requires an increased understanding of cellular and molecular changes as they occur in intact living animal models. We have begun to address these needs by developing a method of labeling tumor cells through constitutive expression of an optical reporter gene, noninvasively monitoring cellular proliferation in vivo using a sensitive photon detection system. A stable line of HeLa cells that expressed a modified firefly luciferase gene was generated, proliferation of these cells in irradiated severe combined immunodeficiency (SCID) mice was monitored. Tumor cells were introduced into animals via subcutaneous, intraperitoneal and intravenous inoculation and whole body images, that revealed tumor location and growth kinetics, were obtained. The number of photons that were emitted from the labeled tumor cells and transmitted through murine tissues was sufficient to detect 1×103 cells in the peritoneal cavity, 1×104 cells at subcutaneous sites and 1×106 circulating cells immediately following injection. The kinetics of cell proliferation, as measured by photon emission, was exponential in the peritoneal cavity and at subcutaneous sites. Intravenous inoculation resulted in detectable colonies of tumor cells in animals receiving more than 1×103 cells. Our demonstrated ability to detect small numbers of tumor cells in living animals noninvasively suggests that therapies designed to treat minimal disease states, as occur early in the disease course and after elimination of the tumor mass, may be monitored using this approach. Moreover, it may be possible to monitor micrometastases and evaluate the molecular steps in the metastatic process. Spatiotemporal analyses of neoplasia will improve the predictability of animal models of human disease as study groups can be followed over time, this method will accelerate development of novel therapeutic strategies.

High gain avalanche photodiode arrays for DIRC applications

The detection of light emitted in Cherenkov radiators requires fast detector arrays with high sensitivity to short wavelength photons. Photomultiplier tubes, the traditional imaging detectors for short wavelength optical radiation, have limited spatial resolution and require expensive anti-magnetic shielding. We report on the performance of a new, Geiger mode operated, Si micro-avalanche photodiode (/spl mu/APD) array, designed for Cherenkov light imaging applications. We address issues of optical interfacing, speed, and pulse spectra achievable with these /spl mu/APDs. The new /spl mu/APD array provides a high sensitivity detector for applications requiring two dimensional light mapping with single photon sensitivity. These features make it a promising candidate for the detection of Cherenkov light in modern high energy physics experiments.

Study on photon sensitivity of silicon diodes related to materials used for shielding

Abstract Large area silicon diodes used in electronic neutron dosemeters have a significant over-response to X- and gamma-rays, highly non-linear at photon energies below 200 keV. This over-response to photons is proportional to the diode's active area and strongly affects the neutron sensitivity of such dosemeters. Since silicon diodes are sensitive to light and electromagnetic fields, most diode detector assemblies are provided with a shielding, sometimes also used as radiation filter. In this paper, the influence of materials covering the diode's active area is investigated using the MCNP-4A code by estimating the photon induced pulses in a typical silicon wafer (300 μm thickness and 1 cm diameter) when provided with a front case cover. There have been simulated small-size diode front covers made of several materials with low neutron interaction cross-sections like aluminium, TEFLON, iron and lead. The estimated number of induced pulses in the silicon wafer is calculated for each type of shielding at normal photon incidence for several photon energies from 9.8 keV up to 1.15 MeV and compared with that in a bare silicon wafer. The simulated pulse height spectra show the origin of the photon-induced pulses in silicon for each material used as protective cover: the photoelectric effect for low Z front case materials at low-energy incident photons (up to about 65 keV) and the Compton and build-up effects for high Z case materials at higher photon energies. A simple means to lower and flatten the photon response of silicon diodes over an extended X- and gamma rays energy range is proposed by designing a composed photon filter.

In vivo imaging of spontaneous ultraweak photon emission from a rat's brain correlated with cerebral energy metabolism and oxidative stress.

Living cells spontaneously emit ultraweak light during the process of metabolic reactions associated with the physiological state. The first demonstration of two-dimensional in vivo imaging of ultraweak photon emission from a rat's brain, using a highly sensitive photon counting apparatus, is reported in this paper. It was found that the emission intensity correlates with the electroencephalographic activity that was measured on the cortical surface and this intensity is associated with the cerebral blood flow and hyperoxia. To clarify the mechanism of photon emission, intensity changes from whole brain slices were examined under various conditions. The removal of glucose from the incubation medium suppressed the photon emission, and adding 50 mM potassium ions led to temporal enhancement of emission and subsequent depression. Rotenone (20 microM), an inhibitor of the mitochondrial electron transport chain, increased photon emission, indicating electron leakage from the respiratory chain. These results suggest that the photon emission from the brain slices originates from the energy metabolism of the inner mitochondrial respiratory chain through the production of reactive oxygen. Imaging of ultraweak photon emission from a brain constitutes a novel method, with the potential to extract pathophysiological information associated with neural metabolism and oxidative dysfunction of the neural cells.

Aging and rejuvenation of a TMAE+methane multiwire photon detector

A UV sensitive multiwire photon detector has been tested as a possible candidate for the HERA-B RICH detector. The main obstacle to using such a TMAE+Methane filled gas detector in a high rate experiment appears to be rapid aging in the form of prohibitive loss of chamber gas gain. A special circuit has been designed for heating the anode wires in-situ with elevated currents, thus evaporating the polymer deposits. Heating almost completely recovers the initial gain, but this rejuvenation is unfortunately of short duration. Nevertheless, the cells exposed to periodic heat treatments have an average gain considerably higher than the non-heated cells.

Hadrons in the nuclear medium

In this talk I first discuss predictions based on QCD sum rules and on hadronic models for the properties of vector mesons in the nuclear medium. I then describe possible experimental signatures and show detailed predictions for dilepton invariant mass spectra with a special emphasis on nuclear reactions involving elementary incoming beams and nuclear targets. I will in particular illustrate that the sensitivity of pion and photon induced reactions to in-medium effects is nearly as large as that of heavy-ion reactions,

Photon detection with high gain avalanche photodiode arrays

The detection of light emitted in fast scintillating fibers and Cerenkov radiators used for fiber calorimetry and tracking applications in high energy colliders, requires fast detector arrays with high sensitivity to short wavelength photons. Photomultiplier tubes, the traditional imaging detectors for short wavelength optical radiation, have limited spatial resolution and require expensive anti-magnetic shielding. We report on short wavelength sensitivity improvement and detection efficiency performance for a novel p-n junction planar structure silicon avalanche photodiode (APD) array, operated in Geiger mode. The APD array provides a high sensitivity detector for applications requiring the detection of light spatial distributions with single photon sensitivity.

A position-sensitive superheated emulsion chamber for three-dimensional photon dosimetry

A position-sensitive detector chamber is introduced for the three-dimensional (3D) dosimetry of photon-emitting brachytherapy sources. The detector is based on an extremely fine suspension of monochloropentafluoroethane droplets emulsified in a gel. The droplets are highly superheated at room temperature and their evaporation can be triggered by photon interactions, leading to the formation of microscopic bubbles. Thus, when photon-emitting brachytherapy sources are inserted into the detector, bubble distributions form around them, enabling visualization of the radiation field. The tissue-equivalent emulsifier gel is highly viscous and keeps the bubbles immobilized at the location of their formation. Bubbles can then be imaged by nuclear magnetic resonance or optical scanning techniques. After the imaging, the detector can be pressurized in order to recondense the bubbles to the liquid phase. In a few minutes, the device is annealed and ready to be used again for repeated measurements improving the counting statistics. The photon sensitivity of the monochloropentafluoroethane droplets was determined with highly filtered, quasi-monochromatic x-ray beams and radionuclide sources. The air-kerma response presents a broad maximum at low energies, due to the relatively high effective atomic number of the halocarbon molecule. A prototype chamber was built and successfully tested: bubble distributions deriving from the insertion of a source were imaged by means of a slice-selective 3D gradient-echo technique. These experiments confirm the potential and viability of this new approach to 3D photon dosimetry.

The ISPA-tube and the HPMT, two examples of a new class of photodetectors: the hybrid photo detectors

A new class of photodetectors, the Hybrid Photo Detectors (HPD), is coming up as a major breakthrough in photodetection. The principle of the HPD is to couple a fully depleted silicon diode or silicon diode array to a photocathode in a vacuum tube. At present HPD's with one diode or a few diodes (Hybrid Photo Multiplier Tubes, HPMT) are commercially available (DEP, The Netherlands, and Hamamatsu, Japan) and a position sensitive photon detector (the Imaging Silicon Pixel Array tube, ISPA tube) is under development in our group (in collaboration with the RD-19 collaboration at CERN). We shall present the main features of HPD's and then our main results and applications for both HPMT's and ISPA tubes on photon counting, on imaging and on high energy physics applications.