TANGO Array.: 1. The instrument

TANGO Array is an air shower experiment which has been constructed in Buenos Aires, Argentina. It was commissioned during the year 2000 becoming fully operational in September, 2000. The array consists of four water Cherenkov detectors enclosing a geometrical area of $\text{∼30,000}\text{m}{}^2$ and its design has been optimized for the observation of Extended Air Showers produced by cosmic rays near the “knee” energy region $\text{∼4×10}{}^{15}\text{eV}$. Three of the detectors have been constructed using 12,000-l stainless-steel tanks, and the fourth has been mounted in a smaller, 400-l plastic container. The detectors are connected by cables to the data acquisition room, where a very simple system, which takes advantage of the features of a four-channel digital oscilloscope, was set for data collection. This data collection setup allows a fully automatic experiment control which does not require operator intervention. It includes monitoring, data logging, and daily calibration of all detectors. This paper describes the detectors and their associated electronics, and details are given on the data acquisition system, the triggering and calibration procedures, and the operation of the array. Examples of air shower traces, recorded by the array, are presented.     

TANGO Array.: 2. Simulations

The angular and energy resolutions of the TANGO Array were obtained using extensive Monte Carlo simulations performed with a double purpose: (1) to determine the appropriate parameters for the array fitting to the desired range of sensitivity (the knee energy region), and (2) to construct a reliable shower database required for reference in the analysis of experimental data. The AIRES code, with the SIBYLL hadronic collision package, was used to simulate Extended Air Showers produced by primary cosmic rays (assuming protons and iron nuclei), with energies ranging from 10¹⁴ to 10¹⁸ eV. These data were fed into a realistic code which simulates the response of the detectors (water Cherenkov detectors), including the electronics, pickup noise, and the signal attenuation in the connecting cables. The trigger stage was considered in the simulations in order to estimate the trigger efficiency of the array and to verify the accuracy of the reconstruction codes. This paper delineates the simulations performed to obtain the expected behavior of the array, and describes the simulated data. The results of these simulations suggest that we can expect an error in the energy of the primary cosmic-ray of ∼60% of the estimated value and that the error in the measurement of the direction of arrival can be estimated as ∼4°. The present simulations also indicate that unambiguous assignments of the primary energy cannot be obtained because of the uncertainty in the nature of the primary cosmic ray.     

Study of wavelength-shifting chemicals for use in large-scale water Cherenkov detectors

Cherenkov detectors employ various methods to maximize light collection at the photomultiplier tubes (PMTs). These generally involve the use of highly reflective materials lining the interior of the detector, reflective materials around the PMTs, or wavelength-shifting sheets around the PMTs. Recently, the use of water-soluble wavelength-shifters has been explored to increase the measurable light yield of Cherenkov radiation in water. These wave-shifting chemicals are capable of absorbing light in the ultraviolet and re-emitting the light in a range detectable by PMTs. Using a 250 L water Cherenkov detector, we have characterized the increase in light yield from three compounds in water: 4-Methylumbelliferone, Carbostyril-124, and Amino-G Salt. We report the gain in PMT response at a concentration of 1 ppm as 1.88±0.02 for 4-Methylumbelliferone, stable within 0.5% over 50 days, 1.37±0.03 for Carbostyril-124, and 1.20±0.02 for Amino-G Salt. The response of 4-Methylumbelliferone was modeled, resulting in a simulated gain within 9% of the experimental gain at 1 ppm concentration. Finally, we report an increase in neutron detection performance of a large-scale (3.5 kL) gadolinium-doped water Cherenkov detector at a 4-Methylumbelliferone concentration of 1 ppm.     

Light concentrators for the Sudbury Neutrino Observatory

There is an important and growing class of elementary particle detectors which are characterized by a large sensitive volume (thousands of tonnes), very low radioactive backgrounds, and rely on the emission of light for particle detection. Water Cherenkov detectors come into this category; they have a large mass of water as the sensitive medium. Particles are detected when they interact with the water and produce Cherenkov light, so detection efficiency relies on having a huge light sensitive area at the periphery of the detector. The most cost-effective way of achieving this is by placing light concentrators on large photomultiplier tubes (PMTs). This paper describes the work carried out on light concentrators for the PMTs in the Sudbury Neutrino Observatory, a 1000 tonne heavy water Cherenkov detector. We discuss the advantages of using light concentrators, summarize the optical theory of non-imaging light concentration, and describe in detail the development and manufacture of the concentrators themselves.     

Large-scale gadolinium-doped water Cherenkov detector for nonproliferation

Fission events from Special Nuclear Material (SNM), such as highly enriched uranium or plutonium, can produce simultaneous emission of multiple neutrons and high-energy gamma-rays. The observation of time correlations between any of these particles is a significant indicator of the presence of fissionable material. Cosmogenic processes can also mimic these types of correlated signals. However, if the background is sufficiently low and fully characterized, significant changes in the correlated event rate in the presence of a target of interest constitutes a robust signature of the presence of SNM. Since fission emissions are isotropic, adequate sensitivity to these multiplicities requires a high efficiency detector with a large solid angle with respect to the target. Water Cherenkov detectors are a cost-effective choice when large solid angle coverage is required. In order to characterize the neutron detection performance of large-scale water Cherenkov detectors, we have designed and built a 3.5 kL water Cherenkov-based gamma-ray and neutron detector, and modeled the detector response in Geant4 [1]. We report the position-dependent neutron detection efficiency and energy response of the detector, as well as the basic characteristics of the simulation.     

The effect of cathode bias (field effect) on the surface leakage current of CdZnTe detectors

Surface resistivity is an important parameter of multi-electrode CZT detectors such as coplanar-grid, strip, or pixel detectors. Low surface resistivity results in a high leakage current and affects the charge collection efficiency in the areas near contacts. Thus, it is always desirable to have the surface resistivity of the detector as high as possible. In the past the most significant efforts were concentrated to develop passivation techniques for CZT detectors. However, as we found, the field-effect caused by a bias applied on the cathode can significantly reduce the surface resistivity even though the detector surface was carefully passivated. In this paper we illustrate that the field-effect is a common feature of the CZT multi-electrode detectors, and discuss how to take advantage of this effect to improve the surface resistivity of CZT detectors.     

Trackless ring identification and pattern recognition in Ring Imaging Cherenkov (RICH) detectors

An algorithm for identifying rings in Ring Imaging Cherenkov (RICH) detectors is described. The algorithm does not make use of tracking information to seed ring centres. The algorithm is necessarily Bayesian and makes use of a Metropolis-Hastings Markov chain Monte Carlo sampler to locate the rings. In particular, the sampler employs a novel proposal function whose form is responsible for significant speed improvements over earlier trackless methods. The method is optimised for finding multiple overlapping rings in detectors similar to the LHCb experiment's “RICH2” detector.     

A fast detector for single-shot positron annihilation lifetime spectroscopy

When an intense sub-nanosecond positron pulse impinges upon a target, a pulse of γ-rays is created which can yield information concerning electron–positron pairs just prior to annihilation. Many conventional γ-ray detectors are unable to exploit the timing information contained within such pulses, and we describe here the development of a fast detector that is able to do so. Using a single-crystal PbF₂ Cherenkov radiator coupled to a fast photomultiplier tube (PMT), we have produced a detector with a time response of 4 ns (primarily determined by the PMT response), as well as a low-efficiency detector with a sub-nanosecond response. Since 511 keV photons produce very little Cherenkov light, the problem of photomultiplier saturation is mitigated and this detector is therefore well suited to single-shot positron annihilation lifetime spectroscopy (SSPALS) measurements.     

High speed classification of individual bacterial cells using a model-based light scatter system and multivariate statistics

We describe a model-based instrument design combined with a statistical classification approach for the development and realization of high speed cell classification systems based on light scatter. In our work, angular light scatter from cells of four bacterial species of interest, Bacillus subtilis, Escherichia coli, Listeria innocua, and Enterococcus faecalis, was modeled using the discrete dipole approximation. We then optimized a scattering detector array design subject to some hardware constraints, configured the instrument, and gathered experimental data from the relevant bacterial cells. Using these models and experiments, it is shown that optimization using a nominal bacteria model (i.e., using a representative size and refractive index) is insufficient for classification of most bacteria in realistic applications. Hence the computational predictions were constituted in the form of scattering-data-vector distributions that accounted for expected variability in the physical properties between individual bacteria within the four species. After the detectors were optimized using the numerical results, they were used to measure scatter from both the known control samples and unknown bacterial cells. A multivariate statistical method based on a support vector machine (SVM) was used to classify the bacteria species based on light scatter signatures. In our final instrument, we realized correct classification of B. subtilis in the presence of E. coli,L. innocua, and E. faecalis using SVM at 99.1%, 99.6%, and 98.5%, respectively, in the optimal detector array configuration. For comparison, the corresponding values for another set of angles were only 69.9%, 71.7%, and 70.2% using SVM, and more importantly, this improved performance is consistent with classification predictions.     

Trigger electronics of the new Fluorescence Detectors of the Telescope Array Experiment

The Telescope Array Project is an experiment designed to observe Ultra High Energy Cosmic Rays via a “hybrid” detection technique utilizing both fluorescence light detectors (FDs) and scintillator surface particle detectors (SDs). We have installed three FD stations and 507 SDs in the Utah desert, and initiated observations from March 2008. The northern FD station reuses 14 telescopes from the High Resolution Fly's Eye, HiRes-I station. Each of the two southern FD stations contains 12 new telescopes utilizing new FADC electronics. Each telescope is instrumented with a camera composed of 256 PMTs. Since the detectors are composed of many PMTs and each PMT detects fluorescence photons together with the vast amount of night sky background, a sophisticated triggering system is required. In this paper, we describe the trigger electronics of these new FD stations. We also discuss performance of the FDs with this triggering system, in terms of efficiencies and apertures for various detector configurations.     

EAGLE—the central European Array for Gamma Levels Evaluation at the Heavy Ion Laboratory of the University of Warsaw

The EAGLE array (European Array for Gamma Levels Evaluations) has been designed as a multi-configuration detector set-up for in-beam nuclear spectroscopy studies at the Heavy Ion Laboratory of the University of Warsaw. The array can accommodate a maximum of 30 Compton suppressed Ge detectors coupled to various ancillary devices, such as a 4π inner ball consisting of 60 BaF₂ crystals, 30 element 4π silicon detector array, compact scattering chamber equipped with 110 PIN diodes placed at backward angles and a conversion-electron spectrometer.     

Rejection of Surface Background in Thermal Detectors

A challenging aspect of the next generation detector for rare events searches (i.e. neutrinoless double beta decay and dark matter searches) is the reduction of the background in the region of interest that can mimic the expected signals. In the field of thermal detectors, which have a leading role in rare events searches thanks to their excellent energy resolution and to the wide choice of absorber materials, the background coming from surface contaminations is frequently dominant. A background surface rejection detector is a scintillation-based approach for tagging this type of background. We discuss the innovative application of this technique in non-scintillating thermal detectors. We will report on the performances of the prototype bolometric detector, realized to prove the feasibility of this new technique.     

A hadron-blind detector

The development of highly efficient solid photocathodes, compatible with high-gain gaseous detectors, has opened the possibility to build threshold Cherenkov counters with a good hadron rejection, with a minimum amount of matter, and a time resolution of the order of the nanosecond. We discuss the properties of a hadron-blind detector, with a granularity of a few millimetres. The study of the background sources shows that a rejection power of the order of 99% can be achieved for high-energy hadrons. It permits instantaneous multi-hadron rejection and, combined to a fast electromagnetic calorimeter, can ensure on-line electron selection, even when they are produced close to hadronic jets. It could permit the operation of lepton-tracking detectors in a magnetic field, in very high hadronic backgrounds.     

Charge transfer in the presence of a layer of trapping centers in semiconductor SiC ionizing radiation detectors

The formation of a response signal in the presence of a layer of trapping centers in semiconductor SiC ionizing radiation detectors is considered on the basis of a new model. Since the trapping layer is situated near the detector surface, nuclear particles that possess long tracks partly generate a charge behind this layer. Under certain conditions, the proposed model leads to a paradoxical situation, in which the signal decreases with increasing bias voltage. The application of the model to results obtained using ion-doped 4H-SiC detectors provides a qualitative explanation for the experimentally observed increase in the signal with the temperature in the region of saturation with respect to the bias voltage (i.e., under the conditions of total charge transfer), which were previously unclear.     

高效液相色谱测定啤酒瓶盖内衬垫DEHP的含量

建立了高效液相色谱测定啤酒瓶盖内衬垫中邻苯二甲酸二(2-乙基己基)酯(DEHP)含量的方法。通过改变样品的前处理方法及检测的色谱条件,以保留时间来定性,外标法来定量,对实验结果进行优化。实验得到:样品在经过无水乙醇浸泡处理,流动相甲醇与水的体积比为95∶5,紫外检测器检测波长为275 nm时,能获得的分离效果最佳;在该分析条件下,DEHP标准曲线呈现很好的线性关系,相关系数均大于0.998,精密度RSD<1.5%,DEHP在无水乙醇中浸泡的溶出量为24.45%,在95%甲醇中浸泡的溶出量为15.97%。     

Measurement of the tunnel rate in SIS' tunnel junctions as function of bias voltage

Cryogenic detectors with superconducting tunnel junctions can provide an energy resolution improved by at least one order of magnitude compared with standard semiconductor detectors. While the detection principle was already demonstrated many years ago, the past years were dedicated to the transition from the laboratory sample to practical detectors. Our most favored detector design gives rise to tunnel junctions with electrodes of unequal energy gaps. In such hetero tunnel junctions bias conditions can be established which cause a negative signal current. We report the experimental verification of this effect, and we discuss the yield of charge signal of cryogenic detectors based upon superconducting tunnel junctions.     

Bicell Pyroelectric Optical Detector Made from a Single LiNbO(3) Domain-Reversed Electret

Using electric-field poling at room temperature, we selectively reversed the direction of the spontaneous polarization in a 200-mum-thick, z-cut LiNbO(3) electret to produce a bicell pyroelectric detector. The detector required only a single set of electrodes, one electrode on the front surface and one on the back surface. Microphonic noise that is typical of monocell pyroelectric detectors is reduced in the present device. Our spatial uniformity data indicate that the optical response of one half of the bicell detector area was equal to and opposite the other half within 1.2%. The microphonic suppression of the bicell pyroelectric detector was less than -36 dB from 10 to 50 Hz and less than -118 dB at 35 Hz of that of a reference monocell pyroelectric detector. The substrate thickness is significantly greater than those of other domain-engineered pyroelectric detector designs and allows us to build practical, large-area detectors for radiometric applications.     

Correlation-sensitive adaptive sequence detection

In high density magnetic recording, noise samples corresponding to adjacent signal samples are heavily correlated as a result of front-end equalizers, media noise, and signal nonlinearities combined with nonlinear filters to cancel them. This correlation significantly deteriorates the performance of detectors at high densities. In this paper, we propose a novel sequence detector that is correlation sensitive and adaptive to the nonstationary signal sample statistics. We derive the correlation-sensitive maximum likelihood detector. It can be used with any Viterbi-like receiver (e.g., partial response maximum likelihood, fixed delay tree search, multilevel decision feedback equalization) that relies on a tree/trellis structure. Our detector adjusts the metric computation to the noise correlation statistics. Because these statistics are nonstationary, we develop an adaptive algorithm that tracks the data correlation matrices. Simulation results are presented that show the applicability of the new correlation-sensitive adaptive sequence detector     

Second European Workshop on Low Temperature Detectors: Annecy, France, 2–6 May 1988

The workshop was concluded with a comprehensive summary and discussion (Pretzel, MPI, Munich, FRG). Clearly there is a growing interest in the development of low temperature devices for neutrino and dark matter detectors. Other applications are spectroscopic ultra-high spatial resolution X-ray imaging, optical and infrared radiation detectors, cold neutron and other low energy particle detectors, and phonon detectors. However, it is also clear that the fundamental experiments that were the original motivation of this research are a long way from realization. Slow but steady progress is being made on all fronts. In the process this research is enhancing an understanding of the particle/radiation interactions and revealing important phenomena in cryogenic devices. Each of the technologies under development has individual strengths and weaknesses so no single device is expected to solve all the detection problems. The state of the current research is still exploratory. It appears that at least two to three years of research will be required to demostrate the viability of one or more of these ideas. At that point, full-scale development of one or more detector systems will hopefully begin to address the many fundamental problmes posed at this workshop.     

The evaluation of an improved method of gas-freeing an aviation fuel storage tank

A new technique is described for the gas-freeing of large storage tanks used for aviation fuel. The technique involves the use of natural and mechanical ventilation, together with an air-driven pump for removal of liquid residues form the irregular bottom of the tank. Am assessment was made by gas-freeing a 4500m{inf3} (1 M gal) tank in which the atmosphere was monitored using portable flammable gas detectors and checked by the analysis of samples using chromatography. The new technique was much quicker than the traditional method of gas-freeing and was more controlled so that a work schedule could be devised in advance with confidence.The portable gas detectors were shown to under-estimate gas concentrations; it is thus imperative that the concentration levels at which various inspection and maintenancem operations are permitted in tanks should reflect the likely errors of the instrument used to measure concentration.