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.     

Detection of Cherenkov light emission in liquid argon

Detection of Cherenkov light emission in liquid argon has been obtained with an ICARUS prototype, during a dedicated test run at the Gran Sasso Laboratory external facility. Ionizing tracks from cosmic ray muons crossing the detector active volume have been collected in coincidence with visible light signals from a photo-multiplier (PMT) immersed in liquid argon. A 3D reconstruction of the tracks has been performed exploiting the ICARUS imaging capability. The angular distributions of the tracks triggered by the PMT signals show an evident directionality. By means of a detailed Monte Carlo simulation we show that the geometrical characteristics of the events are compatible with the hypothesis of Cherenkov light emission as the main source of the PMT signals.     

ALTAI: computational code for the simulations of TeV air showers as observed with the ground-based imaging atmospheric Cherenkov telescopes

Ground-based atmospheric Cherenkov telescopes are proven to be effective instruments for observations of very high-energy (VHE) γ-radiation from celestial objects. For effective use of such technique one needs detailed Monte Carlo simulations of γ-ray- and proton/nuclei-induced air showers in the Earth atmosphere. Here we discuss in detail the algorithms used in the numerical code ALTAI developed particularly for the simulations of Cherenkov light emission from air showers of energy below 50 TeV. The specific scheme of sampling the charged particle transport in the atmosphere allows the performance of very fast and accurate simulations used for interpretation of the VHE γ-ray observations.     

A high-granularity scintillator calorimeter readout with silicon photomultipliers

We report on the design, construction and performance of a prototype for a high-granularity tile hadronic calorimeter for a future international linear collider detector. Scintillating tiles are read out via wavelength-shifting fibers that guide the scintillation light to a novel photodetector, the silicon photomultiplier. A prototype has been tested using a positron test beam at DESY. The results are compared with a reference prototype calorimeter equipped with multichannel vacuum photomultipliers. Detector calibration, noise, linearity and stability are discussed, and the energy response in a 1–6 GeV positron beam is compared with simulations. The present results demonstrate that the silicon photomultiplier is well-suited as photodetectors in calorimeters and thus has been selected for the construction of a calorimeter prototype to operate in hadron beams.     

A detector for filtering γ-ray spectra from weak fusion–evaporation reactions out of strong background and for Doppler correction: The recoil filter detector, RFD

A detector has been designed and built to assist in-beam γ-ray spectroscopy with fusion–evaporation reactions. It measures with high efficiency the evaporation residues that recoil out of a thin target into the angular interval from 1.8° to 9.0° at an adjustable distance of 1000–1350 mm from a target, in coincidence with γ-rays detected in a Ge-detector array. This permits filtering of such γ-rays out of a much stronger background of other reaction products and scattered beam. Evaporation residues are identified by their time-of-flight and the pulse height using a pulsed beam. The velocity vector of the γ-emitting recoil is also measured in the event-by-event mode, facilitating to correct the registered γ-ray energy for the Doppler shift, with the resulting significant improvement of the energy resolution. The heavy-ion detection scheme uses emission of secondary electrons caused by the recoiling ions when hitting a thin foil. These electrons are then electrostatically accelerated and focused onto a small scintillator that measures the summed electron energy, which is proportional to the number of electrons. The detector is able to operate at high frequency of the order of 1 MHz and detect very heavy nuclei with as low kinetic energy as 5 MeV. The paper describes the properties of the detector and gives examples of measurements with the OSIRIS, GAREL+ and EUROBALL IV γ-ray spectrometers. The usefulness of the technique for spectroscopic investigations of nuclei with a continuous beam is also discussed.     

The central pixel of the MAGIC telescope for optical observations

The MAGIC telescope has been designed for the observation of Cherenkov light generated in Extensive Air Showers initiated by cosmic particles. However, its 17 m diameter mirror and optical design makes the telescope suitable for direct optical observations as well. In this paper, we report about the development of a system based on the use of a dedicated photo-multiplier (PMT) for optical observations. This PMT is installed in the centre of the MAGIC camera (the so-called central pixel). An electro-to-optical system has been developed in order to transmit the PMT output signal by an optical fibre to the counting room, where it is digitized and stored for off-line analysis. The performance of the system using the optical pulsation of the Crab nebula as calibration source is presented. The time required for a 5σ detection of the Crab pulsar in the optical band is less than 20 s. The central pixel will be mainly used to perform simultaneous observations of the Crab pulsar both in the optical and γ-ray regimes. It will also allow for periodic testing of the precision of the MAGIC timing system using the Crab rotational optical pulses as a very precise timing reference.     

CdWO4 scintillator as a compact gamma ray spectrometer for planetary lander missions

The objective of this work is to develop a gamma ray spectrometer (GRS) suitable for use on planetary rover missions. The main characteristics of this detector are low weight, small volume low power and resistance to cosmic ray radiation over a long period of time. We describe a 3 cm diameter by 3 cm thick CdWO₄ cylindrical scintillator coupled to a PMT as a GRS for the energy region 0.662–7.64 MeV. Its spectral performance and efficiency are compared to that of a CsI(Tl) scintillator 2.5 cm diameter by 6 cm thick coupled to a 28 mm×28 mm PIN photodiode. The comparison is made experimentally using ¹³⁷Cs, ⁶⁰Co, 6.13 MeV gamma rays from a ¹³C(,γn)O¹⁶* source, 7.64 MeV thermal neutron capture gamma rays emitted from iron bars using a ²⁵²Cf neutron source, and natural radioactivity 1.46 MeV ⁴⁰K and 2.61 MeV ²³²Th gamma rays. We use a Monte Carlo method to calculate the total peak efficiency of these detectors and the full energy, first and second escape peak efficiencies. The experimental and calculated results agree well. We investigated the usefulness of these detectors for a GRS on a Mars lander mission. Although both detectors meet desired specifications, it was found that CdWO₄ has advantages over CsI(Tl) being a more compact detector of higher efficiency. Using a shaping amplifier of 24 ms, CdWO₄ spectrometer exhibited a 6.8% FWHM at 662 keV. At 6.13 MeV, CdWO₄ detector possesses an intrinsic total and full energy peak efficiencies of 16.7% and 6.3%, respectively. These efficiencies are nearly a factor of 1.6 and 4 greater than the corresponding efficiencies of the CsI(Tl) detector.

A proposed gamma ray spectroscopy system to be placed on a rover, consists of a central detector surrounded by a Compton suppressor shield. The central detector is a cylindrical CdWO₄ detector and the Compton suppressor shield is made of segmented CdWO₄, coupled to PIN photodiodes. The shield also prevents thermal neutron activation of the central detector.     

Amadeus - a new type of large area scintillation detector with position-, energy- and time-of-flight determination

The Amadeus detector consists of a scintillator plate, a Plexiglas plate and a matrix of photomultiplier tubes oriented opposite to the particle direction. A charged particle traversing the scintillator emits scintillation light which is seen only by the nearby phototubes. In a first approximation the sum of the pulse height seen by these tubes is proportional to the deposited energy and the center of gravity of the detected light distribution corresponds to the particle impact position. This approximation allows a fast determination of particle parameters with sufficient accuracy for online applications. In connection with the time-of-flight the particle identification and the complete determination of the four-momentum vector is given. For the off-line analysis a more refined algorithm can be used resulting in energy-, position- and time-of-flight resolution values in the range of σ_x = 3–5 mm σ_E = 3–5 MeV, σ₁ = 150–250 ps, respectively.     

Simple photomultiplier tube internal-gating method for use in subnanosecond time-resolved spectroscopy

We propose a simple photomultiplier tube (PMT) internal-gating method for use in the field of subnanosecond time-resolved spectroscopy. In the proposed method, we control two dynodes in the PMT by applying a gate signal whose pulse width is Tg. When controlling the mth and the n(> m)th dynodes, a resolution time delta t is approximately given by delta t = Tg-(n-m) tau, where tau is a transit time of a lump of secondary electrons traveling between the two dynodes in the PMT. In principle, the resolution time delta t shorter than the pulse width Tg of the gate signal can be easily obtained. From a fundamental performance test, we found that a subnanosecond resolution time delta t = 0.31 ns was obtained for the case of m = 2 and n = 5. To demonstrate the effectiveness of the proposed method, we carried out a time-resolved spectroscopic measurement of emission obtained from a white-light-emitting diode (LED) driven by a nanosecond current pulse.     

Effect of pulse time gate on CsI(T1) light output

The mass and charge identification of secondary particles with Z < 4 by a large CsI(T1) scintillation detector is performed using pulse shape analysis and time-of-flight methods. The dependence of the light output on E, A and Z is studied in the energy range of 1–20 MeV/A and special attention is paid to the integration time of the photomultiplier anode signal. It is found that the behaviour of the calibration curves strongly depends on the choice of the integration time interval.     

Phase-modulation fluorometer using a dynode-voltage burst-modulated photomultiplier tube

We propose a new scheme for a phase-modulation fluorometer (PMF) in which a photomultiplier tube (PMT) is used as a photo detector whose gain is modulated sinusoidally with a burst signal of period T and duty ratio 0.5. The carrier wave of the burst modulation signal is synchronized with an incident fluorescence signal. In order to modulate the gain of the PMT, one of the dynodes in the PMT was deeply biased and the burst signal was superimposed. Because the fluorescence signal is converted to a direct current (dc) signal by the PMT internal modulation, we can make the value of the load resistance of the PMT relatively large under the condition tau < or = T/2, where tau is a time constant of a low-pass filter attached to the output of the PMT. The proposed scheme brings about advantages in sensitivity and noise immunity in detecting weak fluorescence in comparison with those of the conventional PMF. The combined technique of the burst modulation of the gain of the PMT and the alternating current (ac) signal detection alleviates the influence of the background light.     

An accurate redetermination of the Sn binding energy

The energy of well-known strong γ line from ¹⁹⁸Au, the “gold standard”, has been modified in the light of new adjustments in the fundamental constants and the value of 411.80176(12) keV was determined, which is 0.29 eV lower than the latest 1999 value. An energy calibration procedure for determining the neutron binding energy, B_n, from complicated (n, γ) spectra has been developed. A mathematically simple minimization function consisting only of terms having as parameters the coefficients of the energy calibration curve (polynomial) is used. A priori information about the relationships among the energies of different peaks on the spectrum is taken into account by a Monte-Carlo simulation. The procedure was used in obtaining B_n for ¹¹⁸Sn. The γ-ray spectrum from thermal neutron radiative capture by ¹¹⁷Sn has been measured on the IBR-2 pulsed reactor. γ-rays were detected by a 72 cm³ HPGe detector. For a better determination of B_n it was important to determine B_n for ⁶⁴Cu. This value was obtained from two γ-spectra. One spectrum was measured on the IBR-2 by the same detector. The other spectrum was measured with a pair spectrometer at the Brookhaven High Flux Beam Reactor. From these two spectra, B_n for ⁶⁴Cu was determined to be equal to 7915.52(8) keV. This result essentially differs from the previous value of 7915.96(11) keV. The mean value of the two most precise results of the B_n for ¹¹⁸Sn, was determined to be 9326.35(9) keV. The B_n for ⁵⁷Fe was determined to be 7646.08(9) keV.     

Characterization of an Al-STJ-based X-ray detector with monochromatized synchrotron radiation

The characterization of an Al-STJ-based detector with Pb absorber was performed with monochromatized synchrotron radiation. Detector response was measured in the energy range from 3 to 10 keV. A small non-linearity of the signal pulse height was detected, probably due to the escape of recombination phonons from the detector. The non-linearity can be described by a second order polynomial function. Additionally, detector signals were recorded while an X-ray beam of 50 μm diameter was directed to several locations on and near the absorber. For a well-aligned beam, detector artefacts are of at least two orders of magnitude lower intensity than the absorber events.     

The observation of Cherenkov light from the decay of 40K

In the observation of Cherenkov light produced by cosmic rays in the ocean, illuminations produced by ⁴⁰K should be considered as a background to a photomultiplier. We have measured the pulse height distribution and the counting rate of Cherenkov light from ⁴⁰K using a solution with a higher concentration of potassium salts than sea water and compared them with expected values.     

纯碘化铯(CsI)晶体的发光与光衰减特性研究

纯碘化铯晶体是一种具有快衰减闪烁特性的新型辐射探测材料, 在伽马射线、中子和其它辐射探测技术中有重要的应用前景。本研究以Bridgman法所生长的纯碘化铯为对象, 分别研究了该晶体的透射光谱以及在紫外光、连续X射线、脉冲X射线和宇宙射线激发下的发射光谱、时间响应特性和沿晶体生长方向不同部位的光输出分布特点。实验测得晶体的吸收边为240 nm, 激发和发射波长分别位于241 nm和318 nm, 发光衰减时间分别为2~3 ns和18~25 ns。以尺寸为30 mm×30 mm×200 mm的晶体籽晶端和尾端与PMT耦合所测得的光输出分别是143 p.e/MeV和127 p.e./MeV, 尽管晶体两端的光输出存在12.6%的差异, 但没有观察到衰减时间长于100 ns的慢分量。这些性能进一步证明纯碘化铯晶体具有作为快闪烁体的优势。     

Chemical sensing systems using xerogel-based sensor elements and CMOS photodetectors

We present the first example of an integrated complementary metal-oxide-semiconductor (CMOS) photodetector coupled with a solid-state xerogel-based thin-film sensor to produce a compact chemical sensor system. We compare results using two different CMOS-based detector systems to results obtained by using a standard photomultiplier tube (PMT) or charge-coupled device (CCD) detector. Because the chemical sensor elements are governed by a Stern-Volmer relationship, the Stern-Volmer quenching constant is used as the primary comparator between the different detectors. All of the systems yielded Stern-Volmer constants from 0.042 to 0.049 O/sub 2/%/sup -1/. The results show that the CMOS detector system yields analytical data that are comparable to the CCD- and PMT-based systems. The disparity between the data obtained from each detector is primarily associated with the difference in how the signals are obtained by each detector as they presently exist. We have also observed satisfactory reversibility in the operation of the sensor system. The CMOS-based system exhibits a response time that is faster than the chemical sensor element's intrinsic response time, making the CMOS suitable for time-dependent measurements. The CMOS array detector also uses less than 0.1% the power in comparison to a standard PMT or CCD. The combined xerogel/CMOS system represents an important step toward the development of a portable, efficient sensor system.     

Microstructural research on hot strips of low carbon steel produced by a compact strip production line under different thermal histories

Coupons with the same composition and thickness (4.0 mm nominal gauge) obtained from hot strips of low carbon steel underwent a series of investigations to analyze the microstructural characteristics and mechanisms responsible for their differences in mechanical properties. Two different industrial technologies were adopted, although the strips used in this research were produced on the same Compact Strip Production (CSP) line. One of the strips was produced with a routine γ→ CSP thermal history, but the other with a γ→→γ* conventional thermal history. The only difference between them was that one technology had a →γ* thermal history. Different specimens of both types of strips were prepared for metallographic observation, tensile tests, electron back-scattered diffraction tests and positron annihilation technique tests. Experimental results showed that the differences in mechanical properties could be ascribed to dissimilarities not only in the grain size and textural components but also in dislocation density.     

Study of TOF PET using Cherenkov light

This work investigates the possibilities of improving the measurements of arrival time difference of the two 511 keV photons arising from annihilation of a positron in positron emission tomography (PET). The new technique of detecting the prompt Cherenkov light, produced by absorption of the annihilation photon in a suitable crystal, could considerably improve the image quality. A simple apparatus with PbF₂ crystals and microchannel plate photomultipliers (MCP PMTs) has been constructed and coincidence resolutions of 71 ps FWHM and 95 ps FWHM have been achieved with 5 and 15 mm thick crystals, respectively. Simulation calculations are in agreement with the experimental findings.     

Production of positron annihilation radiation by cosmic-rays near sea level

Production of positron annihilation radiation by cosmic-rays in Al, Fe, Sn and Pb is measured by means of a triggered HPGe detector. The equipment is located in Belgrade, at an absolute height of 125 m a.s.l. The production rate per unit mass is found to be proportional to the square of the atomic number of the material divided by its mass number, with the proportionality constant equal to 8.1(3)×10⁻⁶ s⁻¹ g⁻¹.