A. Stoykov

A Time-resolution study with a plastic scintillator read out by a Geiger-mode Avalanche Photodiode

In this work we attempt to establish the best time resolution attainable with a scintillation counter consisting of a plastic scintillator read out by a Geiger-mode Avalanche Photodiode. The measured time resolution is inversely proportional to the square root of the energy deposited in the scintillator, and scales to σ = 18ps at 1MeV. This result competes with the best ones reported for photomultiplier tubes.

Evaluation of two thermal neutron detection units consisting of ZnS/ 6 LiF scintillating layers with embedded WLS fibers read out with a SiPM

Two single channel detection units for thermal neutron detection are investigated in a neutron beam. They consist of two ZnS/(LiF)-Li-6 scintillating layers sandwiching an array of WLS fibers. The pattern of these units can be repeated laterally and vertically in order to build up a one-dimensional position sensitive multi-channel detector with the needed sensitive surface and with the required neutron absorption probability. The originality of this work arises from the fact that the WLS fibers are read out with SiPMs instead of the traditionally used PMTs or MaPMTs. The signal processing system is based on a photon counting approach. For SiPMs with a dark count rate as high as 0.7 MHz, a trigger efficiency of 80% is achieved together with a system background rate lower than 10(-3) Hz and a dead time of 30 mu s. No change of performance is observed for neutron count rates of up to 3.6 kHz

Chiral Induction in Lyotropic Liquid Crystals: Insights into the Role of Dopant Location and Dopant Dynamics†

The pitch P of the helix can be directly observed in the polarizing optical microscope as the periodic pattern of the “fingerprint texture”. Chiral induction in liquid crystals (LCs) is one of the most sensitive methods for the detection of chirality. The unique chirality effects in LCs have been studied widely, including the molecular induction mechanism in thermotropic LCs and in a self-assembled twodimensional model system. For LLCs, however, the molecular induction mechanism in the N* phase has been a matter of discussion for more than 20 years. There are two proposed mechanisms: a) a dispersive chiral interaction between dopants in adjacent micelles (the dopant should preferentially be located at the micellar surface), and b) a steric dopant–amphiphile interaction yielding distorted micelles (in this case the solubilization of the chiral dopant within the micelle should be favorable). 10, 11] The temperature dependence of the pitch P(T) is expected to differ for the two mechanisms: in (a), P increases linearly with T, whilst in (b), P may decrease hyperbolically (with T ; see Supporting Information). Experimental studies on the chiral induction mechanism include pitch measurements of varied guest–host systems 7] and X-ray diffraction, however, the latter has not provided clear evidence of distorted micelles. The pitch was found to depend on the chemical composition of the LLC host phase, the temperature, the dopant concentration and, in particular, the chemical nature of the dopant. A general correlation between properties of the chiral dopant and its chiral induction power in a host phase has not yet been established for LLCs, in contrast to the molecular concepts developed for thermotropic LCs (see, for example, Ref. [1] and Ref. [12]), which are also important for LLCs, as we will discuss later. A crucial point of discussion regarding LLCs, especially in view of the suggested mechanisms, is the actual location of the chiral dopants in the N* phase: within the apolar core of the micelle or at the micellar surface. The dopant location is proposed to play an important role for the chiral induction power, 13] but locating the dopants experimentally has not yet been successful. Recently, a magnetic resonance method suitable for studying dopants present at low concentrations, avoidedlevel-crossing muon spin resonance (ALC-mSR), was used to reveal the dopant location in lamellar LLC phases. The ALCmSR technique involves the formation of a radical by the addition of muonium, Mu, a light hydrogen isotope (mH = 9mMu) with a positive muon m + as the nucleus, to an unsaturated bond (Scheme 1). The time-integrated muon spin polarization, which is proportional to the muon decay asymmetry A, is measured as a function of an external magnetic field. Resonances occur when there is coupling between eigenstates of the three-spin-=2 system composed of the radical electron, the muon, and the proton bound to the same carbon as the muon. The resonance field Bres is determined by the hyperfine coupling constants of the radical; these coupling constants are, among other factors, sensitive to the polarity of the surroundings, and Bres is shifted to higher values with increasing polarity. The polarity of the local environment and thus the location in the LLC is determined by comparing Bres in the LLC with the values in a Figure 1. Schlieren texture and model of the nematic (N) LLC host phase with disk-like micelles (left). Fingerprint texture and model of the chiral nematic (N*) phase; micelles represent the helical modulation of the director n with pitch P induced by doping the host phase with 4.37% R-MA (right).

μ-SR investigations in silicon

Abstract Results on the temperature dependence of the residual polarization of negative muons in silicon with phosphorus ( 4.5×10 18 , 2.3×10 15 , and 3.2×10 12 cm −3 ) and aluminium (2.4×1018 and 2×10 14 cm −3 ) impurities are presented. The muon spin rotation (μSR) experiments were carried out in a magnetic field of 0.2 T and in the temperature range 4.2–300 K. In all investigated samples a relaxation of the muon spin and a shift of the spin-precession frequency were observed. The frequency shift (relative to the room-temperature value) amounts to 7×10−3 at 15 K. In the sample with a high concentration of phosphorus impurity ( 4.5×10 18 cm −3 ) damped and undamped components of the muon spin polarization were observed at T K . Hyperfine interaction between the magnetic moments of the muon and that of the electron shell of the muonic atom (acceptor centre – μAl) is estimated on the basis of the muon spin precession frequency shift data. The temperature dependence of the spin-lattice relaxation rate of the magnetic moment of the shallow acceptor centre in silicon in the absence of external stress is determined for the first time. It is found that the relaxation rate is well approximated by the power function ν(T)=CTq, where the parameter q lies between 2 and 3.

A 16-ch module for thermal neutron detection using ZnS:

Abstract A scalable 16-ch thermal neutron detection system has been developed in the framework of the upgrade of a neutron diffractometer. The detector is based on a ZnS:6LiF scintillator with embedded WLS fibers which are read out with SiPMs. In this paper, we present the 16-ch module, the dedicated readout electronics, a direct comparison between the performance of the diffractometer obtained with the current 3He detector and with the 16-ch detection module, and the channel-to-channel uniformity.

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The High-Field μSR instrument is a highly challenging project under realization at the Swiss Muon Source [1] of the Paul Scherrer Institut (PSI, Switzerland). The detector system of the new spectrometer has to satisfy strict requirements on the time resolution and compactness. Muon-spin precession signals with frequencies of up to 1.3GHz in magnetic fields up to 9.5T have to be detected, the reduction of the signal amplitude should not exceed 50%. This requires an accuracy of better than 140 ps (sigma) in measuring the muon-positron time correlations – a time resolution unprecedented for such high fields. The small spiraling radius of the decay positrons in high fields (∼ 1 cm in 9.5T) sets restrictions on the maximal radial dimension of the detector. Preservation of the 10 ppm uniformity of the magnetic field at the sample position requires all detector components located in the vicinity of the sample to be non-magnetic. R&D work on the detector development for the High-Field project at PSI has started in 2004. It was realized that the required time resolution can hardly be achieved within the “standard” detector technology using photomultiplier tubes (PMTs), the limiting factors being attenuation and broadening of the light pulses in the indispensable light guides. Other potentially promising photosensors have been evaluated and the choice was made in favor of Geiger-mode Avalanche Photodiodes (G-APDs) [2]. These novel solid-state photodetectors deliver performance similar to that of PMTs, being at the same time insensitive to magnetic fields, compact, and non-magnetic (when choosing an appropriate packaging). The potential of the G-APD based detector technology for μSR and its reliability have been proven in [3, 4, 5]. The found technical solutions and the gained experience constituted an essential ground for working out the concept of the detector system of the High-Field μSR instrument.

LiF scintillator with embedded WLS fibers coupled to SiPMs and its dedicated readout electronics

Abstract We present a digital signal processing system based on a photon counting approach which we developed for a thermal neutron detector consisting of ZnS(Ag):6LiF scintillating layers read out with WLS fibers and SiPMs. Three digital filters have been evaluated: a moving sum, a moving sum after differentiation and a digital CR-RC4 filter. The performances of the detector with these filters are presented. A full analog signal processing using a CR-RC4 filter has been emulated digitally. The detector performance obtained with this analog approach is compared with the one obtained with the best performing digital approach.

High-Field μSR Instrument at PSI: Detector Solutions

Temperature-dependent remanent polarization of negative muons in a silicon crystal doped with phosphorus (3.2 × 1012, 2.3 × 1015, and 4.5 × 1018 cm−3) and aluminum (2 × 1014 and 2.4 × 1018 cm−3) was examined. Measurements were made over the temperature range 4–300 K in a magnetic field of 2000 G perpendicular to the muon spin. Temperature dependence of the relaxation rate was determined for the magnetic moment of a shallow Al acceptor center in a nondeformed silicon sample, and the hyperfine interaction constant was estimated for the interaction between the magnetic moments of muon and electron shell of the muonic mAl atom in silicon.

Digital signal processing for a thermal neutron detector using ZnS(Ag):6LiF scintillating layers read out with WLS fibers and SiPMs

Avoided level crossing muon spin resonance (ALC-muSR) has been used to study the cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the discotic liquid crystal HAT6 (2,3,6,7,10,11-hexahexyloxytriphenylene) in the crystalline (Cr) phase, the hexagonal columnar mesophase (Col(h)) and isotropic (I) phase. In the Cr phase unpaired electron spin density can be transferred from the radical to neighboring HAT6 molecules depending on the overlap of their pi-systems and hence on the relative orientation of the triphenylene rings. The two Delta(1) resonances in the ALC-muSR spectra of the Cr phase indicate that the neighboring HAT6 molecules have two preferred orientations with respect to the radical: one which results in negligible spin density transfer and a second where 17% of the unpaired spin density is transferred. The ALC-muSR spectra in Col(h) and I phases are substantially different from those of the Cr phase in that there are two narrow resonances superimposed on an extremely broad and intense resonance. The narrow resonances are due to highly mobile radicals located in the aliphatic region between the columns and the broad resonance is due to radicals incorporated within the columns of HAT6 molecules. The large width and amplitude of this resonance indicates that the radicals within the columns are undergoing rapid electron spin relaxation but the mechanism that causes this relaxation is unknown.

µ−spin rotation study of the temperature-dependent relaxation rate of acceptor centers in silicon

The residual polarization of negative muons in crystal silicon samples with phosphorus (P: 1.6×1013 cm−3) and antimony (Sb: 2×1018 cm−3) impurities is investigated. The measurements are made in a 1000 G magnetic field oriented in a direction transverse to the muon spin in the temperature range 4–300 K. The relaxation rate and shift of the precession frequency in the silicon sample with the phosphorus impurity are measured more accurately than previously. It is found that in antimony-doped silicon the acceptor center µA1 at temperatures below 30 K can be in both ionized and neutral states. The experimental data are interpreted on the basis of spin-lattice relaxation of the magnetic moment of an acceptor center, formation of acceptor-donor pairs, and recombination of charge carriers at the acceptor. Preliminary measurements showed a nonzero residual polarization of negative muons in germanium.