Alternative Scintillating Materials for the CRESST Dark Matter Search

The CRESST Dark Matter Search aims to detect Dark Matter particles via their elastic scattering off nuclei in scintillating target crystals. A?simultaneous measurement of a phonon and light signal allows to efficiently discriminate the dominant beta and gamma background from expected nuclear recoil signals. For a given light detector and reflector, it is the scintillation efficiency of the crystal which determines the lowest energy for which the active background discrimination is still efficient. Crystals with a higher light output therefore can improve the overall sensitivity of the experiment. In this context, measurements of the light yield of CsI (undoped) and CdWO₄ crystals at milli-Kelvin temperature are presented. Furthermore we report on first results of measurements of these crystals in conventional CRESST configuration.     

Scintillation properties of SrF2 and SrF2-Ce3+ crystals

This Letter presents the results of measuring scintillation properties of pure SrF₂ crystals and crystals activated by various concentrations of Ce³⁺ ions. The light yield of these materials is compared to that of the known scintillators NaI-Tl and CaF₂-Eu²⁺. Strontium fluoride crystals activated with Ce³⁺ ions are found to be characterized by high light yield and to be promising materials for use in scintillation detectors employed for γ-ray well logging.     

Growth and electrical characterization of the lead magnesium niobate-lead titanate (PMN-PT) single crystals for piezoelectric devices

The lead magnesium niobate-lead titanate is one of the new generations of piezoelectric materials with outstanding properties. Single crystals of 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) show superior properties as compared to piezoelectric ceramics and piezoelectric films in device applications. Large size crystals are required for specific applications. Previously, transparent and high quality PMN-PT single crystals have been grown in our laboratory using the high temperature flux method. The size of the obtained crystals was small and varied from 2 to 8 mm³, mostly showing regular prismatic shape. In present attempt, PMN-PT crystals are grown from the vertical gradient freeze method with no flux using congruent melt. Processing conditions and growth parameters have been optimized to increase the size and yield of the perovskite crystals including cooling rate, soak time and soak temperature. The size of the grown crystals obtained by this method is very large relatively and varied from 5 to 30 mm³. The microstructure of the as grown single crystals is investigated by scanning electron microscopy. Piezoelectric properties of the grown crystals are also investigated. PMN-PT plates show excellent piezoelectric properties. Samples were poled under an applied electric field of 5 kV/cm. Dielectric properties at a frequency of 1 kHz are examined. Finally, the variation of pyroelectric coefficient with temperature is studied. The grown PMN-PT crystals show typical relaxor dielectric properties.     

Neutron Scattering Facility for the Measurement of Light Quenching Factors of Dark Matter Detectors at Low Temperatures

The light yield of scintillating crystals which is quantified by light Quenching Factors (QFs) strongly depends on the kind of interaction in the crystal. For Dark Matter experiments like CRESST the precise knowledge of QFs is crucial for the discrimination of background events from possible WIMP signals. At the tandem accelerator of the Maier-Leibnitz-Laboratorium (MLL) in Garching a low-temperature scattering facility was set up, which in its current phase aims at the determination of the QFs of O, Ca, and W in CaWO₄ crystals as used in the CRESST experiment. A CRESST detector module consists of a 300?g CaWO₄ target crystal operated as a phonon detector and a separate silicon-on-sapphire (SOS) light detector to detect the corresponding scintillation light simultaneously. In order to disentangle the light yield corresponding to O, Ca and W recoils, monoenergetic neutrons (11 MeV) produced by the accelerator are scattered off an especially developed CRESST-like detector module, which is operated at mK temperatures in a dilution refrigerator. Arrays of liquid-scintillator detectors placed at fixed scattering angles allow one to identify the recoiling nuclei by a neutron time-of-flight measurement. The unique facility is suited for the characterization of different detector materials and will be a powerful tool also for the future multi-material experiment EURECA. We report on the experimental approach, the low-temperature setup and present first results.     

Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition

Ion-induced photon emission (IPE) during bombardment of magnesium aluminate spinel crystals MgO·nAl₂O₃ by 20 keV Ar⁺ ions was studied. The dependence of the yield of particles in specific excited states on the fluence of incident ions in the range of (0.1–1.6)×10¹⁷ ions/cm² was measured. It was shown that yield of magnesium and aluminum atoms and ions in most excited states do not depend (or slightly depend) on the fluence of ion bombardment. An exception was found for yields of Mg⁺ ions in the 4s ²S excited state and Al atoms in the 5p ²P⁰ excited state leading to emission lines at 292.8 and 669.6 nm, respectively. The yield of particles in these excited states drastically decreases at the start of ion bombardment. Analysis of these results and published data on the bombardment-induced surface modification of spinel crystals allows to elucidate the role of crystal structure and chemical bonding in the formation of some excited states. The dependence of excited state yield (except of Mg and Al indicated above) from spinel crystals of different composition MgO·nAl₂O₃ (n=1.0, 1.5, 2.0, and 2.5) does not reflect quantitatively the variation of the calculated bulk concentration of constituent atoms in these targets.     

Thermal expansion of crystals of lithium isotopes

The parameters of interatomic interaction in crystals of isotopes of lithium ⁷Li and ⁶Li are determined for both the minimal and maximal values of Debye temperature. These parameters and the Debye crystal model are used to calculate the temperature dependences of the coefficient of thermal expansion (CTE) and of the lattice parameter from 0 K to the melting temperature of these crystals. It is demonstrated that the temperature dependences of CTE for crystals of ⁷Li and ⁶Li intersect at temperature T _x = 230–300 K so that, under conditions of melting, the CTE for crystal of light isotope is higher than that for crystal of heavy isotope of lithium.     

A comparison of performance between organic scintillation crystals and moderated He-based detectors for fission neutron detection

Direct detection of fast neutrons using organic scintillators is one alternative to moderated thermal neutron detectors deployed to detect fission neutrons—a relevant question in light of dwindling ³He supplies. Recent developments in materials science have demonstrated the capability to grow larger crystals in reasonable times. In light of these developments, this study compares the relative performance of a ³He-based neutron module from a commercially available portal monitor with a theoretical organic scintillator of similar overall size. Stilbene serves as a benchmark with its performance estimated from a combination of energy deposition modeled by radiation transport calculations and an assumption of the lowest neutron energy at which pulse shape discrimination can effectively separate neutron and gamma-ray events. Before intrinsic detection efficiencies on par with moderated detector systems can be achieved, the results point to the need for further advances including significant increases in detector size, especially thickness, and/or lower pulse shape discrimination thresholds.     

Effects of grain size on cyclic plasticity and fatigue crack initiation in nickel

Fatigue experiments were conducted on polycrystalline nickel of two grain sizes, 24 and 290 μm, to evaluate the effects of grain size on cyclic plasticity and fatigue crack initiation. Specimens were cycled at room temperature at plastic strain amplitudes ranging from 2.5×10⁻⁵ to 2.5×10⁻³. Analyses of the cyclic stress–strain response and evolution of hysteresis loop shape indicate that the back stress component of the cyclic stress is significantly affected by grain size and plastic strain amplitude, whereas these parameters have little effect on friction stress. A nonlinear kinematic hardening framework was used to study the evolution of back stress parameters with cumulative plastic strain. These are related to substructural evolution features. In particular, long range back stress components are related to persistent slip bands. The difference in cyclic plasticity behavior between the two grain sizes is related to the effect of grain size on persistent slip band (PSB) morphology, and the effect this has on long range back stress. Fine grain specimens had a much longer fatigue life, especially at low plastic strain amplitude, as a result of the influence of grain size on fatigue crack initiation characteristics. At low plastic strain amplitude (2.5×10⁻⁴), coarse grain specimens initiated cracks where PSBs impinged on grain boundaries. Fine grain specimens formed cracks along PSBs. At high plastic strain amplitude (2.5×10⁻³), both grain sizes initiated cracks at grain boundaries.     

Effects of packaging SrI2(Eu) scintillator crystals

Recent renewed emphasis placed on gamma-ray detectors for national security purposes has motivated researchers to identify and develop new scintillator materials capable of high energy resolution and growable to large sizes. We have discovered that SrI₂(Eu) has many desirable properties for gamma-ray detection and spectroscopy, including high light yield of ∼90,000 photons/MeV and excellent light yield proportionality. We have measured <2.7% FWHM at 662 keV with small detectors (<1 cm³) in direct contact with a photomultiplier tube, and ∼3% resolution at 662 keV is obtained for 1 in.³ crystals. Due to the hygroscopic nature of SrI₂(Eu), similar to NaI(Tl), proper packaging is required for field use. This work describes a systematic study performed to determine the key factors in the packaging process to optimize performance. These factors include proper polishing of the surface, the geometry of the crystal, reflector materials and windows. A technique based on use of a collimated ¹³⁷Cs source was developed to examine light collection uniformity. Employing this technique, we found that when the crystal is packaged properly, the variation in the pulse height at 662 keV from events near the bottom of the crystal compared to those near the top of the crystal could be reduced to <1%. This paper describes the design and engineering of our detector package in order to improve energy resolution of 1 in.³-scale SrI₂(Eu) crystals.     

Scintillating and particle discrimination properties of selected crystals for low-temperature bolometers: from LiF to BGO

Selected crystals have been investigated at 20 mK for their properties of scintillation produced under irradiation. A scintillation signal is found in preliminary studies of YAP:Ce, GSO:Ce, CaF₂:Eu and Al₂O₃:Ti crystals thanks to high sensitivity light detectors. The full mixed “light-and-heat” bolometric technique has been applied to further characterize materials already employed in rare event researches: CaWO₄, BGO, undoped Al₂O₃ (sapphire). All these crystals show powerful discrimination properties for /γ or nuclear recoil/γ detection, with a special mention to undoped Al₂O₃, hitherto untested, which revealed an unexpected strong light emission. A weaker light signal has been found on LiF and TeO₂ bolometers too, sufficient to separate alpha from gamma events in these crystals.     

Characterization of Yb:YAG and Yb:YAP scintillators by means of LAAPD at temperature around 100 K

A new class of scintillators based on charge-transfer luminescence of the Yb³⁺ ion is being investigated for the last few years. The main prospect for these scintillators is in neutrino physics. The crystals manifest maximum light output at temperatures 100 K<T<150 K. In this range Large Area Avalanche Photodiodes (LAAPD) are the best photodetectors. In this work Yb(25%):YAG and Yb(5%):YAP scintillators were characterized by means of a 16 mm diameter API LAAPD at temperatures around 100 K. Light yield and energy resolution were determined. Light yield non-proportionality was detected for all the crystals comparing light output at 661.6 and 59.6 keV peaks.     

Analysis of transient effects of two-level atom in laser light

We are concerned with an analytic separation approach of the optical Bloch equations in the case of the two-level atom interacting with a classical light. The conditions that permit a complete separation of these equations are extracted in a natural manner and the exact solutions are obtained. A theoretical comparison with Torrey method is given. In this context, we find that the transient regime, valid from the instant of the application of the laser on atom, is significant for the evolution of forces and the dynamics before the steady-state regime will be reached. The case of Eu^3?+ ion moving in plane-light wave and standing-wave is treated with some details. The implications of the results for operating atoms and ions using laser light are discussed.     

Observations on dual-ended readout of 100 mm long LYSO crystals

We are investigating using dual-ended readout of axially oriented long thin scintillator crystals in detectors for a compact geometry, small ring diameter animal PET system. The axial position of interaction is determined from the light sharing between two photodetectors at opposite ends of the crystal. We examine the light output, energy resolution and axial spatial resolution of 1.5-5×2×100 mm³ polished LYSO crystals by irradiating with an electronically collimated beam of 511 keV photons oriented perpendicular to the long axis and read out at either end by position sensitive photomultiplier tubes (PSPMTs). Three reflector materials, namely Teflon, 3 M enhanced specular reflector (ESR) and black paint are examined for the 2×2×100 mm³ crystal size. The light output increases and energy resolution improves with the crystal cross-section. Generally, the spatial resolution worsens with increase in crystal cross-section. For the 2×2×100 mm³ crystal size, the mean energy resolutions of the photopeak over the nine irradiation positions were 14.4±0.4%, 16.0±1.2% and 28.3±2.1% with mean spatial resolutions of 7.0±1.0, 9.4±3.3 and 26.0±5.0 mm using ESR, Teflon and black paint, respectively. ESR reflector gave the best light output, energy and axial spatial resolutions. These characterization results of PSPMT-based dual-ended long LYSO crystals will be useful in the design of detector modules for a highly compact geometry preclinical PET system using this detector technology.     

The effect of the skin thickness and spherulite size on the mechanical properties of injection mouldings

In this work are studied the relationships between the microstructure and the mechanical properties of an injection moulded propylene-ethylene copolymer. Distinct microstructures were obtained by processing, through a moulding programme that includes the variation of the injection and the mould temperatures and the injection flow rate. They were characterized by the skin ratio (measured by polarised light microscopy) and the spherulite size (evaluated by small angle light scattering system). Tensile tests were carried out at two different constant loading velocities: 2 mm/min (3.33 × 10^–5 m/s) and 3 m/s, in order to assess the initial modulus, the yield stress, the strain and the energy at break. The results are presented in terms of the relationships between the chosen microstructural parameters and the selected tensile properties. The skin thickness is evidenced as an important microstructural feature. The role of the core spherulite size is secondary or even negligible. The results also show that other microstructural parameters must be considered to establish more general microstructure-properties relationships.     

Defect-and-impurity state of type Ib diamond single crystals of cubic habit

Type Ib diamond single crystals of size to 5–6 mm and to 2.4 carats in weight have been grown at high pressures and temperatures. The defect-and-impurity state and dislocation structures of these crystals have been studied using the IR and optical microscopies as well as the method of the selective etching. To produce type Ib crystals of cubic habit has been made possible by the minimization of the growing temperature. Defect regions in the form of a cone with the basis 0.2–1.8 mm in diameter and 0.5–2.5 mm in height are contained by these crystals. The study of the cone-shaped defect regions using the selective etching showed that at the exposure on the faces the etching pits are of the tetragonal shape and the dislocation density in them exceeds the density of dislocations in crystals that were grown under the usual conditions by 70 to 100 times. The observed defect regions are formed in the course of the diamond crystals growth as the temperature decreases by ~ 30–35°C at the crystallization front because of the increasing heat sink in the direction of a seed crystal.     

Scintillation and optical properties of Pb-doped YCa4O(BO3)3 crystals

This communication reports optical properties and radiation responses of Pb²⁺ 0.5 and 1.0 mol%-doped YCa₄O(BO₃)₃ (YCOB) single crystals grown by the micro-pulling-down (μ-PD) method for neutron scintillator applications. The crystals had no impurity phases according to the results of X-ray powder diffraction. These Pb²⁺-doped crystals demonstrated blue-light luminescence at 330 nm because of Pb²⁺¹S₀-³P_0,1 transition in the photoluminescence spectra. The main emission decay component was determined to be about 250-260 ns under 260 nm excitation wavelength. When irradiated by a ²⁵²Cf source, the relative light yield of 0.5% Pb²⁺-doped crystal was about 300 ph/n that was determined using the light yield of a reference Li-glass scintillator.     

On the growth and properties of single crystal LaB4

A technique has been developed for the growth of single crystals of lanthanum tetraboride LaB₄, from solution in La. Under optimum conditions, crystal size is limited by the size of the crucible. Lattice parameters, thermal expansion coefficient, and microhardness are reported and the growth facets have been identified. An initial check on the electron emitting properties have shown them to be comparable with those of LaB₆.     

Luminescence and scintillation properties of Cs3BiCl6 crystals

The optical and scintillation properties of Cs₃BiCl₆ single crystals were characterized, and absorption properties were investigated for thin films. The thin films showed two absorption bands at ∼220 nm and 330 nm; these bands can be attributed to the ¹A_1g → ¹T_1u and ¹A_1g → ³T_1u transitions of Bi³⁺ ions, respectively. A luminescence band was observed at ∼390 nm in the photoluminescence spectra; this band can be attributed to the ³T_1u → ¹A_1g transition of Bi³⁺ ions. In the X-ray-induced radioluminescence spectrum, in addition to the luminescence band at 390 nm, another band was observed at 600–700 nm; this band was tentatively attributed to the radiative recombination of self-trapped excitons. The decay-time constants of photoluminescence and scintillation decay were of the order of nanoseconds. The scintillation light yield was 800 photons/MeV. The results indicate that Cs₃BiCl₆ has a fast scintillation decay and a relatively poor light yield.     

Acoustic Properties of Solid 4He in the Limit of Zero Impurity

We have studied the elasticity of solid ⁴He in relation with its possible supersolidity. For this we have measured acoustic resonance frequencies in a 1 cm³ cell filled either with polycrystals or with single crystals of ⁴He. We have observed a large stiffening at low temperature as first observed by Day and Beamish in polycrystals. The ³He impurity content has been varied from 300 ppb to 0.4 ppb. When kept in equilibrium with liquid helium, single crystals should be impurity free. In these crystals, a large stiffening is observed, which should not be the result of the pinning of dislocations by impurities.     

Effects of added c-BN seed crystals on the reaction sintering of c-BN accompanied by a conversion from h-BN to c-BN

Reaction sintering behaviour of c-BN which is accompanied by a conversion from h-BN to c-BN was investigated under high pressure (7 GPa) and temperature (1700°C) conditions for 30 to 60min. A high conversion yield of c-BN in the sintered compact was attained by adding fine-grained c-BN seed crystals (particle size 0.5 to 8m) to h-BN powder in the presence of 1 wt% NH₄NO₃ as a catalyst. An induced transformation from h-BN to c-BN occurs over a large surface area of c-BN seed crystals, which results in the formation of direct interparticle bonding between c-BN grains in the sintered compact. A fully dense sintered compact of c-BN (bulk density 98% theoretical) was obtained from the specimen of 70wt% h-BN with 30wt% added c-BN crystals having a particle size of 0.5m. This c-BN compact had an average microhardness of 5100 kg mm^–2 and a specific dielectric constant of 10.0 at a frequency of 1 MHz.