Thermal neutron detection with Ce doped LiCaAlF6 single crystals

Cerium-doped LiCaAlF₆ (Ce:LiCAF) crystals have been studied as scintillators in application to thermal neutron detection. Three crystals: high-doping Ce:LiCAF, low-doping Ce:LiCAF with 50% enrichment of ⁶Li (both 10 mm×10 mm×2 mm, rectangular) and high-doping Ce:LiCAF with 95% enrichment of ⁶Li (Ø50.8 mm×2 mm, discus) coupled to Photonis XP5300B PMT, were tested. The response of these crystals to neutrons emitted from a paraffin moderated ²³⁸PuBe source has been investigated. Thermal neutron peaks have been found at a Gamma Equivalent Energy (GEE) of ∼2.5 MeV for high-doping Ce:LiCAF (50% ⁶Li), ∼2 MeV for low-doping Ce:LiCAF (50% ⁶Li) and ∼1.9 MeV for high-doping Ce:LiCAF (95% ⁶Li). The light output of Ce:LiCAF was also measured (175-250 phe/MeV from sample to sample). Lithium-6 glass GS20 from Saint Gobain was used as a reference scintillator (Ø50 mm×2 mm, circle). Relative neutron efficiency, normalized to that of GS20 lithium glass, as well as gamma-neutron intrinsic efficiency for all tested samples was calculated. Intrinsic efficiency on thermal neutron detection for small Ce:LiCAF samples was estimated at about 32-35% of that of GS20 and for large Ce:LiCAF sample as about 82% of that of GS20.     

Development of an energy-binned photon-counting detector for X-ray and gamma-ray imaging

The aim of this study is to develop an energy-binned photon-counting (EBPC) detector that enables us to provide energy information of x-rays with a reasonable count statistics. We used Al-pixel/CdTe/Pt semiconductor detectors, which had an active area of 8 mm×144 mm and consisted of 18 modules aligned linearly. The size of a CdTe detector module was 8 mm×8 mm and the thickness of the CdTe crystal was 1 mm. Each module consisted of 40×40 pixels and the pixel size was 200 μm×200 μm. We applied the bias voltage of −500 V to the Pt common electrode. The detector counted the number of x-ray photons with four different energy windows, and output four energy-binned images with pixel depths of 12, 12, 11 and 10 bits at a frame rate of 1200 Hz (300 Hz×4 energy bins). The basic performance of the detector was evaluated in several experiments. The results showed that the detector realized the photon counting rate of 0.4×10⁶ counts/sec/pixel (10⁷ counts/sec/mm²), energy resolution 4.4% FWHM at 122 keV. The integral uniformity of the detector was about 1% and the differential uniformity was about 1%. In addition, the image quality was examined with a resolution chart and step-wedge phantoms made of aluminum and polymethyl methacrylate. And we compared the quality of an acquired image with that acquired with an energy integration detector. The results of these experiments showed that the developed detector had desirable intrinsic characteristics for x-ray photon counting imaging.     

Coded source neutron imaging with a MURA mask

In coded source neutron imaging the single aperture commonly used in neutron radiography is replaced with a coded mask. Using a coded source can improve the neutron flux at the sample plane when a very high L/D ratio is needed. The coded source imaging is a possible way to reduce the exposure time to get a neutron image with very high L/D ratio. A 17×17 modified uniformly redundant array coded source was tested in this work. There are 144 holes of 0.8 mm diameter on the coded source. The neutron flux from the coded source is as high as from a single 9.6 mm aperture, while its effective L/D is the same as in the case of a 0.8 mm aperture. The Richardson-Lucy maximum likelihood algorithm was used for image reconstruction. Compared to an in-line phase contrast neutron image taken with a 1 mm aperture, it takes much less time for the coded source to get an image of similar quality.     

Linac based photofission inspection system employing novel detection concepts

Rapiscan Systems is developing a LINAC based cargo inspection system for detection of special nuclear material (SNM) in cargo containers. The system, called Photofission Based Alarm Resolution (PBAR) is being developed under a DHD/DNDO Advanced Technology Demonstration (ATD) program. The PBAR system is based on the Rapiscan Eagle P9000 X-ray system, which is a portal system with a commercial 9 MeV LINAC X-ray source. For the purposes of the DNDO ATD program, a conveyor system was installed in the portal to allow scanning and precise positioning of 20 ft ISO cargo containers.The system uses a two step inspection process. In the first step, the basic scan, the container is quickly and completely inspected using two independent radiography arrays: the conventional primary array with high spatial resolution and a lower resolution spectroscopic array employing the novel Z-Spec method. The primary array uses cadmium tungstate (CdWO₄) detectors with conventional current mode readouts using photodiodes. The Z-Spec array uses small plastic scintillators capable of performing very fast (up to 10⁸ cps) gamma-ray spectroscopy. The two radiography arrays are used to locate high-Z objects in the image such as lead, tungsten, uranium, which could be potential shielding materials as well as SNM itself.In the current system, the Z-Spec works by measuring the energy spectrum of transmitted X-rays. For high-Z materials the higher end of the energy spectrum is more attenuated than for low-Z materials and thus has a lower mean energy and a narrower width than low- and medium-Z materials.The second step in the inspection process is the direct scan or alarm clearing scan. In this step, areas of the container image, which were identified as high Z, are re-inspected. This is done by precisely repositioning the container to the location of the high-Z object and performing a stationary irradiation of the area with X-ray beam. Since there are a large number of photons in the 9 MV Bremsstrahlung spectrum above the photofission “threshold” of about 6 MeV, the X-ray beam induces numerous fissions if nuclear material is present. The PBAR system looks for the two most prolific fission signatures to confirm the presence of special nuclear materials (SNM). These are prompt neutrons and delayed gamma rays. The PBAR system uses arrays of two types of fast and highly efficient gamma ray detectors: plastic and fluorocarbon scintillators. The latter serves as a detector of fission prompt neutrons using the novel threshold activation detector (TAD) concept as well as a very efficient delayed gamma ray detector. The major advantage of TAD for detecting the prompt neutrons is its insensitivity to the intense source related backgrounds.The current status of the system and experimental results will be shown and discussed.     

Pyroelectric properties of arrayed BaTiO3 system thick film for uncooled IR detector

The Sr and Ca added to BaTiO₃ in order to shift transition temperature near room temperature. The donor (Yb₂O₃) and acceptor (MnCO₃) impurities were added to the (Ba,Sr,Ca)TiO₃ powder for the improvement of structural and electrical properties. The (Ba,Sr,Ca)TiO₃ powder was made by sol-gel method and the thick films were fabricated by screen-printing. We fabricated array type thick films. The 1 mm × 3 mm array thick films were arranged 2 × 8. Relative dielectric constant and dielectric loss of Yb₂O₃ 0.1 mol% doped (Ba,Sr,Ca)TiO₃ array thick film were 1068 and 2.8%, respectively at Curie temperature, 44 °C. Pyroelectric coefficient and F.M.D* showed 21.7 × 10⁻⁹ C/cm² K and 3.2 × 10⁻⁹ C cm/J, respectively.     

Mechanical behavior of laser micro-machined bulk 6H–SiC diaphragms

Mechanical behavior of laser micro-machined monolithic hexagonal silicon carbide (6H–SiC) diaphragms was investigated to determine the effects of laser processing. Square diaphragms with a nominal size of 1.5 mm × 1.5 mm were fabricated from bulk 6H–SiC wafers using a Q-switched Nd:YAG laser operating at a wavelength of 1064 nm, an average power of 0.35W, a pulse repetition rate of 3 kHz, and a pulse width of 100 ns. These parameters were chosen, based on previous experiments, to minimize surface roughness. Analysis of laser-machined diaphragms revealed that the average thickness of a diaphragm was 151 μm which is composed of two layers. One is a soft, black layer with a thickness of about 83 μm consisting of silicon, oxygen, and carbon. The other layer was a hard, virgin SiC layer with a thickness of 68 μm. The diaphragms were subjected to micro-hardness indentation tests to obtain load versus deflection curves. The data was validated using Timoshenko’s analytical model for maximum deflection of a thin plate under concentrated loading with hinged and clamped boundary conditions. Experimental measurements of the deflection were found to be slightly higher than those predicted by the analytical model. The variations in the thickness of the diaphragms, homogeneity of the elastic properties of the laser micro-machined SiC, and possibly inappropriate boundary conditions during testing of the diaphragms chiefly account for the deviations between the experimental results and the analytical model.     

Optical properties and gamma-ray response of Czochralski grown Pr:Lu3Al5O12 scintillating garnet crystals with different Pr content

Pr:LuAG single crystalline scintillators with different Pr³⁺ concentration, 0.1, 0.18, and 0.22 mol% were grown by the Czochralski method. The crystals were cut to dimensions of 2.2 × 2.2 × 15 mm³ and polished, simulating sensors for Positron Emission Tomography (PET). Their absorption coefficients were examined, and the absorption strength was found to be proportional to the Pr concentration. The α-ray induced emission spectra of the samples demonstrated two emission lines peaking at 310 and 370 nm. The emission intensities in the radio luminescence spectra were also proportional to the Pr content. The absolute light yields and intrinsic energy resolution under γ-ray irradiation were evaluated at +20, 0, and −20 °C using avalanche photodiode as a photodetector. Pr 0.22% doped crystal had strongest light output of 16 400 ph/MeV, and its intrinsic energy resolution was around few % at several hundred keV. When coupled with PMT, the decay time was around 25 ns, and it was almost independent on concentration.     

Small pixel CZT detector for hard X-ray spectroscopy

A new small pixel cadmium zinc telluride (CZT) detector has been developed for hard X-ray spectroscopy. The X-ray performance of four detectors is presented and the detectors are analysed in terms of the energy resolution of each pixel. The detectors were made from CZT crystals grown by the travelling heater method (THM) bonded to a 20×20 application specific integrated circuit (ASIC) and data acquisition (DAQ) system. The detectors had an array of 20×20 pixels on a 250 μm pitch, with each pixel gold-stud bonded to an energy resolving circuit in the ASIC. The DAQ system digitised the ASIC output with 14 bit resolution, performing offset corrections and data storage to disc in real time at up to 40,000 frames per second. The detector geometry and ASIC design was optimised for X-ray spectroscopy up to 150 keV and made use of the small pixel effect to preferentially measure the electron signal. A ²⁴¹Am source was used to measure the spectroscopic performance and uniformity of the detectors. The average energy resolution (FWHM at 59.54 keV) of each pixel ranged from 1.09±0.46 to 1.50±0.57 keV across the four detectors. The detectors showed good spectral performance and uniform response over almost all pixels in the 20×20 array. A large area 80×80 pixel detector will be built that will utilise the scalable design of the ASIC and the large areas of monolithic spectroscopic grade THM grown CZT that are now available. The large area detector will have the same performance as that demonstrated here.     

Crashworthiness characteristics investigation of silk/epoxy composite square tubes

This research concentrates on the evaluation of crashworthiness characteristics of natural silk/epoxy composite square tubes energy-absorbers. Composite laminate specimens were subjected to static axial compression load and experimental evaluation of the energy absorption capability of silk/epoxy composite. Specimens were in the form of square cross-sections with the dimension of 80 mm × 80 mm and a radius curvature of 5 mm. The variables in the experiment were the length of the tubes built 50 mm, 80 mm and 120 mm. Meanwhile, the thickness of the walls, consisting of laminates of silk/epoxy of 12, 24 and 30 plies, correspond to equivalent wall thickness of 1.7 mm, 3.4 mm and 4.2 mm, respectively. The parameters measured were the total absorbed energy (E_total), and the crash force efficiency (CFE). E_total is the measure of the amount of energy that the structure can withstand without failure and thus is a measure of its strength, while CFE gives a quantitative indication of the mode of failure of the composites. The mode of failure was observed using photography.     

Performance test of Si PIN photodiode line scanner for thermal neutron detection

Thermal neutron imaging using Si PIN photodiode line scanner and Eu-doped LiCaAlF₆ crystal scintillator has been developed. The pixel dimensions of photodiode are 1.18 mm (width)×3.8 mm (length) with 0.4 mm gap and the module has 192 channels in linear array. The emission peaks of Eu-doped LiCaAlF₆ after thermal neutron excitation are placed at 370 and 590 nm, and the corresponding photon sensitivities of photodiode are 0.04 and 0.34 A/W, respectively. Polished scintillator blocks with a size of 1.18 mm (width)×3.8 mm (length)×5.0 mm (thickness) were wrapped by several layers of Teflon tapes as a reflector and optically coupled to the photodiodes by silicone grease. JRR-3 MUSASI beam line emitting 13.5 meV thermal neutrons with the flux of 8×10⁵ n/cm² s was used for the imaging test. As a subject for imaging, a Cd plate was moved at the speed of 50 mm/s perpendicular to the thermal neutron beam. Analog integration time was set to be 416.6 μs, then signals were converted by a delta-sigma A/D converter. After the image processing, we successfully obtained moving Cd plate image under thermal neutron irradiation using PIN photodiode line scanner coupled with Eu-doped LiCaAlF₆ scintillator.     

Organic nonlinear optical crystal 4-hydroxy-N-methyl 4-stilbazolium besylate synthesis and characterization

Novel organic nonlinear optical material 4-hydroxy-N-methyl 4-stilbazolium besylate has been synthesized and 7 mm × 5 mm × 3 mm size crystals were grown by slow evaporation technique. Single crystal X-ray diffraction analysis reveals that the crystal lattice is monoclinic. From powder X-ray diffraction analysis the diffraction planes have been indexed. Fourier transform infrared spectrum for our sample confirms the presence of functional groups in the grown crystal. Second harmonic efficiency was determined using Kurtz powder method in comparison with urea to confirm the nonlinearity of the material. The results are discussed in detail.     

Performance measurements from LYSO scintillators coupled to a CMOS position sensitive SSPM detector

We have designed a 5×5 mm² position sensitive solid-state photomultiplier (PS-SSPM) using a complementary metal-oxide-semiconductor (CMOS) process that provides imaging capability on the micro-pixel level. The PS-SSPM has 11,664 micro-pixels total, with each having an active area and micro-pixel pitch of 30×30 μm² and 44.3 μm, respectively. The PS-SSPM was then examined for its performance characteristics such as its energy and spatial resolution, and LYSO scintillator array imaging capabilities. When coupled to 5×5×3 mm³ LYSO, the energy resolution at 511 keV (²²Na) was measured as a function of bias, and corrected for the PS-SSPM non-linear output. The resolution is 14% (FWHM) at 511 keV with 30 V bias. The LYSO coincidence timing resolution was 9.4 ns (FWHM) at 511 keV. Spatial resolution studies were conducted using a focused (∼30 μm beam spot diameter) pulsed 635 nm diode laser. Scintillator array imaging studies were conducted at 511 keV using a 6×6 LYSO array, having 500 μm pixels (530 μm pitch) and 5 mm tall.     

Study of optimum growth condition of Zinc Cadmium Thiocyanate (ZCTC) single crystals in silica gel

Some bimetallic thiocyanate complexes exhibit excellent nonlinear optical properties. Single crystals of one such material Zinc Cadmium Thiocyanate ZnCd(SCN)₄ abbreviated as ZCTC have been grown in Silica gel using gel technique by the process of diffusion. Colorless transparent crystals of size 12 mm × 2 mm × 1.3 mm have been obtained. Structural analysis has been carried out in powder form and for single crystal using X-ray diffractometer. Single crystal X-ray diffraction analysis reveals that the crystal lattice of ZCTC is tetragonal. From powder X-ray diffraction analysis the diffraction planes have been identified and unit cell parameters are determined.     

Deposition of CdS thin films onto Si(111) substrate by PLD with femtosecond pulse

The effect of laser fluence (laser incident energy in the range of 0.5-1.5 mJ/pulse with the same laser spot size of 0.5 mm × 0.7 mm) on the structural quality and optical properties synthesized by femtosecond pulsed-laser deposition has been studied. The structural quality and optical properties of the deposited CdS thin films were investigated by X-ray diffraction, atomic force microscopy and photoluminescence measurement. The studies revealed an improvement in the structural quality and optical properties of the CdS thin films with increasing the laser fluence in some range. However, too high laser fluence could lead to the structural quality and optical properties of the CdS thin films to degrade. We defined the optimum laser incident energy was around 1.2 mJ/pulse. And the kinetic energy of the plasma produced by femtosecond laser strongly affects the structure and properties of the deposited CdS thin films.     

An investigation into the effects of graphite particles on the damping behavior of ZA-27 alloy composite material

The objective of present work is to investigate the effect of macroscopic graphite particles on damping behavior of ZA-27 alloy composites. Compo casting technique was used to prepare the composites, in which the graphite particles were used to reinforce ZA-27 alloy, in varying fractions in order to arrive at optimum graphite reinforcement for bearing applications and establish a correlation of experimental readings. The experimental method used was the cantilever technique with Dynamic Mechanical Analyzer to evaluate damping properties. Standard strips of size 60 mm × 10 mm × 1 mm thick were tested by cantilever method of vibration tests at various temperatures from ambient to 300 °C. It was observed that the damping capacity of the material increased with increasing temperature and fractions of graphite particles.     

Growth, spectroscopic properties and laser performance of Nd-doped potassium ytterbium tungstate laser crystal

The 5 at.% Nd³⁺-doped potassium ytterbium tungstate (Nd³⁺:KYb(WO₄)₂, hereafter Nd:KYbW) laser crystal with the dimension up to 28 mm × 15 mm × 12 mm was grown by the top seeded solution growth (TSSG) method. The infrared spectrum of crystal sample was measured, and the vibrational peaks were assigned. According to the absorption and emission spectra of crystal sample, the absorption and emission cross-sections are 16.03 × 10⁻²⁰ cm² at 808 nm and 10.72 × 10⁻²⁰ cm² at 1067 nm, respectively. The fluorescence life of ⁴F_3/2 energy level is 196.33 μs, and the fluorescence branching ratio for the ⁴F_3/2-⁴I_11/2 transition at 1067 nm is 55.74%. The energy transfer between Nd³⁺ and Yb³⁺ ions was observed from the fluorescence spectra pumped by 808 and 980 nm LD sources and the Stark levels of Yb³⁺ in Nd:KYbW crystal were determined. Highly efficient laser output up to 305 mW of Nd:KYbW crystal at 1067 nm has been achieved under pumping by a CW 808 nm laser diode at room temperature. The optical-optical conversion efficiency is 33.9% and the slope efficiency is 46.8%.     

Growth, thermal and mechanical properties of new nonlinear optical barium bis-paranitrophenolate paranitrophenol tetrahydrate single crystal

Barium bis paranitrophenolate paranitrophenol tetrahydrate, a new semiorganic nonlinear optical single crystal has been grown by slow evaporation solution growth technique at room temperature of 30 °C. Crystal of dimensions of 29 mm × 11 mm × 5 mm was obtained in a period of 30 days. X-ray diffraction analysis reveal the newness of the crystal structure belonging to the orthorhombic class with lattice parameters a = 19.899(5) Å, b = 28.019(8) Å, c = 10.745(4) Å and α = β = γ = 90°. The grown crystal is examined for its nonlinear optical nature with Kurtz powder technique after being sieved for particle sizes between 5 and 100 μm and analyzed for its thermal and mechanical properties. The effective nonlinear optical coefficient being 16 times greater than that of KDP crystal, good thermal stability up to 120 °C with the Meyer's constant n < 2 helps fashion the crystal towards device geometry.     

Large-area SnO2: F thin films by offline APCVD

In this paper, we reported the successful preparation of fluorine-doped tin oxide (FTO) thin films on large-area glass substrates (1245 mm × 635 mm × 3 mm) by self-designed offline atmospheric pressure chemical vapor deposition (APCVD) process. The FTO thin films were achieved through a combinatorial chemistry approach using tin tetrachloride, water and oxygen as precursors and Freon (F-152, C2H4F2) as dopant. The deposited films were characterized for crystallinity, morphology (roughness) and sheet resistance to aid optimization of materials suitable for solar cells. We got the FTO thin films with sheet resistance 8-11 Ω/□ and direct transmittance more than 83%. X-ray diffraction (XRD) characterization suggested that the as-prepared FTO films were composed of multicrystal, with the average crystal size 200-300 nm and good crystallinity. Further more, the field emission scanning electron microscope (FESEM) images showed that the films were produced with good surface morphology (haze). Selected samples were used for manufacturing tandem amorphous silicon (a-Si:H) thin film solar cells and modules by plasma enhanced chemical vapor deposition (PECVD). Compared with commercially available FTO thin films coated by online chemical vapor deposition, our FTO coatings show excellent performance resulting in a high quantum efficiency yield for a-Si:H solar cells and ideal open voltage and short circuit current for a-Si:H solar modules.     

Superconducting magnet for K-500 cyclotron at VECC, Kolkata

K-500 superconducting cyclotron is in the advanced stage of commissioning at VECC, Kolkata. Superconducting magnet is one of the major and critical component of the cyclotron. It has been successfully fabricated, installed, cooled down to 4.2 K by interfacing with LHe plant and energized to its rated current on 30th April, 2005 producing magnetic field of 4.8 T at median plane of cyclotron. The superconducting magnet (stored energy of 22MJ) consists of two coils (α and β), which were wound on a sophisticated coil winding machine set-up at VECC. The superconducting cable used for winding the coils is multi filamentary composite superconducting wire (1.29 mm diameter) having 500 filaments of 40 μm diameter Nb-Ti in copper matrix which is embedded in OFHC grade copper channel (2.794 mm × 4.978 mm) for cryogenic stability. The basic structure of coil consists of layer type helical winding on a SS bobbin of 1475 mm ID × 1930 mm OD × 1170 mm height. The bobbin was afterwards closed by SS sheet to form the LHe chamber. The total weight of the coil with bobbin was about 6 tonne and the total length of the superconducting cable wound was about 35 km. Winding was done at very high tension (2000 PSI) and close tolerance to restrict the movement of conductor and coil during energization. After coil winding, all four coils (two each on upper and lower half of median plane of cyclotron) were banded by aluminium strip (2.7 mm × 5 mm) at higher tension (20,000 PSI) to give more compressive force after cool down to 4.2 K for restricting the movement of coil while energizing and thereby eliminating the chances of quench during ramping of current.After completion of coil winding by October, 2003, cryostat assembly was taken up in house. The assembly of cryostat (13 tonne) with support links (9 Nos.) refrigeration port, instrumentation port, helium vapour cooled current loads, etc. was completed by June, 2004. Meanwhile assembly of magnet frame was taken up and the cryostat was positioned in the magnet frame with proper alignment by August, 2004. After installation of cryostat on magnet, the cryostat was connected to the helium refrigerator/liquefier, having refrigeration capacity of 200 W and 100 l/h in liquefier mode with LN2 pre-cooling. The cryogenic delivery system supplying the liquid helium and liquid nitrogen to the superconducting magnet was successfully commissioned in November, 2004. The cool down of the cryostat to 10 K took around 8 days following which the LHe was filled in the cryostat (300 l) on 15th January, 2005. Subsequently the superconducting coils (α and β) were energized by two DC current regulated power supplies (20 V, 1000 A, 10 ppm stability) with slow and fast dump resistors connected externally across the superconducting coils for protection of coils at the time of power failure and quench.The paper describes the intricacies involved in coil winding, winding set-up, assembly of cryostat, cooling down the superconducting coils, filling by LHe and energization to rated current. The paper also highlights the operating experience of superconducting magnet and related test results.