Enhanced variant designs and characteristics of the microstructured solid-state neutron detector

Silicon diodes with large aspect ratio perforated microstructures backfilled with ⁶LiF show a dramatic increase in neutron detection efficiency beyond that of conventional thin-film coated planar devices. Described in this work are advancements in the technology with increased microstructure depths and detector stacking methods to increase thermal neutron detection efficiency. The highest efficiency devices thus far have delivered over 37% intrinsic thermal neutron detection efficiency by device-coupling stacking methods. The detectors operate as conformally diffused pn junction diodes with 1 cm² square-area. Two individual devices were mounted back-to-back with counting electronics coupling the detectors together into a single dual-detector device. The solid-state silicon device operated at 3 V and utilized simple signal amplification and counting electronic components. The intrinsic detection efficiency for normal-incident 0.0253 eV neutrons was found by calibrating against a ³He proportional counter and a ⁶LiF thin-film planar semiconductor device. This work is a part of on-going research to develop solid-state semiconductor neutron detectors with high detection efficiencies and uniform angular responses.     

Microchannel plate special nuclear materials sensor

Nova Scientific Inc., is developing for the Domestic Nuclear Detection Office (DNDO SBIR #HSHQDC-08-C-00190), a solid-state, high-efficiency neutron detection alternative to ³He gas tubes, using neutron-sensitive microchannel plates (MCPs) containing ¹⁰B and/or Gd. This work directly supports DNDO development of technologies designed to detect and interdict nuclear weapons or illicit nuclear materials. Neutron-sensitized MCPs have been shown theoretically and more recently experimentally, to be capable of thermal neutron detection efficiencies equivalent to ³He gas tubes. Although typical solid-state neutron detectors typically have an intrinsic gamma sensitivity orders of magnitude higher than that of ³He gas detectors, we dramatically reduce gamma sensitivity by combining a novel electronic coincidence rejection scheme, employing a separate but enveloping gamma scintillator. This has already resulted in a measured gamma rejection ratio equal to a small ³He tube, without in principle sacrificing neutron detection efficiency. Ongoing improvements to the MCP performance as well as the coincidence counting geometry will be described. Repeated testing and validation with a ²⁵²Cf source has been underway throughout the Phase II SBIR program, with ongoing comparisons to a small commercial ³He gas tube. Finally, further component improvements and efforts toward integration maturity are underway, with the goal of establishing functional prototypes for SNM field testing.     

Boron-coated straws as a replacement for He-based neutron detectors

US and international government efforts to equip major seaports with large area neutron detectors, aimed to intercept the smuggling of nuclear materials, have precipitated a critical shortage of ³He gas. It is estimated that the annual demand of ³He for US security applications alone is more than the worldwide supply. This is strongly limiting the prospects of neutron science, safeguards, and other applications that rely heavily on ³He-based detectors. Clearly, alternate neutron detection technologies that can support large sensitive areas, and have low gamma sensitivity and low cost must be developed.We propose a low-cost technology based on long copper tubes (straws), coated on the inside with a thin layer of ¹⁰B-enriched boron carbide (¹⁰B₄C). In addition to the high abundance of boron on Earth and low cost of ¹⁰B enrichment, the boron-coated straw (BCS) detector offers distinct advantages over conventional ³He-based detectors, and alternate technologies such as ¹⁰BF₃ tubes and ¹⁰B-coated rigid tubes. These include better distribution inside moderator assemblies, many-times faster electronic signals, no pressurization, improved gamma-ray rejection, no toxic or flammable gases, and ease of serviceability.We present the performance of BCS detectors dispersed in a solid plastic moderator to address the need for portal monitoring. The design adopts the outer dimensions of currently deployed ³He-based monitors, but takes advantage of the small BCS diameter to achieve a more uniform distribution of neutron converter throughout the moderating material. We show that approximately 63 BCS detectors, each 205 cm long, distributed inside the moderator, can match or exceed the detection efficiency of typical monitors fitted with a 5 cm diameter ³He tube, 187 cm long, pressurized to 3 atm.     

Room temperature deposited vanadium oxide thin films for uncooled infrared detectors

We demonstrate the room temperature deposition of vanadium oxide thin films by pulsed laser deposition (PLD) technique for application as the thermal sensing layer in uncooled infrared (IR) detectors. The films exhibit temperature coefficient of resistance (TCR) of 2.8%/K implies promising application in uncooled IR detectors. A 2-D array of 10-element test microbolometer is fabricated without thermal isolation structure. The IR response of the microbolometer is measured in the spectral range 8-13 μm. The detectivity and the responsivity are determined as ∼6×10⁵ cm Hz^1/2/W and 36 V/W, respectively, at 10 Hz of the chopper frequency with 50 μA bias current for a thermal conductance G∼10-3 W/K between the thermal sensing layer and the substrate. By extrapolating with the data of a typical thermally isolated microbolometer (G∼10⁻⁷ W/K), the projected responsivity is found to be around 10⁴ V/W, which well compares with the reported values.     

Sub-Kelvin cooler configuration study for the Background Limited Infrared Submillimeter Spectrometer BLISS on SPICA

The Background Limited Infrared Submillimeter Spectrometer (BLISS) is an instrument proposed for the Japanese space borne telescope mission SPICA. The BLISS concept is a suite of grating spectrometers which combine to cover the 40-400 μm range at resolving power R∼700 with detector sensitivity approaching the natural photon background limits. To achieve the high sensitivity, the BLISS detectors require cooling to 50 mK, well below the 1.7 K cold stage provided on the SPICA spacecraft. We present a thermal architecture for BLISS that includes a thermal intercept stage actively cooled to a temperature in between the 1.7 K cold tip and the detector stage at 50 mK. This architecture requires, essentially, two coolers; one to cool the intercept stage from 1.7 K and one to cool the detectors from the intercept stage temperature to 50 mK. We compared several configurations of flight-heritage coolers to cool the intercept and detector stages. Of the various configurations studied, a continuous adiabatic demagnetization refrigerator (ADR) for each stage has the highest maturity, lowest heat dump at 1.7 K and total mass comparable to other approaches. Other options, such as a Herschel ³He sorption cooler-ADR hybrid and the recently demonstrated closed cycle version of the dilution cooler on Planck are also feasible for BLISS on SPICA.     

Detection of SNM by delayed gamma rays from induced fission

The Pulsed Neutron Interrogation Test Assembly (PUNITA) is an experimental device for research in NDA methods and field applicable instrumentation for nuclear safeguards and security applications. PUNITA incorporates a standard 14-MeV (D-T) pulsed neutron generator inside a large graphite mantle. The generator target is surrounded by a thick tungsten filter with the purpose to increase the neutron output and to tailor the neutron energy spectrum. In this configuration a sample may be exposed to a relatively high average thermal neutron flux of about (2.2±0.1)×10³ s⁻¹ cm⁻² at only 10% of the maximum target neutron emission. The sample cavity is large enough to allow variation of the experimental setup including the fissile sample, neutron and gamma detectors, and shielding materials.The response from SNM samples of different fissile material content was investigated with various field-applicable scintillation gamma detectors such as the 3×2 in. LaBr₃ detector. Shielding in the form of tungsten and cadmium was applied to the detector to improve the signal to background ratio. Gamma and neutron shields surrounding the samples were also tested for the purpose of simulating clandestine conduct. The energy spectra of delayed gamma rays were recorded in the range 100 keV-9 MeV. In addition time spectra of delayed gamma rays in the range 3.3-8 MeV were recorded in the time period of 10 ms-120 s after the 14-MeV neutron burst. The goal of the experiment was to optimize the sample/detector configuration including the energy range and time period for SNM detection. The results show, for example, that a 170 g sample of depleted uranium can be detected with the given setup in less than 3 min of investigation. Samples of higher enrichment or higher mass are detected in much shorter time.     

Crystal growth and thermal neutron scintillation response of Ce doped KLiYF5

K⁶Li(Y_1−xCe_x)F₅ (x = 0.003, 0.02) single crystals were grown from the melt using the precise atmosphere control type Micro-Pulling-Down (μ-PD) method to examine their potential as a new thermal neutron scintillators. The grown crystals were single-phase materials as confirmed by XRD. The crystals demonstrated 40-60% transmittance above 320 nm and Ce³⁺ 5d-4f luminescence observed around 340 nm when exited by α-ray. The radio luminescence measurements under thermal neutron excitation (²⁵²Cf) demonstrated the light yield of 890 (Ph/neutron) and the decay time excited by α-ray exhibited 20 and 259 ns.     

Hygroscopicity and bulk thermal expansion in Y2W3O12

Negative thermal expansion material, Y₂W₃O₁₂ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with a = 10.098(1) Å, b = 13.315(3) Å, c = 9.691(4) Å. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3-6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of >100 μm, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained (2-5 μm) ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100-300 K and an average linear thermal expansion co-efficient of −9.65 × 10⁻⁶/K is observed. The effect of water on the thermal expansion behavior of this system is discussed.     

PKUNIFTY: A neutron imaging facility based on an RFQ accelerator

A neutron imaging facility, PKUNIFTY, based on a radio frequency quadrupole (RFQ) accelerator-driven compact neutron source, presently under construction at the Peking University, is described. It consists of a deuteron linear accelerator, a neutron target-moderator-reflector assembly, and a thermal neutron imaging system. Neutrons are generated via the deuteron-beryllium reaction with an expected fast-neutron yield of 3×10¹² n/s. The thermal neutron flux on the imaging plane is 5×10⁵ n/cm²/s at a nominal L/D ratio of 50, and the L/D ratio can be selectable over a range of 25-200. The corresponding n/γ ratio is close to or higher than 1×10¹⁰ n/cm²/Sv. The field of view is 20 cm×20 cm at 2 m downstream of the collimator entrance aperture where the thermal neutron flux uniformity is better than 7%. The effective Cd ratio can be tuned by using a light chopper and neutron time-of-flight technique without excessive sacrifice of the thermal neutron flux.     

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.     

Neutron diffraction measurements at the INES diffractometer using a neutron radiative capture based counting technique

The global shortage of ³He gas is an issue to be addressed in neutron detection. In the context of the research and development activity related to the replacement of ³He for neutron counting systems, neutron diffraction measurements performed on the INES beam line at the ISIS pulsed spallation neutron source are presented. For these measurements two different neutron counting devices have been used: a 20 bar pressure squashed ³He tube and a Yttrium-Aluminum-Perovskite scintillation detector. The scintillation detector was coupled to a cadmium sheet that registers the prompt radiative capture gamma rays generated by the (n,γ) nuclear reactions occurring in cadmium. The assessment of the scintillator based counting system was done by performing a Rietveld refinement analysis on the diffraction pattern from an ancient Japanese blade and comparing the results with those obtained by a ³He tube placed at the same angular position. The results obtained demonstrate the considerable potential of the proposed counting approach based on the radiative capture gamma rays at spallation neutron sources.     

Beryllium neutron activation detector for pulsed DD fusion sources

A compact fast neutron detector based on beryllium activation has been developed to perform accurate neutron fluence measurements on pulsed DD fusion sources. It is especially well suited to moderate repetition-rate (<0.2 Hz) devices, such as the plasma focus or Z-pinch. The detector comprises a beryllium metal sheet sandwiched between two large-area xenon-filled proportional counters. A methodology for calculating the absolute response function of the detector using a “first principles” approach is described. This calibration methodology is based on the ⁹Be(n,α)⁶He cross-section, energy calibration of the proportional counters, and numerical simulations of neutron interactions and beta-particle paths using MCNP5. The response function R(E_n) is determined over the neutron energy range 2-4 MeV. The count rate capability of the detector has been studied and the corrections required for high neutron fluence measurements are discussed. For pulsed DD neutron fluencies >3×10⁴ cm⁻², the statistical uncertainty in the fluence measurement is better than 1%. A small plasma focus device has been employed as a pulsed neutron source to test two of these new detectors, and their responses are found to be practically identical. Also the level of interfering activation is found to be sufficiently low as to be negligible.     

Fabrication and characterization of large size LiF/CaF2:Eu eutectic composites with the ordered lamellar structure

As alternative candidates for the ³He neutron detectors, ⁶LiF/CaF₂:Eu eutectic composites were fabricated and their scintillation properties were evaluated. Large size LiF/CaF₂:Eu eutectic composites of 58 mm diameter and 50 mm thickness were produced by Bridgman method. The composites had a finely ordered lamellar structure along the solidification direction. The lamellar structure was controlled by the direction and the rate of solidification, and it was optimized to improve the scintillation properties. Better results were achieved when thinner lamellar layers were aligned along the scintillation light path.     

Neutron detection gamma ray sensitivity criteria

The shortage of ³He has triggered the search for effective alternative neutron detection technologies for national security and safeguards applications. Any new detection technology must satisfy two basic criteria: (1) it must meet a neutron detection efficiency requirement, and (2) it must be insensitive to gamma-ray interference at a prescribed level, while still meeting the neutron detection requirement. It is the purpose of this paper to define measureable gamma ray sensitivity criteria for neutron detectors. Quantitative requirements are specified for: intrinsic gamma ray detection efficiency and gamma ray absolute rejection. The gamma absolute rejection ratio for neutrons (GARRn) is defined, and it is proposed that the requirement for neutron detection be 0.9<GARRn<1.1 at a 10 mR/h exposure rate. An example of the results from a ³He based neutron detector is provided showing that this technology can meet the stated requirements. Results from tests of some alternative technologies are also reported.     

A CdTe detector with a Gd converter for thermal neutron detection

A CdTe detector with a Gd converter has been developed and investigated as a neutron detector for neutron imaging. The fabricated Gd/CdTe detector with the 25 μm thick Gd was designed on the basis of simulation results of thermal neutron detection efficiency and spatial resolution. The energy resolution of the Gd/CdTe detector is less than 4 keV, which is enough to discriminate neutron capture gamma rays from background gamma emission. The Gd/CdTe detector shows the detection of neutron capture gamma ray emission in the ¹⁵⁵Gd(n, γ)¹⁵⁶Gd, ¹⁵⁷Gd(n, γ)¹⁵⁸Gd and ¹¹³Cd(n, γ)¹¹⁴Cd reactions and characteristic X-ray emissions due to conversion-electrons generated inside the Gd film. The observed efficient thermal neutron detection with the Gd/CdTe detector shows its promise in neutron radiography application.     

He and BF3 neutron detector pressure effect and model comparison

Radiation detection systems for homeland security applications must possess the capability of detecting both gamma rays and neutrons. The radiation portal monitor systems that are currently deployed use a plastic scintillator for detecting gamma rays and ³He gas-filled proportional counters for detecting neutrons. Proportional counters filled with ³He are the preferred neutron detectors for use in radiation portal monitor systems because ³He has a large neutron cross-section, is relatively insensitive to gamma-rays, is neither toxic nor corrosive, can withstand extreme environments, and can be operated at a lower voltage than some of the alternative proportional counters. The amount of ³He required for homeland security and science applications has depleted the world supply and there is no longer enough available to fill the demand. Thus, alternative neutron detectors are being explored.Two possible temporary solutions that could be utilized while a more permanent solution is being identified are reducing the ³He pressure in the proportional counters and using boron trifluoride gas-filled proportional counters. Reducing the amount of ³He required in each of the proportional counters would decrease the rate at which ³He is being used; not enough to solve the shortage, but perhaps enough to increase the amount of time available to find a working replacement. Boron trifluoride is not appropriate for all situations as these detectors are less sensitive than ³He, boron trifluoride gas is corrosive, and a much higher voltage is required than what is used with ³He detectors. Measurements of the neutron detection efficiency of ³He and boron trifluoride as a function of tube pressure were made. The experimental results were also used to validate models of the radiation portal monitor systems.     

Evaluation of a LiI(Eu) neutron detector with coincident double photodiode readout

Previous work showed that enriched ⁶Li halide scintillation crystal is a good candidate for portable neutron-sensitive detectors. Photodiode readout is a good alternative to PMT in compact devices. These detectors are often required to work in presence of a strong gamma background. Therefore, great discrimination against gamma rays is crucial. Because of the high Q-value of the ⁶Li(n,α)³H reaction, the light yield of a neutron capture signal corresponds to 3-4 MeV gamma equivalent in spite of the quenching effect of heavily charged particles. As a result, energy discrimination is quite effective against gamma signals generated in thin crystals. However, direct gamma interactions inside the photodiode can create pulses whose amplitude is large enough to interfere with thermal neutron peak. This study shows an innovative design based on coincident readout to solve this problem. In this design, two photodiodes are attached on both sides of the LiI crystal. The output signal is only accepted when both photodiodes give out coincident output. The method is proved to effectively suppress background in the neutron window in a 420 mR/h ¹³⁷Cs field down to the level of natural background.     

Effect of carrier gas on anode performance of Si thick-film electrodes prepared by gas-deposition method

Silicon thick-film electrodes for Li-ion battery anodes were prepared by a gas-deposition method using carrier gases of Ar and He. We investigated the effect of the carrier gases on film morphology and anode performance. The root mean square roughness of the films formed by He gas was twice larger than that formed by Ar gas. The Si electrode obtained by He gas exhibited a larger film thickness up to 4 μm, a higher initial capacity of 2280 mA h g^− 1, and larger capacities until the 40th cycle compared with other electrodes. The better performance is probably attributed to a formation of many interspaces within the thick film. It is suggested that these interspaces enhanced the contact area between active material and electrolyte, which enabled an efficient alloying/dealloying reaction of Li-Si.     

High-resolution neutron microtomography with noiseless neutron counting detector

The improved collimation and intensity of thermal and cold neutron beamlines combined with recent advances in neutron imaging devices enable high-resolution neutron radiography and microtomography, which can provide information on the internal structure of objects not achievable with conventional X-ray imaging techniques. Neutron detection efficiency, spatial and temporal resolution (important for the studies of dynamic processes) and low background count rate are among the crucial parameters defining the quality of radiographic images and tomographic reconstructions. The unique capabilities of neutron counting detectors with neutron-sensitive microchannel plates (MCPs) and with Timepix CMOS readouts providing high neutron detection efficiency (∼70% for cold neutrons), spatial resolutions ranging from 15 to 55 μm and a temporal resolution of ∼1 μs—combined with the virtual absence of readout noise—make these devices very attractive for high-resolution microtomography. In this paper we demonstrate the capabilities of an MCP-Timepix detection system applied to microtomographic imaging, performed at the ICON cold neutron facility of the Paul Scherrer Institute. The high resolution and the absence of readout noise enable accurate reconstruction of texture in a relatively opaque wood sample, differentiation of internal tissues of a fly and imaging of individual ∼400 μm grains in an organic powder encapsulated in a ∼700 μm thick metal casing.     

Neutronic design and simulated performance of Peking University Neutron Imaging Facility (PKUNIFTY)

The Peking University Neutron Imaging Facility (PKUNIFTY) is a Radio Frequency Quadruple (RFQ) accelerator based system. The fast neutrons are produced by 2 MeV deuterons bombarding beryllium target. The moderator, reflector, shielding and collimator have been optimized with Monte-Carlo simulation to improve the neutron beam quality. The neutrons are thermalized in water cylinder of Φ26×26 cm² with a polyethylene disk in front of Be target. The size of deuteron beam spot is optimized considering both the thermal neutron distribution and the demand of target cooling. The shielding is a combination of 8 cm thick lead and 42 cm thick boron doped polyethylene. The thermal neutrons are extracted through a rectangular inner collimator and a divergent outer collimator. The thermal neutron beam axis is perpendicular to the D⁺ beam line in order to reduce the fast neutron and the γ ray components in the imaging beam. When the neutron yield is 3×10¹² n/s and the L/D is 50, the thermal neutron flux is 5×10⁵ n/cm²/s at the imaging plane, the Cd ratio is 1.63 and the n/γ ratio is 1.6×10¹⁰ n/cm²/Sv.