Yigang Yang

High detection efficiency neutron sensitive microchannel plate

Direct addition of neutron-absorbing atoms into MCP glass, such as 10B and 155,157Gd, without changing the remainder of MCP fabrication process, can make MCP sensitive to neutrons. Since the much larger cross section of 157Gd, the MCP doped with natGd2O3 is more efficient than one doped with enriched 10B2O3, theoretical indicates that the detection efficiency of a MCP doped with 3 mole% of natGd2O3 is equivalent even superior to a MCP doped with 20 mole% of 10B2O3. In this paper a 50 mm diameter rimless format 10 μm pore diameter MCP doped with 3 mole% natGd2O3 was fabricated, but the MCP glass doped with 20 mol% of natB2O3 failed to pass through the corrosive chemical processing necessary in MCPs manufacture. A primary experimental tests proved the MCP doped with 3 mole% natGd2O3 is capable of imaging thermal neutrons with 35% detection efficiency. A potential of enhancement on detection efficiency should be achieved for a optimized geometry MCP with an appropriate selection between bias angle, open area ratio and length to diameter ratio.

Fusion of X-ray Imaging and Photoneutron Induced Gamma Analysis for Contrabands Detection

A 7 MV LINAC based photoneutron interrogating system has been setup to fulfill the demand of contrabands detection in homeland security. Both X-ray imaging and photoneutron induced γ-ray analysis are used to extract the information of inspected materials. 480 CsI detectors of 5 mm (height) x 10 mm (width) x 20 mm (length) are used to form the detector array to measure the attenuation information of penetrating X-rays. 16 NaI(Tl) detectors of 127 mm (height) x 127 mm (diameter) are used to register the photoneutron induced γ-ray spectra of inspected materials. 2-dimensional elemental distributions of H, N, Fe and Cl are extracted by calculating the area of 2.223 MeV, 10.829 MeV, 7.64/9.298 MeV and 1.165/1.951/1.959 MeV γ-ray peaks in the spectra measured by NaI(Tl) arrays respectively. Mixed materials like iron, salt, water, melamine and sugar are scanned to test the contrabands detection capability. The images of X-ray attenuation and 4 elemental 2-dimensional distributions are fused together to separate suspicious materials. Areas with high concentrations of nitrogen and chlorine are easily identified and can indicate the potential existence of illicit substances.

Materials identification by X-ray and photoneutron transmission

In this paper, we present the method research of materials identification based on X-ray and photoneutron transmission. X-ray is produced by 7MeV LINAC and photoneutron is emitted as the byproduct of X-ray when beryllium convertor is used to convert X-ray to neutron. The different attenuation coefficients of X-ray and neutron in materials are used to form a factor, named V, to discriminate materials. The main problems of MIXPT (materials identification by X-ray and photoneutron transmission) come from (1) the interference of X-ray pulses to the detection of photoneutrons and (2) varied V value of materials originated by continuous spectra of X-rays and photoneutrons when penetrating materials; To the first problem, a thermal neutron detector is designed to detect the fast penetrating photoneutron. With the aid of neutron moderation substances surrounding BF3 detector, fast photoneutron is thermalized and timely delayed thus could be detected free of X-ray pulses interference and with high detection efficiency. A time window of 950μs width after each X-ray pulse is carefully determined according to both simulation and experimental research to measure the penetrating photoneutron. To the second problem, the result of simulation with MCNP5 shows that each material has a definite curve of V versus mass per unit area and could be discriminated from other materials effectively. The experimental results of polyethylene, aluminium, iron, copper, lead and explosive simulant also confirm these curves;

Study of boron-lined straw-tube detector array for neutron scattering measurement

Boron-Lined neutron detector may be a substitute of 3He proportional counter in the field of thermal neutron detection. We present in this paper the simulation results of the detection efficiency and energy response of the boron-lined detector. Thickness of natural or enriched B4C, which is used as the neutron converter to emit charged particles to form signals in proportional gas, is researched to achieve the optimized efficiency for a single detector or the whole detector array. The relationships between the rate of spurious events caused by scattering and row number or thickness of the substrate of the detectors are also researched.

Research of boron lined honey-neutron detector realized with atomic layer deposition

Boron lined gaseous detector is one of the alternative detector types that possibly replace the helium-3 counter. The most important problem in the realization of boron-lined gaseous detector is how to plate thin boron film on the surface of large area substrate. Though methods of PVD (physical vapor deposition) can plate B4C layer effectively with good thickness uniformity and adhesion, they suffer from the drawback that the area of substrate, typically 0.1 m2 in once coating process, is far smaller than required in SANS (small angle neutron scattering) spectrometry. ALD (Atomic layer deposition) technique, which is a kind of CVD (chemical vapor deposition) method, can plate boron on the surface of large aspect ratio (at most 2000:1) substrate, so it can accommodate large area substrates. Honeycomb detector, which is composed of many small honeycomb cells with 6 mm side length, has very large surface area and is placed inside the reaction chamber of ALD machine to deposit boron layer on the surface of it to realize high detection efficiency. BBr3 and H2O are used as precursors to form B2O3 layer and diethyl zinc and H2O are used as precursors to form ZnO film, the passivation layer. 0.04 mg/cm2 of B2O3 is deposited after 2250 cycles of growth. The boron lined honeycomb then is filled with 95% argon + 5% CO2 to be a proportional counter. Its neutron sensitivity is tested under the thermal neutron field provided by a photoneutron source. The preliminary result shows that the neutron sensitivity of the honeycomb detector would be 1.26 × 10-2 cm2 (for one cell) and 1.15 cm2 (for the whole detector).

Research on a Neutron Detector With a Boron-Lined Honeycomb Neutron Converter

A new design of the boron-lined gaseous neutron detector composed of a boron-lined honeycomb neutron converter and an electron multiplier is proposed in this paper. The motivation for this research was to decrease the manufacturing difficulty and improve the robustness of the boron-lined gaseous neutron detector. The numerous anode wires in the traditional designs were removed, and the gas electron multiplier (GEM) was used as the electron multiplier. To drive the ionized electrons produced inside the honeycomb structure out to the incident surface of the GEM, a drift electric field was applied inside the holes of the honeycomb structure. The design principles of this detector were discussed. Geant4, Maxwell11, and Garfield9 were used to estimate the neutron absorption efficiency and the electron migration process. A prototype detector was constructed and experimentally evaluated. Both the simulation and experimental results indicate that this detector has the potential to be used in the applications of small angle neutron scattering for scientific research, and to replace the currently used 3He detectors, which have the trouble of very limited supply of 3He gas.

A capture-gated fast neutron detection method

To address the problem of the shortage of neutron detectors used in radiation portal monitors (RPMs), caused by the 3He supply crisis, research on a cadmium-based capture-gated fast neutron detector is presented in this paper. The detector is composed of many 1 cm × 1 cm × 20 cm plastic scintillator cuboids covered by 0.1 mm thick film of cadmium. The detector uses cadmium to absorb thermal neutrons and produce capture γ-rays to indicate the detection of neutrons, and uses plastic scintillator to moderate neutrons and register γ-rays. This design removes the volume competing relationship in traditional 3He counter-based fast neutron detectors, which hinders enhancement of the neutron detection efficiency. Detection efficiency of 21.66% ± 1.22% has been achieved with a 40.4 cm × 40.4 cm × 20 cm overall detector volume. This detector can measure both neutrons and γ-rays simultaneously. A small detector (20.2 cm × 20.2 cm × 20 cm) demonstrated a 3.3 % false alarm rate for a 252Cf source with a neutron yield of 1841 n/s from 50 cm away within 15 s measurement time. It also demonstrated a very low (<0.06%) false alarm rate for a 3.21×105 Bq 137Cs source. This detector offers a potential single-detector replacement for both neutron and the γ-ray detectors in RPM systems.

The research of high detection efficiency boron lined detector with a honeycomb neutron converter

We proposed a new design of a boron lined neutron detector with separated neutron converter and electron multiplier to simplify the manufacturing process. The comprehensive simulations about the detection efficiency have been performed with the aid of MCNP5, Maxwell and Garfield. The neutron detection efficiency with one row of detector unit could achieve 47%@25.3meV with the natB layer and can reach up to 80%@25.3meV with five rows of detector units, implying that this detector could be used in the applications of small-angle neutron scattering for scientific research or radiation portal monitor for homeland security.

Realization and test of position sensitive boron coated straw-tube

To deal with the shortage of 3He gas, which is widely used in thermal neutron detection, the boron coated straw-tubes have been developing to replace the 3He proportional counters. We present in this paper the realization process of a straw tube with 1 m long and the diameter is 4 mm, with 1 μm B4C layer coated on the inner surface to convert neutrons to charged particles.[1] The parameters, high voltage, working gas and the diameter of the anode wire, are researched to realize the best performance of the straw tube. 7.27 mm, the average position resolution of a single tube, is achieved with the straw tube with 15 μm-diameter karma anode wire (1.33μΩ · m at 20°C), with 1atm pure argon as working gas and working under the voltage of 730 V in the tentative experimental research. The linearity of positions measured by charged division read out and the real positions can achieve 99.992%.

Measurement of Atomic Number by MV X-Ray Scattering Spectra Analysis

In order to measure atomic number (Z) of materials, we present a Z identification method based on MV X-ray scattering spectra analysis in this paper. Scattering photons, which include positron annihilation photons, bremsstrahlung photons and Compton scattered photons, are generated by interactions of X-rays with matters and carry their Z information. A LaBr3(Ce) detector is used for its short resolving time (<; 100 ns) to measure scattered photons to alleviate pulse pileup problem in the 5 μs X-ray pulse duration. ADC with high sampling rate of 120 MHz and high resolution of 14-bit is used to digitize signal waveform from the preamplifier of the LaBr3(Ce) detector. An offline algorithm is designed to reconstruct scattering spectra. Appropriate shielding structure is set up to reduce the number of transmission photons from accelerator target and scattering photons in environment that enter the detector. These designs ensure effective acquisition of scattering photons in the intense pulsed radiation field. We investigated 10 materials with Z from 13 to 82. Preliminary experimental results show that Z of different materials can be determined successfully.