Study of natural diamond detector spectrometric properties under neutron irradiation

Natural diamond detector (NDD) performance was studied up to a neutron fluence of 10¹⁵ neutron/cm². The variations of the NDD spectrometric response to incident α-particles from ²⁴¹Am source after exposure to fast neutron fluences up to 3×10¹⁶ n/cm² were examined. No significant variations up to the level of 10¹⁴ n/cm² were observed. Degradation of charge collection efficiency at higher fluences is reported. No remarkable increase of the NDD leakage current and count rate change had been observed up to a neutron fluence of 3×10¹⁶ n/cm². The charge collection efficiency variations of neutron irradiated diamond spectrometer were studied ex situ under γ-rays, β-radiation and visible light excitation. Charge collection efficiency restoration up to 75% level and the NDD performance stabilization by extrinsic low-intensity visible light (550 nm<λ<800 nm) or intrinsic excitations have been demonstrated.     

Measurements of the proton light output function of the organic liquid scintillator NE213 in several detectors

When using an organic liquid scintillator such as NE213 for neutron spectrometry, the light output as a function of proton energy is needed in order to unfold the neutron spectrum from the scintillator's pulse height distribution. We have measured this function for several detectors over the range 1.5–16 MeV approximately, using monoenergetic neutrons from the Harwell 5 MV Van de Graaff accelerator. Results were obtained for a wide variety of sizes and shapes of the scintillator cell, and were found to be essentially in agreement within errors. The results were also compared with those of several other workers (amongst whom there is considerable disagreement). Below 10 MeV, there is excellent agreement with one worker and moderate or poor agreement with others; above 10 MeV, agreement is moderate in all cases. We conclude that workers wishing to unfold neutron spectra from NE213 pulse height distributions would be advised to make measurements with their own particular detector configuration, rather than use published functions.     

Neutron field measurements at the 590 MeV ring cyclotron of the Paul Scherrer Institute

A complete characterization of the neutron field was performed at 3 representative areas around the 590 MeV Ring cyclotron of the Paul Scherrer Institute. The neutron spectra were measured with a Bonner spheres system, ²⁰⁹Bi and ²³²Th fission track detectors. Their shapes are very different according to the location; neutron energies up to about 200 MeV were recorded. The dosimetry was performed with various active instruments: 2202D, LB 6411, LINUS, nm 500, nm 500X, HANDI, REM 500. The comparison between the H*(10) values determined by different systems is presented and discussed.     

Application of Neural Networks for unfolding neutron spectra measured by means of Bonner Spheres

A Neural Network structure has been used for unfolding neutron spectra measured by means of a Bonner Sphere Spectrometer set. The present work used the “Stuttgart Neural Network Simulator” as the interface for designing, training and validation of a MultiLayer Perceptron network. The back-propagation algorithm was applied. The Bonner Sphere set chosen has been calibrated at the National Physical Laboratory, United Kingdom, and uses gold activation foils as thermal neutron detectors. The neutron energy covered by the response functions goes from 0.0001 eV to 10 MeV. Two types of neutron spectra were numerically investigated: monoenergetic and continuous. Good results were obtained, indicating that the Neural Network can be considered an interesting alternative among the neutron spectrum unfolding methodologies.     

Neutron and gamma spectra measurements and calculations in benchmark spherical iron assemblies with 252Cf neutron source in the centre

The neutron and gamma spectra measurements have been made for benchmark iron spherical assemblies with the diameter of 30, 50 and 100 cm. The ²⁵²Cf neutron sources with different emissions were placed into the centre of iron spheres. In the first stage of the project, independent laboratories took part in the leakage spectra measurements. The proton recoil method was used with stilbene crystals and hydrogen proportional counters. The working range of spectrometers for neutrons is in energy range from 0.01 to 16 MeV, and for gamma from 0.40 to 12 MeV. Some adequate calculations have been carried out. The authors propose to carefully analyse the leakage mixed neutron and gamma spectrum from iron sphere of diameter 50 cm and then adopt that field as standard.     

Measurements of photon spectra in mixed fields at a nuclear installation

This paper describes the measurements of photon spectra in mixed neutron/photon radiation fields at a few locations in a nuclear reactor. The measurements were performed inside the containment of reactor 4 at the Swedish reactor site Ringhals, with a Ge-detector (4%). The measurements were carried out as a part of a EURADOS project in co-operation with the Swedish authorities and the reactor operating company. The measurements showed that a large fraction of the photons are high-energy photons (up to 7.6 MeV). This implies that GM-based photon detectors will overread in these fields since this type of detector generally overestimates the ambient dose equivalent in 6–7 MeV photon fields. The measurements also indicated that the photon field was almost isotropic, which in turn implies that the effective dose as well as the personal dose equivalent will be lower than the ambient dose equivalent.     

Response of a BaF2 scintillation detector to quasi-monoenergetic fast neutrons in the range of 45 to 198 MeV

We have studied the neutron response of a scintillation detector consisting of a 14 cm long, hexagonal-shaped BaF₂-crystal with an inner diameter of 8.75 cm coupled to an EMI9821QB photomultiplier tube. The detector was exposed to calibrated quasi-monoenergetic neutron fields obtained from ⁷Li(p,n)⁷Be reactions. The measurements were performed at neutron energies of 45, 60, 96, 147 and 198 MeV as given by the energies of the incident protons. The experimental pulse-height spectra of the BaF₂-detector are compared with Monte Carlo simulations using the FLUKA code. The detection efficiency of the BaF₂-detector in the energy range of 45–198 MeV was determined as a function of the discriminator threshold and compared to the literature data. At neutron energies above 100 MeV the detection efficiency of the BaF₂-detector was found to be a factor of two higher than that of an NE213-detector of comparable size.     

Determination of light output function and angle dependent correction for a stilbene crystal scintillation neutron spectrometer

In addition to liquid NE213 scintillators also stilbene solid crystals are applied traditionally for fast neutron spectrometry. A proper evaluation of experimental data provides a precise determination of the nonlinear light output function for the given scintillator/photomultiplier combination, and for stilbene additionally an adequate correction of the anisotropy effect. Calibration experiments with monoenergetic neutrons (1.2, 2.5, 5.0, 13.95, 14.8, 19.0 MeV) and various neutron incidence angles were carried out at the accelerator facility of the PTB Braunschweig for two cylindrical scintillators (∅30 mm×25 mm, ∅10 mm×10 mm). An improved analytic light output function as well as an adequate angle dependent correction function were derived.     

Neutron flux measurements with a Li2B4O7 crystal

The effect of neutron irradiation on a lithium tetraborate (Li₂B₄O₇, LBO) single crystal has been investigated. The crystals of high optical quality are found to be quite stable under high neutron fluence. This study shows that LBO crystals can be used as a proportional counter for neutron fluxes of the order 10⁹ cm⁻² s⁻¹ and higher. The detectors fabricated were found to have a sensitivity of ∼3×10⁻¹⁸ A (nv)⁻¹.     

In-phantom spectra and dose distributions from a high-energy neutron therapy beam

In radiotherapy with external beams, healthy tissues surrounding the target volumes are inevitably irradiated. In the case of neutron therapy, the estimation of dose to the organs surrounding the target volume is particularly challenging, because of the varying contributions from primary and secondary neutrons and photons of different energies. The neutron doses to tissues surrounding the target volume at the Louvain-la-Neuve (LLN) facility were investigated in this work. At LLN, primary neutrons have a broad spectrum with a mean energy of about 30 MeV. The transport of a 10×10 cm² beam through a water phantom was simulated by means of the Monte Carlo code MCNPX. Distributions of energy-differential values of neutron fluence, kerma and kerma equivalent were estimated at different locations in a water phantom. The evolution of neutron dose and dose equivalent inside the phantom was deduced. Measurements of absorbed dose and of dose equivalent were then carried out in a water phantom using an ionization chamber and superheated drop detectors (SDDs). On the beam axis, the calculations agreed well with the ionization chamber data, but disagreed significantly from the SDD data due to the detector's under-response to neutrons above 20 MeV. Off the beam axis, the calculated absorbed doses were significantly lower than the ionization chamber readings, since gamma fields were not accounted for. The calculated data are doses from neutron-induced charge particles, and these agreed with the values measured by the photon-insensitive SDDs. When exposed to the degraded spectra off the beam axis, the SDD offered reliable estimates of the neutron dose equivalent.     

Measurement of neutron energy spectra from 15 to 150 MeV using stacked liquid scintillators

A multiple liquid scintillator system for measuring the energy spectrum of a neutron beam in the range 15–150 MeV is described. Two or more slabs of NE213 scintillator (13×13×7 cm³) are stacked behind one-another and only events in which a neutron interacts in the upstream scintillator are analysed. The system is designed to minimise the escape of forward recoil protons from the detecting media. Test measurements and Monte Carlo simulations of the detector response to quasi-monoenergetic neutron beams of energies 62.5 and 97.5 MeV are presented.     

Development of fast neutron profiling method

We discuss the effects of neutron scattering and γ-ray background in fast neutron imaging and the method to reduce them. As a profiling device, a combination of an imaging plate (IP) and a polypropylene film (CH₂) has been employed in this study. Good profiles were obtained by employing appropriate neutron energy, a CH₂ thickness and geometry for accelerator-based fast neutrons (5–14 MeV). The neutron flux was ∼3.5×10⁴ cm⁻² s⁻¹ in the CH₂–IP position. Furthermore, we are designing the device using a position-sensitive photomultiplier in order to improve the signal-to-noise ratio by obtaining the information of pulse height for particle selection together with position.     

Photon spectrometry in thermal neutron standard field

An NE213 liquid scintillation counter (5.08 cm in diameter and 5.08 cm long) with an LiF filter was used to measure the energy distribution of photons mixed in a thermal neutron field. The response function matrix of photons in an energy range up to 10 MeV was calculated by the EGS4/PRESTA code and properly folded with a resolution function. Pulse height spectra measured with a set of reference γ-ray sources were compared to the calculated response function and agreed very well for all reference γ-ray sources. The GRAVEL and MIEKE codes from the HEPRO program were used to unfold measured pulse height spectra. Energy distributions obtained by the unfolding were applied to evaluate the effective dose equivalent of photons mixed in a thermal neutron field.     

Room temperature PL characterization of micro and nanocrystalline diamond grown by MPCVD from Ar/H2/CH4 mixtures

A photoluminescence (PL) study at room temperature was accomplished as a complement to well-established structural and morphological characterization techniques such as μ-Raman, FTIR, XRD, XPS or SEM. Considering the wide electronic band gap of pure diamond (5.45 eV), the near ultraviolet excitation (325 nm) from an HeCd laser source was selected. The observed nanocrystalline diamond (NCD) and microcrystalline CVD diamond (MCD) samples were obtained by microwave plasma (MPCVD) from hydrogen poor Ar/H₂/CH₄ mixtures. The PL spectrum of both NCD and MCD samples is dominated by the 1.681 eV emission with significant intensity and energy variations. The well-known 1.681 eV band related to the Si-vacancy colour centre is much more pronounced in MCD. In addition, for NCD, the band shifts to higher energies with thickness, suggesting two mechanisms for the silicon incorporation: co-deposition from the plasma and diffusion from the substrate. The samples were further characterized by μ-Raman spectroscopy, X-ray diffraction and scanning electron microscopy, structurally and morphologically.     

High-energy neutron reference fields for the calibration of detectors used in neutron spectrometry

Quasi-monoenergetic reference neutron beams in the energy range between 20 and 100 MeV have been produced and characterized with a proton recoil telescope, a scintillation spectrometer, a ²³⁸U fission chamber and a Bonner sphere spectrometer. The beams are well suited for the calibration of detectors used in neutron spectrometry. A new method is described which reduces the correction for the contribution from low-energy neutrons present in the beams.     

Development of an inconel self powered neutron detector for in-core reactor monitoring

The paper describes the development and testing of an Inconel600 (2 mm diameter×21 cm long) self-powered neutron detector for in-core neutron monitoring. The detector has 3.5 mm overall diameter and 22 cm length and is integrally coupled to a 12 m long mineral insulated cable. The performance of the detector was compared with cobalt and platinum detectors of similar dimensions. Gamma sensitivity measurements performed at the ⁶⁰Co irradiation facility in 14 MR/h gamma field showed values of −4.4×10⁻¹⁸ A/R/h/cm (−9.3×10⁻²⁴ A/γ/cm²-s/cm), −5.2×10⁻¹⁸ A/R/h/cm (−1.133×10⁻²³ A/γ/cm²-s/cm) and 34×10⁻¹⁸ A/R/h/cm (7.14×10⁻²³ A/γ/cm²-s/cm) for the Inconel, Co and Pt detectors, respectively. The detectors together with a miniature gamma ion chamber and fission chamber were tested in the in-core Apsara Swimming Pool type reactor. The ion chambers were used to estimate the neutron and gamma fields. With an effective neutron cross-section of 4b, the Inconel detector has a total sensitivity of 6×10⁻²³ A/nv/cm while the corresponding sensitivities for the platinum and cobalt detectors were 1.69×10⁻²² and 2.64×10⁻²² A/nv/cm. The linearity of the detector responses at power levels ranging from 100 to 200 kW was within ±5%. The response of the detectors to reactor scram showed that the prompt response of the Inconel detector was 0.95 while it was 0.7 and 0.95 for the platinum and cobalt self-powered detectors, respectively. The detector was also installed in the horizontal flux unit of 540 MW Pressurised Heavy Water Reactor (PHWR). The neutron flux at the detector location was calculated by Triveni code. The detector response was measured from 0.02% to 0.07% of full power and showed good correlation between power level and detector signals. Long-term tests and the dynamic response of the detector to shut down in PHWR are in progress.     

Charpy impact test on A508-3 steel after neutron irradiation

Reactor pressure vessel (RPV) is the critical un-changeable component of the reactor during its service lifetime, which prevents the radioactive leak of the nuclear power plant core. The irradiation test (about 10 × 10¹⁹ cm^− 2, E > 1 MeV) in research reactor of the pressure vessel material was carried out, and the charpy impact test has been carried out before and after the neutron irradiation. Analysis of the impact energy and the fracture morphology has been done to estimate the embrittlement due to neutron irradiation. It shows that the main effects of neutron irradiation to fracture are the crack initiation and stable expansion process. And there also are a small amount of intergranular fracture in the unstable crack expansion after neutron irradiated which aware us pay more attention to the increasing intergranular fracture behavior of higher neutron fluence level after 60a nuclear power plant operation.     

Thermal conductivity of Cu/diamond composites prepared by a new pretreatment of diamond powder

Cu/diamond composites were fabricated by spark plasma sintering (SPS) after the surface pretreatment of the diamond powders, in which the diamond particles were mixed with copper powder and tungsten powder (carbide forming element W). The effects of the pretreatment temperature and the diamond particle size on the thermal conductivity of diamond/copper composites were investigated. It was found that when 300 μm diamond particles and Cu–5 wt.% W were mixed and preheated at 1313 K, the composites has a relatively higher density and its thermal conductivity approaches 672 W (m K)⁻¹.     

High rate deposition of a-Si:H and a-SiNx:H by VHF PECVD

Very High Frequency (VHF) plasma enhanced chemical vapour deposition (PECVD) has been applied to hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiN_x:H) films for thin film transistors (TFTs) fabrication. The effect of the excitation frequency on the deposition rate and the film quality of both films has been investigated. The films were prepared by VHF (30 MHz∼50 MHz) and HF (13.56 MHz) plasma enhanced CVD.High deposition rates were achieved in the low pressure region for both a-Si:H and a-SiN_x:H depositions by the use of VHF plasma. The maximum deposition rates were 180 nm/min for a-Si:H at 50 MHz and 340 nm/min for a-SiN_x:H at 40 MHz. For a-SiN_x:H films deposited in VHF plasma, the optical bandgap, the hydrogen content and the [Si–H]/[N–H] ratio remain almost constant regardless of an increase in deposition rate. The increase of film stress could be limited to a lower value even at a high deposition rate. The TFTs fabricated with VHF PECVD a-Si:H and a-SiN_x:H films showed applicable field effect mobility. It is concluded that VHF plasma is useful for high rate deposition of a-Si:H and a-SiN_x:H films for TFT LCD application.     

Stabilizing diamond surface conductivity by phenol-formaldehyde and acrylate resins

H-terminated undoped nano-crystalline diamond films of 200 nm thickness are deposited by microwave plasma chemical vapor deposition on fused silica substrates seeded by a diamond powder. The films exhibit surface conductivity 10^? 7 (Ω/□)^? 1. Phenol-formaldehyde and acrylate resins are spin-coated on the diamond films in the thickness of 0.2–1.7 μm. After the coating, the surface conductivity changes by ? 12% to + 52% compared to a bare diamond surface. It also exhibits significantly higher temporal stability. These effects are attributed to an encapsulation of the surface conductive channel from the ambient and to an electrostatic field of molecular dipoles in the resins.