Low Neutron Flux Research Articles

Combined X-ray Diffraction Analysis and Quantum Chemical Interpretation of the Effect of Thermal Neutrons on the Geometry and Electronic Properties of CdTe

Objective: Substantiation of the result of the interaction of thermal neutrons with CdTe crystals by quantum-chemical methods and comparison of the experimental results with the results of quantum-chemical calculations. Methods: Single crystal samples of cadmium telluride (CdTe) were obtained by a modified Bridgman method. The plates cut from the single crystal were subjected to mechanical grinding with abrasive paper of the type P1,200-P4,000, mechanical polishing with silver paste. To clean the surface of the plate from contamination, it was washed in ethyl alcohol. In the experimental part of the work, the influence of low thermal neutron fluxes on the structural parameters of CdTe was studied. The samples were irradiated with a thermal neutron flux in the range from 2.64×107 neutron/cm2 to 1.85×109 neutron/cm2. To study the structure of CdTe crystals, the method of X-ray structural analysis was used, using a DRON-3 X-ray diffractometer. In the second part of the work, computer modeling of the process of interaction of thermal neutrons on CdTe crystals was carried out. Results: X-ray diffraction analysis of the crystals showed that as a result of irradiation with low thermal neutron fluxes, the structural parameters of CdTe improve, as evidenced by the ordering of the crystallites. In addition, in this region of the thermal neutron flux the resistance of the crystals decreases. Conclusion: Calculations of the crystal lattice parameters of CdTe are in good agreement with experimental data. The electronic and optical properties of CdTe have been studied. A decrease in the band gap after irradiation with thermal neutrons has been shown.

Advancing Neutron Detection: Fabrication, Characterization, and Performance Evaluation of Self‐Powered PIN BGaN/GaN Superlattice‐Based Neutron Detectors

Solid state semiconductor based neutron detectors have the potential to be energy efficient and compact, making them suitable for applications where low power consumption and size constraints are important considerations. Herein, neutron detection devices based on PIN structures consisting of BGaN/GaN superlattice (SL) are demonstrated. These SL structures enable to incorporate significant boron (B) content and achieve good crystalline quality epilayers crucial for better neutron detection. Further, by leveraging the built‐in electric field generated by the PIN structure, these devices can be operated without any applied bias, simplifying overall operation and enabling a more compact size system for detection. Their performance is evaluated by measuring real‐time current response under neutron irradiation (IN) and without it (ID). The neutron induced current density (ΔJ = JN − JD) is determined, reaching an impressive value of 0.67 pA cm−2 (two times JD) under thermal neutron flux of 1.2 × 104 n cm−2 s−1 without biasing, demonstrating their self‐powered capability. They exhibit a linear response to varying thermal neutron flux levels. Additionally, the detectors successfully detect low thermal neutron fluxes down to 300 n cm−2 s−1, showcasing their potential for diverse applications, including in low neutron environments, screening nuclear warheads, and preventing illegal trafficking of radiological materials.

Environmentally resilient data collecting system optimized for measuring low density neutron flux

In this work we detailed present the data collecting system (DCS), designed and optimized for use with a helium neutron counter even under very challenging environmental conditions. The DCS, in addition to its simplicity, allows for the flexible configuration of multiple DCS units into a single measurement system and can also interface with various types of detectors and different data acquisition systems as per the experimental requirements. The main advantages of the presented DCS include its autonomy, remote control capability, and modularity. The pulse shape analysis procedure employed within the DCS enhances measurement sensitivity, which is crucial for detecting extremely low neutron fluxes, such as those in underground laboratories.

Online Dark Count Rate Measurements in 150 nm CMOS SPADs Exposed to Low Neutron Fluxes

Online Dark Count Rate Measurements in 150 nm CMOS SPADs Exposed to Low Neutron Fluxes

The Random Ray Method Versus Multigroup Monte Carlo: The Method of Characteristics in OpenMC and SCONE

The Random Ray Method (TRRM) is a recently developed approach to solving neutral particle transport problems based on the Method of Characteristics. While the method previously has been implemented only in closed-source or limited-functionality codes, this work describes its implementation in two open-source Monte Carlo codes: OpenMC and SCONE. The random ray implementations required small modifications to the existing Multigroup Monte Carlo (MGMC) solvers, offering a rare venue for redundant, fine-grained, “apples-to-apples” speed and accuracy comparisons between transport methods. To this end, TRRM and MGMC solvers are evaluated against each other using each code’s native capabilities on reactor eigenvalue problems with different degrees of energy discretization. On the C5G7 benchmark (featuring only seven energy groups), TRRM achieves a maximum pin power error comparable to or lower than that of MGMC for a given run time. On a problem with 69 energy groups, MGMC is found to scale more efficiently, obtaining a lower pin power error for a given run time. However, the defining difference between the two transport methods is found to be their vastly different uncertainty distributions. Specifically, TRRM is found to maintain similar levels of accuracy and uncertainty throughout the simulation domain whereas MGMC can exhibit orders-of-magnitude greater errors in areas of the problem that feature low neutron flux. For instance, TRRM provided an up to 373 times speed advantage compared with MGMC for computing the flux in low-flux regions in the moderator surrounding the C5G7 core.

A neutron beam monitor based on ceramic gas electron multiplier with high spatial resolution for low flux measurements.

Neutron scattering instruments play an important role in studying the inner structure of materials. A neutron beam monitor is a detector commonly used in a neutron scattering instrument. The detection efficiency for most neutron beam monitors is quite low (10-4-10-6). However, in some experiments with a low neutron flux, such as small angle neutron scattering (SANS) and inelastic neutron scattering experiments, a neutron beam monitor with a higher detection efficiency (∼1% for thermal neutrons) is required to reduce the duration of the experiment. To meet this requirement, a ceramic gas electron multiplier-based neutron beam monitor equipped with a 1 µm 10B4C neutron converter was developed in this study. Its performance was determined both experimentally and in simulations. The detection efficiency in the wavelength range of 1.8-5.5 Å was measured experimentally and was confirmed by the simulation results. An algorithm based on event selection and position reconstruction was developed to improve the spatial resolution to about 1mm full-width-half-maximum. The wavelength spectrum was measured in beamline 20 (BL20) and agreed well with the results obtained using a commercial monitor. The maximum counting rate was 1.3MHz. The non-uniformity over the whole 100 × 100mm2 active area was determined to be 1.4%. Due to the excellent performance of this monitor, it has been used in several neutron instruments, such as the SANS and the High-Energy Direct-Geometry Inelastic Spectrometer instruments in the China spallation neutron source.

Measurement of dosimetrical cross sections with 14.05 MeV neutrons from compact neutron generator

This paper deals with the measurement of the differential dosimetry cross sections using a small compact D-T neutron generator with 14.05 ± 0.08 MeV neutron emission (1 × 108 n/s into 4π). In spite of low neutron flux, it is possible to get measurable gamma-ray activities by irradiating a larger amount of target material placed in close geometry. The experimentally determined cross sections are in good agreement with the cross sections in the IRDFF-II dosimetry library. The comparison with other nuclear data libraries was performed as well. It is worth noting that the mean standard deviation in IRDFF-II library is about 4 %, while in the case of other data libraries they are from 5.5 % to 7.5 %. This result can be understood as a validation of IRDFF-II using 14.05 MeV neutrons and also a confirmation of the applicability of small compact generators in the measurement of activation cross sections.

Advancing neutron diffraction for accurate structural measurement of light elements at megabar pressures

Over the last 60 years, the diamond anvil cell (DAC) has emerged as the tool of choice in high pressure science because materials can be studied at megabar pressures using X-ray and spectroscopic probes. In contrast, the pressure range for neutron diffraction has been limited due to low neutron flux even at the strongest sources and the resulting large sample sizes. Here, we introduce a neutron DAC that enables break-out of the previously limited pressure range. Key elements are ball-bearing guides for improved mechanical stability, gem-quality synthetic diamonds with novel anvil support and improved in-seat collimation. We demonstrate a pressure record of 1.15 Mbar and crystallographic analysis at 1 Mbar on the example of nickel. Additionally, insights into the phase behavior of graphite to 0.5 Mbar are described. These technical and analytical developments will further allow structural studies on low-Z materials that are difficult to characterize by X-rays.

Measurement of the 241Am(n,γ) cross section at the n_TOF facility at CERN

The neutron capture cross section of 241Am is an important quantity for nuclear energy production and fuel cycle scenarios. Several measurements have been performed in recent years with the aim to reduce existing uncertainties in evaluated data. Two previous measurements, performed at the 185 m flight-path station EAR1 of the neutron time-of-flight facility n_TOF at CERN, have permitted to substantially extend the resolved resonance region, but suffered in the near-thermal energy range from the unfavorable signal-to-background ratio resulting from the combination of the high radioactivity of 241Am and the rather low thermal neutron flux. The here presented 241Am(n,γ) measurement, performed with C6D6 liquid scintillator gamma detectors at the 20 m flight-path station EAR2 of the n_TOF facility, took advantage of the much higher neutron flux. The current status of the analysis of the data, focussed on the low-energy region, will be described here.

Laser-Driven Collimated Neutron Sources Based on Kinematic Focusing

Neutron beams are unique tools to probe samples in many fields, such as biology medicine and fusion research. Access to such sources is traditionally limited to large particle accelerators. However, laser-driven pulsed neutron sources have attracted attention in recent years due to their compactness and cost savings. Although, in practical applications, the major obstacle is terribly low neutron flux due to the isotropic scattering. Here we propose a roadmap for laser-driven neutron sources based on ``kinematic focusing,'' which is ideal for producing highly collimated neutron beams and leads to the neutron-flux enhancement by more than 1 order of magnitude. Such a good collimation is conducive to the detection of samples located several meters from neutron sources, and it becomes feasible to place sensitive detectors adjacent to the neutron sources without the necessity of heavy shielding. A theoretical model for kinematic focusing and supporting two-dimensional particle-in-cell simulations are presented. It is shown that a well-collimated neutron beam, with the maximal divergence angle of about ${30}^{\ensuremath{\circ}}$, is generated by bombardment of a hydrogen target with ${\mathrm{Li}}^{3+}$ beams, which come from a metallic lithium foil irradiated by a short laser pulse at an intensity of $5.5\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.2em}{0ex}}\mathrm{W}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. This proposed method can be achieved by present-day petawatt laser facilities.

A thermal neutron detection system based on boron-coated straw detector with prompt gamma coincidence

Accurate measurement of the neutron flux is of crucial importance for rare event search experiments in underground laboratories. The intrinsic radioactive background of the detector becomes the limiting factor for the detection of extremely low neutron flux. In this study, a thermal neutron detection system aimed at low flux measurement was developed based on boron-coated straw (BCS) neutron detectors. The neutron events can be distinguished from the detector background by coincidence measurement of neutron and the prompt gamma ray. With state-of-the-art BCS neutron detectors and NaI(Tl) gamma detector, a system sensitivity of ∼120 cps/nv was achieved, comparable to that of the commonly used 3He counters. Based on the selected coincidence criteria, the background events were rejected to 0.1% with 45.3% of the neutrons preserved. The background accidental coincidence count rate of the system was measured as 2.3×10-5 cps, corresponding to a lower limit of measurable thermal neutron flux of 1.9×10-7 n/cm2/s. The performance of the system can be further improved by using other gamma scintillator with lower neutron absorption (such as BGO) and adding extra shielding for ambient gamma rays.

L/D study via low-flux neutron imaging with TRAPY

The teaching and research reactor AKR-2 of the Technische Universität Dresden (TUD) is a neutron source with a very low neutron flux and provides very difficult radiation field conditions for neutron radiography. On the one hand the radiation field contains both neutrons and gamma particles, on the other hand the neutron field covers the complete energy spectrum of the reactor, i.e. from the thermal Maxwell spectrum to the (fast) fission spectrum. In this context, neutron imaging with thermal neutrons was carried out at the AKR-2 with the aim of demonstrating the feasibility of this method in the sense of a proof of concept at a very low-flux neutron source with difficult neutron field conditions. This investigation is a follow-up to a previous study using a low-budged imaging system called DELCam that showed promising results and therefore motivated to continue this investigation with a state-of-the-art imaging setup called TRAPY. In the framework of this investigation, an L/D study with a tilted cadmium double-edge has been conducted to characterize the new imaging setup and to determine a useful relation between an acceptable L/D and exposure time for imaging at AKR-2.Furthermore, due to the difficult radiation field conditions at the AKR-2, a more robust estimator and a pragmatic deviation measure are needed. In this context, the median and the median absolute deviation (MAD) seem to be appropriate. Finally, neutron imaging at the AKR-2 (as a representative of a very low neutron flux facility) is a useful tool not only to introduce this non-destructive method to the next generation of scientists and engineers, but also for research where low spatial resolution and long exposure time is acceptable. This makes very low neutron flux facilities, such as the AKR-2, interesting for preparatory measurements for high-flux imaging and thus contributes to increasing the availability of the neutron imaging method in research.

Design and Characterization of a Neutron-Friendly Lithium-Ion Battery Coin Cell for Extreme Fast-Charging

Lithium-ion batteries (LIBs) have profoundly advanced the development of electric vehicles (EV). However, one of the remaining bottlenecks in their widespread deployment is the long charging time typically required for commercial LIBs. There is a global push towards extreme fast charging (XFC) of EV batteries to reduce their charging times to 10-15 minutes.1 However, XFC causes severe degradation in the electrochemical performance of LIBs, which is mainly attributed to irreversible Li plating on the surface of the graphite anode rather than intercalation of Li ions inside the graphite as the battery is fast-charged.2 Since Li plating has been identified as one of the major XFC degradation mechanisms Bs, understanding its origin and spatial heterogeneity across three dimensions (3D) on porous graphite anodes as a function of time necessitates a non-destructive characterization platform to differentiate between graphite and Li.Neutron-based imaging provides the sensitivity to differentiate graphite from Li due to the large difference in their total neutron cross-sections (Li:C ~ 73:11 barns). However, high energy X-rays enable penetrating the metallic battery components. e.g., Cu, Al. We tested the feasibility of simultaneous neutron and X-ray tomography (NeXT) on the BT-2 imaging beamline at National Institute of Standards and Technology Center for Neutron Research.4 Our spatial resolution for both X-rays and neutrons was ~15-20 μm. For ex-situ Li plating detection, we imaged 1) pristine, and 2) a cycled graphite anode.5 For cycled anode, we disassembled the battery pouch cells in their fully discharged condition and harvested the anode after 450 cycles of 9C charging and C/2 discharging. For data analysis, we used 2D bivariate histogram phase segmentation to differentiate different battery materials such as Cu, graphite, etc. However, low X-ray energy (40keV) combined with the flat sample geometry of the graphite strips made it challenging to remove beam hardening artifacts from copper and, therefore, to segment lithium from graphite. In addition, to determine NeXT feasibility for in-situ imaging, we also characterized a fully assembled, uncycled single-layer battery pouch cell. However, the high neutron attenuation by the hydrogen in EC: EMC (3:7) electrolyte and hydrogenous polymeric materials in the Celgard separator made it particularly difficult to segment graphite anode from the separator and positively identify interfaces.In this presentation, we will outline the design and characterization of a neutron-friendly LIB coin cell for extreme fast-charging. Our design is based on a standard 2032 format coin cell with custom modifications to address the dual criteria of electrochemistry and neutron imaging, i.e., good neutron transmission and relative contrast from a functional LIB at 6-C charging rate. We will present three advantages of coin cell geometry: 1) it provides more uniform attenuation throughout the entire tomography scan thus, reducing artifacts; 2) multiple coin cells can be stacked on top of each other for high throughput battery imaging; 3) it reduces the amount of expensive deuterated materials needed while developing the optimum cell configurations. In addition, we will address the need of a deuterated EC: DMC electrolyte in a fluorinated separator for the neutron-friendly battery design (Figure 1). Since the total neutron cross-section of deuterium and fluorine are on the orders of magnitude lower than that of hydrogen, we anticipate that these modifications will increase neutron transmission through the battery, thus improving signal-to-noise ratio and reducing acquisition time especially at low neutron flux facilities. Secondly, they will result in a cleaner electrolyte-separator-anode interface for the detection of plated lithium because of absence of hydrogen. Finally, we will show how these modifications affect the electrochemical performance of our LIB coin cells to determine the onset of Li plating at 6-C charging rate. i-e., capacity fade vs. number of cycles.

Small-angle neutron scattering geometry with ring-shaped collimation for compact neutron sources

The neutron flux of compact neutron sources is low, making it difficult to measure scattered neutrons with a typical small-angle neutron scattering (SANS) geometry. A SANS geometry with ring-shaped collimation (r-SANS) was thus developed to enable experiments to be conducted using low-flux neutron source facilities. In this geometry, circumferentially collimated beams hit a sample. The scattered neutron flux is high on the ring center axis because scattered neutrons with each nominal scattering angle are superimposed on each point of the ring center axis. A 3He point detector – which has higher neutron counting efficiency compared with that of a typical scintillation detector and can also properly distinguish neutron from gamma events – is set on the ring center axis. These features make high signal-to-noise ratio experiments possible. Monte Carlo (MC) simulation studies were conducted to evaluate the characteristics of the r-SANS geometry. The results show that oblique incident neutrons, which cause cross-talk, asymmetrically smear the scattering intensity, and increase the incident neutrons. To study the feasibility of the r-SANS geometry, it was tested at the Kyoto University Accelerator-driven Neutron Source (KUANS), which is one of the most compact and has one of the lowest neutron fluxes among available neutron beam facilities. Two types of porous silica gels, HC-N and C-500HG, were examined. As reference, the same samples were analyzed using a small-angle X-ray scattering (SAXS) spectrometer under a typical pinhole geometry. To consider the smearing effect of the r-SANS geometry, MC simulations with SAXS experimental results were conducted. The simulated results agree with the r-SANS experimental results. In addition, to show the consistency of the scattering model parameters determined via r-SANS with those via SAXS, a parameter survey was performed, revealing reasonable agreement between the results.

Verification and Validation of the Geant4 Monte Carlo Code Toolkit for DEMO TBR Evaluations

Verification and Validation analyses of the Monte Carlo code toolkit Geant4 were conducted with nuclear data from the JEFF-3.3 library to show the suitability for fusion applications. We present benchmarking against data of the engineering design oriented HCPB (Helium Cooled Pebble Bed) Tritium Breeder Module (TBM) mock-up experiment taken from the SINBAD compilation and nuclear analyses in comparison with MCNP of the DEMO reactor equipped with a HCPB breeding blanket. Missing functionalities for Geant4 were developed such as a track length detector with multiple energy bins and tally multiplication, reflective boundaries, and statistical checks in a post-processing step. The activation foil measurements in the TBM mock-up revealed Geant4 underestimates the uncollided and fast neutron flux increasingly with penetration depth, while being consistent with MCNP for the low energy neutron flux. Geant4 underestimates the experimental tritium activity in the breeder pellets by up to 2.7% more than MCNP, which is within the experimental uncertainty. The DEMO analyses yielded a 0.97% lower Tritium Breeding Ratio (TBR) of Geant4 than MCNP for a homogenized breeder blanket, and a 1.46% higher TBR of Geant4 than MCNP for a mostly heterogenic blanket. This good agreement with MCNP demonstrates Geant4’s suitability for DEMO TBR evaluations.

Position-Sensitive Detector in Neutron Tomography and Material Characterization

Position-sensitive detectors (PSDs) have been used in neutron tomography and nanostructural characterization of material using neutrons that scattered from a monochromator of highly oriented pyrolytic graphite type, placed directly in the neutron beam in the Small Angle Neutron Scattering (SANS) facility at the Malaysian Nuclear Agency. PSD uses the 3He(n,p) reaction to detect neutrons. Because of the 3He high neutron cross section of 5333 b at 25.3 meV, a PSD is suitable for neutron detection across its axial direction at a low neutron flux of 103 cm−2‧s−1. Because of its insensitivity toward gamma radiation, the signals from the PSD for real neutrons are relatively easy to analyze. This paper discusses the use of a PSD in neutron tomography and nanostructural characterization of material in the SANS facility at the Malaysian Nuclear Agency.

Detection and Characterisation of Low Dense Charges Inside Metallic Devices Used in Space Applications by Neutron Radiography

Neutron radiography (NR) is a non-destructive evaluation method for detecting the presence of hydrogenous and other neutron absorbing compounds present inside sealed metal enclosures. Most of the explosives used in pyro devices are hydrogenous or contains boron. Detection of such compounds at its location becomes very critical when it has a specified function to perform such as cable cutting in satellites, stage separation in launch vehicles etc. Hence NR is a mandatory inspection technique for many of the pyro devices used in launch vehicles and satellites. Absence of explosive will lead to failure in the predefined action to be performed by the device. The paper concentrates on detection and the possible extent of characterisation of various neutron absorbing compounds when they are present in metal shells of variable sizes. The experiments are carried out using a low-flux deuterium–tritium (accelerator based) neutron source, a moderator-collimator set up developed indigenously to suit the low flux source and a 12 bit cooled CCD based neutron imaging camera. The study shows typical grayscale value ranges for various compounds present in same volume inside the metallic shell of variable wall thickness. Experiments carried out with specimens made of SS304 material having microscopic absorption cross-section of 13.9 barns per molecule and Aluminium of 1.62 barns per molecule shows the effect of variable microscopic cross-sections offered by the metallic cases and variable wall thickness on the grayscale value obtained. Attenuation chart for various compounds when present inside metallic casings are prepared which shows a typical grayscale value range for various thicknesses. Results also show a difficulty in distinguishing compounds having similar range of macroscopic cross-sections. From the present study, it can be established that it is possible to distinguish low dense materials such as chalk powder of macroscopic cross-section 0.13 cm−1 from compounds such as sucrose of macroscopic cross-section 1.63 cm−1 when present inside metal devices. The study focuses on the limitations of an accelerator based low flux neutron radiography setup.

Development of new small-angle neutron scattering geometry with ring-shaped collimated beam for compact neutron source

Small-angle neutron scattering (SANS) is an important tool to investigate material properties in nanometer to micrometer scale. The opportunity to conduct SANS experiments is, however, limited because of the low number of available SANS beam lines. Compact neutron sources are expected to play a significant role to increase neutron scattering facilities including SANS beam lines. The problem is that the flux of compact neutron sources can be very low, which makes it difficult to measure scattered neutrons from a sample. A SANS geometry with ring-shaped collimated beam (r-SANS) is developed to conduct SANS experiments at very low flux neutron sources. By using ring-shaped collimated neutrons to hit a large sample, the scattered neutron flux becomes high on the ring center line because the scattered neutrons with each scattered angle overlaps on each point of the ring center line. By setting a 3He point detector on the center line and shielding the surrounding of the small detection area well, high signal to noise ratio experiments are possible. In this paper, we show the concept of this new geometry and a preliminary experimental result of a glassy carbon sample taken with the r-SANS geometry constructed at Kyoto University proton Accelerator Neutron Source (KUANS).

A Large Volume (1m3) Spherical Proportional Counter for Atmospheric Neutron Measurements

A large volume (1m3) spherical proportional counter has been developed at CEA/Saclay, for low flux neutron measurements. Neutrons can be measured successfully, with high sensitivity, using 3He gas in the detector. The proton and tritium energy deposition in the drift gaseous volume, from the reaction 3He(n,p)3H, can provide the neutron spectra from thermal neutrons up to several MeV. The α resolution of the detector has been measured using 222Rn gas, in order to have the response of the total active volume of the detector. The three α peaks from 222Rn decay and 222Rn daughters decay have been measured with FWHM less then 2%, with gas mixture Ar 98% and CH4 2% at a pressure of 200 mbar. The neutron sensitivity has been tested inserting in the sphere 100mgr of 3He. The 0.765 MeV peak of p and 3H is well separated form the cosmic ray background after irradiation with 252Cf neutron source. The ground level atmospheric thermal neutrons have been also measured after several hours of exposure. The very low noise, the limited wall effect, the high resolution and the high sensitivity are some of the advantages of the detector.

Underground Low Flux Neutron Background Measurements in LSM Using a Large Volume (1m3) Spherical Proportional Counter

A large volume (1m3) spherical proportional counter has been developed at CEA/Saclay, for low flux neutron measurements. The high voltage is applied to a small sphere 15mm in diameter, located in the center of the counter and the wall of the counter is grounded. Neutrons can be measured successfully, with high sensitivity, using 3He gas in the detector. The proton and tritium energy deposition in the drift gaseous volume, from the reaction 3He(n,p)3H, can provide the neutron spectra from thermal neutrons up to several MeV.The detector has been installed in the underground laboratory in Modane (LSM) to measure the neutron background. The sphere has been has been filled with gas mixture of Ar + 2% CH4 +3gr He-3, at 275 mbar. The thermal neutron peak is well separated from the cosmic ray and gamma background, permitting of neutron flux calculation. Other potential applications requiring large volume of about 10 m in radius are described in detail in reference