Boron Counter Tube ##BEST##
The Photonis boron-lined proportional counters (CPNB) are intended to measure neutron radiations in a source range of nuclear reactors and in all neutron source applications where safety is crucial. They operate at temperatures up to 200C.
boron counter tube
The Boron 10 is a solid layer deposited by Chemical Vapor Deposition (CVD) for Photonis counters that are qualified for the safety instrumentation start-up channels of more than 100 NPP in the world. Thin Boron 10 layers obtained by CVD process give unequaled life time and pulse resolution.
There is usually an option to produce audible clicks representing the number of ionization events detected. This is the distinctive sound associated with handheld or portable Geiger counters. The purpose of this is to allow the user to concentrate on manipulation of the instrument while retaining auditory feedback on the radiation rate.
The intended detection application of a Geiger counter dictates the tube design used. Consequently, there are a great many designs, but they can be generally categorized as "end-window", windowless "thin-walled", "thick-walled", and sometimes hybrids of these types.
End-window Geiger counters are still used as a general purpose, portable, radioactive contamination measurement and detection instrument, owing to their relatively low cost, robustness and relatively high detection efficiency; particularly with high energy β-particles. However, for discrimination between α- and β-particles or provision of particle energy information, scintillation counters or proportional counters should be used. Those instrument types are manufactured with much larger detector areas, which means that checking for surface contamination is quicker than with a Geiger counter.
This is necessary as the low-pressure gas in the tube has little interaction with higher energy photons. However, as photon energies decrease to low levels there is greater gas interaction, and the direct gas interaction increases. At very low energies (less than 25 keV) direct gas ionisation dominates, and a steel tube attenuates the incident photons. Consequently, at these energies, a typical tube design is a long tube with a thin wall which has a larger gas volume, to give an increased chance direct interaction of a particle with the fill gas.
Above these low energy levels, there is a considerable variance in response to different photon energies of the same intensity, and a steel-walled tube employs what is known as "energy compensation" in the form of filter rings around the naked tube, which attempts to compensate for these variations over a large energy range. A chrome steel Geiger-Müller tube is about 1% efficient over a wide range of energies.
A variation of the Geiger tube is used to measure neutrons, where the gas used is boron trifluoride or helium-3 and a plastic moderator is used to slow the neutrons. This creates an alpha particle inside the detector and thus neutrons can be counted.
While "Geiger counter" is practically synonymous with the hand-held variety, the Geiger principle is in wide use in installed "area gamma" alarms for personnel protection, as well as in process measurement and interlock applications. The processing electronics of such installations have a higher degree of sophistication and reliability than those of hand-held meters.
There is a particular type of gamma instrument known as a "hot spot" detector which has the detector tube on the end of a long pole or flexible conduit. These are used to measure high radiation gamma locations whilst protecting the operator by means of distance shielding.
Particle detection of alpha and beta can be used in both integral and two-piece designs. A pancake probe (for alpha/beta) is generally used to increase the area of detection in two-piece instruments whilst being relatively light weight. In integral instruments using an end window tube there is a window in the body of the casing to prevent shielding of particles. There are also hybrid instruments which have a separate probe for particle detection and a gamma detection tube within the electronics module. The detectors are switchable by the operator, depending the radiation type that is being measured.
Cosmic-Ray neutron sensors are widely used to determine soil moisture on the hectare scale. Precise measurements, especially in the case of mobile application, demand for neutron detectors with high counting rates and high signal-to-noise ratios. For a long time Cosmic Ray Neutron Sensing (CRNS) instruments have relied on 3He as an efficient neutron converter. Its ongoing scarcity demands for technological solutions using alternative converters, which are 6Li and 10B. Recent developments lead to a modular neutron detector consisting of several 10B-lined proportional counter tubes, which feature high counting rates via its large surface area. The modularity allows for individual shieldings of different segments within the detector featuring the capability of gaining spectral information about the detected neutrons. This opens the possibility for active signal correction, especially useful when applied to mobile measurements, where the influence of constantly changing near-field to the overall signal should be corrected. Furthermore, the signal-to-noise ratio could be increased by combining pulse height and pulse length spectra to discriminate between neutrons and other environmental radiation. This novel detector therefore combines high-selective counting electronics with large-scale instrumentation technology.
Figure 2. Detection principle of a proportional counter. Neutron conversion into ionizing radiation takes place in either the gas phase (1a) or in solid material (1b). (1a) indicates the 3He and (1b) the 10B conversion processes. The fragments of the conversion process are emitted in opposite directions. The ionization trace is indicated in yellow. An electric field between the tube wall and the axial wire accelerates the generated electrons toward the wire. In the vicinity of the wire, the electron's kinetic energy reaches the gas ionization energy and charge multiplication takes place (2). The resulting pulse is then read out by charge sensitive amplifiers. 1(c) Indicates other types of radiation that may induce a signal. The thickness of the tracks indicate the ionization energy deposition (see section 3.2.1).
Figure 5. Schematic drawing of the interplay between surface area and response function. The cross sections through two combinations of a rectangular moderator and cylindrical neutron counter at thermal efficiency of 50% are shown. Both figures are not to scale. A typical neutron track after thermalization through the inside of the moderator is shown in blue with the color saturation indicating the absorption probability by the converter material. (A) Maximizes the response function but features the smallest surface area possible, while (B) has a large surface area compared to the neutron counter dimension. Configuration (B) therefore has a large neutron flux impinging the moderator but a low response function because of the lower probability for a counter transect.
Figure 6. Setup of the large-area boron-lined detector for mobile measurements. Six base units are shown, assembled in two rows with two units each and two units on top of the back row. Each base unit is equipped with moderator sheets on three sides.
Figure 8. Pulse shape analysis for a boron-lined detector at a counting gas pressure of 1 bar. (a) Shows the pulse height spectrum of 6,200 detected events. (b) Displays a pulse generated by the readout electronics corresponding to a neutron event and indicates how pulse height and length are determined. Pulse length corresponds to the time interval during which the pulse exceeds a certain threshold voltage level. The scatter plot (c) depicts the two-dimensional pulse data of the detected events. (d) Shows the event pulse length data as a histogram. (a,d) Are the projections of (c) to the pulse height and length axis, respectively. The blue events could be identified as neutrons whereas the orange cluster contains both electrons and neutrons. These regions can be separated by appropriate thresholds depicted by the red and black line in (c).
Figure 10. Pulse shape analysis for a 3He proportional counter. (A) Shows the pulse length and height plot and (B) the projected pulse height spectrum.
The global network of neutron monitors comprises predominantly of the monitor standardised by Carmichael in 1964, the NM-64. The design of these existing monitors and their instrumentation have changed very little over the last sixty years. For example, their neutron detectors rely on gas filled proportional counters that are either filled with highly toxic boron trifluoride (BF3) or helium-3 (3He). Almost the entire global supply of 3He is derived from a waste product of nuclear weapons programmes and, with the termination of such programmes and reducing nuclear weapons stockpile, the supply of 3He has become limited. Consequently, 3He supply became strictly controlled in 2008 and its price has fluctuated since. In some cases, new neutron monitors have reverted to BF3 filled counter tubes when the price of 3He has been at a premium. Helium-3 filled proportional counters are also used extensively in radiation portal monitors deployed for homeland security and non-proliferation; objectives which have increased significantly over the last two decades. The reduced production and increased demand for 3He has led to concerns over its supply and provided the research motivation for alternative neutron detection methods which are viable in terms of sensitivity, stability and gamma-rejection for certain applications. One of these alternative technologies is based on boron-coated straws (BCS) manufactured and supplied by Proportional Technologies, Inc (PTI). The technology is built on a patented low-cost technology that enables long copper tubes, known as 'straws', to be coated on the inside with a thin layer of 10B-enriched boron carbide (10B4C). Thermal neutrons captured in the 10B are converted into secondary particles, through the 10B(n, α) reaction. The straws can be of various diameter (circa 4 mm to 15 mm), length (up to 2 m) and shape (round, star or pie) to increase the surface area of 10B. Multiple straws can be packed inside a 1" diameter aluminium tube acting as a single drop-in replacement for traditional 3He detectors or individually distributed directly throughout the moderating medium, thus increasing efficiency by detecting the thermal neutrons at the point that they are created. BCS-based detectors are widely used in systems for homeland security, safeguards and neutron imaging in direct exchange for 3He tubes. This study aims to design a neutron monitor utilising BCS technology that is cheaper, more compact and produces comparable results to the existing network of NM-64 monitors. Monte Carlo simulations using the MCNP radiation transport code to model several BCS-based solutions and an NM-64 computational benchmark are reported. These models are validated experimentally using a standard PTI portal monitor (PTI-110-NDME) to determine its efficiency, dieaway, deadtime and gamma rejection using a combination of bare 252Cf, AmLi and 137Cs sources. The PTI-110-NDME consists of a 12" x 5" x 1 m high density polyethylene (HDPE) slab with thirty 15-mm diameter straws, 93 cm active length, embedded uniformly throughout the moderator. Funded by UK Research & Innovation (UKRI), this research is part of the Space Weather Instrumentation, Measurement, Modelling and Risk (SWIMMR) programme. 041b061a72