This device relates generally to an apparatus for tracking cosmic ray muons through an underground sensor to develop an image of subsurface density above the sensor (muon radiography), and to use multiple sensors to build a 3D model of density (muon tomography).
The present invention relates to a gas electron multiplier (GEM) and, to a cylindrical GEM using the interaction between radiation and gas, as well as a radiation detector using multiple, concentric cylindrical GEMs.
Exploitation of underground resources (e.g., mineral deposits, oil reservoirs) employs varied geophysical methods to detect, image, and monitor underground regions of interest. Many of the devices and systems used are large. There are numerous designs of borehole detectors. To be useful, a borehole detector that fits inside a very narrow aperture borehole (e.g., a NQ diamond drill bore, 76 mm diameter) must have very good spatial resolution for detecting the location of a muon at various points as it passes through the sensor. This is because the “lever arm” of the detector is very small, less than a few centimeters in total if the detector enclosure thickness and gap to the borehole is considered. What is needed then is a muon detector with very fine spatial resolution.
The muon detector must also be cost-effective. Very expensive systems e.g., such as those based on silicon pixel technology, are not amenable to deployment down drill holes where there is a high probability of losing the detector due to deformation of the hole.
A type of gaseous avalanche electron multipliers for the detection of ionising radiation are often referred to as Gas Electron Multipliers detectors. GEM detectors have been extensively used in high energy physics experiments such as hadron colliders. They represent a significant improvement over conventional detectors such as multi-wire proportional counters.
Conventional GEMs have been used to detect radiation, such as charged particles, gamma rays, and neutrons. When the radiation, which is the detection target, enters such a GEM, it uses electron avalanche effects to multiply electrons released from gas atoms because of the interaction between radiation and a gas and enables to detect the radiation as an electrical signal.
GEM detectors and systems have two electrodes that consist of two thin sheets of conductive material that are overlain on both sides of a polymer foil. An etching method develops narrow regularly spaced holes through both conductive sheets and the middle polymer layer. The conductive sheets are held at different electrical potentials, with a difference of a few 10's to 100's of volts. The very narrow gap between the sheets then creates a very strong electric field that is mostly axial within the etched holes.
A GEM detector has a drift region with an ionizing gas. A charged particle passing through the drift region liberates some electrons from the ionizing gas. The liberated electrons then drift towards the electrode leaving an ionization trail. As some of the electrons pass through the gap between the conductive sheets, they are accelerated greatly by the very large electric field and generate further ionization in the gas and thus a large enhancement in the drift current (an electron avalanche).
A GEM detector has an induction region between GEM layers or between the last GEM layer and the sense PCB. In this last induction region, the macroscopic electrical current induces a mirror current on the pickup wires at the bottom of the induction region, which is how the passage of charged particles is then ultimately measured. Multiple layers of GEM electrodes create multiplicative growth in the “gain” of current from the very few ionization electrons liberated in the drift region.
A GEM detector device has pairs of orthogonal readout electrodes that are used to measure the location of the ionization trail. The device has orthogonal sets of X and Y coordinate strips along its axes. By interpolating the measured current across neighbouring sets of X and Y coordinate strips, the absolute position of a charged particle when it passed through the plane can be determined with sub-millimeter precision.
Multi-layer GEM detectors in planar and other configurations have been used to detect radiation in particle collider beamlines.
An embodiment for a cylindrical apparatus for detecting cosmic ray muons using gas electron multiplier (GEM) technology where readout electrodes are oriented in a helical pattern around the apparatus is disclosed. The apparatus is relatively inexpensive to manufacture, affords very high spatial resolution, and can be fit inside a narrow aperture borehole.
The GEM detector is composed of one or more, concentric, coaxial, cylindrical GEM tracking layers. Each GEM layer has two conductive layers 20, 30, and an insulating layer 10 between the two conductive layers. Each layer has perforations that are coaligned among the layers. The three layers with perforations 75 are collectively a foil. Typically, the diameter of each perforation 75 is similar to the thickness of the layers.
In an embodiment, the GEM detector has one concentric cylindrical triple-GEM tracking layers, where each layer has a total active length of 300 cm (comprised of multiple segments of 50 cm stacked together).
The anode plane of the detector is a multi-layer kapton/copper flexible circuit with helical clockwise and counter-clockwise strips. In an embodiment, the GEM detector may be operated with an approximate Ar:C4H10 90:10 gas mixture.
In an embodiment the apparatus 1 has one GEM layer and helically wound conductive strips for readout of the amplified ionization current. The apparatus has an insulating medium 10, inner and outer conductive layers 20, 30 surrounding the insulating medium 10, and an inner layer 50 having readout electrodes 60 on an inner printed circuit board (PCB) 70. The conductive layers have perforations 75 in them for amplification of the ionization charge. In this Figure the perforations 75 are exaggerated for illustration purposes. They are typically closely spaced and have small diameter such that the GEM layer resembles a mesh foil that is only partially opaque.
When in use, an electron avalanche forms in the region between the conductive layers, within the perforations (holes) of the insulating layer.
In an embodiment, the printed circuit board 70 has one or more layers of conductive strips 80 that are wrapped helically, clockwise and counter-clockwise, around the cylindrical body of the inner layer from the bottom to the top of the apparatus in a helical configuration. In an embodiment, the PCB has two layers of conductive strips that are wrapped exactly once around the cylinder between the two surfaces of the device. The two layers of conductive strips on the PCB are electrically isolated from one another. In an embodiment the PCB is constructed with a flexible material.
Referring to
Supposing that there are m windings in the clockwise and n windings in the counter-clockwise direction (m=4, n=3 in
These (m+n) possible locations are resolved by using the two-sided measurement to determine where along the set of pickup wires the muon-initiated electron avalanche induced a mirror current. This provides an additional measurement with associated uncertainty indicated by the gradient band. If the uncertainty is narrower than the pitch between the (m+n) possible solutions, then the actual position at which the muon impacts one side of the cylindrical apparatus (the black dot) is uniquely determined.
During operation, a data acquisition system “reads out”, or measures the current pulse characteristics (e.g., voltage and time) to determine the impact location of a muon-induced electron avalanche along a pickup wire.
The conductive strips 80 are read out at one or both ends of the detector. In the case of double-sided readout, the relative charge—based on the resistive properties of the strips—and/or relative timing information for readouts on both ends can be further used to interpolate the position of the induced signal from a charged particle along a particular strip.
In an embodiment, the apparatus has the helix windings >1 for each helix, and the ambiguity of multiple crossings is resolved using the relative charge and/or timing measurements from both ends of each strip.
Charged particles (such as muons) passing through the ionizing gas liberate electrons which drift towards the GEM layer. Large electron multiplication (electron avalanche) occurs in the 1 or more GEM layers. This avalanche drifts towards the conductive strips on the printed circuit board which are held at a lower potential. The drift current induces a fast response on the readout strips, which can be digitized. By measuring the voltage response on multiple neighbouring strips within any one of the helical bundles, an average position of the pulse across those strips may be interpolated. Then, the intersection of the average strip between the counter wound helices provides a spatial position measurement for where a charged particle (muon) trajectory pierced the cylinder wall. Since the positions of the strips and the system geometry is known, these yield fully 3D space point measurements on piercing the wall of the detector at entry and exit for each charged particle that passes through the detector. The (minimally) two space points provide a straight-line trajectory that can be used to generate a radiographic intensity image, which can be combined from multiple other such sensors to perform muon tomographic analysis.
This application is a continuation of Patent Cooperation Treaty (PCT) application No. PCT/CA2021/051509 having an international filing date of 26 Oct. 2021 which in turn claims priority from, and for the purposes of the United States, the benefit under 35 USC 119 in relation to U.S. application No. 63/105,811 filed 26 Oct. 2020. All of the applications referred to in this paragraph are hereby incorporated herein by reference.
Number | Date | Country | |
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63105811 | Oct 2020 | US |
Number | Date | Country | |
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Parent | PCT/CA2021/051509 | Oct 2021 | US |
Child | 18306180 | US |