Patent ID: 12259507

DETAILED DESCRIPTION

Detectors that can be used for measuring the intensity of cosmogenic neutrons near the land surface have been known for decades (Knoll, 2000), and dedicated sensors for measuring soil moisture have been around for approximately ten years (Zreda et al., 2008, 2012). Such sensors, which we call Cosmogenic Neutron Sensors (CNS), measure the local intensity of neutrons in an energy band that is dictated by the design of the sensor. An unmoderated, or bare, thermal neutron detector is sensitive to thermal neutrons having a median energy of 0.025 eV. Adding a plastic moderator around the thermal neutron detector shifts the energy sensitivity of the thermal neutron detector to neutrons having higher energies. An energy band may be selected to optimize the ability of the sensor to detect neutrons that have maximum sensitivity to water in the material near the surface. The standard CNS (Zreda et al., 2012) has 2.5 cm of plastic surrounding the bare detector. It measures neutrons in energy band 1 eV to 1000 eV, which is sensitive to hydrogen content.

The intensity of cosmogenic fast neutrons displays a complex spatial pattern around the neutron measuring device, with contributions that vary with direction and distance. The current neutron sensors count neutrons coming from all directions and distances and produce a single integral value of neutron intensity; the directional and distance information is lost in the integration process. That measured value is converted to an integrated value of soil moisture by means of a calibration function.

The distance that contributes 86.5% (1-e-2) of neutrons counted is considered the effective measurement range of a cosmogenic sensor, or its “footprint”. The physical spatial neutron distribution that reflects the probability of a measured neutron originating from the material at a given distance is called the spatial sensitivity function of the neutrons. In the very common case of radial symmetry, the function is referred to as the radial sensitivity function. When the cosmogenic method for measuring soil moisture was developed, its developers determined from neutron transport modeling that the radial sensitivity function follows exponential decay with distance (Desilets and Zreda, 2013; “conventional” inFIG.1). But recent modeling results (Kohli et al., 2015; Schrön et al., 2017b), as well as our own field measurements, suggest a different radial sensitivity that can be approximated by double-exponential functions.

FIGS.1A-1Bare graphs showing the radial sensitivity functions for conventional and revised models of cosmogenic soil moisture probes. The normalized horizontal weight for the conventional model is shown in broken lines, while the normalized horizontal weight for the revised model is shown in solid lines. The boundaries of fractional contribution100are indicated below the curves in bothFIGS.1A and1B. The conventional radial sensitivity function of Desilets and Zreda (2013) is an exponential curve, and the revised radial sensitivity function of Schron et al. (2017b) is a double exponential curve.FIG.1Bshows that both conventional and revised models are similar at radial distances greater than 10 meters.FIG.1Ashows that the models are very different at shorter distances from 0 meters to about 5 meters.

InFIG.1A, the boundaries of fractional contribution100indicate that, up to a distance just under 3 meters, the neutron contribution to the total measurement of a CNS is about 25%. Thus, the neutrons that come from short distances contribute a large fraction of the total measured neutron intensity, thereby interfering with wide-area measurement. InFIG.1B, the boundaries of fractional contribution100indicate that another 25% of the neutrons in the total measurement come from distances less than 50 meters, while another 25% comes from distances between that point and about 120 meters.

Critically, the contribution to the total neutron count is not uniform within the footprint, with a large proportion coming from the first few meters around a detector. This important fact was not well understood among the community of CNS experts. The ability separately to measure local and wide-area neutrons would represent a powerful improvement to the CNS technique. However, such measurement discrimination is impossible with the currently-used cosmogenic soil moisture measuring devices, as they are not capable of discerning which neutrons have contributed to the total measurement.

FIG.2is an illustration of the neutron sources incident upon a neutron detector210. A neutron detector210is shown oriented above a measurement surface200. In use, the measurement surface200may be the ground, construction, a body of water, and the like. Neutrons are incident upon the detector210from overhead, from the wide area below the detector210, and from the local area below the detector210.

The measured neutron intensity contains neutrons coming directly from above the detector210, also called overhead neutrons201. These overhead neutrons201, when corrected for the effects of solar activity, atmospheric pressure, latitude and longitude, and other conditions, constitute a constant background in all measurements. They have no history of interactions with soil water, and are therefore generally undesirable. Removal of overhead neutrons201may decrease the noise, and thus improve the sensitivity and accuracy of the neutron sensor210to changes in soil moisture. This in turn improves the signal-to-noise ratio and increases the dynamic range of the CNS method.

The measured neutron intensity also contains a significant amount of neutrons coming from areas below the detector210that are several meters or hectometers away from the detector210, also called wide area neutrons202. Although the area enclosed within less than 10 m radius around the detector contributes approximately one-third to the total number of neutrons measured, a significant remainder of the detected neutrons are wide area neutrons.

The measured neutron intensity also contains a significant amount of neutrons coming from areas below the detector210that are near the detector210, also called local area neutrons203. The neutrons coming from below the detector210can distort the measurement of wide area average moisture value.

This is important for at least two reasons. First, an accurate measurement of a wide area measurement surface should minimize the local area neutrons203. If wide-area measurement is desired, the neutrons coming from below should be reduced or substantially eliminated. Second, an accurate measurement of a local area measurement surface should minimize the wide area neutrons202. High sensitivity to local area neutrons203can be used to measure near-field (within meters) soil moisture. However, because far-field neutrons (those beyond a few meters from the detector210) contribute approximately 70% to the total neutrons measured, they have to be blocked if near-field measurement is desired.

Furthermore, the neutrons coming from below and above the detector201,203make up approximately 50% of the measured total neutrons, although this number depends on soil moisture and other local conditions. The remaining ˜50% are wide area neutrons202coming from the sides from distances between a few meters and a few hectometers. These numbers represent average distributions of neutrons coming from above, below, and to the side of the detector210. There may be some directional overlapping of neutrons from any direction. For example, a portion of neutrons201may not hit the detector210from above, but may enter from the side. A portion of neutrons202may hit the detector210from above rather than from the side. This representation of neutrons201,202, and203and their directions with respect to the detector210merely indicates that neutrons are probabilistically likely to come from these respective directions. Given these distributions, the ability preferentially to select these neutrons is desirable, even required, for proper wide-area measurement of soil moisture.

FIG.3Ais an illustration of the prior art cosmogenic neutron sensor5. The moderated cosmogenic neutron sensor5of the prior art may include a hydrogen-sensitive neutron detector1. The hydrogen-sensitive neutron detector1may include a thermal neutron detector surrounded by a neutron moderator, as shown inFIG.3B. The neutron moderator may be made from a moderating material, such as polyethylene, that makes the thermal neutron detector sensitive to epithermal neutrons in a desired energy range. The signal from hydrogen-sensitive neutron detector1may be transmitted via a cable2to electronic modules and data logger3. A power supply4may provide power to the hydrogen-sensitive neutron detector1. As shown inFIG.2, above, the prior art cosmogenic neutron sensor5may receive neutrons from overhead, from a wide area, and from a local area.

FIG.3Bis an illustration of exemplary prior art hydrogen-sensitive neutron detectors310,320,330. Hydrogen-sensitive neutron detector310includes a thermal neutron detector301, which is a gas proportional counter. Thermal neutron detector301may include a moderator302surrounding the thermal neutron detector301, with a space303in between. The space303may generally be a vacuum or an air-filled space. Hydrogen-sensitive neutron detector320includes a thermal neutron detector304, which is a lithium foil detector. Thermal neutron detector304may include a moderator302surrounding the thermal neutron detector304, with a space303in between. Hydrogen-sensitive neutron detector330includes a fast neutron detector305, which may be a scintillator detector. Other hydrogen-sensitive neutron detectors may be included the scope of this disclosure.

It should be noted that, in the drawings, areas shown with parallel line hatching indicate neutron shields. Areas shown with cross hatching indicate moderators. And areas shown with stippling indicate constant hydrogen materials.

FIG.4is an illustration of neutrons201,202,203impinging upon the prior art moderated cosmogenic neutron sensor5. Cosmogenic neutrons400propagate from space to objects and soil on earth. The cosmogenic neutrons400may become overhead neutrons201, which impinge upon the moderated cosmogenic neutron sensor5without interacting with any objects or soil. The cosmogenic neutrons400may interact with objects or hydrogen in soil and become wide area or local area neutrons202,203. Wide and local area neutrons202,203, along with overhead neutrons201, may propagate toward and impinge upon the moderated cosmogenic neutron sensor5, reaching the thermal neutron sensor5and causing a measurement to be made from all directions of the neutron sources. The moderated cosmogenic neutron sensor5may detect the total intensity of all neutrons impinging upon the cosmogenic neutron sensor5.

FIG.5Ais an illustration of a cosmogenic neutron detector510in accordance with a first exemplary embodiment of the present disclosure. The cosmogenic neutron detector510includes a hydrogen-sensitive neutron detector505orientable above a measurement surface500. A neutron shield512is positionable on the hydrogen-sensitive neutron detector505. The neutron shield512is positioned to interact with at least a portion of cosmogenic neutrons201,202,203propagating in a direction of the hydrogen-sensitive neutron detector505.

The hydrogen-sensitive neutron detector505may be any suitable type of neutron detector, including gas-proportional detectors with moderator, scintillation neutron detectors, semiconductor neutron detectors, and others. The hydrogen-sensitive neutron detector505may respond to energies between 0 and 2 MeV, or any subrange thereof. The hydrogen-sensitive neutron detector505may be sized and shaped to detect cosmogenic neutrons over a desired area or with a desired sensitivity. The hydrogen-sensitive neutron detector505may include a moderator to make the hydrogen-sensitive neutron detector505sensitive to a desired range of neutrons. The hydrogen-sensitive neutron detector505may detect neutrons from all directions without discrimination.

The term “neutron detector”505in this disclosure may refer to any suitable type of hydrogen-sensitive neutron detector, with or without a moderator, as some neutron sensors do not require moderated neutrons. For ease of representation in the drawings, the neutron detector505may generally be shown as a box. However, the box is a diagrammatic representation only; the neutron detector505may actually include one or more tube detectors, sheet detectors, or moderators.

The neutron detector505is orientable above a measurement surface500. The neutron detector505may be oriented in any suitable direction, whether vertically or horizontally, depending on the desired use. The measurement surface500may be an area or surface below the cosmogenic neutron sensor510of any size, elevation, and material. In one example, the measurement surface500may be an area of land having dirt, soil, rocks, water, urban construction, or some combination thereof. The measurement surface500may have a local area, a wide area, and an intermediate area. The local area may be a portion of the measurement surface500located immediately below the physical footprint of the neutron detector505, and often, the radial location below and within several meters of the cosmogenic neutron sensor510. In one example, the local area may be located within 1, meter, 2, meters, 3, meters, 5, meters, or greater of the cosmogenic neutron sensor510. The wide area may be a portion of the measurement surface500located further away from the cosmogenic neutron sensor510. In one example, the wide area may begin where the local area ends, and may continue to the extent of the measurement surface. For instance, if, hypothetically, the local area is located within about 5 meters of the cosmogenic neutron sensor510, the wide area may begin at about 5 meters from the cosmogenic neutron sensor510and may continue until the end of the measurement area. The intermediate area may be a portion of the measurement surface500located between the local area and the wide area. For example, the intermediate area may include a portion of the local area and a portion of the wide area.

The neutron detector505may be oriented above the measurement surface500in use. Where the measurement surface500is generally land or water, this means that the neutron detector505may be positioned vertically or horizontally above the measurement surface500. This may cause overhead neutrons201to propagate toward the cosmogenic neutron sensor510substantially from above, while local area and wide area neutrons203,202propagate toward the cosmogenic neutron sensor510substantially from below and from the sides, respectively.

The neutron shield512may be positionable on the neutron detector505. The shield512interacts with neutrons201,202,203propagating in a direction of the neutron detector505by causing them to be substantially blocked. This may prevent the neutrons201,202,203from reaching the neutron detector505. For the purposes of this disclosure, “interact” means to prevent a neutron from reaching the neutron detector505in the measurable energy range. Therefore, when the shield512interacts with the neutrons201,202,203propagating in a direction of the neutron detector505, the shield512causes those neutrons to either fail to reach the neutron detector505, or to fail to reach the neutron detector505with a measurable strength, which, in the field of neutron detector, is equivalent to preventing them from reaching the neutron detector505. In this way, the neutrons that interact with the shield512are blocked or prevented from being detected by the neutron detector505.

The shield512may be made from any suitable material for slowing or reflecting neutrons, such as a plastic like high density polyethylene (HDPE) or ultra-high molecular weight polyethylene (UHMW). Various combinations of a neutron moderator such as HDPE and neutron filter such as cadmium can act as the neutron shield as well.

The shield512may be positioned to interact with at least a portion of cosmogenic neutrons201,202,203propagating in a direction of the neutron detector505. Depending on use, a user may wish to shield the neutron detector505from neutrons propagating from one or more sources or directions. For example, a user wishing to measure only the local area neutrons203may wish to shield the neutron detector505from wide area and overhead neutrons202,201. A user wishing to measure only wide area neutrons202may wish to shield the neutron detector505from local area and overhead neutrons203,201. A user wishing to measure only neutrons from an intermediate area may wish to shield the neutron detector505from a portion of local area neutrons203and a portion of wide area neutrons202. A user wishing to reduce the noise floor of the cosmogenic neutron sensor510may wish to shield the neutron detector505from overhead neutrons201in combination with other shielding patterns.

The location and sizing of the shield512may be dependent on the size of the neutron detector505, the height of the detector above the measurement surface500, the size of the measurement surface500, or some combination thereof. In one example, a shield512positioned to interact with local area neutrons203may be located below the neutron detector505and may be at least the size of the lower side of the detector505. In one example, the shield512may extend past the lower side of the detector505to interact with neutrons from a greater distance away. For wide area sensing, the shield512may be somewhat larger than the lower side of the detector505. The size of the shield512below may influence what percentage of neutrons from below are blocked. In another example, a shield512positioned to interact with wide area neutrons202may cover the sides of the neutron detector505not facing the measurement surface500. In another example, a shield512positioned to interact with overhead neutrons201may be located above the neutron detector505and may be at least the size of the upper side of the detector505. In one example, the shield512may extend past the upper side of the detector501to interact with neutrons from a greater angle above the detector505.

FIG.5Ashows the shield512interacting with neutrons from many different potential sources. Cosmogenic neutrons400propagate down from space and toward the measurement surface500. Overhead neutrons201reach the shield512and are slowed or deflected when interacting with the shield material. Cosmogenic neutrons400that reach the measurement surface500interact with hydrogen in the surface material and propagate upward toward the cosmogenic neutron sensor510. Wide area neutrons202and local area neutrons203reach the shield512and are slowed or deflected when interacting with the shield material. In one example, neutrons that do not interact with the shield512reach the moderator and the neutron detector505and are counted. In this way, only neutrons from particular and desired sources are detected by the cosmogenic neutron sensor510.

The feasibility of a shield512has been confirmed by field experiments and neutron transport modeling over areas with contrasting water content. One of the experiments was a transect across a water tank surrounded by soil obtained using a standard moderated detector shown inFIGS.3and4followed by the cosmogenic neutron sensor shown inFIG.6, below. The results, shown inFIG.7, below, show that the standard detector is highly sensitive to local neutrons, and thus is not a good wide-area detector, whereas the detector with neutron shield below has a much reduced sensitivity to local neutrons and thus is a good wide-area detector. Other operating examples are discussed below.

The cosmogenic neutron sensor510may include additional electronic components, such as a power source, communications interface, control hardware, and the like. For portable detectors510, the power source may be a battery or solar power. The communications interface may allow a user to collect and retrieve neutron data from the cosmogenic neutron sensor510. The communications interface may include communications hardware, such as data ports, antennas, and the like, and may be accessed by wired or wireless communication. The control hardware may allow a user to operate and troubleshoot the device.

FIGS.5B-5Eare illustrations of cosmogenic neutron sensors510in accordance with a first exemplary embodiment of the present disclosure.

FIG.5Bshows a cosmogenic neutron sensor510with a wide area configuration. Neutrons203propagating from a local area below the cosmogenic neutron sensor510are blocked from reaching the neutron detector by a shield512, while wide area neutrons202and overhead neutrons201reach the neutron detector.

FIG.5Cshows a cosmogenic neutron sensor510with a wide and local area configuration. Neutrons201propagating from overhead are blocked from reaching the neutron detector by a shield512, while wide area neutrons202and local area neutrons203reach the neutron detector.

FIG.5Dshows a cosmogenic neutron sensor510with a wide area and noise reduction configuration. Neutrons201,203propagating from overhead and from a local area below the cosmogenic neutron sensor510are blocked from reaching the neutron detector by a shield512, while wide area neutrons202reach the neutron detector.

FIG.5Eshows a cosmogenic neutron sensor510with a local area and noise reduction configuration. Neutrons201,202propagating from overhead and from a wide area below the cosmogenic neutron sensor510are blocked from reaching the neutron detector by a shield512, while local area neutrons203reach the neutron detector.

FIG.6is an illustration of a wide area cosmogenic neutron sensor610, in accordance with the first exemplary embodiment of the present disclosure. The cosmogenic neutron sensor610may include a neutron detector505. The neutron detector505may be orientable above a measurement surface500. A neutron shield612may be positioned to interact with cosmogenic neutrons propagating from a local area of the measurement surface500below the neutron detector505.FIG.6shows a cross-sectional illustration of the cosmogenic neutron sensor610. The neutron detector505is oriented above a measurement surface500. The neutron shield612is positioned on the neutron detector505at a bottom side of the detector603. In one example, the neutron shield612may be made from HDPE and may be about 15 centimeters in thickness. In the example shown inFIG.6, the neutron shield612covers the entirety of the bottom side of the detector603and extends outward past the neutron detector505. This may provide shielding from the local area of the measurement surface500that is both directly below the neutron detector505and somewhat farther away. In another example, the neutron shield612may be smaller, covering only the bottom side of the detector603.

This exemplary cosmogenic neutron sensor610may allow a user to measure only cosmogenic neutrons propagating from overhead and from a wide area of the measurement surface500. This may allow the cosmogenic neutron sensor610higher sensitivity to wide area measurements, as the substantial contribution of neutrons from the local area of the measurement surface500will not be measured due to the neutron shield612. In operation, this wide area-type cosmogenic neutron sensor610may be used at some height above the measurement surface500so as to detect neutrons from a broad area of the measurement surface500.

FIG.7is an exemplary transect graph700of neutron intensity measured using the prior art moderated cosmogenic neutron detector and the cosmogenic neutron sensor610ofFIG.6. The two sensors were used to measure neutron intensity over a local area while being moved over a land measurement surface, over a water measurement surface, and back over a land measurement surface. The results of the prior art moderated cosmogenic neutron detector are shown with circular plot points as a function of position, while the results of the cosmogenic neutron sensor610ofFIG.6are shown with square plot points as a function of position. The space between lines701indicates where the measurement surface was water. The space outside of lines701indicates where the measurement surface was land. The plot for the prior art sensor shows that it is sensitive to local area neutrons, and therefore shows a large change at the boundary between the water and the dry land. This is because the local area neutron contribution from a water measurement surface is lower than the local area neutron contribution from a land measurement surface. In contrast, the plot for the cosmogenic neutron sensor610shows no statistically relevant sensitivity to local neutrons. The neutron intensity count remains nearly constant as the cosmogenic neutron sensor610moves from land to water and back to land.

FIG.8is an illustration of a local area cosmogenic neutron sensor810, in accordance with the first exemplary embodiment of the present disclosure. The cosmogenic neutron sensor810may include a neutron detector505having a thermal neutron detector501inside. The neutron detector505may be orientable above a measurement surface500. A neutron shield812may be positioned to interact with cosmogenic neutrons propagating from an area above the neutron detector505and a wide area below the neutron detector505. The neutron shield812is positioned on the neutron detector505to shield the neutron detector505everywhere except for the bottom side of the detector803. In one example, the neutron shield812may be made from HDPE and may be about 15 centimeters in thickness. In the example shown inFIG.8, the neutron shield812covers the entirety of the detector501except for the bottom side of the detector803. This includes a top side of the detector and horizontal sides of the detector. This may provide shielding from overhead and wide areas of the measurement surface500.

This exemplary cosmogenic neutron sensor810may allow a user to measure only cosmogenic neutrons propagating from a local area of the measurement surface500. This may allow the cosmogenic neutron sensor810higher sensitivity to local area measurements, as the substantial contribution of neutrons from the wide area of the measurement surface500and neutrons from overhead will not be calculated due to the neutron shield812. In operation, this local area-type cosmogenic neutron sensor810may be used at a short height above the measurement surface500so as to detect neutrons from a specific surface area of the measurement surface500.

FIG.9is an exemplary transect graph900of neutron intensity measured using the prior art moderated cosmogenic neutron detector and the cosmogenic neutron sensor810ofFIG.8. The feasibility of this cosmogenic neutron sensor810was confirmed by field experiments and neutron transport modeling over areas with contrasting water content. One was a transect across a water tank sunken into soil in which the prior art moderated detector was used, followed by the cosmogenic neutron sensor810. As a control, the cosmogenic neutron sensor810was also used over the same soil area without the water tank present. The space between lines901indicates where the measurement surface was water, while the space outside lines901indicates where the measurement surface was land. The circle plot points show the baseline neutron intensities obtained by the cosmogenic neutron sensor810before the tank was installed. They indicate uniform soil moisture conditions along the land-water transect. The triangle plot points show measurements made with the cosmogenic neutron sensor810. The cosmogenic neutron sensor810is sensitive to local neutrons and shows a change at the boundary between the water and the soil of nearly a factor of two. The square plot points show measurements made with the prior art moderated detector that sees both local neutrons and neutrons from far away. This is why the prior art moderated data show a smaller change between the water and the surrounding soil. The neutron contribution from far-away distances is greatly reduced by the cosmogenic neutron sensor810, which detects and measures predominantly local area neutrons.

With the cosmogenic neutron sensor810at the measurement surface, its measurement footprint is similar to the physical size of the detector or its physical footprint, e.g., the length and width spatial dimensions of the detector. In the example shown inFIG.9, this is approximately 0.5 meters. With the detector raised above the measurement surface, the measurement footprint increases significantly to the order of meters or tens of meters, depending on the height at which the cosmogenic neutron sensor810is placed. This is due to the fact that as the detector is raised above the measurement surface, the measurement footprint increases due to the increased angular area of neutron interaction below the detector, e.g., where the higher the detector is raised, a greater number of neutrons arriving on angular paths can be detected.

The results show that the prior art detector is capable of measuring only a small portion of the contrast between water and land, and thus is not a good local area detector. In contrast, the cosmogenic neutron sensor810has a much increased sensitivity to local neutrons and thus is a good small-area detector. This shows a much improved performance of the cosmogenic neutron sensor810with the neutron shield812around the entire detector501except at the bottom surface803in measuring local area neutrons. This measurement is substantially less affected by the wide area neutrons than measurements made with the prior art moderated detector. Essentially, the cosmogenic neutron sensor810measures substantially only the neutrons shown inFIG.1A.

FIG.10Ais an exemplary graph1000of a calibration function measured using the cosmogenic neutron sensor ofFIG.8. The cosmogenic neutron sensor810was used to measure neutron intensity over solid surfaces with different soil water contents. The results were then normalized to the results of measured neutron intensity over water located in the same area, as inFIG.9. The resultant curve was generally a match to the calibration curve of a standard prior art cosmogenic neutron sensor.

The feasibility of the cosmogenic neutron sensor to measure moisture across the full range of values, from dry soil to water, has been demonstrated by measurements over numerous sites with water content variable between a few percent by volume and 100% (water), assessed independently by taking soil samples, drying them in an oven, and computing water content from the water loss by drying. The results show a clear correlation between known soil water content and the neutron intensity, thus showing the feasibility of the detector.

FIG.10Bis an exemplary illustration of a wide area calibration site. To calibrate a wide area cosmogenic neutron sensor1020, such as the one discussed inFIG.6, above, a local area cosmogenic neutron sensor may be used to sample the moisture content in a number of local areas1010-1016within the wide area. The moisture content of the local areas1010-1016may be averaged or otherwise processed in order to calibrate the wide area cosmogenic neutron sensor1020. This is discussed further inFIG.17B, below.

FIG.11is an illustration of a cosmogenic neutron sensor1110with an overhead neutron shield1112, in accordance with the first exemplary embodiment of the present disclosure. The cosmogenic neutron sensor1110may include a neutron detector505. The neutron detector505may be orientable above a measurement surface500. A neutron shield1112may be positioned to interact with cosmogenic neutrons propagating from an area above the neutron detector505. The neutron shield1112is positioned on the neutron detector505to shield the neutron detector505from a top of the detector1103. In one example, the neutron shield1112may be made from HDPE and may be about 15 centimeters in thickness. In the example shown inFIG.11, the neutron shield1112covers the entirety of the top of the detector1103and extends further horizontally past the top of the detector1103. This may provide shielding from overhead neutrons. In one example, the neutron shield1112may only cover the top of the detector1103without extending further, depending on the intended use of the cosmogenic neutron sensor1110.

This exemplary cosmogenic neutron sensor1110may allow a user to measure only cosmogenic neutrons propagating from local and wide areas of the measurement surface500. This may allow the cosmogenic neutron sensor1110higher sensitivity to these measurements, as the substantial contribution of neutrons from overhead will not be calculated due to the neutron shield1112. Essentially, the unimportant neutrons contributing to the noise of the detector1110may be blocked.

FIG.12is an exemplary graph1200of the neutron intensity as a function of neutron shield thickness measured using the cosmogenic neutron sensor1110ofFIG.11. Four neutron shields1112of increasing thickness were tested. The greatest decrease in neutron intensity is shown using shields of up to about 10 centimeters thick. Thus, a thickness of about 10 or more centimeters may be sufficient to block a reasonable number of overhead neutrons from reaching the neutron detector505.

FIGS.13A-13Bare illustrations of wide area cosmogenic neutron sensors1310,1320, in accordance with the first exemplary embodiment of the present disclosure.

FIG.13Ashows a cosmogenic neutron sensor1310for detecting wide area neutrons. The cosmogenic neutron sensor1310may include a neutron detector505. The neutron detector505may be orientable above a measurement surface500. Neutron shields1312,1313may be positioned on the neutron detector505. Upper neutron shield1312may be positioned to interact with cosmogenic neutrons propagating from an area above the neutron detector505. Lower neutron shield1313may be positioned to interact with cosmogenic neutrons propagating from a local area below the neutron detector505. The neutron shields1312,1313are positioned on the neutron detector505to shield the neutron detector505at a top and bottom of the detector. In one example, the neutron shield1312may be made from HDPE and may be about 15 centimeters in thickness. In the example shown inFIG.13A, the neutron shields1312,1313cover the entirety of the top and bottom of the neutron detector505and extend further horizontally past the neutron detector505. This may provide shielding from overhead neutrons and local area neutrons below the neutron detector505. In one example, the neutron shields1312,1313may only cover the top and bottom of the neutron detector505without extending further, depending on the intended use of the cosmogenic neutron sensor1310.

FIG.13Bshows a cosmogenic neutron sensor1320modified to weigh less than the cosmogenic neutron sensor1310ofFIG.13A. The cosmogenic neutron sensor1320may include a neutron detector1305. The neutron detector1305may include a neutron detector501and a moderating material located around a portion of the neutron detector501. The moderating material, or moderator, may be positioned to moderate cosmogenic neutrons propagating from a wide area of the measurement surface500below the neutron detector505. Portions of the neutron detector505that will be shielded may not have the moderating material in order to reduce the detector1305's weight. The neutron detector505may be orientable above a measurement surface500. Neutron shields1322,1323may be positioned on the neutron detector505. The neutron shields1322,1323may be cadmium or a like material, and the size of the shields1322,1323may cover only the top and bottom of the neutron detector505. This may allow the cosmogenic neutron sensor1320to be smaller and more lightweight than the detector described relative toFIG.13A.

FIG.14is an illustration of an intermediate area cosmogenic neutron sensor1410, in accordance with the first exemplary embodiment of the present disclosure. The cosmogenic neutron sensor1410may include a neutron detector505. The neutron detector505may be orientable above a measurement surface500. The cosmogenic neutron sensor1410may have a neutron shield1412on the neutron detector505and a neutron shield1422below the neutron detector505. The neutron shield1412on the neutron detector505may interact with neutrons propagating from overhead and from a portion of a wide area below the neutron detector505. The neutron shield1422may interact with neutrons propagating from a portion of a local area below the neutron detector505. The bottom side of the thermal neutron detector1403may not be directly covered by the neutron shield1412,1422. An air gap1402may separate the neutron detector505and the lower neutron shield1422. The neutron shield1412,1422may be positioned to allow cosmogenic neutrons propagating from an intermediate area of the measurement surface500below the neutron detector505, as the neutron shield elements1412,1422are spaced apart vertically. The neutron shield1412,1422reduces the local area neutron contribution, wide area neutron contribution, and overhead neutron contribution. However, neutrons propagating toward the neutron detector505from an intermediate distance are not blocked from hitting the detector501. The scale of observation can be controlled by changing the size of the air gap1402between the lower neutron shield1422and the upper neutron shield1412, as well as by adjusting the sizes of the neutron shields1412,1422.

FIG.15is a flow chart showing a method for detecting cosmogenic neutrons, in accordance with the first exemplary embodiment of the present disclosure. It should be noted that any process descriptions or blocks in flow charts should be understood as representing modules, segments, portions of code, or steps that include one or more instructions for implementing specific logical functions in the process, and alternate implementations are included within the scope of the present disclosure in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present disclosure.

Step1510includes providing a neutron detector and a neutron shield positionable on the neutron detector. The neutron detector and neutron shield may be the same neutron detector and neutron shield described relative toFIGS.5-14, above.

Step1520includes orienting the neutron detector above a measurement surface. The measurement surface may be any desired measurement surface, including land, water, urban construction, and the like. The neutron detector may be oriented above the measurement surface by any suitable means. In one example, the neutron detector may be placed on a number of legs or a stand. In another example, the neutron detector may be a handheld device that is oriented above the measurement surface by a user of the device. In another example, the neutron detector may be attached to a vehicle, such as an automobile, airplane, or drone. The vehicle may hold the neutron detector above the measurement surface while also moving the neutron detector about the measurement surface. This may be particularly helpful for local area measurements made over a large area. In another example, the neutron detector may be attached to an aircraft, drone, satellite, tower, or tall building.

Step1530includes positioning the neutron shield to interact with at least a portion of cosmogenic neutrons propagating in a direction of the neutron detector. The neutron shield may be positioned in one or more places on the neutron detector in order to interact with cosmogenic neutrons propagating from one or more particular directions. For example, the neutron shield may be positioned on a top or upper portion of the neutron detector in order to interact with overhead neutrons. The neutron shield may be positioned on a bottom or lower portion of the neutron detector in order to interact with local area neutrons propagating from below the neutron detector. The neutron shield may be positioned on or around a side portion of the neutron detector in order to interact with wide area neutrons propagating from below the neutron detector. The positions of the shields may be combined as well. A neutron shield may be positioned on a top portion of the neutron detector and a bottom portion of the neutron detector in order to interact with cosmogenic neutrons propagating from overhead and from a local area below the neutron detector. Only a portion of a side of the neutron detector may be covered by the neutron shield. Any other combinations and permutations of the neutron shield location may be considered within the scope of this disclosure.

Step1540includes measuring a quantity of cosmogenic neutrons impinging upon the neutron detector. For stationary sensors, measurements may be made over the course of several minutes to one hour. For mobile sensors, measurements may be made over the course of several seconds to several minutes. Software may record the various measurements and process them to convert a neutron count rate to a moisture content value.

The method may further include any other features, components, or functions disclosed relative to any other figure of this disclosure.

FIG.16is an illustration of the cosmogenic neutron sensor1610in use during calibration, in accordance with a second exemplary embodiment of the present disclosure. The cosmogenic neutron sensor1610may be the local area-type detector discussed relative toFIG.8, above. The cosmogenic neutron sensor1610may include neutron detector505having a neutron shield1612positioned to interact with neutrons propagating from a wide area of the measurement surface500below the neutron detector505. In one example, the neutron shield1612may also be positioned to interact with neutrons propagating from above the measurement surface500so as to decrease the noise of the cosmogenic neutron sensor1610. Thus, the cosmogenic neutron sensor1610may primarily measure the intensity of neutrons propagating from a local area below the neutron detector505.

The cosmogenic neutron sensor1610may be calibrated by using the detector1610to record specific measurements, then calculating a calibration curve. This is described further inFIGS.17A-17B, below.FIG.16shows the cosmogenic neutron sensor1610oriented above a first measurement surface500, which may be a body of water, as well as above a second measurement surface1650, which may be a constant hydrogen content surface. The constant hydrogen content surface material may be one or more material with precisely engineered hydrogen content. For example, the material may be a combination of mineral grains mixed with water to achieve the specific hydrogen content. In another example, a material may be fabricated by combining particular amounts of mineral grains, for example, quartz sand, with amounts of HDPE particles, to make a homogenous mixture that can be pressed or melted into a solid material. The hydrogen content of the material will remain constant over time; thus, the material can be used to calibrate the cosmogenic neutron sensor1610. Multiple different materials representing a full range of hydrogen content or moisture content may be used to calibrate a cosmogenic neutron sensor1610over the range.

FIG.17Ais a flow chart showing a method of calibrating a local area cosmogenic neutron sensor for soil moisture detection, in accordance with a second exemplary embodiment of the present disclosure. Traditionally, a calibration function for the standard cosmogenic soil moisture probe is obtained using measured neutron intensity and independently measured soil moisture to calculate a parameter. Two or more calibrations at different soil moisture values are needed to define the curve, and the calibration measurements must be performed by heating the samples in an oven and measuring the mass of water removed in order to obtain the moisture content of the soil. This can be an inconvenient, labor-intensive process. The method of calibrating a local area cosmogenic neutron sensor described herein allows calibration tests to be made and verified from within the cosmogenic neutron sensor. The objective of the method is to find the relationship between local area soil moisture and local area neutron intensity. In order to do that, neutron intensity may be measured at a number of local area sites with varying soil moisture contents or using a number of artificial materials with varying, but known, hydrogen content. The resulting calibration function may be of the form: N=f(SM), where N is neutron intensity, and f(SM) is a function of the soil moisture.

Step1710includes providing a neutron detector and a neutron shield positioned to interact with cosmogenic neutrons propagating from a wide area of a measurement surface below the neutron detector. In one example, the neutron shield may also be positioned to interact with cosmogenic neutrons propagating from above the neutron detector. In other words, the cosmogenic neutron sensor may be a local area-type detector for measuring neutrons propagating toward the neutron detector from a local area below the detector.

Step1720includes orienting the neutron detector above a first measurement surface. This may be done as described inFIG.15, above. The first measurement surface may generally be a body of water. The neutron detector may be oriented at a suitable height above the first measurement surface to make an accurate measurement.

Step1730includes measuring the neutron intensity of the first measurement surface. The neutron intensity may be recorded and stored, either onboard the cosmogenic neutron sensor, or on connected memory. Step1730may also include measuring the moisture content of the first measurement surface using an alternative method, such as oven drying, capacitive, resistive, core sampling method, and the like. This may provide verification for the measured neutron intensity of the first measurement surface as a function of the soil moisture. If the moisture content of the first measurement surface is already known—for instance, if the first surface is water or an artificial surface having a known moisture content—then using an alternative method to measure the moisture content of the first measurement surface may be redundant.

Step1740includes calibrating a cosmogenic neutron sensor based on the measured neutron intensity of the first measurement surface and at least one additional data point. In one example, the at least one additional data point may be a measurement of the neutron intensity from at least a second measurement surface. The at least second measurement surface must be a different measurement surface than the first measurement surface. The second measurement surface may have a different moisture content from the first measurement surface. Third and subsequent measurements may have different levels of moisture content as well. The first and subsequent measurements of neutron intensity at the first and subsequent measurement surfaces may be correlated to first and subsequent moisture content values, either as described above or using other correlation methods. For instance, a first measurement may be correlated with a first moisture content value using an alternative measurement method. Subsequent measurements may be correlated with subsequent moisture content values using the same alternative measurement method. This may be repeated a number of times in order to provide a sufficient amount of data to define a calibration curve. The moisture content values may be used to define the calibration curve as a function of moisture content.

When the at least one additional data point is measured from at least a second measurement surface, calibrating may include measuring the neutron intensity of the at least second measurement surface. In one example, the first measurement surface may be a body of water, and a second measurement surface may be a constant hydrogen content surface. The body of water may provide a calibration measurement for a measurement surface with 100% moisture content, while the constant hydrogen content surface may provide a calibration measurement for a measurement surface with a specific and known moisture content. Additional measurements may be made with constant hydrogen content surfaces of other specific and known moisture contents. For example, the first measurement may be with a surface having 100% moisture content, while a second may be made with a surface having 75% moisture content, and a third may be made with a surface having 10% moisture content.

Calibrating may further include defining a calibration curve based on the first and at least second measurements. The calibration curve may be defined based on any suitable analytical techniques, depending on the number of measurements, the accuracy desired, and the intended use of the cosmogenic neutron detector. For example, regression analysis may be combined with the double exponential curve of the radial sensitivity function to calculate the calibration parameters of two or more measurements. The cosmogenic neutron sensor may be calibrated based on the defined calibration curve.

In one example, the at least one additional data point may be a known calibration curve for the cosmogenic neutron sensor. For instance, if a calibration curve is already known, then the cosmogenic neutron sensor may only require a single measurement of neutron intensity over a surface. The single measurement may be correlated to a moisture content value and fit to the known calibration curve, and the cosmogenic neutron sensor may be calibrated according the to fit.

In one example, the cosmogenic neutron sensor may be oriented on a soil surface, wherein the bottom of the cosmogenic neutron sensor touches the soil surface. The neutron intensity of the first measurement surface may be measured to a desired precision. The cosmogenic neutron sensor may be removed, and soil samples from the measured area may be collected at depths between 0 centimeters and 30 centimeters to allow for gravimetric water content measurements. At least one additional data point may be obtained from at least a second measurement. At least one of the additional data points may be a measurement made over a water surface. The moisture content of the soil samples may be determined by an alternative method. A calibration curve may then be defined based on the measurements of the neutron intensities and the corresponding measurements of the moisture content of the local areas.

FIG.17Bis a flow chart showing a method of calibration of a wide area cosmogenic neutron sensor for soil moisture detection, in accordance with the second exemplary embodiment of the present disclosure.

Step1750includes determining at least two local area calibration functions, each determined by: providing a hydrogen-sensitive neutron detector and a neutron shield positioned to interact with cosmogenic neutrons propagating from a wide area of a measurement surface below the hydrogen-sensitive neutron detector; orienting the hydrogen-sensitive neutron detector above a first measurement surface; and calibrating a cosmogenic neutron sensor based on the defined measured neutron intensity of the first measurement surface and at least one additional data point to produce a local area calibration data point.

In one example, a local area cosmogenic neutron sensor may be used as described in steps1710-1740. The first measurement surface may be a local area portion of a wide area measurement surface. The at least one additional data point may be a measurement from a second surface, which may be a different local area having a different moisture content or a material having a known moisture or hydrogen content. Any additional measurements may be made from yet different local areas of the measurement surface. The at least one additional data point may alternatively be a known calibration curve for the cosmogenic neutron sensor. The cosmogenic neutron sensor may be calibrating based on the measured neutron intensity of the first measurement surface and the at least one additional data point. This may produce a local area calibration function.

The local area calibration function may be determined for at least two local area points. As shown inFIG.10B, local area calibration functions may be determined from a number of local areas within a wide area to ultimately be measured.

Step1760includes calibrating a wide area cosmogenic neutron sensor based on the at least two determined local area calibration functions and a weighting function. The wide area cosmogenic neutron sensor may be a wide area cosmogenic neutron sensor described herein, or it may be a prior art cosmogenic neutron sensor. In one example, the weighting function may include a spatial sensitivity function. The wide area calibration curve may be defined based on an average of the at least two local area calibration functions, as this may indicate the average moisture content over the wide area. This calibration curve may be used to calibrate a wide area cosmogenic neutron sensor for the wide area measurement surface.

In one example, the method may be performed using a local area cosmogenic neutron sensor and a wide area cosmogenic neutron sensor. The local area cosmogenic neutron sensor may be used to calibrate the wide area cosmogenic neutron sensor. The wide area cosmogenic neutron sensor may be placed above a first surface. In one example, this point may be the center of a wide area measurement surface. The wide area measurement surface may be a circular area roughly 400 meters in diameter. The wide area cosmogenic neutron sensor may be used to make a wide area measurement for the remaining duration of the method. The local area cosmogenic neutron sensor may be used to measure the neutron intensity, and subsequently, the moisture content, of a local area within the wide area measurement surface. A local area moisture content measurement may be made for a different local area within the wide area measurement surface. A number of local area measurements may be made in order to sufficiently compute an accurate and precise average soil moisture over the wide area measurement surface. In one example, this may be 10 or more local area measurements. Preferably, this will be at least 18 local area measurements, depending on the side of the wide area measurement surface.

The average wide area soil moisture may be determined from the local area measurements. The wide area cosmogenic neutron sensor may finish making the wide area neutron intensity measurement. An average neutron intensity for the wide area may be determined for the measurement interval. A calibration function may be determined based on the average local area moisture measurements and the average neutron intensity for the wide area. This calibration function may be weighted by a weighting function, which may be the spatial or radial sensitivity function.

In one particular example, a wide area cosmogenic neutron detector may be used to make a measurement. A local area cosmogenic neutron detector may be calibrated as discussed relative toFIG.17A, above. The local area cosmogenic neutron detector may be used to measure neutron intensity at18sites within a 200 meter radius of the wide area cosmogenic neutron sensor. The local area neutron intensity measured at the18sites may be converted to moisture level values using the calibration function determined during the local area cosmogenic neutron detector calibration. A few grams of soil from each of the local area sites may be collected. A soil composite may be made by mixing 1-2 grams of each of the collected soil samples together to form a relatively homogenous mixture. The lattice water or hydrogen content of the soil composite may be measured. The soil organic carbon of the soil composite may be measured. The average moisture content may be determined from the above measurements. A calibration function for the wide area cosmogenic neutron sensor may be determined, and the sensor may be calibrated.

The usual calibration of such sensors involves taking numerous soil samples and measuring their hitherto unknown water content in the laboratory. The number of samples has to be sufficient to capture the spatial variability of soil moisture; Zreda et al. (2012) suggested taking 108 samples. The samples are dried in laboratory oven and their water content is calculated from the difference between wet soil mass and its dry mass. The process is laborious, expensive and at many locations difficult or impossible to conduct because of the presence of stones or rock outcrops in the soil. The new example above uses the local area cosmogenic neutron sensor to replace sampling and processing of soil. The measured local neutron intensities taken at many points inside the wide area measurement footprint are combined to produce an average value over the wide area sensor's measurement footprint. That value is used to calculate the calibration parameters.

FIG.18is an illustration of a cosmogenic neutron sensor1810having overhead and wide area cadmium neutron shields1812, in accordance with the first exemplary embodiment of the present disclosure. The cosmogenic neutron sensor1810may include a neutron detector501, which may be the same as the neutron detector301shown inFIG.3B, above. Moderating material1805may be positioned on the neutron detector501and at a bottom portion of the neutron detector501. The moderating material1805, or moderator, may be positioned to moderate cosmogenic neutrons propagating from a local area of the measurement surface500below the neutron detector501. A cadmium neutron shield1812may be positioned on the neutron detector501to interact with neutrons propagating from above the neutron detector501and from a wide area of the measurement surface500below the neutron detector501. The incorporation of a cadmium neutron shield1812in place of additional moderating material1805around the top and sides of the cosmogenic neutron sensor1810may cause the cosmogenic neutron sensor1810to weigh less than other examples discussed herein. This may be particularly useful for cosmogenic neutron sensors1810for use in airplanes, drones, satellites, and other aerial vehicles, as a reduced payload may be required for the cosmogenic neutron sensor1810to be able to be used with these vehicles.

This is a cadmium improvement of the prior art moderated detector discussed inFIGS.3A-3B, above. The moderator that is on all sides of the detectors inFIG.3Bmay be reduced to one moderator slab1805placed below the neutron detector501and surrounded by a cadmium sheet1812. Epithermal neutrons coming from below enter the space enclosed by cadmium1812, undergo moderation in the moderator1805and become thermal neutrons that are counted by the neutron detector501. Epithermal neutrons coming from all other directions enter the space enclosed by cadmium1812, reflect off the moderator slab1805, become thermalized, and are counted by the neutron detector501. The main advantage of this example is a significant reduction of mass of the instrument, which is important in aerial neutron sensing using drones where payload is a limiting factor.

FIG.19is an illustration of a local area cosmogenic neutron sensor1910with a neutron shield1922and a cadmium foil layer1912, in accordance with the first exemplary embodiment of the present disclosure. The cosmogenic neutron sensor1910may include a neutron detector501oriented above a measurement surface500. A layer of cadmium foil1912may be positioned around the neutron detector501, leaving a gap space1900. The gap space1900may contain air, or it may be a vacuum. A neutron shield1922may be positioned on the layer of cadmium foil1912. The cadmium foil and neutron shield1912,1922may be positioned to interact with neutrons propagating from overhead and from a wide area of the measurement surface500below the neutron detector501.

FIG.20is an illustration of a local area cosmogenic neutron sensor2010with a skirt-style neutron shield2012, in accordance with the first exemplary embodiment of the present disclosure. The local area cosmogenic neutron sensor2010may include a neutron detector501positioned above a measurement surface500. A skirt-style neutron shield2012may be positioned on the neutron detector501to interact with overhead and wide area neutrons. The sides of the neutron shield2012may extend vertically below the neutron detector501toward the measurement surface500. This may enhance the neutron shield2012's effectiveness in blocking wide area neutrons by narrowing the cosmogenic neutron sensor2010's local area measurement footprint. In turn, the resolution of the cosmogenic neutron sensor2010may be increased.

FIG.21is an illustration of a local area, thermal cosmogenic neutron sensor2110, in accordance with the first exemplary embodiment of the present disclosure. In particular,FIG.21illustrates a high spatial resolution technique with a cosmogenic-neutron soil moisture sensor (CNS) which has a high spatial resolution in the thermal neutron energy range for local area detection. A standard CNS is sensitive to neutrons in an energy band well above the thermal neutron energy range, e.g., well above 0.5 eV. A thermal neutron detector near the land surface is sensitive to cosmogenic neutrons that have been fully thermalized by hydrogen in the environment including hydrogen in the soil below the land surface. This is called a thermal cosmogenic-neutron sensor (CNS). It measures neutrons in an energy band well below that of the standard CNS.

As shown inFIG.21, the thermal CNS2110is a high spatial resolution thermal CNS2110which comprises a thermal neutron detector2120having a generalized neutron shield2130positioned above the detector2120, on any or all sides of the detector2120, and/or above the detector2120and on all lateral sides of the detector2120, as is depicted inFIG.21. The thermal CNS2110is positioned an elevated distance D off a ground surface2142using a stand structure2150, where the soil2144or other materials below the ground surface2142have hydrogen atoms2146which thermalize cosmogenic neutrons to produce thermal neutrons2102. The generalized neutron shield2130can prevent neutrons from passing therethrough. In particular, the generalized neutron shield2130can effectively stop thermal neutrons (0.5 eV)2102, epithermal neutrons (0.5 eV-1000 eV)2104, and fast neutrons (1,000 eV or more)2106, that impinge upon the generalized neutron shield2130, i.e., from the top and sides thereof as shown inFIG.21from reaching the thermal neutron detector2120positioned within or interior of the generalized neutron shield2130. The energy of blocked neutrons ranges from 0 eV to 20 MeV and can include any subrange within this larger range. The generalized neutron shield2130can be constructed from any material that prevents neutrons from entering the thermal neutron detector2120, whereby the material absorbs and/or deflects neutrons arriving at the thermal neutron detector2120from locations above and to the sides thereof, or otherwise prevents neutrons from entering the thermal neutron detector2120. As an example, a neutron shield for neutrons in the specified energy ranges may include a hydrogenated material of sufficient thickness to moderate and, ultimately, absorb neutrons. Examples of hydrogenated materials include HDPE, UHMW, water, and paraffin, but other materials may also be used. The generalized neutron shield2130may be constructed from a given material with sufficient thickness to substantially block neutrons from entering the thermal neutron detector2120from a location where the generalized neutron shield2130is located, such that the only neutrons that can reach the thermal neutron detector2120are those that arrive from the intended measurement surface2140. As an example, when the generalized neutron shield2130is formed from a hydrogenated material such as HDPE, the thickness of the shield ranges from 0.25 inches to 24 inches.

A thermal CNS2110with the generalized neutron shield2130will be sensitive to thermal neutrons2102in a local area measurement surface2140below the thermal neutron detector2120. InFIG.21, the measurement surface2140may be the local area under the thermal neutron detector2120, including directly below the thermal neutron detector2120and below but angularly offset from the thermal neutron detector2120. Being sensitive to the local area means that the thermal CNS2110can achieve a high spatial resolution, and it may be referred to as a local area, thermal CNS2110. In contrast to the local area, neutrons from the wide area, including thermal neutrons2102, epithermal neutrons2104, and fast neutrons2106will be shielded by the generalized neutron shield2130.

The thermal neutron detector2120of the local area, thermal CNS2110is sensitive to thermal neutrons2102traveling upward from the land surface2142in a local area2140. If the local area, thermal CNS2110is positioned directly on the land surface2142, i.e., without a stand structure2150, its measurement range on the land surface2142will be at a minimum and based on the size of the thermal neutron detector2120itself, since the thermal neutrons2102being detected will only come from directly below the spatial footprint of the thermal neutron detector2120. However, as the local area, thermal CNS2110is raised above the land surface2142, its region of sensitivity on the land surface2142will increase due to the widening angle from which it can receive angularly traveling neutrons, but the sensitivity will still be substantially within the local area of the thermal neutron detector2120. As an example, a typical height range for the local area, thermal CNS2110above the land surface2142may be from zero feet to 50 feet, or more. Often, it may be desirable to position the local area, thermal CNS2110a spaced distance off the ground surface2142using a stand structure, such that there is an air gap between the ground surface2142and the bottom of the thermal neutron detector2120.

It is noted that, in general, thermal neutrons2102, epithermal neutrons2104, and neutrons with higher energies, such as fast neutrons2106, emanate upward from the land surface2142. The thermal neutron detector2120of the local area, thermal CNS2110is only sensitive to the energy band of the thermal neutrons2102and not epithermal and higher energy neutrons2104,2106. Epithermal neutrons2104and higher energy neutrons, such as fast neutrons2106, with high probability, pass through the thermal neutron detector2120without detection or interaction when they are not blocked by the shield2130.

When the generalized neutron shield2130is made from a moderating material, then epithermal neutrons2104and fast neutrons2106, which have higher energies of above 0.5 eV, emanating upward from the land surface2142may be moderated down to the thermal energy range (less than 0.5 eV) and backscatter from the generalized neutron shield2130into the thermal neutron detector2120, as shown at2104A and2106A. This process is problematic since the local area, thermal CNS2110is intended to measure only thermal environmental neutrons and the neutron flux from these unwanted higher energy neutrons emanating from the land surface2142in the local area which are moderated and back scattered into the detector2120can interfere with measurement of the thermal environmental neutrons.

Accordingly, to remove unwanted neutron flux from the back scattered epithermal and higher energy neutrons, a thermal neutron filter2160may be placed between the moderating generalized neutron shield2130and the thermal neutron detector2120, as shown inFIG.21. The thermal neutron filter2160may be constructed from various materials, including, for example, cadmium, gadolinium, and boron-10 in various forms including boron carbide. The thermal neutron filter2160acts to absorb and remove local area epithermal neutrons2104and higher energy environmental neutrons, such as fast neutrons2106, that have been moderated to the thermal range and backscattered by the generalized neutron shield2130in the direction of the thermal neutron detector2120. Higher energy neutrons which are not moderated by the generalized neutron shield2130, pass through the thermal neutron detector2120, and therefore, the thermal neutron filter2160may not be necessary in situations where higher energy neutrons are not moderated.

It should be emphasized that the above-described embodiments of the present disclosure, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) of the disclosure without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present disclosure and protected by the following claims.