CN107064993B - Method for neutron detection based on time difference - Google Patents
Method for neutron detection based on time difference Download PDFInfo
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Abstract
The application discloses a method for neutron detection based on time difference. The neutron detection method comprises the steps of reacting incident neutron flow by using a boron layer to generate secondary particles; the secondary particles are ionized in the working gas; under the action of an electric field generated by the anode filament surface, electrons drift to the anode filament surface and an avalanche phenomenon occurs near the anode filament surface; the two surfaces of the anode filament surface are respectively provided with a parallel readout filament surface, the readout channels of the upper and lower readout filament surfaces are provided with a delay block unit, the time difference of time signals at two ends of the delay block unit is calculated to determine the neutron position, and the upper and lower readout filament surfaces are respectively defined as the X direction and the Y direction so as to determine the two-dimensional coordinates of the incident neutron flow. According to the method, the position of the incident neutron is determined by measuring the time difference of signals at two ends of the delay block unit, and the reading filament surfaces arranged on the upper surface and the lower surface of the anode filament surface are respectively defined as the two-dimensional coordinates of the incident neutron in the X direction and the Y direction, so that the number of reading paths of electronics is greatly reduced.
Description
Technical Field
The application relates to the field of neutron detection, in particular to a method for neutron detection based on time difference.
Background
Both neutrons and X-rays are powerful tools for humans to explore the microstructure of matter. Since 1895, when X-rays were discovered, the internal structure of a substance was studied by utilizing the characteristics of X-rays such as diffraction and scattering, and great efforts were made. The application of neutrons and neutron scattering makes people's knowledge of the microstructure of a substance more and more different, and unlike X-rays which mainly interact with electron clouds around atoms, neutrons do not basically interact with electron clouds but mainly interact with nuclei in the substance. In addition, neutrons are uncharged, have magnetic moments, strong penetrability, can distinguish light elements, isotopes and neighboring elements, have characteristics of non-destructiveness and the like, so that the neutron scattering technology becomes one of ideal probes for researching the structure and the dynamic property of a substance, and is a powerful means for detecting the microstructure and the atomic motion of the substance in multidisciplinary research.
The commonly used neutron detectors mainly comprise GEM detectors based on boron coating technology3Proportional counting tube and multi-wire proportional chamber for He gas and based on He gas6Li scintillator detectors, and the like. Wherein the most used is based on3He gas detector, mainly3He proportional counter array and3he multifilar proportional chamber. In contrast to other types of detectors,3the He gas detector has the advantages of high detection efficiency, good position resolution, good n/gamma resolution, strong radiation resistance and the like, and can be manufactured in a large area. Early on based on3The detector of He gas is mainly3He proportional counter tube array due to3Neutron detectors for He tubes with non-uniform efficiency, for increasing the proximity of the tube wallEfficiency of detection, often requiring filling with high air pressure3He gas, the outer wall effect is more obvious, and the position resolution is also poor. Relative to3An array of He tubes is provided,3he multi-filament proportional chamber is convenient for large-area production, and3he is used in a small amount, and the position resolution is high. Therefore, the temperature of the molten metal is controlled,3the He gas multi-wire proportional chamber is the first choice for a neutron detector with high positioning accuracy.
However, at present, internationally3The shortage of He gas leads to the rapid increase of the price of the detector, and the GEM detector based on the boron coating technology cannot achieve a large-area thin standard structure in China due to the limitation of the GEM film process, so that a novel neutron detector needs to be independently developed.
Disclosure of Invention
The application aims to provide a method for neutron detection based on time difference.
The following technical scheme is adopted in the application:
the application discloses a method for neutron detection based on time difference, which comprises the following steps,
(1) reacting the incident neutron flow by using the boron layer to generate secondary particles;
(2) the secondary particles are ionized in the working gas to generate electrons;
(3) under the action of an electric field generated by the anode filament surface, electrons drift to the anode filament surface and an avalanche phenomenon occurs near the anode filament surface;
(4) the upper surface and the lower surface of the anode filament surface are respectively provided with a parallel readout filament surface, the upper readout filament surface and the lower readout filament surface are respectively composed of a plurality of readout filaments which are parallel to each other, the readout filament direction of the upper readout filament surface is parallel to the anode filament direction of the anode filament surface, the readout filament directions on the two readout filament surfaces are vertical, every two readout filaments on each readout filament surface are a readout channel, each readout channel is provided with a delay block unit, the delay block units on each readout filament surface are respectively led out by two paths of signals after being connected in series, and the upper readout filament surface and the lower readout filament surface are totally four paths of signals; and respectively defining the upper and lower read-out filament surfaces as an X direction and a Y direction, taking the time signal of the anode filament surface as a starting signal, taking four-way signals X1, X2, Y1 and Y2 of the upper and lower read-out filament surfaces as position signals, and determining the incident position of the neutron by calculating the time difference of the time signals at two ends of the delay block unit.
The method further comprises the steps of amplifying the time signal of the anode filament surface and four paths of signals of the upper and lower read filament surfaces by using a fast time preamplifier, screening by using a constant ratio timer, and then performing TDC acquisition.
Preferably, the working gas is argon and carbon dioxide at one atmosphere.
The working gas is a mixed gas of argon and carbon dioxide, secondary particles generated after neutrons react with boron mainly generate primary ionization with Ar molecules in the mixed gas, and carbon dioxide is used as a catalytic gas and mainly prevents a detector from being ignited to damage detector components.
Preferably, the sensing wires used on the upper sensing wire surface and the lower sensing wire surface are gold-plated tungsten wires with a diameter of 25 μm, and the spacing between the sensing wires is 1 mm.
Preferably, the anode wire used on the anode wire surface is a gold-plated tungsten wire with the diameter of 25 μm, and the distance between the anode wires is 2 mm.
Preferably, the delay block units adopted by the upper readout silk surface and the lower readout silk surface are respectively provided by a delay block chip, and the delay block chip is composed of a plurality of delay block units connected in series.
It should be noted that the neutron detection method of the present application is actually performed on a neutron detector newly developed in the present application. The neutron detector comprises at least one neutron detection unit, wherein the neutron detection unit sequentially consists of a first boron-coated entrance window, a first readout filament surface, an anode filament surface, a second readout filament surface and a second boron-coated entrance window which are parallel to each other; the first reading wire surface is formed by arranging a plurality of parallel gold-plated tungsten wires in a plane, the two gold-plated tungsten wires are one path of reading channel, each path of reading channel is provided with a delay block unit, and all the delay block units of the first reading wire surface are connected in series and then are led out by two paths of signals; the anode wire surface is formed by arranging a plurality of parallel gold-plated tungsten wires in a plane, and the gold-plated tungsten wires on the anode wire surface have the same direction as the gold-plated tungsten wires on the first reading wire surface; the second reading wire surface is formed by arranging a plurality of parallel gold-plated tungsten wires in a plane, similarly, two gold-plated tungsten wires are one path of reading channel, each path of reading channel is provided with a delay block unit, and all the delay block units of the first reading wire surface are connected in series and then led out by two paths of signals; and the gold-plated tungsten wire on the second reading wire surface is perpendicular to the gold-plated tungsten wire on the anode wire surface.
The gold-plated tungsten wires on the anode wire surface and the gold-plated tungsten wires on the first reading wire surface have the same direction, which means that the gold-plated tungsten wires on the two surfaces are parallel to each other. The gold-plated tungsten wires on the second reading wire surface are perpendicular to the gold-plated tungsten wires on the anode wire surface, namely the gold-plated tungsten wires on the two surfaces are perpendicular to each other; of course, the two faces of the second readout filament and the anode filament are also parallel, so the gold-plated tungsten filaments of the two are only perpendicular in space and have no intersection point.
The utility model provides a neutron detector, during the use, first scribble boron entrance window and second scribble boron entrance window and connect negative high voltage, positive high voltage is connected to the positive pole silk face, and the neutron converts into electronic signal behind the 10B layer of scribbling boron entrance window. Two reading filament surfaces, namely cathode filament surfaces, are read at two ends respectively, and the reading filament surfaces of the two gold-plated tungsten filaments which are vertical to each other are defined as an X axis and a Y axis of a plane respectively, so that the incident position of neutrons can be determined. That is, one neutron detection unit outputs five signals in total, one is a time signal of an anode wire as a start time T signal, and the other four are x1, x2, y1 and y2 of a cathode wire as position signals. The T signal is a common time signal and the location of the neutron incidence is determined by recording the time difference with the time signal on the other cathode. In one implementation mode of the application, five paths of signals are amplified by using a fast time preamplifier, and the pre-amplified signals are screened by a constant ratio timer and then enter a TDC (time division multiplexing) for collection; the position of the incident neutron is thus determined by calculating the time difference between the position signal and the door open signal.
It should be noted that, in the first readout filament surface and the second readout filament surface, two gold-plated tungsten filaments are used as one readout channel, and each readout channel is provided with a delay block unit; it can be understood that if one gold-plated tungsten wire is used as one path for reading, the position resolution is better, but more delay block units are needed, which introduces more noise and affects the position resolution, and in comprehensive consideration, two gold-plated tungsten wires are preferably used as one path of reading channel in the present application; of course, the two ways can be judged according to the actual test.
The anode wire plays a critical role in the detector, after neutrons react with the boron layer, generated secondary particles are ionized in working gas, electrons after ionization drift to the vicinity of the anode wire under the action of field intensity to generate an avalanche phenomenon, the smaller the diameter of the anode wire is, the larger the field intensity of the vicinity of the wire is, the more easily the electrons generate the avalanche phenomenon, the larger the gain is, but considering the stability of the detector, the too thin wire is easy to break, and the anti-interference capability is poor, and the detector generally adopts a 25-micron gold-plated tungsten wire as the anode wire. Considering the noise influence on the detector caused by the small distance between the anode wires and the larger capacitance, the diameter of the anode wires and the distance keep about 1 percent of corresponding relation.
In the neutron detector, the interval between the first boron-coated entrance window and the first readout filament surface is 6-8mm, the interval between the first readout filament surface and the anode filament surface is about 3mm, the interval between the anode filament surface and the second readout filament surface is about 3mm, and the interval between the second readout filament surface and the second boron-coated entrance window is 6-8 mm. The first readout silk surface and the second readout silk surface are respectively provided with 50-100 readout channels.
It should be noted that, the distance between the first boron-coated entrance window and the first readout filament surface is a drift region, after neutrons react with boron, the generated stimulation particles alpha and 7Li ionize with working gas in the drift region, the range of the alpha and 7Li in the working gas is about 4-6 mm, if the drift region is small, the energy of secondary particles cannot be completely deposited, and the signal is small, so that the distance between the first boron-coated entrance window and the first readout filament surface is generally selected to be 6-8mm, which is suitable.
The more the read channels of the detector are, the more the position resolution is accurate, but for the delay block reading mode, the more the channels are, the more the delay units are adopted, because the delay units are composed of some resistors, capacitors and inductors, the signal attenuation and reflection phenomena are caused, the signal-to-noise ratio of the detector is poor, and the position resolution is poor, so for the read wire with the distance of 1mm, one wire is read out in one way, the position resolution is better, but 100 delay units are needed, the introduced noise is more, the position resolution is influenced, 2 wires are read out in one way, the position resolution is poorer, but only 50 delay units are needed, and the quality of the two modes needs to be judged according to the actual test conditions. In one implementation of the present application, for a detector with an effective area of 100 × 100mm, typically 2mm and one channel, 50 delay units are needed, and for a detector with an effective area of 200 × 200mm, typically 2mm and 4mm channels are used for comparison, to see which design has better position resolution, and 100 or 50 delay units are needed. In the preferred design of the present application, the initial consideration is to use 50 delay units, and of course, the delay units can be adjusted according to the experimental results to achieve the purpose of optimization.
In the neutron detector of the application, the characteristic impedance of the delay block units adopted on the first readout filament surface and the second readout filament surface is matched with the design of the preamplifier. The delay block unit is used for realizing the time difference of the arrival of the signals at two ends; the preamplifier is arranged on two paths of signals led out after the delay block units are connected in series, the preamplifier is used for amplifying the signals of the two sections of the delay block, the signals are large enough to be acquired by electronics, the arrival time difference of the signals at the two ends can be seen more obviously, and the more accurate time difference measurement is realized. It can be understood that the delay block unit directly makes the signals at the two ends have time difference, and the preamplifier amplifies the signals to measure the time difference; therefore, the characteristic impedance of the delay block unit is matched with the design of the preamplifier, so that the loss of the original signal can be reduced or avoided, and the further amplification can be realized
In the application, the delay block unit plays a critical role in the performance of the neutron detector. The delay block chip can save experimental space and ensure the consistency of delay time between signals output from the multi-wire proportional chamber to the preamplifier. The utility model provides an adopted 1507-50C Delay block chip that Data Delay Device company produced in an implementation, every Delay block chip has ten Delay block units, the Delay time of every Delay block unit is 5ns, equivalent inductance is 1 mu H for L, equivalent capacitance is 25pF for C, a Delay block chip has 14 feelers, wherein link to each other with reading out the silk has 10, input or output all the way, it is unsettled all the way, last two ways are the angle of earthing, in the neutron detector design of this application, last two ways are earthed through the contact of metal screw. The delay block unit positioning method is essentially to convert position information into time measurement, so the scale of the corresponding relationship between delay time and position is very important. In one implementation of the present application, a rectangular pulse is generated by a signal generator to scale the delay time difference and the positional relationship prior to assembly of the neutron detector.
The utility model provides a neutron detector, including range upon range of parallel arrangement's 3-5 neutron detecting element in an implementation, two adjacent neutron detecting element share one scribble boron incident window to, in two adjacent neutron detecting element, the gold-plated tungsten filament trend of the anode filament face of two is perpendicular.
It should be noted that the detection efficiency of a neutron detection unit for neutrons is limited, and in an implementation manner of the present application, the detection efficiency is only about 4%; the detection efficiency of neutrons can be effectively improved after the neutron detection units are stacked, the detection efficiency of 3-5 neutron detection units can reach about 15%, and the using requirement of neutron detection can be met. The specific structure of stacking more than two neutron detection units comprises a first boron-coated entrance window, a first readout silk surface, a first anode silk surface, a second readout silk surface, a second boron-coated entrance window, a third readout silk surface, a second anode silk surface, a fourth readout silk surface, a third boron-coated entrance window which are parallel to each other in sequence, and the like.
The cathode window of the neutron detector adopts the boron-coated cathode window, namely the boron-coated incident window, so that neutrons can be well detected and used as the neutron detector; it can be understood that if the cathode window of the detector adopts a common film, the X-ray can also be detected, other conditions are not changed, and only the cathode window needs to be changed.
The beneficial effect of this application lies in:
according to the method for detecting neutrons based on time difference, the position of incident neutrons is determined by measuring the signal time difference at two ends of the delay block unit, and the readout filament surfaces arranged on the upper surface and the lower surface of the anode filament surface are defined as the X direction and the Y direction respectively, so that the two-dimensional coordinates of the incident neutrons are determined, and the number of readout paths of electronics is greatly reduced.
Drawings
FIG. 1 is a schematic structural diagram of a neutron detector according to an embodiment of the present application;
FIG. 2 is a schematic circuit diagram of a neutron detector of an embodiment of the present application;
FIG. 3 is a graph showing the effect of the diameter of the gold-plated tungsten wire on the anode wire surface on the gain in the neutron detector according to the embodiment of the present invention, wherein the results are sequentially from left to right for 20 μm, 25 μm and 30 μm of the diameter of the gold-plated tungsten wire;
FIG. 4 is a graph showing the effect of the tension of a gold-plated tungsten wire on the anode wire surface on the gain in the neutron detector according to the embodiment of the present application;
fig. 5 is a graph of a neutron detector according to an embodiment of the present application, in which a time difference of a delay block unit is measured, and scales of a time difference and a position of a filament surface are read, where a line 1 is a scale relation between a delay time difference in an X direction and a distance, and a line 2 is a scale relation between a delay time difference in a Y direction and a distance;
FIG. 6 is a result of a test of the influence of delay block units on the rising edge of a signal in a neutron detector according to an embodiment of the present application;
FIG. 7 is a plateau curve of a neutron detector of an embodiment of the present application;
FIG. 8 is a gain of a neutron detector of an embodiment of the present application;
FIG. 9 is an energy resolution of a neutron detector of an embodiment of the present application;
FIG. 10 is a test result of the effect of negative high voltage on the detector of the cathode window in the neutron detector of the embodiment of the present application;
FIG. 11 is a graph showing the positional resolution of the sensing filament surface where the gold-plated tungsten filament is perpendicular to the anode filament surface in the neutron detector according to the embodiment of the present application;
FIG. 12 is a view showing the positional resolution of the faces of the readout filaments in which the gold-plated tungsten filament is parallel to the anode filament face in the neutron detector according to the embodiment of the present application;
fig. 13 is a result of detecting a two-dimensional distribution image of incident particles by the neutron detector according to the embodiment of the present application.
Detailed Description
The present application will be described in further detail with reference to specific examples. The following examples are intended to be illustrative of the present application only and should not be construed as limiting the present application.
Examples
The method for neutron detection based on time difference comprises the following steps,
(1) reacting the incident neutron flow by using the boron layer to generate secondary particles;
(2) the secondary particles are ionized in the working gas to generate electrons;
(3) under the action of an electric field generated by the anode filament surface, electrons drift to the anode filament surface and an avalanche phenomenon occurs near the anode filament surface;
(4) the upper surface and the lower surface of the anode filament surface are respectively provided with a parallel readout filament surface, the upper readout filament surface and the lower readout filament surface are respectively composed of a plurality of readout filaments which are parallel to each other, the readout filament direction of the upper readout filament surface is parallel to the anode filament direction of the anode filament surface, the readout filament directions on the two readout filament surfaces are vertical, every two readout filaments on each readout filament surface are a readout channel, each readout channel is provided with a delay block unit, the delay block units on each readout filament surface are respectively led out by two paths of signals after being connected in series, and the upper readout filament surface and the lower readout filament surface are totally four paths of signals; and respectively defining the upper and lower read-out filament surfaces as an X direction and a Y direction, taking the time signal of the anode filament surface as a starting signal, taking four-way signals X1, X2, Y1 and Y2 of the upper and lower read-out filament surfaces as position signals, and determining the incident position of the neutron by calculating the time difference of the time signals at two ends of the delay block unit.
The neutron detection method of the present example is performed in the neutron detector provided by the present example, and the neutron detector of the present example includes a neutron detection unit, as shown in fig. 1, the neutron detection unit is sequentially composed of a first boron-coated entrance window 11, a first readout filament surface 12, an anode filament surface 13, a second readout filament surface 14, and a second boron-coated entrance window 15, which are parallel to each other; the first reading wire surface is formed by arranging a plurality of parallel gold-plated tungsten wires in a plane, the two gold-plated tungsten wires are one path of reading channel, each path of reading channel is provided with a delay block unit, and all the delay block units of the first reading wire surface are connected in series and then are led out by two paths of signals; the anode wire surface is formed by arranging a plurality of parallel gold-plated tungsten wires in a plane, and the gold-plated tungsten wires on the anode wire surface have the same direction as the gold-plated tungsten wires on the first reading wire surface; the second reading wire surface is formed by arranging a plurality of parallel gold-plated tungsten wires in a plane, similarly, two gold-plated tungsten wires are one path of reading channel, each path of reading channel is provided with a delay block unit, and all the delay block units of the first reading wire surface are connected in series and then led out by two paths of signals; and the gold-plated tungsten wire on the second reading wire surface is perpendicular to the gold-plated tungsten wire on the anode wire surface.
The first reading wire surface, the anode wire surface and the second reading wire surface are all made of gold-plated tungsten wires with the diameter of 25 mu m, the distance between the gold-plated tungsten wires of the first reading wire surface and the second reading wire surface is 1mm, and the distance between the gold-plated tungsten wires of the anode wire surface is 2 mm. In the present example, the first readout filament surface, the anode filament surface and the second readout filament surface are all formed by forming a through hole on the circuit board, the size of which is equivalent to that of the boron-coated area of the boron-coated incident window, and welding the gold-plated tungsten filament in parallel on the through hole to form the readout filament surface or the anode filament surface. The outer frames of the through holes of the first readout silk surface and the second readout silk surface circuit board are used for fixing the delay block units; the first readout filament surface, the anode filament surface and the second readout filament surface are all fixed on a shielding shell of the neutron detector through a circuit board outer frame. The interval between the first boron-coated entrance window and the first readout filament surface is 8mm, the interval between the first readout filament surface and the anode filament surface is 3mm, the interval between the anode filament surface and the second readout filament surface is 3mm, and the interval between the second readout filament surface and the second boron-coated entrance window is 8 mm. The first boron-coated entrance window and the second boron-coated entrance window are respectively formed by coating 10B on a base material, the base material is made of a material with small neutron scattering, and specifically, an aluminum foil with the thickness of 50 microns is adopted in the embodiment; the coating method adopted in the embodiment is magnetron sputtering. The first readout filament side has 50 readout channels and the second readout filament side has 100 readout channels. In this example, the delay block units used on the first readout filament surface and the second readout filament surface are provided by a delay block chip; the delay block chip is composed of a plurality of delay block units which are connected in series, and each path of reading channel on the reading silk surface is respectively connected with a corresponding delay block unit on the delay block chip.
The neutron detector circuit schematic diagram structure of this example is shown in fig. 2, the readout filament surfaces above and below the anode filament surface, i.e. the first readout filament surface and the second readout filament surface, are respectively used for reading the X direction and the Y direction, one end of the readout filament readout channel is connected to the delay block unit, the other end is connected to the end a of the 10M Ω resistor, the end B of the 10M Ω resistor is connected to the screw hole, since the screw 21 is inserted into the screw hole and connected to the detector housing, the end B of the 10M Ω resistor is also connected to the detector housing, and the detector housing is equivalent to the ground. The A end of the anode wire is connected with the filter circuit and the positive high voltage, and the B end is directly suspended.
In this example, the first and second boron-coated entrance windows have an effective area of 100mm x 100mm and a boron-coated thickness of 2-3 μm, in this example 3 μm. The circuit board for fixing the gold-plated tungsten wire to form the anode wire surface is an anode wire frame, the whole size of the circuit board is 162mm multiplied by 162mm, the diameter phi of 4 holes is 7mm on the periphery, the distance between the centers of the adjacent holes is 132mm, and the distance between the center of each hole and the edge of the circuit board is 15mm, so that the circuit board is used for fixing the anode wire surface. The circuit board for fixing the gold-plated tungsten wire to form the first reading wire surface or the second reading wire surface is a reading wire frame, the whole size of the circuit board is 162mm multiplied by 162mm, the diameter of 4 holes on the periphery of the reading wire frame is 7mm, the distance between the centers of adjacent holes is 132mm, and the distance between the center of each hole and the edge of the circuit board is 15mm, so that the reading wire frame is fixed. All parts and processes of the neutron detector adopt domestic equipment and products, and the localization of the neutron detector is realized without depending on the support of foreign products.
A first boron-coated entrance window,The first reading silk surface, the anode silk surface, the second reading silk surface and the second boron-coated entrance window are mutually parallel and fixed on a sealed detector shell, and the detector shell provides a sealed environment to ensure that the detection is carried out in a working gas atmosphere; the detector shell not only provides a detection environment and avoids detection from being influenced by the outside, but also has the function of preventing neutrons from escaping. The ground antenna of the delay block is in contact with the detector shell through a metal screw to be grounded, a double-layer O ring is adopted for sealing, and the U-shaped pipe and the bubble bottle are adopted for testing the sealing performance of the detector shell, so that the sealing performance of the detector shell is guaranteed. The working gases used in this example were Ar and CO2。
When the neutron detector is used, the first boron-coated entrance window and the second boron-coated entrance window are connected with negative high voltage, the anode filament surface is connected with positive high voltage, neutrons are converted into secondary charged particles such as alpha and 7Li after passing through a 10B layer of the boron-coated entrance window, and the secondary particles are ionized in gas to generate electronic signals. Two reading filament surfaces, namely cathode filament surfaces, are read at two ends respectively, and the reading filament surfaces of the two gold-plated tungsten filaments which are vertical to each other are defined as an X axis and a Y axis of a plane respectively, so that the incident position of neutrons can be determined. That is, one neutron detection unit outputs five signals in total, one is a time signal of an anode wire as a start time T signal, and the other four are x1, x2, y1 and y2 of a cathode wire as position signals. The T signal is a common time signal and the location of the neutron incidence is determined by recording the time difference with the time signal on the other cathode. In one implementation mode of the application, five paths of signals are amplified by using a fast time preamplifier, and the pre-amplified signals are screened by a constant ratio timer and then enter a TDC (time division multiplexing) for collection; the position of the incident neutron is thus determined by calculating the time difference between the position signal and the door open signal.
In the preamplifier of the embodiment, two preamplifier boards are buckled on a preamplifier main board, one surface of the preamplifier board, which is spliced with a detector, is a 'B surface', and the other surface of the preamplifier board, which is spliced with the detector, is an 'A surface'; the connection of the B surface is realized through 8 LEMO joints, and the model is EPB.00.250. NTN; the motherboard size was 279.4mm by 152.4mm, and the relative size of the 8 LEMOs was: the distance between the left and right rows of LEMO is 150mm, and the distance between the upper and lower adjacent LEMO rows in the same row is 8 mm. The scheme of the front-placed mainboard shielding case is as follows: the shield is designed as a rectangular parallelepiped of about 359.4mm by 232.4mm by 100 mm. The shielding shell is required to have: the diameter of the 5 fixing holes is 1.6mm, the fixing holes are used for fixing the main board in the shielding cover, the diameter of the 8 LEMO fixing holes is 6.8mm, the LEMO heads are used for extending out of the base of the shielding cover, and all cables of the front amplification main board comprise power lines, signal lines and RJ45 network lines and are led out from a larger circular opening of about 5cm, which is formed in the surface A.
On the filament surface of the neutron detector, except the anode filament surface, the reading filament surface is acted by the electrostatic attraction force of the anode filament surface, and the gap between the reading filament surface and the anode filament surface in the central area of the filament chamber tends to become narrow in the large-scale filament chamber, so that the non-uniformity of the electric field intensity is caused, and the non-uniformity of the gain is further caused. Therefore, the maximum perturbation experienced by a gold-plated tungsten wire is closely related to its tension. The influence of the diameter and the tension of the anode wire on the gain of the detector is simulated and calculated by adopting a Garfield software package, and the result is shown in fig. 3 and 4, wherein fig. 3 shows the influence of the diameter of the gold-plated tungsten wire on the gain, specifically, the gold-plated tungsten wires with the diameters of 20 mu m, 25 mu m and 30 mu m are calculated, the abscissa is the high voltage of the anode wire, and the ordinate is the gain of the detector; FIG. 4 is a graph of the effect of tension on gain for a 25 μm diameter gold plated tungsten wire, with the wire tension on the abscissa and the detector gain on the ordinate. The results show that the larger the effect of the gold-plated tungsten wire on the gain with increasing diameter, considering the 25 μm diameter of the anode wire used in this example. The results in fig. 4 show that the higher the tension applied to the gold-plated tungsten wire, the lower the gain, but too low the tension will cause the gold-plated tungsten wire to sag under the influence of gravity, which affects the performance of the detector, and the experimental result shows that the ultimate tension of the gold-plated tungsten wire with a diameter of 25 μm is 100g, and 30g is selected in this example in order to reduce the possibility of wire breakage caused by vibration. In addition, the selection of the working gas of the detector has certain influence on position resolution, and the currently common mixed gas mainly comprises Ar and CO2。
In the neutron detector of the embodiment, the propagation time of signals is required, the delay time of cables with different lengths is different, and the propagation speed of the electric signals in the coaxial cable is about 5ns/m in general. According to the characteristic, all the readout strips of the detector can be connected together one by one through a delay unit with fixed delay time, induction signals on all the readout strips on a readout plane after electron avalanche amplification are overlapped together after delay and simultaneously propagate to two directions, an anode signal is used as common trigger, the time difference of the propagation of the readout plane induction signals to two ends is recorded, then the equivalent distance is converted, the position of the electron avalanche can be obtained, and the incident position of particles is further obtained. The embodiment adopts 1507-50C Delay block chips produced by Data Delay Device company, each Delay chip has ten Delay units, the Delay time of each Delay unit is 5ns, the equivalent inductance is L ═ 1 muH, the equivalent capacitance is C ═ 25pF, one Delay unit has 14 antennae, wherein, 10 are connected with the readout strip/wire, one input or output, one is suspended, the last two are grounding antennae, in the design of the detector, the grounding is realized by the contact of metal screws.
Calibration of delay unit
The delay line positioning method essentially converts position information into time measurement, so that the scale of the corresponding relationship between delay time and position is very important. Before the detector is assembled, the delay time difference and the position relation are calibrated by generating a rectangular pulse by a signal generator. The difference in arrival times of the signals at the two ends of the delay block is measured for each read channel. The pulse frequency of the signal generator is 1kHz, the amplitude is 5mV, the width is 100ns, and the rise time is 5 ns. The X direction has 50 delay units, and the scale relation of delay time difference-distance is that X is 0.095t + 25.17; the Y direction is 100 delay units, the scale relationship of delay time difference-distance is Y equal to 0.0829t +51.38, where t is the signal time difference between two ends of the delay block, as shown in fig. 5, the abscissa is the signal time difference between two ends of the delay block, and the ordinate is the channel position.
The delay block reading mode is adopted, the position information is mainly determined by the time difference of the measurement signals after passing through the delay block, and the measurement of the arrival time of the signals is very important. In this example, 3 delay blocks are arbitrarily selected, and the influence of the delay blocks on the rising edge of the signal is tested, as shown in fig. 6, the abscissa is the rising edge of the input signal, and the ordinate is the delay time of the delay blocks on the signal.
Testing of detector performance
The detector adopts matched Ortec charge pre-amplification and main amplification, the amplification factor is 100, the integration time is about 1 mu s, and the signal of the anode wire is tested to judge whether the detector works normally and whether the avalanche phenomenon is generated near the anode wire. The result shows that the amplitude of the anode wire signal is about 200mV, and the rising edge is about 4 mus.
The counting rate plateau curve is a curve of the counting rate of the detector along with the change of the high voltage under the same flux of the incident particles, the test result of the embodiment is shown in fig. 7, the abscissa is the anode wire high voltage, the ordinate is the counting, and as a result, the whole plateau length is 300V, and the working range of the anode wire is 1950V-2250V.
An anode wire signal of the detector is firstly amplified by a charge sensitive preamplifier, the amplified signal directly enters an oscilloscope, a waveform peak value is obtained by an oscilloscope acquisition program based on LabVIEW, the gain and energy resolution of the detector can be obtained, and a test result shows that the gain of the detector is within 10 within the optimized voltage given by a plateau curve3～104As shown in fig. 8, the abscissa is anode wire high voltage, and the ordinate is detector gain; the energy resolution is about 23%, as shown in fig. 9, the abscissa is the anode wire high voltage, and the ordinate is the energy resolution of the detector.
In order to reduce the material content in the detector, the cathode window is temporarily replaced by gold-plated tungsten wire, fig. 10 shows the effect of the cathode high voltage on the detector, the abscissa shows the cathode high voltage, and the ordinate shows the energy resolution of the detector. Along with the increase of negative high voltage of the cathode, the energy resolution is gradually stable, meanwhile, when the negative high voltage of the cathode is 0, the original electrons still can drift to the vicinity of the anode wire under the strong action of the field to generate an avalanche phenomenon, but the signal-to-noise ratio is poor, an escape peak and a full energy peak cannot be completely separated, and the detector has good prospect on the whole.
Position resolution is one of the most important indexes of the detector, and whether the position resolution is good or not is the key for checking whether the reading method is feasible or not. In order to improve the position resolution of the detector, an oscilloscope Lecroy Wavepro7100 with a high sampling rate and a set of oscilloscope data acquisition system based on LabVIEW are adopted, and the arrival time of a signal is acquired through a constant ratio discriminator.
To measure the position resolution of the detector, a collimating slit with a width of 0.3mm, a length of 10mm and a depth of 10mm was made.55The Fe X-ray enters the single-layer neutron detector through the slit, and the position resolution of different positions in the X and Y directions is measured, as shown in fig. 11 and 12, the abscissa is the incident position, and the ordinate is the measurement position and the position resolution. The results show that the detector has better position resolution in the middle and slightly worse ends, but both are within 2 mm. And the detector has good position linearity. It should be noted that if a set of dedicated electronics is configured to reduce the noise of the detector, the position resolution will be further improved; for example, all the currently used preamplifiers are commercially available, the internal resistance and capacitance of the preamplifiers are fixed, the preamplifiers cannot be completely matched with the characteristic impedance of the delay block unit of the commercially available delay block chip, and the signal of the delay block is attenuated to a certain extent; therefore, if specially designed, the factors such as the characteristic impedance and cut-off frequency of the delay block unit can be taken into consideration, the loss of the preamplifier to the signal of the delay block unit can be reduced to the maximum extent, and the resolution can be improved.
The main purpose of a two-dimensional position-sensitive detector is to obtain a two-dimensional distribution image of incident particles. Two-dimensional imaging capability is also a key indicator of detectors. In order to examine the two-dimensional imaging capability of the detector, a regular pattern is manufactured in the example, the width of the slit is 0.5mm, the distance between the slits is 5mm, and the regular pattern passes through55The reconstructed two-dimensional image can be clearly seen by the uniform irradiation of the Fe X-ray, as shown in fig. 13. Fig. 13 shows a prepared regular pattern real object picture in the lower left corner, and a two-dimensional pattern reconstructed by the neutron detector in this example in the upper right corner; therefore, the neutron detector of the embodiment has good two-dimensional imaging capability, and the feasibility of a delay line reading scheme is verified.
The foregoing is a more detailed description of the present application in connection with specific embodiments thereof, and it is not intended that the present application be limited to the specific embodiments thereof. For those skilled in the art to which the present application pertains, several simple deductions or substitutions may be made without departing from the concept of the present application, and all should be considered as belonging to the protection scope of the present application.
Claims (4)
1. A method for neutron detection based on time difference is characterized in that: comprises the following steps of (a) carrying out,
(1) reacting the incident neutron flow by using the boron layer to generate secondary particles;
(2) the secondary particles are ionized in the working gas to generate electrons;
(3) under the action of an electric field generated by the anode filament surface, electrons drift to the anode filament surface and an avalanche phenomenon occurs near the anode filament surface;
(4) the upper surface and the lower surface of the anode filament surface are respectively provided with a parallel readout filament surface, the upper readout filament surface and the lower readout filament surface are respectively composed of a plurality of readout filaments which are parallel to each other, the readout filament direction of the upper readout filament surface is parallel to the anode filament direction of the anode filament surface, the readout filament directions on the two readout filament surfaces are vertical, every two readout filaments on each readout filament surface are a readout channel, each readout channel is provided with a delay block unit, the delay block units on each readout filament surface are respectively led out by two paths of signals after being connected in series, and the upper readout filament surface and the lower readout filament surface are totally four paths of signals; respectively defining an upper readout filament surface and a lower readout filament surface as an X direction and a Y direction, taking a time signal of an anode filament surface as a starting signal T, taking four-way signals X1, X2, Y1 and Y2 of the upper readout filament surface and the lower readout filament surface as position signals, wherein the T signals are common time signals, and determining the incident position of a neutron by recording the time difference between the time signals on the upper readout filament surface and the time signals on the lower readout filament surface;
the method also comprises the steps of amplifying a time signal of the anode filament surface and four paths of signals of an upper read filament surface and a lower read filament surface by adopting a fast time preamplifier, screening by adopting a constant ratio timer, and then performing TDC acquisition;
the electronic system of the preamplifier adopts a structural mode of a mother-son board, two preamplifier boards are buckled on a main board, one surface of the preamplifier board, which is spliced with the detector, is a 'B surface', and the other surface of the preamplifier board, which is spliced with the detector, is an 'A surface'; the connection of the B surface is realized through 8 LEMO joints, and the model is EPB.00.250. NTN; in the 8 LEMO joints, the distance between the left and right rows of LEMO joints is 150mm, and the distance between the upper and lower adjacent LEMO joints in the same row is 8 mm; the mainboard is shielded by a cuboid shielding case, and the shielding case is provided with 5 holes for fixing the mainboard and holes for 8 LEMO connectors to extend out of a base of the shielding case; all cables of the mainboard comprise power lines, signal lines and RJ45 network cables, and the cables are led out from a circular opening which is arranged on the surface A and is about 5 cm;
the working gas is argon and carbon dioxide at one atmosphere.
2. The method of claim 1, wherein: the reading wires adopted by the upper reading wire surface and the lower reading wire surface are gold-plated tungsten wires with the diameter of 25 mu m, and the distance between the reading wires is 1 mm.
3. The method of claim 1, wherein: the anode wire adopted on the anode wire surface is a gold-plated tungsten wire with the diameter of 25 mu m, and the distance between the anode wires is 2 mm.
4. The method of claim 1, wherein: the delay block units adopted by the upper readout silk surface and the lower readout silk surface are respectively provided by a delay block chip, and the delay block chip is composed of a plurality of delay block units connected in series.
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