CN106970412A - A kind of MCP neutron detectors based on polyethylene - Google Patents

Abstract

The invention relates to a neutron detector for measuring neutron time spectrum in an ultrafast pulsed neutron radiation field, in particular to a method for generating secondary electrons in an MCP by using the recoil proton incidence generated by the reaction of neutrons and polyethylene And multiply the output pulse current signal, so as to realize the detector for neutron measurement. It is used to overcome the defect of slow time response of neutron detectors in the prior art. It includes an outer cylinder, a high-voltage cable head, a signal cable head, an incident window sealed and fixed on the front surface of the outer cylinder, an exit window sealed and fixed on the rear end surface of the outer cylinder, and a switch that is sequentially fixed in the outer cylinder along the propagating direction of incident neutrons. Target, secondary electron multiplier and ceramic sheet, the conversion target is a polyethylene sheet, the secondary electron multiplier is a number of MCP sheets stacked, and the two sides of the secondary electron multiplier are respectively loaded with the high-voltage cable head The cathode and anode of the signal cable head are arranged on the outside of the ceramic sheet.

Description

A MCP neutron detector based on polyethylene

technical field

The invention relates to a neutron detector for measuring neutron time spectrum in an ultrafast pulsed neutron radiation field, in particular to a method for generating secondary electrons in an MCP by using the recoil proton incidence generated by the reaction of neutrons and polyethylene And multiply the output pulse current signal, so as to realize the detector for neutron measurement.

Background technique

When a high-energy electron beam of ps level is incident on a thick metal target, neutrons with a pulse width of ns level or even sub-ns level will be generated, which has the characteristics of low neutron intensity and wide neutron energy distribution range. Through inertial confinement fusion (ICF), ultrafast neutrons with pulse widths in the sub-ns range can also be produced. In order to measure ultrafast pulsed neutrons, neutron detectors are required to have ultrafast time response and high neutron sensitivity.

At present, the commonly used detectors for measuring pulsed neutrons include scintillation film detectors, scintillation fiber detectors and slit fission detectors.

Scintillation film detectors and scintillation fiber detectors both detect neutrons by using the recoil protons produced by neutrons in the scintillator to excite the scintillator to emit light. The time response of the photomultiplier tube used is also in the ns order, which determines that the time response of the detector is above ns. The slit fission detector uses PIN to detect the fission fragments produced by neutron-induced fission target fission to realize the detection of neutrons, and the time response of PIN is usually more than ten ns.

Contents of the invention

The purpose of the present invention is to provide a polyethylene-based MCP (microchannel plate) neutron detector to overcome the defect of slow time response of neutron detectors in the prior art.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A kind of MCP neutron detector based on polyethylene, its special feature is that it includes an outer cylinder, a high-voltage cable head, a signal cable head, an incident window arranged on the front surface of the outer cylinder, an exit window arranged on the rear end surface of the outer cylinder, and A conversion target, a secondary electron multiplier and a ceramic sheet are sequentially fixed in the outer cylinder along the propagating direction of the incident neutron, the conversion target is a polyethylene sheet, and the secondary electron multiplier is a plurality of stacked MCP The cathode and anode of the high-voltage cable head are loaded on both sides of the secondary electron multiplier device respectively, and the signal collection end of the signal cable head is arranged outside the ceramic sheet.

Further, the secondary electron multiplier device is one, two or three MCP chips.

Further, a splint and a strut are arranged inside the outer cylinder, the conversion target and the secondary electron multiplier device are fixed on the splint, and the splint is fixed on the exit window through the strut.

Further, the distance between the splint and the exit window is 40 mm, and the splint is made of polytetrafluoroethylene.

Further, the incident window and the exit window are circular flange structures, and the center is a thin window recessed inward.

Further, the signal cable head and the high-voltage cable head are sealed and fixed on the exit window, the number of the signal cable head is one, and the number of the high-voltage cable head is two.

Further, the outer cylinder is also provided with a vacuum port.

Further, the external cylinder is cylindrical in shape.

The beneficial effect of the present invention relative to prior art is:

1. The present invention has ultra-fast time response, about 100 ps. The MCP sheet used in the present invention has a time response of less than 100 ps, and the recoil protons produced by the polyethylene sheet have a movement time of less than 30 ps in the polyethylene sheet, which determines that the neutron detector of the present invention has an ultra-fast time response .

2. The present invention has higher fast neutron sensitivity. If two pieces of MCP sheets are used, the gain can reach 10 ⁶ , the cross-section of polyethylene sheet and neutron elastic scattering is larger, and the number of recoil protons produced is more, and the double recoil proton produced in the channel of MCP sheet The number of secondary electrons can reach 3-4, so the invention can realize higher fast neutron sensitivity, which can reach 10 ^-15 C·cm ² .

3. The invention is convenient for installation and replacement of polyethylene sheets and MCP sheets.

Description of drawings

Fig. 1 is the structural representation of embodiment;

Figure 2 is a partial enlarged schematic diagram of the conversion target, the secondary electron multiplier device and the ceramic sheet;

Fig. 3 is the relationship diagram of the secondary electron yield produced by a single recoil proton on the inner wall of the MCP sheet produced by neutrons of different energies acting on polyethylene sheets of different thicknesses through Monte Carlo simulation calculation;

Fig. 4 is the output proton yield relation graph that adopts Monte Carlo method simulation calculation to obtain different energy neutrons and different thickness polyethylene sheets to produce;

Fig. 5 is the relationship diagram of the secondary electron yield generated by the different energy neutrons of unit intensity acting on the inner wall of the MCP sheet with different thicknesses by using the Monte Carlo method simulation calculation;

Fig. 6 is a schematic diagram of the neutron sensitivity theoretical calculation results of this embodiment when two MCP sheets with a gain of 2.25×10 ⁶ are used;

Fig. 7 is the average time-of-flight schematic diagram of recoil protons in the polyethylene sheet obtained by Monte Carlo simulation;

Among them: 1-outer cylinder, 2-incidence window, 3-exit window, 4-strut, 5-splint, 6-polyethylene sheet, 7-MCP sheet, 8-signal cable head, 9-high voltage cable head, 10 -Vacuum suction port, 11-copper sheet 1, 12-copper ring, 13-ceramic sheet, 14-copper sheet 2, 15-neutron beam.

detailed description

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figures 1-2, the embodiment provides a polyethylene-based MCP neutron detector, including an outer cylinder 1, an incident window 2 sealed and fixed on the front end of the outer cylinder 1, and an exit window 2 sealed and fixed on the rear end of the outer cylinder 1. Window 3, pole 4 fixed on the exit window 3, splint 5 installed on the pole 4, polyethylene sheet 6, copper sheet 11, MCP sheet 7, copper ring 12, ceramics fixed on the splint 5 Sheet 13 and copper sheet 2 14, signal cable head 8 and high-voltage cable head 9 fixed on the exit window 3, vacuum suction port 10 arranged on the side of outer cylinder 1, copper sheet 11 and copper wire of the inner core of high-voltage cable head 9 Connection, the copper ring 12 is connected with the copper wire of the high-voltage cable head 9 shell, and the copper sheet 14 is connected with the copper wire of the signal cable head 8; the outer cylinder 1 is a cylinder made of stainless steel, with an outer diameter of Φ120mm, an inner diameter of Φ112mm, and a 83mm; the incident window 2 and the exit window 3 at the front and rear ends of the outer cylinder 1 are circular flange structures, the diameter of the flange is Φ150mm, and the center of the flange is a thin window with a thickness of 0.5mm and a diameter of 60mm; the support rod 4 is made of stainless steel, and the length 5cm, the number is 3, and two adjacent poles 4 are evenly installed on the exit window 3 at a distance of 120°; the splint 5 is a polytetrafluoroethylene plate with a diameter of Φ80mm and a thickness of 5mm, and the middle of the splint 5 is a diameter of Φ16mm or Φ21mm There are 6 round holes with a diameter of Φ2mm and 3 screw holes with a diameter of Φ0.5mm around the round hole. The splint 5 is installed on the pole 4, and the splint 5 is used to fix the polyethylene sheet 6 and copper sheet 11 , MCP sheet 7, copper ring 12, ceramic sheet 13 and copper sheet 14; the diameter of polyethylene sheet 6 is Φ16mm or Φ21mm, and the thickness is determined according to neutron sensitivity requirements; MCP sheet 7 has a channel diameter of 6 μm and a channel spacing of Two microchannel plates with a thickness of 8 μm, a thickness of 0.38 mm, a chamfer angle (the angle between the channel of the MCP sheet 7 and the vertical axis of the two ends of the MCP sheet) of 6°, and an outer diameter of Φ16 mm or Φ25 mm; copper sheet 11 and copper sheet 2 The diameter of 14 is Φ16mm or Φ25mm, and the thickness is 1mm; the outer diameter of copper ring 12 is Φ16mm or Φ25mm, the inner diameter is Φ12mm or Φ21mm, and the thickness is 1mm; the signal cable head 8 is used to lead the detection signal from the inside of the vacuum chamber to the chamber on the external signal reading instrument; the high-voltage cable head 9 is used to guide the high voltage from the high-voltage source outside the vacuum chamber to the MCP sheet 7 in the chamber; the inner diameter of the vacuum pumping port 10 is 16mm, and the distance between the center of the pumping port and the exit window 3 is 40mm, which is used to evacuate the inside of the vacuum chamber; a resistor with a resistance value of 50MΩ is also set between the copper ring 12 and the high-voltage cable head 9, and a resistance value is also set between the copper sheet 14 and the signal cable head 8 The resistance is 100kΩ; the outer cylinder 1 is sealed with the entrance window 2 and the exit window 3 through a rubber ring or a knife edge to ensure that the dynamic vacuum of the detector vacuum chamber is less than 1×10 ^-4 Pa; the inner surface of the outer cylinder 1 should be polished Minimizes outgassing on inner surfaces.

The working principle of the neutron detector is: the recoil protons produced by the reaction between the neutron beam 15 and the polyethylene sheet 6 are incident on the MCP sheet 7 to generate secondary electrons and multiply the output pulse current signal, thereby realizing neutron detection. Measurement.

Figure 3 is the secondary electron yield produced by a single recoil proton produced by neutrons of different energies acting on polyethylene targets of different thicknesses on the inner wall of the MCP sheet through simulation and calculation using the Monte Carlo method. It can be seen from the figure that a single recoil proton produced by a neutron with an energy of 0.2MeV to 0.25MeV produces the largest amount of secondary electrons on the inner wall of the MCP, about 3 to 4.

Fig. 4 is the outgoing proton yield produced by the interaction of neutrons with different energies and polyethylene with different thicknesses obtained through simulation and calculation by the Monte Carlo method. It can be seen from the figure that for polyethylene with a thickness of 0.1 mm to 1.5 mm, when the energy of incident neutrons is higher than a certain value, the output of protons does not change much with the energy of neutrons; for polyethylene with a thickness of 1.5 mm or more, The greater the energy of the incoming neutrons, the greater the yield of outgoing protons.

Fig. 5 is the secondary electron yield generated by neutrons of different energies per unit intensity acting on polyethylene targets of different thicknesses on the inner wall of the MCP sheet obtained by Monte Carlo simulation. It can be seen from the figure that when the thickness of the polyethylene target is less than 2.0mm, the yield of secondary electrons reaches a maximum when the neutron energy is a certain value; when the thickness of the polyethylene target is greater than 2.0mm, the yield of secondary electrons increases with increases with the increase of sub-energy.

Fig. 6 is the calculation result of the neutron sensitivity of the detector, which is obtained by using two MCP chips with a gain of 2.25×10 ⁶ . It can be seen from the figure that when the thickness of the polyethylene target is less than 2.0mm, the neutron sensitivity of the detector reaches the maximum when the neutron energy is a certain value; when the thickness of the polyethylene target is greater than 2.0mm, the neutron sensitivity of the detector increases with increases with neutron energy. When the thickness of the polyethylene target is greater than 1.0 mm, the sensitivity of the detector to neutrons with energy above 1 MeV is greater than 1.0×10 ^-15 C·cm ² .

Figure 7 shows the average flight time of recoil protons in polyethylene obtained by Monte Carlo simulation calculation. From the calculation results, it can be known that the average flight time of recoil protons is not greater than 30 ps.

Claims (8)

1. a kind of MCP neutron detector based on polyethylene, it is characterized in that: comprise urceolus, high-voltage cable head, signal cable head, the entrance window that is sealed and fixed on the front end face of urceolus, the exit window that is sealed and fixed on the rear end face of urceolus, And a conversion target, a secondary electron multiplier device and a ceramic sheet fixedly arranged in sequence along the propagating direction of incident neutrons in the outer cylinder, the conversion target is a polyethylene sheet, and the secondary electron multiplier device is stacked For several MCP sheets, the cathode and anode of the high-voltage cable head are respectively loaded on both sides of the secondary electron multiplier device, and the signal collection end of the signal cable head is arranged outside the ceramic sheet. 2. A polyethylene-based MCP neutron detector according to claim 1, characterized in that: the secondary electron multiplier device is one or two or three MCP sheets. 3. a kind of MCP neutron detector based on polyethylene according to claim 2, is characterized in that: also be provided with splint and strut in described outer cylinder, described conversion target and secondary electron multiplier device are fixed on On the splint, the splint is fixed on the exit window through the support rod. 4. A polyethylene-based MCP neutron detector according to claim 3, characterized in that: the distance between the splint and the exit window is 40 mm, and the splint is made of polytetrafluoroethylene. 5. A polyethylene-based MCP neutron detector according to claim 4, characterized in that: the incident window and the exit window are circular flange structures, and the center is a thin window recessed inward. 6. A kind of polyethylene-based MCP neutron detector according to claim 5, characterized in that: the signal cable head and the high-voltage cable head are sealed and fixed on the exit window, the number of the signal cable head is 1, The number of high-voltage cable heads is 2. 7. A polyethylene-based MCP neutron detector according to claim 6, characterized in that: the outer cylinder is also provided with a vacuum port. 8. A polyethylene-based MCP neutron detector according to claim 7, characterized in that: the external cylinder is cylindrical in shape.