JP2009115818A - Cold neutron image detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a neutron image detector excelling in position resolution of 500 microns or less and position uniformity of neutron sensitivity. <P>SOLUTION: The neutron image detector with high neutron detecting position resolution is achieved by constituting a liquid helium-3 detector for detecting a neutron by filling liquid helium-3 in a pressure-proof chamber that can be cooled to a low temperature and detecting a proton or a triton discharged when helium-3 catches the neutron, by a cold corpuscular beam detecting element arranged in the pressure-proof chamber, wherein the liquid helium detector is operated at an operation temperature T of 3.19 K or below, and its depletion layer width is set to 5.8 microns or less when using an InSb semiconductor detecting element as the cold corpuscular beam detecting element, and set to 7.3 microns or less when using an Si semiconductor detecting element as the cold corpuscular beam detecting element. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a neutron detector using helium 3 cooled to a low temperature and a neutron image detector having a high position resolution. The present invention relates to a neutron detector that can be used even at a low temperature, and also relates to a neutron image detector excellent in high neutron detection position resolution and uniformity of neutron detection sensitivity. For this reason, this detector is used for neutron scattering experiments or neutron radiography that require high position resolution. It can also be used for neutron scattering experiments where it is necessary to cool the sample to a low temperature and measure the scattered neutrons at close range.

  Until now, as a neutron position detector that captures the incident position of neutrons in one or two dimensions, a wire chamber as shown in FIG. 1 in which a plurality of wires are stretched in helium 3 gas at about 10 atm has been developed. Generally used (for example, refer nonpatent literature 1). This wire chamber is used in many neutron scattering experiments due to its high detection efficiency, low background, high neutron / gamma ray discrimination, and high detection sensitivity uniformity. The position resolution of this wire chamber is usually about 2-3 mm at best.

  On the other hand, a microstrip type neutron detector as shown in FIG. 2 has been developed as a neutron gas detector with improved position resolution of the above-described wire chamber (see, for example, Non-Patent Document 2). In this detector, an anode and a cathode electrode of several tens of microns are stretched on an insulating substrate at a pitch of several hundred microns using a lithography technique, and a positional resolution of 0.4 mm is realized.

  In addition, attempts have been made to detect secondary particles with a wire electrode using liquid helium 3, but the gas amplification factor has not increased and has not been realized yet (for example, see Non-Patent Document 3).

V. Radeka et al. , BNLreport # 650032, E.M. Gatti et al, Nucl. Instrum. & Meth. 163, 83 (1979) A. Oed et al, Nucl. Instrum. & Meth. A263,351 (1988) Radiation, No. 26, 3 (2000)

  The position resolution of prior art neutron gas image detectors is still not sufficient. In order to achieve high position resolution, it is essential to shorten the range of secondary particles (protons and tritons) generated as a result of the neutron capture reaction as much as possible. Methods for shortening the range include increasing the sealing pressure and adding a gas having a high molecular weight. However, when detecting these particles using gas amplification, there is a major problem in reducing the gas amplification factor. It was difficult.

  Further, in order to reduce the range of the secondary particles to several tens of microns, helium 3 gas of about 1000 atm is required at room temperature, and the restriction due to the pressure resistance of the gas chamber is large and difficult to realize.

  In the present invention, helium 3 is cooled to a low temperature and used, the helium 3 nucleus density is increased without increasing the gas pressure, and the secondary particles to be discharged are detected by a particle detector that can be used at a low temperature. An object of the present invention is to provide a low-temperature neutron image detector provided with

  In order to achieve the above object, the present invention uses the following solutions.

  A cold particle beam in which helium 3 gas is enclosed inside a pressure-resistant chamber that can be cooled to a low temperature, and protons (p) or tritons (T) that are released when helium 3 captures neutrons are arranged inside the pressure-resistant chamber. A helium 3 gas detector that detects neutrons by detecting with a detecting element is constructed, and its operating temperature T is 3.19K or more and 100K or less, and the sealed helium 3 gas pressure satisfies the condition of P> T / 2.77. Operate with gas pressure.

  A neutron gas detector having a neutron sensitivity is constructed by enclosing liquid helium 3 gas inside the cryogenic particle detection element and a pressure-resistant chamber, and its operating temperature T is 3.19K or less. Make it work.

  An InSb semiconductor detection element and a Si semiconductor detection element are used as the low temperature particle detection element, and the depletion layer width is set to be equal to or less than the range of protons and tritons to remove background such as gamma rays.

  When helium 3 gas reservoir installed at room temperature is connected to the pressure resistant chamber, the gas pressure inside the pressure resistant chamber can be automatically adjusted to the set value, improving operability and maintaining low temperature Ensuring safety against pressure problems that are anticipated.

  By realizing a high density state of helium 3 gas using a low temperature environment, a high neutron detection position resolution of 500 microns or less can be obtained.

  In addition, when helium 3 in a gas state is used at the same time, the neutron image detector has a very uniform neutron detection efficiency.

  When liquid helium 3 is used, it is not necessary to adjust the gas pressure required in the gas state, and the neutron sensitivity of the detector is uniquely determined only by the operation of determining the operating temperature. A highly neutron image detector is realized.

  Since the depletion layer thickness of the semiconductor element used is sufficient and necessary for neutron detection, a high signal-to-noise ratio can be obtained and the neutron-to-background ratio can be improved.

  By pre-sealing a predetermined amount of gas in a reservoir tank with a predetermined gas resistance and capacity installed at room temperature, the detector can be detected at a predetermined operating condition only by the action of cooling the detector to the operating temperature. Since the instrument can be easily set, the operability is improved, the reliability and reproducibility of the neutron detection sensitivity is improved, and the safety can be ensured.

It is a figure which shows the conventional wire chamber type neutron detector. It is a figure which shows the conventional microstrip type | mold neutron detector. It is a figure which shows the neutron image detector (side view) which consists of helium 3 gas and the semiconductor detection element which can detect particle | grains. It is a figure which shows the neutron image detector (plan view) which consists of helium 3 gas and the semiconductor detection element which can detect particle | grains. It is a figure which shows the calculation result of the gas pressure dependence of the proton (574 KeV) range in helium 3 gas in the temperature range of 3.3K to 300K. It is a figure which shows the neutron image detector (side view) which consists of InSb semiconductor detection element which can detect helium 3 gas and particle | grains. It is a figure which shows the neutron image detector (plan view) which consists of helium 3 gas and the InSb semiconductor detection element which can detect particle | grains. It is a figure which shows the neutron signal measurement result in the gas pressure of 0.012 atm of the neutron image detector which consists of helium 3 gas and an InSb semiconductor detection element which can detect particles. It is a figure which shows the neutron wave height spectrum measurement result in the gas pressure of 0.012-12atm of the neutron image detector which consists of helium 3 gas and an InSb semiconductor detection element which can detect particles. It is a figure which shows the neutron wave height spectrum measurement result in the operating temperature of 1.6K of the neutron image detector consisting of the liquid helium 3 and the InSb semiconductor detection element which can detect particle | grains. It is a figure which shows the neutron image detector (side view) which consists of helium 3 gas and the semiconductor detection element group which can detect particle | grains. It is a figure which shows the neutron image detector (plan view) which consists of helium 3 gas and the semiconductor detection element group which can detect particles. It is a figure which shows the calculation result by the length of the detection element one side and the filling rate of the distance between particle beam detection semiconductor elements. It is a figure which shows the calculation result of the proton range in an InSb semiconductor detection element. It is a figure which shows the calculation result of the proton range in Si semiconductor detection element. It is a figure which shows the block diagram of a superconducting tunnel junction element. It is a figure which shows the neutron imaging detector using the liquid helium 3 and a superconducting tunnel junction element. It is a figure which shows the calculation result of the energy provided to the superconducting tunnel junction element of a proton (574 keV) and a Triton (191 keV). It is a figure which shows the neutron image detector (side view) which consists of helium 3 gas and a superconducting tunnel junction detection element group which can detect particle | grains. It is a figure which shows the neutron image detector (plan view) which consists of helium 3 gas and a superconducting tunnel junction detection element group which can detect particles. It is a figure which shows the calculation result of the distance between superconducting tunnel junction detection elements. It is a figure which shows the calculation result of the tank capacity | capacitance required for a neutron image detector at the time of using helium 3 gas. It is a figure which shows the calculation result of the tank capacity | capacitance required for the reservoir tank of a neutron image detector at the time of using liquid helium.

First, the operation principle of the neutron detector of the present invention will be described. FIG. 3 shows a side view of the neutron detector according to the present invention, and FIG. 4 shows a plan view. A cold particle beam detection element is installed in a pressure-resistant gas chamber that can be cooled, and helium 3 is enclosed in the space. Neutrons are captured by the nuclear reaction ³ He (n, p) T reaction with helium 3, and protons and tritons that are secondary particles are ejected in opposite directions. Neutrons are detected by detecting either one of the emitted protons and tritons that enter the low-temperature particle beam detection element. When the cold particle beam detection elements are arranged as an array, the neutron incident position is specified by determining which position the detection element is incident on.

  Since the position resolution of this detector is determined by the range of protons having a long range among the secondary particles, reducing this range is the key to realizing high position resolution. Since the range of protons (574 keV) in helium 3 at room temperature and 1 atm is 54 mm, a gas pressure as high as 108 atm is required at room temperature in order to realize a range of 500 microns, that is, a positional resolution of about 500 microns. It becomes. It is very difficult to manufacture a gas chamber that maintains such a high gas pressure.

  Here, when the gas pressure is P, the volume is V, and the temperature is T, it is known that the ideal gas follows Boyle-Charles' law (PV / T = constant). According to this equation, when a gas of the same pressure is sealed in a state where the temperature is 1/10, 10 times the amount of gas is sealed, and as a result, a nuclear density of 10 times is realized. Specifically, the gas density state realized at a gas pressure of 100 atm at normal temperature (300K) is realized at only 1.1 atm at 3.3K. A gas chamber that can withstand such a gas pressure can be easily manufactured. Helium 3 is a rare gas having a boiling point of 3.19 K, a critical temperature of 3.3 K, and a critical pressure of 1.1 atm. Therefore, helium 3 exists in a gas state even at a critical temperature or higher than the critical pressure.

The proton range R (mm) at each temperature T (K) when only helium 3 is filled is given by the following equation with the gas pressure being P (atm).
R = 54 / [P · 300 / T] (mm)
In order to achieve a position resolution of 500 microns or less at each operating temperature, the gas pressure P satisfies the following conditions.
P> 0.36T
FIG. 5 shows the pressure dependence of the range calculated at each temperature.

  Example 1 will be described with reference to FIG. 6 (side view) and FIG. 7 (plan view). The neutron detector of the present invention comprises a particle beam detection element capable of detecting particles at a low temperature, a pressure-resistant chamber, and helium 3 gas, and has an operating temperature of 3.19K to 100K, and The operation is performed under the condition that the sealed helium 3 gas pressure is set to a pressure equal to or higher than the value obtained by multiplying the operating temperature (unit: K) by 0.36.

The size of the InSb element used as the particle beam detection element is 5 × 7 × 0.4 tmm ³ , and the element is fixed by In solder on a Cu plate having a thickness of 1.0 mm. This element was fixed in a SUS304 chamber having a pressure resistance of 15 atm by stycast. The InSb element used was manufactured using a non-doped InSb wafer manufactured by Sumitomo Electric. A rectifying electrode was formed by an Au / Pd layer mesa having a diameter of 3 mm and a thickness of 4 nm formed on the InSb element, and the element and the signal line were connected by a gold wire. This element was operated with no bias because of noise caused by leakage current. The thickness of the neutron absorption layer formed by the chamber and the surface of the InSb element is ˜3.6 mm. The entire chamber was immersed in liquid helium, and the InSb element and helium 3 gas were cooled to 4.2K. In this case, the gas pressure that achieves a position resolution of 0.5 mm was calculated to be 1.5 atm or more.

In the JAERI remodeled reactor No. 3 T1-4 port (maximum neutron flux 10 ⁵ n / cm ² / s), this detector was irradiated with thermal neutrons, and the neutron detection characteristics were examined. Neutrons were incident from the back of the InSb element having a thickness. A signal from the element was taken out to the outside by a signal line passing through a gas pipe having a length of about 1000 mm and connected to a preamplifier (Camberra 2003BT) installed at room temperature.

  FIG. 8 shows the output signal pulse waveform of the preamplifier observed when the gas pressure of 0.012 atm is sealed. It was confirmed that the InSb element satisfactorily captured secondary particles at a rise time of ~ 80 nsec.

  The output signal from the charge amplifier was waveform-shaped with a shaping amplifier (Camberra 2001) with a time constant of 1.5 microseconds, and the wave height distribution was measured with MCA. FIG. 9 shows the result of measuring the change in the pulse height spectrum when the gas pressure is changed from 0.012 to 12.4 atm. Protons could be clearly captured at any gas pressure.

  As a result, it was confirmed that neutrons (protons) could be detected at a gas pressure satisfying the condition of P> 0.36 × T at a temperature of 4.2 K.

  Example 2 is characterized in that a detector is composed of a particle beam detection element capable of detecting particles at a low temperature, a pressure-resistant chamber, and liquid helium, and is operated at an operating temperature T of 3.19K or less. This is related to the neutron detector.

  A neutron detector similar to the structure shown in FIGS. 6 and 7 was used, and the entire detector was cooled to 1.6K. Thereafter, helium 3 in a liquid state was introduced into the chamber to try to detect neutrons.

  FIG. 10 shows a wave height spectrum when an external pressure of 0.92 atm is applied to the liquid helium 3. From this, it was confirmed that neutrons (protons) can be detected well even when helium-3 liquid is used as a neutron converter.

  Example 3 relates to a low-temperature neutron image detector that uses an array detection element composed of two or more elements as a low-temperature particle beam detection element and detects the incident position of neutrons.

  FIG. 11 shows a detection element group (side view) in which two or more low-temperature semiconductor detection elements are arrayed, and FIG. 12 shows a plan view.

It is essential that the length L of one side of the detection element is smaller than the range R of protons generated as a result of the neutron capture reaction. Therefore, it is assumed that the length of one side of the element satisfies L <R. Here,
L is determined from the condition of L <500 microns.

The distance d between the elements is determined by L determined based on the above conditions and the effective area occupied by the entire detector.
Now, the effective area ratio x (x <1) is expressed as x = L ² / (L + 2d) ²
If d is defined, d is given by the following equation using x.
d = L / 2 · (1-a) / a
Here, a = x ^0.5 .

  FIG. 13 shows a calculation result of the inter-element distance d required when the effective area ratio is set with respect to the size L of one side of the element. From these equations and drawings, the size and arrangement interval of the array detection elements to be arranged can be determined.

  As an example, when the operating temperature of the neutron detector is 4.2 K and the enclosed gas pressure is 2 atmospheres, the proton range is 372 microns. Therefore, when the size of one side of the array detection element is 300 microns and the detection elements are arranged in an array at intervals of 43.6 microns, a low-temperature neutron image detector having an effective area ratio of 0.6 can be manufactured.

  Example 4 relates to a low-temperature neutron detector characterized in that an InSb semiconductor detection element is used as a particle beam detection element usable at a low temperature, and the depletion layer width is 5.8 microns or less.

  In the neutron detector, radiation other than neutrons (for example, X-rays, gamma rays, etc.) is preferably insensitive to these because it becomes noise in the background. On the other hand, it is important that protons generated by neutron absorption give the total energy in the semiconductor element and increase the signal-to-noise ratio. From this point of view, it can be seen that there is an optimum value for the depletion layer thickness of the semiconductor detection element to be used.

  Since the radiation absorption efficiency is proportional to the thickness of the depletion layer, the thickness of the depletion layer of the semiconductor detection element is preferably as thin as possible from the viewpoint of reducing the background. Therefore, the thickness of the depletion layer of the semiconductor element group needs to be equal to or less than the thickness at which protons generated by the neutron capture reaction impart all energy in the semiconductor element.

  FIG. 14 shows the proton energy dependence of the proton range in the InSb device. From this, it can be seen that the gamma ray background can be reduced by setting the depletion layer of the InSb element to 5.8 microns or less from the range in which the proton (574 keV) generated by the neutron capture reaction imparts the total energy. .

  Example 5 relates to a low-temperature neutron detector characterized in that a Si semiconductor detection element is used as a particle beam detection element usable at a low temperature, and the depletion layer width is 7.3 microns or less.

  In addition, in the neutron detector, radiation other than neutrons (for example, X-rays and gamma rays) is preferably insensitive to these because they become noise in the background. On the other hand, it is important that protons generated by neutron absorption give the total energy in the semiconductor element and increase the signal-to-noise ratio. From this point of view, it can be seen that there is an optimum value for the depletion layer thickness of the semiconductor detection element to be used.

  Since the radiation absorption efficiency is proportional to the thickness of the depletion layer, the thickness of the depletion layer of the semiconductor detection element is preferably as thin as possible from the viewpoint of reducing the background. Therefore, the thickness of the depletion layer of the semiconductor element group needs to be equal to or less than the thickness at which protons generated by the neutron capture reaction impart all energy in the semiconductor element.

  As a Si semiconductor detector that can be used at a low temperature, a low temperature compatible ULTRA / Si detector series manufactured by ORTEC, USA can be used.

  FIG. 15 shows the proton energy dependence of the proton range in the Si device. From this, it can be seen from the range in which the proton (574 keV) generated by the neutron capture reaction imparts the total energy, the gamma ray background can be reduced by setting the Si element to 7.3 microns or less as a depletion layer.

  Example 6 relates to a low-temperature neutron detector characterized by using a superconducting tunnel junction element as a low-temperature particle detection element.

  The superconducting tunnel junction element is a radiation detector operating at a low temperature, and has a sandwich structure in which two superconductors are sandwiched by a thin insulating film having a thickness of about 1 nanometer as shown in FIG. The radiation absorbed in the superconductor generates a large number of signal electrons there, and these electrons tunnel between the superconducting films, thereby inducing a signal between the two superconductors. This is the radiation detection principle. Usually, niobium having a superconducting critical temperature as high as 10K is used as the superconducting film. Therefore, in principle, it operates at a temperature of 10 K or less, which is the critical temperature of niobium, but it is usually operated at a temperature of 1/2 to 1/3 or less from the viewpoint of thermal noise.

As an example, it is assumed that a superconducting tunnel junction element having a size of 178 microns square designed with a thickness of two niobium films of 200 nm and a critical current density of 100 A / cm ² is used. As shown in FIG. 17, the device is installed in a cryostat, cooled to 0.4 K, and suppresses the Josephson current flowing independently of the X-ray signal. Used by applying a magnetic field.

  FIG. 18 shows the result of calculating how much energy is imparted to the superconducting film by protons and triton generated as a result of neutron absorption in helium 3. In the figure, the horizontal axis represents the thickness of the niobium superconducting film, and the vertical axis represents the energy applied by protons or tritons. From this figure, if the thickness of the niobium film is 100 nm or more, the amount of energy imparted by protons and tritons is 6 keV or more, which is a value of one digit or more of the detection limit, so that protons and tritons can be sufficiently detected. Recognize. Therefore, it can be used for neutron detection.

  In the above, an example of a superconducting tunnel junction element having a SIS type (superconductor / insulating film / superconductor) structure has been described. However, superconducting of a NIS type (normal conductor / insulating film / superconductor) structure is described. The same can be done with a tunnel junction element.

  Example 7 relates to a low-temperature neutron image detector characterized by detecting an incident position of neutrons using an array detection element composed of two or more superconducting tunnel junction elements as a low-temperature particle beam detection element. Is.

  FIG. 19 is a side view of a detection element group in which two or more low-temperature particle beam detection elements are arrayed, and FIG. 20 is a plan view.

It is essential that the length L of one side of the detection element is smaller than the range R of protons generated as a result of the neutron capture reaction. Therefore, it is assumed that the length of one side of the element satisfies L <R. Here,
L is determined from the condition of L <500 microns.

The distance d between the elements is determined by L determined based on the above conditions and the effective area occupied by the entire detector.
Now, the effective area ratio x (x <1) is expressed as x = L ² / (L + 2d) ²
If d is defined, d is given by the following equation using x.
d = L / 2 · (1-a) / a
Here, a = x ^0.5 .

  FIG. 21 shows the calculation result of the inter-element distance d required when the effective area ratio is set with respect to the size L of one side of the element. From these equations and drawings, the size and arrangement interval of the array detection elements to be arranged can be determined.

  As an example, when the operating temperature of the neutron detector is 4.2 K and the enclosed gas pressure is 2 atmospheres, the proton range is 372 microns. Therefore, when the size of one side of the array detection element is 300 microns and the detection elements are arranged in an array at intervals of 43.6 microns, a low-temperature neutron image detector having an effective area ratio of 0.6 can be manufactured.

  In Example 8, a helium 3 gas reservoir installed at room temperature is connected to a pressure-resistant chamber, and the gas pressure inside the pressure-resistant chamber can be automatically adjusted to a set value. The present invention relates to a detector or a cryogenic neutron image detector.

Consider a case where the sealed helium 3 is in a gas state. Volume V _{res of the} reservoir tank to be installed at room temperature when the pressure when operating the neutron image detector having the capacity V ₁ (cm ³ ) is P ₁ (atm) and the temperature is T ₁ (K) And the pressure _Pres are given by the following equations from Boyle-Charles law.
V _res = (300 / T ₁ ) (P ₁ / P _res ) V ₁
Here, _Pres is determined by the pressure resistance of the reservoir tank. FIG. 22 shows the reservoir tank capacity V _res normalized with the detector capacity V ₁ calculated for the set pressure P _{1 of the} helium 3 for operating the detector when the pressure resistance of the reservoir tank is 10 atm. Show. Thus, if the required operating conditions of the neutron image detector can be determined by using the above equation, the capacity of the reservoir tank can be determined using the same drawing and equation.

As an example, when the pressure when the neutron image detector having a capacity of 1 cm ³ is operated is 2 atmospheres, the temperature is 4.2 K, and the pressure resistance of the reservoir tank is 10 atmospheres, a reservoir tank having a capacity of 60 cm ³ is connected and the temperature is low. It was confirmed that the neutron image detector was automatically adjusted to the required operating gas pressure by cooling to 4.2K.

The reservoir tank capacity when liquid helium 3 is sealed can be calculated in the same manner. In this case, it is simplified by setting the density of the liquid helium 3 to 0.08 (g / cm ³ ), and is calculated by the following formula.
V _res > 664 (V ₁ / P _res )
FIG. 23 shows the result of calculating the reservoir tank capacity V _res normalized by the detector capacity V ₁ as a function of the pressure resistance of the reservoir tank according to the above equation.

  From these results, it was confirmed that if the operating temperature and operating pressure of the neutron image detector according to the present invention can be determined, the minimum required reservoir tank capacity can be calculated by determining the gas pressure resistance.

As examples, the neutron image detector according to capacity 66.4Cm ³ liquid helium 3, if the withstand voltage of the reservoir tank is set to 10 atm, capacity connects the reservoir tank 66.4Cm ³ cold neutron image detector 3. It was confirmed that the operating gas pressure was automatically adjusted to the required level by cooling to 19K or lower.

Claims (4)

  Liquid helium 3 is sealed inside a pressure-resistant chamber that can be cooled to a low temperature, and protons or tritons released when helium 3 captures neutrons are detected by a low-temperature particle beam detector arranged inside the pressure-resistant chamber. Thus, a liquid helium 3 detector for detecting neutrons is constructed, the operating temperature T is operated at 3.19 K or less, an InSb semiconductor detection element is used as the low temperature particle beam detection element, and the depletion layer width is 5 Low temperature neutron image detector characterized by being 8 microns or less.   Liquid helium 3 is sealed inside a pressure-resistant chamber that can be cooled to a low temperature, and protons or tritons that are released when helium 3 captures neutrons are detected by a low-temperature particle beam detector arranged inside the pressure-resistant chamber. A liquid helium 3 detector for detecting neutrons, operating at an operating temperature T of 3.19 K or less, using a Si semiconductor detector as the low temperature particle detector, and a depletion layer width of 7. A cryogenic neutron image detector characterized by being 3 microns or less.   The low-temperature neutron image detector according to claim 1 or 2, wherein an array detection element composed of two or more elements is used as the low-temperature particle beam detection element to detect the incident position of neutrons.   The helium 3 gas reservoir installed at room temperature is connected to the pressure resistant chamber so that the gas pressure inside the pressure resistant chamber can be automatically adjusted to a set value. The cryogenic neutron image detector according to claim 1.

Images