What Is a Neutron Detector?

A neutron detector is a device that detects neutrons by using charged particles generated by the interaction of neutrons with boron or uranium to ionize a gas or activate the material itself after neutron irradiation. Neutron detectors are widely used in reactor nuclear power measurement or core neutron fluence rate measurement. ^[1]

A neutron is an uncharged particle. The working principle of a neutron detector is: When a neutron reacts with a certain nucleus, it emits charged particles. When the charged particles move in the gas, they ionize the gas. The level of the neutron fluence rate is determined by measuring the amount of gas ionization. For example, neutrons react with (n, ) of B to emit alpha particles; or neutrons react with U to generate fission fragments.

The following figure shows the working mechanism of gas ionization. The curve shows the relationship between the number of ions collected on the electrode and the voltage between the electrodes after the gas between the two electrodes is ionized by the ray. The I region indicates that the generated ions were partially recombined before being collected by the electrode. As the voltage increases, the probability of ion recombination becomes lower and lower. Region indicates that all the generated ions are collected by the electrode, which is called the saturated ionization current region. In a fairly wide range of voltage changes, the number of ions collected is only proportional to the ray density and is basically independent of the voltage, that is, a "ping" appears. The ionization chamber works within this "ping" range. The voltage continues to increase, and a considerable electric field strength is formed near the electrode. When the ions move toward the electrode, a relatively large amount of energy is obtained in the electric field, causing secondary ionization of the gas and forming a gas discharge. For example, in area , the output pulse remains proportional to the original total ionization at this time, which is called proportional counting area. A detector based on this characteristic is called a proportional counter tube. Flora is limited proportional area. The gas in zone is avalanche ionized, and the amplification factor is so large that the output pulse has nothing to do with the original total ionization. Each ionization, that is, each pulse, the number of ions collected by the electrode is a constant. A detector made using this feature is called a Geiger Miller counting tube. Region VI is a continuous discharge region.

When the neutron fluence rate is very small, it will be difficult to measure a small ionization current with an ionization chamber. In this case, the interference of gamma rays and residual radioactivity becomes more significant. The counting tube emits discontinuous pulses. Counting tubes are required to output a current pulse for each ionization event, they can be amplified, and their counting rate can be measured with a suitable counting rate meter. Because the signal is pulsed, the insulation problem of the "chamber" is not critical, and usually requires more than 10. The pulse height mainly depends on the capacitance and resistance of the system used by the counting tube. For the internal structure of the counting tube, it is best to minimize the capacitance and the distance between the electrodes for the range of the ionization event. To distinguish unwanted ionization events, a simpler method is to use pulse height discrimination.

(1) BF3 proportional counting tube

An electrode is formed by a metal circular tube, and an insulated thin wire is suspended along the central axis of the circular tube as another electrode, and the tube is filled with BF ₃ gas. Neutrons interact with alpha particles emitted by B to ionize the gas. The polarization voltage applied to the thin wire is quite high, about 3000 V, which can generate secondary ionization to increase the number of ion pairs generated and improve sensitivity. Generally, each neutron can produce 3 counts per square centimeter. This counting tube is the most sensitive of the commonly used pulse counting tubes. The pulse height generated by neutrons is about 100 times the pulse height generated by -rays, so the interference of background can be reduced by pulse height discrimination. The maximum counting rate of the counting tube is 5 × 10n / s, but the gamma ray intensity of its working environment is generally below 1 Sv / h. BF ₃ proportional counter can increase sensitivity by increasing the concentration of B. Figure 3 shows a sectional view of a proportional counter tube.

(2) Boron-coated proportional counting tube

The solid boron is coated on the wall of the counting tube. Its shell is made of aluminum, the shell is filled with argon, and an electrode with a thin wire is installed at the center. Connect it to a high-voltage power supply to obtain the higher electric field strength required for gas discharge. This kind of counting tube has a relatively long life, and it also recovers well after high neutron fluence rate irradiation. However, the boron-coated layer can absorb some alpha particles, affect the energy of the alpha particles entering the filling gas, and cause some fluctuations in the output pulse height, so the effect of pulse height discrimination is worse.

(3) Fission counting tube

The fission phenomenon of uranium can also be applied to the counting tube. The electrode of the counting tube is coated with uranium, and the tube is filled with an inert gas. After neutron irradiation, the debris produced by uranium fission is very high in energy and has sufficient ionization capacity to generate appropriate pulse amplitudes. When the capacitance is 100pF, the pulse amplitude is generally in the range of 0.1 to 1 mV. The alpha radioactivity of uranium can also cause background count rates. This effect can also be reduced with pulse height discrimination. The sensitivity of the fission counting tube is about 0.2 counts per unit neutron fluence rate. The important characteristic of the fission counter tube is that it can work in a strong gamma ray field (up to 10 Sv / h); and it can be designed to work at a temperature of up to 900 ° C. ^[2]

The ionization chamber is mostly a cylindrical parallel electrode, which is enclosed in a metal box or housing. The joint is insulated to minimize leakage. Typical resistance between the electrode and the box is on the order of 10 ₁₂ or higher. If the signal level is expected to be low, a shield structure can be used for insulation. This structure consists of an insulated shield ring or cylinder surrounding each wire. The shield ring remains equipotential to the corresponding electrode. The electrodes must be designed and assembled so that there is a uniform electric field in the sensitive area of the ionization chamber. An auxiliary electrode can be used to help achieve uniformity of the electric field.

(1) Boron ionization chamber

Used to measure thermal neutrons. Fill the ionization chamber with BF3 gas or apply boron to each electrode. In the latter case, the gas filled in the container is generally hydrogen. Both the container and the electrode are made of pure aluminum. After neutron irradiation, the neutron and boron undergo (n, ) reaction, and the particles produced have an energy of about 2.5 MeV, which can ionize hydrogen or BF ₃ to generate an electric current. The applied voltage of the ionization chamber is about 200 ~ 500V. The neutron sensitivity of the ionization chamber is 10- 10- A / (n / cm · s), and the sensitivity of -ray is 10- 10- A / (Sv / h). The neutron sensitivity of the ionization chamber can be adjusted by: selecting the area of the electrode coated with boron, selecting the pressure of the aerated body, and selecting the concentration of the B isotope.

(2) -ray compensation ionization chamber

One of the problems with the boron ionization chamber is its lack of selectivity. It can detect any ionizing radiation, so it may cause considerable measurement errors under strong gamma fields. The ionization chamber is divided into two equal parts, one part is sensitive to neutrons and gamma rays, and the other part only acts on gamma rays. If you try to make the currents in the two ionization chambers flow in opposite directions, the current obtained is only proportional to the neutron fluence rate. The volume compensation is not affected by changes in the gamma-ray energy spectrum and intensity, and its compensation degree is adjusted to 97% to 98% at the factory, and it cannot be adjusted again when in use. Another method is voltage compensation. By adjusting the voltage of the compensation electrode from the outside to change the distribution of power lines to change the effective volume of the two parts of the ionization chamber, the degree of compensation for gamma rays (called voltage compensation) is adjusted. The chamber can be extended by about two orders of magnitude.

(3) Long neutron ionization chamber

In large reactors, in order to measure the axial power imbalance of the core, a long neutron ionization chamber with a length equal to the core height is used. The interior is composed of two or more sections of boron ionization chambers, corresponding to the upper and lower halves of the core, and the measured signals are processed on the circuit to measure the axial power distribution.

(4) Fission ionization chamber

An electrode is arranged in a sealed stainless steel container filled with argon. Uranium is deposited on the electrode, and fission fragments generated by the interaction between neutron and uranium generate ionization in the ionization chamber. The lowest level of neutron fluence rate that can be measured in a fission ionization chamber is limited by spurious currents caused by the natural alpha decay of uranium at the electrode in the ionization chamber.