JPH0335636B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for selecting BF ₃ proportional counter tubes for detecting neutrons, and particularly to a method for selecting counter tubes with good radiation resistance.

BF ₃ proportional counters are widely used for low neutron flux measurements because they are highly sensitive. However, its life characteristics are not very good; simply irradiating the counter with radiation without putting it into operation will cause almost no deterioration of the characteristics, but when it is put into operation, the characteristics will deteriorate relatively quickly even if the external radiation level is not very high. The drawback was that deterioration occurred, which caused problems in neutron measurement.

The above-mentioned external radiation is not limited to neutrons, but also applies to gamma rays, and the output pulse of the counter gradually becomes smaller.

The present invention was made to improve the poor radiation resistance of the BF ₃ proportional counter, and was discovered through our extensive experiments. The selection method is to irradiate the BF ₃ proportional counter with thermal neutrons of 10 ¹⁰ to ^{10 11} nvt while applying a high DC voltage, and select those with good characteristics to determine the radiation resistance of the BF ₃ proportional counter. is being improved.

The thermal neutron flux for irradiation may be 10 ⁶ nv or less.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

Figure 1 shows the change in the gas amplification factor of the BF ₃ proportional counter when thermal neutrons are irradiated with the BF ₃ proportional counter in operation, that is, with a high DC voltage applied. The BF ₃ proportional counter used in the experiment has an anode wire outer diameter of 0.025 mm and a cathode inner diameter of 21.8 mm.
BF ₃ gas filling pressure is 350Torr. The operating voltage is usually selected between 1.1KV and 1.9KV, and in this experiment, 1.7KV was applied during irradiation, and the irradiation neutron flux was approximately ¹⁰⁴ nV.The gas amplification factor on the vertical axis is the value at 1.7KV. be. The gas amplification factor is irradiation amount of 2×10 ⁹ nvt.
It is divided into D, which uniformly decreases up to the point, but then continues to decline, and A, B, and C, which recover. The amount of radiation at which recovery begins varies slightly depending on the counter. However, this variation is small and does not pose any problem in improving the characteristics. It can be seen from the figure that the gas amplification factors of samples A, B, and C whose characteristics have been restored are stable with respect to subsequent irradiation. The horizontal axis of the figure is shown logarithmically, and after recovery, approximately
This shows that the gas amplification factor characteristics are stable even with irradiation of 1.5×10 ¹¹ nvt or more. Although not shown, the characteristics are stable even if the same irradiation is performed unless the BF ₃ proportional counter is put into operation.

Furthermore, Fig. 2 shows the deterioration and recovery behavior of the BF ₃ proportional counter tube when it is irradiated with neutrons while a voltage is applied, using pulse height distribution characteristics. The magnitude of the neutron signal pulse from the counter is not constant and usually has a distribution as shown by curve 1. The steeply rising portion on the left side of FIG. 2 is due to noise in the measurement circuit. The initial wavelength distribution shown in curve 1 has a sharp peak, the wave height is large, and it is well separated from the noise of the measurement circuit, so that neutrons can be counted correctly. 2.0× shown in curve 2
The wave height distribution characteristic after irradiation with 10 ⁹ nvt collapses to the smaller wave height side, and in the wave height distribution characteristic (curve 3) after irradiation with 5.1×10 ⁹ nvt, the deterioration has progressed further. 2.0×
It can be seen that with 10 ⁹ nvt irradiation (curve 2), the neutron signal pulses with small wave heights can no longer be separated from the measurement circuit noise. However, as the neutron irradiation continues, the wave height distribution gradually approaches the initial state. Figure 3 is 1.8×
This figure shows in detail the changes in the pulse height distribution characteristics of a counter tube that has recovered after being irradiated with 10 ¹¹ nvt, that is, after it has been degraded. It can be seen that the initial deterioration as seen in FIG. 2 is not observed, and the characteristics are stable.

From the above, it can be seen that when a high voltage is applied to a BF proportional counter tube in the operating state and irradiated with thermal neutrons, that is, when the thermal neutron counting operation continues, the output pulse height will be reduced at a relatively low irradiation dose (10 ⁸ to ^{10 9} nvt). However, if the BF ₃ proportional counter is activated in advance and irradiated with thermal neutrons of 10 ¹⁰ to 10 ¹¹ nvt to select those whose characteristics have recovered, then the neutron counting operation is performed. It was found that even at extremely high irradiation doses, that is, the total count value, the characteristics were stable.

As mentioned above, this invention was made based on our experimental results, and the BF ₃ proportional counter tube was irradiated with thermal neutrons of 10 ¹⁰ to 10 ¹¹ nvt while a high voltage was applied. Therefore, at this point, we have selected a product with good characteristics (characteristics A, B, and C in Figure 1 are shown).
If a BF ₃ proportional counter tube is selected, it will continue to operate stably without showing any deterioration in characteristics during subsequent use. Furthermore, since the upper limit of the irradiation amount is set, the remaining life of the BF ₃ proportional counter will not be unnecessarily shortened.

According to the invention described above, before the BF ₃ proportional counter is used, the characteristics once degraded by irradiation with thermal neutrons of 10 ¹⁰ to 10 ¹¹ nvt while a predetermined DC high voltage is applied to the electrodes. recovered BF ₃
Radiation resistance can be improved by selecting proportional counters.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the change in gas amplification factor when the BF ₃ proportional counter is in operation and irradiated with thermal neutrons. Figure 2 is a characteristic diagram when the BF ₃ proportional counter is irradiated with neutrons with voltage applied. A wave height distribution characteristic diagram showing the state of deterioration and recovery. FIG. 3 is a wave height distribution characteristic diagram of a counter tube that has recovered after deterioration.

Claims (1)

[Claims] 1 BF ₃ where BF ₃ gas is sealed in a tube equipped with an electrode
Before using the proportional counter, the electrodes are irradiated with thermal neutrons of 10 ¹⁰ to 10 ¹¹ nvt while a predetermined DC high voltage is applied to select BFs whose characteristics have recovered once they have deteriorated. ₃ How to sort proportional counter tubes.