JPH0425514B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improving the radiation resistance of a BF ₃ proportional counter for detecting neutrons.

In general, BF ₃ proportional counters are easy to obtain with high sensitivity and are widely used for measuring low neutron fluxes. However, its life characteristics are not very good, and if the counter is not put into operation and simply irradiated with radiation, its characteristics will hardly deteriorate, but if the counter is put into operation, the external radiation level is not very high. However, its characteristics deteriorate relatively quickly, resulting in problems in neutron measurement.

It is known that the external radiation described above is not limited to neutrons, but also gamma rays, which cause similar characteristic deterioration. As a method to improve this radiation life characteristic, simply irradiating the counter with radiation without putting it into operation will hardly cause any deterioration of the characteristics.
A method has been reported in which gamma ray irradiation is performed at a dose rate of at least 10 ⁵ Rad/hr and at least 10 ⁶ Rad by activating the BF ₃ proportional counter, that is, by applying a high DC voltage. (British Patent No. 1267196) This is because after the gamma ray irradiation described above, the characteristics of the BF ₃ proportional counter are less likely to deteriorate. However, this method requires a very strong gamma ray source to obtain a high dose rate of ₁₀ ⁵ Rad/hr or more, which is not easy to implement. Our detailed and extensive experimental results have revealed that the drawback is that the remaining life of the counter tube after irradiation is shortened.

Therefore, the present invention aims to eliminate the above-mentioned drawbacks of the conventional art. Before using a BF ₃ proportional counter tube in which BF ₃ gas is sealed in a tube equipped with electrodes, a predetermined DC high voltage is applied to the electrodes. By irradiating gamma rays or X-rays with an irradiation dose rate of 10 ³ R/hr or less with an irradiation dose of 10 ⁴ to 2 × 10 ⁵ R,
This is an attempt to improve the radiation resistance without shortening the remaining life by selecting those whose characteristics have recovered once they have deteriorated.

An embodiment of the present invention will be described below.

Figure 1 shows the changes in the gas amplification factor of the BF ₃ proportional counter when gamma rays are irradiated with the BF ₃ proportional counter in operation, that is, with a high DC voltage applied, while varying the irradiation dose rate. It is something. The counter tube used in the experiment has an anode wire outer diameter of 0.025 mm, a cathode inner diameter of 21.8 mm, and a BF ₃ gas filling pressure of 550 T _prr . The operating voltage is usually chosen between 1.5KV and 2.2KV, and in this experiment, 2KV was applied during irradiation. The gas amplification factor on the vertical axis is the value at 2KV, and is measured under the same gamma ray dose rate as the irradiation dose rate. This is why the initial value of the gas amplification factor is a little low at high dose rates, which is due to the space charge effect in the counter.
Now, from the figure, the gas amplification factor of the counter tube is approximately determined by the cumulative dose, regardless of the irradiation dose rate, and is approximately 2×10 ³ R.
It can be seen that there are two types: those that decrease uniformly up to , and then recover around 10 ⁴ R, and those that continue to decrease. Although the cumulative dose value at which recovery begins varies, those that recover are stable for subsequent irradiations. Numerous irradiation experiments have also shown that the ratio of bifurcation into two types does not depend on the irradiation dose rate. Although not shown, the characteristics are stable for the same irradiation unless the counter is put into operation.

Figure 2 shows the counts that fall into a predetermined wave height (channel), and shows the deterioration and recovery of a BF ₃ tube that deteriorates and recovers due to gamma ray irradiation from the perspective of wave height distribution characteristics. It can be seen that the wave height distribution characteristic deteriorates monotonically until around 1600, and then recovers. Although not shown in the figure for the BF ₃ tube that does not recover, the wave height distribution characteristics remain the same even after continued irradiation.
It continues to deteriorate as shown by the curve in the figure.

From the above, all BF ₃ proportional counters have a relatively low integrated dose (approximately 100 ~
1000R) shows a deterioration phenomenon in which the signal pulse height _becomes smaller.
After irradiating with 10 ⁴ R of gamma rays, determining their characteristics, and selecting those whose characteristics have recovered, they show no deterioration and exhibit stable characteristics even when operated in a gamma ray environment, even at very high integrated doses. It turns out that. The method of the present invention is particularly effective when measuring neutrons in a field where gamma rays are present, such as when applied to a nuclear reactor.

Like the UK patent mentioned above to restore properties.
A high dose of 10 ⁶ Rad or more or a high dose rate of 10 ⁵ Rad/hr or more is not necessary, and in fact, irradiation that exceeds the necessary will shorten the remaining life. Furthermore, a very strong gamma ray source is required, and implementation is difficult. It's not easy.

In addition, although the example in which the applied voltage was 2 KV was shown above, the applied voltage may be any other voltage value as long as it is within the operating voltage range of the BF ₃ proportional counter tube. Similar results were obtained in experiments where the applied voltage was actually 1.8KV.

Furthermore, although gamma rays were used as the radiation source in the above, gamma rays and high-energy X-rays are physically the same electrode wave, and it is clear that X-rays may also be irradiated.

As described above, this invention was made based on extensive experimental results, and the irradiation dose rate was ^{10 3} _R /
The irradiation dose of gamma rays or X-rays less than hr is 10 ⁴ ~
By irradiating with 2×10 ⁵ R, we selected those whose characteristics had recovered once they had deteriorated, so we could use those with good characteristics and ensure that their characteristics would not deteriorate even in a strong gamma ray environment. This has the advantage of being able to select BF ₃ proportional counters that continue to operate stably. In particular, since the upper limit of the irradiation amount is set to a low amount, the remaining life of the BF ₃ proportional counter will not be unnecessarily shortened.

[Brief explanation of drawings]

FIG. 1 is a graph showing, on a semi-logarithmic scale, the change in gas amplification factor when gamma rays are irradiated with the BF ₃ proportional counter in operation. FIG. 2 is a graph showing, on a linear scale, changes in the wave height distribution characteristics during the gamma ray irradiation.

Claims (1)

[Claims] 1 BF ₃ where BF ₃ gas is sealed in a tube equipped with an electrode
Before using the proportional counter, check the irradiation dose rate while applying a specified DC high voltage to the above electrodes.
If the irradiation dose is less than 10 ³ R/hr of gamma rays or
By irradiating with 10 ⁴ to 2×10 ⁵ R, we select those whose characteristics have recovered once they have deteriorated. BF ₃
How to select comparison counters.