Patent Description:
For example, neutron position detectors are used in applications in an accelerator facility in which neutrons are irradiated on a sample to be examined and the characteristics of the sample are examined by detecting the scattering of the neutrons, etc..

A neutron position detector includes a position-sensitive neutron detection proportional counter (PSD) as a neutron position detector, a processing circuit that calculates the neutron incident position by processing a charge output from the neutron position detector, etc..

A neutron position detector includes a tubular enclosure used as a cathode; an anode is located at the axial center inside the enclosure; and a gas that includes a <NUM>He gas and an additive gas are sealed inside the enclosure. Then, when a neutron enters the enclosure, the <NUM>He inside the gas has a nuclear reaction with the neutron that produces a proton and tritium; the proton and the tritium travel through the gas and ionize the surrounding gas; and the ionized charge is collected by the anode. Then, the incident position of the neutron is detected in a processing circuit based on the output charge from the two ends of the anode.

In such a neutron position detector, the additive gas degrades and the life is easily reduced when the neutron intensity is high.

It is therefore desirable for a neutron position detector to have a longer life while ensuring the position resolution, i.e., the detection accuracy of the incident position of the neutron.

The invention is directed to provide a neutron position detector that can have a longer life while ensuring the position resolution, i.e., the detection accuracy of the incident position of the neutron.

According to the present invention, there is provided a neutron position detector as set out in independent claim <NUM>. Advantageous developments are defined in the dependent claims.

An embodiment will now be described with reference to the drawings.

As shown in <FIG>, a neutron position detection device <NUM> includes a neutron position detector <NUM>, a high-voltage power supply <NUM>, and a processing circuit <NUM>. The processing circuit <NUM> includes preamplifiers 14a and 14b, an AD converter <NUM>, an arithmetic unit <NUM>, etc..

Also, the neutron position detector <NUM> is a one-dimensional position-sensitive neutron detection proportional counter (PSD). The neutron position detector <NUM> includes a tubular enclosure <NUM> that is a cathode, an anode <NUM> located at the axial center of the enclosure <NUM>, terminal parts 22a and 22b located at the two ends of the enclosure <NUM>, and a gas <NUM> sealed inside the enclosure <NUM>.

The enclosure <NUM> has a circular tubular shape that is long in the axial direction and sealed at two ends. A sealed space <NUM> is provided inside the enclosure <NUM>.

The anode <NUM> is a resistive core wire (a resistive metal wire) having a constant resistance value per unit length. The anode <NUM> is located along the axial center inside the enclosure <NUM>; and the two ends of the anode <NUM> are linked to and electrically connected to the terminal parts 22a and 22b.

The terminal parts 22a and 22b are located at the two ends of the enclosure <NUM> in an insulated state with respect to the enclosure <NUM>. The two ends of the anode <NUM> are linked to and electrically connected to the terminal parts 22a and 22b.

The gas <NUM> is sealed in the sealed space <NUM> of the enclosure <NUM>. The gas <NUM> includes a <NUM>He gas that is ionized by absorbing the neutrons, and an additive gas that is added to the <NUM>He gas.

The partial pressure of the <NUM>He gas is arbitrarily set according to the specification of the neutron detection efficiency and is, for example, in the range of <NUM> to <NUM> atm (<NUM><NUM><NUM> - <NUM><NUM><NUM> Pa).

The additive gas includes nitrogen as a quenching gas that is a molecular gas, and an argon for reducing the ranges of protons and tritium that are reaction products of the neutrons and the <NUM>He gas. In such a case, the relationship of d×pN2 > <NUM> is favorable, where d is the inner diameter of the enclosure expressed in cm, and pN2 is the partial pressure of nitrogen expressed in atm (<NUM> atm = <NUM> Pa). Also, the relationship is favorable when the partial pressure of the argon added to the <NUM>He is greater than the partial pressure of nitrogen. It is favorable for the partial pressure of the argon added to the <NUM>He to be in the range of <NUM> to <NUM> atm, (<NUM>-<NUM> Pa).

Also, the composition of the gas <NUM> is such that the partial pressure of the <NUM>He gas and the partial pressure of the additive gas are set so that the total of the ranges of the proton and tritium inside the gas <NUM> is, for example, in the range of <NUM> to <NUM>.

Also, the high-voltage power supply <NUM> applies an operating voltage between the anode <NUM> and the enclosure <NUM> that is the cathode. For example, the operating voltage is set to the range of <NUM> to <NUM> kV so that the output charge from the anode <NUM> is, for example, <NUM> to <NUM> pC.

Also, the preamplifiers 14a and 14b of the processing circuit <NUM> respectively convert the output charges from the two ends of the neutron position detector <NUM> (hereinbelow, called the two detector ends) into electrical signals and output the electrical signals. The preamplifiers 14a and 14b include coupling capacitors 30a and 30b that cut high-voltage components applied to the neutron position detector <NUM>, op-amps 31a and 31b that convert the output charges after cutting high-voltage components into prescribed electrical signals, etc. To adapt to an increase of the operating voltage of the neutron position detector <NUM>, a higher capacitance and lower distortion due to lower impedance may be realized by connecting two of each of the coupling capacitors 30a and 30b in parallel. Furthermore, it is favorable to use JFET input operational amplifiers as the op-amps 31a and 31b to suppress the operation delay distortion to a minimum.

Also, the AD converter <NUM> converts the electrical signals (the analog signals) of the two detector ends output from the preamplifiers 14a and 14b into digital signals (waveform signals). The AD converter <NUM> includes an element having a resolution of not less than <NUM> bits. For example, the AD converter <NUM> may include an element having a resolution of <NUM> bits.

Also, the arithmetic unit <NUM> determines the wave heights from the waveform data of the electrical signals of the two detector ends digitized by the AD converter <NUM> and calculates the incident position of the neutron in the axial direction of the neutron position detector <NUM> based on the ratio of the wave heights.

An operation of the neutron position detection device <NUM> will now be described.

The operating voltage is applied between the anode <NUM> and the enclosure <NUM> that is the cathode by the high-voltage power supply <NUM>.

Then, as shown in <FIG>, when a neutron n enters the enclosure <NUM>, a nuclear reaction occurs between the neutron n and the <NUM>He gas (n + <NUM>He → p + T + <NUM> keV), and a proton p and tritium T that are reaction products are produced. "A" shown in <FIG> is the position at which the nuclear reaction occurred and is the position at which the proton p and the tritium T were produced.

As shown in <FIG>, the proton p has about <NUM> keV of energy, and the tritium T has <NUM> keV of energy; and the proton p and the tritium T travel through the gas <NUM> in mutually-opposite directions to gradually stop due to the loss of energy due to colliding with the atoms and molecules of the surrounding gas <NUM>. When the proton p and the tritium T collide with the gas <NUM>, a portion of the energy of the proton p and the tritium T is transferred to the gas <NUM> to ionize the gas <NUM> and generate a charge e.

The generated charge e is collected by the anode <NUM> due to the electric field formed between the anode <NUM> and the enclosure <NUM> that is the cathode. Thereby, the output charges that are output from the two ends of the anode <NUM> are of a ratio corresponding to the distances to the two ends of the anode <NUM> from the collection position of the charge e in the anode <NUM>.

The output charges that are from the two detector ends (the two ends of the anode <NUM>) are converted into electrical signals by the preamplifiers 14a and 14b; and the electrical signals of the two detector ends output from the preamplifiers 14a and 14b are converted into digital signals (waveform signals) by the AD converter <NUM>.

In the arithmetic unit <NUM>, the wave heights are determined from the waveform data of the electrical signals of the two detector ends digitized by the AD converter <NUM>; and the incident position of the neutron n in the axial direction of the neutron position detector <NUM> is calculated based on the ratio of the wave heights.

The additive gas that is used in the neutron position detector <NUM> will now be described.

The neutron position detector <NUM> is one type of proportional counter. For stable operation of the proportional counter, a molecular gas other than the <NUM>He gas that causes the nuclear reaction with the neutron n is added. For example, as in <NPL>, an object of adding the molecular gas is to absorb the ultraviolet rays produced when the ionized <NUM>He ions recombine, thereby stabilizing the operation of the proportional counter.

A gas having such an effect is generally called a quenching gas. It is possible to use any gas that absorbs ultraviolet rays, and although methane (CH<NUM>), carbon dioxide (CO<NUM>), and carbon tetrafluoride (CF<NUM>) are widely used as commercial products, nitrogen, hydrogen, etc., can also be used. For example, the specification of <CIT> includes an example in which nitrogen is used as a quenching gas in a proportional counter.

On the other hand, to date, there are no products that use nitrogen as the quenching gas of the neutron position detector <NUM>; and many products use carbon dioxide or carbon tetrafluoride.

Reasons that nitrogen is not used in the prior art the quenching gas of the neutron position detector <NUM> will now be described.

Although the neutron position detector <NUM> is a detector for performing the position detection of the neutron n, the accuracy of the position detection, i.e., the position resolution, which is an important item of the detector performance, is affected by the ranges of the proton p and the tritium T that are reaction products through the gas <NUM>.

As shown in <FIG>, the charge e is generated in a range from the position A at which the proton p and the tritium T were produced until the proton p and the tritium T stop. Because the masses and the energies are not equal between the proton p and the tritium T, the ranges from the position A at which the nuclear reaction occurred until stopping are different from each other. Therefore, as shown in <FIG>, the centroid of the charge e made by the proton p and the tritium T is shifted toward the proton p-side from the position A at which the nuclear reaction occurred. Accordingly, the position A at which the nuclear reaction occurred and the centroid of the charge e are shifted. Also, the directions in which the proton p and the tritium T travel are random.

Therefore, when it is assumed that many neutrons n react at one point of the neutron position detector <NUM>, the centroid of the charge e created in the gas <NUM> is not at one point, but is spread over a range having a correlation with the ranges of the proton p and the tritium T.

In the neutron position detection device <NUM> that uses the neutron position detector <NUM>, the incident position of the neutron n is detected by determining the centroid of the charge e; therefore, the detection accuracy, i.e., the position resolution, of the incident position of the neutron n is affected more as the ranges of the proton p and the tritium T increase.

Accordingly, to increase the position resolution, it is sufficient to reduce the ranges of the proton p and the tritium T; to this end, it is necessary to use a heavy additive gas or increase the partial pressure of the additive gas.

However, nitrogen has little effect on reducing the ranges of the proton p and the tritium T. The relationship between the gas type and the ranges of the proton p and the tritium T are shown in the table of <FIG>. The ranges of the proton p and the tritium T per atmosphere of gas at <NUM> are shown in the table of <FIG>.

It can be seen from the table of <FIG> that carbon tetrafluoride and carbon dioxide have high effects of reducing the ranges of the proton p and the tritium T and are excellent as additive gases of the neutron position detector <NUM>.

On the other hand, although nitrogen functions as a quenching gas, the ranges of the proton p and the tritium T are long, therefore making it necessary to increase partial pressure to obtain the desired ranges of the proton p and the tritium T.

When, as done in the present invention, nitrogen is added as the additive gas of the neutron position detector <NUM>, compared to carbon tetrafluoride or carbon dioxide, the output charge for the applied operating voltage is markedly reduced.

The ratio of the output charge for the applied operating voltage decreases as the partial pressure of the added nitrogen increases, therefore making it necessary to apply a higher operating voltage to obtain the desired output charge.

The table of <FIG> shows the relationship between the gas type and the operating voltage. As an example, the table of <FIG> shows the increase amount of the operating voltage when increasing the additive gas by <NUM> atm (<NUM> Pa) in the product of a typical neutron position detector <NUM> (cathode diameter φ1/<NUM> = <NUM>, anode diameter φ of <NUM>, pressure of <NUM>He gas of <NUM> atm (<NUM><NUM><NUM> Pa).

It can be seen from the table of <FIG> that nitrogen has a higher operating voltage than the other gases. As the operating voltage of the neutron position detector <NUM> is increased, problems occur in that there is a risk that discharge may occur between the enclosure <NUM> and the anode <NUM>, the withstand voltage of the elements used in the processing circuit <NUM> may be exceeded, etc..

From the above reasons <NUM> and <NUM>, when added as the additive gas of the neutron position detector <NUM>, nitrogen functions as a quenching gas but has a weak ability to reduce the ranges of the proton p and the tritium T; therefore, when the partial pressure of nitrogen is increased to reduce the ranges of the proton p and the tritium T, then the operating voltage undesirably increases. Accordingly, until now, nitrogen has not been applied to products as an additive gas of the neutron position detector <NUM>.

In contrast, in the neutron detector <NUM> of the present invention the additive gas includes nitrogen as a quenching gas and argon as a gas that reduces the ranges of the proton p and the tritium T that are reaction products of the neutron n and the <NUM>He gas.

Although nitrogen is not used as quenching gas in neutron position detectors <NUM> of the prior art described above due to the lengths of the ranges of the proton p and the tritium T and/or the high operating voltage, the long life of nitrogen is an example of its advantages. The triple bond of the nitrogen molecule has the characteristic of being not easy to break because the bond energy is greater than those of the double bond of carbon dioxide and the single bond of carbon tetrafluoride.

Also, the disadvantages of nitrogen such as the lengths of the ranges of the proton p and the tritium T and the high operating voltage are eliminated by adding argon together with nitrogen.

In such a case, when using the neutron position detector <NUM> in practice, it is favorable, as done in the present invention, for the partial pressure of nitrogen to have the relationship of d×pN2 > <NUM> expressed in atm·cm, where d, expressed in cm, is the inner diameter of the enclosure <NUM>, and pN2 is the partial pressure of nitrogen expressed in atm (<NUM> atm = <NUM> Pa). It is necessary to increase the partial pressure of nitrogen when the inner diameter of the enclosure <NUM> is narrow because the ultraviolet rays must be absorbed within a short distance when the inner diameter is narrow. When the partial pressure of nitrogen is less than the partial pressure determined by the formula above, there is a possibility that the absorption of the ultraviolet rays may not be sufficient, the operation may become unstable, or discharge may occur at a low voltage.

Also, it can be seen from the table of <FIG> that the ranges of the proton p and the tritium T of argon are substantially equal to those of nitrogen.

However, the applied voltage at which the same output charge is obtained is drastically different between argon and nitrogen. It can be seen from the table of <FIG> that when argon and nitrogen are added with the same partial pressure, the effect on the operating voltage of argon is about <NUM>/<NUM> of that of nitrogen. Accordingly, argon is superior to nitrogen as an additive gas for reducing the ranges of the proton p and the tritium T. That is, even when the partial pressure of argon is increased to reduce the ranges of the proton p and the tritium T, the effect on the operating voltage is small. In such a case, it is favorable for the relationship to be such that the partial pressure of argon is greater than the partial pressure of nitrogen to reduce the ranges of the proton p and the tritium T.

However, the partial pressure of argon cannot be increased limitlessly because compared to nitrogen, argon has the disadvantage of having high sensitivity to gamma rays and having an effect on the operating voltage that is not completely negligible. Also, although the ranges of the proton p and the tritium T decrease as the partial pressure of argon increases, the position resolution of the neutron position detector <NUM> is not determined only by the ranges of the proton p and the tritium T, and is also affected by thermal noise of the electrical circuit system and the S/N ratios of the preamplifiers; and the position resolution is not limitlessly improved only by increasing the partial pressure of argon. On the other hand, when the pressure of argon is too low, the ranges of the proton p and the tritium T are not reduced, and the position resolution that is important in the neutron position detector <NUM> is not improved.

Accordingly, in practice, the partial pressure of argon has an appropriate range that is favorably <NUM> to <NUM> atm. The total of the ranges of the proton p and the tritium T is substantially inversely proportional to the pressure and is <NUM> at <NUM> atm (<NUM> Pa) of argon and <NUM> at <NUM> atm (<NUM> Pa). On the other hand, when using the neutron position detector <NUM> in practice, the position resolution is in the range of <NUM> to <NUM>; and setting argon to <NUM> to <NUM> atm (<NUM>-<NUM> Pa) would actually realize a significant improvement of the position resolution. Although the ranges of the proton p and the tritium T are reduced by setting the partial pressure of argon to be greater than this range, other factors (the thermal noise of the electrical circuit, etc.) prevail, the position resolution is not improved, the gamma ray sensitivity is increased, the operating voltage is increased, and only disadvantages become pronounced. On the other hand, when the subdivision of argon is less than this range, the effect of adding argon is small, and the obtained advantages are meager.

Thus, in the neutron position detector <NUM> of the embodiment, because the additive gas includes nitrogen as a quenching gas and argon as a gas that reduces the ranges of the proton p and the tritium T that are reaction products of the neutron n and the <NUM>He gas, a longer life is possible while ensuring the position resolution, i.e., the detection accuracy of the incident position of the neutron n.

Claim 1:
A neutron position detector (<NUM>), comprising:
an enclosure (<NUM>) used as a cathode, the enclosure (<NUM>) being tubular;
an anode (<NUM>) located at an axial center inside the enclosure (<NUM>); and
a gas (<NUM>) sealed inside the enclosure (<NUM>),
the gas (<NUM>) including a <NUM>He gas and an additive gas,
characterized in that
the additive gas including
nitrogen as a quenching gas,
argon as a gas reducing a range of a reaction product of a neutron and the <NUM>He gas, and
having a relationship of <MAT>
where d is an inner diameter of the enclosure (<NUM>) expressed in cm, and pN2 is a partial pressure of the nitrogen expressed in atm (<NUM> atm = <NUM> Pa).