JPS5912963B2 - Method for measuring particle size of radioactive substances - Google Patents

Description

[Detailed Description of the Invention] 15 The present invention relates to a method for measuring the particle size of radioactive particles by directly utilizing the ionization phenomenon of radiation, and is suitable for automation of measurement and can significantly shorten measurement time and improve measurement accuracy. The present invention relates to a particle size measurement method that can be used to measure particle size.

In addition, in the present invention, "particle size" in the particle size measurement method means "diameter of a single particle." This method is useful for uniformity tests of nuclear fuel and other radioactive materials, such as fuel pellet tests and scattered powder particle size measurements, or when it is desired to determine the location and particle size of 25 radioactive materials, such as body and clothing monitoring. This is a suitable method.

Conventionally, when measuring the particle size of radioactive particles, various types of photographic plates (nuclear plates, X-ray plates, instant development film) are used to closely contact the radioactive particles, and the radiation emitted by each individual radioactive particle is measured. Radioactivity was evaluated by measuring the photosensitive diameter with the naked eye or by counting the number of tracks in the vicinity, and calculating the particle size.

However, this method has the problem that there are many errors when evaluating radioactivity, and it is difficult to estimate the error.Also, since it is measured with the naked eye, there are also problems such as individual differences. Furthermore, it is not possible to determine whether the actual radioactivity is present unless measurements are taken several times, and preliminary exposure is required to determine the actual exposure time. Considering the work time required to determine the particle size, it can be said that there is no immediate response. Furthermore, since there is no discrimination ability for energy, including the case of instant phenomenon films, 234
It was not possible to discriminate between different nuclides such as Pu and 241Am, and furthermore, it was not possible to extract information as an electrical signal, so subsequent processing was troublesome. Various devices have been known to measure radioactivity using the ionization phenomenon of radiation.
cannot be measured.

The purpose of the present invention is to eliminate the drawbacks of the prior art as described above, to significantly shorten measurement time, to improve measurement accuracy and reliability of measurement data, and to enable measurement and subsequent data processing to be performed electrically. An object of the present invention is to provide a method for measuring the particle size (particle size of a single particle) of a radioactive substance that can be carried out online.

In order to achieve this object, the present invention is configured to directly utilize the ionization phenomenon of radiation to measure the particle size of radioactive particles. An embodiment of the present invention will be described in detail below.

First, the detector used is a radiation detector capable of energy discrimination. An example of this is shown in FIGS. 1 and 2. In this radiation detector, strip-shaped electrodes with a width of about 1 μm to 511 μm (to give a specific example, width 5, gap 0.511) are arranged perpendicularly above and below the semiconductor detector 1. A method of extracting position signals in the X direction and Y direction (so-called checkerboard method) is used. To extract signals from the strip-shaped electrodes, the same resistor 2 is installed between each electrode, the charge is divided by the resistor, and a signal containing position information is extracted from both ends of the resistor connected to the detector. I'm trying to make it happen. The semiconductor material used for the prototype was n-type, with a diameter of 5211φ, a thickness of 211, and a specific resistance of 1.7×.
1040hm-? , diameter 76uφ, thickness 300
μm1 specific resistance 10000hm-? It is manufactured as a surface barrier type detector. Such a detector 1
It is generally preferable to place the device in the storage container 3 to prevent noise from outside.

A sample 5 is placed on the sample holding table 4 of the storage container 3, and the detector 1 is brought into close contact with the sample 5 through the contamination prevention film 6. The membrane 6 is provided to prevent radioactive contamination and to bring the detector 1 and the sample 5 as close as possible to prevent deterioration of resolution due to an increase in the distance between them. The signal obtained from such a detector 1 is recognized and stored by a position information analysis device as shown in FIG.

The charges collected within the detector 1 are collected on respective electrodes. As mentioned above, the electrode on one side is connected to the resistor 2a,
Since the electrodes on the other side are connected to the resistors 2b, the collected charges are guided to the resistors 2a and 2b.
The charges collected through the resistor 2a are summed in a summing circuit 10, resulting in a quantity QE representing the total amount of energy lost by the radioactive particles within the detector 1. In lines 11 and 12, charges QY and Qx containing positional information in the Y direction and X direction are generated, respectively.
By performing the divisions Y/QE and QX/QE, pulses having wave heights corresponding to the positions on the X and Y axes can be generated on the lines 14 and 15.

The pulse passes through an interface circuit 16 for impedance and timing junctions, and a subsequent multiple pulse height analyzer 17 measures the pulse height, allowing the location of the radiation incident to be determined. Incidentally, the block indicated by the reference numeral 18 is a single channel pulse height analyzer. This allows selective counting of a specific nuclide (such as 239Pu) by outputting a signal only when an energy pulse of the specific nuclide is received, but this is not essential. Now, in the above system, the pulse signal generated by the charges collected by the resistors 2a and 2b includes energy information and position information.

What is the amount of charge corresponding to the total amount of radiation energy emitted by radioactive particles? Then, on the right side of the resistor 2a (1-
X/1. )QO, and since a charge of (X/L)QO is generated on the left side, the addition circuit 10 adds the charge to Q. can be found. Also, the magnitude of the signal changes depending on the nuclide and position of the radioactive material, but in the division circuit 13?
By dividing by the value of , the position value can be normalized and the accurate attachment position can be determined. In other words, if we assume that there is a particle at the position (X, Y) when looking at the detector two-dimensionally, the radiation emitted from the particle at a specific position (X, Y) will be transmitted between the detector 1 and the sample 5. Since the two are in close contact with each other, the radiation enters the detector directly above the particle and the position and radioactivity of the particle can be measured.

Therefore, even if many different radioactive particles are present in the sample 5, the radioactivity of each individual particle can be measured independently, and the measurement results can be taken out as electrical signals. Next, the diameter DA (μm) of the radioactive particle is determined by the radioactivity equation (1) of the particle obtained in this way.
seek.

Note that the diameter of a radioactive particle refers to the chemical form of radioactive substances (for example, PuO2 for 239Pu as described later) and other non-radioactive substances depending on the type of radionuclide determined by radioactivity measurement. This refers to the diameter of aggregated particles expressed in metric units, assuming that their density is the theoretical density and that they are spherical. \
- ” However, λ is the decay constant, M is the mass number of the radionuclide contained in the radioactive particle, A is Avogadro's number, ρ is the density of the radioactive particle, f is the proportion of the radionuclide contained in the radioactive particle (weight ratio) ), χ is radioactivity (Dpm).

Note that in equation (1), the geometric effect is assumed to be 50%. This is due to the arrangement in which the detector 1 is placed in close contact with the sample 5. Further, the density ρ of the radioactive particles and the proportion (weight ratio) f of the radioactive nuclides contained in the radioactive particles are determined separately by chemical analysis or the like, or estimated from the used nuclides. Percentage of radionuclides contained in radioactive particles (
Weight ratio) f is the ratio of the weight of the radionuclide contained in the radioactive particle (single particle) to the weight of the radioactive particle (single particle) whose particle size is to be measured. The single particle present at the specific position (X, Y) above consists only of UO2, and 2% by weight of it
When is radioactive U235O2 and the remaining 98% by weight is non-radioactive U238O2, f is as follows according to the above definition. That is, f = [weight (W) of radionuclide contained in radioactive particle (single particle) to be measured]/[weight (m) of radioactive particle (single particle) to be measured] 0r). Become.

In addition, when the radioactive particles to be measured are UO2 containing non-radioactive substances such as binders, and radioactive U2
35O2 content is 2.0 weight of total particle weight? If it is(
However, 2.0% by weight does not refer to UO2,
This is the ratio to the weight of the entire particle including the binder etc.

) Again, similar to the example above.

As mentioned above, f indicates the abundance ratio of radionuclides in the radioactive particles, and is a factor used to correct the diameter of the radioactive particles.

Further, the above equation (1) can be derived as follows.

, the conversion from the radioactivity χDpm to the particle diameter can be calculated using a formula that calculates the mass and diameter from the radioactivity of the radioactive particle, assuming that the particle is spherical. Here, N is the number of atoms of the radionuclide present in the radioactive particle, DN is the number of atoms that decay in time Dt, W is the weight of the radionuclide in the radioactive particle (g), and V is the volume of the radioactive particle (CTiL). do.

The symbol ``ya'' is the same as that described above. The decay of radionuclides contained in radioactive particles can be written as N-AW/
Because it is M, it is.

On the other hand, the weight W of the radionuclide in the radioactive particle is also (
-DN/Dt) is the decay rate (Dps), so (-D
N/Dt) = χ = 60 (Dpm), the equation (4) becomes, and the above equation (1) can be derived.

Regarding f and ρ, when handling radioactive materials, their component ratio, specific gravity, weight, etc. are generally controlled under very strict conditions, so the composition of radioactive particles as described above must always be accurately grasped by managers. Therefore, it is extremely easy to find the value of f.

In addition, if the compositional density is unknown, it may be determined separately by chemical analysis or the like. For example, 1 dpm at a certain point on the detector
Suppose that radioactivity is detected.

In this case, the radioactive particles attached to the sample are removed from PuO2 (
239Pu). With conventional methods, it was not possible to measure 239Pu particles separately from the alpha rays emitted by 222Rn and its daughter nuclides, which exist in the general air. Therefore, it takes at least three days to confirm the presence of 239Pu. However, by using a detector capable of energy discrimination, it becomes possible to measure without leaving the sample, and the time can be shortened from 3 days to 1 week.

The particle size of the plutonium particles is determined by assuming the density of radioactive particles (in this case, 11.45 g/Cd)
It can be calculated. The diameter DA of the radioactive particle is (1)
From the formula, it is 1.1 μm. Similarly, for other radioactive particles, the radioactivity of each particle can be measured, so
The particle size of the radioactive particles can be determined by equation (1).

Therefore, in addition to monitoring the body and clothing, this method also makes it possible to determine the particle size of each substance in cases where uranium and plutonium are mixed, such as in fuel rods. In the above embodiment, the detector 1 has narrow metal electrodes deposited on the upper and lower surfaces of the semiconductor so as to be perpendicular to each other, and the carriers ionized by the radiation incident inside the semiconductor are directly above and below, respectively. The charge is collected on a certain strip-shaped electrode, and the charge is collected using an externally attached resistor.
Although the position and energy are detected using the signal, the present invention is not limited to only such a detector.

For example, in the case of strip-shaped electrodes, it is possible to obtain (X, Y) coordinates and energy information by connecting an amplifier to each electrode in the X direction and Y direction and measuring the coincidence of X and Y. can. Alternatively, the electrode on the semiconductor surface itself may be made of a resistor, and a position analysis device similar to that of the above embodiment may be connected. In addition, in the case of a proportional counter or a GM counter, the radioactivity of individual particles can be measured in the same way by using each electrode as having the same function as the strip-shaped electrode in a semiconductor detector. In the case of scintillation detectors, individual radioactivity can be measured in the same way as semiconductor detectors by processing the scintillator into strips or using a resistive charge splitting method in the part that detects luminescence. However, in consideration of the two requirements of simplification of the device configuration and good measurement accuracy, the above embodiment is preferable. Since the present invention is constructed as described above, compared to conventional methods, the present invention has the effect of significantly shortening the measurement time and improving measurement accuracy and reliability of measurement data.

In addition, the background is low, which improves the certainty of determining that the measured data is emitted from radioactive materials, and since all measurements can be performed electrically, data processing after data collection can be performed electronically. It can be performed online, saves labor, is free from human error, and can organize measurement results in a short time.

[Brief explanation of drawings]

Figure 1 is a sectional view of a detector suitable for use in the present invention, Figure 2 is a cross-sectional view of a detector suitable for use in the present invention;
The figure is an explanatory diagram thereof, and FIG. 3 is a block diagram of an apparatus suitable for carrying out the method of the present invention. 1... Semiconductor detector, 2, 2a, 2b...
...Resistor, 3...Storage container, 4...
Sample holding stand, 5...sample, 6...contamination prevention membrane.

Claims (1)

[Claims] 1. A radiation position detector capable of energy discrimination is brought into close contact with a sample through a contamination prevention film to detect the nuclide of a radioactive substance existing at a specific position and its radioactivity as an electrical signal. , dA={(M×10^1^1/π・λ・A・ρ・f
)×x}^1^/^3 However, λ: Decay constant M: Mass number of radionuclides contained in radioactive particles A: Avogadro's number ρ: Density of radioactive particles f: Mass number of radionuclides contained in radioactive particles A method for measuring the particle size of a radioactive substance in which the diameter of the radioactive particle is calculated from the following formula: ratio (weight ratio) χ: radioactivity (dpm) d: diameter of radioactive particle (μm).