METHOD FOR DETERMINING 224Ra IN SEDIMENT BY USING PULSE IONIZATION CHAMBER EMANOMETER

Abstract

Disclosed is a method for determining .sup.224Ra in a sediment by using a pulse ionization chamber emanometer, which belongs to the technical field of analysis and measurement. A pulse ionization chamber emanometer (PIC), a new emanometer, is used. Based on the half-life characteristics of different radon isotopes, one can separate the .sup.220Rn activity from the total counts by dual counting. The resulting .sup.220Rn measurement then can be used to determine the .sup.224Ra activity in sediment according to the principle of secular radioactive equilibrium.

Claims

1. A method for determining .sup.224Ra in a sediment by using a pulse ionization chamber emanometer, comprising steps of (1) placing a sediment standard sample of .sup.224Ra in a sample tray, connecting the sample tray with an air pump and the pulse ionization chamber emanometer, to form an enclosed test system; turning on the air pump and circulating gas in the test system for at least 5 minutes, such that a radioactive equilibrium between .sup.220Rn gas released from the sediment standard sample and .sup.224Ra in the sediment standard sample is reached, and performing a first continuous measurement for an activity of Rn in the test system, to obtain a sum of counting rates of .sup.222Rn and .sup.220Rn, represented by C.sub.1, in cpm; turning off the air pump, closing inlet valve(s) and outlet valve(s) of the pulse ionization chamber emanometer, and leaving the pulse ionization chamber emanometer to stand for at least 5 minutes, such that .sup.220Rn in the pulse ionization chamber emanometer completely decays and disappears, and performing a second continuous measurement for an activity of Rn in the pulse ionization chamber emanometer, to obtain a counting rate of .sup.222Rn, represented by C.sub.2, in cpm; calculating a counting rate of .sup.224Ra in the sediment standard sample according to equation 1;
C.sub.p=C.sub.d=C.sub.1−C.sub.2  equation 1, in equation 1, C.sub.p represents the counting rate of .sup.224Ra in the sediment standard sample, and C.sub.d represents the counting rate of .sup.220Rn in the sediment standard sample; (2) repeating step (1) by using different sediment standard samples with a .sup.224Ra activity gradient to obtain counting rates of .sup.224Ra in different sediment standard samples with a .sup.224Ra activity gradient; plotting a standard curve of activities of .sup.224Ra versus counting rates, in which the counting rates of .sup.224Ra in the different sediment standard samples are set as ordinate, and activities of .sup.224Ra in the different sediment standard samples are set as abscissa; and (3) performing a measurement on a sediment sample according to step (1) to obtain a counting rate of .sup.224Ra, and calculating the .sup.224Ra activity of the sediment sample according to the standard curve obtained in step (2).

2. The method as claimed in claim 1, wherein the sediment sample to be tested and the sediment standard samples are in same type.

3. The method as claimed in claim 1, wherein an air flow rate provide by the air pump is in the range of 0.5-3 L/min.

4. The method as claimed in claim 1, wherein each of the sediment sample to be tested and the sediment standard samples independently has a moister content of 0-70 wt %.

5. The method as claimed in claim 1, wherein the first continuous measurement is performed for 0.5 to 6 hours.

6. The method as claimed in claim 1, wherein the second continuous measurement is performed for 0.5 to 4 hours.

7. The method as claimed in claim 1, further comprising calculating a relative standard deviation of the activity of .sup.224Ra in the sediment sample to be tested after step (3).

8. The method as claimed in claim 7, wherein calculating the relative standard deviation of the activity of .sup.224Ra in the sediment sample to be tested is performed by a process comprising calculating a standard deviation of the sum of counting rates of .sup.222Rn and .sup.220Rn obtained from the first continuous measurement according to equation 2 and equation 3, $\begin{array}{cc}{\sigma }_1=\frac{\sqrt{N_1}}{N_1}\times C_1,& \mathrm{equation}2\end{array}$ $\begin{array}{cc}{N}_1=C_1\times T_1,& \mathrm{equation}3\end{array}$ in equations 2 and 3, σ.sub.1 represents the standard deviation of the sum of counting rates of .sup.222Rn and .sup.220Rn obtained from the first continuous measurement, in cpm; N.sub.1 represents a counting value in the first continuous measurement, in counts; T.sub.1 represents time for the first continuous measurement, in minute; calculating a standard deviation of the counting rate of .sup.222Rn obtained from the second continuous measurement according to equation 4 and equation 5, $\begin{array}{cc}\mathrm{\sigma 2}=\frac{\sqrt{N_2}}{N_2}\times C_2,& \mathrm{equation}4\end{array}$ $\begin{array}{cc}{N}_2=C_2\times T_2,& \mathrm{equation}5\end{array}$ in equations 4 and 5, σ.sub.2 represents the standard deviation of the counting rate of .sup.222Rn obtained from the second continuous measurement, in cpm; N.sub.2 represents a counting value in the second continuous measurement, in counts; T.sub.2 represents time for the second continuous measurement, in minute; and calculating the relative standard deviation of the activity of .sup.224Ra according to equation 6, represented by δ in equation 6; $\begin{array}{cc}\delta =\frac{\sqrt{{\sigma }_1^2+{\sigma }_2^2}}{C_1-C_2}\times 100\%.& \mathrm{equation}6\end{array}$

9. The method as claimed in claim 1, wherein each of the sediment sample to be tested and the sediment standard samples independently has a mass of 1-60 g.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 shows the schematic diagram of the device used in some embodiments of the present disclosure.

[0035] FIG. 2 shows a schematic flow chart of the method for determining .sup.224Ra in a sediment according to some embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The present disclosure provides a method for determining .sup.224Ra in a sediment by using a pulse ionization chamber emanometer, comprising steps of [0037] (1) placing a sediment standard sample of .sup.224Ra in a sample tray, connecting the sample tray with an air pump and the pulse ionization chamber emanometer, to form an enclosed test system; [0038] turning on the air pump and circulating gas in the test system for at least 5 minutes, such that a radioactive equilibrium between .sup.220Rn gas released from the sediment standard sample and .sup.224Ra in the sediment standard sample is reached, and performing a first continuous measurement for an activity of Rn in the test system, to obtain a sum of counting rates of .sup.222Rn and .sup.220Rn, represented by C.sub.1 in cpm; [0039] turning off the air pump, closing inlet valve(s) and outlet valve(s) of the pulse ionization chamber emanometer, and leaving the pulse ionization chamber emanometer to stand for at least 5 minutes, such that .sup.220Rn in the pulse ionization chamber emanometer completely decays and disappears, and performing a second continuous measurement for an activity of Rn in the pulse ionization chamber emanometer, to obtain a counting rate of .sup.222Rn, represented by C.sub.2 in cpm; [0040] calculating a counting rate of .sup.224Ra in the sediment standard sample according to equation 1;

C.sub.p=C.sub.d=C.sub.1−C.sub.2  equation 1, [0041] in equation 1, C.sub.p represents the counting rate of .sup.224Ra in the sediment standard sample, and Ca represents the counting rate of .sup.220Rn in the sediment standard sample; [0042] (2) repeating step (1) by using different sediment standard samples with a .sup.224Ra activity gradient to obtain counting rates of .sup.224Ra in different sediment standard samples with a .sup.224Ra activity gradient; [0043] plotting a standard curve of activities of .sup.224Ra versus counting rates, in which the counting rates of .sup.224Ra in the different sediment standard samples are set as ordinate, and .sup.224Ra activities in the different sediment standard samples are set as abscissa; and [0044] (3) performing a measurement on a sediment sample according to step (1) to obtain a counting rate of .sup.224Ra (cm) and calculating the .sup.224Ra activity of the sediment sample according to the standard curve obtained in step (2).

[0045] As shown in FIG. 1, in the method according to the present disclosure, the sediment standard sample of .sup.224Ra is first placed in the sample tray, and the sample tray is connected with the air pump and the pulse ionization chamber emanometer, to form an enclosed test system.

[0046] In the present disclosure, there is no special type limitation on the pulse ionization chamber (PIC) emanometer, and any PIC well known in the art may be used. In the present disclosure, there is no special requirements on the connection method of the sample tray, the air pump and the pulse ionization chamber emanometer, and the connection method well known in the art may be adopted, which is common knowledge in the art. In the present disclosure, there is no special requirement on the type of the sediment standard sample, for example clay and silt. In some embodiments of the present disclosure, the sediment standard sample has a moisture content of 0-70 wt %. Those skilled in the art could select an appropriate moisture content according to the particle size of the sediment standard sample. In some embodiments of the present disclosure, the sediment standard sample has a mass of 1-60 g, preferably 10-50 g, and further preferably 20-40 g.

[0047] According to the present disclosure, after the enclosed test system is formed, the air pump is turn on, and the gas in the test system is circulated for at least 5 minutes, such that a radioactive equilibrium between .sup.220Rn gas released from the sediment standard sample and .sup.224Ra in the sediment standard sample is reached, and a first continuous measurement is performed for an activity of Rn in the test system, to obtain a sum of counting rates of .sup.222Rn and .sup.220Rn, represented by C.sub.1, in cpm.

[0048] In some embodiments of the present disclosure, the air pump is run to provide a constant flow rate of 0.5 to 3.0 L/min, and preferably 1.0 to 2.0 L/min.

[0049] The α decay of .sup.224Ra produces a gaseous daughter .sup.220Rn. Since the half-life of .sup.224Ra (T.sub.1/2=3.66 days) is much longer than that of its daughter .sup.220Rn (T.sub.1/2=55.6 s), According to the principle of secular radioactive equilibrium, the activity of .sup.224Ra is the same as that of .sup.220Rn after five-fold time of the half-life of .sup.220Rn (i.e., five minutes). There are four natural radioactive radium isotopes in nature, namely .sup.228Ra (T.sub.1/2=5.75 years), .sup.226Ra (T.sub.1/2=1620 years), .sup.224Ra (T.sub.1/2=3.66 days) and .sup.223Ra (T.sub.1/2=11.4 days), and three types of Rn isotopes would be produced when they decay, namely .sup.222Rn (T.sub.1/2=3.83 days), .sup.220Rn (T.sub.1/2=55.6 s) and .sup.219Rn (T.sub.1/2=3.96 s). Since the object measured in the ionization chamber are positive and negative charges generated by ionizing air with alpha particles, the three nuclides .sup.222Rn, .sup.220Rn and .sup.219Rn could not be directly distinguished by the PIC. Therefore, in the first continuous measurement, the result measured by PIC is the sum of the radon isotopes in the gas (.sup.222Rn, .sup.220Rn and .sup.219Rn). Since the content of .sup.223Ra in the sediment is extremely low and the half-life of .sup.219Rn is very short, the influence of .sup.219Rn on the .sup.220Rn measurement in the sediment could be ignored during the measurement. Therefore, the result obtained in the first continuous measurement is the sum of the activities (counting rates) of .sup.222Rn and .sup.220Rn.

[0050] In the present disclosure, the counting rate is expressed in unit of cpm, i.e., counts per minute, which is the number of decays of radioactive isotopes per minute observed by the instrument, which corresponding to the activity of the radioactive element at the corresponding stage of the measurement by the PIC emanometer.

[0051] In the present disclosure, there is no special requirements on the upper limit of the time for the gas circulation.

[0052] In some embodiments of the present disclosure, the first continuous measurement is performed for 0.5 to 6 hours, and preferably 2 to 4 hours. A longer time for the first continuous measurement would result in a higher counting value, thereby causing a smaller measurement error at this stage. The time for the first continuous measurement could be selected according to the measurement error requirement. The calculation of the error will be described in detail below.

[0053] According to the present disclosure, after obtaining the sum of counting rates of .sup.222Rn and .sup.220Rn (C.sub.1), inlet valve(s) and outlet valve(s) of the pulse ionization chamber emanometer are closed, and the pulse ionization chamber emanometer is left to stand for at least 5 minutes such that the .sup.220Rn in the pulse ionization chamber emanometer completely decays and disappears, and a second continuous measurement is performed for an activity of Rn in the pulse ionization chamber emanometer, to obtain a counting rate of .sup.222Rn, represented by C.sub.2, in cpm.

[0054] In some embodiments of the present disclosure, leaving the pulse ionization chamber emanometer to stand is performed for 5 to 10 minutes.

[0055] In some embodiments of the present disclosure, the second continuous measurement is performed for 0.5 to 4 hours, and preferably 1 to 3 hours. A longer time for the second continuous measurement results in a higher counting value, thereby causing a smaller measurement error at this stage. The time for the second continuous measurement could be chosen according to the measurement error requirement. The calculation of the error will be described in detail below.

[0056] According to the present disclosure, after obtaining the sum of counting rates of .sup.222Rn and .sup.220Rn (C.sub.1) and the counting rate of .sup.222Rn (C.sub.2), the counting rate of .sup.224Ra in the sediment standard sample is calculated according to equation 1,

C.sub.p=C.sub.d=C.sub.1−C.sub.2  equation 1,

[0057] in equation 1, C.sub.p represents the counting rate of .sup.224Ra in the sediment standard sample, and C.sub.d represents the counting rate of .sup.220Rn in the sediment standard sample.

[0058] According to the present disclosure, the above steps are repeated by using different sediment standard samples with a .sup.224Ra activity gradient to obtain counting rates of .sup.224Ra in different sediment standard samples with a .sup.224Ra activity gradient. A standard curve of activities of .sup.224Ra versus counting rates is plotted, in which the counting rates (cpm) of .sup.224Ra in the different sediment standard samples are set as ordinate, and activities of .sup.224Ra in the different sediment standard samples are set as abscissa.

[0059] In the present disclosure, there is no special requirements on the progress for plotting the standard curve. According to the present disclosure, after plotting the standard curve, a linear equation of activities of .sup.224Ra versus counting rates and a correlation coefficient (R.sup.2) are obtained, and a correlation coefficient (R.sup.2) much closer to 1 indicates a higher accuracy of the measurement method according to the present disclosure. In some embodiments of the present disclosure, the linear equation is shown in equation 7:

C.sub.1−C.sub.2=kA.sub.standard+b  equation 7,

in equation 7, (C.sub.1-C.sub.2) equals to the counting rate of .sup.224Ra, A.sub.standard represents the activity of .sup.224Ra in the standard sample, k represents the instrument efficiency, and b represents the background value of the .sup.224Ra in the sediment sample.

[0060] According to the present disclosure, after obtaining the standard curve of activities of .sup.224Ra versus counting rates, a measurement is performed on the sediment sample to be tested according to the above-mentioned steps, to obtain the counting rate of .sup.224Ra in the sediment sample to be tested, and the activity of .sup.224Ra in the sediment sample to be tested is calculated according to the standard curve of activities of .sup.224Ra versus counting rates.

[0061] In some embodiments of the present disclosure, the sediment sample to be tested and the sediment standard samples are in same type. For example, if the sediment standard sample is a silty sediment, the sample to be tested is also a silty sediment.

[0062] In some embodiments, the activity of .sup.224Ra in the sediment sample to be tested is calculated according to the linear equation which corresponds to the standard curve, and the activity of .sup.224Ra calculated is the theoretically calculated activity. In the present disclosure, the activity of .sup.224Ra in the sediment sample to be tested (C.sub.theory) is calculated according to equation 8,

C.sub.theory=(C.sub.1−C.sub.2)/k  equation 8.

[0063] In order to ensure the accuracy of the measurement, the method according to the present disclosure further comprises calculating a relative standard deviation of the activity of .sup.224Ra in the sediment sample to be tested.

[0064] In some embodiments of the present disclosure, calculating the relative standard deviation of the activity of .sup.224Ra in the sediment sample to be tested is performed by a process comprises the following steps: [0065] calculating a standard deviation of the sum of counting rates of .sup.222Rn and .sup.220Rn obtained from the first continuous measurement according to equation 2 and equation 3,

[00004]$\begin{array}{cc}{\sigma }_1=\frac{\sqrt{N_1}}{N_1}\times C_1,& \mathrm{equation}2\end{array}$$\begin{array}{cc}{N}_1=C_1\times T_1,& \mathrm{equation}3\end{array}$ [0066] in equations 2 and 3, σ.sub.1 represents the standard deviation of the sum of counting rates of .sup.222Rn and .sup.220Rn obtained from the first continuous measurement, in cpm; N.sub.1 represents a counting value in the first continuous measurement, in counts; T.sub.1 represents time for the first continuous measurement, in minute; [0067] calculating a standard deviation of the counting rate of .sup.222Rn obtained from the second continuous measurement according to equation 4 and equation 5,

[00005]$\begin{array}{cc}\mathrm{\sigma 2}=\frac{\sqrt{N_2}}{N_2}\times C_2,& \mathrm{equation}4\end{array}$$\begin{array}{cc}{N}_2=C_2\times T_2;& \mathrm{equation}5\end{array}$ [0068] in equations 4 and 5, σ.sub.2 represents the standard deviation of the counting rate of .sup.222Rn obtained from the second continuous measurement, in cpm; N.sub.2 represents a counting value in the second continuous measurement, in counts; T.sub.2 represents time for the second continuous measurement, in minute; and [0069] calculating the relative standard deviation of the activity of .sup.224Ra according to equation 6, represented by δ in equation 6,

[00006]$\begin{array}{cc}\delta =\frac{\sqrt{{\sigma }_1^2+{\sigma }_2^2}}{C_1-C_2}\times 100\%.& \mathrm{equation}6\end{array}$ [0070] In the present disclosure, the relationship between the actual activity of .sup.224Ra in the sediment sample to be tested and the theoretical activity therefore is represented by C.sub.actual=C.sub.theory×(1±δ).

[0071] The method for determining .sup.224Ra in a sediment by using a pulse ionization chamber emanometer according to the present disclosure will be described in detail below in conjunction with the examples, but they should not be construed as limiting the scope of the present disclosure.

Example 1

[0072] Six standard samples of silty sediments with a known .sup.224Ra activity gradient was provided. An experimental device as shown in Figure was used. The moister contents of the standard samples of sediments were adjusted to 30 wt %. An air pump was turned on, and the flow rate was adjusted to 1 L/min. The gas was circulated in the system for 5 minutes, such that a radioactive equilibrium between .sup.220Rn gas released from the sediment standard sample and .sup.224Ra in the sediment standard sample was reached, and a first continuous measurement for Rn in the test system was performed for 2 hours, obtaining the sum of the counting rates of .sup.222Rn and .sup.220Rn (i.e. C.sub.1). The air pump was turned off. The inlet valve(s) and outlet valve(s) of the PIC were closed, and the PIC was left to stand for at least 5 minutes, such that the .sup.220Rn in the PIC completely decayed and disappeared. A second continuous measurement for the activity of Rn in the PIC was performed for 2 hours, obtaining the counting rate of the .sup.222Rn (C.sub.2). The difference between the two measurement results (i.e. C.sub.1−C.sub.2) was the counting rate of .sup.220Rn in the system, i.e. the counting rate of .sup.224Ra. A standard curve of activities of .sup.224Ra versus counting rates was plotted, in which the counting rates of .sup.224Ra in the different sediment standard samples are set as ordinate, and activities of .sup.224Ra in the different sediment standard samples are set as abscissa. The standard curve was fitted, obtaining a linear equation and R.sup.2 value, the linear equation being shown as equation 9,

C.sub.1−C.sub.2=0.2A.sub.standard+1.6  equation 9.

[0073] 20 g of silty sediment to be tested was provided and placed into a sample tray, and subjected to a measurement according to the same procedure and conditions as the sediment standard samples. The moisture content of the sediment was adjusted the same as the standard samples, i.e., 30 wt %. The air pump was turned on, and the flow rate was adjusted to 1 L/min. The gas in the system was circulated for 5 minutes. A first continuous measurement for Rn in the test system was performed for 2 hours, obtaining a total counting rate of Rn (C.sub.1) of 5.0 cpm. The inlet valve(s) and outlet valve(s) of the PIC were closed. The PIC emanometer was left to stand for 5 minutes, such that the .sup.220Rn in the PIC completely decayed and disappeared. A second continuous measurement was performed for 2 hours for the activity of .sup.222Rn in the PIC, obtaining the counting rate of .sup.222Rn (C.sub.2) of 0.8 cpm. The difference between the two measurement results was the counting rate of the .sup.220Rn in the system, i.e. 5.0 cpm-0.8 cpm=4.2 cpm. Since the .sup.224Ra in the sediment and the .sup.220Rn in the system were in a secular radioactive equilibrium, the counting rate of .sup.224Ra by the instrument was 4.2 cpm. For the silty sediment samples, the standard sample had an efficiency of 0.2 cpm/dpm with R.sup.2 larger than 0.99. According to equation 9, the theoretical activity of .sup.224Ra in the sediment sample was calculated to be 1.05 dpm/g, i.e. 4.2 cpm/0.2 (cpm/dpm)/20 g=1.05 dpm/g.

[0074] Error Calculation:

[0075] The error of total radon in the first continuous measurement: 5 cpm×120 min=600 counts, the standard deviation of the counting rate (σ.sub.1) was 0.2 cpm, i.e. σ.sub.1=

[00007]$\frac{\sqrt{600}}{600}\times 5=0.2\mathrm{cpm};$

[0076] The error of .sup.222Rn in the second continuous measurement: 0.8×120 min=96 counts, the standard deviation of the counting rate (σ.sub.2) was 0.08 cpm, i.e.

[00008]$02=\frac{\sqrt{96}}{96}\times 0.8=0.08\mathrm{cpm};$

[0077] Therefore, the relative standard deviation of .sup.224Ra (δ) was 5%, i.e.

[00009]$\delta =\frac{\sqrt{{0.2}^2+{0.08}^2}}{4.2}\times 100\%=5\%.$

[0078] Therefore, the actual activity of.sup.224Ra in the sediment sample (C.sub.actual) was 1.05±0.05 dpm/g, i.e. C.sub.actual=1.05 dpm/g×(1±0.05)=1.05±0.05 dpm/g.

[0079] The above are only the preferred embodiments of the present disclosure. It should be pointed out that for those skilled in the art, without departing from the principles of the present disclosure, several improvements and modifications could be made. And the improvements and modifications shall fall within the scope of the present disclosure.