Method for measuring neutron-induced fission yield

Abstract

The invention relates to a neutron-induced fission yield determination method, belongs to the technical field of nuclear data determination, and solves the technical problems of low fission yield measurement efficiency and poor precision. The determination method comprises the following steps: selecting isotope carrier nuclides, manufacturing an irradiation target, carrying out neutron irradiation, chemically separating and purifying target isotopes, determining the nuclear number ratio of an activated product to a fission product in a purified sample, and calculating the yield of the fission product. The determination method is used for determination of neutron-induced fission yield and theoretical research and application of fission nuclear reaction.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear data measurement, and in particular relates to a method for measuring neutron-induced fission yield. Background Art

[0002] The nucleus is composed of protons and neutrons, and its mass number is the sum of the number of protons and neutrons that make it up. A nucleus with a larger mass number is called a heavy nucleus. A heavy nucleus that can undergo a fission nuclear reaction under the action of a neutron is called a fissile nucleus, and the fission nuclear reaction that occurs is called a neutron-induced fission nuclear reaction. Fission usually produces two nuclei with medium mass numbers, which are called fission fragments. The fission fragments are in a highly excited state and are often de-excited by emitting neutrons (prompt neutrons) and gamma rays (prompt gamma). The fission fragments that have emitted prompt neutrons and prompt gamma rays are called primary fission products. Most of the primary fission products are radioactive nuclides, which form a decay chain by a series of radioactive decays to stable nuclei. The nuclides generated after the primary fission products undergo radioactive decay are called secondary fission products. The primary fission products and secondary fission products are collectively referred to as fission products. There are about 500 nuclides of fission products discovered so far, including stable nuclides and short-lived radioactive nuclides with a half-life of less than 1 second. The mass numbers of fission products range from 66 to 172, belonging to more than 40 elements from nickel to erbium. The yield of fission products refers to the probability of generating the fission product during fission, which can be divided into independent yield and cumulative yield. Independent yield refers to the probability of directly generating the fission product during fission, and cumulative yield is the sum of its independent yield and the independent yield of all its precursor decay parents. Fission yield reflects the nature of nuclear fission, plays an important role in the theoretical research and application of fission nuclear reactions, and is an important nuclear data.

[0003] Atomic nuclei will also undergo neutron activation reactions such as (n, γ) and (n, 2n) under the action of neutrons, and the activation products are isotopes of the atomic nucleus (nuclides with the same number of protons but different mass numbers). The probability of nuclear reaction between incident neutrons and atomic nuclei (called target nuclei) is often expressed by cross sections.

[0004] A carrier is another substance that can carry a certain trace amount of a substance to participate in a certain chemical or physical process. According to the properties of the carrier and the substance it carries, it can be divided into isotopic carriers and non-isotopic carriers.

[0005] The measurement of fission yield includes relative measurement method and absolute measurement method. The former only needs to measure the amount of each fission product produced after fission, which is mainly used to measure the relative yield between products; the latter needs to measure the amount of fission product produced and the fission number of fissionable nuclei at the same time, and the ratio of the two is the yield.

[0006] Among the absolute measurement methods of fission yield, the classic measurement method is to determine the fission number with a neutron activation indicator and the content of the fission products by radiochemical method, thereby calculating the fission yield. In the classic measurement method, the neutron activation indicator used to determine the fission number is one nuclide, and the carrier added to separate and analyze specific fission products is another nuclide. The activation rate of the activation indicator and the chemical recovery rate of the carrier need to be measured separately, which has problems such as low experimental efficiency and low measurement accuracy. Summary of the invention

[0007] In order to overcome the shortcomings of low efficiency and poor precision in fission yield measurement, the present invention proposes a method for measuring neutron-induced fission yield.

[0008] The technical solution adopted by the present invention to solve the technical problem is:

[0009] A method for determining neutron-induced fission yield,

[0010] (1) Select isotope carrier nuclides. According to the fission products whose yields are to be measured, select appropriate isotopes as carrier nuclides. When selecting carrier nuclides, consider that their neutron activation reaction cross sections are suitable and that the activated products are easy to measure accurately.

[0011] (2) Preparation of irradiation targets. According to the irradiation conditions of the neutron source, a suitable irradiation target is prepared. The number of nuclides of carrier nuclides and fissile nuclides in the irradiation target is accurately known and is evenly mixed.

[0012] (3) Perform neutron irradiation. Place the above irradiation target on a neutron source and irradiate. Assume that the neutron flux rate is constant at φ and the fission cross section of the fissile nucleus is σ f , the activation cross section of the carrier nuclide is σ a The decay constant of the activated product is λ a , the independent yield of fission product i is y i , the decay constant is λ i , irradiation time is t R , and the irradiation time t R is short enough that the amount of secondary fission products generated by the decay of the precursor of fission product i during this period can be ignored. At the end of irradiation, the number of nuclides of fission product i and activation product a in the irradiated target are:

[0013] N i =N f0 ·σ f ·φ·y i ·[1-exp(-λ i ·t R )] / λ i (1)

[0014] N a =Na0 ·σ a ·φ·[1-exp(-λ a ·t R )] / λ a (2)

[0015] Where N i is the number of fission product i in the irradiated target at the end of irradiation, N f0 is the initial number of fissile nuclei in the irradiated target, N a0 and N a are the initial number of nuclei of the carrier and the number of nuclei of the activated product at the end of irradiation. The fission cross section, activation cross section and decay constant are all known nuclear data, which can be evaluation data or experimental measurement values.

[0016] (4) Chemical separation and purification of target isotopes. At time t after the irradiation, the target isotopes are purified from the mixture of fissionable nuclides, carrier nuclides and fission products of the irradiated target using an appropriate chemical separation method. Assuming that the isotope exchange between the carrier isotopes and fission product i is complete, and the fission product i generated by the decay of the precursor of fission product i can be ignored, the number of nuclides of fission product i and carrier activation product in the purified sample are:

[0017] N i =Y C ·N f0 ·σ f ·φ·y i ·exp(-λ i ·t)·([1-exp(-λ i ·t R )] / λ i (3)

[0018] N a =Y C ·N a0 ·σ a ·φ·exp(-λ a ·t)·[1-exp(-λ a ·t R )] / λ a (4)

[0019] Where Y C It is the chemical recovery rate of the activation product and the fission product i during the chemical separation and purification process, and the two are the same.

[0020] (5) Determine the ratio of the number of activation products to the number of fission products i in the purified sample. According to equations (3) and (4), the ratio R is:

[0021]

[0022] (6) Calculate the yield of fission product i. According to formula (5), the independent yield of fission product i is:

[0023]

[0024] When the precursors of fission product i have all decayed into the fission product at the moment of chemical separation, the yield calculated using the above formula is the cumulative yield.

[0025] The beneficial effects of the present invention are:

[0026] A method for determining neutron-induced fission yield, using activation product tracers of a carrier and the chemical recovery rates of its isotopic fission products, which are identical under conditions of complete isotope exchange, and determining the fission yield by measuring the ratio of activation product to fission product without separately measuring the chemical recovery rate.

[0027] A method for determining neutron-induced fission yield, in which the activation reaction share of a carrier indicates the fission reaction share of a fissile nucleus, and the fission yield is determined by determining the ratio of activation products to fission products without determining the fission number of the fissile nucleus.

[0028] A method for measuring neutron-induced fission yield, in which a carrier nuclide and a fissile nuclide are mixed to prepare an irradiation target, the neutron fields of the two nuclides are completely consistent, and the uncertainty of yield measurement introduced by the difference in neutron fields can be avoided.

[0029] A method for determining neutron-induced fission yield, in which a carrier and a fission product are isotopes of each other, and there is no need to add multiple tracers. The target of the radiochemical separation process is unique, and the separation process can be simplified and the speed of radiochemical separation can be increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of determining the neutron-induced fission yield in an embodiment of the present invention;

[0031] Figure 2 It is the target fission product of Example 1 of the present invention 132m Schematic diagram of the decay chain in which Sb is located;

[0032] Figure 3 It is the target fission product of Example 2 of the present invention 129 Schematic diagram of the decay chain in which Sb is located. DETAILED DESCRIPTION

[0033] The following is combined with Figure 1 The present invention is further described in detail with reference to the accompanying drawings and specific embodiments.

[0034] Embodiment 1

[0035] A method for determining the yield of neutron-induced fission, using the principle of isotope dilution, irradiating an irradiation target with accurately known amounts of fissile nuclides and isotope carrier nuclides on a neutron source, separating and purifying the isotopes of the carrier nuclides by chemical methods, determining the ratio of fission products in the isotopes to carrier activation products, and calculating the cumulative yield or independent yield of the relevant fission products.

[0036] Determination of thermal neutron induced radiation in reactors 239 When Pu fission 132m Independent yield of Sb.

[0037] (1) Select the isotope carrier. Thermal neutron induced 239 The main fission products of Sb generated during Pu fission and their yield evaluation values ​​(ENDF) and decay constants are shown in Table 1. According to the measurement purpose and economy, natural Sb was selected as the carrier, that is, natural 121 Sb (isotopic abundance 57.21%) and 123 Sb (isotopic abundance is 42.79%) as 132m Isotope carrier nuclide of Sb, the activation product of the two 122 Sb and 124 The fission yield of Sb is extremely low and the amount produced by fission can be ignored.

[0038] Table 1 Thermal neutron induced 239 Information on the main fission products of Sb generated during Pu fission

[0039]

[0040] (2) Preparation of irradiation targets. Use the reactor's rabbit-running device to irradiate the sample, and select a mixed solution of plutonium nitrate and antimony nitrate with known concentrations to prepare the irradiation target. 239 A mixed solution of plutonium nitrate with a Pu concentration of 0.1 mg / mL and antimony nitrate with a Sb concentration of 10 mg / mL is weighed with a 1 / 10,000 balance and added to the irradiation target and sealed. The fission nuclei in the irradiated target are 239 The mass of Pu is 0.1 mg, the mass of Sb element is 10 mg, 239 Pu, 121 Sb, 123 The nuclide numbers of Sb are accurately known, which are:

[0041]

[0042] (3) Perform neutron irradiation. The above irradiation target is placed on the reactor rabbit device and neutron irradiated in pulse mode. When irradiating one pulse, the total neutron injection at the rabbit irradiation position is φ = 1×10 14 n cm -2 , 239The fission cross section of Pu is σ f =747.83b, carrier isotope 121 Sb, 123 The (n,γ) activation cross sections of Sb are σ Sb121ng =5.77b,σ Sb123ng =3.875b.

[0043] Fission products 132m Sb and activation products 122 Sb, 124 The decay data of Sb are shown in Table 1, and the fission products 132m Sb, 129 The Sb-related decay chain is shown in Figure 2 .

[0044] Depend on Figure 2 It can be seen that: 132m Sb has no precursor nuclide, so the number of nuclei measured after irradiation is its independent yield y Sb132m Contribution

[0045] Taking the irradiation time as the “zero” time, according to formula (1) and formula (2), at the end of irradiation, the fission products 132m The number of Sb nuclei is 1.86×10 10 ×y Sb132m , activated product 122 Sb and 124 The number of Sb nuclei is 1.63×10 10 and 8.19×10 9 .

[0046] (4) Chemical separation and purification of target isotopes. After irradiation, t = 2 min, the isotopes of Sb are purified from the irradiated target using a suitable chemical separation method. The chemical recovery rate of the separation process is Y C , in the purified samples 122 Sb and 132m The number of Sb nuclei are:

[0047]

[0048] The ratio of the two is:

[0049]

[0050] (5) Determine the ratio of the number of activation products to fission products i in the purified sample. The purified sample is immediately measured on a high purity germanium (HPGe) detector. 132m Sb, 132 Sb and activation products 122 Sb, 124 The characteristic gamma rays of Sb that can be measured and analyzed are shown in Table 2.132m Sb and 132 Some characteristic gamma rays of Sb cannot be distinguished, so 132m The Sb measurement energy peak is selected at 150.6keV. 122 The 564.2keV radiation of Sb and 132m The peak area ratio S of the 150.6keV ray of Sb can be used to calculate the nuclear number ratio of the two at the time of measurement:

[0051]

[0052] Where S is the peak area of ​​the corresponding ray, t r is the clock time of spectrum measurement, p γ is the emission probability of the corresponding γ-ray, and ε is the relative detection efficiency.

[0053] Table 2 γ-ray energy and emission probability of each target Sb isotope

[0054]

[0055] (6) Calculate the yield of fission product i. By making equation (13) and equation (14) equal, we can calculate the fission product i. 132m The independent production of Sb is:

[0056]

[0057] Embodiment 2

[0058] Determination of thermal neutron induced radiation in reactors 239 When Pu fission 129 Cumulative production of Sb.

[0059] (1) Selecting an isotope carrier. This step is the same as in Example 1.

[0060] (2) Making an irradiation target. This step is the same as that in Example 1.

[0061] (3) Perform neutron irradiation. Place the above irradiation target on the reactor rabbit device and irradiate with neutrons for 10 seconds. The neutron injection rate at the rabbit irradiation position is 1×10 13 n cm -2 s -1 , 239 The fission cross section of Pu is σ f =747.83b, carrier isotope 121 Sb, 123 The (n,γ) activation cross sections of Sb are σ Sb121ng =5.77b,σ Sb123ng =3.875b. Fission products 129 Sb and activation products 122 Sb,124 The decay data of Sb are shown in Table 1, and the fission products 129 The Sb-related decay chain is shown in Figure 3 .

[0062] The start time of irradiation is recorded as the “zero” time. Since the irradiation time is short, the irradiation period 129 The decay of Sb can be ignored. Figure 3 It can be seen that: 129 The half-life of the precursor nuclide of Sb and its own half-life (T 1 / 2 =4.37h) are shorter than those of the previous one, with the longest being 414s. After irradiation and cooling for 1h, all precursor nuclides decay into 129 Sb, measured at this time 129 The number of Sb nuclei can be regarded as its cumulative yield Y Sb129 According to formula (1) and formula (2), the fission products in the irradiated target after cooling for 1 hour are 129 The nuclide number of Sb is 1.61×10 10 ×Y Sb129 , activated product 122 Sb and 124 The number of Sb nuclei is 1.63×10 10 and 8.19×10 9 .

[0063] (4) Chemical separation and purification of target isotopes. After cooling for 1 hour, the isotopes of Sb were purified from the irradiated target using a suitable chemical separation method. The chemical recovery rate of the separation process was Y C , in the purified samples 122 Sb and 129 The number of Sb nuclei are:

[0064] N Sb122 =1.63×10 10 ×Y C (16)

[0065] N Sb129 =1.61×10 10 ×Y Sb129 ×Y C (17)

[0066] The ratio of the two is:

[0067]

[0068] (5) Determine the ratio of the number of activation products to fission products i in the purified sample. The purified sample is immediately placed on a high purity germanium (HPGe) detector for measurement. As shown in Table 2, 122 Sb and 129The Sb measurement energy peaks are 564.28keV and 812.8keV respectively. 122 The 564.2keV radiation of Sb and 129 The peak area ratio of Sb's 812.8keV ray can give the nuclear number ratio of the two:

[0069]

[0070] (6) Calculate the yield of fission product i. By making equation (18) and equation (19) equal, we can calculate the fission product i. 129 The cumulative production of Sb is:

[0071]

[0072] Embodiment 3

[0073] Fissile nuclides include but are not limited to 233 U. 235 U. 238 U. 239 Pu, 241 When fissionable nuclides interact with neutrons of a certain energy, light-mass nuclei, i.e., fission products, can be generated through nuclear fission reactions.

[0074] Isotope carrier nuclides refer to natural or artificial nuclides that are isotopes of certain fission products (regardless of their specific chemical composition). When carrier nuclides interact with neutrons of a certain energy, stable or radioactive isotopes, i.e., activation products, can be generated through neutron activation reactions such as (n, γ) and (n, 2n).

[0075] The measurement of the isotope ratio relationship between fission products and activation products includes but is not limited to radioactivity spectrum and mass spectrometry measurement methods.

[0076] Fission yield refers to the cumulative yield or independent yield of a certain fission product generated when a fissionable nuclide undergoes neutron-induced nuclear fission.

Claims

1. A method for determining neutron-induced fission yield, characterized in that: The following steps are involved: Step 1, select the isotope carrier nuclide: According to the fission product whose yield is to be measured, its isotope is selected as the carrier nuclide under the condition that its neutron activation reaction cross section is suitable and the activation product can be accurately measured; Step 2: Make the irradiation target: According to the irradiation conditions of the neutron source, an irradiation target is manufactured; after the irradiation target is manufactured, the number of nuclides of the carrier nuclides and the fissile nuclides in the irradiation target is known; Step 3, neutron irradiation: neutron irradiation is performed on an irradiation target using a neutron source; Step 4: Chemical separation and purification of target isotopes: At time t after the irradiation is completed, the target isotope is purified from the mixture of fissile nuclides, carrier nuclides and fission products of the irradiated target by chemical separation method to obtain a purified sample; Step 5, determining the nuclear number ratio of activation products to fission products in the purified sample: According to the nuclear numbers of the activation product a and the fission product i in the purified sample of the purified target isotope, the nuclear number ratio R of the activation product a and the fission product i in the purified sample is calculated; Step 6, calculate the yield of fission products: According to the nuclear number ratio R of the activated product a to the fission product i in the purified sample obtained at time t after the end of neutron irradiation, the independent yield or cumulative yield of fission product i is calculated. When the precursor of fission product i has decayed into the fission product at the time of chemical separation, it is the cumulative yield.

2. The method for determining neutron-induced fission yield according to claim 1, characterized in that: The step 1 further comprises: Neutron induced 239 The fission yield of Pu is the object of measurement. When the fission product is 132m Sb, with its isotope 121 Sb and 123 Sb is the carrier nuclide.

3. The method for determining neutron-induced fission yield according to claim 2, characterized in that: The step 2 further comprises: A mixed solution of plutonium nitrate and antimony nitrate is used as the irradiation target; based on the plutonium concentration in the plutonium nitrate solution and the antimony concentration in the antimony nitrate solution, the nuclides in a certain amount of irradiation target are calculated. 239 Pu, 121 Sb, 123 The number of Sb nuclei.

4. The method for determining neutron-induced fission yield according to claim 3, characterized in that: In the plutonium nitrate solution, 239 The concentration of Pu is 0.1 mg / mL; the concentration of Sb in antimony nitrate solution is 10 mg / mL; in 1g of irradiated target solution, the nuclide 239 Pu, 121 Sb, 123 The number of Sb nuclei is: 2.52×10 17 , 2.83×10 19 , 2.11×10 19 .

5. The method for determining neutron-induced fission yield according to claim 2, characterized in that: The step 5 further comprises: The sample was purified and the number of nuclei of activation product a and fission product i was measured using a high purity germanium (HPGe) detector.

6. The method for determining neutron-induced fission yield according to claim 1, characterized in that: The step 5 further comprises: The number of nuclei of the activation product a and the fission product i of the purified sample is measured by radioactivity spectrum method or mass spectrometry method.

7. The method for determining neutron-induced fission yield according to claim 1, characterized in that: The step 6 further comprises: The independent yield or cumulative yield of fission product i is calculated as follows: In formula (6), y i is the independent or cumulative yield of fission product i, N f0 is the initial number of fissile nuclei in the irradiated target, N a0 is the initial number of cores of the carrier, σ f is the fission cross section of the fissile nucleus, σ a is the activation cross section of the carrier nuclide, λ a is the decay constant of the activated product, λ i is the decay constant, t R is the irradiation duration.

8. The method for determining neutron-induced fission yield according to claim 1, characterized in that: The fissile nuclide may be 233 U, or 235 U, or 238 U, or 241 Am.

9. The method for determining neutron-induced fission yield according to claim 1, characterized in that: The neutron source is a reactor rabbit device.

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