Am-Li neutron source preparation method based on molecular assembly

Abstract

According to the Am-Li neutron source preparation method based on molecular assembly, metal-organic framework (MOFs) molecular assembly serves as a core, Am < 3 + > and Li < + > are accurately connected through coordinate bonds to achieve field assembly of a neutron source, controllable release is achieved, the raw material utilization rate and the neutron yield are high, and the method is suitable for large-scale production. The used raw materials Am (NO), LiCOs, ligands and the like are all in a non-radioactive source state, can be transported independently, do not need lead shielding, are low in transportation cost, do not need to be reported in advance, and are adaptive to portable scenes such as the field and the site.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of neutron source preparation, and particularly relates to an Am-Li neutron source preparation method based on molecular assembly. BACKGROUND

[0002] A neutron source is a core equipment in the fields of nuclear physics research, medical diagnosis, industrial nondestructive testing, security and anti-terrorism, and its performance (neutron yield, energy spectrum stability, portability) directly determines the adaptability of the application scene. At present, the mainstream neutron sources can be divided into five categories according to the generation principle. Among them, the radioisotope neutron source becomes the first choice for portable scenes (such as field geological exploration and on-site security detection) because of its "simple structure, low cost and no need for external power supply", but it has the common problems of "fixed yield, difficult energy spectrum control and high transportation risk"; the accelerator neutron source can accurately control the energy, but it needs high-voltage acceleration equipment (such as a 1-10 MeV proton accelerator), and the volume is usually more than 10 m³, which is only suitable for fixed laboratory scenes.

[0003] Among the existing radioisotope neutron sources, the Am-Be source (neutron yield about 2.3 x 10 6 n / s・Ci) and the Am-Li source (neutron yield about 2.0 x 10 6 n / s・Ci) are most widely used. Both of them adopt the process of "mechanical mixing of Am powder and target material (Be / Li) + metal shell packaging" (such as the "compact Am-Be neutron source preparation method" disclosed in Chinese patent CN201810234567.1); the theoretical basis of this process is the "alpha particle bombardment target nucleus nuclear reaction", that is, the Am241 decay releases alpha particles, which react with Li nuclei to produce neutrons through the 7 Li (α, n) 10B reaction, but there are obvious limitations in actual application: first, mechanical mixing depends on stirring equipment, which is easy to cause Am particle agglomeration (the agglomerated particle size can reach 5-10 µm), resulting in uneven collision probability of alpha particles and Li target, and the neutron yield fluctuation range reaches ±15%; second, after packaging, it is a three-class radioactive source, which needs to meet the "three-class packaging" requirements of the "Regulations on the Safety Management of Radioactive Material Transport" (such as the thickness of the lead shielding layer ≥5 mm), and the transportation cost is 40-60% higher than that of ordinary goods, and it needs to be reported and approved by the environmental protection department in advance. SUMMARY

[0004] The purpose of the present application is to provide an Am-Li neutron source preparation method based on molecular assembly to solve the problems raised in the background art.

[0005] To achieve the above-mentioned purpose, the present application provides the following technical scheme: an Am-Li neutron source preparation method based on molecular assembly, characterized by comprising the following steps: S1: raw material pretreatment: three reagents with purity greater than 99.9% are weighed, and the molar ratio of Am (NO3) 3·6H2O: Li2CO3: H2BDC-NH2 is 2:4:3; each is dissolved with a solvent, and the solvent volume is matched according to the following ratio: Am (NO3) 3·6H2O is dissolved with N,N-dimethylformamide, and the volume ratio of Am (NO3) 3·6H2O to N,N-dimethylformamide is 1:100; Li2CO3 is dissolved with 0.1 mol / L dilute nitric acid, and the volume ratio of Li2CO3 to dilute nitric acid is 1:25; H2BDC-NH2 is dissolved with N,N-dimethylformamide, and the volume ratio of H2BDC-NH2 to N,N-dimethylformamide is 1:100; S2: MOFs assembly: mix the three solutions obtained in step S1 and transfer to a hydrothermal reaction kettle, seal the hydrothermal reaction kettle at an internal pressure of 0.3 MPa, and then place it in an oven, set the oven to 5℃ / min to 100℃ and keep for 20h; after the reaction is completed, cool to room temperature at 2℃ / min, then take out the hydrothermal reaction kettle and place it in a centrifuge, centrifuge at 8000rpm for 15min, then collect the precipitate; S3: the precipitate collected in step S2 is washed with N,N-dimethylformamide and then with ethanol, and the washed precipitate is placed in a 60℃ vacuum drying oven for 8h to obtain Am-Li-MOFs crystals; S4: place the Am-Li-MOFs crystals in the reaction container for neutron release, when it is necessary to stop the neutron release, add ethylenediaminetetraacetic acid solution with a molar ratio of 1:1 to the reaction container to terminate the reaction, wherein the concentration of the ethylenediaminetetraacetic acid solution is 0.01-0.1 mol / L.

[0006] Preferably, when dissolving Am (NO3) 3·6H2O and N,N-dimethylformamide, an ultrasonic device is used for ultrasonic dispersion for 25min; when dissolving H2BDC-NH2 and N,N-dimethylformamide, the solution is heated to 60℃ and stirred for 30min, and the solubility of the solute is improved by the above operation.

[0007] Preferably, in step S4, when the neutron is released, the neutron yield is calculated by the following formula: , wherein, is the neutron yield, is the total number of Am³⁺ in Am-Li-MOFs, is the decay constant of Am-241, Cross section for 7Li (α, n) 10B reaction, α particle flux, taking 1, α particle utilization, taking 0.95, 97% is the error coefficient of the theoretical yield and the actual yield of neutrons.

[0008] Compared with the prior art, the beneficial effects of the present application are: 1. The present application can transport Am (NO3)3, Li2CO3, ligand and other "non-radioactive source state" raw materials separately, assemble them on site, does not need lead shielding, can reduce the packaging weight to below 0.5kg, reduce the transportation cost, and does not need to be reported in advance, suitable for portable scenes such as field and site; 2. The present application realizes atomic-level uniform distribution of Am³⁺ and Li⁺ through molecular assembly, and the α particle utilization rate is increased to 90-95%, and the actual neutron yield is 2.7-2.8×10 6 n / s・Ci, which is 35-40% higher than the prior art, meeting the needs of high-sensitivity detection scenes (such as neutron imaging of trace explosives); 3. The hydrothermal reaction and centrifugal separation adopted by the present application are mature processes, and the equipment cost is only 1 / 100 of that of an accelerator neutron source; at the same time, it supports "multiple assembly-disassembly", (Am³⁺ after EDTA disassembly can participate in MOFs assembly again), and the raw material utilization rate is increased to more than 90%, reducing the use cost. BRIEF DESCRIPTION OF DRAWINGS

[0009] Figure 1 is the process flow chart of the Am-Li neutron source preparation method provided by the embodiment of the present application; Figure 2 is the scatter plot of the change of neutron yield with time when the neutron release is terminated; Figure 3 is the graph of the change of neutron yield deviation with ultrasonic dispersion time when Am(NO3)3・6H2O is dissolved. DETAILED DESCRIPTION

[0010] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0011] Please refer to Figures 1-3The application provides a technical solution: an Am-Li neutron source preparation method based on molecular assembly. The technology takes metal-organic framework (MOFs) molecular assembly as the core, precisely connects Am 3+ and Li + through coordination bonds, and realizes on-site assembly and controllable release of the neutron source. 1. Raw material system design Am source: Am(NO3)3·6H2O (purity ≥ 99.9%) is used. Am 3+ has the highest stability in aqueous solution (hydrolysis is usually 1.2×10 -5 -6 per year), and can avoid side reactions with other ions; Li source: Li2CO3 (purity ≥ 99.99%) is used. After being dissolved in dilute nitric acid, Li⁺ is generated, which has strong combination ability with the ligand and no interference of impurity ions; Ligand: 2-amino terephthalic acid (H2BDC-NH2) is selected. The amino group (-NH2) in the molecular structure of the ligand is an electron-donating group, which can enhance the coordination ability of the ligand and metal ions (the coordination bond energy is increased to 80-90 kJ / mol, which is 30-40% higher than that of the ligand without amino group), and the carboxyl group (-COOH) can form bidentate coordination with Am 3+ and Li + to construct a three-dimensional porous MOFs structure; Competitive ligand: ethylenediaminetetraacetic acid (EDTA, concentration 0.01-0.1 mol / L) is used. The coordination constant (lgK=23.2) of EDTA with Am 3+ is much larger than that of H2BDC-NH2 (lgK=12.5), which can competitively disassemble the MOFs structure.

[0012] 2. Molecular assembly process steps. The process flow chart is shown in Figure 1 . S1: raw material pretreatment: 0.1 mmol of Am (NO3)3·6H2O with a purity greater than 99.9% was weighed into 10 mL of N,N-dimethylformamide (using a specific gravity bottle method according to the volume ratio of Am (NO3)3·6H2O to N,N-dimethylformamide of 1:100), and ultrasonic dispersion was performed for 25 min using an ultrasonic device with a power of 300 w; 0.2 mmol of Li2CO3 with a purity greater than 99.9% was weighed into 5 mL of 0.1 mol / L dilute nitric acid solution and stirred until completely dissolved (using a specific gravity bottle method according to the volume ratio of Li2CO3 to dilute nitric acid of 1:25); 0.15 mmol of H2BDC-NH2 was weighed into 15 mL of N,N-dimethylformamide (using a specific gravity bottle method according to the volume ratio of H2BDC-NH2 to N,N-dimethylformamide of 1:100), and the solution was heated to 60°C and stirred for 30 min; S2: MOFs assembly: the three solutions obtained in step S1 were mixed and transferred to a hydrothermal reaction kettle, the hydrothermal reaction kettle was sealed at an internal pressure of 0.3 MPa and placed in an oven, the oven was set to increase the temperature to 100°C at a rate of 5°C / min and maintain for 20 h; after the reaction was completed, the hydrothermal reaction kettle was cooled to room temperature at a rate of 2°C / min, then taken out and placed in a centrifuge, and centrifuged at a speed of 8000 rpm for 15 min to collect the precipitate; S3: the precipitate collected in step S2 was washed with N,N-dimethylformamide and then with ethanol, and the washed precipitate was placed in a 60°C vacuum drying oven for 8 h to obtain Am-Li-MOFs crystals; S4: Am-Li-MOFs crystals were placed in a reaction container for neutron release, the reaction container used in this embodiment was a sealed container made of 316L material, and the leakage rate of the sealed container was ≤1×10 -9 Pa·m 3 / s, in the reaction container, Am 3+ decay released α particles uniformly bombarded Li + in the MOFs pores, and 7 Li (α, n) 10 B reaction produced neutrons; when it was necessary to stop the neutron release, an ethylenediaminetetraacetic acid (EDTA) solution with a molar ratio of 1:1 to Am-Li-MOFs was added to the reaction container, the EDTA competitively combined with Am 3+ , separated Am 3+ from Li + (the distance exceeded the range of α particles 0.05 mm), and the MOFs structure was completely disassembled within 30-60 min, and the reaction was terminated, during the termination of the reaction, the neutron yield changed with time as Figure 2As shown, the concentration of the ethylenediaminetetraacetic acid solution is 0.01-0.1 mol / L.

[0013] Neutron yield calculation during neutron release: Based on nuclear reaction dynamics, the formula for calculating neutron yield (Y) is as follows:

[0014] Parameter definitions and values: The total number of Am³⁺ atoms in Am-Li-MOFs is calculated based on the amount of raw materials used: 0.1 mmol Am(NO3)3・6H2O corresponds to =0.1×10 -3 ×6.02×10 23 =6.02×10 19 indivual The decay constant of Am-241 = ln2 / T 1 / 2 =0.693 / (432.2×365×24×3600)=5.08×10 -11 / s : 7 Li (α, n) 10 The cross-section of reaction B shows that the alpha particle energy released by Am-241 is 5.486 MeV, corresponding to... =0.12b=0.12×10 -28 m 2 Alpha particle flux; MOF porosity 50-60%; unimpeded alpha particle transport. = 1 (All α particles reach the vicinity of Li⁺) α-particle utilization rate, molecular assembly to achieve atomic-level uniform distribution of Am³⁺ and Li⁺. = 0.95.

[0015] Error Handling: Actual neutron yield may deviate due to incomplete ligand dissolution. This invention addresses this by using ultrasonic dispersion during the dissolution of Am(NO3)3·6H2O to ensure uniform solvent dissolution. The deviation in actual neutron yield can be controlled to within 3%. Figure 3 As shown, the actual neutron yield needs to be multiplied by an error factor of 97% based on the theoretical neutron yield. Substituting all the data into the calculation yields the actual neutron yield. ≈2.9×10 6 n / s・Ci.

[0016] Although the present application has been described in detail with reference to the foregoing embodiments, the technical solutions recorded in the foregoing embodiments can be modified, or some of the technical features can be replaced by equivalent features, by those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing an Am-Li neutron source based on molecular assembly, characterized in that: Includes the following steps: S1: Raw material pretreatment: Weigh three reagents, each with a purity greater than 99.9%, according to the molar ratio of Am(NO3)3·6H2O:Li2CO3:H2BDC-NH2 = 2:4:3; dissolve them separately in solvents, with the solvent volumes proportioned as follows: Am(NO3)3·6H2O is dissolved in N,N-dimethylformamide, and the volume ratio of Am(NO3)3·6H2O to N,N-dimethylformamide is 1:100; Li2CO3 is dissolved in 0.1 mol / L dilute nitric acid, and the volume ratio of Li2CO3 to dilute nitric acid is 1:25; H2BDC-NH2 is dissolved in N,N-dimethylformamide, and the volume ratio of H2BDC-NH2 to N,N-dimethylformamide is 1:100; S2: MOF assembly: The three solutions obtained in step S1 are mixed and transferred to a hydrothermal reactor. The hydrothermal reactor is sealed with an internal pressure of 0.3 MPa and placed in an oven. The oven is heated to 100°C at 5°C / min and kept at that temperature for 20 hours. After the reaction is completed, the temperature is lowered to room temperature at 2°C / min. The hydrothermal reactor is then removed and placed in a centrifuge. The precipitate is collected after centrifugation at 8000 rpm for 15 minutes. S3: The precipitate collected in step S2 was washed with N,N-dimethylformamide and then with ethanol. The washed precipitate was dried in a vacuum drying oven at 60°C for 8 hours to obtain Am-Li-MOFs crystals. S4: Place the Am-Li-MOFs crystals into the reaction vessel to release neutrons. When it is necessary to stop the neutron release, add an ethylenediaminetetraacetic acid solution with a molar ratio of 1:1 to Am-Li-MOFs to terminate the reaction. The concentration of the ethylenediaminetetraacetic acid solution is 0.01-0.1 mol / L.

2. The method for preparing an Am-Li neutron source based on molecular assembly according to claim 1, characterized in that: When dissolving Am(NO3)3・6H2O with N,N-dimethylformamide, ultrasonic dispersion was performed for 25 minutes using an ultrasonic device; when dissolving H2BDC-NH2 with N,N-dimethylformamide, the solution was heated to 60°C and stirred for 30 minutes.

3. The method for preparing an Am-Li neutron source based on molecular assembly according to claim 1, characterized in that: In step S4, during neutron release, the neutron yield is calculated using the following formula: In the formula, Y represents the neutron yield. The total number of Am³⁺ atoms in Am-Li-MOFs. The decay constant of Am-241, for 7 Li (α, n) 10 The cross section of reaction B. Let α be the alpha particle flux, with a value of 1. The alpha particle utilization rate is set to 0.95, and 97% is the error coefficient between the theoretical and actual neutron yield.

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