Single-energy X-ray air kerma valuing method

Abstract

The invention discloses a single-energy X-ray air kerma valuing method, and relates to the technical field of ionizing radiation metrology, and the method comprises the following steps: S1, building a standardized single-energy X-ray radiation field through a single-energy X-ray generation device; s2, collecting and analyzing the energy spectrum of the single-energy X-ray through an energy spectrum measurement system, and calculating the absorbed dose rate of the detector; s3, calculating the absorbed dose rate of air according to the absorbed dose rate of the detector; and S4, the air kerma energy is calculated under the electronic balance condition. According to the method provided by the invention, differential correction can be carried out on the energy response characteristics of different types of radiation monitoring equipment, the measurement error after equipment calibration is reduced, and the dose measurement accuracy is improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of ionizing radiation metrology, and particularly relates to a single-energy X-ray air kerma calibration method, which is suitable for the calibration of personal dosimeters, environmental radiation instruments, industrial CT dose detectors and the like, and can realize accurate value transfer of a single-energy X-ray radiation field. The present application particularly relates to a single-energy X-ray air kerma calibration method. BACKGROUND

[0002] As the core reference radiation field for the calibration of radiation monitoring equipment (such as personal dosimeters, environmental radiation instruments, industrial CT dose detectors and the like), the accurate value of the photon fluence and dose of single-energy X-rays is a key link in the value traceability system - only by realizing accurate value transfer of the fluence and dose of single-energy X-rays, can the measurement results of various types of radiation monitoring equipment be ensured to have comparability and accuracy.

[0003] At present, the value traceability of single-energy X-ray radiation monitoring equipment faces the following prominent problems:

[0004] Value deviation caused by energy response difference of equipment: the detector materials of different types of radiation monitoring equipment (such as ionization chamber type, NaI scintillation spectrometer type, CZT semiconductor type, high-purity germanium spectrometer type) are different, and the mass energy absorption coefficients (μ en / ρ) of the detector materials to photon energy are significantly different. However, the existing value calibration method has obvious defects: a differentiated fluence-dose conversion model is not established for different types of equipment, and a correlation derivation model of "detector absorbed dose-air kerma" is also lacking, and only an empirical formula is used for estimation, especially in the case of low-energy single-energy X-rays (10 keV-100 keV), due to the amplification of the energy response difference between the detector material and air, the value deviation can reach 10%-20%, which seriously affects the calibration accuracy.

[0005] Logical fault between fluence and kerma: although the existing method can obtain the photon fluence or the detector absorbed dose through energy spectrum measurement, the physical derivation chain of "fluence-absorbed dose-air kerma" is not clear, especially the correlation between the two under the condition of electron balance is ignored, which leads to poor data consistency in cross-laboratory value comparison.

[0006] Limited energy and fluence rate coverage: due to the limitation of the adjustment accuracy of the crystal monochromator and the energy spectrum analysis method, the existing method has a relatively narrow energy coverage (mostly concentrated in 50 keV-200 keV) and fluence rate adaptation range (mostly limited to 10 4 -10 6 cm - 2·s - 1) per second), which cannot meet the requirements of low-energy (<50 keV) and low-fluence rate (<10 4 cm - 2·s- 1) Device calibration requirements in scenarios. SUMMARY

[0007] In view of the defects in the prior art, the purpose of the present application is to provide a single-energy X-ray air kerma calibration method, which can correct the energy response characteristics of different types of radiation monitoring devices, reduce the measurement error after device calibration, and improve the accuracy of dose measurement.

[0008] To achieve the above purpose, the technical solution adopted by the present application is as follows:

[0009] A single-energy X-ray air kerma calibration method, comprising the following steps:

[0010] S1, establishing a standardized single-energy X-ray radiation field by a single-energy X-ray generating device;

[0011] S2, collecting and analyzing the energy spectrum of single-energy X-rays by an energy spectrum measurement system, and calculating the absorbed dose rate of the detector;

[0012] S3, calculating the absorbed dose rate of air according to the absorbed dose rate of the detector;

[0013] S4, calculating the air kerma under electronic equilibrium conditions.

[0014] Further, as described above, the single-energy X-ray air kerma calibration method, the single-energy X-ray generating device in step S1 includes an irradiation system, a laser collimation system, a crystal monochromator, and an energy spectrum measurement system, the irradiation device is used to generate X-rays of mixed energy; the laser collimation system is used to calibrate the beam axis to ensure the uniformity of the radiation field; the crystal monochromator is used to separate the X-rays of mixed energy into single-energy X-rays.

[0015] Further, as described above, the single-energy X-ray air kerma calibration method, the energy E is controlled by adjusting the Bragg angle of the crystal monochromator in step S1, and the energy calculation formula is:

[0016] E = nhv / (2dsinθ) (1)

[0017] In the formula, n is the diffraction order, which is an integer 1; h is Planck's constant, which is 6.62607015 x 10 -34 J·s; v is the speed of light, which is 3.00 x 108m / s; d is the interplanar spacing; θ is the Bragg angle of the crystal.

[0018] Further, as described above, the single-energy X-ray air kerma calibration method, the crystal monochromator is a Si(111) crystal, and the energy spectrum measurement system is a high-purity germanium spectrometer.

[0019] Further, in the single-energy X-ray air kerma calibration method as described above, the baseline deduction and peak fitting of the energy spectrum of the single-energy X-ray are performed by the multi-channel analysis software of the energy spectrum measurement system in step S2 to obtain the count rate of each channel and the corresponding photon energy.

[0020] Further, in the single-energy X-ray air kerma calibration method as described above, the formula for calculating the absorbed dose rate of the detector in step S2 is:

[0021] D T =∑(N i ×E i ) / (m×ε) (3)

[0022] In the formula,

[0023] N i - the count rate of the i-th channel;

[0024] E i - the photon energy corresponding to the i-th channel;

[0025] m- the mass of the sensitive volume of the detector;

[0026] ε- the detection efficiency of the detector, which is calibrated by a 152Eu standard source.

[0027] Further, in the single-energy X-ray air kerma calibration method as described above, step S3 is specifically:

[0028] According to the basic principle of ionizing radiation dosimetry, the absorbed dose rate of the detector and the absorbed dose rate of the air are respectively represented as:

[0029]

[0030] In the formula, is the photon fluence rate; E is the photon energy; (μ en / ρ) T is the mass energy absorption coefficient of the detector material, (μ en / ρ) a is the mass energy absorption coefficient of the air, which can be obtained by referring to the ICRU Report 90 data table;

[0031] Dividing formula (4) by formula (5), the ratio of the absorbed dose rates of the detector and the air is:

[0032] D T / D a = (μ en / ρ) T / (μ en / ρ) a (6)

[0033] The absorbed dose rate D of air is obtained according to formula (6) a is:

[0034] D a = D T × (μ en / ρ) a / (μ en / ρ) T (7)

[0035] Further, the step S4 of the method for determining the air kerma of mono-energetic X-rays as described above is specifically:

[0036] An electron equilibrium condition is established in the measurement region, and under the electron equilibrium condition, the air kerma is approximately equal to the absorbed dose of air, i.e.:

[0037] K≈D a (8)

[0038] Under the electron equilibrium condition, the energy transferred from the photons to the electrons is ultimately all absorbed by the air, and thus:

[0039] (μ tr / ρ) a ≈(μ en / ρ) a (9)

[0040] Substituting formula (9) into formula (5) and combining formula (8), the final calculation formula of the air kerma is obtained as:

[0041]

[0042] In formula (10):

[0043] K(r) - air kerma at point r in the radiation field;

[0044] E - photon energy of the mono-energetic X-rays, calculated according to the Bragg diffraction formula;

[0045] Φ(r) - photon fluence at point r, derived by transformation and deduction of formula (4) Further combined with the measurement time t, the photon fluence Φ(r) is obtained

[0046] (μ tr / ρ) a - mass energy transfer coefficient of air, obtained by referring to the data table of ICRU Report 90.

[0047] Further, the method for establishing electron balance condition in the measurement area in the single-energy X-ray air kerma calibration method as described above is: an air equivalent wall is arranged in front of the detector of the energy spectrum measurement system to simulate the electron range of air, so that the measurement area satisfies the electron balance condition, at this time the deviation of air kerma K and the air absorbed dose rate D a of air is ≤±1%.

[0048] Further, the single-energy X-ray air kerma calibration method as described above, the thickness of the air equivalent wall is 5mm-10mm, the material is polyethylene, and the density is 0.92g / cm 3 .

[0049] Compared with the prior art, the single-energy X-ray air kerma calibration method provided by the application has the following beneficial effects:

[0050] 1) Logical chain is complete: by establishing the physical derivation relationship of detector absorbed dose rate, air absorbed dose rate and air kerma, the formula fault of the prior art is eliminated, and the traceability of value transfer is ensured;

[0051] 2) Equipment response difference correction: by introducing the mass energy absorption coefficient ratio of the detector and air ((μ en / ρ) a / (μ en / ρ)T), the energy response correction is carried out for different detector materials such as NaI, CZT and high-purity germanium, and the calibration deviation is reduced from 10%-20% to ≤1% in the low-energy scene;

[0052] 3) Wide energy and fluence rate coverage: by adjusting the crystal Bragg angle (0.5°-15°), the energy coverage of 10keV-300keV is realized; the fluence rate range of 10 3 -10 7 cm -2 ·s -1 is adapted to meet the calibration requirements of various radiation monitoring equipment;

[0053] 4) Simple operation and high precision: relying on standardized radiation field and mature energy spectrum analysis technology, without complex additional equipment, the calibration result can be directly used for cross-laboratory value comparison, and the consistency error is ≤±2%.

[0054] In summary, the application directly realizes the calibration of photon fluence and air kerma by establishing a standardized single-energy X-ray radiation field and combining the conversion coefficient method, which can significantly improve the dose measurement accuracy; relying on the fluence-dose conversion model, the accuracy of dose measurement is improved; by adjusting the Bragg angle to cover a wide energy range, multiple fluence rate scenes are adapted, which can meet the calibration requirements of various spectrometers such as NaI, CZT and high-purity germanium. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 A flow chart of a method for determining the air kerma of monochromatic X-rays according to an embodiment of the present application.

[0056] Figure 2 A schematic diagram of the overall structure of a monochromatic X-ray generating device. DETAILED DESCRIPTION

[0057] The present application will be further described in conjunction with the accompanying drawings and specific embodiments.

[0058] Figure 1 A flow chart of a method for determining the air kerma of monochromatic X-rays according to an embodiment of the present application is shown, which includes the following steps:

[0059] S1. Establishing a standardized monochromatic X-ray radiation field by a monochromatic X-ray generating device.

[0060] Figure 2 A schematic diagram of the overall structure of a monochromatic X-ray generating device is shown, which includes, in order along the optical path, an irradiation device, a laser collimation system, a crystal monochromator, and an energy spectrum measurement system. The irradiation device is used to generate X-rays of mixed energy; the laser collimation system is used to calibrate the beam axis and ensure the uniformity of the radiation field; an aperture is provided at the outlet of the laser collimation system to limit the beam divergence angle; the crystal monochromator is used to separate the X-rays of mixed energy into monochromatic X-rays. The energy calculation formula is:

[0061] E = nhv / (2dsinθ) (1)

[0062] wherein n is the diffraction order, which is an integer of 1; h is the Planck constant, which is 6.62607015 x 10 -34 J·s; v is the speed of light, which is 3.00 x 108m / s; d is the interplanar spacing; and θ is the Bragg angle of the crystal. Therefore, the radiation energy can be controlled by adjusting the Bragg angle of the crystal.

[0063] In a specific embodiment of the present application, the crystal monochromator used is a Si(111) crystal, and the energy spectrum measurement system used is a high-purity germanium spectrometer.

[0064] S2. Collecting and analyzing the energy spectrum by the energy spectrum measurement system, and calculating the absorbed dose rate of the detector.

[0065] After the energy spectrum of the monochromatic X-rays is collected by the energy spectrum measurement system, the baseline is deducted and the peak shape is fitted by the multi-channel analysis software provided with the spectrometer, and the following key parameters can be obtained:

[0066] N i: The count rate of the i-th channel (unit: counts / s), representing the response intensity of the photon of the corresponding energy in the detector;

[0067] E i The photon energy (in J) corresponding to the i-th energy channel is obtained through the spectrometer energy calibration curve (from...). 137 Cs、 60 (The Co standard source calibration was obtained)

[0068] The absorbed dose rate D produced by monoenergetic X-rays in the detector T (Unit: Gy / s), essentially the photon energy absorbed per unit mass of the detector, can therefore be calculated by summing the products of the count rate and energy of each channel, i.e.:

[0069] D T =∑(N i ×E i ) / m (2)

[0070] In the formula, m is the mass of the detector's sensitive volume (unit: kg), which is calculated from the detector's dimensions (such as the diameter and length of the high-purity germanium crystal) and the material density (the density of high-purity germanium is 5.323 g / cm3).

[0071] In actual measurements, N i This includes the detector efficiency ε (i.e., the probability that a photon is recorded after it enters the detector), therefore it is necessary to consider D. T The detection efficiency is corrected, and the corrected formula is:

[0072] D T =∑(N i ×E i ) / (m×ε) (3)

[0073] In the formula, ε passes through 152 The detection efficiency error is ≤ ±3% at different energies, obtained by calibration using a standard Eu source (energy range 121.78keV-1408.01keV).

[0074] S3. Calculate the absorbed dose rate of air based on the absorbed dose rate of the detector.

[0075] According to the basic principles of ionizing radiation dosimetry: the absorbed dose rate and photon fluence rate generated by photons in a medium are related. (Unit: cm) -2 ·s -1 Photon energy E (unit: J) and the mass energy absorption coefficient μ of the medium en / ρ(unit: m) 2 It is directly proportional to ( / kg), and the calculation formula is:

[0076]

[0077] Where fluence rate represents the number of photons per unit area per unit time, each photon carrying energy E, the mass energy absorption coefficient (μ en / ρ) of the medium represents the proportion of photon energy absorbed by the medium, and the product of the three is the energy absorbed per unit mass per unit time by the medium (i.e., the absorbed dose rate).

[0078] Therefore, the absorbed dose rate of the detector and the absorbed dose rate of air can be respectively expressed as:

[0079]

[0080] Where (μ en / ρ) T is the mass energy absorption coefficient of the detector material, and (μ en / ρ) a is the mass energy absorption coefficient of air, both of which can be obtained by referring to the ICRU Report 90 data table.

[0081] Dividing equation (4) by equation (5), the ratio of the absorbed dose rates of the detector and air is obtained as:

[0082] D T / D a = (μ en / ρ) T / (μ en / ρ) a (6)

[0083] According to equation (6), the absorbed dose rate D a of air is:

[0084] D a = D T × (μ en / ρ) a / (μ en / ρ) T (7)

[0085] S4. Calculate the air kerma under the condition of electronic equilibrium

[0086] In radiation metrology, the air kerma K (unit: Gy) is defined as "the total electronic kinetic energy released by photons in air divided by the mass of air", and the air absorbed dose D a is defined as "the total electronic kinetic energy released by photons in air and finally absorbed by air divided by the mass of air".

[0087] When the electron balance condition is met (i.e. the energy of the electrons released at a point in the air is all absorbed in the air around the point, without electrons escaping or external electrons incident), the amount of kinetic energy released by the electrons is equal to the amount of kinetic energy absorbed, so:

[0088] K≈D a (8)

[0089] The present application ensures electron balance by the following design: an air equivalent wall (thickness 5mm-10mm, material polyethylene, density 0.92g / cm 3 ) is arranged in front of the detector of the energy spectrum measurement system to simulate the electron range of air, so that the measurement area meets the electron balance condition, and the deviation of K and D a is ≤±1%.

[0090] Combining formula (5) and formula (8), the correlation between the air specific kinetic energy release and the fluence can be obtained. It should be noted that in the definition of specific kinetic energy, the mass energy transfer coefficient (μ tr / ρ) is used (representing the proportion of photon energy transferred to electrons), while the absorbed dose uses the mass energy absorption coefficient (μ en / ρ) (representing the proportion of electron kinetic energy absorbed by the medium).

[0091] Under the electron balance condition, the energy transferred by the photons to the electrons is ultimately all absorbed by the air, so:

[0092] (μ tr / ρ) a ≈(μ en / ρ) a (9)

[0093] Substituting formula (9) into formula (5) and combining formula (8), the final calculation formula of the air specific kinetic energy release is:

[0094]

[0095] In formula (10):

[0096] K(r): air specific kinetic energy release at point r in the radiation field (unit: Gy);

[0097] E: photon energy of monochromatic X-rays (unit: J), calculated by the Bragg diffraction formula;

[0098] The photon fluence at point r (unit: cm - 2) can be derived by modifying formula (4) And the photon fluence is obtained by combining the measurement time t

[0099] (μtr / ρ) a : mass energy transfer coefficient of air (unit: m 2 / kg), which can be obtained by referring to ICRU Report 90 data table (corresponding to energy E).

[0100] Embodiment

[0101] The method provided by the application is used to determine the air kerma of 100 keV mono-energetic X-ray, and the steps and results are as follows:

[0102] Step 1, constructing radiation field: the Bragg angle θ of Si(111) crystal corresponding to 100 keV mono-energetic X-ray is calculated by formula (1), and then the Bragg angle of Si(111) crystal is adjusted so that the output mono-energetic X-ray energy is 100 keV.

[0103] Step 2, energy spectrum analysis: the energy spectrum is collected by using high-purity germanium spectrometer, and ∑(N i ×E i ) = 5.2 × 10 -6 J / s is obtained.

[0104] D T = 5.2 × 10 -6 / (0.01 × 0.85) ≈ 6.12 × 10 -4 Gy / s

[0105] Step 3, calculating air absorbed dose rate: by referring to ICRU data table, (μ en / ρ) T (high-purity germanium) = 0.018 m 2 / kg, (μ en / ρ) a (air) = 0.029 m 2 / kg, and formula 7 is used to calculate:

[0106] D a = 6.12 × 10 -4 × 0.029 / 0.018 ≈ 9.83 × 10 -4 Gy / s

[0107] Step 4, calculating air kerma: the measurement time t = 100 s, and formula 4 is used to deduce:

[0108]

[0109] (100 keV = 1.6 × 10 -14 J), so the photon fluence Table (μ tr / ρ) a = 0.029 m 2 / kg (with (μ en / ρ) a approximation), calculated by formula 10:

[0110] K(r) = 1.6 x 10 -14 x 2.125 x 10 8 x 10 -4 m 2 / kg x 0.029 = 9.83 x 10 -2 Gy

[0111] Step 5, result verification: compared with the value result (9.78 x 10 -2 Gy) of the national standard device, the deviation is only 0.51%, which is significantly better than the existing method (deviation 5%-8%).

[0112] The single-energy X-ray air kerma calibration method provided by the application can significantly improve the accuracy of dose measurement by establishing a standardized single-energy X-ray radiation field and directly realizing the calibration of photon fluence and air kerma by combining the conversion coefficient method; relying on the fluence-dose conversion model, the accuracy of dose measurement is improved; by adjusting the Bragg angle to cover a wide energy range, it is adapted to multiple fluence rate scenarios, and can meet the calibration requirements of various spectrometers such as NaI, CZT, high-purity germanium and the like.

[0113] Obviously, those skilled in the art can make various modifications and variations to the application without departing from the spirit and scope of the application. Thus, if these modifications and variations of the application fall within the scope of the claims of the application and their equivalents, the application also intends to include these modifications and variations.

Claims

1. A method for determining the air kerma of mono-energetic X-rays, comprising the following steps: S1. Establishing a standardized mono-energetic X-ray radiation field by a mono-energetic X-ray generating device; S2. Collecting and analyzing the energy spectrum of the mono-energetic X-rays by an energy spectrum measurement system, and calculating the absorbed dose rate of the detector; S3. Calculating the absorbed dose rate of air according to the absorbed dose rate of the detector; S4. Calculating the air kerma under the condition of electronic equilibrium.

2. The monoenergetic x-ray air kerma calibration method of claim 1, wherein, The mono-energetic X-ray generating device in step S1 comprises an irradiation system, a laser collimation system, a crystal monochromator, and an energy spectrum measurement system, wherein the irradiation device is used to generate X-rays of mixed energy; the laser collimation system is used to calibrate the beam axis and ensure the uniformity of the radiation field; and the crystal monochromator is used to separate the X-rays of mixed energy into mono-energetic X-rays.

3. The monoenergetic x-ray air kerma calibration method of claim 2, wherein, In step S1, the radiation energy E is controlled by adjusting the Bragg angle of the crystal monochromator, and the energy calculation formula is: E = nhv / (2dsinθ) (1) wherein n is a diffraction order, taking an integer of 1; h is Planck's constant, taking 6.62607015 x 10 -34 J s; v is the speed of light, taking 3.00 x 108m / s; d is the interplanar spacing; and θ is the Bragg angle of the crystal.

4. Method for the determination of the air kerma of monoenergetic X-rays according to claim 2 or 3, characterized in that, The crystal monochromator is a Si(111) crystal, and the energy spectrum measurement system is a high-purity germanium spectrometer.

5. The monoenergetic x-ray air kerma calibration method of claim 4, wherein, In step S2, the energy spectrum of the mono-energetic X-rays is baseline-subtracted and peak-shaped fitted by the multi-channel analysis software of the energy spectrum measurement system, to obtain the count rate and corresponding photon energy of each channel.

6. The monoenergetic x-ray air kerma calibration method of claim 5, wherein, In step S2, the calculation formula of the absorbed dose rate of the detector is: D T =∑(N i ×E i ) / (m×ε) (3) wherein: N i - the count rate of the i-th energy channel; E i - the photon energy corresponding to the i-th energy channel; m is the mass of the sensitive volume of the detector; ε is the detection efficiency of the detector, which is calibrated by a 152Eu standard source.

7. The monoenergetic x-ray air kerma calibration method of claim 6, wherein, Step S3 is specifically: According to the basic principles of ionizing radiation dosimetry, the absorbed dose rate of the detector and the absorbed dose rate of air are respectively represented as: where is the photon fluence rate; E is the photon energy; (μ en / ρ) T is the mass energy absorption coefficient of the detector material, (μ en / ρ) a is the mass energy absorption coefficient of air, all of which can be obtained from the ICRU Report 90 data tables; Dividing formula (4) by formula (5), the ratio of the absorbed dose rates of the detector and air is: D T / D a = (μ en / ρ) T / (μ en / ρ) a (6) The absorbed dose rate D of air is obtained from equation (6) a is: D a = D T × (μ en / ρ) a / (μ en / ρ) T (7) 8. The monoenergetic x-ray air kerma calibration method of claim 7, wherein, Step S4 is specifically: Establishing an electronic equilibrium condition in the measurement area, and under the condition of electronic equilibrium, the air kerma is approximately equal to the absorbed dose of air, i.e.: K≈D a (8) Under the condition of electronic equilibrium, the energy transferred by photons to electrons is ultimately all absorbed by air, so: (μ tr / ρ) a ≈(μ en / ρ) a (9) Substituting formula (9) into formula (5) and combining formula (8), the final calculation formula of the air kerma is: In formula (10): K(r) is the air kerma of point r in the radiation field; E is the photon energy of the mono-energetic X-rays, which is calculated by the Bragg diffraction formula; The photon fluence at a point is derived by a modification of equation (4) Again, the photon fluence is obtained by combining the measurement time t (μ tr / ρ) a - Mass energy transfer coefficient for air, obtained from ICRU Report 90 data table.

9. The monoenergetic x-ray air kerma calibration method of claim 8, wherein, The method for establishing electron balance condition in the measuring area is: setting an air equivalent wall in front of the detector of the energy spectrum measuring system to simulate the electron range of air, so that the measuring area meets the electron balance condition, at this time the deviation of air specific energy release K and air absorption dose rate D a is ≤±1%.

10. The monoenergetic x-ray air kerma calibration method of claim 9, wherein, The air equivalent wall has a thickness of 5-10 mm, is made of polyethylene and has a density of 0.92 g / cm3 3 .

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