CN103311086B - Multi-layer cavity ionization chamber - Google Patents

Abstract

The invention belongs to the technical field of ionizing radiation dose measurement, and discloses a multilayer cavity ionization chamber. The ionization chamber consists of a basic thin-wall ionization chamber and a series of concentric chamber walls with different thicknesses, and is characterized in that the ionization chambers with the chamber walls with different thicknesses can be obtained by selecting proper concentric chamber walls to combine. The ionization chamber has the characteristics of simple operation and accurate measurement result.

Description

Multi-layer cavity ionization chamber

Technical field

The invention belongs to technical field of ionization radiation dose measurement, be specifically related to a kind of multi-layer cavity ionization chamber.

Background technology

Ionization chamber measuring technique is one of basic fundamental of radiation dose measurement.Usual ionization chamber is made up of high-pressure stage, collector and protection pole, and its structure can adopt plate shaped, spherical, cylindrical etc., and inner punching, with suitable gas medium, is air under normal circumstances.By imposing suitable operating voltage, forming uniform electric field in ionization chamber inside, realizing being ionized the electronics of medium and the charge-trapping of ion, thus reaching radiometric object.For spheric (ionization) chamber, spherical center is collector, forms high-pressure stage by the spherical housing of suitable material, is also the locular wall of ionization chamber simultaneously.

Air kerma is the important physical amount characterizing radiation dose size.The air kerma of ionization chamber to photon is utilized to measure.Its cardinal principle is that the locular wall of photon and ionization chamber interacts, and produces secondary electron, and secondary electron causes gas medium ionization in sensitive volume, and ionization chamber collector collects electronics or cation.Such measuring principle requires that the thickness of locular wall must meet the requirement of so-called secondary charged particle balance, to make measurement result accurately and reliably.

Due to usually measure X, these photons of gamma-rays energy all at below 2MeV, so the structure of traditional spheric (ionization) chamber is fixing, chamber wall thickness is about 600mg/cm ², this thickness measure energy is that the measurement of the photon of below 2MeV can meet secondary charged particle balance requirement.When measured photon energy is increased to more than 2MeV, traditional ionization chamber structure just can not realize required measurement object, main cause is along with the range of the increase secondary charged particle of photon energy adds a lot, if meet the requirement of secondary charged particle balance requirement, the thickness of ionization chamber locular wall just must be increased.Such as, for the photon of (6 ~ 7) MeV, need chamber wall thickness by common about 600mg/cm ²be increased to 4g/cm ²above.

In addition, during measurement to the air kerma of high-energy photon, the effective attenuation curve of high-energy photon in the ionization chamber wall material of different-thickness must be measured, and then obtain air kerma.1999, German PTB disclosed and measures effective attenuation curve with the spheric (ionization) chamber of one group of different wall in document " Calibrationofpersonalandareadosimetersinhigh-energyphoto nfields ".But this method existing defects, if the ionization chamber adopting one group of different wall, will inevitably bring uncertainty because of the difference of sensitive volume, must carry out the calibration of sensitive volume to the ionization chamber of each thickness.In addition, multiple ionization chamber need be equipped with when measuring different-energy scope photon air kerma.

Summary of the invention

The present invention is directed to the problem existing for prior art, propose a kind of multi-layer cavity ionization chamber, this ionization chamber is by the photon of the measurement in a closed series different-energy scope of different wall, simple to operate and measurement result is accurate.

In order to solve the problem, the present invention is achieved by the following technical solutions:

Multi-layer cavity ionization chamber, this ionization chamber is made up of the homocentric sphere hull shape locular wall of a basic thin-wall chamber and a series of different-thickness, key is, can for different measuring objects, the i.e. energy range of photon, choose suitable spherical shell shape locular wall and carry out combining the ionization chamber obtaining there is different-thickness locular wall, to meet the requirement of high-energy photon secondary charged particle balance.

Described basic thin-wall chamber is primarily of collector, high-pressure stage, protection pole composition.Wherein, collector is positioned at the center of multi-layer cavity ionization chamber, for collecting electronics or cation; High pressure is ionization chamber ground floor locular wall very; The effect of protection pole ensures electric fields uniform; Collector, high-pressure stage, protection pole constitute a basic ionization chamber measuring unit.

Described a series of different-thickness spherical shell is concentric with ionization chamber ground floor locular wall, and each spherical shell is connected to form by the mode of close-fitting by two hemisphere spherical shells, the inwall of concentric spherical shell and the wall contacts of front one deck locular wall.The material of spherical shell and ionization chamber ground floor locular wall is graphite.

Beneficial effect of the present invention is:

(1) measure accurately.Because the sensitive volume in multi-layer cavity ionization chamber is fixing in different wall anabolic process, therefore avoid the measure error adopting different ionization chamber to bring in sensitive volume difference.

(2) improve operating efficiency.When adopting the ionization chamber of one group of different wall to measure effective attenuation curve, must correct the sensitive volume of each ionization chamber, and adopt multi-layer cavity ionization chamber, when measuring effective attenuation curve, avoid the step that sensitive volume is corrected, thus simplify working procedure, improve measurement efficiency.

(3) cost has been saved.Only need process the locular wall of a basic ionization chamber measuring unit and a series of different-thickness, and not need to process multiple ionization chamber, effectively save cost.

Accompanying drawing explanation

Fig. 1 multi-layer cavity ionization chamber structural representation;

1, ionization chamber collector; 2, ionization chamber protection pole; 3, connecting rod; 4, ionization chamber ground floor locular wall (high-pressure stage); 5, ionization chamber second layer locular wall; 6, ionization chamber n-th layer locular wall.

Embodiment

Below in conjunction with embodiment and Figure of description, the present invention is further elaborated.

Multi-layer cavity ionization chamber; this ionization chamber is primarily of the locular wall composition of collector, high-pressure stage, protection pole and a series of different-thickness; key is; can for different measuring objects; the i.e. energy range of photon; choose suitable locular wall and carry out combining the ionization chamber obtaining there is different-thickness locular wall, to meet the requirement of secondary charged particle balance.

Described collector is positioned at the central authorities of multi-layer cavity ionization chamber, for collecting electronics or cation; Described high pressure is ionization chamber ground floor locular wall very; The effect of described protection pole ensures electric fields uniform; Collector, high-pressure stage, protection pole constitute a basic ionization chamber measuring unit.

Described a series of different-thickness spherical shell is concentric with ionization chamber ground floor locular wall, and each spherical shell is connected to form by the mode of close-fitting by two hemisphere spherical shells, the inwall of concentric spherical shell and the wall contacts of front one deck locular wall.The material of spherical shell and ionization chamber ground floor locular wall is graphite.

Embodiment 1

The present embodiment is 1m place in reference point, is that the air kerma of the high-energy photon of 6.62MeV is measured to average energy.

First, select the thin-wall chamber that basic, its ground floor chamber wall thickness is 4mm, and material is graphite, and the sensitive volume of elementary cell is 50cm ³.? ¹³⁷in Cs Reference radiation field, ionization chamber is calibrated, obtain ionization chamber and exist ¹³⁷calibration factor N in Cs Reference radiation field _e, according to N _emake ionization chamber ionization current with the change curve of chamber wall thickness and effective attenuation curve and according to following formula:

$N_D=\frac{1}{\mathrm{\ρ}\·v}\·\left(\frac{W}{e}\right)\·{\left[{\left(\frac{{\mathrm{\μ}}_{\mathrm{en}}}{\mathrm{\ρ}}\right)}_{w,a}\right]}_E=N_E\left(\frac{W}{e}\right)\·{\left[{\left(\frac{{\mathrm{\μ}}_{\mathrm{en}}}{\mathrm{\ρ}}\right)}_{w,a}\right]}_E\·\left[{\left(\frac{S}{\mathrm{\ρ}}\right)}_{a,w}\right]$

Calculate N _dvalue.K is calculated by the effective attenuation curve recorded _mand β ^-1and required wall thickness gross thickness, in the present embodiment, required locular wall gross thickness is 52mm.

Then, the wall thickness of change to different-thickness according to the gross thickness obtained and effective attenuation curve combines, the locular wall selecting thickness to be respectively 7mm, 2mm, 3mm, 4mm, 14mm, 18mm altogether carries out combination according to order from inside to outside and obtains multi-layer cavity ionization chamber, to meet the requirement of charged particle equilibrium.Wherein, the connected mode of every two hemisphere spherical shells is that draw-in groove connects.

Finally, the multi-layer cavity ionization chamber obtained is placed in the high energy light subfield that average energy is 6.62MeV, and is 1m place in reference point, air kerma is measured, measure the integral charge Q in 100s.By k _mand β ^-1substitute into formula $K_{a,C}=N_D\·Q\·k_{\mathrm{Tp}}{\left(\frac{{\mathrm{\μ}}_{\mathrm{en}}}{\mathrm{\ρ}}\right)}_{a,w}\·{\left(\frac{S}{\mathrm{\ρ}}\right)}_{w,a}\·k_m\·{\mathrm{\β}}^{-1}$ In calculate air collides Kerma and final according to formula calculating air kerma is 8.5 μ Gy/h.

Determined major parameter is measured as table 1 according to above.

Table 1 calculating K _athe main parameter used

This ionization chamber is that the air kerma measurement result of the high-energy photon of 6.62MeV is comparatively accurate measuring average energy, and its Related combined standard uncertainty is 3.2%.

Embodiment 2

Identical with the measuring object in embodiment 1, change be that the connected mode of hemispherical chamber wall is for being threaded.Measuring air kerma is 8.5 μ Gy/h, and relative uncertainty degree is 3.3%.

The concrete principle that multi-layer cavity ionization chamber provided by the invention measures air kerma is:

To high-energy photon measure core formula as shown in Equation 1:

$K_a=\left(\frac{{\mathrm{\μ}}_{\mathrm{tr}}}{{\mathrm{\μ}}_{\mathrm{en}}}\right)\·K_{a,C}...\left(1\right)$

Can finding out from formula 1, obtaining air kerma K by measuring impingement air Kerma _{a, C}, and K _{a, C}can be obtained by formula 2, as follows:

$K_{a,C}=N_D\·Q\·k_{\mathrm{Tp}}{\left(\frac{{\mathrm{\μ}}_{\mathrm{en}}}{\mathrm{\ρ}}\right)}_{a,w}\·{\left(\frac{S}{\mathrm{\ρ}}\right)}_{w,a}\·k_m\·{\mathrm{\β}}^{-1}...\left(2\right)$

photon, to the ratio of ionization room air with chamber wall materials mass energy absorption coefficient, is constant;

K _m: the scattering of ionization chamber chamber wall materials and the absorption correction factor;

chamber wall materials organizes ability ratio with air, is constant.

β: the modifying factor considering the active centre that secondary electron produces in locular wall.

Q: the integral charge recorded in measurement time t, directly measures and obtains;

K _{t, p}: air pressure, temperature correction factor, directly measure and obtain;

N _d: absorption of air dose factor.

N _dcalculating as shown in Equation 3:

$N_D=\frac{1}{\mathrm{\ρ}\·v}\·\left(\frac{W}{e}\right)\·{\left[{\left(\frac{{\mathrm{\μ}}_{\mathrm{en}}}{\mathrm{\ρ}}\right)}_{w,a}\right]}_E=N_E\left(\frac{W}{e}\right)\·{\left[{\left(\frac{{\mathrm{\μ}}_{\mathrm{en}}}{\mathrm{\ρ}}\right)}_{w,a}\right]}_E\·\left[{\left(\frac{S}{\mathrm{\ρ}}\right)}_{a,w}\right]...\left(3\right)$

In formula: for constant; for at energy be E photon field in photon to the ratio of ionization chamber chamber wall materials with the mass energy absorption coefficient of air, be constant; N _e: ionization chamber is the calibration factor in the photon field of E at energy, can be by ⁶⁰co or ¹³⁷cs reference radiation calibration obtains.

As can be seen from formula 1,2,3, the key measuring air kerma rate obtains parameter k _mand β ^-1.? ⁶⁰co or ¹³⁷cs reference radiation calibration obtains N _eafter, ionization chamber ionization current can be made with the change curve of chamber wall thickness and effective attenuation curve, also can obtain N simultaneously _d.K is calculated again according to effective attenuation curve _mand β ^-1.Usually doing effective attenuation curve is obtain with the ionization chamber measurement of one group of different wall, but due to the sensitive volume difference in each ionization chamber, needs the correction each ionization chamber being carried out to sensitive volume just can make effective attenuation curve, reduce measurement efficiency.Adopt multi-layer cavity ionization chamber of the present invention then to avoid this to correct.

By k _mand β ^-1substitute in formula (2) and calculate air collides Kerma and finally calculate air kerma according to formula (1).

Claims (4)

1. multi-layer cavity ionization chamber, this ionization chamber is made up of the concentric locular wall of a basic thin-wall chamber and a series of different-thickness, it is characterized in that, chooses concentric locular wall and carry out combining the cavity ionization chamber obtaining having different chamber's wall thickness; The structure of described concentric locular wall is two hemisphere hull shape locular walls, and two hemispherical chamber wall are connected by the mode of close-fitting; The inwall of described concentric locular wall and the wall contacts of front one deck locular wall; Described chamber wall materials is air-equivalent material.

2. multi-layer cavity ionization chamber according to claim 1, is characterized in that, described one basic thin-wall chamber mainly comprises collector, high-pressure stage and protection pole.

3. multi-layer cavity ionization chamber according to claim 2, is characterized in that, described collector is positioned at the center of multi-layer cavity ionization chamber, for collecting electronics or cation; High pressure is ionization chamber ground floor locular wall very; After described protection pole is positioned at collector, its effect ensures electric fields uniform.

4. multi-layer cavity ionization chamber according to claim 1, is characterized in that, described air-equivalent material is graphite.