ionizationThe chamber uses ionizationradiationThe ionization effect of ionizing radiation measurement detector, also known as ion chamber, Taiwan and other places called ionization box.The ionization chamber consists ofpotentialAnd the medium between them.Ionizing radiation generates ionized ion pairs in the medium. Under the effect of electric field, positive and negative ions drift towards the negative and positive respectively, forming ionization current.Since the ionization current is proportional to the intensity of radiation, the intensity of ionizing radiation can be obtained by measuring the current.
According to the type of medium, ionization chamber can be divided into air ionization chamber, liquid ionization chamber and solid ionization chamber;According to the measurement method, it can be divided into pulse ionization chamber and current ionization chamber.There are many kinds of ionization chambers. In addition to the above classification, they can also be classified according to application, radiation type, shape, gas type and pressure, measurement object, measured calibration amount, etc.For example: absorption dose ionization chamber, electron beam ionization chamber, high pressure well type ionization chamber, alpha pulse ionization chamber, compensated neutron ionization chamber, (treatment level) ionization chamber for radiation measurement of radiotherapy, etc.
The ionization chamber is the earliestNuclear radiation detector。It is also used to detect ionizing radiationGas detector。The principle of the gas detector is that when the detector is subjected toradialDuring irradiation, the radiation interacts with the molecules in the gas to generate an electron and aPositive ionConstituention pair。These ions diffuse freely to the surrounding area.diffusion process Electrons and positive ions can recombine and form againNeutral molecule。However, if a DC polarization voltage V is applied to the collecting electrode and high voltage electrode constituting the gas detectorelectric field, then electrons and positive ions will be pulled to the positive and negative poles respectively and collected.With the gradual increase of polarization voltage V, the working state of the gas detector will change from the composite area, saturation area, proportional area, limited proportional area, Geiger area (G - M area) to the continuous discharge area.
classification
Announce
edit
The classification of ionization chambers is extremely complicated. Below are some examples of common classifications.
According to the measured dose rate or dose size
1) The sensitive volume of the treatment level ionization chamber is about 0.01 ml to 5.0 ml, and the measured dose rate range is 1 mGy/min to 10 Gy/min;
2) Diagnostic horizontal ionization chamber, the sensitive volume is about 5ml to 500ml, the measured dose rate range is 0.1 mGy/min to 10 Gy/min, but the dose is only 0.1 mGy to 0.1Gy;
3) Protective horizontal ionization chamber, the sensitive volume is about 30ml to 3L, and the measured dose rate range is 0.1 μ Gy/h to 1 Gy/h;
4) The sensitive volume of the environmental level ionization chamber is about 3 to 100 liters. In order to reduce the volume, the gas pressurization design is often adopted, and the measured dose rate range is 1 μ Gy/h to 10 μ Gy/h;
By measured object
1) α ionization chamber, β ionization chamber, or α, β ionization chamber. Some walls of the ionization chamber are very thin, and α, β particles or their secondary electrons can pass through to reach the sensitive volume.Sometimes, it is also designed to place α and β sources directly into the ionization chamber;
2) The wall of γ ionization chamber usually has a certain thickness, which reduces the response to α and β particles;
3) The neutron ionization chamber used for neutron dose measurement is usually designed as a double ionization chamber.The basic principle is that the graphite ionization chamber mainly responds to photons and has no response to neutrons, while the plastic ionization chamber responds to both photons and neutrons. The neutron dose is calculated as the difference between the responses of the two ionization chambers;
4) Electron beam ionization chamber, a parallel plate ionization chamber specially used for accelerator electron beam measurement;
5) Bragg peak ionization chamber is designed to measure the Bragg peak of heavy ion beam.
According to the shape of ionization chamber
1) Shell ionization chamber: the measured volume of the ionization chamber is 0.1cmthreeTo 1.0 cmthreeAnd is surrounded by a hard outer electrode installed on the support rod.The measured volume is usually symmetrical around the axis of the support rod, and the axis of symmetry is perpendicular to the axis of the radiation beam when used.There are two types of shell ionization chamber, namely, the (sleeve) type ionization chamber and the spherical type ionization chamber.The finger (sleeve) type ionization outdoor electrode is a hard cylindrical shape, with one end closed and the other end mounted on the support rod.The outer electrode of the spherical ionization chamber is a hard spherical wall mounted on the support rod.
2) Parallel plate ionization chamber: the measured volume of the ionization chamber is 0.01cmthreeTo 0.5 cmthreeAnd is surrounded by two parallel electrodes.When in use, the two parallel electrode surfaces are perpendicular to the axis of the radiation beam.
3) Ventilated ionization chamber: the structure of the ionization chamber enables the air in the measured volume to exchange freely with the atmosphere.Therefore, the influence of air density change needs to be corrected.
4) Sealed ionization chamber: the structure of the ionization chamber limits the passage between the air and the atmosphere within the measurement volume, fully ensuring that the response of the ionization chamber is independent of the changes in environmental conditions within the time period specified by the manufacturer.
5) Unprotected ionization chamber: the protective conductor around the central signal conductor in the cable of the ionization chamber is terminated in the cable and does not extend into the rod or body of the ionization chamber device.
6) Partial protection ionization chamber: the protective conductor in the cable of the ionization chamber around the central signal conductor extends into the rod or body of the ionization chamber device, but does not enter the air of the ionization chamber.
7) Protective ionization chamber: the protective conductor of the rod or body of the ionization chamber device is connected with the protective electrode, and the protective electrode contacts the air in the ionization chamber.
According to measurement theory
1) Graphite cavity ionization chamber;
2) Absolute measurement free air ionization chamber.[1]
working principle
Announce
edit
The so-called ionization chamber is a gas detector working in the saturation zone, so the saturation zone is also called the ionization chamber zone.If the appropriate polarization voltage is selected,Compound effectIt can be ignored and there is no collision amplification. At this time, it can be considered thatradialInitial generatedion pairJust all are collected to form ionization current.ThisionizationThe current is proportional toRay intensity。The monitoring detector of accelerator generally adopts ionization chamber.standarddosimeterThe ionization chamber is also used as the measuring element.The current of the ionization chamber can be measured with a highly sensitiveelectrometerMeasurement.
It is easy to see that the ionization chamber is mainly composed of collecting electrode and high voltage electrode, and there is gas between collecting electrode and high voltage electrode.Different from other gas detectors, the ionization chamber is generally composed of air at about one atmospheric pressureSensitive volume, this part can be completely connected with the outside world, or it can be closed.It is surrounded by an electrode made of conductive air equivalent material or tissue equivalent material, and its center is a collecting electrode,BipolarFormation by applying certain polarization voltage betweenelectric field。To make the collectedionizationAll ions form ionization current to reduce leakage loss, and protective electrodes need to be added between the collecting electrode and the high-voltage electrode.
When Xradial、Gamma rayWhen the ionization chamber is irradiated, the photon interacts with the ionization chamber material and is mainly generated on the chamber wallSecondary electron。Secondary electron ionizesAir ionizationIonized ions move towards the collecting electrode under the action of electric field, and the ions reaching the collecting electrode are collected to form ionization current signal and output to the measuring unit.[1]
Alpha rayIs a kind ofCharged particle flowBecause it is charged, it is easy to cause ionization wherever it goes.Alpha rayWith strong ionization ability, this property can be used.It also brings some damage, and has a great ability to damage tissues in the human body.Because of its large mass and poor penetration ability, its range in the air is only and centimeters, which can be blocked by a piece of paper or healthy skin.
Beta rayIt is also a high-speed charged particle. Its ionization ability is much smaller than that of alpha rays, but its penetration ability is larger than that of alpha rays. However, its range is shorter than that of X and gamma rays, and it is easy to bealuminum foil、organic glassAnd other materials.
X-rays and gamma rays have roughly the same properties, without the length of radio waveselectromagnetic wave, so they are collectively calledphoton。Both of them have strong penetrability, so special attention should be paid to accidental radiation protection.
ionizing radiationRadiation workers are those who exist in nature, but artificial radiation has spread in all fields, and are specialized in the production, use and research of ionizing radiation.Occupations related to radiation include:
Exploration, mining, smelting and finishing of nuclear industry system and raw materials, nuclear fuel andreactorProduction, use and research;Cultivate new varieties of agricultural radiation, ensure vegetables and fruits, and store grain;Medical XradialFluoroscopy, photographic diagnosisradioactivitynuclideMeasurement of human organs, radiation treatment of tumors, etc;Various accelerators, ray generators andelectron microscopeElectronic fast welding machine, color TVKinescope, high-voltage electronic tubes, etc.[2]
main performance
Announce
edit
sensitivity
Generally speaking,ionizationThe sensitivity of the chamber depends on the air quality in the ionization chamber.Since the air pressure in the ionization chamber is approximately one atmospheric pressure, it can also be said that its sensitivity is proportional to the air volume, so this volume is also called“Sensitive volume”。along withionizationThe chamber volume and sensitivity increased.
Energy response
As mentioned above,ionizationThe response (sensitivity) of the chamber is proportional toAir kermaRate（Exposure rate）It shall not be affected by other factors, for example, it shall not change with the change of energy and temperature.However, because the ionization chamber itself cannot be completely made of air, it cannot be completely equivalent to air. When the radiation energy changes, the response (sensitivity) of the ionization chamber will also change. This characteristic is called energy response.
For ionization chambers for dose measurement, energy response is a very important performance parameter, while for ionization chambers for dose monitoring, although energy response is also concerned, it is not very important.
Electronic balance
In the interaction between accelerator radiation and air, accelerator photons cannot directly causeionizationInstead, energy is lost through photoelectric absorption, Compton scattering and electron pair generation, and secondary electrons are generated.Although the primary electrons of the accelerator cause ionizationAir ionizationIt is mainly secondary electrons.Accelerator photons or primary electrons first produce secondary electrons in the interaction with matter, and as an ionization chamber, the secondary electrons entering the air cavity of the ionization chamber are mainly generated in the wall of the ionization chamber.Because of the material of the wallDensity ratioThe air is much larger, and more electrons are generated, so as the wall thickness increases, the air enters the ionization chamberSensitive volumeWhen the thickness of the ionization chamber wall increases to a certain extent, the barrier effect of the ionization chamber wall on the secondary electrons begins to be obvious, and eventually makes the secondary electrons entering the sensitive volume equal to the secondary electrons escaping from the sensitive volume. We call this state "electron balance", or "electron completion".In a broad sense, the so-called electron balance means that the energy of the secondary electron entering the measuring volume element is equal to the energy of the secondary electron leaving the volume element.WhenradialWhen the energy of is high, the energy of the secondary electron is also high, the thickness of the penetrated material increases, and the thickness to reach the electron balance also increases.
In general, as long as the thickness of the material surrounding the collected volume of air is greater than the maximum range of secondary electrons, the electron balance condition can be basically met.Let's analyze it in a little more detail.
For common or radiationcharged particle equilibrium The required thickness of the medium is the secondary electron range generated by it.Table 1 shows some XradialThe attenuation coefficient in water and the range of secondary electrons.
Table 1. Attenuation coefficient of X-ray in water and secondary electron range
Maximum energy of X-ray
(MeV)
Linear weakening coefficient
（cm）
Maximum range of secondary electron
（cm）
zero point zero two
zero point seven nine six
zero point zero zero zero eight
zero point zero five
zero point two two six
zero point zero zero four two
zero point one
zero point one seven one
zero point zero one four
zero point five
zero point zero nine seven
zero point zero nine seven
one
zero point zero seven one
zero point three three
two
zero point zero four nine
zero point eight four
four
zero point zero three four
zero point nine three
six
zero point zero two seven
one point two seven
eight
zero point zero two four
one point six nine
ten
zero point zero two two
two point zero three
fifteen
zero point zero one nine
two point two eight
twenty
zero point zero one eight
two point nine five
Compound loss
As mentioned above, if the appropriate polarization voltage is selected, the composite effect can be ignored.However, the recombination loss is not only related to the polarization voltage, but also related toionizationroomSensitive volumeThe ionization density of the air in is related to the dose rate.Due to ion recombination, the charge collection efficiency in the cavity is not high, and a correction factor is required.
If studied in detail, the ionization chamberCompound effectIt is related to its shape, collection voltage, and the rate at which radiation generates electric charges.When measuring the accelerator, the radiation is pulsed and instantaneousRadiation dose rateIt is far greater than the average dose rate, so the composite correction factor becomes very important.For continuous irradiation (γradialThe beam) recombination effect is generally very small.To verifyionizationThe compound loss of the chamber can reduce the polarization voltage to 0.5 times the normal value. If the current value is greater than 99.5% of the normal polarization voltage, the problem is not serious;If the current value is only 99.0% of the normal polarization voltage, it must be taken seriously;If smaller, measures must be taken.
However, the larger the volume of the ionization chamber, the greater the disturbance to the surrounding, which affects the measurement of the radiation field.In the measurement of radiation field, the influence of disturbance is difficult to be determined quantitatively.Therefore, the volume of the ionization chamber used for radiation field measurement of radiotherapy cannot be too large, usually less than 1cm.
althoughionizationThe chamber can be calibrated separately, but in most cases, the ionization chamber and its measuring unit are calibrated as a whole.[3]
