Patent Description:
Gamma ray dose (rate) meters and dose equivalent (rate) meters are widely used in military, national defense and civil fields, are extremely important tools for guaranteeing the security of nuclear facilities, gamma ray devices, relative workers and the public. To ensure the accuracy and reliability of performance and measured values thereof, they should be verified or calibrated periodically according to the Metrology Law of the People's Republic of China and correlative regulations.

Gamma dose (rate) meters should be verified and calibrated on gamma air kerma secondary standard devices containing secondary standard reference radiation constituted by isotope radiation sources according to the requirements of the national standard GB/T <NUM>-<NUM> "X and gamma reference radiation for calibrating dose meters and dose rate meters and for determining their response as a function of photon energy- Part <NUM>:--Radiation characteristics and production methods", GB/T12162. <NUM>-<NUM> "X and gamma reference radiation for calibrating dose meters and dose rate meters and for determining their response as a function of photon energy, Part <NUM>: Dosimetry for radiation protection over the energy ranges <NUM> keV to <NUM> MeV and <NUM> MeV to <NUM> MeV", and JJG393-<NUM> "Verification Regulation of X and Gamma Radiation Dose Equivalent (Rate) Meters and Monitors Used in Radiation Protection". In the process of the verification and calibration work, a secondary standard reference radiation should be verified by using an air kerma measurement standard instrument to obtain the air kerma conventional true value at the point of test of the secondary standard reference radiation; then the reference point disposed on the probe of the dosimeter being verified is accurately positioned in the secondary standard reference radiation as required. And measurement is performed to obtain the calibration factor K: <MAT>, wherein, Kair,c is the gamma air kerma (rate) measured or calculated by the standard instrument at the experiment point of the secondary standard reference radiation, i.e., the conventional true value of gamma air kerma (rate) at the experiment point, and Mc ' is the indicate value of the dosimeter being verified.

When a gamma air kerma secondary standard device is built, the dimension of reference radiation influencing the dose value, the scattering rays from the shielding wall and the ground, the radiation area of the ray beams, and the non-uniformity of the irradiation area should be designed scientifically, and be tested and verified through detailed experiments, so as to check whether the standard reference radiation meets the requirements. In accordance with relevant standards, the dimension of the standard reference radiation meeting above requirements shall not be smaller than <NUM>×<NUM>×<NUM>, and the dose rate of gamma rays of the isotope radiation source shall cover the range from µGy/h to mGy/h. Such standard reference radiation cannot be removed no matter in volume or in weight including a shielding building or the like, which leads that all gamma ray dosimeters must be delivered to metrology technology institutions possessing standard reference radiation at fixed sites for verification or calibration. Dosimeters, for the purposes of radiation security monitoring on nuclear power plant reactors and relevant nuclear facilities, are impossible or difficult to be dismounted, and cannot be periodically verified or calibrated by scientific methods and technologies and proper devices yet. Thus it brings hidden danger for radiation security.

One way to realize on-site and in-situ verification or calibration for gamma ray dosimeters is to reduce the spatial volume and the weight of an at least <NUM>×<NUM>×<NUM> standard reference radiation, which prescribed by the standards, till dismount facilitated. However, reducing the spatial volume of the reference radiation inevitably leads to increase of scattering components in the radiation. Thus the dose contribution rate of the scattered rays in the small-scale reference radiation exceeds <NUM>%, which does not comply with the requirements of existing standards, influences response of the The publication<CIT> discloses a prior art dosimeter and results a calibration error. system and method for an in-situ calibration of radiation monitors.

In order to solve the problem that there is no scientific or proper method for verifying and calibrating gamma ray dosimeters, the present invention provides a method for determining a conventional true value of air kerma [kinetic energy released per unit mass] which is characterized by including the features of claim <NUM>.

Specifically, step <NUM> includes the following specific steps:.

Further, step <NUM> includes the following specific steps:.

Specifically, step <NUM> includes the following steps:.

Further, in step 33A, the proper energy interval ΔE refers to:
ΔE=<NUM>/(128x2z)keV, wherein <NUM>≤z≤<NUM>, z is an integer.

Specifically, step 33C includes the following specific steps:.

Specifically, in step 34A, the specific method for obtaining a prediction model Kij=f<NUM>(Ψij,Kj') of Kij by adopting a support vector machine regression method includes the steps as follows: radial basis function being selected as the kernel function in the model training process. , Parameters of the kernel function being determined by a cross validation method ,. in the training process , the sample data (Kij,Ψij,Kj')(x×y)×(m+<NUM>) being divided into training sets and testing sets according to a proper proportion; and when test error is not more than <NUM>%, ending the training, and determining the prediction model Kij=f<NUM>(Ψij,Kj').

Still further, the proper proportion refers to that the proportion of the training set to the testing set is more than or equal to <NUM>: <NUM>.

The beneficial effects of the present invention are that when determining the air kerma conventional true value in a small-scale reference radiation with a prediction model Kij=f<NUM>(Sij,Kj'), PCA reasonably extracts the feature components which characterize the dose features in the MRR well. Meanwhile the dimension of sample data applied for model training is reduced significantly and the model training efficiency is improved. SVM, as a multivariate linear regression method, is suitable for small sample modeling. The compatibility of the prediction model trained by SVM, the accuracy of the prediction value and the universality of the model are excellent. The method deducts the disturbance from the scattering gamma rays caused by the small-scale reference radiation and probe in determining the air kerma conventional true value at the point of test. The air kerma conventional true value determined by the method is equal to that determined according to national standards GBfT <NUM>-<NUM>, GBfT <NUM>-<NUM> and JJG393-<NUM>. Based on the determination method present in this invention, verification or calibration devices and equipments for radiation protection dosimeters with proper weight and volume can be designed and manufactured as skid-mounted, vehicle-mounted, hand-propelled or other removable type, which are suitable for the in-situ verification or calibration of various gamma ray dosimeters and security monitoring dosimeters.

Among them, <NUM> is a shielding box, <NUM> is a dosimeter being verified, <NUM> is incident rays, <NUM> is a radiation source, <NUM> is a test hole, <NUM> is a point of test, <NUM> is a monitor point, <NUM> is an incident hole, and <NUM> is a gamma spectrometer.

The technical solution of the present invention will be described in detail below in combination with the embodiment and the accompanying drawings.

An air kerma conventional true value determining method of the present invention is that: firstly, establishing a small-scale reference radiation (MRR), which comprises a shielding box and a gamma spectrometer. The shielding box is positioned horizontally and an incident hole is set on the side thereof for incidence of incident rays. The point of test is set in the direction of the incident rays in the shielding box. There is also a test hole on the upper surface of the shielding box, by which the probe of a dosimeter being verified can be put into the shielding box. The reference point of the probe should be coincident with the point of test. Further a monitor point is set in the shielding box. The shielding box is segmented into two parts by one plane perpendicular to the connecting line of the incident hole and the point of test. The monitor point is located at the part adjacent to the incident hole in the shielding box where directly irradiate incident rays are avoided. A gamma spectrometer is disposed in the shielding box. The reference point on the probe of the spectrometer is coincident with the monitor point and the probe is fixed in the shielding box. Next, a proper radiation source and source intensity being selected to provide incident rays for the shielding box. And then a plurality of gamma ray dosimeters to be selected as samples for training a prediction model of the air kerma conventional true value at the point of test. Lastly, the probes of dosimeters being verified are arranged at the point of test, then measuring the scattering gamma spectrum with a gamma spectrometer, with the prediction model and the gamma spectrum as input, the air kerma conventional true value at the point of test is obtained.

In this embodiment, the structural schematic diagram of the small-scale reference radiation (MRR) is shown as <FIG>, the small-scale reference radiation comprises a shielding box <NUM> with a dimension not more than <NUM> meters and a gamma spectrometer <NUM>. The shielding box <NUM> is positioned horizontally and provided with an incident hole <NUM> on the side thereof for the incidence of incident rays <NUM>. A point of test <NUM> is arranged in the direction of the incident rays <NUM> in the shielding box <NUM>, a test hole <NUM> is further provided on the upper surface of the shielding box <NUM>, by which a probe of a dosimeter 2being verified can be put into the shielding box <NUM> and can make the reference point of the probe being coincident with the point of test <NUM>. A monitor point <NUM> is further arranged in the shielding box <NUM>, the shielding box <NUM> is segmented into two parts by one plane that is perpendicular to the connecting line of the incident hole <NUM> and the point of test <NUM>, and the monitor point <NUM> is located at the part adjacent to the incident hole <NUM> in the shielding box <NUM> and at the position not directly irradiated by the incident rays <NUM>. The gamma spectrometer <NUM> is disposed in the shielding box <NUM>. The reference point on the probe thereof is coincident with the monitor point <NUM>, and is fixed in the shielding box <NUM>.

In this embodiment, the shielding box <NUM> could be a cube with a sectional size of <NUM> meter, e.g., a <NUM>×<NUM>×<NUM> sized cube, and could also be a cuboid or in other shape, the specific size being determined by the total weight of the MRR which allowed by the intended use. The incident hole <NUM> could be located in the center position of the side of the shielding box, the point of test <NUM> could also be located in the geometrical center of the shielding box, and the monitor point <NUM> is generally located at the inner bottom of the shielding box <NUM>.

In use, the specific method includes the following steps:.

This step includes the following specific steps:.

The method may further include the following step, which is not part of the present invention :
Obtaining a correction factor ω = Kij/Rij in combination with the indicate value Rij of the dosimeter being verified.

The energy response characteristic of the dosimeter being verified can also be obtained via the above method by switching the radiation sources with different energies; or the angle response data of dosimeter being verified can be obtained by rotating the probe thereof, and other verification items stipulated in JJG393-<NUM> can also be realized.

A specific example is as follows:
A radioisotope source <NUM>Cs is selected as the radiation source <NUM> in this embodiment to provide a radiation ray source for the small-scale reference radiation (MRR), and a calibration device for calibration of a gamma ray radiation protection instrument is constructed, the structure thereof being shown in <FIG>. According to the requirement of radiation protection, the shielding box is made of a material such as lead, tungsten alloy or the like with a proper thickness, thus ensuring personnel security when the device is used.

The shielding box <NUM> is in the shape of a cube having a side length of <NUM>, and the geometric center thereof is set as a point of test <NUM>. The incident hole <NUM> having the diameter of <NUM> and used for the incidence of gamma rays is arranged in the geometric center of the side which is close to the radiator (the radiation source <NUM>), of the shielding box <NUM>. The shielding box <NUM> is segmented into two parts by one plane that is perpendicular to the connecting line of the incident hole <NUM> and the point of test <NUM>, and the monitor point <NUM> is located at the part adjacent to the incident hole <NUM> on the bottom center line of the shielding box <NUM> and spaced <NUM> from the projection point at the point of test <NUM> on the bottom; the test hole <NUM> having the diameter of <NUM> is arranged at the top of the shielding box <NUM>, and is used for putting the probe of the instrument <NUM> being verified; scattering gamma ray spectrum in the box are measured by an Inspector1000 portable gamma spectrometer of Canberra company, the reference point of the probe of the gamma spectrometer <NUM> is aligned with the monitor point <NUM> on the bottom of the shielding box <NUM>, and the probe of the gamma spectrometer <NUM> is fixed.

The activity degree of the <NUM>Cs radioisotope source is 1Ci, the incident ray beam <NUM> is provided for the shielding box <NUM> via the device <NUM> such as a radiator or the like, and the attenuation times of the incident ray beam <NUM> is adjusted according to the source intensity of the radiation source and the range of the dosimeter being verified. The times of an attenuator is adjusted according to the range of common gamma ray dose (rate) meter and dose equivalent (rate) meter to obtain five experiment source intensities Vj,(j=<NUM>,<NUM>,. ,<NUM>), and the range of the dose rate is 65µGy/h-<NUM>.

An experiment is carried out according to the method, a prediction model of the air kerma conventional true value at the point of test <NUM> is obtained, and the specific implementation steps are as follows:.

The prediction model is trained on a Matlab software platform for the Windows7 system. And the version of the Matlab software is 2012a. A radial basis function <MAT> is selected as the kernel function of the model by calling a least squares support vector machine toolbox (LS-SVMlab Toolbox User's Guide version <NUM>) in the platform. And the parameter σ<NUM> of the kernel function and the regularization parameter c are determined by an L-fold cross validation method. L is set to be equal to <NUM>. And the data sample (Kij,Ψij,Kj')<NUM>×(m+<NUM>) is allocated to a training set and a testing set according to a proportion of <NUM>: <NUM>. And the training is ended when the test error is less than or equal to <NUM>%. The prediction model of Kij is Kij=F[(Ψij,Kj'), (Ψ,K")]'×α+b is finally acquired, wherein F is the kernel function, a and b are parameters of the model, Ψij is the principle component vector of the energy spectrum Sij when the dosimeter being verified is introduced into the shielding box, And Kj is the air kerma value at the point of test of the shielding box when no probe is introduced under the source intensity, Ψ' and K" are sample data of the principle component vector of the energy spectrum for training the model and air kerma sample data at the point of test in the shielding box when no probe is introduced. In combination with the function Ψij=f<NUM>(Sij), the model can be expressed as Kij=F[(f<NUM>(Sij),Kj'), (Ψ',K")]'×α+b, i.e., Kij=f<NUM>(Sij,Kj').

When the BH3103A gamma ray dose rate meter <NUM> being verified is calibrated, a probe of the BH3103A is put into the shielding box, and the reference point of the probe is coincident with the point of test <NUM> of the MRR; a proper radiation source intensity Vj is determined according to the range of the BH3103A in a manner of selecting an attenuator or the like so that the indicate value of the BH3103A is nearby the midpoint of the calibration range, scattering gamma spectrum measured by the gamma spectrometer <NUM> are recorded, the principle component vector Ψi of the spectrum data is extracted and introduced into the prediction model established Kij=F[(Ψij,Kj'), (Ψ',K")]'×α+b, the air kerma conventional true value Kij at the point of test <NUM> of the MRR under such condition is <NUM>. 27µGy/h, the mean R of indicate values of the five dosimeters is <NUM>. 82µGy/h, a calibration factor is obtained according to formula <MAT>, and calibration of the dosimeter is thus realized.

The aforesaid description is only an example for realizing the present invention, and the present invention can be realized in multiple ways. For example, the shape of small-scale reference radiation MRR is not limited to a cube, A MRR in other shape such as a cuboid or the like does not influence the effect of the present invention, and the methods of introducing gamma rays via the shielding box and limiting the gamma rays into a small closed space are all implementations of the present invention; the point of test and the monitor point are not limited to the positions in the embodiment, as long as they are located in the MRR, can fulfill the purposes required by the claims and do not influence the effect of the present invention; as for the SVM method for establishing the prediction model Kij=f<NUM>(Sij,Kj') of the air kerma conventional true value at the point of test in the MRR, the SVM method has multiple forms and is rapidly developed, the SVM regression mode is not limited to the least squares support vector machine LS-SVM used in this embodiment, and other modes of SVM, C-SVM, v-SVM and the like adopting an SMO (Sequential Minimal Optimization) algorithm are available for fulfilling the purpose of establishing the prediction model Kij=f<NUM>(Sij,Kj') of the air kerma conventional true value at the point of test in the MRR.

Other than one <NUM>Cs cesium source for calibration of the gamma ray dose (rate) dosimeter in this embodiment, <NUM>Cs, <NUM>Am and <NUM>Co sources and the method introduced in the present invention can also be simultaneously adopted to obtain the indicators of energy response, angle response and the like of the gamma ray dose (rate) dosimeter. An X ray machine serving as a ray source and the method of the present invention can also be adopted for verification and calibration of gamma and X ray dose (rate) dosimeters.

Claim 1:
A method for determining a conventional true value of air kerma [kinetic energy released per unit mass] , comprising the following steps:
Step <NUM>: establishing a small-scale radiation field comprising
a shielding box (<NUM>) with a side length of not more than <NUM> meters;
• providing an incident hole (<NUM>) on a side of the shielding box for incidence of incident rays (<NUM>) ;
• arranging a check point (<NUM>) within the shielding box (<NUM>) ;
• providing a test hole (<NUM>) on the upper surface of the shielding box (<NUM>) ; arranging a dose feature point (<NUM>) within the shielding box (<NUM>) ;
• segmenting the shielding box (<NUM>) into two parts by a plane perpendicular to the connecting line of the incident hole (<NUM>) and the check point (<NUM>) and that contains the check point (<NUM>), the dose feature point (<NUM>) being located at a part close to the incident hole (<NUM>) in the shielding box (<NUM>) and at a position not directly irradiated by the incident rays (<NUM>) ;
• arranging a gamma spectrometer (<NUM>) in the shielding box (<NUM>) , a reference point on a probe thereof being superposed with the dose feature point (<NUM>) and the gamma spectrometer being fixed in the shielding box (<NUM>) ;
Step <NUM>:
• selecting a proper radiation source and source strength and
• providing incident rays (<NUM>) for the shielding box (<NUM>)
Step <NUM>:
• selecting a plurality of gamma ray dosimeters as experiment for establishing a prediction model of the air kerma conventional true value at the check point (<NUM>);
Step <NUM> :
• arranging a probe of an instrument to be detected at the check point (<NUM>)
• recording scattering gamma spectra measured by the gamma spectrometer and
• introducing the scattering gamma spectra to the prediction model to obtain an air kerma conventional true value.