RU2418306C1 - Method of correcting scintillation detector signals - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of correcting temperature-dependent signals generated by a scintillation detector involves absorption of ionising radiation on a scintillator, converting optical radiation into a sequence of electrical pulses, measuring their amplitude which is proportional to energy of the absorbed radiation, simultaneous measurement of temperature inside the detector, calculating the average temperature value over the measurement time, determining the calibration factor based on the average temperature using a mathematical function which is a polynomial with predefined parametres, determining the true value of energy of the absorbed radiation based on the obtained calibration factor and generating the detector output signal based on the said value.

EFFECT: stable quality of correcting the detector output signal.

2 cl

Description

The invention relates to the field of measuring equipment, and in particular to methods for stabilizing the readings of scintillation detectors when working in a wide range of changes in ambient temperatures, in particular when working in the field.

When developing highly sensitive dosimetric and radiometric equipment using scintillation detection units based on NaI (T1) crystals and photomultiplier tubes (PMTs) designed to operate in a wide range of operating temperatures from minus 20 ° С to plus 40 ° С, it is necessary to provide a temperature compensation system for the detected energy spectrum.

The absence of thermal compensation leads to a shift in the measured gamma-ray spectrum when the ambient temperature changes from normal conditions, which in turn worsens the metrological characteristics of these measuring instruments or completely excludes the possibility of their intended use in operating temperature operating conditions.

To this end, the following methods are applied:

a) temperature control of the measuring path;

b) electronic stabilization using an LED placed between the scintillator and the PMT;

c) electronic stabilization using a reference radionuclide source of gamma radiation located outside or inside the scintillator;

d) the instrument is re-calibrated using exemplary spectrometric radionuclide sources of gamma radiation at an ambient operating temperature different from normal temperature.

Thermostating significantly complicates the design, requires increased energy consumption, increases the cost of development, manufacture and release of equipment.

Electronic stabilization using an LED requires the purchase of an expensive scintillation unit with a built-in LED, which stabilizes only the electronic path and does not take into account the temperature dependence of the scintillator itself. In addition, the emission of the LED is not completely independent of temperature.

For effective temperature correction of the entire measuring path, including the scintillator, using a radionuclide source, you need a source with "hard" gamma radiation, preferably monochromatic, and activity that provides the statistical load necessary for correction. Using this method leads to an increase in the background of the device and, as a result, to a significant deterioration in the sensitivity of the device, roughening the measurement range.

Re-grading can only be carried out under stationary operating conditions, provided that it can be carried out and that the necessary set of radionuclide sources is included in the package. In the field, this procedure is not possible due to their operating conditions.

A known method of temperature correction of scintillation detector readings (US patent No. 7592587, publ. 09/22/2009), including the absorption of ionizing radiation on a scintillator, converting light radiation into a sequence of electrical pulses whose parameters are proportional to the energy of absorbed radiation, determining the calibration coefficient from the pulse shape parameter by a given mathematical function, which is a polynomial, determining the true value of the energy of the absorbed quantum, taking into account the Nogo calibration factor to generate an output signal based on the calibrated signal detector.

In this case, the mathematical function must be calculated individually for each detector.

This method is the closest to the proposed technical essence.

The disadvantage of this method is the lack of stability of the quality of the correction, since due to the natural process of aging of the crystal, a change in the parameters of the pulse shape occurs. The operation of the crystal under conditions of increased radiation load leads to the same effect. In addition, during prolonged operation, a change in the parameters of the electronic path occurs, and first of all, the characteristics of the PMT. Therefore, the use for a long time in the process of correction of the output signal of the detector of the initially measured parameters of the pulse shape leads to a decrease in the quality of correction.

The technical result of the invention is to ensure the stability of the quality of the correction of the output signal of the detector.

The specified technical result is achieved by the fact that the method of correction of temperature-dependent signals generated by a scintillation detector includes the absorption of ionizing radiation on a scintillator, converting light radiation into a sequence of electrical pulses, measuring the hardware value of their amplitude proportional to the energy of absorbed radiation, simultaneously measuring the temperature inside the detector, calculation of the average temperature during the measurement, determination of the calibration coefficient coefficient, taking into account the average temperature, for a given mathematical function, which is a polynomial, with predefined parameters, determining the true value of the energy of absorbed radiation, taking into account the obtained calibration coefficient, generating the detector output signal based on it.

In this case, to determine the polynomial parameters, the relative displacement of the positions of the total absorption peaks in the energy spectrum is used depending on the temperature inside the detector measured by the temperature sensor.

The main distinguishing feature of the method is the use of a directly measured temperature value in the calculation of the calibration coefficient, which is not associated with a temporary change in the properties of the scintillator and the parameters of the electronic circuit, which ensures the stability of the correction quality of the detector output signal.

The proposed method can be implemented in detectors, which, along with typical nodes, such as a scintillator, photomultiplier tube (PMT), and data conversion and processing tools, contain a temperature sensor (for example, AD-7314) installed directly after the PMT, as close as possible to the longitudinal axis devices.

At the same time, the means for converting and processing data contain at least amplifiers, an amplitude-to-digital converter (ADC), and a controller structurally located in the detector itself or made in the form of an external control unit (for example, a PC).

It was established experimentally that when the temperature changes in the range from -20 ° С to + 40 ° С, the hardware value of the energy of the total absorption peaks shifts, which can reach ± 10% of the positions of the peaks at normal temperature (20 ± 5 ° С).

Studies were conducted using gamma-ray detectors PGI-1.

The positions ( _{Еmeasurements} ) of the five peaks of total energy absorption were determined: 122 keV for cobalt-57, 662 keV for cesium-137, 898 and 1836 keV for yttrium-88 and 2614 keV for thorium-232 in the recorded energy spectra and average readings temperature sensor at eight temperatures in the heat chamber (minus 20 ° С, minus 10 ° С, 0 °, plus 10 ° С, plus 20 ° С, plus 25 ° С, plus 30 ° С and plus 40 ° С).

Then the deviations E _meas. relative to known full energy absorption peak values (F _ist.), determined at a temperature of 20 ± 1 ° C.

The experimental results are shown in table 1, from which it can be seen that the values of the relative deviations of the energy of the total absorption peaks for each temperature are the same for the entire energy range of 0.1-3 meV.

Table 1 The values of the relative deviations of the energy of the absorption peaks for each temperature Temperature ° C Total Absorption Peak Energy (keV) T 122 662 898 1836 2614 42 0.9267 0.9285 0.9294 0.9281 0.928 30,2 0.9722 0.9728 0.9713 0.9703 0.9715 26 0,9901 0.9896 0.986 0.986 0.9881 21 one one one one one 11.2 1,0278 1,0276 1,0271 1,031 1,0284 1,2 1,0556 1,0407 1,0408 1,0502 1,046 -10.2 1,0421 1,0298 1,0321 1,0491 1,0384 -15.8 1,0278 1.014 1,0147 1,0344 1,024

Further processing of the obtained data made it possible to obtain the averaged values of the calibration coefficients K = E _meas. / E _source for the entire temperature range and the value of the maximum error in determining the calibration coefficient ΔK / K (%) for each temperature, are given in table 2, from which it follows that for each temperature the value of the calibration coefficient can be considered constant for the entire energy range, since its measurement error is not exceeds 1.1%.

table 2 Values of the error in determining the calibration coefficient T, ° С TO ΔK ΔK / K,% 42 0.928 -0.001 0.1 30,2 0.9716 -0.001 0.1 26 0.988 ± 0.002 0.2 21 one 0 0 11.2 1,0284 0.003 0.3 1,2 1,0466 0.009 0.8 -10.2 1,0383 ± 0.011 one -15.8 1,023 ± 0.011 1,1

Using the obtained calibration coefficients, the least squares method was used to determine the parameters of the analytical function K _T = f (T), which was independent of the energy of gamma radiation. It can be represented with an accuracy of 1-2% by the following polynomial of the third order

where T is the temperature sensor averaged during the measurement of the energy spectrum, ° C.

The experimentally obtained formula (1) is stored in the memory of the data processing device and used in the measurement process for temperature correction of the detector output signal.

The specific values of the coefficients of the polynomial and its degree are determined by the specified range of operating temperatures and the design features of the detector.

Similar experiments were carried out on three PGI-1 detectors. The data obtained indicate that the parameters of the polynomial can be determined on the lead samples of the series and, while maintaining the elemental base and design, are extended to all detectors made using the same technology.

In view of the foregoing, the proposed method for correcting temperature-dependent signals generated by a scintillation detector is as follows.

When the detector is turned on in an environment suggesting the presence of ionizing, for example, gamma radiation, it is absorbed by the scintillator, the received light is converted by a photomultiplier into a sequence of electrical pulses, amplified, digitalized by the ADC and the controller measures the hardware value of their amplitude proportional to the energy absorbed quanta.

At the same time, with a measurement frequency of 1-2 seconds, the temperature inside the detector is measured by means of a temperature sensor installed in it. The controller calculates the average temperature during the measurement and determines the calibration coefficient, taking into account the measured temperature according to a given mathematical function (1), which is a polynomial, in this case, of the third order, with parameters predefined by the above procedure. Taking into account the obtained calibration coefficient, the true value of the energy of the absorbed quanta is determined and an output signal is generated based on the calibrated detector signal.

Since the claimed method uses only the energy of absorbed quanta and temperature, which are independent of the scintillator characteristics and electronic path parameters, to calculate calibration coefficients, the proposed correction method ensures the stability of the results regardless of the duration of use of the detector.

Thus, the proposed correction method allows fairly correctly to take into account the influence of temperature on the readings of radiation monitoring devices.

The practical application of the proposed method showed a decrease in the error introduced by the temperature change from 10% to 1.5%.

Claims (2)

1. A method of correction of temperature-dependent signals generated by a scintillation detector, including the absorption of ionizing radiation on a scintillator, converting light radiation into a sequence of electrical pulses, measuring the hardware value of their amplitude proportional to the energy of absorbed radiation, simultaneously measuring the temperature inside the detector, calculating its average value for measurement time, determination of the calibration coefficient taking into account the average temperature for a given mathematical eskoy function representing a polynomial with predetermined parameters to determine the true value of the absorbed radiation energy with the obtained calibration factor to generate at its output based on the detector signal. 2. The method according to claim 1, characterized in that to determine the polynomial parameters, the relative displacement of the positions of the total absorption peaks in the energy spectrum is used depending on the temperature inside the detector measured by the temperature sensor.