HJ 1129-2020Technical Specifications for Determination of emitting Radionuclides in Soil by In-situ HPGe Spectrometry (English PDF)

3.2 Energy resolution energy resolution

The detector's ability to distinguish incident gamma rays with different but very similar energies. Energy resolution and incident gamma ray energy Regarding, for a single energy ray with a specified energy, it is often expressed by the half-width of the full-energy absorption peak of the energy, keV.

3.3 Full width at half maximum

In the pulse height spectrum, the distance between the abscissas of two points at half the peak of a single peak, keV. The half-height width is also recorded as FWHM.

Note. If the peak in the energy spectrum is composed of several peaks superimposed, each peak has a half-width.

3.4 Field-of-view of a detector

Characterizes the amount of the detector's measurement of the surface soil range, that is, 90% of the non-scattered rays measured by the detector come from this range the soil inside. Generally expressed by the radius of the measuring soil range, m.

3.5 Lower limit of detection (LLD)

With a given degree of confidence, the in-situ high-purity germanium spectrometer can detect the lowest activity concentration or the smallest deposition flux.

3.6 in-situ measurement Do not make any physical or chemical property treatment or change of the object to be measured, and the measurement is carried out at the site where it is located.

3.7 Detection height

The vertical height of the geometric center of the high purity germanium crystal from the ground surface, m.

3.11 Effective front area

A parameter that characterizes the detection efficiency of the detector's all-energy absorption peak, that is, the ray beam with energy E is incident parallel to the axis of the detector in the case of the unit fluence rate, the all-around absorption peak count rate, m2.

3.12 Angle correction factor

For the ray with energy E, the in-situ high-purity germanium spectrometer is parallel to the measurement along the axis of the detector during actual measurement on the ground surface in the field.

When the line is incident, the unit the ratio of the total energy absorption peak count rate obtained from the ray fluence rate, the quantity of dimension one.

6.1 Distribution principle

In general, choose an open and flat area, and there should be no interfering objects (such as building Buildings, woods, etc.), see Appendix A for the field of view Rs of the detector. When measuring the deposition flux of radionuclides, self-release should be selected Since the deposition of radionuclides, have been distributed in the soil in undamaged areas (for example. no agricultural activities).

6.2 Record of measurement conditions

Record the geographic coordinates of the measurement point, as well as the description of the meteorological conditions, topography, etc.

6.3 Checking the status of the instrument

Before measuring, check whether the crystal temperature of the high-purity germanium detector reaches the working temperature. After it is at working temperature, turn it on again Bias power supply of high purity germanium spectrometer, check the energy resolution of the instrument.

6.4 Installation of on-site high purity germanium spectrometer

In the middle of the selected measurement area, the high-purity germanium detector is stably installed on the measurement bracket, the end face of the detector Face down. Adjust the height of the detector so that it is 1 m away from the surface. Connect all cables of the high purity germanium spectrometer electronics.

6.5 Measurement settings

Set the measurement time according to the count rate of the all-powerful absorption peak of the characteristic peak of the nuclides to be measured in the field measurement to make the area of interest the counting statistics fluctuate within 10%.

6.6 Collection of energy spectrum

Collect energy spectrum within a predetermined time. After stopping the measurement, store the collected energy spectrum.

6.7 Energy spectrum analysis Use the energy spectrum analysis software to view the collected energy spectrum. If there are clearly distinguishable peaks in the energy spectrum, set the Set a suitable area of interest and read the net peak area of the all-around absorption peak; if there is no clearly distinguishable peak in the energy spectrum, Measure the energy of the characteristic peak of the nuclide and the energy resolution of the detector, set the area of interest, and read the integrated area of the all-energy absorption peak.

7.1 Principle

For in-situ high-purity germanium spectrometers, it is difficult to have large-area -ray standard fields with known different depth distributions for detection efficiency.

Rate calibration. Therefore, it is necessary to adopt a theoretical model of the distribution of radionuclides in the soil according to the physical parameters of the soil and air.

And combine the characteristics of the detector to calculate the detection efficiency. The detection efficiency of the in-situ high-purity germanium spectrometer depends on the radionuclide in the soil the depth distribution in the soil, the height of the detector, the nature of the detector, the energy of the ray and the absorption properties in the soil and air Other factors can be calculated using the following Beck formula.

7.2 Method for calculating detection efficiency

7.2.1 Method overview

It can be seen from 7.1 that the fluence activity ratio F, the angle correction factor W and the effective front area are three unrelated physical the product of the quantity can get the detection efficiency of the on-site high-purity germanium spectrometer . Among them, the fluence activity ratio F and the angle correction factor W and the radiation the mass relaxation depth of sex nuclides is related, and the determination of the mass relaxation depth is a prerequisite for the calculation of detection efficiency. Already Knowing the mass relaxation depth , carry out the detection efficiency calculation according to the following steps. (1) Calculate the soil according to the theoretical model the fluence activity of the radionuclide is higher than F. (2) Use standard source experimental measurement or Monte Carlo simulation to calculate the detector Area before effect . (3) The theoretical model calculates the angular distribution of the primary fluence rate () at the height of the detector;

Empirical measurement or Monte Carlo calculation of the detector's angular response factor R(); joint primary fluence rate angular distribution () and detection the angle response factor R() of the device is calculated to obtain the angle correction factor W.

7.2.2 Determination of mass relaxation depth

Generally speaking, in the newly settled radioactive ash, the radionuclides are evenly distributed on the soil surface, = 0 g.cm-2;

Natural radionuclides are uniformly distributed in the soil, = g.cm-2.Nuclides in radioactive fall ash over time Continuously infiltrate the soil depth, and the activity concentration decreases exponentially with the increase of soil depth z, 0 g.cm-2