HPGe-detector
Remark: Supplied by German Federal Office of Radiation Protection.
Purpose
To calibrate airbourne gamma spectrometer with HPGe detector for aerial survey.
Discussion
To be able to estimate the soil contamination due to individual radionuclides the gamma spectrometer must be calibrated. This calibration procedure is similar to the procedure given for an in situ gamma spectrometry (Procedure Dla).
The detector calibration factor (Cf) that is the ratio of the net peak area count rate (Rf) of a characteristic gamma line to the surface concentration (As) of a given radionuclide can be expressed as a product of three factors that can be determined separately:
where Cf =
f RO * As detector calibration factor
response factor; peak counting rate due to a unit primary photon flux density of energy E incident on the detector along the detector axis (normal to the detector face)
angular correction factor; required to account for detector angular response geometrical factor; total photon flux density at the detector position per unit
concentration or inventory of the radionuclide.
The complete formalism to calculate the detector calibration factor for in situ spectrometry is given in [10, 14] and can also be used for the calibration of airbourne gamma ray
spectrometer systems using HPGe-detectors.
Response factor Ro/O is determined by laboratory measurements using calibrated point sources. The angular correction factor Rf/Ro is determined by the combination of laboratory measurements of detector angular sensitivity and the model calculation of angular distribution of flux density and finally the geometrical factor O/As is calculated for different altitudes using the method described in [14].
In order to evaluate the soil contamination, knowledge of the distribution of the radionuclides in the soil is essential. In most cases the distribution of artificial radionuclides in soil could be described by an exponential decrease with depth. The relevant parameter in this case is the relaxation mass parameter a. Time dependent distribution of radionuclides in soil and
migration in deeper soil layers play a minor role just after reactor accident. Assuming surface distribution the radionuclide ground concentrations may be underestimated by a factor of up to 2 [14].
Equipment/Supplies
> Airbourne gamma ray spectrometer
> Certified reference point sources
Stepl
Perform spectrometer energy calibration using procedure D2a.
Step 2
Determine the detector response factor Ro/O using Procedure Dla, Step 3.
Step 3
Determine the angular correction factor Rf/R« using Procedure Dla, Step 4.
Step 4
Take the geometrical factor O/As for the peak in the spectrum at energy E and the flying altitude from the curves in Figure A3, which correspond to surface contamination with no penetration into the soil for different flying altitudes commonly used for aerial monitoring.
The geometrical factors for other altitudes can be determined by interpolation between the curves or by using the formalism in [10, 14].
StepS
Calculate detector calibration factors at different energies by multiplying the corresponding three factors:
c =R j _ R o . _ i .
f R0 (j) As
Plot Cf vs. energy for different flying altitudes on log-log scale and fit the data points by smooth curves. Record all Cf points and save diagram in spectrometer logbook.
Spectrometer calibration for aerial monitoring Procedure A6a, Pg. 3 of 3
FIGURE A3
GEOMETRICAL FACTOR <j)/As AS A FUNCTION OF PHOTON ENERGY IN CASE OF SURFACE RADIONUCLIDE DISTRIBUTION FOR DIFFERENT FLYING ALTITUDES
0.60
0.00 100 1000 10000
Energy [keV]
Discussion
Aerial monitoring is regarded as most appropriate method for a rapid search over large areas.
In this case Nal(Tl) detectors are the most favourite detectors. However, systems based on pressurized ionization chamber, proportional counters, GM detectors or other suitable dose rate meters may be also used.
The detector response as a function of the photon energy and the angle of incidence (including possible shielding of the helicopter itself) has to be known. The system has to be calibrated for aerial monitoring prior to use. If a spectrometric system is used use Procedure D2a for energy calibration and Procedure A6a for detector calibration factor. Alternatively if pressurized ionization chamber, proportional counter, GM tube or other suitable dose rate monitor is used a simplified calibration procedure can be applied. In that case rapid and simple calibration of the detector efficiency for a given geometry can be performed experimentally by flying over suitable reference sources.
The following points should be considered when planning any search activities:
i. Suspected source locations should be surveyed first; within these search areas priority should be given to populated areas.
ii. Flight line spacing and altitudes depend on the activity and number of sources and the sensitivity of the monitoring system,
iii. Navigational capabilities of the aircraft or helicopter, iv. Communication between the ground teams and the crew.
In general there are several different flying possibilities depending on objectives of survey.
For survey with helicopters the following search patterns may be used:
Parallel Track Search — PT:
Description: Parallel straight tracks with a length of some kilometers with a distance between the tracks of 300 m.
Applicability: Survey in flat or hilly landscape.
Line Search — LS:
Description: Flight along specified lines (e.g. roads, railway tracks, rivers) with parallel tracks with a distance of 300 m.
Applicability: Survey along traffic lines.
Contour Search — CSV
Description: Flight along geographic contour lines.
Applicability: Survey in deeply cut or mountainous terrain.
Source monitoring by aerial survey Procedure A 7, Pg. 2 of 7
Penetration Search — PS:
Description: Flights from landmark to landmark on different tracks.
Applicability: Rapid detection of contaminated area borders.
For search activities in general see [2].
It is difficult to give detailed procedures for all possible types of airbourne monitoring
systems. Therefore, only a generic procedure is given here. In practice this procedure must be revised and customized for the specific spectrometer system or dose rate monitor used.
Equipment/Supplies
> Common equipment to all teams (Checklist AO)
> Airbourne gamma monitoring system (Checklist A3)
> Helicopter or fixed wing aircraft
Step 1
1.1. Receive an initial briefing and initial assignments from the Environmental Analyst/Radiological Assessor.
1.2. Obtain appropriate equipment using Checklists AO and A3.
1.3. Check the instruments using Checklist AO.
1.4. Obtain information on weather conditions and forecast.
1.5. Obtain information on the area to be surveyed.
1.6. Receive information on coordinate system used from Environmental Analyst/Radiological Assessor.
Step 2
According to the instructions from the Environmental Analyst/Radiological Assessor:
2.1. S et the alarms of self-reading do simeters.
2.2. Wear appropriate personal protective equipment.