Radiometrics theoretical background (R25)
Contents
1. M P and Richardson L M 1990 Portable calibration sources for airborne gamma ray spectrometers Expl Geophys 21 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 3 Library Help Top 4 Back gt Background corrections introduction Radiation not originating from the ground is regarded as background The background corrections are by far the largest and most difficult correction to make There are three main sources of background radiation radioactivity due to the aircraft and its contents cosmic radiation e atmospheric radon 7 2Rn and its daughter products Radon cosmic and aircraft background radiation all originate in the atmosphere and in the aircraft and its equipment The shapes of these spectra are determined from calibration of the airborne acquisition system Calibration is achieved using a series of test flights The data from these calibrations is used to compute the various coefficients or spectra which are required to perform the radiometric corrections Aircraft background radiation is due to the contamination of the aircraft structure and equipment and the detector itself Primary cosmic radiation from outside our solar system and from the sun reacts with atoms and molecules in the upper atmosphere and generates a complex secondary radiation This radiation reacts with the air aircraft and detector to produce the cosmic gamma ray backg2. calculate the dead time for the cosmic channel differently INTREPID has a facility for independent dead time correction in the cosmic channel See Live Dead time introduction for more information about dead time correction You can perform cosmic background correction either as a full spectrum correction before photo peak extraction or as a window correction after photo peak extraction 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 4 Library Help Top 4 Back gt Radon background estimation Library Help Top You can estimate the radon background with the full spectrum analysis technique known as the spectral ratio method For gamma ray counts above the Compton continuum the low energy 7 4Bi photopeak at 0 609 MeV for atmospheric radiation suffers far less attenuation relative to the 7 4Bi peak at 1 76 MeV than is the case for radiation from uranium in the ground Since thorium and potassium sources do not contribute appreciably to these peak count rates you can use them to estimate the contributions of radon and uranium to the observed spectrum Before performing the radon background estimation you must adjust the spectra for dead time See Live Dead time introduction energy calibrate them See Energy calibration introduction and remove the cosmic and aircraft background components See immediately above Minty et al used portable calibration s
3. height of the aircraft above ground level for fluctuations in temperature and pressure We use the formula p CoPi lm ti to Po where h effective height hm measured height to 273 15 Po 1013 25 t measured temperature degrees C p measured air pressure millibars In practice the exponential attenuation correction for height does not yield meaningful results for data which is collected above about 250 metres It is common practise to ignore data obtained from heights above this level 1 For AGSO s system current values of u are for total count integral u 0 006560 for potassium u 0 007550 for uranium u 0 005570 for thorium u 0 005570 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 10 Library Help Top 4 Back gt Sensitivity conversions theory Library Help Top If you make traverses over a test strip of known ground concentrations in potassium uranium and thorium and compare your spectrometer data with the known concentrations you can establish a relationship between the count rates recorded by your spectrometer and the actual ground concentrations You can thus establish the sensitivity of the gamma ray spectrometer in terms of counts per unit concentration per unit time You can use this sensitivity to scale the corrected radiometric count rate data to give ground concentrations This provides a degree of data indep
4. INTREPID User Manual Radiometrics theoretical background R25 1 Library Help Top lt 4 Back gt Radiometrics theoretical background R25 Top Multi channel spectra processing performs the following operations on multi channel airborne radiometric data Spectral noise cleaning e Live time Dead time correction Energy calibration correction for photo peak drift Aircraft cosmic and radon background correction Once the spectra have been successfully adjusted standard potassium uranium thorium and total count window data are extracted by integrating each spectrum over the appropriate energy range Live Dead time introduction The dead time of a gamma ray spectrometer is effectively the time taken by the equipment to analyse a single gamma ray during this period the equipment is busy and cannot analyse any other gamma rays You can calculate the true count N from the measured count n if the dead time T is known using the approximation N n 1 nT You can measure the dead time for your spectrometer empirically The live time of a gamma ray spectrometer is the time in a given time interval that the spectrometer is available to accept data The live time L will normally be related to the dead time T by the formula L 1 nT In some systems there may be factors other than the count rate contributing to a reduction in the live time For example housekeeping functions such as on line energy calibration may cause th
5. d encompassing the energy bounds of the spectrometer you are using normally 0 0 3 0 MeV The five required calibration spectra are as follows Aircraft the aircraft background spectrum in counts 100 seconds Cosmic the cosmic spectrum corresponding to a count rate of 100 counts second in the 3 6 MeV cosmic channel Standard a typical spectrum at the nominal survey altitude Radon a typical spectrum due to Radon at the nominal survey altitude 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 7 Library Help Top lt 4 Back gt Uranium the spectrum due to uranium from the ground at the nominal survey altitude INTREPID only uses the shape of the Standard Radon and Uranium calibration spectra so the actual count rates for these spectra in the calibration spectra file are immaterial The calibration spectra file must be in ASCII format and have the extension asc INTREPID will ignore blank lines and lines starting with We have included an abbreviated sample calibration spectra file in this manual See Example of calibration spectra file in Radiometrics file formats R26 You can use this as a model for creating your own file INTREPID also has a library of example calibration spectra files in the config calibration_spectra dir You can use these pre existing calibrations files if you cannot derive them for your own data Standard 3 corrections theory The el
6. e spectrometer to be unavailable for a short period Dead time correction for systems with these features is not accurate enough Such systems instead measure live time using the spectrometer and record it with the spectral data See INTREPID 256 channel spectrum format in Radiometrics file formats R26 For systems using live time the true count is given by N n L INTREPID can perform either dead or live time corrections Your spectrometer may behave differently for data in the cosmic channel 38 6 MeV so you may wish to perform dead time correction in the cosmic channel differently or omit it INTREPID provides a facility for this See Cosmic background corrections for details about the cosmic background correction Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 2 Library Help Top 4 Back gt Energy calibration introduction Library Help Top INTREPID has a standard set of spectrum data with peaks and troughs typical of the radiometric data you have produced The standard spectrum depends on the aircraft and the spectrometer You must determine them empirically for the spectrometer and aircraft that you are using Count rate peaks and troughs occur at energies that you can use as landmarks for checking the measured energies of your data It compares your data with this standard to check and adjust the accuracy of the energies re
7. ements Potassium K Uranium U and Thorium Th emit gamma rays These elements occur in most rocks and soils The gamma rays are of different energies which you can detect using a differential spectrometer You can process and analyse this data to provide information about the abundances of the three elements in the rock or soil Gamma rays are attenuated by rock and overburden As a result approximately 90 of the radiation comes from the top 150 to 200 mm of rock or 300 to 400 mm of soil cover Gamma rays are also attenuated by the air between the source and the aircraft The radiation is reduced by half in passing through each 100 to 130 metres of air Compton scattering in the source air and the detector modifies the energy distribution of the gamma rays The spectrometer in the aircraft also measures a fairly constant background radiation level which is due to sources other than the ground cosmic aircraft and atmospheric radon gas See Multi channel gamma ray spectrometric processing C07 for a full discussion of background radiation dead time and spectrum calibration corrections as well as calculation of the Potassium Uranium Thorium and Total Count data We wish to remove the effects of height attenuation and Compton scattering and end up with count rates in the Potassium Uranium and Thorium channels which are directly proportional to the abundance of the three elements at the Earth s surface The S3GSC process performs
8. endence from survey parameters such as crystal volume and survey flying height The best estimates for K U and Th are obtained using spectral smoothing NASVD MNF followed by conventional 3 channel processing It is standard practice to report total count in units of nGy h the equivalent air absorbed dose rate at 1m above the ground 2012 Intrepid Geophysics lt 4 Back gt
9. kground Let L and H be the radon contribution to the low energy and high energy peak count rates respectively Similarly let L and H be the uranium contribution to the low energy and high energy peak count rates Then Lop Le Ly and Hob H Hy Also since we can assume that the shape of the uranium and radon components are constant for a particular altitude we have L c H 8 and Ly c2H 4 where c4 and cg are constants that can be determined directly from the radon and uranium spectra respectively Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 6 Library Help Top lt 4 Back gt Equations 1 to 4 give L Lop 2 Hop 1 9 c1 5 You can then use the value of L with the radon calibration spectrum to calculate the radon background spectrum correction and subtract it from the observed spectra It is most important to correctly perform the energy calibration of the spectra The Bi2 4 0 609 MeV photopeak is relatively narrow and you need to energy calibrate the observed spectra to a high degree of precision To obtain sufficiently good statistics to calculate the radon background INTREPID sums the spectra over a defined interval default 100 readings It then uses Equation 5 to calculate the amplitude of the radon background spectrum and subtracts it from each individual spectrum over the summed interva
10. l INTREPID repeats the procedure until it reaches the end of the line For the purposes of radon background estimation you can specify energy windows surrounding the 2 4Bi 0 609 MeV and 1 76 MeV photopeaks INTREPID s default Radon background calculation intervals are 0 54 0 68 MeV for the low energy L window and 1 65 1 96 MeV for the high energy H window INTREPID also supports radon removal using the older upward looking detector method Standard 3 K U Th channel data generation introduction After you have performed the corrections and calibration on the multichannel data you can extract the potassium thorium and uranium count rates The count rate for each element is found by integrating Summing the counts over a defined energy range or window The International Atomic Energy Agency IAEA recommends the following energy windows for the three elements Potassium 1 370 1 570 MeV Uranium 1 660 1 860 MeV Thorium 2 410 2 810 MeV Using this method for each spectrum i e each integration period data point you can produce a complete set of K Th and U data for your survey The calibration spectra Library Help Top To carry out these adjustments on multi channel spectra you need to derive a set of calibration spectra empirically from the aircraft and spectrometer that you are using The calibration spectra set must consist of five individual spectra all 256 channel an
11. ources to measure pure U Th and K spectra over a range of altitudes and Minty and Richardson derived aircraft and cosmic spectra for the AGSO system from high altitude calibration flights You can derive the radon spectrum from calibration flights at survey altitude over water and in the presence of radon The following figure shows the Rn spectrum and the U Th and K spectral components relative to a low energy 0 54 0 68 MeV window centred on the 2 4Bi 0 609 MeV photopeak and a high energy 1 65 1 96 MeV window centred on the 214Bi 1 76 MeV photopeak 1 ibid 2 Minty B R S and Richardson L M 1989 Calibration of the BMR airborne gamma ray spectrometers upward looking detector February 1989 Bureau of Mineral Resources Australia Record 1989 8 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiomeirics theoretical background R25 5 Library Help Top lt 4 Back gt The important feature of these windows is that only the radon and uranium components contribute significantly to gamma ray counts above the Compton continuum You can estimate these count rates the peak count rates by integrating under each peak but above the straight lines used to approximate the Compton continuum Relative counts per channel Energy MeV Let Lop and Hop be the respective observed peak count rates in the low energy and high energy peaks after correcting the spectrum for aircraft and cosmic bac
12. ported by your spectrometer INTREPID superimposes your data on the standard spectrum and looks for the landmark peaks and troughs If your landmarks occur at a different energy INTREPID will adjust your reported energy range to conform with the standard spectrum In this way INTREPID calibrates your spectrometer Rather than process the whole spectrum you can examine a number of energy ranges which contain the best landmarks for the calibration process These are called energy windows You can specify the energy windows directly or use the default set provided by INTREPID INTREPID performs the calibration process one traverse line at a time It integrates the spectra for the line and aligns the data in the energy windows with that of the standard spectrum INTREPID calculates the calibration the corrected high and low bounds for the whole spectrum for the whole traverse line and then updates all of the individual spectra in the line using this calibration You can find a full description of this energy calibration technique in Minty et al The technique involves iteratively adjusting the energy bounds of the spectrum using double quadratic minimisation It does not alter any of the count rates To ensure that the quadratic minimisation converges the initial estimation for the energy bounds must be fairly close to the true values You must specify a nominal spectra energy range for this initial estimation 1 Minty B R S Morse
13. round Atmospheric Rn and its daughter products specifically 7 4Bi and 7 Pb are the major contributors to the background Rn is very mobile and can escape into the atmosphere from soils and rock fissures in response to the pumping action of changing temperatures and pressures Its daughter products 214Bi and 2 4Pb attach to airborne aerosols and dust particles and their distribution is thus a function of air movements and wind patterns Aircraft background corrections The aircraft background radiation spectrum is constant It can be performed either as a full spectrum correction before photo peak extraction or as a window correction after photo peak extraction Cosmic background corrections Library Help Top In the lower atmosphere this radiation has a constant energy distribution but it decreases in amplitude with decreasing altitude The amplitude of the cosmic spectrum is linearly related to the count rate in the 3 6 MeV range the so called cosmic channel If you know the cosmic spectrum corresponding to a cosmic channel base count rate you can use the cosmic channel count rate for each spectrum to calculate the cosmic background spectrum and subtract it from the spectrum Since the cosmic channel has an energy range outside that of the spectral data you wish to use you may have different arrangements for collecting this data or your spectrometer may behave differently in the 3 6 MeV range You may therefore wish to
14. st them to a standard distance above ground level You would typically use the nominal terrain clearance of your aircraft as this standard distance In theory you might consider the geometry of the source whilst calculating this height correction in order to use a more complex attenuation formula In practice the simpler process of applying a uniform exponential correction provides a satisfactory height correction as long as the ground clearance varies within reasonable limits 1 For the AGSO system a 000518 h 0 45500 B 000804 h 0 44100 y 000976 h 0 80500 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 9 Library Help Top Library Help Top 4 Back gt Our correction is based on the formula Ng Neo 2 where No height corrected count N height uncorrected count Nec from the stripping correction See Compton scattering correction stripping theory h height above ground measured and corrected for temperature and pressure if possible see below H nominal flying height u attenuation coefficient different for each energy of the gamma rays ug Uy Upp The attenuation of the gamma rays with height in reality relates to the mass of the column of air below the aircraft This depends on the air pressure and temperature If air temperature and pressure data are available then you can adjust the measured
15. the height correction and Compton scattering correction often called stripping or energy stripping of the Potassium Uranium Thorium and Total Count data Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Radiometrics theoretical background R25 8 Library Help Top lt 4 Back gt Compton scattering correction stripping theory Compton scattering contributes to the uranium and potassium count rates Scattered thorium radiation contributes counts to the lower energy uranium and potassium channels Scattered uranium contributes to the potassium channel only The coefficients for correction depend on altitude the window widths employed size number and spacing of radiation detectors etc Our stripping equations are as follows Neqh Nop Ney Ny a Nth Nex Nx a B Nepp y Ney 1 where N is the measured count rate for element p Nc is the corrected count rate for element p a B y are stripping ratios a Mab Ca B mgh F Cp y mhtc where m slope change per metre of ratio x Cx ground level value of ratio x h measured clearance height above ground Height correction theory It is in practice impossible for an aircraft to maintain a constant ground clearance At different heights radiation from the ground suffers different attenuations due to variations in the mass of air it has to pass through It is easier to compare data within a survey if you adju
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