2 Methods
2.7 Radioactivity
The rare case in which resistivity was logged in a borehole and a percussion hole at the same spot is displayed in Fig. 2.27. In both holes, the resistivity was logged by the 16" normal down to a depth of 4.5 m.
The differences in the resitivities of silt, clay and water-filled gravel range about 350 Om, but only in the borehole log are their boundaries so distinct that depths can be determined exactly. The percussion log has many more steep anomalous peaks, which permit no precise decision as to where the boundaries are situated.
52 2 Methods EXPLANATION
e
Lung cancer attributed to these IX particles Long-lived, geochemically independent Measured in NURE airborne surveysFig. 2.28. 238U decay series
The measuring unit of radioactivity is the becquerel (Bq), which is named after the French physicist who received the Nobel Prize in 1896 for the detection of radioactivity. One becquerel corresponds to one radioactive decay per second.
Formerly the unit curie (Ci) was used (Table 2.5). 1 Ci equals 3.7' 10 \0 Bq.
Therefore, even very weak radiation will be reported in large figures and the public may be frightened by an overestimated nuclear danger!
The number of atomic nuclei that disintegrate per second conveys nothing about the effectiveness of radioactive rays. The measure for this is the energy dose or the amount of absorbed radiation received by the radiated material.
Three kinds of radioactive doses are distinguished: < the energy dose D, the ionic dose J, and the equivalent dose H. They are different for a-, f3- and
~radiation. The time dependence of the dose is expressed in units of dose performance ( dose wattage).
The units of measurement presented in Table 2.5 are used mainly to describe and compare the damages inflicted by radioactive exposure on living creatures.
Most important is the equivalent dose H, counted in mSv (millisievert), which gauges the dose of radiation a human body may encounter. The threshold values listed in Table 2.6 should not be exceeded.
The ionizing effect of radioactive radiation is used by the Geiger-Mueller counter to count the number of ionizations by the sudden collapse of a static voltage or potential field. Nowadays, a more precise and sophisticated method is favored. It is based on the property of crystals of sodium jodide to sparkle under radiation (radiophotolumincence). The brightness that is created is picked up by the photodiodes of scintillation counters.
It is worthwhile converting the scintillation values into Bq. The scaling into counts per second (cps) is less favorable, since this unit depends on the size ofthe crystal. Surveys by different instruments are then difficult to compare, unless they are calibrated in a test pit.
Table 2.S. Units of Radiometric Doses Dose
class
Energy
Ionic
Equivalent E =energy m =mass
Mark
D
J
H
Definition
D=~ M
J= Q m H=D·q
rad = radiation absorbed dose C =Coulomb
Unit
Old New
rad
R
rem
Gy(Gray) C kg
Sv (Sievert) R =Roentgen
rem = Roentgen equivalent q = evaluation factor for radioactive absorption of biologic bodies
Table 2.6. Threshold values of radiation doses for human organs gonads, uterus, red bone marrow
bone surface, skin all other organs and tissue
0.3 mSv 1.8 mSv 0.9 mSv
Conversion
I Gy= 1-J = IOOrad kg
I
;g
= 3876RI Sv = I kg = 100 rem J
Special scintillation counters are able to measure the abundant energy of radia- tion by the strength ofthe sparkles, which is recorded in MeV. Most common are the channels 1.46 MeV, 1.76 MeV and 2.62 for the natural radionuclides 4°K, 238U and 232Th, which allow the differentiation of the three elements.
Radiometric surveys are carried out in the field in dense grids. The scintillo- meters are either held close above the surface of the earth (0.2-1.0 m), or placed in shallow percussion holes 0.5 -I, m deep. Even airborne surveys are common from helicopters or airplanes, which fly very low. The results are presented most- ly in colored isoline maps orland in radiometric sections of the f"intensity.
This method is used to locate buried radioactive waste and the f"radiation that emerges from radioactive rain as fallout. But also geological rock complexes, which contain radioactive minerals like granit or uraniferous lodes, can be pro- spected. Radiometric results can tell about the spread of radioactive material on the surface or very near to the surface. Radionuclides that sit deeper than 3 m in the ground may already be out of reach. Some but not all domestic waste sites dis- play weak f"activity, which can be used for detection.
Radon surveys count the a-radiation that is emitted by the decay of the radon isotope. The datum is the content of radon gas of the ground air. A limited amount of ground air is pumped out of 0.5 -l-m depth, after a hole has been sunk by per-
54 2 Methods cussion. The number of radioactive decay events is measured at the air sample by an a-scintillometer or an ionization chamber.
The emittance of radon by contaminated waste or by natural rock is based upon geochemical and geodynamical processes. Radon enrichment of the soil indicates the presence of preferred ascent paths. This can be leaking seals at dumps or natural steep dipping structures, like fracture or fissure zones. In any case, gases contaminated by radon will follow the path that is paved by higher permeabilities of rocks or materials. Radon surveys are therefore well adapted to investigate planned waste disposal sites in hard rock for potential transport paths of conta- minants.