The radioactivity program at the GSC has undergone more than 60 years of evolution (Darnley, 1991). The development of equipment and methods to measure natural radioactivity has been a Canadian specialty since the early 1930's, triggered by the discovery of radium at Great Bear Lake (NWT) in 1930.

The 'Geiger counter' was the original radiation detector, recording the total count rate from all energy levels of radiation. Ionization chambers [1] and Geiger counters were first adapted for field use in the 1930's. The first transportable Geiger-Müller counter (weighing 25 kg) was constructed at the University of British Columbia in 1932. H.V. Ellsworth of the GSC built a lighter weight, more practical unit in 1934. Subsequent models were the principle instruments used for uranium prospecting for many years [2]. In the early 1960's, R. Doig completed geophysical studies with a 10 kg portable gamma-ray spectrometer [3] designed and constructed at the GSC and McGill University. With proper calibration, this spectrometer was capable of providing chemical concentrations of potassium, uranium & thorium.

The Geiger counter has evolved into a sophisticated 256-channel spectrometer [4] that can be tuned to measure radiation from specific elements. It is used by geologists and geophysicists for ground followup on anomalies and other areas of interest [5] observed from the maps produced from the aerial surveys. In this picture, a spectrometer is being used with a pocket-sized Global Positioning System (GPS) receiver to determine the location of the site [6] & [7].

[1] [2] [3]

[4] [5] [6]

[7]

The use of airborne detectors to prospect for radioactive minerals was first proposed by G.C. Ridland, a geophysicist working at Port Radium in 1943. In 1947, the earliest recorded trial of airborne radiation detectors (ionization chambers and Geiger counters) was conducted by Eldorado Mining & Refining, Ltd. (a Canadian Crown Corporation). The first patent for a portable gamma-ray spectrometer was filed by Professors Pringle, Roulston & Brownell of the University of Manitoba in 1949, the same year as they tested the first portable scintillation counter on the ground and in the air in northern Saskatchewan.

In 1955, the GSC carried out the first long-range aerogeophysical reconnaissance survey, flying over the Canadian Arctic Islands carrying a magnetometer and a scintillometer. In the late 1950's, A.F. Gregory of the GSC commenced a series of experiments that led to the development, 10 years later, of the GSC's high sensitivity airborne gamma-ray spectrometry system. Designed and constructed to meet GSC specifications at Atomic Energy of Canada, Ltd., by a team headed by Q. Bristow, its performance revolutionized the practice of airborne gamma-ray spectrometry, making it possible to do 'geochemistry from the air'. The calibration standards and data reduction procedures that were established were subsequently recommended for worldwide use by the International Atomic Energy Agency.

The development of the gamma-ray spectrometer in the early 1960's and its introduction into aircraft systems (requiring a significant increase in crystal volume) in the mid-60's marked a new era. In 1967 the first airborne gamma-ray surveys were done with a Vertol helicopter [8]. Such full-spectrum systems allowed the measurement of discrete 'windows' within the spectrum of gamma-ray energies [9], to determine concentrations of individual radioelements (in particular, K, U, Th) and thus to begin mapping rocks by virtue of their characteristic radioelement signatures.

Systematic airborne radioactivity mapping of Canada began in 1969 using the GSC's Skyvan aircraft [10], CF-GSC. The Skyvan aircraft has a large carrying capacity and is particularly suitable for flying airborne gamma-ray spectrometry surveys at relatively slow speeds (~180 km/h) and at low levels (~130 m height above ground). On the other hand, the Space Shuttle [11] is considerably less suited to this application.

The Skyvan's data acquisition system (Bristow, 1979) consisted of a Data General Nova 1220 computer [12] and an array of 14 NaI detectors (total volume: 50 L), 2 of which are 'upward-looking' [13]. In 1995, the acquisition system was replaced with an Exploranium GR820 spectrometer, using the same array of detectors.

A national program of gamma-ray spectrometry was initiated in 1969 and aerial surveys were carried out with the Skyvan between 1968 and 1995. The federal-provincial Uranium Reconnaissance Program (URP, 1975-1979), directed by A.G. Darnley of the GSC, gave a major boost to progress by making it possible to employ commercial contractors and requiring them to meet the new high sensitivity standards. The URP provided reconnaissance-level coverage (5 km & 25 km line spacing) of more than a third of the Canadian Shield and Maritimes. Since 1979, these surveys have continued at a slower, but steady pace. Some of the surveys have been done by contractors, using both fixed-wing and helicopter systems, shown here being calibrated using transportable calibration pads [14].

[8] [9] [10]

[11] [12] [13]

[14]

The initial justification for the extensive use of the Airborne Gamma-Ray Spectrometry (AGRS) technique was to aid in uranium exploration, although it was clear from the early 1960's that it had wider applications. Over the years, major advances have been made in the presentation and interpretation of data. The early airborne scintillometer surveys provided rough total count contour maps. But these instruments measured only 'total radioactivity' from all sources, and were used primarily for uranium exploration. In the 1980's, colour display of data provided the means of producing ternary radioelement maps, that have proven very effective in the presentation of data for geological mapping, multi-element mineral exploration, and environmental studies. Variations in the absolute and relative abundances of the radioelements potassium, uranium and thorium provide valuable indicators relevant to bedrock and surficial geological mapping and to exploration for a wide range of mineral commodities. In the 1990's, gamma-ray spectrometry surveys have proven to be a very effective tool for mineral exploration, resulting in the discovery of several deposits of significant economic value [15] (Gandhi et al, 1996).

The AGRS method also has unique capabilities for quantitative environmental monitoring [16] (Grasty, 1983) and radionuclide identification in nuclear accidents. In 1978, a Soviet nuclear-powered satellite fell to earth in northern Canada. The GSC airborne gamma-ray spectrometer was a major factor in the success of Operation Morning Light, the joint Canada-US operation to locate and recover radioactive debris from that accident. Environmental geochemistry has also become a subject of global concern and airborne gamma-ray spectrometry is an essential tool in its study.

Airborne gamma-ray spectrometry is now accepted as a technique with worldwide applications for geological mapping, mineral exploration & environmental monitoring.

[15] [16]

We acknowledge contributions to system/application development, airborne and ground data collection/compilation/production since the late 1960s: