Speaker

Shri Pravin Kumar Sharma, Scientific Officer/H, Head Physics Group, AMD, Hyderabad obtained his M.Sc. degree in Physics from Meerut University in 1980 and Post Graduate Diploma in Instrumentation from Regional Engineering College, Kurukshetra in 1983. He has joined Atomic Minerals Directorate, Hyderabad in August 1984 as a Scientific Officer/C, after completion of 27th batch of BARC Training School in Nuclear Science and Technology at BARC, Mumbai. He has worked as Incharge of Regional Physics Laboratories of Central Region, Nagpur and Eastern Region, Jamshedpur.

He has immensely contributed to uranium exploration activities in Central and Eastern India. He has studied the behaviour of radon in soil gas and water samples in various field areas to find out the buried uranium occurrences/deposits. He was also associated with DAE coordinated BRNS project on ‘Radon mapping of the country’ and estimated radon/thoron and their progeny concentration in the dwellings of various areas under Central Region, AMD, Nagpur.

He has fabricated Transportable Calibration Pads for the calibration of Portable Gamma Ray Spectrometers (PGRS) and on board Airborne Gamma Ray Spectrometer (AGRS).These pads can be transported to the air base where airborne surveys are in progress.

Apart this, he was made Convener of International Geo-Science Programme (IGCP-571), Radon, Health and Natural Hazards. He has organized annual meetings, workshop of National Working Group Members (NWG) and specialists in Radon measurements work related to Radon measurements in the Dwellings and Radon measurements for Earth quake prediction, Doctors (Oncologists) under the programme. He has also organized training programmes for NWG members, Scientists of various departments and Research Scholars of various universities.

He has published twenty papers on various topics like Radon Emanometry in uranium exploration, Radon measurement in dwellings, Characterisation of geological materials by radiometric techniques, Active borehole logging techniques in uranium exploration, Radioisotopes, Mossbauer Spectroscopy in uranium exploration and Physical protection of nuclear material and facilities.

Presently Shri Sharma is leading the Physics Group, AMD, Hyderabad as Head Physics Group and guiding seven Regional Physics Laboratories at New Delhi, Banglore, Jamshedpur, Shillong, Jaipur, Nagpur , Hyderabad and Radiaton Standard analysis,’ Laboratory at Hyderabad.

Title: RADIOMETRIC METHODS IN URANIUM EXPLORATION

Exploration of any mineral to its exploitation is a time consuming and costly affair. This is also true in the case of naturally occurring radioactive minerals. The detection and estimation of these elements in the field should be as fast as possible. There are three naturally occurring radioactive elements Uranium, Thorium and Potassium. It is advantageous to tap their radioactive properties. We can detect, estimate radioactive element by measurement of their alpha, beta and gamma ray emissions. Radioactive methods are cost effective, speedy and easily provide onsite results which are very advantageous in planning and changing the exploration strategies at the field itself. Gamma ray surveys are preferred for field investigation compared to alpha, beta emissions. By measuring gamma radiation, detection is possible at distances of a few hundred of metres in air or a few tens of centimetres in water. Instrument response is almost instantaneous, and measurements can be made from a detector which is in motion. Thus any moving vehicle, ship or aircraft may provide a platform from which measurements can be made. With proper precautions the grade of in situ material can be estimated without the necessity to remove samples. Radiometric techniques actually access the quantity of radioactive element in situ which is to be exploited in future rather than taken a very small quantity of sample for chemical analysis for assigning the grade. A survey of the radon content in soil air may be undertaken by direct measurement of the alpha radiation emitted by radon.

Consideration is given to the selection of promising areas which may be favourable for uranium occurrence (indicated by existing geological and geophysical information). Detailed geological appraisal of the selected area from existing geologic knowledge is carried out in an attempt to reduce the areas to be examined.

Geiger-Muller tube was invented in the 1920s led to the first portable instrument designed for field survey use in the early 1930s. Because the Geiger-Muller tube is primarily sensitive to radiation and is a very inefficient gamma-ray radiation detector, early attempts to use Geiger-Muller tubes for airborne surveys were unsatisfactory. However, the cheapness, compactness and ruggedness of Geiger-Muller tubes made them very suitable for ground surveys. These characteristics, combined with the relative insensitivity of the tubes to temperature change, made them particularly suitable for borehole logging.

It was the discovery of high efficiency thallium-activated sodium iodide in 1940 which made possible most of the present day gamma-ray spectrometry survey measurement techniques. A scintillation detector provides a much more efficient response to gamma radiation than a Geiger-Muller tube and possesses the very important advantage of being able to discriminate between gamma radiations of different energies.

Potassium has a simple form of radioactive decay. Only one 40K of the several natural isotopes of potassium is radioactive. It has a relative isotopic abundance of only 0.0118%. Natural uranium consists principally of two isotopes 238U and 235U of which the first is the most abundant (99.27%) and is the only one of concern under field survey conditions. The radioactive decay of 238U is complex and passes through 14 steps, each with characteristic disintegration and daughter products before it reaches the final stable end product 206Pb. However, it is most important to realize that the principal gamma emission is associated with 214Pb and 214Bi and not directly with 238U. Many of the problems associated with the effectiveness and interpretation of gamma-ray surveys stem from this fact. 214Pb and 214Bi are respectively the eighth and ninth daughter products in the decay series. 232Th is the principal isotope of natural thorium and like 238U has a complex decay process before reaching 208Pb. The strongest gamma emitter is the ninth in the decay series, 208Tl.

Under surface and near-surface geological conditions a closed chemical system may not exist. The process of weathering provides for both the introduction and removal of material. The fact that the decay series of uranium and thorium, especially the former, proceed through a series of different phases, with different chemical and physical properties, makes it probable that under weathering conditions some chemical and physical dispersal of parent and daughter products will take place. This causes disequilibrium in the decay series, and the intermediate and end products will not be present at any one point in the proportions predicted by the laws of radioactive decay. Under these conditions a measurement of the abundance of a decay product (Gamma Survey) will not necessarily provide a reliable measurement of the abundance of the parent element.

Disequilibrium is also dependent on the sample volume studied. A small hand specimen is much more likely to show extreme disequilibrium than will a large bulk sample, or an in situ measurement on a large volume of material. The degree of disequilibrium is not easily established with direct field gamma ray measurements, although it can be determined in a number of ways in the laboratory, either by comparing chemical estimates with estimates based on decay product radioactivity, measuring the radioactivity of different decay products, or by measuring234 Pa gamma of 1.001 MeV of uranium group by High resolution HPGe detector.

In the general case, airborne surveys should precede ground surveys. Ground surveys are made primarily as a follow-up to airborne surveys in order to locate, identify and, if necessary, sample anomalous features found from the airborne survey.

The zone of influence obtained with a steadily held portable instrument is bowl-shaped, and its intercept with the terrain surface is a circle (‘circle of investigation’) - Over solid rock the 90% sample volume has a thickness of about 30cm just below the point o f detection. For the uranium window of a portable spectrometer, the radii of the 50% and the 90% circles of investigation are approximately given by R50 ~ 1.6 h and R90 ~ 6.9 h respectively where h is the elevation of the detector probe. The approximate proportionality between sample radius and detector elevation shows that a detector carried on the back produces several hundred times more areal coverage than a detector placed directly on the ground.

The area sampled by an airborne survey system is an oval strip whose length is somewhat greater than the moving distance of the aircraft during a counting period. The width of the strip increases with the survey altitude, but is not proportional to the latter because of the greater attenuation of gamma rays with Oblique angles of incidence. For the high-sensitivity spectrometer flown by AMD by fixed wing aircraft, using a terrain clearance o f ~ 120 metres, 50 per cent of the counts in the thorium window can be ascribed to a strip o f ~ 130 metres in width.

The calibration of the radiometric survey instruments is essential to access the potential of survey area with reliability. Laboratory gamma ray spectrometers are calibrated with IAEA reference materials. Atomic Minerals Directorate has constructed both fixed and transportable calibration facilities for the calibration of portable and airborne gamma ray spectrometers as per IAEA guidelines. Calibration facility has also been constructed for the bore hole logging system and shielded detector logging in AMD.

Thus, radiometric techniques are used in uranium exploration programme at all stage of prospecting, drilling for proving the extension of ore bodies and mining.