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Experiment - prospecting for uranium ore

Why

Because the rocks are pretty and because small pieces of radioactive minerals are useful for testing of radiation detectors.

Geological background

There are many areas where deposits of uranium ore can be found. In areas with significant deposits there are usually mines present, though many are already abandoned as the deposits are exhausted and mining is not economical anymore.

To get to the ore, and to get the ore out, large quantities of unwanted rocks have to be mined out and then dumped to nearby heaps. These heaps are readily accessible sources of pretty samples, either the ore itself or various accompanying minerals - as no sorting process is hundred-percent accurate, so something always slips through and then waits for an aspiring geologist.

One of the heaps is present near the town of Pribram, where one of the richest deposits was mined. The ore is present there as pitchblende, a black mineral occuring in veins in rocks or as nodular clusters on their surfaces, a mixed oxide of uranium.

In contrast to non-radioactive minerals, uranium ore samples are announcing their presence with a fairly intense and penetrating gamma radiation. Even pretty small rocks can be sensed through a (thinner) layer of other rocks, with a sufficiently sensitive Geiger counter or a scintillator crystal.

Equipment

A Geiger counter with a pancake probe was used as the detector. For easier manipulation, to avoid the need to crawl on the ground, a broomstick was borrowed from a corners-sweeping broom; its head was detachable and its angle was adjustable by a screw.

Two holes were drilled to the end of the broomstick. Corresponding holes were drilled to the Geiger's case and flat square M5 nuts were hot-glued to them. The counter was attached to the handle with a pair of M5 bolts.

A cable was connected to the headphones output of the counter, and led along the broomstick to its other end. A headset was connected there.

Search procedure

The background radiation on the waste heaps is significantly higher than the background with less radioactive composition. (Well, duh.) Despite that, the activity of pieces of ore, also called here "hot rocks", is higher by orders of magnitude. They act as point sources (actually area-sources, but in comparison with the area being checked their own area is negligible). A detector with large-area window, whether a pancake probe or a scintillator, is desirable.

An efficient way to locate the hot spots is slowly walking forward, sweeping the ground with slow, wide arcs, not unlike locating mines with a metal detector. The ore specimens will announce their presence under the detector with significantly increased ticking; even from some distance they can be heard by increased presence of characteristic close-coincidence double-ticks that sound like cracks.

Once the area with the specimen is closely located, within about 10x10 cm, the individual rock has to be isolated. That can be done either by putting the counter a bit to the side, so it is less affected by the hot spot, and taking the rocks one by one, moving them close to the counter's window. The tick rate increases rapidly when the hot one is held near it. The rock is then placed into its specimen container. A quick sweep over the combed-through area is repeated to make sure there was only one radiation source present there.

Another method for isolating the sample, suitable for spot with many smaller rocks, uses taking a handful of the rocks at a time and checking them all at once, throwing them away if none is active. When a handful (or a shovelful - just a bit, thin layer on the shovel's surface) contains a source and the detector sings a merry song in its vicinity, dividing to half and then half again and checking each half can quickly eliminate most of the dud rocks.

The aluminium shield over the probe's mica window was kept closed. Alpha radiation has too small range to be useful for larger-distance detection and the intensity difference of alpha+beta+gamma vs higher-energy beta+gamma when the shield is closed is not worth both the risk of damaging the expensive tube and the hassle with opening and closing the shield.

The large aperture of the detector used is beneficial for scanning wide areas, but detrimental for precise location of an individual rock. A method for finer locating of a nearby point source would require a shield to make the aperture much smaller.

Specimen handling

The rocks aren't all that dangerous, a short casual contact can be tolerated. (The miners spent much more time between much more rocks of purer composition, and the main hazard for them was the radon emitted by the rocks and its airborne decay products. This hazard can be eliminated by good ventilation of the mines and, for larger amount of stones, the sample storage area.) For prolonged storage, maintaining a distance between personnel and the specimens is advised; few meters is generally sufficient. In any case, do not store them under your bed.

The rocks as picked are quite dirty. Washing with a mild deterent, maybe with a very mild abrasive, using a soft brush, is suggested for getting clean surfaces for further specimen evaluation or processing. As the brush ends to spray around droplets of suspension of dirt in water, care is advised to not the droplets onto things like toothbrushes or glasses, or carefully wash them afterwards; uranium has significant chemical toxicity, comparable with lead.

As the prettier deposits with recognizable grape-like structures of uranium oxide globules are fairly fragile, wrapping them into a soft tissue paper or other protective medium for both transport and storage is strongly advised.

To provide a size reference, the specimens were photographed on graph paper with 5x5 mm grid.

TODO

Images


Rocks in their natural habitat

Rocks in their natural habitat

Rocks in their natural habitat

Searching for hot spots

Searching for hot spots

Searching for hot spots

Searching for hot spots

Isolating a hot rock

Searching for hot spots

Isolating a hot rock

Digging for a hot rock

Uranium ore samples group 1, top side

Uranium ore samples group 1, top side

Uranium ore samples group 1, bottom side

Uranium ore samples group 2, top side

Uranium ore samples group 2, top side

Uranium ore samples group 2, bottom side

Detail of pitchblende clusters

Detail of pitchblende clusters, small sample

Detail of a pitchblende vein

Detail of a pitchblende vein

Detail of a pitchblende vein

Detail of a pitchblende vein

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