Thallium (Tl)

Thallium announced itself as a colour. In March 1861 William Crookes was examining residue from a sulfuric acid works, hoping to find tellurium, and put a sample into a flame in front of a spectroscope. A single brilliant green line appeared where no known element had one. He named the element after it, from the Greek thallos, a green shoot — the fresh growth on a twig in spring.

Crookes did not do much with the discovery for a year. Claude-Auguste Lamy, in Lille, found the same green line in similar residue in 1862 and pushed further: he separated enough of the element to establish that it was a soft, heavy, silvery metal, and cast a small ingot of it.

Two medals for one element

The collision happened in public. At the 1862 International Exhibition in London, Lamy showed his ingot and the jury awarded him a medal for it. Crookes objected loudly that he had found the element and published first, and the organisers resolved it by issuing a second medal — Crookes for the discovery of the element, Lamy for producing it in quantity.

It is one of the tidier outcomes in the history of disputed discoveries, and it maps onto a real distinction. Crookes had a spectral line and a name. Lamy had a metal. The data card above carries Crookes, as the conventional credit does, and the fuller answer is that neither man's contribution would have been sufficient on its own.

Why the body cannot tell thallium from potassium

Thallium's singly charged ion is 1.5 ångströms across; potassium's is 1.4. They carry the same charge and behave, to the machinery of a living cell, as the same thing.

The consequence is that thallium is not merely absorbed but actively pumped. The sodium-potassium ATPase that every animal cell uses to maintain its ionic balance transports Tl⁺ in place of K⁺ and does so with roughly ten times the affinity, so thallium concentrates inside cells rather than being excreted. Once there it disrupts every potassium-dependent process, binds to sulfhydryl groups in mitochondrial enzymes, and interferes with riboflavin metabolism.

Thallium salts are also colourless, odourless, tasteless and freely soluble in water. The combination is what earned the element the nickname "inheritance powder", and it is why thallium's story is so heavily criminal.

The symptoms, and the novel that recognised them

Thallium poisoning is difficult to diagnose because its early course looks like a dozen commoner illnesses: nausea and vomiting, then over the following week an unusually painful peripheral neuropathy, characteristically burning pain in the soles of the feet. The distinguishing sign arrives late. Around two to three weeks in, the hair falls out.

Graham Frederick Young poisoned colleagues at a photographic instrument works in Hertfordshire in 1971, killing two, and was convicted the following year; he had a documented fascination with the element and had used it before, as a teenager, on his own family.

The most-cited episode runs the other way. Agatha Christie, who had worked in a hospital dispensary during both world wars, made thallium the murder weapon in The Pale Horse in 1961 and described the progression carefully, hair loss and all. In 1977 a nurse at Hammersmith Hospital named Marsha Maitland was caring for a nineteen-month-old child with a baffling illness, recognised the pattern from having read the novel, and suggested thallium to the medical team. The diagnosis was confirmed and the child survived. Prussian blue — ferric hexacyanoferrate — is the licensed antidote, and it works by trapping thallium in the gut so that the body stops reabsorbing what it has already excreted into the bile.

Thallium sulfate was sold as a household rodenticide and ant poison for decades, which put a lethal, tasteless, water-soluble salt under a great many kitchen sinks. The United States withdrew it from consumer sale in 1965 and banned all uses in 1972; most countries followed.

The poison is the tracer

Because thallium follows potassium into cells, an injected trace of radioactive thallium-201 goes where potassium goes — preferentially into heart muscle that is being perfused and is still alive. Cells that are ischaemic take up less; cells that are dead take up none.

That made thallium-201 the foundation of myocardial perfusion imaging for a generation. Its 73-hour half-life and low-energy emissions suit the job, and it has one property that its technetium-based successors lack: thallium redistributes. Over a few hours it washes out of healthy myocardium and continues to accumulate in tissue that is poorly perfused but viable, so a repeat image distinguishes muscle worth revascularising from scar. Technetium agents are brighter and have taken most of the routine work, but thallium's redistribution keeps it in use for viability assessment.

The crystal in nearly every gamma detector

Thallium's largest non-medical role is one almost nobody notices, because it is present at about a tenth of a per cent.

A sodium iodide crystal on its own is a poor scintillator. Doped with thallium, it becomes the workhorse gamma detector of the twentieth and twenty-first centuries: the thallium ions create luminescent centres that convert the energy of an absorbed gamma ray into a flash of visible light bright enough for a photomultiplier to count. NaI(Tl) is what sits inside handheld survey meters, borehole logging tools, portal monitors at ports and borders, and the gamma cameras in nuclear medicine departments. Cesium iodide doped the same way does similar work in CT detectors and particle physics.

Two other uses trade on thallium's heavy, polarisable ions:

• KRS-5, a mixed thallium bromide-iodide crystal, transmits infrared radiation out past 40 micrometres, far beyond the range of ordinary optical materials, and is used for windows and internal-reflection elements in spectroscopy.
• Thallium-barium-calcium-copper oxide held the record for the highest superconducting transition temperature — around 125 K — from 1988 until mercury-based cuprates overtook it in 1993.

A neutrino detector made of rock

Thallium has two stable isotopes, and their proportions vary enough between natural sources that the tables give its atomic weight as a range with two endpoints.

The heavier one supports a remarkable proposal. Thallium-205 can capture a low-energy neutrino and become lead-205, which is otherwise absent from nature. The mineral lorandite, a thallium arsenic sulfide, occurs in workable quantity at exactly one place on Earth — the Allchar deposit in North Macedonia — and it has been sitting there for around four million years. Any lead-205 in it is a record of the solar neutrino flux integrated over that entire period, which no live detector can provide, since every running experiment measures only the Sun as it is now. The LOREX proposal to read that record has been pursued on and off for four decades; the difficulty is measuring a handful of lead-205 atoms per gram against everything else in the rock.

No deposit anywhere is mined for thallium alone. What reaches the market is skimmed in small amounts off the flue dusts of pyrite roasting and off lead and zinc smelting, which is also how the element enters the environment — coal combustion and smelting are its main anthropogenic sources, and thallium contamination of soil and water downwind of such operations is a documented and persistent problem.