When an alpha particle tears through a gas, it leaves behind not a continuous track but a chain of individual ionizations: tiny flashes where electrons are knocked off atoms. In a living cell, such flashes damage DNA, and the denser they are packed into a clump, the more severe the damage. An instrument that could count these clumps in a volume the size of DNA itself has remained a bulky laboratory monster. The new work takes the first step toward fitting such a counter into a box the size of a palm.
The detector is built around a plate of ordinary circuit board or acrylic, in which seven holes, each less than a millimeter in diameter, have been drilled. Above the plate, a weak electric field slowly drifts ions toward the holes, and inside each hole the field is sharply amplified. When an ion enters such a hole, it triggers an avalanche: a cascade of new charges that boosts the signal enough to be seen on an oscilloscope. In the experiment, alpha particles from an americium-241 source passed through propane at a pressure one hundred times lower than atmospheric. At such rarefaction, millimeter distances in the gas correspond to nanometer distances in water, so a hole 0.8 millimeters in diameter "sees" a volume that would occupy just eight nanometers in living tissue: roughly the width of three turns of the DNA helix.
The results showed that a single alpha particle indeed produces several pulses arriving almost simultaneously. For the acrylic plate, the average number of pulses reached nearly two per particle, and arrival times fell within fifteen microseconds: exactly the time ions need to travel from their birthplace to the holes. This means the comb collects ions not randomly, but from the same clump. However, the expected dozens of pulses did not appear: the plate caught only a small fraction of the ions, because many settled on the dielectric surface and charged it. The charging altered the electric field, and the instrument's sensitivity drifted from measurement to measurement.
Despite its modest efficiency, the work has for the first time shown that a multi-hole structure can extract ions from a single nanometer-sized volume. Until now, compact nanodosimeters did not exist at all: every instrument capable of measuring ionization cluster distributions took up entire tables and required complex electronics. Now there is a prototype that can be shrunk further, by adding independent readout from each hole and replacing propane with nitrogen so the film does not grow. If such instruments become stable enough, they could check beam quality in proton therapy and assess radiation hazards in space, right on the spot, without bulky setups.
The comb with widely spaced teeth does not yet comb out the entire clump, but it has already shown that each tooth can work on its own. The next step is to make the teeth independent and coat them with a material that does not fear charge. Then the ion counter will become not a laboratory curiosity, but a working tool in the hands of a physician or a radiation protection engineer.
At a propane pressure of 1 mbar, a hole with a diameter of 0.8 mm corresponds to a volume in water with a diameter of about 8.4 nm, which is roughly the width of three turns of DNA.