When a radiation particle flies through a cell, it leaves behind a chain of damage, stripping electrons from atoms. These stripped electrons gather into clumps, and the denser the clump, the more severe the damage to DNA. Until now, instruments that could resolve such clumps were huge and complex. New work shows how to make such an instrument small.

The plate has seven holes drilled in it, each smaller than a millimeter. Above the plate, a weak electric field slowly drifts ions toward the holes. Inside a hole, the field intensifies sharply, and an ion that enters triggers an avalanche of charges, making the signal strong enough to be seen on a screen. In the experiment, alpha particles passed through a rarefied propane gas. At such low pressure, a millimeter in the gas corresponds to a few nanometers in living tissue, so a hole "sees" a volume the size of three turns of DNA. The device did catch several pulses from a single particle, but so far only a small fraction of what was expected. Ions settled on the plate and charged it, causing the sensitivity to drift.

The problem is familiar to anyone who has combed hair in dry weather: the comb accumulates charge and starts to repel what it was supposed to gather. In the detector, a polymer film that forms from propane and settles on the surface causes similar trouble. But the main thing has been achieved: it has been shown that the holes can act as independent teeth of a comb. If they are made independent and the gas is replaced, the device will become stable. Then it could be used in hospitals to check proton therapy beams or in space to assess radiation hazards.