Greg visited on Wednesday 2 December, a bitterly cold, windy day in Chicago. Today for the first time the pile was supposed to go critical. Greg was there to observe the experiment on behalf of his boss, General Groves. He hinted jovially to anyone who asked that Groves feared an explosion and had deputed Greg to take the risk for him. In fact Greg had a more sinister mission. He was making an initial assessment of the scientists with a view to deciding who might be a security risk.

Security on the Manhattan Project was a nightmare. The top scientists were foreigners. Most of the rest were left-wingers, either Communists themselves or liberals who had Communist friends. If everyone suspicious was fired there would be hardly any scientists left. So Greg was trying to figure out which ones were the worst risks.

Enrico Fermi was about forty. A small, balding man with a long nose, he smiled engagingly while supervising this terrifying experiment. He was smartly dressed in a suit with a waistcoat. It was mid-morning when he ordered the trial to begin.

He instructed a technician to withdraw all but one of the control rods from the pile. Greg said: ‘What, all at once?’ It seemed frighteningly precipitate.

The scientist standing next to him, Barney McHugh, said: ‘We took it this far last night. It worked fine.’

‘I’m glad to hear it,’ said Greg.

McHugh, bearded and podgy, was low down on Greg’s list of suspects. He was American, with no interest in politics. The only black mark against him was a foreign wife: she was British – never a good sign, but not in itself evidence of treachery.

Greg had assumed there would be some sophisticated mechanism for moving the rods in and out, but it was simpler than that. The technician just put a ladder up against the pile, climbed halfway up it, and pulled out the rods by hand.

Speaking conversationally, McHugh said: ‘We were originally going to do this in the Argonne Forest.’

‘Where’s that?’

‘Twenty miles south-west of Chicago. Pretty isolated. Fewer casualties.’

Greg shivered. ‘So why did you change your minds and decide to do it right here on Fifty-seventh Street?’

‘The builders we hired went on strike, so we had to build the damn thing ourselves, and we couldn’t be that far away from the laboratories.’

‘So you took the risk of killing everyone in Chicago.’

‘We don’t think that will happen.’

Greg had not thought so, either, but he did not feel so sure now, standing a few feet away from the pile.

Fermi was checking his monitors against a forecast he had prepared of radiation levels at every stage of the experiment. Apparently the initial stage went according to plan, for he now ordered the last rod to be pulled halfway out.

There were some safety measures. A weighted rod hung poised to be dropped into the pile automatically if the radiation rose too high. In case that did not work, a similar rod was tied to the gallery railing with a rope, and a young physicist, looking as if he felt a bit silly, stood holding an axe, ready to cut the rope in an emergency. Finally three more scientists called the suicide squad were positioned near the ceiling, standing on the platform of the elevator used during construction, holding large jugs of cadmium sulphate solution, which they would throw on to the pile, as if dousing a bonfire.