“That’s what I’m saying to you. But those are the measurements.”

“But a situation like that leaves the nucleus over fifty neutrons short. You can’t have plutonium-186. You couldn’t squeeze ninety-four protons into one nucleus with only ninety-two neutrons and expect it to hang together for even a trillion-trillionth of a second.”

“That’s what I’m telling you, Doc,” said Tracy, patiently.

And then Hallam stopped to think. It was tungsten he was missing and one of its isotopes, tungsten-186, was stable. Tungsten-186 had 74 protons and 112 neutrons in its nucleus. Could something have turned twenty neutrons into twenty protons? Surely that was impossible.

“Are there any signs of radioactivity?” asked Hallam, groping somehow for a road out of the maze.

“I thought of that,” said the technician. “It’s stable. Absolutely stable.”

“Then it can’t be plutonium-186.”

“I keep telling you, Doc.”

Hallam said, hopelessly, “Well, give me the stuff.” Alone once more, he sat and looked at the bottle in stupefaction. The most nearly stable isotope of plutonium was plutonium-240, where 146 neutrons were needed to make the 94 protons stick together with some semblance of partial stability.

What could he do now? It was beyond him and he was sorry he had started. After all, he had real work begging to be done, and this thing—this mystery—had nothing to do with him. Tracy had made some stupid mistake or the mass spectrometer was out of whack, or— Well, what of it? Forget the whole thing! Except that Hallam couldn’t do that. Sooner or later, Denison would be bound to stop by and, with that irritating half-smile of his, ask after the tungsten. Then what could Hallam say? Could he say, “It isn’t tungsten, just as I told you.”

Surely Denison would ask, “Oh, and what is it, then?” and nothing imaginable could have made Hallam expose himself to the kind of derision that would follow any claim that it was plutonium-186. He had to find out what it was, and he had to do it himself. Clearly, he couldn’t trust anyone.