A Technological and Scientific Appendix to Postblogging Technology, November 1950: Little Neutrons

 

Between 1959 and 1969, old time science fiction writers Frederick Pohl and Jack Williamson collaborated on three loosely-connected novels set in Hoyle's steady state universe, eventually collected as The Starchild Trilogy. Here's a review that characterises them as bad books, but "odd . . . [and] offer[ing] a remarkable level of wacky fun." The conceit is that all of that hydrogen spontaneously appearing out of nothing in interstellar space is food for space-coral called "fusorians," which are the basis of a deep-space pyramid of life sustained by biologically-mediated nuclear fusion. By which I am probably being too kind to the amount of handwaving involved in Pohl and Willliamson's "science." Still, this was my first exposure to the steady state universe, and Pohl and Williamson incidentally put a lot more emphasis on the Hoyle/Chandrekesar theory's potential for explaining the synthesis of the heavy elements than most accounts of their cosmology do. The synthesis of the heavy elements is a bit of a problem in most cosmological models. It is ascribed to the more esoteric kinds of deep space collisions these days, which is certainly more likely than monocellular space-life synthesising plutonium out of spontaneously-generated protons, given that neutron stars actually exist. That being said, it might actually be more plausible than the notion that the universe's entire supply of heavy elements was produced in as many neutron star collisions as a fifteen-billion-year-old universe has had time for.

Time's coverage of Steady State cosmology  in the 23 November issue is a better jumping off point here than strange science fiction novels, but Steady State is, we now know, drawing dead. On the other hand, Bruno Pontecorvo's September 1950 defection, gets into the 6 November issue, brings in the neutrino. Not only that, we have the veiled announcement of the Savannah River Plant. It was at this just-opened nuclear reactor site that Clyde Cowan and Frederick Reines would confirm the existence of the elusive beast in the 1954 Cowan-Reines experiment, published in Science in July of 1956. 

Neutrinos are yet another of the arcane zoo of subatomic particles that we have to find room for in the standard model because otherwise the reactions we can observe, don't really make sense. In particular, once the Curies began measuring radioactivity, they identified "alpha, beta, and gamma radiation. Beta radiation turned out to be which turned out to be the spontaneous decay of neutrons, either bound in atomic nucleii or free-floating, and turned out to fail to conserve energy unless a new particle were postulated to fill out the decay equation: 

n0
 → 
p+
 + 
e−
 + 
ν
e

The theoretical properties of little  ν turned out to include having little to no mass, being electrically neutral, and unaffected by the strong force. I feel like subatomic particles are allowed to just decide which rules they're going to follow, but that's probably because I am old and uncomfortable with the freedoms of today's fundamental building blocs of matter. The point is that they're hard to find. Pontecorvo came up with a way of detecting them, at least in the dense neutrino fluxes produced by nuclear reactors and spent a few postwar years at Chalk River trying to spot one. His method didn't work out very well, because it turns out that nuclear reactors produce antineutrinos, not neutrinos, but was rehabilitated later to detect solar neutrinos, and work was discontinued shortly after Pontecorvo's departure for Harwell, where he worked until a security clearance downgrade had him set for an academic career at Liverpool, to which fate the Soviet Union was apparently preferable. 

Meanwhile, Cowen and Reines had been working at Los Alamos. Nuclear devices produce an even denser neutrino flux than nuclear reactors, and according to their own account they were trying to see neutrinos in atomic blasts when they were packed off to Hanford instead. It seems as though getting a look into the neutrino flux of an atomic blast might be more than merely academic  concern, especially given that there is now a flourishing field of neutrino astronomy that proposes to understand what is going on in the core of the Sun and in potentially incipient supernovas by looking at their neutrino emissions. 

My wild speculations aside, Cowan and Reines were soon moved from Hanford to the new, du Pont-managed Savannah River Site. This plant was the AEC's answer to the ever-escalating demand for plutonium and tritium for the atomic and hydrogen bomb programmes. Plutonium and tritium began to ship from Savannah in 1956, by which time the Cown-Reines experiments were complete. In other words, this work came very, very early in the history of the facility. The official history of SRS is extremely sparse, but on the other hand it isn't clear to me that it had to be anything particularly special for the experiment to work out. The full discussion over at Wikipedia notes that the SRS site had better shielding against cosmic radiation than the Hanford site, being 11m from the reactor and 12m underground. Physically the detection apparatus consisted of  two tanks with a total of about 200 liters of water with about 40 kg of dissolved cadmium dichloride. The water tanks were sandwiched between three scintillator layers which contained 110 five-inch (127 mm) photomultiplier tubes. The scintillators were supposed to light up when inverse beta decay

produced a spark when the flying electron from a neutrino colliding with a hydrogen atom bound in a water molecule, close enough to a free proton as makes no difference, hit a scintillator. Meanwhile, the free neutron might hit an atom of Cadmium-108, producing Cadmium-109m that would quickly decay into Cadmium-108 plus a gamma ray that would activate the scintillators. Co-occurrence of the two distinct scintillations to a statistically significant extent would indicate the existence of the neutrino.  Cadmium chloride sounds pretty exotic, and cadmium is widely used in nuclear applications due to its unusually high neutron capture cross-section (I think I have the jargon right.) That said, cadmium chloride was formerly widely used as areagent for the production of Cadmium Yellow, a paint pigment. So it is not as though it was hard to come by. 

Between access to the SRS, the experimental set up involved, and the amount of number crunching required to demonstrate that the event had occurred at all, I am comfortable with calling the Cowan-Reines experiment "big physics," and the fact that both men were working for Los Alamos at the time very strongly suggests that the experiment was believed to have national security implications. I assume that they have to do with  using neutrino emissions to get a better understanding of what was going on in the exploding bomb. This in turn suggests that the panic over Pontecorvo's defection was more than just another episode of Cold War hysteria. His work at Harwell was in nuclear reactor design with a special interest in the "slow" neutrons. I'm not an atomic physicist and cannot draw a line between low-energy neutrons and neutrino emissions, but it sure seems like there would be a connection given that neutrinos were conceived in the first place as a way of explaining the variable results of beta decay, including the fact that some emitted neutrons are fast (and useless), while others are slow and useful. Maybe one day the nuclear weapon designers will come clean, and we will know. When they do, I hope they will also publish results that overthrow the Standard Model and let humanity loose in the Reefs of Space. 

A man can dream, anyway.